travel by time machine

Is Time Travel Possible?

We all travel in time! We travel one year in time between birthdays, for example. And we are all traveling in time at approximately the same speed: 1 second per second.

We typically experience time at one second per second. Credit: NASA/JPL-Caltech

NASA's space telescopes also give us a way to look back in time. Telescopes help us see stars and galaxies that are very far away . It takes a long time for the light from faraway galaxies to reach us. So, when we look into the sky with a telescope, we are seeing what those stars and galaxies looked like a very long time ago.

However, when we think of the phrase "time travel," we are usually thinking of traveling faster than 1 second per second. That kind of time travel sounds like something you'd only see in movies or science fiction books. Could it be real? Science says yes!

This image from the Hubble Space Telescope shows galaxies that are very far away as they existed a very long time ago. Credit: NASA, ESA and R. Thompson (Univ. Arizona)

How do we know that time travel is possible?

More than 100 years ago, a famous scientist named Albert Einstein came up with an idea about how time works. He called it relativity. This theory says that time and space are linked together. Einstein also said our universe has a speed limit: nothing can travel faster than the speed of light (186,000 miles per second).

Einstein's theory of relativity says that space and time are linked together. Credit: NASA/JPL-Caltech

What does this mean for time travel? Well, according to this theory, the faster you travel, the slower you experience time. Scientists have done some experiments to show that this is true.

For example, there was an experiment that used two clocks set to the exact same time. One clock stayed on Earth, while the other flew in an airplane (going in the same direction Earth rotates).

After the airplane flew around the world, scientists compared the two clocks. The clock on the fast-moving airplane was slightly behind the clock on the ground. So, the clock on the airplane was traveling slightly slower in time than 1 second per second.

Credit: NASA/JPL-Caltech

Can we use time travel in everyday life?

We can't use a time machine to travel hundreds of years into the past or future. That kind of time travel only happens in books and movies. But the math of time travel does affect the things we use every day.

For example, we use GPS satellites to help us figure out how to get to new places. (Check out our video about how GPS satellites work .) NASA scientists also use a high-accuracy version of GPS to keep track of where satellites are in space. But did you know that GPS relies on time-travel calculations to help you get around town?

GPS satellites orbit around Earth very quickly at about 8,700 miles (14,000 kilometers) per hour. This slows down GPS satellite clocks by a small fraction of a second (similar to the airplane example above).

GPS satellites orbit around Earth at about 8,700 miles (14,000 kilometers) per hour. Credit: GPS.gov

However, the satellites are also orbiting Earth about 12,550 miles (20,200 km) above the surface. This actually speeds up GPS satellite clocks by a slighter larger fraction of a second.

Here's how: Einstein's theory also says that gravity curves space and time, causing the passage of time to slow down. High up where the satellites orbit, Earth's gravity is much weaker. This causes the clocks on GPS satellites to run faster than clocks on the ground.

The combined result is that the clocks on GPS satellites experience time at a rate slightly faster than 1 second per second. Luckily, scientists can use math to correct these differences in time.

If scientists didn't correct the GPS clocks, there would be big problems. GPS satellites wouldn't be able to correctly calculate their position or yours. The errors would add up to a few miles each day, which is a big deal. GPS maps might think your home is nowhere near where it actually is!

In Summary:

Yes, time travel is indeed a real thing. But it's not quite what you've probably seen in the movies. Under certain conditions, it is possible to experience time passing at a different rate than 1 second per second. And there are important reasons why we need to understand this real-world form of time travel.

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Is time travel possible? Why one scientist says we 'cannot ignore the possibility.'

A common theme in science-fiction media , time travel is captivating. It’s defined by the late philosopher David Lewis in his essay “The Paradoxes of Time Travel” as “[involving] a discrepancy between time and space time. Any traveler departs and then arrives at his destination; the time elapsed from departure to arrival … is the duration of the journey.”

Time travel is usually understood by most as going back to a bygone era or jumping forward to a point far in the future . But how much of the idea is based in reality? Is it possible to travel through time? 

Is time travel possible?

According to NASA, time travel is possible , just not in the way you might expect. Albert Einstein’s theory of relativity says time and motion are relative to each other, and nothing can go faster than the speed of light , which is 186,000 miles per second. Time travel happens through what’s called “time dilation.”

Time dilation , according to Live Science, is how one’s perception of time is different to another's, depending on their motion or where they are. Hence, time being relative. 

Learn more: Best travel insurance

Dr. Ana Alonso-Serrano, a postdoctoral researcher at the Max Planck Institute for Gravitational Physics in Germany, explained the possibility of time travel and how researchers test theories. 

Space and time are not absolute values, Alonso-Serrano said. And what makes this all more complex is that you are able to carve space-time .

“In the moment that you carve the space-time, you can play with that curvature to make the time come in a circle and make a time machine,” Alonso-Serrano told USA TODAY. 

She explained how, theoretically, time travel is possible. The mathematics behind creating curvature of space-time are solid, but trying to re-create the strict physical conditions needed to prove these theories can be challenging. 

“The tricky point of that is if you can find a physical, realistic, way to do it,” she said. 

Alonso-Serrano said wormholes and warp drives are tools that are used to create this curvature. The matter needed to achieve curving space-time via a wormhole is exotic matter , which hasn’t been done successfully. Researchers don’t even know if this type of matter exists, she said.

“It's something that we work on because it's theoretically possible, and because it's a very nice way to test our theory, to look for possible paradoxes,” Alonso-Serrano added.

“I could not say that nothing is possible, but I cannot ignore the possibility,” she said. 

She also mentioned the anecdote of  Stephen Hawking’s Champagne party for time travelers . Hawking had a GPS-specific location for the party. He didn’t send out invites until the party had already happened, so only people who could travel to the past would be able to attend. No one showed up, and Hawking referred to this event as "experimental evidence" that time travel wasn't possible.

What did Albert Einstein invent?: Discoveries that changed the world

Just Curious for more? We've got you covered

USA TODAY is exploring the questions you and others ask every day. From "How to watch the Marvel movies in order" to "Why is Pluto not a planet?" to "What to do if your dog eats weed?" – we're striving to find answers to the most common questions you ask every day. Head to our Just Curious section to see what else we can answer for you. 

Time travel could be possible, but only with parallel timelines

Assistant Professor, Physics, Brock University

Disclosure statement

Barak Shoshany does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.

Brock University provides funding as a member of The Conversation CA-FR.

Brock University provides funding as a member of The Conversation CA.

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Have you ever made a mistake that you wish you could undo? Correcting past mistakes is one of the reasons we find the concept of time travel so fascinating. As often portrayed in science fiction, with a time machine, nothing is permanent anymore — you can always go back and change it. But is time travel really possible in our universe , or is it just science fiction?

Read more: Curious Kids: is time travel possible for humans?

Our modern understanding of time and causality comes from general relativity . Theoretical physicist Albert Einstein’s theory combines space and time into a single entity — “spacetime” — and provides a remarkably intricate explanation of how they both work, at a level unmatched by any other established theory. This theory has existed for more than 100 years, and has been experimentally verified to extremely high precision, so physicists are fairly certain it provides an accurate description of the causal structure of our universe.

For decades, physicists have been trying to use general relativity to figure out if time travel is possible . It turns out that you can write down equations that describe time travel and are fully compatible and consistent with relativity. But physics is not mathematics, and equations are meaningless if they do not correspond to anything in reality.

Arguments against time travel

There are two main issues which make us think these equations may be unrealistic. The first issue is a practical one: building a time machine seems to require exotic matter , which is matter with negative energy. All the matter we see in our daily lives has positive energy — matter with negative energy is not something you can just find lying around. From quantum mechanics, we know that such matter can theoretically be created, but in too small quantities and for too short times .

However, there is no proof that it is impossible to create exotic matter in sufficient quantities. Furthermore, other equations may be discovered that allow time travel without requiring exotic matter. Therefore, this issue may just be a limitation of our current technology or understanding of quantum mechanics.

The other main issue is less practical, but more significant: it is the observation that time travel seems to contradict logic, in the form of time travel paradoxes . There are several types of such paradoxes, but the most problematic are consistency paradoxes .

A popular trope in science fiction, consistency paradoxes happen whenever there is a certain event that leads to changing the past, but the change itself prevents this event from happening in the first place.

For example, consider a scenario where I enter my time machine, use it to go back in time five minutes, and destroy the machine as soon as I get to the past. Now that I destroyed the time machine, it would be impossible for me to use it five minutes later.

But if I cannot use the time machine, then I cannot go back in time and destroy it. Therefore, it is not destroyed, so I can go back in time and destroy it. In other words, the time machine is destroyed if and only if it is not destroyed. Since it cannot be both destroyed and not destroyed simultaneously, this scenario is inconsistent and paradoxical.

Eliminating the paradoxes

There’s a common misconception in science fiction that paradoxes can be “created.” Time travellers are usually warned not to make significant changes to the past and to avoid meeting their past selves for this exact reason. Examples of this may be found in many time travel movies, such as the Back to the Future trilogy.

But in physics, a paradox is not an event that can actually happen — it is a purely theoretical concept that points towards an inconsistency in the theory itself. In other words, consistency paradoxes don’t merely imply time travel is a dangerous endeavour, they imply it simply cannot be possible.

This was one of the motivations for theoretical physicist Stephen Hawking to formulate his chronology protection conjecture , which states that time travel should be impossible. However, this conjecture so far remains unproven. Furthermore, the universe would be a much more interesting place if instead of eliminating time travel due to paradoxes, we could just eliminate the paradoxes themselves.

One attempt at resolving time travel paradoxes is theoretical physicist Igor Dmitriyevich Novikov’s self-consistency conjecture , which essentially states that you can travel to the past, but you cannot change it.

According to Novikov, if I tried to destroy my time machine five minutes in the past, I would find that it is impossible to do so. The laws of physics would somehow conspire to preserve consistency.

Introducing multiple histories

But what’s the point of going back in time if you cannot change the past? My recent work, together with my students Jacob Hauser and Jared Wogan, shows that there are time travel paradoxes that Novikov’s conjecture cannot resolve. This takes us back to square one, since if even just one paradox cannot be eliminated, time travel remains logically impossible.

So, is this the final nail in the coffin of time travel? Not quite. We showed that allowing for multiple histories (or in more familiar terms, parallel timelines) can resolve the paradoxes that Novikov’s conjecture cannot. In fact, it can resolve any paradox you throw at it.

The idea is very simple. When I exit the time machine, I exit into a different timeline. In that timeline, I can do whatever I want, including destroying the time machine, without changing anything in the original timeline I came from. Since I cannot destroy the time machine in the original timeline, which is the one I actually used to travel back in time, there is no paradox.

After working on time travel paradoxes for the last three years , I have become increasingly convinced that time travel could be possible, but only if our universe can allow multiple histories to coexist. So, can it?

Quantum mechanics certainly seems to imply so, at least if you subscribe to Everett’s “many-worlds” interpretation , where one history can “split” into multiple histories, one for each possible measurement outcome – for example, whether Schrödinger’s cat is alive or dead, or whether or not I arrived in the past.

But these are just speculations. My students and I are currently working on finding a concrete theory of time travel with multiple histories that is fully compatible with general relativity. Of course, even if we manage to find such a theory, this would not be sufficient to prove that time travel is possible, but it would at least mean that time travel is not ruled out by consistency paradoxes.

Time travel and parallel timelines almost always go hand-in-hand in science fiction, but now we have proof that they must go hand-in-hand in real science as well. General relativity and quantum mechanics tell us that time travel might be possible, but if it is, then multiple histories must also be possible.

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Time Machines

Recent years have seen a growing consensus in the philosophical community that the grandfather paradox and similar logical puzzles do not preclude the possibility of time travel scenarios that utilize spacetimes containing closed timelike curves. At the same time, physicists, who for half a century acknowledged that the general theory of relativity is compatible with such spacetimes, have intensely studied the question whether the operation of a time machine would be admissible in the context of the same theory and of its quantum cousins. A time machine is a device which brings about closed timelike curves—and thus enables time travel—where none would have existed otherwise. The physics literature contains various no-go theorems for time machines, i.e., theorems which purport to establish that, under physically plausible assumptions, the operation of a time machine is impossible. We conclude that for the time being there exists no conclusive no-go theorem against time machines. The character of the material covered in this article makes it inevitable that its content is of a rather technical nature. We contend, however, that philosophers should nevertheless be interested in this literature for at least two reasons. First, the topic of time machines leads to a number of interesting foundations issues in classical and quantum theories of gravity; and second, philosophers can contribute to the topic by clarifying what it means for a device to count as a time machine, by relating the debate to other concerns such as Penrose’s cosmic censorship conjecture and the fate of determinism in general relativity theory, and by eliminating a number of confusions regarding the status of the paradoxes of time travel. The present article addresses these ambitions in as non-technical a manner as possible, and the reader is referred to the relevant physics literature for details.

• 1. Introduction: time travel vs. time machines

2. What is a (Thornian) time machine? Preliminaries

3. when can a would-be time machine be held responsible for the emergence of ctcs, 4. no-go results for (thornian) time machines in classical general relativity theory, 5. no-go results in quantum gravity, 6. conclusion, other internet resources, related entries, 1. introduction: time travel vs. time machine.

The topic of time machines is the subject of a sizable and growing physics literature, some of which has filtered down to popular and semi-popular presentations. [ 1 ] The issues raised by this topic are largely oblique, if not orthogonal, to those treated in the philosophical literature on time travel. [ 2 ] Most significantly, the so-called paradoxes of time travel do not play a substantial role in the physics literature on time machines. This literature equates the possibility of time travel with the existence of closed timelike curves (CTCs) or worldlines for material particles that are smooth, future-directed timelike curves with self-intersections. [ 3 ] Since time machines designate devices which bring about the existence of CTCs and thus enable time travel, the paradoxes of time travel are irrelevant for attempted “no-go” results for time machines because these results concern what happens before the emergence of CTCs. [ 4 ] This, in our opinion, is fortunate since the paradoxes of time travel are nothing more than a crude way of bringing out the fact that the application of familiar local laws of relativistic physics to a spacetime background which contains CTCs typically requires that consistency constraints on initial data must be met in order for a local solution of the laws to be extendable to a global solution. The nature and status of these constraints is the subject of ongoing discussion. We will not try to advance the discussion of this issue here; [ 5 ] rather, our aim is to acquaint the reader with the issues addressed in the physics literature on time machines and to connect them with issues in the philosophy of space and time and, more generally, with issues in the foundations of physics.

