# I GR -- about spacetime and time travel

1. May 6, 2017

### SD das

if GR is correct about spacetime (expanding of space) then i think time travel in back (back to the past) is impossible...

suppose before 2 billion years ago (from now) the total space (volume) of universe was "X" so, it's clear that the space (volume) of the present universe must be greater than "X" I.e X+Y where y is increased volume (space).
now, suppose Mr.Q is trying to travel back in time (2billion year before from now). so, can he travel back in that time? it's answer is No .because Mr.Q has some Volume

( X+Y= X + volume of Mr.Q+ Z, where Y=Z+vol.of Mr.Q )

and volume of past is fixed and so he can't travel back (there is no extra space for mr.Q in past)

So what do you think?

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2. May 6, 2017

### phinds

You misunderstand expansion. Bound objects such as galactic clusters, galaxies, solar systems, planets, you, atoms, etc do not expand.

3. May 6, 2017

### rootone

Reverse time travel is generally considered to be not possible, even if math does not rule it out.
Reasonable logic says that if you could do that you would change the past.
So therefore you cannot exist in the future which you claim to have traveled from.

Last edited: May 6, 2017
4. May 6, 2017

### Brian E

I am no expert! Could there not exist a situation in theory, where you when travelling back in time, would create one more timeline. So that you in theory could exist on parallel timelines. Anyway this is how I imagine a multiverse.

5. May 6, 2017

### rootone

I am not convinced of multiverse ideas, but not rejecting,
Seems a bit over the top as a description of reality, heh, I'm off from here for now,
Probably back at the end of May.

6. May 6, 2017

### Brian E

I am not convinced either and I am far from being an expert. In my opinion you already have to accept some far out descriptions of reality if you accept relativitytheory and quantum mechanics to be true. Once these theories were considered nonsense. PS: I agree that my point is purely speculation and that it is far from beeing backed up by facts.

7. May 6, 2017

### strangerep

Both are now supported by vast amounts of experimental evidence. Reality does not care whether you consider it "far out" (i.e., weird).

But the multiverse? Not even testable.

Last edited: May 7, 2017
8. May 6, 2017

### Brian E

I could have expressed myself badly, but that is 100% what I tried to say, thank you for helping me I guess :-)

9. May 15, 2017

### Denis

Unfortunately this is the way how relativity and quantum theory is teached and presented to laymen. Such presentations seem to follow the rule "the more strange, the better".

And this cannot be corrected, at least here, because presenting interpretations of GR or QT which are less far out of common sense is evil philosophy/metaphysics, and not allowed here.

10. May 15, 2017

### phinds

If by "here" you are referring to PF, then whether things are "far out of common sense" has no bearing whatsoever on whether they are acceptable for discussion here. What DOES have a bearing is whether or not they are based on science.

Last edited: May 15, 2017
11. May 15, 2017

### Denis

The Lorentz ether interpretation is as much based on science as the Minkowski interpretation. But it is essentially not allowed to discuss it here. Even those points where above interpretations differ in their physical predictions (say, that in the Lorentz ether it would not be possible to prove Bell's theorem, because it does not forbid in principle hidden FTL effects).

12. May 15, 2017

### Brian E

I am a layman and you are right when you say that my point is based on some understanding og Lorentz. I did not understand Lorentz and his transformations as controversial. I still don't, but now I know :-)

13. May 15, 2017

### Denis

Sounds like a misunderstanding. The Lorentz transformations are not at all controversial. But Lorentz has also defended an interpretation which contained an ether in classical Newtonian space and absolute time, which caused a distortion of rulers and clocks in such a special way that the whole picture is Lorentz-symmetric, so that it becomes impossible to identify, with our distorted rulers and clocks, what is that system of rest of the ether. This theory is a possible interpretation of special relativity, it makes for all observable effects the same predictions as special relativity, and therefore is nothing but another interpretation of special relativity. This interpretation is named "Lorentz ether". Discussing this interpretation is, for some reasons I don't understand, not allowed here.

14. May 15, 2017

### Brian E

Thank you!! Your answer makes me very curious. I am not competent to debate if those theories should be allowed in here. I am glad it came up, it makes me wanna go google for some Lorentz ether :-)

15. May 15, 2017

### UsableThought

16. May 15, 2017

### Denis

Oh, thanks, I see that discussion is allowed:

17. May 15, 2017

### craigi

Hypothetically, there are solutions to Einstein's Field Euqations which permit travel to a previous time. You need to look into wormholes. Whether or not information can travel through a wormhole would depend upon a theory of Quantum Gravity.

There is indeed a paradox associated with it but it can be resolved by an interpretation of Quantum Mechanics whereby travel is to another spacetime. More discussion is in a paper by David Deutsche, if you can find it.

Also one of Wheeler's students explored this and published a book on the subject. Search for something along the lines of 'physics of the stargate'.

I'm sure that it's no longer an active area of theoretical research and it is considered purely hypothetical, but not impossible.

