Exploring Time Travel into the Past

[JesseM]

Few physicists know the complex math of general relativity.

I doubt it's true that "few" physicists know it--all cosmologists would have to, as should any physicist studying theories of quantum gravity. It may be that most physicists are required to study the basics as part of a general physics education in graduate school, I'm not really sure.

I'm not sure whether it's required in What is meant with time-travel? Does this mean that we can run the movie backwards? Can we go back to the stone age?

What does "run the movie backward" mean? The idea of slicing the block of ice into cross-sections and using them as frames in a movie was just a sort of schematic way of thinking about the difference between our ordinary notion of moving time and spacetime in relativity, the block of ice with the strings embedded in it is really the fundamental thing, just as spacetime as a whole with various worldlines embedded in it is really the fundamental thing in general relativity. You can sometimes do a "foliation" on spacetime which means cross-sectioning 4D spacetime into a series of 3D "spacelike hypersurfaces", but this isn't possible for all possible spacetimes, and in spacetimes where you can there's no unique way to do it (so two events which are part of the same hypersurface in one foliation would be part of different hypersurfaces in another).

But for any worldline, you can at least talk about the "proper time" experienced along that worldline, ie the time that would be read on a clock that followed that worldline. So backwards time travel in this context would mean that the clock's worline loops around in such a way that when it reads 4:00, it passes next to the point on the worldline where it read 3:00.

Anyway, here is a very good argument against time travel.

http://mb-soft.com/public/time.html

That's a pretty ill-informed argument actually. For example, they say:

Speculate now, that time travel into the past was possible, and that people a thousand years in our future learned how to do it and control it. For the moment, let's take a big example, that they decided to transport a medium sized hill from their time to ours, a thousand years earlier.

No matter WHAT process might be involved, it is certainly clearly necessary to know exactly where each atom was at some specific instant in time. This would be an astoundingly difficult accomplishment, but we will assume that those people a thousand years in our future would have figured out some way to do it.

Since we would need to somehow re-construct the structure of the hill, we would certainly need to know the position of each atom REALLY accurately. If two atoms were supposed to be right next to each other (as in a compound, like hydrogen and oxygen forming water molecules), The location of the atoms would obviously have to be known to about 10-12 cm (or one one-trillionth of a centimeter). This figure comes from nuclear physics, where this is a common distance of interaction between individual atoms.

But time travel as envisioned by physicists does not involve an object disappearing from one time and being reconstructed in another, like the transporter in star trek--instead, it involves a physical object taking a continuous trip through a twisty region of spacetime which results in it ending up in the past, there are no discontinuities where it disappears or reappears. Since the entire rest of the section of that paper is based on this disappear/reappear premise, it doesn't make sense. The paper also assumes that history could be "changed" by the presence of a time traveler, but in the "static spacetime" view, whatever the time traveler does in the past should have been part of history all along, so it wouldn't have changed anything. See this thread for a discussion of how paradoxes can be avoided by postulating that only self-consistent histories would be allowed by the laws of physics.

 

[quantumdude]

No you have to back up your ideas if you think time travel is really a possibility.

I didn't say anything about it one way or the other. The statement I quoted from you is that few physicists understand GR. I know that you are wrong about that.

 

[Myriad209]

By far the majority of physicists consider time travel as pseudoscience. There just isn't any experimental evidence that it happens.

That isn't true, many concepts in physics work forward and backward in time and it DOES happen in a few cases.

 

[JesseM]

That isn't true, many concepts in physics work forward and backward in time and it DOES happen in a few cases.

To say that an antiproton is mathematically equivalent to a proton moving backwards in time is not really evidence of time travel. True time travel is only possible in general relativity, and there's no evidence that this actually happens, or that this will still be possible once a theory of quantum gravity is found.

 

[El Hombre Invisible]

To say that an antiproton is mathematically equivalent to a proton moving backwards in time is not really evidence of time travel. True time travel is only possible in general relativity, and there's no evidence that this actually happens, or that this will still be possible once a theory of quantum gravity is found.

Why do you assume that the mathematical equivilence of an antiparticle to a particle traveling backwards in time (or vice versa) is not a consequence of its physical truth? If an electron exists in a certain point in space in the future and another in the past, why would it be any less true to say that it is a positron traveling from the future (further away from the big bang) to the past (nearer to it) than to say that it is an electron traveling from the past to the future? It is in the future and in the past and at every point in time in between. That's its trajectory.

 

[loz]

Time travel probably violates causality and energy conservation:

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While I cannot cannot argue with your statement about time travel violating causality I disagree that it violates energy conservation. Since space (here, there) and time (past, present and future ) could be regarded as within the same system, then energy conservation is not an issue. There are numerous experiments showing energy borrowing although the greater the energy borrowed, it seems, the quicker it must be repayed.

 

[JesseM]

Why do you assume that the mathematical equivilence of an antiparticle to a particle traveling backwards in time (or vice versa) is not a consequence of its physical truth?

I don't assume it's not, I'd just say it's not necessary to view it that way, thus this cannot be taken as "evidence" of time travel. After all, in Newtonian physics a ball falling down in a vacuum is mathematically equivalent to a time-reversed version of the same ball moving upwards in a vacuum after being thrown, but I don't think many people would say that this proves Newtonian physics allows backwards time travel.

 

[PeteSF]

Time travel probably violates causality and energy conservation

What is causality, exactly, and has anyone proved it can't be violated?

 

[Starship]

What is causality, exactly, and has anyone proved it can't be violated?

Causality says that every effect must have a cause (e.g, energy is needed to do work). What we have (and what we are of course) is a proof that causality is never violated.

 

[JesseM]

Causality says that every effect must have a cause (e.g, energy is needed to do work).

"Energy is needed to do work" would not really be an example of what physicists mean by causality--causality as they define it deals not with abstract concepts but with specific events, causality basically means that an event can only affect other events that are in its future light cone. Also, I don't think causality would demand that every event be caused by other events--I'm pretty sure quantum randomness is not considered to violate causality, for example.

What we have (and what we are of course) is a proof that causality is never violated.

There is no proof that causality is never violated. A universe that allowed closed timelike curves could still be perfectly lawlike since it would still be constrained by the laws of general relativity, it's just that events could be affected by both events in their past and events in their future. Of course there is also no evidence that causality is violated, and most physicists would probably guess that the most basic laws of nature (quantum gravity) will end up preserving causality.