Paradox mongers can be reassured in that if a paradox is lost in shifting the focus from time travel itself to time machines, then a paradox is also gained: if it is possible to operate a time machine device that produces CTCs, then it is possible to alter the structure of spacetime such that determinism fails; but by undercutting determinism, the time machine undercuts the claim that it is responsible for producing CTCs. But just as the grandfather paradox is a crude way of making a point, so this new paradox is a crude way of indicating that it is going to be difficult to specify what it means to be a time machine. This is a task that calls not for paradox mongering but for scientifically informed philosophizing. The present article will provide the initial steps of this task and will indicate what remains to be done. But aside from paradoxes, the main payoff of the topic of time machines is that it provides a quick route to the heart of a number of foundations problems in classical general relativity theory and in attempts to produce a quantum theory of gravity by combining general relativity and quantum mechanics. We will indicate the shape of some of these problems here, but will refer the interested reader elsewhere for technical details.

There are at least two distinct general notions of time machines, which we will call Wellsian and Thornian for short. In The Time Machine , H. G. Wells (1931) described what has become science fiction’s paradigmatic conception of a time machine: the intrepid operator fastens her seat belt, dials the target date—past or future—into the counter, throws a lever, and sits back while time rewinds or fast forwards until the target date is reached. We will not broach the issue of whether or not a Wellsian time machine can be implemented within a relativistic spacetime framework. For, as will soon become clear, the time machines which have recently come into prominence in the physics literature are of an utterly different kind. This second kind of time machine was originally proposed by Kip Thorne and his collaborators (see Morris and Thorne 1988; Morris, Thorne, and Yurtsever 1988). These articles considered the possibility that, without violating the laws of general relativistic physics, an advanced civilization might manipulate concentrations of matter-energy so as to produce CTCs where none would have existed otherwise. In their example, the production of “wormholes” was used to generate the required spacetime structure. But this is only one of the ways in which a time machine might operate, and in what follows any device which affects the spacetime structure in such a way that CTCs result will be dubbed a Thornian time machine . We will only be concerned with this variety of time machine, leaving the Wellsian variety to science fiction writers. This will disappoint the aficionados of science fiction since Thornian time machines do not have the magical ability to transport the would-be time traveler to the past of the events that constitute the operation of the time machine. For those more interested in science than in science fiction, this loss is balanced by the gain in realism and the connection to contemporary research in physics.

In Sections 2 and 3 we investigate the circumstances under which it is plausible to see a Thornian time machine at work. The main difficulty lies in specifying the conditions needed to make sense of the notion that the time machine “produces” or is “responsible for” the appearances of CTCs. We argue that at present there is no satisfactory resolution of this difficulty and, thus, that the topic of time machines in a general relativistic setting is somewhat ill-defined. This fact does not prevent progress from being made on the topic; for if one’s aim is to establish no-go results for time machines it suffices to identify necessary conditions for the operation of a time machine and then to prove, under suitable hypotheses about what is physically possible, that it is not physically possible to satisfy said necessary conditions. In Section 4 we review various no-go results which depend only on classical general relativity theory. Section 5 surveys results that appeal to quantum effects. Conclusions are presented in Section 6.

The setting for the discussion is a general relativistic spacetime \((\mathcal{M},g_{ab})\) where \(\mathcal{M}\) is a differentiable manifold and \(g_{ab}\) is a Lorentz signature metric defined on all of \(\mathcal{M}\). The central issue addressed in the physics literature on time machines is whether in this general setting it is physically possible to operate a Thornian time machine. This issue is to be settled by proving theorems about the solutions to the equations that represent what are taken to be physical laws operating in the general relativistic setting—or at least this is so once the notion of a Thornian time machine has been explicated. Unfortunately, no adequate and generally accepted explication that lends itself to the required mathematical proofs is to be found in the literature. This is neither surprising nor deplorable. Mathematical physicists do not wait until some concept has received its final explication before trying to prove theorems about it; indeed, the process of theorem proving is often an essential part of conceptual clarification. The moral is well illustrated by the history of the concept of a spacetime singularity in general relativity where this concept received its now canonical definition only in the process of proving the Penrose-Hawking-Geroch singularity theorems, which came at the end of a decades long dispute over the issue of whether spacetime singularities are a generic feature of solutions to Einstein’s gravitational field equations. [ 6 ] However, this is not to say that philosophers interested in time machines should simply wait until the dust has settled in the physics literature; indeed, the physics literature could benefit from deployment of the analytical skills that are the stock in trade of philosophy. For example, the paradoxes of time travel and the fate of time machines are not infrequently confused in the physics literature, and as will become evident below, subtler confusions abound as well.

The question of whether a Thornian time machine—a device that produces CTCs—can be seen to be at work only makes sense if the spacetime has at least three features: temporal orientability, a definite time orientation, and a causally innocuous past. In order to make the notion of a CTC meaningful, the spacetime must be temporally orientable (i.e., must admit a consistent time directionality), and one of the two possible time orientations has to be singled out as giving the direction of time. [ 7 ] Non-temporal orientability is not really an obstacle since if a given general relativistic spacetime is not temporally orientable, a spacetime that is everywhere locally the same as the given spacetime and is itself temporally orientable can be obtained by passing to a covering spacetime. [ 8 ] How to justify the singling out of one of the two possible orientations as future pointing requires a solution to the problem of the direction of time, a problem which is still subject to lively debate (see Callender 2001). But for present purposes we simply assume that a temporal orientation has been provided. A CTC is then (by definition) a parameterized closed spacetime curve whose tangent is everywhere a future-pointing timelike vector. A CTC can be thought of as the world line of some possible observer whose life history is linearly ordered in the small but not in the large: the observer has a consistent experience of the “next moment,” and the “next,” etc., but eventually the “next moment” brings her back to whatever event she regards as the starting point.

As for the third condition—a causally innocuous past—the question of the possibility of operating a device that produces CTCs presupposes that there is a time before which no CTCs existed. Thus, Gödel spacetime, so beloved of the time travel literature, is not a candidate for hosting a Thornian time machine since through every point in this spacetime there is a CTC. We make this third condition precise by requiring that the spacetime admits a global time slice \(\Sigma\) (i.e., a spacelike hypersurface without edges); [ 9 ] that \(\Sigma\) is two-sided and partitions \(\mathcal{M}\) into three parts—\(\Sigma\) itself, the part of \(\mathcal{M}\) on the past side of \(\Sigma\) and the part of \(\mathcal{M}\) on the future side of \(\Sigma\)—and that there are no CTCs that lie on the past side of \(\Sigma\). The first two clauses of this requirement together entail that the time slice \(\Sigma\) is a partial Cauchy surface , i.e., \(\Sigma\) is a time slice that is not intersected more than once by any future-directed timelike curve. [ 10 ]

Now suppose that the state on a partial Cauchy surface \(\Sigma_0\) with no CTCs to its past is to be thought of as giving a snapshot of the universe at a moment before the machine is turned on. The subsequent realization of a Thornian time machine scenario requires that the chronology violating region \(V \subseteq \mathcal{M}\), the region of spacetime traced out by CTCs, [ 11 ] is non-null and lies to the future of \(\Sigma_0\). The fact that \(V \ne \varnothing\) does not lead to any consistency constraints on initial data on \(\Sigma_0\) since, by hypothesis, \(\Sigma_0\) is not intersected more than once by any timelike curve, and thus, insofar as the so-called paradoxes of time travel are concerned with such constraints, the paradoxes do not arise with respect to \(\Sigma_0\). But by the same token, the option of traveling back into the past of \(\Sigma_0\) is ruled out by the set up as it has been sketched so far, since otherwise \(\Sigma_0\) would not be a partial Cauchy surface. This just goes to underscore the point made above that the fans of science fiction stories of time machines will not find the present context of discussion broad enough to encompass their vision of how time machines should operate; they may now stop reading this article and return to their novels.

Figure 1. Misner spacetime

As a concrete example of these concepts, consider the \((1 + 1)\)-dimensional Misner spacetime (see Figure 1 ) which exhibits some of the causal features of Taub-NUT spacetime, a vacuum solution to Einstein’s gravitational field equations. It satisfies all three of the conditions discussed above. It is temporally orientable, and a time orientation has been singled out—the shading in the figure indicates the future lobes of the light cones. To the past of the partial Cauchy surface \(\Sigma_0\) lies the Taub region where the causal structure of spacetime is as bland as can be desired. But to the future of \(\Sigma_0\) the light cones begin to “tip over,” and eventually the tipping results in CTCs in the NUT region.

The issue that must be faced now is what further conditions must be imposed in order that the appearance of CTCs to the future of \(\Sigma_0\) can be attributed to the operation of a time machine. Not surprisingly, the answer depends not just on the structure of the spacetime at issue but also on the physical laws that govern the evolution of the spacetime structure. If one adopts the attitude that the label “time machine” is to be reserved for devices that operate within a finite spatial range for a finite stretch of time, then one will want to impose requirements to assure that what happens in a compact region of spacetime lying on or to the future of \(\Sigma_0\) is responsible for the CTCs. Or one could be more liberal and allow the would-be time machine to be spread over an infinite space. We will adopt the more liberal stance since it avoids various complications while still sufficing to elicit key points. Again, one could reserve the label “time machine” for devices that manipulate concentrations of mass-energy in some specified ways. For example, based on Gödel spacetime—where matter is everywhere rotating and a CTC passes through every spacetime point—one might conjecture that setting into sufficiently rapid rotation a finite mass concentration of appropriate shape will eventuate in CTCs. But with the goal in mind of proving negative general results, it is better to proceed in a more abstract fashion. Think of the conditions on the partial Cauchy surface \(\Sigma_0\) as encoding the instructions for the operation of the time machine. The details of the operation of the device—whether it operates in a finite region of spacetime, whether it operates by setting matter into rotation, etc.—can be left to the side. What must be addressed, however, is whether the processes that evolve from the state on \(\Sigma_0\) can be deemed to be responsible for the subsequent appearance of CTCs.

The most obvious move is to construe “responsible for” in the sense of causal determinism. But in the present setting this move quickly runs into a dead end. For if CTCs exist to the future of \(\Sigma_0\) they are not causally determined by the state on \(\Sigma_0\) since the time travel region \(V\), if it is non-null, lies outside the future domain of dependence \(D^+ (\Sigma_0)\) of \(\Sigma_0\), the portion of spacetime where the field equations of relativistic physics uniquely determine the state of things from the state on \(\Sigma_0\). [ 12 ] The point is illustrated by the toy model of Figure 1 . The surface labeled \(H^+ (\Sigma_0)\) is called the future Cauchy horizon of \(\Sigma_0\). It is the future boundary of \(D^+ (\Sigma_0)\), [ 13 ] and it separates the portion of spacetime where conditions are causally determined by the state on \(\Sigma_0\) from the portion where conditions are not so determined. And, as advertised, the CTCs in the model of Figure 1 lie beyond \(H^+ (\Sigma_0)\).

Figure 2. Deutsch-Politzer spacetime

Thus, if the operation of a Thornian time machine is to be a live possibility, some condition weaker than causal determinism must be used to capture the sense in which the state on \(\Sigma_0\) can be deemed to be responsible for the subsequent development of CTCs. Given the failure of causal determinism, it seems the next best thing to demand that the region \(V\) is “adjacent” to the future domain of dependence \(D^+ (\Sigma_0)\). Here is an initial stab at such an adjacency condition. Consider causal curves which have a future endpoint in the time travel region \(V\) and no past endpoint. Such a curve may never leave \(V\); but if it does, require that it intersects \(\Sigma_0\). But this requirement is too strong because it rules out Thornian time machines altogether. For a curve of the type in question to reach \(\Sigma_0\) it must intersect \(H^+ (\Sigma_0)\), but once it reaches \(H^+ (\Sigma_0)\) it can be continued endlessly into the past without meeting \(\Sigma_0\) because the generators of \(H^+ (\Sigma_0)\) are past endless null geodesics that never meet \(\Sigma_0\). [ 14 ] This difficulty can be overcome by weakening the requirement at issue by rephrasing it in terms of timelike curves rather than causal curves. Now the set of candidate time machine spacetimes satisfying the weakened requirement is non-empty—as illustrated, once again, by the spacetime of Figure 1 . But the weakened requirement is too weak, as illustrated by the \((1 + 1)\)-dimensional version of Deutsch-Politzer spacetime [ 15 ] (see Figure 2 ), which is constructed from two-dimensional Minkowski spacetime by deleting the points \(p_1\)–\(p_4\) and then gluing together the strips as shown. Every past endless timelike curve that emerges from the time travel region \(V\) of Deutsch-Politzer spacetime does meet \(\Sigma_0\). But this spacetime is not a plausible candidate for a time machine spacetime. Up to and including the time \(\Sigma_0\) (which can be placed as close to \(V\) as desired) this spacetime is identical with empty Minkowski spacetime. If the state of the corresponding portion of Minkowski spacetime is not responsible for the development of CTCs—and it certainly is not since there are no CTCs in Minkowski spacetime—how can the state on the portion of Deutsch-Politzer spacetime up to and including the time \(\Sigma_0\) be held responsible for the CTCs that appear in the future?

The deletion of the points \(p_1\)–\(p_4\) means that the Deutsch-Politzer spacetime is singular in the sense that it is geodesically incomplete . [ 16 ] It would be too drastic to require of a time-machine hosting spacetime that it be geodesically complete. And in any case the offending feature of Deutsch-Politzer can be gotten rid of by the following trick. Multiplying the flat Lorentzian metric \(\eta_{ab}\) of Deutsch-Politzer spacetime by a scalar function \(j(x, t) \gt\) produces a new metric \(\eta '_{ab} :=\) j \(\eta_{ab}\) which is conformal to the original metric and, thus, has exactly the same causal features as the original metric. But if the conformal factor \(j\) is chosen to “blow up” as the missing points \(p_1\)–\(p_4\) are approached, the resulting spacetime is geodesically complete—intuitively, the singularities have been pushed off to infinity.

A more subtle way to exclude Deutsch-Politzer spacetime focuses on the generators of \(H^+ (\Sigma_0)\). The stipulations laid down so far for Thornian time machines imply that the generators of \(H^+ (\Sigma_0)\) cannot intersect \(\Sigma_0\). But in addition it can be required that these generators do not “emerge from a singularity” and do not “come from infinity,” and this would suffice to rule out Deutsch-Politzer spacetime and its conformal cousins as legitimate candidates for time machine spacetimes. More precisely, we can impose what Stephen Hawking (1992a,b) calls the requirement that \(H^+ (\Sigma_0)\) be compactly generated ; namely, the past endless null geodesics that generate \(H^+ (\Sigma_0)\) must, if extended far enough into past, fall into and remain in a compact subset of spacetime. Obviously the spacetime of Figure 1 fulfills Hawking’s requirement—since in this case \(H^+ (\Sigma_0)\) is itself compact—but just as obviously the spacetime of Figure 2 (conformally doctored or not) does not.

