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We will adopt the more liberal stance since it avoids various complications while still sufficing to elicit key points. But with the goal in mind of proving negative general results, it is better to proceed in a more abstract fashion. 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. But in the present setting this move quickly runs into a dead end.

Here is an initial stab at such an adjacency condition. But this requirement is too strong because it rules out Thornian time machines altogether. 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 this spacetime is not a plausible candidate for a time machine spacetime. And in any case the offending feature of Deutsch-Politzer can be gotten rid of by the following trick. 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.

The difference takes a bit of explaining. A theorem due to Krasnikov , [Other Internet Resources], a might seem to demonstrate that no relativistic spacetime can count as embodying a Thornian time machine so understood. 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.

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.

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. 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. But Krasnikov has shown that the Geroch 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 a, Minguzzi Are there "no hole" conditions which show the proposed concept of a time machine is not empty? One can verify that J closedness fails in many artificially mutilated examples e.

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 a. 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.

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 b, b. 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.

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.

A small amount of progress has been made on stating and proving precise versions of this conjecture. One way in which strong cosmic censorship can be violated is through the emergence of acausal features. 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. This no-go result does not touch the situation illustrated in Figure 1. By the same token the theorem does not speak to the possibility of operating a Thornian time machine in a spatially closed universe.

The first stems from possible violations of the weak energy condition by stress-energy tensors arising from classical relativistic matter fields see Vollick and Visser and Barcelo But in retrospect, the motivation for this condition seems frayed. Of course, it remains to establish the existence of cases where this entailment holds. 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. Currently loop quantum gravity and string theory are the main research programs aimed at this goal.

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.

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 fortunes were reversed once again by a result of Kay, Radzikowski, and Wald The details of their theorem are too technical to review here, but the structure of the argument is easy to grasp.

The standard renormalization procedure uses a limiting procedure that is mathematically well-defined if, and only if, a certain condition obtains. 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. 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. 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.

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. Actually, a theorem due to Malament 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.

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.

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. 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.

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.

We thank Carlo Rovelli for discussions and John Norton for comments on an earlier draft. Introduction: time travel vs. What is a Thornian time machine? Preliminaries 3. When can a would-be time machine be held responsible for the emergence of CTCs? No-go results for Thornian time machines in classical general relativity theory 5. No-go results in quantum gravity 6. Figure 1. Misner spacetime. Figure 2. Deutsch-Politzer spacetime. Figure 3. A bad choice of initial value surface.

Bibliography Arntzenius, F. Zalta ed. Brightwell, G. Dowker, R. Garcia, J. Henson, and R. Chrusciel, P. The principle concept of the series is that an inventor known only as The Time Traveler has created a Time Machine capable of taking him into the future or the past as he pleases. In the future, he finds a decayed world where Humanity has evolved into separate species: the Eloi and the Morlocks. Wells also wrote another sci-fi novel, When The Sleeper Wakes , which sheds some light on how the world of the Eloi and the Morlocks came to be.

There have been a number of other film and television appearances as well. Additionally, some other authors have written sequels to Wells' original novel, most notably Stephen Baxter 's The Time Ships , an authorized sequel which expands the concepts of the original novel, introducing new races, new scientific explanation of time travel, and new characters.

In , Syfy released Morlocks , a made-for-tv movie that puts a new twist on Wells' original story. This wiki covers all published sources related to The Time Machine, its sequels, movie adaptations, etc. The Time Machine Wiki Explore.

Popular pages. Explore Wikis Community Central. Register Don't have an account? Edit source History Talk 2. Wells' classic novel.

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Use Time Machine, the built-in backup feature of your Mac, to automatically back up your personal data, including apps, music, photos. The Time Machine is a science fiction novella by H. G. Wells, published in The work is generally credited with the popularization of the concept of. Time Machine Inc., established in , is a premier Western Pennsylvania contract machining company. Located in Polk, PA, Time Machine provides superior.