Questions about results of time traveling

[TrentM2833]

I have been writing a lot about time travel and what would happen not only once you time traveled, but when you went to come back. Would that be possible? You might of ended your own timeline by entering another time and therefore, your use to be present timeline has already been written for you and you can't go back to it since you left it. If you could visit the future, does that mean the future is actually already happening but it another dimension of time. The same goes for the past, if you went back to prevent John F Kennedy's death, does that mean the past is also happening right now but in another timeline? Also would this be right in your opinion, even if you went back and prevented presidents deaths, that would have nothing to do with your own present time because yours has already been written and that will only be affecting those of that timeline in that period. This may sound uneducated, but I'm pretty new to this kind of stuff and I'm only 18 years old. I'v been studying physics books and a couple theory of relativity books but I'm still no where close to most of you physicist on here and i want to be one when older after i study it in college at some point and earn a degree for it.

 

[Pengwuino]

The idea of going back in time to save JFK or see the Civil War unfold is non-scientific garbage. Since that form of time travel is not science, there exists no scientific answers to your questions.

 

[TrentM2833]

The idea of going back in time to save JFK or see the Civil War unfold is non-scientific garbage. Since that form of time travel is not science, there exists no scientific answers to your questions.

Very true, so what kind of time travel could be explained by science? Would traveling back to the origin of the universe be more likely or another universe require time travel?

 

[Pengwuino]

I'm surprised you can't answer this question yourself if you've read texts on relativity. What texts have you read?

There are no "other universes" in standard, accepted physics. It's all science fiction at this point.

 

[K^2]

There are space-time geometries that allow you to return into the past. There could, at least hypothetically, exist structures, naturally or created by someone else, that would allow you to return far enough back to alter history. It's very difficult to say how that would work out.

Assuming that existence of these structures does not depend on any changes you can make, because it's the only way to preserve linearity, you can resolve this with quantum mechanics. Without observers, such a QM system trivially resolves itself. With observer being the traveler, it's a little more difficult. In Many-Worlds there are no problems. But how this would look in other interpretations, I'm not sure. What the observer ends up seeing should be no different than MWI prediction, because of equivalence of interpretations, so you can still definitely use that as a guide.

So assuming the space-time structure that allows you to travel into past to change history exists regardless of any changes you can make to history, and we look at scenario where you traveled into the past to change history with help of MWI, what you should observe is that history has changed, and goes along a new path. Contradictions with your own existence and memory would be completely ignored. So even if you are never born in the new history, you don't cease to exist or anything. You would retain memories of your original time-line, and you would not instantly gain memories of new time-line.

What happens when you return to your time might depend on how you do it. If you use SR to simply speed up time flow, you'll definitely arrive in the new time-line. But I'm not sure if that's the same outcome you get if you return using same GR method you traveled into past. I'd have to think about it.

 

[Pengwuino]

K^2, those are rather speculative areas though, correct?

 

[K^2]

K^2, those are rather speculative areas though, correct?

They are good predictions of standard theory under certain assumptions. Whether these assumptions are physically plausible is speculative.

Here is the problem. We know that we can time-travel with General Relativity, but we can only describe object interacting with past self via Quantum Mechanics. I have to make an approximation that space-time curvature doesn't depend on anything I use QM to describe. As soon as the two mix, the theory breaks down.

The reason I can use QM to describe self-interaction is effectively time uncertainty. In fact, you can set up a grandfather paradox experiment with lasers without needing the actual time travel. Take a 50/50 beam splitter. If you hit it from two perpendicular directions at once with coherent beams of equal frequency, you can control which way the outgoing beam goes by matching relative phases. For example, here is a very simple setup with two splitters and two mirrors. (I used double slashes to denote mirrors.) (Edit: You'll have to copy-paste these diagrams into notepad, or some other text editor with fixed-width font. Code tags are meant to resolve that, but apparently, not on this site.)

->----\-->---\
      |      |
      v      v
      |      |
      \-->---\--->-

The final beam goes in just one direction because the phases of the two incoming beams were properly matched.

Now imagine that I've set up something slightly different.

    //--<--\\
     |     |
     v     ^
     |     |
->---\-->--+--->--
     |     |
     v     ^
     |     |
    \\-->--//

Again, beam interferes with itself, but in a more interesting way. The portion sent around the loop can be phase-matched to cancel the beam going around the loop. Optically, not terribly interesting, but if I dial frequency precisely enough, and reduce power sufficiently, which can be done experimentally, I can set it up so that this entire beam is a single photon.

Lets look at it from perspective of a single photon that went down the loop. It goes around, hits the splitter, and prevents itself from going around the loop. Grandfather paradox. Yet, in a setting that can be easily solved even without resorting to QM via simple optics.

So in QM, grandfather paradox, as well as any other time-traveling paradox isn't an issue. We can resolve them quite easily. Collapse and decoherence present problems, but since we are still relying on axioms of QM, we should still be able to pretend that MWI is the "correct interpretation" and even if what we describe isn't actually what happens, from perspective of observer, it will look the same. So under these assumptions, we can say what you will see if you time-travel and change history.

Practically, none of it really matters, of course, The assumptions rely on space-time structures not being man-made, basically. Odds of them existing and you finding one are negligible. And if we start talking about man-made time-travel, we violate the main assumption of GR and QM being separable. To tell us what happens with man-made time-travel, we need quantum gravity, or whatever theory that would combine both.