Imposing the requirement of a compactly generated future Cauchy horizon has not only the negative virtue of excluding some unsuited candidate time machine spacetimes but a positive virtue as well. It is easily proved that if \(H^+ (\Sigma_0)\) is compactly generated then the condition of strong causality is violated on \(H^+ (\Sigma_0)\), which means, intuitively, there are almost closed causal curves near \(H^+ (\Sigma_0)\). [ 17 ] This violation can be taken as an indication that the seeds of CTCs have been planted on \(\Sigma_0\) and that by the time \(H^+ (\Sigma_0)\) is reached they are ready to bloom.

However, we still have no guarantee that if CTCs do bloom to the future of \(\Sigma_0\), then the state on \(\Sigma_0\) is responsible for the blooming. Of course, we have already learned that we cannot have the iron clad guarantee of causal determinism that the state on \(\Sigma_0\) is responsible for the actual blooming in all of its particularity. But we might hope for a guarantee that the state on \(\Sigma_0\) is responsible for the blooming of some CTCs—the actual ones or others. The difference takes a bit of explaining. The failure of causal determinism is aptly pictured by the image of a future “branching” of world histories, with the different branches representing different alternative possible futures of (the domain of dependence of) \(\Sigma_0\) that are compatible with the actual past and the laws of physics. And so it is in the present setting: if \(H^+ (\Sigma_0) \ne \varnothing\), then there will generally be different ways to extend \(D^+ (\Sigma_0)\), all compatible with the laws of general relativistic physics. But if CTCs are present in all of these extensions, even through the details of the CTCs may vary from one extension to another, then the state on \(\Sigma_0\) can rightly be deemed to be responsible for the fact that subsequently CTCs did develop.

A theorem due to Krasnikov (2002, 2003 [Other Internet Resources], 2014a) might seem to demonstrate that no relativistic spacetime can count as embodying a Thornian time machine so understood. Following Krasnikov, let us say that a spacetime condition \(C\) is local just in case, for any open covering \(\{V_{\alpha}\}\) of an arbitrary spacetime \((\mathcal{M}, g_{ab}), C\) holds in \((\mathcal{M}, g_{ab})\) iff it holds in \((V_{\alpha}, g_{ab}|_{V_{\alpha}})\) for all \(\alpha\). Examples of local conditions one might want to impose on physically reasonable spacetimes are Einstein’s gravitational field equations and so-called energy conditions that restrict the form of the stress-energy tensor \(T_{ab}\). An example of the latter that will come into play below is the weak energy condition that says that the energy density is non-negative. [ 18 ] Einstein’s field equations (sans cosmological constant) require that \(T_{ab}\) is proportional to the Einstein tensor which is a functional of the metric and its derivatives. Call a \(C\)-spacetime \((\mathcal{M}', g'_{ab})\) a \(C\)- extension of a \(C\)-spacetime \((\mathcal{M}, g_{ab})\) spacetime if the latter is isometric to an open proper subset of the former; and call \((\mathcal{M}, g_{ab}) C\)- extensible if it admits a \(C\)-extension and \(C\)- maximal otherwise. (Of course, \(C\) might be the empty condition.) Krasnikov’s theorem shows that every \(C\)-spacetime \((\mathcal{M}, g_{ab})\) admits a \(C\)-maximal extension \((\mathcal{M}^{max}, g^{max}_{ab})\) such that all CTCs in \((\mathcal{M}^{max}, g^{max}_{ab})\) are to the chronological past of the image of \(\mathcal{M}\) in \((\mathcal{M}^{max}, g^{max}_{ab})\). So start with some candidate spacetime \((\mathcal{M}, g_{ab})\) for a Thornian time machine, and apply the theorem to \((D^+ (\Sigma_0), g_{ab}|_{D^+ (\Sigma_0)})\). Conclude that no matter what local conditions the candidate spacetime is required to satisfy, \(D^+ (\Sigma_0)\) has extensions that also satisfies said local conditions but does not contain CTCs to the future of \(\Sigma_0\). Thus, the candidate spacetime fails to exhibit the crucial feature identified above necessary for underwriting the contention that the conditions on \(\Sigma_0\) are responsible for the development of CTCs. Hence, it appears as if Krasnikov’s theorem effectively prohibits time machines.

The would-be time machine operator need not capitulate in the face of Krasnikov’s theorem. Recall that the main difficulty in specifying the conditions for the successful operation of Thornian time machines traces to the fact that the standard form of causal determinism does not apply to the production of CTCs. But causal determinism can fail for reasons that have nothing to do with CTCs or other acausal features of relativistic spacetimes, and it seems only fair to ensure that these modes of failure have been removed before proceeding to discuss the prospects for time machines. To zero in on the modes of failure at issue, consider vacuum solutions \((T_{ab} \equiv 0)\) to Einstein’s field equations. Let \((\mathcal{M}, g_{ab})\) and \((\mathcal{M}', g'_{ab})\) be two such solutions, and let \(\Sigma \subset \mathcal{M}\) and \(\Sigma ' \subset \mathcal{M}'\) be spacelike hypersurfaces of their respective spacetimes. Suppose that there is an isometry \(\Psi\) from some neighborhood \(N(\Sigma)\) of \(\Sigma\) onto a neighborhood \(N'(\Sigma ')\) of \(\Sigma '\). Does it follow, as we would want determinism to guarantee, that \(\Psi\) is extendible to an isometry from \(D^+ (\Sigma)\) onto \(D^+ (\Sigma ')\)? To see why the answer is negative, start with any solution \((\mathcal{M}, g_{ab})\) of the vacuum Einstein equations, and cut out a closed set of points lying to the future of \(N(\Sigma)\) and in \(D^+ (\Sigma)\). Denote the surgically altered manifold by \(\mathcal{M}^*\) and the restriction of \(g_{ab}\) to \(\mathcal{M}^*\) by \(g^*_{ab}\). Then \((\mathcal{M}^*, g^*_{ab})\) is also a solution of the vacuum Einstein equations. But obviously the pair of solutions \((\mathcal{M}, g_{ab})\) and \((\mathcal{M}^*, g^*_{ab})\) violates the condition that determinism was supposed to guarantee as \(\Psi\) is not extendible to an isometry from \(D^+ (\Sigma)\) onto \(D^+ (\Sigma^*)\). It might seem that the requirement, contemplated above, that the spacetimes under consideration be maximal, already rules out spacetimes that have “holes” in them. But while maximality does rule out the surgically mutilated spacetime just constructed, it does not guarantee hole freeness in the sense needed to make sure that determinism does not stumble before it gets to the starting gate. That \((\mathcal{M}, g_{ab})\) is hole free in the relevant sense requires that if \(\Sigma \subset \mathcal{M}\) is a spacelike hypersurface, there does not exist a spacetime \((\mathcal{M}', g'_{ab})\) and an isometric embedding \(\Phi\) of \(D^+ (\Sigma)\) into \(\mathcal{M}'\) such that \(\Phi(D^+ (\Sigma))\) is a proper subset of \(D^+ (\Phi(\Sigma))\). A theorem due to Robert Geroch (1977, 87), who is responsible for this definition, asserts that if \(\Sigma \subset \mathcal{M}\) and \(\Sigma ' \subset \mathcal{M}'\) are spacelike hypersurfaces in hole-free spacetimes \((\mathcal{M}, g_{ab})\) and \((\mathcal{M}', g'_{ab})\), respectively, and if there exists an isometry \(\Psi : \mathcal{M} \rightarrow \mathcal{M}'\), then \(\Psi\) is indeed extendible to an isometry between \(D^+ (\Sigma)\) and \(D^+ (\Sigma ')\). Thus, hole freeness precludes an important mode of failure of determinism which we wish to exclude in our discussion of time machines. It can be shown that hole freeness is not entailed by maximality. [ 19 ] And it is just this gap that gives the would-be time machine operator some hope, for the maximal CTC-free extensions produced by Krasnikov’s construction are not always hole free (Manchak 2009b). But Krasnikov (2009) has shown that the Geroch (1977) definition is too strong: Minkowski spacetime fails to satisfy it! For this reason, alternative formulations of the hole-freeness definition have been constructed which are more appropriate (Manchak 2009a, Minguzzi 2012).

Thus, we propose that one clear sense of what it would mean for a Thornian time machine to operate in the setting of general relativity theory is given by the following assertion: the laws of general relativistic physics allow solutions containing a partial Cauchy surface \(\Sigma_0\) such that no CTCs lie to the past of \(\Sigma_0\) but every extension of \(D^+ (\Sigma_0)\) satisfying ________ contains CTCs (where the blank is filled with some “no hole” condition). Correspondingly, a proof of the physical impossibility of time machines would take the form of showing that this assertion is false for the actual laws of physics, consisting, presumably, of Einstein’s field equations plus energy conditions and, perhaps, some additional restrictions as well. And a proof of the emptiness of the associated concept of a Thornian time machine would take the form of showing that the assertion is false independently of the details of the laws of physics, as long as they take the form of local conditions on \(T_{ab}\) and \(g_{ab}\).

Are there "no hole" conditions which show the proposed concept of a time machine is not empty? Let \(J^+(p)\) designate the causal future of \(p\), defined as the set of all points in \(\mathcal{M}\) which can be reached from \(p\) by a future-directed causal curve in \(\mathcal{M}\). The causal past \(J^-(p)\) is defined analogously. Now, we say a spacetime \((\mathcal{M},g_{ab})\) is J closed if, for each \(p\) in \(\mathcal{M}\), the sets \(J^+(p)\) and \(J^-(p)\) are topologically closed. One can verify that J closedness fails in many artificially mutilated examples (e.g. Minkowski spacetime with one point removed from the manifold). For some time, it was thought that a time machine existed under this no-hole condition (Manchak 2011a). But this turns out to be incorrect; indeed a recent result shows that any J closed spacetime \((\mathcal{M},g_{ab})\) of three dimensions or more with chronology violating region \(V \neq \mathcal{M}\) must be strongly causal and therefore fail to have CTCs (Hounnonkpe and Minguzzi 2019). Stepping back, perhaps there are other no-hole conditions which can be used instead to show that the proposed concept of a time machine is not empty. But even if such a project were successful, Manchak (2014a, 2019) has shown that the time machine existence results can be naturally reinterpreted as “hole machine” existence results if one is so inclined. Instead of assuming that spacetime is free of holes and then showing that certain initial conditions are responsible for the production of CTCs, one could just as well start with the assumption of no CTCs and then show that certain initial conditions are responsible for the production of holes. Given the importance of these no hole assumptions to the time machine advocate, much recent work has focused on whether such assumptions are physically reasonable in some sense (Manchak 2011b, 2014b). This is still an open question.

Another open question is whether physically more realistic spacetimes than Misner also permit the operation of time machines and how generic time-machine spacetimes are in particular spacetime theories, such as general relativity. If time-machine spacetimes turn out to be highly non-generic, the fan of time machines can retreat to a weaker concept of Thornian time machine by taking a page from probabilistic accounts of causation, the idea being that a time machine can be seen to be at work if its operation increases the probability of the appearance of CTCs. Since general relativity theory itself is innocent of probabilities, they have to be introduced by hand, either by inserting them into the models of the theory, i.e., by modifying the theory at the level of the object-language, or by defining measures on sets of models, i.e., by modifying the theory at the level of the meta-language. Since the former would change the character of the theory, only the latter will be considered. The project for making sense of the notion that a time machine as a probabilistic cause of the appearance of CTCs would then take the following form. First define a normalized measure on the set of models having a partial Cauchy surface to the past of which there are no CTCs. Then show that the subset of models that have CTCs to the future of the partial Cauchy surface has non-zero measure. Next, identify a range of conditions on or near the partial Cauchy surface that are naturally construed as settings of a device that is a would-be probabilistic cause of CTCs, and show that the subset of models satisfying these conditions has non-zero measure. Finally, show that conditionalizing on the latter subset increases the measure of the former subset. Assuming that this formal exercise can be successfully carried out, there remains the task of justifying these as measures of objective chance. This task is especially daunting in the cosmological setting since neither of the leading interpretations of objective chance seems applicable. The frequency interpretation is strained since the development of CTCs may be a non-repeated phenomenon; and the propensity interpretation is equally strained since, barring just-so stories about the Creator throwing darts at the Cosmic Dart Board, there is no chance mechanism for producing cosmological models.

We conclude that, even apart from general doubts about a probabilistic account of causation, the resort to a probabilistic conception of time machines is a desperate stretch, at least in the context of classical general relativity theory. In a quantum theory of gravity, a probabilistic conception of time machines may be appropriate if the theory itself provides the transition probabilities between the relevant states. But an evaluation of this prospect must wait until the theory of quantum gravity is available.

In order to appreciate the physics literature aimed at proving no-go results for time machines it is helpful to view these efforts as part of the broader project of proving chronology protections theorems , which in turn is part of a still larger project of proving cosmic censorship theorems . To explain, we start with cosmic censorship and work backwards.