Last edited: May 15, 2017
18. May 16, 2017

### David Neves

You are confusing the observable universe with the entire Universe. We can only observe a finite subset of the Universe because if something was farther away than that, the light would not have had time to reach us since the Big Bang. The observable universe is a sphere centered on us, where the radius is the horizon distance, which is the maximum distance light could have traveled since the Big Bang. The volume of space within the observable universe increases as time goes on simply because the length of time since the Big Bang increases as time goes on. This has nothing to do with the expansion of the universe. In the 1920s, Vesto Slipher and Edwin Hubble noticed the redshift of the galaxies, which is explained by saying that the average distance between the galaxies is increasing. Imagine coins glued to surface of a balloon being blown up. The average distance between each coin increases over time. It would not be a good analogy if the galaxies were drawn on the surface of the balloon because then they themselves would expand, which doesn't happen. In the 1990s, by looking at the redshifts of Type Ia supernova, it was found that not only is the universe expanding but the expansion itself is accelerating, which we now attribute to dark energy. Does this mean that as time goes on, the Universe contains "more space"? No! We believe that the Universe is infinite, which means that it always contains the exact same amount of space, which is infinite. When we say that the Universe is expanding, we do not mean that it contains more space as time goes on. We mean the average distance between the galaxies increases as time goes on.

We know that traveling backwards in time is intrinsically impossible, no matter what, because it violates causality, which is famously described by the grandfather paradox. If you went backwards in time, and killed your grandfather, then you would not exist, which means that you would not be able to go backwards in time to kill your grandfather, and thus the paradox.

Despite what others have said here, there is strong evidence for the multiverse. There are many fundamental constants or parameters in the Universe, where if they had slightly different values, life would not be possible. If gravity was either weaker or stronger, life would not be possible. Without the multiverse, how would you explain the following?

In the second and third generation of fermions, the quark with 2/3 charge is heavier than the quark with -1/3 charge. If that were also true in the first generation, which you would logically expect to be the case since that's the pattern, the up quark would be heavier than the down quark, so the proton (uud) would be heavier than the neutron (udd), so then the proton could decay into a neutron, and the proton would be unstable outside the nucleus, so in the early Universe, all protons would quickly decay, and there would be no atoms at all. The most likely explanation is that that indeed is what happens in the vast majority of universes that have a particle spectrum resembling the Standard Model, but those universes have no physicists. We are in a rare exceptional universe in which the pattern is broken, enabling us to be here.

How could else could you possibly explain this without the multiverse?

There are many possible ways that you could have a multiverse. People often think of the Everett Many Words Principle, but that's not the only way. It could be as simple as regions of space that are outside our observable universe. You can have eternal chaotic inflation, where as time goes on, small patches of space suddenly undergo inflation, resulting in a local Big Bang, which results in a new universe, and this process goes on forever. Each universe could occupy a different place within the string landscape, and thus have different low energy physics, different particles, different coupling constants, etc. Another possibility is brane world cosmology where our universe is a 3-brane within higher dimensional space. There reason to assume that there would be only one 3-brane, and if there are many, each will be a different universe.

However, none of this changes the fact that traveling backwards in time is utterly intrinsically impossible, no matter what, because it violates causality.

Time travel has nothing to do with the multiverse. The multiverse has nothing to do with time travel.

The fact that you brought up the multiverse as an argument for time travel makes me suspect that you were using the phrase "traveling backwards in time" to mean "traveling to a universe that coincidentally looks identical to what our universe looked like at some specific point in the past".

Traveling backwards in time is not the same thing as traveling to another universe!

Traveling to another universe is not the same thing as traveling backwards in time!

The only thing that these two different things have in common is that they are both impossible!

First of all, depending on what version of the multiverse that you are invoking, it is unlikely that there is another universe that looks the same as ours did at some point in the past.

Second of all, even if there were such a universe, there would be no way to get there.

Third of all, even if you could, that would still not be time travel. You would be traveling through space to a different place, not a different time.

19. May 16, 2017

### Brian E

David Neves: I am a layman! To present the finetuningproblem as near hard evidence for a multiverse, is that not wrong? There is to my best knowledge other theories trying to explain these things,and also claims, that some of our ideas could be wrong, Like dynamic gravity or changing masses to explain the redshift. and also no other evidence to support the multiverse other than statistics based on our limited knowledge. Is that not a place where Occam's razor should be heard?
Based on my understanding of you arguments it also sems a bit strange, that you try to debunk other theories as wrong, and therefore timetravel to be impossible. Not that I feel we should call timetravel backwards a reality, but to me it seems just as much proved as the multiverse are.

20. May 16, 2017

### David Neves

I agree that the fine tuning of various fundamental parameters in the Universe to have values which are necessary for biological life is evidence for multiverse. Otherwise, you just have to say that is huge coincidence, lucky for us. I agree with that although other people on this forum disagree.

However, at the same time, we know that traveling backwards in time is impossible because it violates causality. We know that is impossible because of the grandfather paradox which I explained earlier.

The multiverse is speculative. Time travel is impossible.