Figure 3. A bad choice of initial value surface

For sake of simplicity, concentrate on the initial value problem for vacuum solutions \((T_{ab} \equiv 0)\) to Einstein’s field equations. Start with a three-manifold \(\Sigma\) equipped with quantities which, when \(\Sigma\) is embedded as a spacelike submanifold of spacetime, become initial data for the vacuum field equations. Corresponding to the initial data there exists a unique [ 20 ] maximal development \((\mathcal{M}, g_{ab})\) for which (the image of the embedded) \(\Sigma\) is a Cauchy surface. [ 21 ] This solution, however, may not be maximal simpliciter, i.e., it may be possible to isometrically embed it as a proper part of a larger spacetime, which itself may be a vacuum solution to the field equations; if so \(\Sigma\) will not be a Cauchy surface for the extended spacetime, which fails to be a globally hyperbolic spacetime. [ 22 ] This situation can arise because of a poor choice of initial value hypersurface, as illustrated in Figure 3 by taking \(\Sigma\) to be the indicated spacelike hyperboloid of \((1 + 1)\)-dimensional Minkowski spacetime. But, more interestingly, the situation can arise because the Einstein equations allow various pathologies, collectively referred to as “naked singularities,” to develop from regular initial data. The strong form of Penrose’s celebrated cosmic censorship conjecture proposes that, consistent with Einstein’s field equations, such pathologies do not arise under physically reasonable conditions or else that the conditions leading to the pathologies are highly non-generic within the space of all solutions to the field equations. A small amount of progress has been made on stating and proving precise versions of this conjecture. [ 23 ]

One way in which strong cosmic censorship can be violated is through the emergence of acausal features. Returning to the example of Misner spacetime ( Figure 1 ), the spacetime up to \(H^+ (\Sigma_0)\) is the unique maximal development of the vacuum Einstein equations for which \(\Sigma_0\) is a Cauchy surface. But this development is extendible, and in the extension illustrated in Figure 1 global hyperbolicity of the development is lost because of the presence of CTCs. The chronology protection conjecture then can be construed as a subconjecture of the cosmic censorship conjecture, saying, roughly, that consistent with Einstein field equations, CTCs do not arise under physically reasonable conditions or else that the conditions are highly non-generic within the space of all solutions to the field equations. No-go results for time machines are then special forms of chronology protection theorems that deal with cases where the CTCs are manufactured by time machines. In the other direction, a very general chronology protection theorem will automatically provide a no-go result for time machines, however that notion is understood, and a theorem establishing strong cosmic censorship will automatically impose chronology protection.

The most widely discussed chronology protection theorem/no-go result for time machines in the context of classical general relativity theory is due to Hawking (1992a). Before stating the result, note first that, independently of the Einstein field equations and energy conditions, a partial Cauchy surface \(\Sigma\) must be compact if its future Cauchy horizon \(H^+ (\Sigma)\) is compact (see Hawking 1992a and Chrusciel and Isenberg 1993). However, it is geometrically allowed that \(\Sigma\) is non-compact if \(H^+ (\Sigma)\) is required only to be compactly generated rather than compact. But what Hawking showed is that this geometrical possibility is ruled out by imposing Einstein’s field equations and the weak energy condition. Thus, if \(\Sigma_0\) is a partial Cauchy surface representing the situation just before or just as the would-be Thornian time machine is switched on, and if a necessary condition for seeing a Thornian time machine at work is that \(H^+ (\Sigma_0)\) is compactly generated, then consistently with Einstein’s field equations and the weak energy condition, a Thornian time machine cannot operate in a spatially open universe since \(\Sigma_0\) must be compact.

This no-go result does not touch the situation illustrated in Figure 1 . Taub-NUT spacetime is a vacuum solution to Einstein’s field equations so the weak energy condition is automatically satisfied, and \(H^+ (\Sigma_0)\) is compact and, a fortiori, compactly generated. Hawking’s theorem is not contradicted since \(\Sigma_0\) is compact. By the same token the theorem does not speak to the possibility of operating a Thornian time machine in a spatially closed universe. To help fill the gap, Hawking proved that when \(\Sigma_0\) is compact and \(H^+ (\Sigma_0)\) is compactly generated, the Einstein field equations and the weak energy condition together guarantee that both the convergence and shear of the null geodesic generators of \(H^+ (\Sigma_0)\) must vanish, which he interpreted to imply that no observers can cross over \(H^+ (\Sigma_0)\) to enter the chronology violating region \(V\). But rather than showing that it is physically impossible to operate a Thornian time machine in a closed universe, this result shows only that, given the correctness of Hawking’s interpretation, the observers who operate the time machine cannot take advantage of the CTCs it produces.

There are two sources of doubt about the effectiveness of Hawking’s no-go result even for open universes. The first stems from possible violations of the weak energy condition by stress-energy tensors arising from classical relativistic matter fields (see Vollick 1997 and Visser and Barcelo 2000). [ 24 ] The second stems from the fact that Hawking’s theorem functions as a chronology protection theorem only by way of serving as a potential no-go result for Thornian time machines since the crucial condition that \(H^+ (\Sigma_0)\) is compactly generated is supposedly justified by being a necessary condition for the operation of such machine. But in retrospect, the motivation for this condition seems frayed. As argued in the previous section, if the Einstein field equations and energy conditions entail that all hole free extensions of \(D^+ (\Sigma_0)\) contain CTCs, then it is plausible to see a Thornian time machine at work, quite regardless ofwhether or not \(H^+ (\Sigma_0)\) is compactly generated or not. Of course, it remains to establish the existence of cases where this entailment holds. If it should turn out that there are no such cases, then the prospects of Thornian time machines are dealt a severe blow, but the reasons are independent of Hawking’s theorem. On the other hand, if such cases do exist then our conjecture would be that they exist even when some of the generators of \(H^+ (\Sigma_0)\) come from singularities or infinity and, thus, \(H^+ (\Sigma_0)\) is not compactly generated. [ 25 ]

Three degrees of quantum involvement in gravity can be distinguished. The first degree, referred to as quantum field theory on curved spacetimes, simply takes off the shelf a spacetime provided by general relativity theory and then proceeds to study the behavior of quantum fields on this background spacetime. The Unruh effect, which predicts the thermalization of a free scalar quantum field near the horizon of a black hole, lies within this ambit. The second degree of involvement, referred to as semi-classical quantum gravity, attempts to calculate the backreaction of the quantum fields on spacetime metric by computing the expectation value \(\langle \Psi \mid T_{ab} \mid \Psi \rangle\) of the stress-energy tensor in some appropriate quantum state \(\lvert\Psi\rangle\) and then inserting the value into Einstein’s field equations in place of \(T_{ab}\). [ 26 ] Hawking’s celebrated prediction of black hole radiation belongs to this ambit. [ 27 ] The third degree of involvement attempts to produce a genuine quantum theory of gravity in the sense that the gravitational degrees of freedom are quantized. Currently loop quantum gravity and string theory are the main research programs aimed at this goal. [ 28 ]

The first degree of quantum involvement, if not opening the door to Thornian time machines, at least seemed to remove some obstacles since quantum fields are known to lead to violations of the energy conditions used in the setting of classical general relativity theory to prove chronology protection theorems and no-go results for time machines. However, the second degree of quantum involvement seemed, at least initially, to slam the door shut. The intuitive idea was this. Start with a general relativistic spacetime where CTCs develop to the future of \(H^+ (\Sigma)\) (often referred to as the “chronology horizon”) for some suitable partial Cauchy surface \(\Sigma\). Find that the propagation of a quantum field on this spacetime background is such that \(\langle \Psi \mid T_{ab} \mid\Psi \rangle\) “blows up” as \(H^+ (\Sigma)\) is approached from the past. Conclude that the backreaction on the spacetime metric creates unbounded curvature, which effectively cuts off the future development that would otherwise eventuate in CTCs. These intuitions were partly vindicated by detailed calculations in several models. But eventually a number of exceptions were found in which the backreaction remains arbitrarily small near \(H^+ (\Sigma)\). [ 29 ] This seemed to leave the door ajar for Thornian time machines.

But fortunes were reversed once again by a result of Kay, Radzikowski, and Wald (1997). The details of their theorem are too technical to review here, but the structure of the argument is easy to grasp. The naïve calculation of \(\langle \Psi \mid T_{ab}\mid\Psi \rangle\) results in infinities which have to be subtracted off to produce a renormalized expectation value \(\langle \Psi \mid T_{ab}\mid\Psi \rangle_R\) with a finite value. The standard renormalization procedure uses a limiting procedure that is mathematically well-defined if, and only if, a certain condition obtains. [ 30 ] The KRW theorem shows that this condition is violated for points on \(H^+ (\Sigma)\) and, thus, that the expectation value of the stress-energy tensor is not well-defined at the chronology horizon.

While the KRW theorem is undoubtedly of fundamental importance for semi-classical quantum gravity, it does not serve as an effective no-go result for Thornian time machines. In the first place, the theorem assumes, in concert with Hawking’s chronology protection theorem, that \(H^+ (\Sigma)\) is compactly generated, and we repeat that it is far from clear that this assumption is necessary for seeing a Thornian time machine in operation. A second, and more fundamental, reservation applies even if a compactly generated \(H^+ (\Sigma)\) is accepted as a necessary condition for time machines. The KRW theorem functions as a no-go result by providing a reductio ad absurdum with a dubious absurdity: roughly, if you try to operate a Thornian time machine, you will end up invalidating semi-classical quantum gravity. But semi-classical quantum gravity was never viewed as anything more than a stepping stone to a genuine quantum theory of gravity, and its breakdown is expected to be manifested when Planck-scale physics comes into play. This worry is underscored by Visser’s (1997, 2003) findings that in chronology violating models trans-Planckian physics can be expected to come into play before \(H^+ (\Sigma)\) is reached.

It thus seems that if some quantum mechanism is to serve as the basis for chronology protection, it must be found in the third degree of quantum involvement in gravity. Both loop quantum gravity and string theory have demonstrated the ability to cure some of the curvature singularities of classical general relativity theory. But as far as we are aware there are no demonstrations that either of these approaches to quantum gravity can get rid of the acausal features exhibited in various solutions to Einstein’s field equations. An alternative approach to formulate a fully-fledged quantum theory of gravity attempts to capture the Planck-scale structure of spacetime by constructing it from causal sets. [ 31 ] Since these sets must be acyclic, i.e., no element in a causal set can causally precede itself, CTCs are ruled out a priori. Actually, a theorem due to Malament (1977) suggests that any Planck-scale approach encoding only the causal structure of a spacetime cannot permit CTCs either in the smooth classical spacetimes or a corresponding phenomenon in their quantum counterparts. [ 32 ]

In sum, the existing no-go results that use the first two degrees of quantum involvement are not very convincing, and the third degree of involvement is not mature enough to allow useful pronouncements. There is, however, a rapidly growing literature on the possibility of time travel in lower-dimensional supersymmetric cousins of string theory. For a review of these recent results and a discussion of the fate of a time-traveller’s ambition in loop quantum gravity, see Smeenk and Wüthrich (2010).

Hawking opined that “[i]t seems there is a chronology protection agency, which prevents the appearance of closed timelike curves and so makes the universe safe for historians” (1992a, 603). He may be right, but to date there are no convincing arguments that such an Agency is housed in either classical general relativity theory or in semi-classical quantum gravity. And it is too early to tell whether this Agency is housed in loop quantum gravity or string theory. But even if it should turn out that Hawking is wrong in that the laws of physics do not support a Chronology Protection Agency, it could still be the case that the laws support an Anti-Time Machine Agency. For it could turn out that while the laws do not prevent the development of CTCs, they also do not make it possible to attribute the appearance of CTCs to the workings of any would-be time machine. We argued that a strong presumption in favor of the latter would be created in classical general relativity theory by the demonstration that for any model satisfying Einstein’s field equations and energy conditions as well as possessing a partial Cauchy surface \(\Sigma_0\) to the future of which there are CTCs, there are hole free extensions of \(D^+ (\Sigma_0)\) satisfying Einstein’s field equations and energy conditions but containing no CTCs to the future of \(\Sigma_0\). There are no doubt alternative approaches to understanding what it means for a device to be “responsible for” the development of CTCs. Exploring these alternatives is one place that philosophers can hope to make a contribution to an ongoing discussion that, to date, has been carried mainly by the physics community. Participating in this discussion means that philosophers have to forsake the activity of logical gymnastics with the paradoxes of time travel for the more arduous but (we believe) rewarding activity of digging into the foundations of physics.

Time machines may never see daylight, and perhaps so for principled reasons that stem from basic physical laws. But even if mathematical theorems in the various theories concerned succeed in establishing the impossibility of time machines, understanding why time machines cannot be constructed will shed light on central problems in the foundations of physics. As we have argued in Section 4, for instance, the hunt for time machines in general relativity theory should be interpreted as a core issue in studying the fortunes of Penrose’s cosmic censorship conjecture. This conjecture arguably constitutes the most important open problem in general relativity theory. Similarly, as discussed in Section 5, mathematical theorems related to various aspects of time machines offer results relevant for the search of a quantum theory of gravity. In sum, studying the possibilities for operating a time machine turns out to be not a scientifically peripheral or frivolous weekend activity but a useful way of probing the foundations of classical and quantum theories of gravity.

• Arntzenius, F. and T. Maudlin, 2009, “Time Travel and Modern Physics,” in E.N. Zalta (ed.), The Stanford Encyclopedia of Philosophy (Spring 2009 Edition), URL = < https://plato.stanford.edu/archives/spr2009/entries/time-travel-phys/ >.
• Brightwell, G., H.F. Dowker, R.S. Garcia, J. Henson, and R.D. Sorkin, 2003, “‘Observables’ in causal set cosmology,” Physical Review D , 67: 08403. [ Preprint available online .]
• Callender, C., 2001, “Thermodynamic Asymmetry in Time,” in E.N. Zalta (ed.), The Stanford Encyclopedia of Philosophy (Spring 2001 Edition), URL = < https://plato.stanford.edu/archives/win2001/entries/time-thermo/ >.
• Chrusciel, P.T. and J. Isenberg, 1993, “Compact Cauchy Horizons and Cauchy Surfaces,” in B.L. Hu and T.A. Jacobson (eds.), Papers in Honor of Dieter Brill: Directions in General Relativity (Volume 2), Cambridge: Cambridge University Press, pp. 97–107.
• Davies, P., 2002a, How to Build a Time Machine , London: Viking Penguin.
• –––, 2002b, “How to Build a Time Machine,” Scientific American , 287(3): 50–55.
• Deutsch, D., 1991, “Quantum Mechanics Near Closed Timelike Lines,” Physical Review D , 44: 3197–3217.
• Earman, J., 1995a, Bangs, Crunches, Whimpers, and Shrieks: Singularities and Acausalities in Relativistic Spacetimes , New York: Oxford University Press.
• –––, 1995b, “Recent Work on Time Travel,” in S.F. Savitt (ed.), Time’s Arrow Today: Recent Physical and Philosophical Work on the Direction of Time , Cambridge: Cambridge University Press, pp. 268–310.
• –––, 1995c, “Outlawing Time Machines: Chronology Protection Theorems,” Erkenntnis , 42: 125–139.
• –––, 1999, “The Penrose-Hawking Singularity Theorems: History and Implications,” in H. Goenner, J. Renn, and T. Sauer (eds.), The Expanding Worlds of General Relativity , Einstein Studies, Vol. 7, Boston: Birkhäuser, pp. 235–267.
• Earman, J., C. Smeenk, and C. Wüthrich, 2009, “Do the Laws of Physics Forbid the Operation of Time Machines?” Synthese , 169: 91–124. [ Preprint available online ]
• Geroch, R., 1977, “Prediction in General Relativity,” in J. Earman, C. Glymour, and J. Stachel (eds.), Foundations of Spacetime Theories (Minnesota Studies in the Philosophy of Science: Volume VIII), Minneapolis, MN: University of Minnesota Press, pp. 81–93.
• Gott, R., 2001, Time Travel in Einstein’s Universe , New York: Houghton Mifflin.
• Greene, B., 2003, The Elegant Universe , New York: W. W. Norton.
• Hawking, S.W., 1992a, “Chronology Protection Conjecture,” Physical Review D , 46: 603–611.
• –––, 1992b, “The Chronology Protection Conjecture,” in H. Sato and T. Nakamura (eds.), Proceedings of the Sixth Marcel Grossmann Meeting on General Relativity , Singapore: World Scientific, pp. 3–13.
• –––, 2001, “Chronology Protection: Making the World Safe for Historians,” in S.W. Hawking et al. (eds.), The Future of Spacetime , New York: W.W. Norton, pp. 87–108.
• Hawking, S.W. and G.F.R. Ellis, 1973, The Large Scale Structure of Space-Time , Cambridge: Cambridge University Press.
• Hawking, S.W. and R. Penrose, 1970, “The Singularities of Gravitational Collapse and Cosmology,” Proceedings of the Royal Society of London A, 314: 529–548.
• Hoefer, C., 2003, “Causal Determinism,” in E.N. Zalta (ed.), The Stanford Encyclopedia of Philosophy (Spring 2003 Edition), URL = < https://plato.stanford.edu/archives/spr2003/entries/determinism-causal/ >.
• Hounnonkpe, R.A. and E. Minguzzi, 2019, “Globally Hyperbolic Spacetimes Can Be Defined Without the ‘Causal’ Condition,” Classical and Quantum Gravity , 36: 197001.
• Kay, B.S., M.J. Radzikowski, and R.M. Wald, 1997, “Quantum Field Theory On Spacetimes with Compactly Generated Cauchy Horizons,” Communications in Mathematical Physics , 183: 533–556.
• Keller, S. and M. Nelson, 2001, “Presentists Should Believe in Time-Travel,” Australasian Journal of Philosophy , 79: 333–345.
• Krasnikov, S., 1999, “Time Machines with Non-Compactly Generated Cauchy Horizons and ‘Handy Singularities’,” in T. Piran and R. Ruffini (eds.), Proceedings of the Eighth Marcel Grossmann Meeting on General Relativity , Singapore: World Scientific, pp. 593–595. [ Preprint available online .]
• –––, 2002, “No Time Machines in Classical General Relativity,” Classical and Quantum Gravity , 19: 4109–4129. [ Preprint available online .]
• –––, 2009, “Even the Minkowski space is holed.” Physical Review D , 79: 124041.
• –––, 2014a, “Corrigendum: No Time machines in Classical General Relativity,” Classical and Quantum Gravity , 31: 079503.
• –––, 2014b, “Time Machines with the Compactly Determined Cauchy Horizon,” Physical Review D , 90: 024067. [ Preprint available online .]
• Malament, D.B., 1977, “The class of continuous timelike curves determines the topology of spacetime,” Journal of Mathematical Physics , 18: 1399–1404.
• Manchak, J.B., 2009a, “Is Spacetime Hole-Free?’ General Relativity and Gravitation , 41: 1639–1643
• –––, 2009b, “On the Existence of ‘Time Machines,” in General Relativity,” Philosophy of Science , 76: 1020–1026.
• –––, 2011a, “No no-go: A remark on time machines,” Studies in History and Philosophy of Modern Physics , 42: 74–76.
• –––, 2011b, “What is a Physically Reasonable Space-Time?” Philosophy of Science , 78: 410–420.
• –––, 2014a, “Time (hole?) machines,” Studies in History and Philosophy of Modern Physics , 48: 124–127.
• –––, 2014b, “On Space-Time Singularities, Holes, and Extensions,” Philosophy of Science , 81: 1066–1076.
• –––, 2019, “A Remark on ‘Time Machines’ in Honor of Howard Stein,” Studies in History and Philosophy of Modern Physics , 67: 111–116.
• Minguzzi, E., 2012, “Causally Simple Inextendible Spacetimes are Hole-Free,” Journal of Mathematical Physics , 53: 062501.
• Monton, B., 2003, “Presentists Can Believe in Closed Timelike Curves,” Analysis , 63: 199–202.
• Morris, M.S. and K.S. Thorne, 1988, “Wormholes in Spacetime and Their Use for Interstellar Travel: A Tool for Teaching General Relativity,” American Journal of Physics , 56: 395-412.
• Morris, M.S., K.S. Thorne, and U. Yurtsever, 1988, “Wormholes, Time Machines, and the Weak Energy Condition,” Physical Review Letters , 61: 1446-1449.
• Nahin, P.J., 1999, Time Machines: Time Travel in Physics, Metaphysics, and Science Fiction , New York: AIP Press, Springer.
• Norton, J., 2008, “The Hole Argument,” in E.N. Zalta (ed.), The Stanford Encyclopedia of Philosophy (Spring 2008 Edition), URL = < https://plato.stanford.edu/archives/spr2008/entries/spacetime-holearg/ >.
• Ori, A., 1993, “Must Time-Machine Construction Violate the Weak Energy Condition?” Physical Review Letters , 71: 2517-2520.
• Politzer, H.D., 1992, “Simple Quantum Systems in Spacetimes with Closed Timelike Curves,” Physical Review D , 46: 4470–4476.
• Rovelli, C., 2004, Quantum Gravity , Cambridge: Cambridge University Press.
• Smeenk, C. and C. Wüthrich, 2010, “Time Travel and Time Machines”, forthcoming in C. Callender (ed.), The Oxford Handbook of Time , Oxford: Oxford University Press.
• Visser, M., 1997, “The Reliability Horizon for Semi-Classical Quantum Gravity: Metric Fluctuations Are Often More Important than Back-Reaction,” Physics Letters B , 115: 8-14.
• –––, 2003, “The Quantum Physics of Chronology Protection,” in G.W. Gibbons, E.P.S. Shellard, S.J. Rankin (eds.), The Future of Theoretical Physics and Cosmology: Celebrating Stephen Hawking’s 60th Birthday , Cambridge: Cambridge University Press, pp. 161–176.
• Visser, M. and C. Barcelo, 2000, “Energy conditions and their cosmological implications,” in U. Cotti, R. Jeannerot, G. Senjanović, and A. Smirnov (eds.), Proceedings of the Third International Workshop on Particle Physics and the Early Universe (COSMO-99), Singapore: World Scientific, pp. 99–112. [ Preprint available online .]
• Vollick, D.N., 1997, “How to Produce Exotic Matter Using Classical Fields,” Physical Review D , 56: 4720–4723.
• Wald, R.M., 1984, General Relativity , Chicago: University of Chicago Press.
• –––, 1994, Quantum Field Theory in Curved Spacetime and Black Hole Thermodynamics , Chicago: University of Chicago Press.
• –––, 1998, “Gravitational Collapse and Cosmic Censorship,” in B.R. Iyer, and B. Bhawal (eds.), Black Holes, Gravitational Radiation and the Universe: Essays in Honor of C. V. Vishveshwara , Dordrecht: Kluwer Academic Publishers, pp. 69–85. [ Preprint available online .]
• Wells, H.G., 1931, The Time Machine , New York: Random House.
• Zwiebach, B., 2004, A First Course on String Theory , Cambridge: Cambridge University Press.

How to cite this entry . Preview the PDF version of this entry at the Friends of the SEP Society . Look up topics and thinkers related to this entry at the Internet Philosophy Ontology Project (InPhO). Enhanced bibliography for this entry at PhilPapers , with links to its database.

• Krasnikov, S., 2003, “ Time Machine (1988-2001) ,” a brief account of the time machine problem; talk given at 11th U.K. Conference on the Foundations of Physics, Oxford, England, 9-13, Sept. 2002.
• Rovelli, C., 2008, “ Loop Quantum Gravity ”, in Living Reviews in Relativity .

determinism: causal | space and time: the hole argument | time: thermodynamic asymmetry in | time travel | time travel: and modern physics

Acknowledgments

We thank Carlo Rovelli for discussions and John Norton for comments on an earlier draft. C.W. acknowledges support by the Swiss National Science Foundation (grant PBSK1-102693).

Copyright © 2020 by John Earman < jearman @ pitt . edu > Christian Wüthrich < christian . wuthrich @ unige . ch > JB Manchak < jmanchak @ uci . edu >

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Can We Travel Through Time to the Past?

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Traveling Into the Past

Black holes and wormholes, causality and alternate realities, wormhole warnings, so, is time travel to the past really possible.

• Ph.D., Physics and Astronomy, Purdue University
• B.S., Physics, Purdue University

Going back in time to visit an earlier era is a fantastic dream. It's a staple of SF and fantasy novels, movies, and TV shows. Who wouldn't like to go back and see the dinosaurs or watch the birth of the universe or meet their great-great grandparents? What could possibly go wrong Could someone travel to a previous era to right a wrong, make a different decision, or even completely alter the course of history? Has it happened? Is it even possible?

There are a lot of questions about travel into the past, but not very many solutions. The best answer science can give us right now is: it's theoretically possible. But, no one has done it. 

It turns out that people time travel all the time, but only in one direction: from the past to the present and moving into the future . Unfortunately, no one has any control over how quickly that time passes and nobody can stop time and continue to live. It seems that time is a one-way street, always moving forward.

This is all right and proper. It also fits with Einstein's theory of relativity because time only flows in one direction—forward. If time flowed the other way, people would remember the future instead of the past. That sounds very counter-intuitive. So, on the face of it, traveling into the past seems to be a violation of the laws of physics.

But not so fast! It turns out that there are theoretical considerations to take into account if somebody wants to build a time machine that goes back to the past. They involve exotic gateways called wormholes, or some science fictional-sounding creation of gateways using a technology not yet available to science. 

The idea of building a time machine, like those often depicted in science fiction films, is likely the stuff of dreams. Unlike the traveler in H.G. Wells's Time Machine, no one has figured out how to build a special carriage that goes from now to yesterday. However, astrophysics gives us one possible pathway: one could possibly harness the power of a black hole to venture through time and space. How would that work?

According to general relativity , a rotating black hole could create a wormhole —a theoretical link between two points of space-time, or perhaps even two points in different universes. However, there's a problem with black holes. They've long been thought to be unstable and therefore un-traversable. However, recent advances in physics theory have shown that these constructs could, in fact, provide a means of traveling through time. Unfortunately, we have almost no idea what to expect by doing so.

Theoretical physics is still trying to predict what would happen inside the wormhole, assuming one could even approach such a place. More to the point, there's no current engineering solution that would allow us to build a craft that would let make that trip safely. Right now, as it stands, once a ship enters the black hole, it's going to get crushed by incredible gravity. The ship, and everyone aboard are made one with the singularity at the heart of the black hole.

But, for the sake of argument, what if it were possible to pass through a wormhole? What would people experience? Some suggest it would probably be a lot like Alice falling through the rabbit hole. Who knows what we would find on the other side? Or in what time frame? Until someone can devise a safe way to make that trip, we aren't likely to find out.

The idea of traveling into the past raises all sorts of paradoxical issues. For instance, what happens if a person goes back in time and kills their parents before they can conceive their child? Lots of dramatic stories have been built around that one. Or, the idea that someone could go back and kill a dictator and change history, or save the life of a famous person. An entire episode of Star Trek was built around that idea.

It turns out that the time traveler effectively creates an alternate reality or parallel universe . So, if someone did travel back and prevent someone else's birth, or murdered someone, a younger version of the victim would never come to be in that reality. And, it might or might not carry on as if nothing had changed. By going back in time, the traveler creates a new reality and would, therefore, never be able to return to the reality they once knew. (If they then tried to travel into the future from there, they would see the future of the new reality, not the one they knew before.) Consider the outcome of the movie "Back to the Future". Marty McFly changes reality for his parents back when they were in high school, and that changes his own reality. He gets back home and finds his parents aren't quite the same as when he left. Did he create a new alternate universe? Theoretically, he did.

This brings us to another issue that is rarely discussed. The nature of wormholes is to take a traveler to a different point in time and space . So if someone left Earth and traveled through a wormhole, they could be transported to the other side of the universe (assuming they are even still in the same universe we currently occupy). If they wanted to travel back to Earth they would either have to travel back through the wormhole they just left (bringing them back, presumably, to the same time and place), or journey by more conventional means. 

Assuming the travelers would even be close enough to make it back to Earth in their lifetimes from wherever the wormhole spat them out, would it still be the "past" when they returned? Since traveling at speeds approaching that of light makes time slow down for the voyager, time would proceed very, very quickly back on Earth. So, the past would fall behind, and the future would become the past... that's the way time works flowing forward ! 

So, while they exited the wormhole in the past (relative to time on Earth), by being so far away it's possible that they wouldn't make it back to Earth at any reasonable time relating to when they left. This would negate the whole purpose of time travel altogether. 

Possible? Yes, theoretically. Probable? No, at least not with our current technology and understanding of physics. But perhaps someday, thousands of years into the future, people could harness enough energy to make time travel a reality. Until that time, the idea will just have to stay relegated to the pages of science-fiction or for viewers to make repeated showings of Back to the Future. 

Edited by Carolyn Collins Petersen .

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Where Does the Concept of Time Travel Come From?

Time; he's waiting in the wings.

The dream of traveling through time is both ancient and universal. But where did humanity's fascination with time travel begin, and why is the idea so appealing?

The concept of time travel — moving through time the way we move through three-dimensional space — may in fact be hardwired into our perception of time . Linguists have recognized that we are essentially incapable of talking about temporal matters without referencing spatial ones. "In language — any language — no two domains are more intimately linked than space and time," wrote Israeli linguist Guy Deutscher in his 2005 book "The Unfolding of Language." "Even if we are not always aware of it, we invariably speak of time in terms of space, and this reflects the fact that we think of time in terms of space."

Deutscher reminds us that when we plan to meet a friend "around" lunchtime, we are using a metaphor, since lunchtime doesn't have any physical sides. He similarly points out that time can not literally be "long" or "short" like a stick, nor "pass" like a train, or even go "forward" or "backward" any more than it goes sideways, diagonal or down.

Related: Why Does Time Fly When You're Having Fun?

Perhaps because of this connection between space and time, the possibility that time can be experienced in different ways and traveled through has surprisingly early roots. One of the first known examples of time travel appears in the Mahabharata, an ancient Sanskrit epic poem compiled around 400 B.C., Lisa Yaszek, a professor of science fiction studies at the Georgia Institute of Technology in Atlanta, told Live Science 

In the Mahabharata is a story about King Kakudmi, who lived millions of years ago and sought a suitable husband for his beautiful and accomplished daughter, Revati. The two travel to the home of the creator god Brahma to ask for advice. But while in Brahma's plane of existence, they must wait as the god listens to a 20-minute song, after which Brahma explains that time moves differently in the heavens than on Earth. It turned out that "27 chatur-yugas" had passed, or more than 116 million years, according to an online summary , and so everyone Kakudmi and Revati had ever known, including family members and potential suitors, was dead. After this shock, the story closes on a somewhat happy ending in that Revati is betrothed to Balarama, twin brother of the deity Krishna. 

Time is fleeting

To Yaszek, the tale provides an example of what we now call time dilation , in which different observers measure different lengths of time based on their relative frames of reference, a part of Einstein's theory of relativity.

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Such time-slip stories are widespread throughout the world, Yaszek said, citing a Middle Eastern tale from the first century BCE about a Jewish miracle worker who sleeps beneath a newly-planted carob tree and wakes up 70 years later to find it has now matured and borne fruit (carob trees are notorious for how long they take to produce their first harvest). Another instance can be found in an eighth-century Japanese fable about a fisherman named Urashima Tarō who travels to an undersea palace and falls in love with a princess. Tarō finds that, when he returns home, 100 years have passed, according to a translation of the tale published online by the University of South Florida . 

In the early-modern era of the 1700 and 1800s, the sleep-story version of time travel grew more popular, Yaszek said. Examples include the classic tale of Rip Van Winkle, as well as books like Edward Belamy's utopian 1888 novel "Looking Backwards," in which a man wakes up in the year 2000, and the H.G. Wells 1899 novel "The Sleeper Awakes," about a man who slumbers for centuries and wakes to a completely transformed London. 

Related: Science Fiction or Fact: Is Time Travel Possible ?

In other stories from this period, people also start to be able to move backward in time. In Mark Twain’s 1889 satire "A Connecticut Yankee in King Arthur's Court," a blow to the head propels an engineer back to the reign of the legendary British monarch. Objects that can send someone through time begin to appear as well, mainly clocks, such as in Edward Page Mitchell's 1881 story "The Clock that Went Backwards" or Lewis Carrol's 1889 children's fantasy "Sylvie and Bruno," where the characters possess a watch that is a type of time machine . 

The explosion of such stories during this era might come from the fact that people were "beginning to standardize time, and orient themselves to clocks more frequently," Yaszek said. 

Time after time

Wells provided one of the most enduring time-travel plots in his 1895 novella "The Time Machine," which included the innovation of a craft that can move forward and backward through long spans of time. "This is when we’re getting steam engines and trains and the first automobiles," Yaszek said. "I think it’s no surprise that Wells suddenly thinks: 'Hey, maybe we can use a vehicle to travel through time.'"

Because it is such a rich visual icon, many beloved time-travel stories written after this have included a striking time machine, Yaszek said, referencing The Doctor's blue police box — the TARDIS — in the long-running BBC series "Doctor Who," and "Back to the Future"'s silver luxury speedster, the DeLorean . 

More recently, time travel has been used to examine our relationship with the past, Yaszek said, in particular in pieces written by women and people of color. Octavia Butler's 1979 novel "Kindred" about a modern woman who visits her pre-Civil-War ancestors is "a marvelous story that really asks us to rethink black and white relations through history," she said. And a contemporary web series called " Send Me " involves an African-American psychic who can guide people back to antebellum times and witness slavery. 

"I'm really excited about stories like that," Yaszek said. "They help us re-see history from new perspectives."

Time travel has found a home in a wide variety of genres and media, including comedies such as "Groundhog Day" and "Bill and Ted's Excellent Adventure" as well as video games like Nintendo's "The Legend of Zelda: Majora's Mask" and the indie game "Braid." 

Yaszek suggested that this malleability and ubiquity speaks to time travel tales' ability to offer an escape from our normal reality. "They let us imagine that we can break free from the grip of linear time," she said. "And somehow get a new perspective on the human experience, either our own or humanity as a whole, and I think that feels so exciting to us." 

That modern people are often drawn to time-machine stories in particular might reflect the fact that we live in a technological world, she added. Yet time travel's appeal certainly has deeper roots, interwoven into the very fabric of our language and appearing in some of our earliest imaginings. 

"I think it's a way to make sense of the otherwise intangible and inexplicable, because it's hard to grasp time," Yaszek said. "But this is one of the final frontiers, the frontier of time, of life and death. And we're all moving forward, we're all traveling through time."

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Originally published on Live Science .

Adam Mann is a freelance journalist with over a decade of experience, specializing in astronomy and physics stories. He has a bachelor's degree in astrophysics from UC Berkeley. His work has appeared in the New Yorker, New York Times, National Geographic, Wall Street Journal, Wired, Nature, Science, and many other places. He lives in Oakland, California, where he enjoys riding his bike. 

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A Scientist Says Time Travel Is Possible With Ring Lasers

So, just find some of those.

• Ronald Mallett is still working on rotating lasers to travel back in time.
• His working theory is based on Albert Einstein’s relativity discussion.
• The prototype laser loop he developed ends up bending time, hypothetically allowing movement back—at the farthest—to the moment the machine was turned on.

Ronald Mallett loves the concept of time travel. He has since he was a kid. At 77, the former University of Connecticut physics professor still isn’t backing down from his theory: A spinning laser loop can bend time in an ongoing way.

“Let’s say you have a cup of coffee in front of you right now,” he explains to The Guardian . “Start stirring the coffee with the spoon. It started swirling around, right? That’s what a rotating black hole does. In Einstein’s theory, space and time relate to each other. That’s why it’s called space-time. So as the black hole is rotating, it’s actually going to cause a twisting of time.”

And you’ve got to start that twisting concept somewhere. “The Wright Brothers didn't just build a plane,” he said a couple of years ago. “First, they actually built a wind tunnel to determine the best configurations for aircraft wings. When it comes to a time machine, we need to build the wind tunnel before we can think about building the plane.”

So Mallett made his prototype machine. He thinks it could work. But it hasn’t yet, and if it does, it doesn’t have the infinite ability to go as far back as he’d like. Instead, he says the dip back in time can only go for as far back as when the loop was created. Think: 2019 .

He’s still hopeful, though. “Let’s suppose that we had already had this device going on some years ago, and now we have medicines that can cure Covid,” he says. “Imagine if we could predict precisely when earthquakes are going to occur, or tsunamis. So, for me, I’ve opened the door to that possibility.”

Tim Newcomb is a journalist based in the Pacific Northwest. He covers stadiums, sneakers, gear, infrastructure, and more for a variety of publications, including Popular Mechanics. His favorite interviews have included sit-downs with Roger Federer in Switzerland, Kobe Bryant in Los Angeles, and Tinker Hatfield in Portland. 

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February 1, 2006

How to Build a Time Machine

It wouldn't be easy, but it might be possible

By Paul Davies

Time travel has been a popular science-fiction theme since H. G. Wells wrote his celebrated novel The Time Machine in 1895. But can it really be done? Is it possible to build a machine that would transport a human being into the past or future?

For decades, time travel lay beyond the fringe of respectable science. In recent years, however, the topic has become something of a cottage industry among theoretical physicists. The motivation has been partly recreational--time travel is fun to think about. But this research has a serious side, too. Understanding the relation between cause and effect is a key part of attempts to construct a unified theory of physics. If unrestricted time travel were possible, even in principle, the nature of such a unified theory could be drastically affected.

Our best understanding of time comes from Einstein's theories of relativity. Prior to these theories, time was widely regarded as absolute and universal, the same for everyone no matter what their physical circumstances were. In his special theory of relativity, Einstein proposed that the measured interval between two events depends on how the observer is moving. Crucially, two observers who move differently will experience different durations between the same two events.

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The effect is often described using the twin paradox. Suppose that Sally and Sam are twins. Sally boards a rocket ship and travels at high speed to a nearby star, turns around and flies back to Earth, while Sam stays at home. For Sally the duration of the journey might be, say, one year, but when she returns and steps out of the spaceship, she finds that 10 years have elapsed on Earth. Her brother is now nine years older than she is. Sally and Sam are no longer the same age, despite the fact that they were born on the same day. This example illustrates a limited type of time travel. In effect, Sally has leaped nine years into Earth's future.

THE EFFECT, KNOWN AS time dilation, occurs whenever two observers move relative to each other. In daily life we don't notice weird time warps, because the effect becomes dramatic only when the motion occurs at close to the speed of light. Even at aircraft speeds, the time dilation in a typical journey amounts to just a few nanoseconds--hardly an adventure of Wellsian proportions. Nevertheless, atomic clocks are accurate enough to record the shift and confirm that time really is stretched by motion. So travel into the future is a proved fact, even if it has so far been in rather unexciting amounts.

To observe really dramatic time warps, one has to look beyond the realm of ordinary experience. Subatomic particles can be propelled at nearly the speed of light in large accelerator machines. Some of these particles, such as muons, have a built-in clock because they decay with a definite half-life; in accordance with Einstein's theory, fast-moving muons inside accelerators are observed to decay in slow motion. Some cosmic rays also experience spectacular time warps. These particles move so close to the speed of light that, from their point of view, they cross the galaxy in minutes, even though in Viewed from such a star, events here would resemble a fast-forwarded video. A black hole represents the ultimate time warp; at the surface of the hole, time stands still relative to Earth. This means that if you fell into a black hole from nearby, in the brief interval it took you to reach the surface, all of eternity would pass by in the wider universe. The region within the black hole is therefore beyond the end of time, as far as the outside universe is concerned. If an astronaut could zoom very close to a black hole and return unscathed--admittedly a fanciful, not to mention foolhardy, prospect--he could leap far into the future.

My Head Is Spinning

SO FAR I HAVE DISCUSSED travel forward in time. What about going backward? This is much more problematic. In 1948 Kurt Gdel of the Institute for Advanced Study in Princeton, N.J., produced a solution of Einstein's gravitational field equations that described a rotating universe. In this universe, an astronaut could travel through space so as to reach his own past. This comes about because of the way gravity affects light. The rotation of the universe would drag light (and thus the causal relations between objects) around with it, enabling a material object to travel in a closed loop in space that is also a closed loop in time, without at any stage exceeding the speed of light in the immediate neighborhood of the particle. Gdel's solution was shrugged aside as a mathematical curiosity--after all, observations show no sign that the universe as a whole is spinning. His result served nonetheless to demonstrate that going back in time was not forbidden by the theory of relativity. Indeed, Einstein confessed that he was troubled by the thought that his theory might permit travel into the past under some circumstances.

Other scenarios have been found to permit travel into the past. For example, in 1974 Frank J. Tipler of Tulane University calculated that a massive, infinitely long cylinder spinning on its axis at near the speed of light could let astronauts visit their own past, again by dragging light around the cylinder into a loop. In 1991 J. Richard Gott of Princeton University predicted that cosmic strings--structures that cosmologists think were created in the early stages of the big bang--could produce similar results. But in the mid-1980s the most realistic scenario for a time machine emerged, based on the concept of a wormhole.

In science fiction, wormholes are sometimes called stargates; they offer a shortcut between two widely separated points in space. Jump through a hypothetical wormhole, and you might come out moments later on the other side of the galaxy. Wormholes naturally fit into the general theory of relativity, whereby gravity warps not only time but also space. The theory allows the analogue of alternative road and tunnel routes connecting two points in space. Mathematicians refer to such a space as multiply connected. Just as a tunnel passing under a hill can be shorter than the surface street, a wormhole may be shorter than the usual route through ordinary space.

The wormhole was used as a fictional device by Carl Sagan in his 1985 novel Contact . Prompted by Sagan, Kip S. Thorne and his co-workers at the California Institute of Technology set out to find whether wormholes were consistent with known physics. Their starting point was that a wormhole would resemble a black hole in being an object with fearsome gravity. But unlike a black hole, which offers a oneway journey to nowhere, a wormhole would have an exit as well as an entrance.

In the Loop

FOR THE WORMHOLE to be traversable, it must contain what Thorne termed exotic matter. In effect, this is something that will generate antigravity to combat the natural tendency of a massive system to implode into a black hole under its intense weight. Antigravity, or gravitational repulsion, can be generated by negative energy or pressure. Negative- energy states are known to exist in certain quantum systems, which suggests that Thorne's exotic matter is not ruled out by the laws of physics, although it is unclear whether enough antigravitating stuff can be assembled to stabilize a wormhole [see Negative Energy, Wormholes and Warp Drive, by Law rence H. Ford and Thomas A. Roman; SCIENTIFIC AMERICAN, January 2000].

Soon Thorne and his colleagues realized that if a stable worm hole could be created, then it could readily be turned into a time machine. An astronaut who passed through one might come out not only somewhere else in the universe but somewhen else, too--in either the future or the past.

To adapt the wormhole for time travel, one of its mouths could be towed to a neutron star and placed close to its surface. The gravity of the star would slow time near that wormhole mouth, so that a time difference between the ends of the wormhole would gradually accumulate. If both mouths were then parked at a convenient place in space, this time difference would remain frozen in.

Suppose the difference were 10 years. An astronaut passing through the wormhole in one direction would jump 10 years into the future, whereas an astronaut passing in the other direction would jump 10 years into the past. By returning to his starting point at high speed across ordinary space, the second astronaut might get back home before he left. In other words, a closed loop in space could become a loop in time as well. The one restriction is that the astronaut could not return to a time before the wormhole was first built.

A formidable problem that stands in the way of making a wormhole time machine is the creation of the wormhole in the first place. Possibly space is threaded with such structures naturally--relics of the big bang. If so, a supercivilization might commandeer one. Alternatively, wormholes might naturally come into existence on tiny scales, the so-called Planck length, about 20 factors of 10 as small as an atomic nucleus. In principle, such a minute wormhole could be stabilized by a pulse of energy and then somehow inflated to usable dimensions.

ASSUMING THAT the engineering problems could be overcome, the production of a time machine could open up a Pandora's box of causal paradoxes. Consider, for example, the time traveler who visits the past and murders his mother when she was a young girl. How do we make sense of this? If the girl dies, she cannot become the time traveler's mother. But if the time traveler was never born, he could not go back and murder his mother.

Paradoxes of this kind arise when the time traveler tries to change the past, which is obviously impossible. But that does not prevent someone from being a part of the past. Suppose the time traveler goes back and rescues a young girl from murder, and this girl grows up to become his mother. The causal loop is now self-consistent and no longer paradoxical. Causal consistency might impose restrictions on what a time traveler is able to do, but it does not rule out time travel per se.

The bizarre consequences of time travel have led some scientists to reject the notion outright. Stephen W. Hawking of the University of Cambridge has proposed a chronology protection conjecture, which would outlaw causal loops. Because the theory of relativity is known to permit causal loops, chronology protection would require some other factor to intercede to prevent travel into the past. What might this factor be? One suggestion is that quantum processes will come to the rescue. The existence of a time machine would allow particles to loop into their own past. Calculations hint that the ensuing disturbance would become self-reinforcing, creating a runaway surge of energy that would wreck the wormhole.

Chronology protection is still just a conjecture, so time travel remains a possibility. A final resolution of the matter may have to await the successful union of quantum mechanics and gravitation, perhaps through a theory such as string theory or its extension, so-called M-theory. It is even conceivable that the next generation of particle accelerators will be able to create subatomic wormholes that survive long enough for nearby particles to execute fleeting causal loops. This would be a far cry from Wells's vision of a time machine, but it would forever change our picture of physical reality.

PAUL DAVIES is a theoretical physicist and professor of natural philosophy at Macquarie University's Australian Center for Astrobiology in Sydney. He is one of the most prolific writers of popular-level books in physics. His scientific research interests include black holes, quantum field theory, the origin of the universe, the nature of consciousness and the origin of life.

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How would time travel affect life as we know it?

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Key Takeaways

• If unrestricted time travel were possible, it would lead to a complete breakdown of the rational order of things.
• The ability to travel to both the past and future would upend our understanding of time.
• Stephen Hawking's "chronology protection hypothesis" suggests there might be natural laws preventing unrestricted time travel.

Science fiction has thoroughly covered the topic of time travel, starting with H.G. Wells' "The Time Machine" in 1895 and continuing right up to modern movies like " Déjà Vu " starring Denzel Washington. But physicists have also explored the nature of time and the plausibility of time travel for more than century, beginning with Albert Einstein's theories of relativity. Thanks to Einstein, scientists know that time slows as moving objects approach the speed of light. Gravity also slows time. This means that, in one sense, all of us can already consider ourselves time travelers in a limited way because we experience a tiny time warp (a difference of only nanoseconds) when we, for example, take a flight on an airplane. But physicists who study time travel today search for plausible ways to create a time warp large enough to allow noticeable travel into the past or future.

In his book "How to Build a Time Machine," physicist Paul Davies writes, "The theory of relativity implies that a limited form of time travel is certainly possible, while unrestricted time travel -- to any epoch, past or future -- might just be possible, too." This astonishing statement begs an important question: If time travel did indeed become a reality, how would it affect our world as we currently experience it?

First, it's important to realize that building a time machine would likely involve enormous expense, and the sheer complexity of such an apparatus would mean only a limited group of time travelers would have access to it. But even a small group of "astronauts" traveling through time and space could conceivably have a tremendous impact on life as we know it today. The possibilities, in fact, seem almost infinite.

Let's begin by assuming that it's possible to create a complete loop in time travel -- that time travelers could travel back into the past and then return to the future (or vice versa). Although scientists view traveling to the future as a much less problematic proposition than traveling to the past, our daily lives wouldn't change much if we could only send time travelers backward or forward in time, unable to recall them to the present. If we could, in fact, complete this loop of time travel, we can conjure up an incredible array of possible effects.

Possibilities and Paradoxes of Time Travel

Time travel turned total mayhem.

Imagine sending a time traveling astronaut 100 years into the future. The time traveler could witness technological advancements that we can only dream of today, much as people at the turn of the 20th century likely couldn't imagine the items we take for granted in 2010, such as iPods or laptop computers. The time traveler could also gain insight into medical advancements, such as new medicines, treatments and surgical techniques. If the time traveler could bring this knowledge backward in time to the present, the time from which he or she came, society could effectively leap forward in terms of its technical and scientific knowledge.

The futuristic time traveler could also bring back knowledge of what lay ahead for the world. He or she could warn of natural disasters, geopolitical conflicts, epidemics and other events of worldwide importance. This knowledge could potentially change the very way we operate. For example, what if a time traveler journeyed into the future and literally saw the effects that automobiles would eventually have on our planet? What if the time traveler witnessed an environment so polluted and damaged that it's unrecognizable? How might that change our willingness to use alternative forms of transportation?

Imagine that time travel became less restricted and more available to a larger population. Perhaps travel into the future would be exploited for personal gain. A futuristic time traveler could draw on knowledge of the stock market to guide his or her investment decisions, effectively using the granddaddy of all insider information to amass a fortune. Militaries might rely on time travel to gain valuable knowledge about the enemy's positioning and resources in future battles. Terrorists could use time travel to scout out the scenes of future attacks, allowing them to carefully plan with precise knowledge of future conditions.

The potential effects seem equally limitless in terms of the less likely possibility of time travel into the past. History books would no longer be based solely on exhaustive research and interpretation of ancient materials. Time travelers could resolve historical debates and verify how things did or didn't happen in the past. Imagine how different our understanding of the world might be if we could say definitively, for example, whether Moses actually parted the Red Sea or whether Lee Harvey Oswald acted alone in killing John F. Kennedy. A journey into the past could prove or disprove religious beliefs or result in face-to-face encounters with people such as Jesus, Buddha, Napoleon or Cleopatra -- or even the time traveler's former self. Perhaps time travelers could even bring back from the past things that had been lost, such as extinct species or dead and long-forgotten languages.

But here it's very important to raise the issue of self-consistent narratives and paradoxes. The concept of self-consistent narratives tells us that anything a time traveler would alter or affect in the past would have to remain consistent with the future from which he or she journeyed. Changing the past would effectively change the future, creating a causal loop. But such causal loops would only pose inherent problems if changes to the past resulted in a future different from the one the time traveler came from.

But perhaps the question of how time travel would affect life as we know it goes deeper than even a discussion of potential paradoxes and causal loops. Perhaps a discussion of specific effects of consequences on life as we know it makes little sense when faced with something that could change everything about the way in which we perceive our world.

Physicist Paul Davies gives a good example of a consistent causal loop in his book "How to Build a Time Machine." A mathematics professor uses a time machine to travel forward in time, where he discovers a new theorem. He returns back to the time he came from and gives one of his particularly gifted students the idea for that theorem. The student goes on to publish the theorem, and it turns out that it was this very student's work that the professor perused during his journey to the future. The narrative here is consistent.

On the other hand, with the grandfather paradox, a time traveler goes back in time and kills his grandfather. But if the time traveler's grandfather dies before the time traveler is born, how can he or she exist at all? And if the time traveler doesn't exist, how could he or she travel back in time to kill granddad?

As physicist Paul Davies describes it, unrestricted time travel -- meaning time travel that could form a complete loop to both the past and future -- would ultimately lead to total mayhem. In his words, "Time travel opens a view of the world that is a sort of madhouse where the rational order of things would no longer work. Under those circumstances, it's very hard to see how ordinary human life could continue."

In a world where the relationship between past, present and future is turned on its head, we would transcend the things that define our lives today. We would lose our notion of how time works, which could be so fundamentally damaging to our worldview that we would no longer care as much about the things that matter to us today: work, finances, making plans with friends and family, shopping -- you name it. These things just wouldn't be relevant in this crazy new world because we'd have a newfound preoccupation with simply making sense of a world without a set chronology -- we wouldn't know the order in which things occur.

It may be beside the point, then, to talk about resolving historical debates, saving endangered species or gaining technological, financial or military insight because those things might very likely fall by the wayside in the strange world that would follow the advent of unrestricted time travel.

As Davies makes clear, none of this fallout would occur from one-way travel. Hitching a one-way ride to the future or even the past (assuming we stick with self-consistent narratives) wouldn't cause this kind of profound reordering of the world as we currently experience it. But closing that loop of travel could be, in a word, disastrous.

Davies points out that science fiction normally focuses on the novelty aspect of time travel. But according to him, "It's not a novelty or a curiosity, it's something that strikes at the very rational basis of how we live and function. It's really hard to imagine that anything could be the same again." In his view, unrestricted time travel could change life as we know it so dramatically that we wouldn't even recognize it. Because chronology would have no meaning, we couldn't easily tell if something happened before or after, was a cause or an effect, and we would lose the ability to predict rationally the outcomes of our actions. In essence, it would be as though we had all gone insane.

These sobering potential effects of time travel have caused some scientists to wonder whether a principle exists in nature that would actually prevent unrestricted time travel, such as Stephen Hawking's "chronology protection hypothesis." This type of "theory of everything" might provide a scientific explanation as to why we could never unhinge the universe as we know it by making unrestricted time travel a reality. Scientists have yet to discover such a theory, but hearing Davies' take on the frightening effects of time travel makes one hope that they find it soon -- even if it means that we won't ever know for sure who killed JFK.

Frequently Asked Questions

How could time travel impact our understanding of history, what are the ethical implications of time travel, lots more information, related articles.

• What is the fourth dimension?
• What does the fourth dimension have to do with time travel?
• Does gravity distort space-time?
• Are there other universes like ours out there?
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• How Special Relativity Works
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More Great Links

• NOVA Online: Time Travel
• Paul Davies' Web site
• ABC Science Online. "The Big Questions: The Riddle of Time." Jan. 17, 2002. (Oct. 7, 2010) http://www.abc.net.au/science/bigquestions/s460740.htm
• Davies, Paul. "How to Build a Time Machine." Penguin Books. 2002.
• Davies, Paul. Personal interview. Oct. 13, 2010.
• PBS Nova. "Sagan on Time Travel." October 1999. (Oct. 7, 2010) http://www.pbs.org/wgbh/nova/time/sagan.html
• Pickover, Clifford. "Time: a traveler's guide." Oxford University Press. 1999.

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Time travel: five ways that we could do it

Cathal O’Connell

Cathal O'Connell is a science writer based in Melbourne.

In 2009 the British physicist Stephen Hawking held a party for time travellers – the twist was he sent out the invites a year later (No guests showed up). Time travel is probably impossible. Even if it were possible, Hawking and others have argued that you could never travel back before the moment your time machine was built.

But travel to the future? That’s a different story.

Of course, we are all time travellers as we are swept along in the current of time, from past to future, at a rate of one hour per hour.

But, as with a river, the current flows at different speeds in different places. Science as we know it allows for several methods to take the fast-track into the future. Here’s a rundown.

1. Time travel via speed

This is the easiest and most practical way to time travel into the far future – go really fast.

According to Einstein’s theory of special relativity, when you travel at speeds approaching the speed of light, time slows down for you relative to the outside world.

This is not a just a conjecture or thought experiment – it’s been measured. Using twin atomic clocks (one flown in a jet aircraft, the other stationary on Earth) physicists have shown that a flying clock ticks slower, because of its speed.

In the case of the aircraft, the effect is minuscule. But If you were in a spaceship travelling at 90% of the speed of light, you’d experience time passing about 2.6 times slower than it was back on Earth.

And the closer you get to the speed of light, the more extreme the time-travel.

Computer solves a major time travel problem

The highest speeds achieved through any human technology are probably the protons whizzing around the Large Hadron Collider at 99.9999991% of the speed of light. Using special relativity we can calculate one second for the proton is equivalent to 27,777,778 seconds, or about 11 months , for us.

Amazingly, particle physicists have to take this time dilation into account when they are dealing with particles that decay. In the lab, muon particles typically decay in 2.2 microseconds. But fast moving muons, such as those created when cosmic rays strike the upper atmosphere, take 10 times longer to disintegrate.

2. Time travel via gravity

The next method of time travel is also inspired by Einstein. According to his theory of general relativity, the stronger the gravity you feel, the slower time moves.

As you get closer to the centre of the Earth, for example, the strength of gravity increases. Time runs slower for your feet than your head.

Again, this effect has been measured. In 2010, physicists at the US National Institute of Standards and Technology (NIST) placed two atomic clocks on shelves, one 33 centimetres above the other, and measured the difference in their rate of ticking. The lower one ticked slower because it feels a slightly stronger gravity.

To travel to the far future, all we need is a region of extremely strong gravity, such as a black hole. The closer you get to the event horizon, the slower time moves – but it’s risky business, cross the boundary and you can never escape.

And anyway, the effect is not that strong so it’s probably not worth the trip.

Assuming you had the technology to travel the vast distances to reach a black hole (the nearest is about 3,000 light years away), the time dilation through travelling would be far greater than any time dilation through orbiting the black hole itself.

(The situation described in the movie Interstellar , where one hour on a planet near a black hole is the equivalent of seven years back on Earth, is so extreme as to be impossible in our Universe, according to Kip Thorne, the movie’s scientific advisor.)

The most mindblowing thing, perhaps, is that GPS systems have to account for time dilation effects (due to both the speed of the satellites and gravity they feel) in order to work. Without these corrections, your phones GPS capability wouldn’t be able to pinpoint your location on Earth to within even a few kilometres.

3. Time travel via suspended animation

Another way to time travel to the future may be to slow your perception of time by slowing down, or stopping, your bodily processes and then restarting them later.

Bacterial spores can live for millions of years in a state of suspended animation, until the right conditions of temperature, moisture, food kick start their metabolisms again. Some mammals, such as bears and squirrels, can slow down their metabolism during hibernation, dramatically reducing their cells’ requirement for food and oxygen.

Could humans ever do the same?

Though completely stopping your metabolism is probably far beyond our current technology, some scientists are working towards achieving inducing a short-term hibernation state lasting at least a few hours. This might be just enough time to get a person through a medical emergency, such as a cardiac arrest, before they can reach the hospital.

In 2005, American scientists demonstrated a way to slow the metabolism of mice (which do not hibernate) by exposing them to minute doses of hydrogen sulphide, which binds to the same cell receptors as oxygen. The core body temperature of the mice dropped to 13 °C and metabolism decreased 10-fold. After six hours the mice could be reanimated without ill effects.

Unfortunately, similar experiments on sheep and pigs were not successful, suggesting the method might not work for larger animals.

Another method, which induces a hypothermic hibernation by replacing the blood with a cold saline solution, has worked on pigs and is currently undergoing human clinical trials in Pittsburgh.

4. Time travel via wormholes

General relativity also allows for the possibility for shortcuts through spacetime, known as wormholes, which might be able to bridge distances of a billion light years or more, or different points in time.

Many physicists, including Stephen Hawking, believe wormholes are constantly popping in and out of existence at the quantum scale, far smaller than atoms. The trick would be to capture one, and inflate it to human scales – a feat that would require a huge amount of energy, but which might just be possible, in theory.

Attempts to prove this either way have failed, ultimately because of the incompatibility between general relativity and quantum mechanics.

5. Time travel using light

Another time travel idea, put forward by the American physicist Ron Mallet, is to use a rotating cylinder of light to twist spacetime. Anything dropped inside the swirling cylinder could theoretically be dragged around in space and in time, in a similar way to how a bubble runs around on top your coffee after you swirl it with a spoon.

According to Mallet, the right geometry could lead to time travel into either the past and the future.

Since publishing his theory in 2000, Mallet has been trying to raise the funds to pay for a proof of concept experiment, which involves dropping neutrons through a circular arrangement of spinning lasers.

His ideas have not grabbed the rest of the physics community however, with others arguing that one of the assumptions of his basic model is plagued by a singularity, which is physics-speak for “it’s impossible”.

The Royal Institution of Australia has an Education resource based on this article. You can access it here .

Related Reading: Computer solves a major time travel problem

Originally published by Cosmos as Time travel: five ways that we could do it

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This man is closer than ever to building the world's first time machine

'Time travel is possible'

A theory of flying clocks

• Twisting space and time

It was a personal tragedy that started the timeline. After Boyd Mallett died of a sudden heart attack in 1955, his 10 year old son, Ronald, made a promise: he would find a way to travel back in time to warn his father of what was going to happen. It was a mission inspired partly by a copy of H.G. Wells' The Time Machine, which Ron discovered a year after his father's passing.

The story follows the narrator's journey into the future, but one line in particular struck Ron: "Scientific people know very well that time is just a kind of space and we can move forward and backward in time just as we can in space." He believed that he could build a fully working time machine to go back in time and so he dedicated his future to proving it.

"For me the sun rose and set on him," says Mallett about his father, a television repairman who was just 33 when he died. Ron kept his research into time travel a secret for many years for fear he might damage his credibility. Sadly, that prevented him from reaching out to people who might have been able to help him.

Now aged 69, Ron Mallett, a physics professor at the University of Connecticut, is totally candid about his research, but he still hasn't reunited with his father and most likely never will. But he has an equation that he believes holds the key to building the first time machine and he might be close to a breakthrough.

Like most time travel theorists - Kip Thorne and his wormholes being one of the best known - Mallett anchors his idea in Einstein's theory of relativity. If you're not already familiar with it, there are two parts you need to know about. The first is called special relativity, which says that the speed of light is constant for everybody. By the theory of special relativity, time will pass slower the faster a person moves. If you alone could travel fast enough to a distant star, you would age slower than everyone else on Earth, arriving back home in a younger state - essentially travelling through time.

"This form of time travel has been actually been achieved on a limited scale using fast-moving planes and hi-speed subatomic particles," says Mallett, referring to an experiment carried out by the US Naval Observatory in 1971. Four clocks were flown twice around the world - eastward and then westward - at the speed of sound. When they were observed on return it was found the airborne clocks had lost a small amount of time compared to the ones that were kept on the ground. Were the clocks put in a rocket flying closer to the speed of light, the effects would be more dramatic.

The second part of Einstein's theory is called general relativity - if you've ever used a GPS unit in your car then you've (possibly unknowingly) experienced its effects. General relativity says that gravity can slow down time - meaning a clock on a satellite runs faster than one on Earth - which is why the frequency standard on each satellite is offset to make it run slower before being launched into space (in fact, satellites have to account for both special and general relativity due to the speed they're moving at, but the magnitudes aren't equal and therefore don't cancel each other out, hence the need for them to be altered before launch).

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If you've seen the film Interstellar, you'll recall how it portrayed the effects of gravitational time dilation on a dramatic scale as the characters' ages fell out of sync with one another.

Einstein believed that light and matter could create gravity. Mallett's breakthrough was a theory that claims if gravity can alter time and light, and light can create gravity, then light can also alter time. So he designed a machine that would use lasers to twist time and bend it back on itself to form a loop. By Einstein's theories, time and space are linked - and so if you affect space you will eventually affect time.

"By using a circulating beam of laser light, I have been able to mathematically show that this can lead to a twisting of space and time," says Ron. "By twisting time into a loop. It could be possible to travel back in time."

Current page: A theory of flying clocks

Hugh Langley is the ex-News Editor of TechRadar. He had written for many magazines and websites including Business Insider, The Telegraph, IGN, Gizmodo, Entrepreneur Magazine, WIRED (UK), TrustedReviews, Business Insider Australia, Business Insider India, Business Insider Singapore, Wareable, The Ambient and more.

Hugh is now a correspondent at Business Insider covering Google and Alphabet, and has the unfortunate distinction of accidentally linking the TechRadar homepage to a rival publication.

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The Best Sci-Fi Time Machines Ever

Whether you prefer phone booths, cars or wearable devices, surely there is a time machine out there for you. We've gathered some of the top time machines from science fiction, in all their wibbily wobbly timey wimey glory.

TARDIS ("Doctor Who")

Perhaps the most famous of time machines , the TARDIS (Time And Relative Dimension In Space), which is disguised as a police phone booth, allows Doctor Who and his companions to jump across eras on planet Earth.

In the new series, they've met many historical figures using this machine, such as the artist Vincent Van Gogh and writers Agatha Christie and Charles Dickens. Doctor Who often uses the TARDIS to try to change events in time, with varying success. [ How Time Travel Works in Sc-Fi (Infographic) ]

DeLorean ("Back to the Future") 

In the " Back to the Future " trilogy, the DeLorean can travel to a time you punch into the car's dashboard as long as the vehicle reaches 88 miles per hour and has sufficient power.

This leads to several complications for the characters, however, when they run out of the plutonium that is usually used to power the vehicle. Much of the first movie revolves around finding an alternative source of power.

Klingon Bird of Prey ("Star Trek IV: The Voyage Home")

While the characters in " Star Trek: The Original Series " are no stranger to time travel, it is in the film "Star Trek IV: The Voyage Home" that they do one of their most famous maneuvers with a captured Klingon Bird of Prey — a slingshot around the sun to nab humpback whales from the 20th century. The Enterprise crew pioneered the manuever with the U.S.S. Enterprise (which we also think is an awesome time machine) in the classic first season episode "Tomorrow Is Yesterday."

The crew's escapades in 1980s San Francisco include Captain James T. Kirk (William Shatner) mastering the use of "colorful metaphor" swearing, and explaining the behavior of Mr. Spock (Leonard Nimoy) by saying Spock used too much "LDS" a few years ago.

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Complicated contraption ("The Time Machine")

Based on the 1895 H.G. Wells novel, this 2002 movie shows Alexander Hartdegen (Guy Pearce) sitting in a complicated device of wheels, levers and light to travel 800,000 years into the future. He planned to test this device to save somebody he loved, but his first flight plunges him into a war he is not prepared to fight.

Phone booth ("Bill & Ted's Excellent Adventure") 

In perhaps the best method ever to get a school assignment done, slackers Bill Preston (Alex Winter) and Ted Logan (Keanu Reeves) use a phone booth to travel back in time and get some first-hand knowledge of historical figures. But things take an unexpected turn when an explosion knocks Napoleon Bonaparte into their wake, dragging the French leader from Austria in 1805 to Southern California in 1988. [ 10 Space Movies to Watch in 2015 ]

Time Displacement Equipment ("The Terminator" movie series) 

Demonstrating how a war can be fought in four dimensions — yes, including time — the Time Displacement Equipment is a central part of "The Terminator" film series. At various moments, different factions in the war between humans and machines use the equipment to send their agents backward or forward in time. The device looks like a big gyroscope, and travelers must be specially coated before entering for it to work properly.

Rocket sled ("Timecop") 

While time travel movies introduce all sorts of paradoxes, the rocket sled in "Timecop" (1994) has an unusual one: The device appears to vanish every time users climb into it and careen toward a wall. But when riders goes back to where they came from, they magically appear in the sled again. This phenomenon has spawned much speculation on the Internet .

Time Turner ("Harry Potter: The Prisoner of Azkaban") 

This innocent-looking pendant is used by Hermione Granger (Emma Watson) to gain more time for her studies. After special approval from a professor at the Hogwarts School of Witchcraft and Wizardry, she uses the device to regularly jump back a few hours in time to take extra classes. Hermione keeps the device secret from her friends Harry Potter (Daniel Radcliffe) and Ron Weasley (Rupert Grint) until it comes in handy for a rescue mission.

The Island ("Lost")

"Lost" is one of those television shows in which nobody really knows what's going on. But a few seasons in, it becomes clear that the island on which the travelers are stranded has time-travel properties, which are sometimes activated via a frozen wheel. But their attempts to change the past usually succeed only in bringing about the same events via other means.

Time Jump Device ("Men In Black III")

While time travel is illegal in the "Men in Black" universe — there are concerns that criminals could use the technology to their advantage — it ends up playing a key role in the plot of "Men in Black III (2012), which includes extensive references to the Apollo 11 launch of July 16, 1969. The hand-held device can be activated by falling with it to the ground from a height.

WABAC machine ("The Rocky and Bullwinkle Show")

This time machine was actually invented by a dog — the super-genius Mr. Peabody — for his pet human boy, Sherman. The duo travel in the time machine together to see important events in history, such as the famous Charge of the Light Brigade during the Crimean War in 1854. The Wayback Machine website on the Internet is an homage to WABAC , which, in turn, was inspired by the popular UNIVAC computers of the 1960s.

Unnamed device ("Primer") 

The time machine in the 2004 movie "Primer" was originally used by the characters to try to make money in the stock market. However, another person discovers the time machine and gets ill from the effects of it, casting doubt on its utility.

Cosmic treadmill ("The Flash") 

In perhaps the ultimate inspiration to exercise, this treadmill will take you back in time — but only if you go at superhuman speed. In one famous comic book , Barry Allen uses the device to go to the 25th century to encounter Professor Zoom, a noted villain in the DC Universe.

Follow Elizabeth Howell  @howellspace , or Space.com  @Spacedotcom . We're also on  Facebook  and  Google+ .   Originally published on  Space.com .

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Elizabeth Howell (she/her), Ph.D., is a staff writer in the spaceflight channel since 2022 covering diversity, education and gaming as well. She was contributing writer for Space.com for 10 years before joining full-time. Elizabeth's reporting includes multiple exclusives with the White House and Office of the Vice-President of the United States, an exclusive conversation with aspiring space tourist (and NSYNC bassist) Lance Bass, speaking several times with the International Space Station, witnessing five human spaceflight launches on two continents, flying parabolic, working inside a spacesuit, and participating in a simulated Mars mission. Her latest book, " Why Am I Taller ?", is co-written with astronaut Dave Williams. Elizabeth holds a Ph.D. and M.Sc. in Space Studies from the University of North Dakota, a Bachelor of Journalism from Canada's Carleton University and a Bachelor of History from Canada's Athabasca University. Elizabeth is also a post-secondary instructor in communications and science at several institutions since 2015; her experience includes developing and teaching an astronomy course at Canada's Algonquin College (with Indigenous content as well) to more than 1,000 students since 2020. Elizabeth first got interested in space after watching the movie Apollo 13 in 1996, and still wants to be an astronaut someday. Mastodon: https://qoto.org/@howellspace

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Giant Freakin Robot

Time Travel Equation Solved By Astrophysicist

Posted: March 25, 2024 | Last updated: March 25, 2024

After a lifetime of pursuing the idea, Physics Professor Ronald Mallett at the University of Connecticut has potentially figured out the theoretical aspects of time travel. Professor Mallett believes that black holes, rotating light, and gravitational pulls may hold the key to exploring time, but it’s all theoretical for now. There are still a lot of hurdles and limitations to handle before time travel can have real, practical applications.

A Life Spent Thinking About Time Travel

Love and loss pushed Professor Mallett into an obsession with time and space. When he was 10 years old, his father passed away from a heart attack. It was his father who nourished his love of science, but H.G. Wells’ book The Time Machine pushed him towards a focus on time travel.

He was hooked from the very first paragraph of the book, “Scientific people know very well that Time is only a kind of Space. And why cannot we move in Time as we move about in the other dimensions of Space?”

That paragraph never left him, and the professor let that time travel question guide him through school and into the Professor Emeritus of Physics position at the University of Connecticut.

Einstein And Black Holes

As he grew up, Professor Mallett spent much of his time on Albert Einstein’s theories about black holes. While his interest in time travel only continued to grow, a potential solution never showed itself. At least, not until the professor ended up in a hospital with a heart condition.

There, lying in the hospital bed, inspiration hit him. Black holes and the gravitational fields they created were the answer to time travel. These gravitational fields had the potential to lead to time loops, which then theoretically could allow people and objects to travel back in time.

Black Holes Manipulating Gravity

While this idea offered an ability to manipulate time, the other problem was how to use these time loops for time travel.

Professor Mallett found this time travel solution much easier than the first problem. Strong and continuous beams of light, like a ring of lasers, with a particular rotation could be used to manipulate gravity and mimic the distorting effects of a black hole.

Though the details are rather complicated, the big time travel picture is a lot simpler to grasp. The professor offers a comparison to help people understand. “Let’s say you have a cup of coffee in front of you. Start stirring the coffee with the spoon. It started to spin, right? That’s what a spinning black hole does. In Einstein’s theory, space and time are related to each other. That’s why it’s called space-time. So when the black hole spins, it will actually cause time to shift.”

Much To Figure Out

Professor Mallett may now have a theory on time travel and a machine to use to make it possible, but that doesn’t mean it will be here in the next few decades.

There’s still a lot to figure out to make such travel practical, such as where the insane amount of energy such a machine would require could come from, and how big the machine would need to be.

There’s also a major constraint on the machine. According to his theories, time travel would only be possible to the very beginning of when the machine was first built. In this way, it’s more like a one-way message service. You can potentially go forward quite a distance, but going back in time is limited by the machine’s creation.

Theoretical Aspects Of Time Travel

The professor has made a huge leap in figuring out the theoretical aspects of time travel, but there’s a lot more to discover and quite a few hurdles and paradoxes to figure out before scientists practically start messing around in time.

Still, the theory is a step in the right direction and does suggest that people can push past what science currently considers possible.

Source: Earth.com

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