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I'm writing a sci-fi novel and I want it to be well founded in science but I'm a bit confused about time travel vs distance travel. Some have said that to travel great distances is really time travel. S, I'm just trying to make sense of all this.

I realize that the light I would see would be relative to the distance from my previous location and in some cases would be light that originated at different times and hence the universe would appear very differently from my previous location. But that doesn't seem like time travel to me (as others have stated here: Could an astronaut find their bearings in the Universe after being transported 6 gigalightyears from Earth?). From my new location, the light coming from earth might only show the earth when it was a fiery hell world, but that is old light. Equating traveling through space with time travel is like saying there is no concurrent anything. Just because I'm looking at very old light, doesn't mean that what I see is the current state, and that current state would exist in the same now that I exist in, even if I can't see it because light takes time to travel. In my world, the characters would be able to talk in real time through a worm hole but that assumes that they have not traveled in time.

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    $\begingroup$ This is a matter of semantics rather than anything else. When you say 'instantly', I think most would understand that as meaning no time duration, so implicitly no time-travel either. $\endgroup$
    – Lee Leon
    Commented Dec 5, 2017 at 9:15
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    $\begingroup$ I'm voting to close this question as off-topic because this is essentially a physics question and would be more appropriate to Physics SE. $\endgroup$
    – StephenG - Help Ukraine
    Commented Dec 5, 2017 at 9:18
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    $\begingroup$ Possible duplicate of Are there any ways to allow some form of FTL travel without allowing time travel? $\endgroup$
    – Mołot
    Commented Dec 5, 2017 at 10:11
  • $\begingroup$ Basic physics: photons don't experience time. For a photon all the events are simultaneous. A photon which left a star six billion light years away reaches Earth instantaneously from the point of view of the photon. $\endgroup$
    – AlexP
    Commented Dec 5, 2017 at 10:57
  • $\begingroup$ What StephenG is trying to say is that as you approach the speed of light time begins to slow. When you reach the speed of light ( c ) time actually stands still, which is why photons does not 'feel' time. Take a look at: en.wikipedia.org/wiki/Time_dilation . $\endgroup$
    – Kickass
    Commented Dec 6, 2017 at 12:15

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Just because I'm looking at very old light,

Light is ageless. There's no such thing as "old light".

doesn't mean that what I see is the current state,

The "current state" is what you can observe.

Any other state is theoretical from an observer's point of view.

and that current state would exist in the same now that I exist in, even if I can't see it because light takes time to travel.

Your mistake in all of this is thinking of time as "absolute". Time, like space, is relative.

One of the many consequences of the theories of special and general relativity is that the concept of simultaneous events is no longer absolute. The concept you need to understand is simultaneity.

These are difficult concepts to grasp, but essentially the only "now" is the now you can observe. Any other "now" is speculative and what one observer sees need not seem to correlate to what another does - at least in human terms. This is why relativity requires serious mathematics to fully grasp - the ideas simply don't work out in a human common-sense way.

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  • $\begingroup$ +1, I don't think it answers the question, but it does make the valid point that the OP is overly simplistic. $\endgroup$
    – Lee Leon
    Commented Dec 5, 2017 at 9:22
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    $\begingroup$ Well the problem is the OP is starting his question from a misunderstanding of time that fundamentally affects the way you define "time travel". The only consistent way to define "past" and "future" is for your own frame - in some frames of reference I might not even exist - not ever (which perhaps explains why my government wants a birth cert every time I so much as ask them a question :-) ). $\endgroup$
    – StephenG - Help Ukraine
    Commented Dec 5, 2017 at 9:33
  • $\begingroup$ of course from the viewpoint / frame of the astronaut he can define "past" and "future". $\endgroup$
    – Henning M.
    Commented Dec 5, 2017 at 17:01
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If you don't actually move there but just happen to be in a place 6 billion light years away, then yes, you haven't moved in time even though you'd not see an earth at all, because of the light's travel time. The nearly formed earth would become visible a good 1,5 billion years later from your position.

If you actually "travel" 6 billion light years in an instance, then you've moved significantly faster than light. This is physically impossible, but theoretically implies that you've gone back in time as well. Excuse me for not calculating the exact amount of time moved back, but you would add that on top of the 6 billion years the light needs to get to your position (all assuming the light can actually reach you without obstruction and is still perceivable there, which is more than unlikely)

For a more formally correct answer, StephenG said all that needs to be said.

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Such rapid movement between inertial reference frames would violate relativity so is not something real that could actually happen in our universe as we understand it today so it’s hard to give a definitive answer.

Assuming that the equations of relativity are basically true and you have somehow hand waved your way around the light barrier then this would have to involve time travel. Assuming that the equations of relativity are flawed in some way then it depends on how they are flawed.

The big problem is that such a scenario assumes that there is some sort of universal time for the whole universe which there is not.

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  • $\begingroup$ We may not understand how it happened, but scientifically we can understand its consequences. The trip can be considered to have been over spacelike hypersurface with a time interval of zero. Ergo, instantaneous travel. Some models of general relativity allow for this. People get too hung up on textbook understandings of special relativity. $\endgroup$
    – a4android
    Commented Dec 5, 2017 at 11:27
  • $\begingroup$ @a4android If this hypersurface is accessabe then faster than light travel is possible therefore there is a problem for relativity when applied to the broader hyper surface. Whilst some models of relativity might allow this some might not and it all depends on the details of how this is resolved. $\endgroup$
    – Slarty
    Commented Dec 5, 2017 at 13:31
  • $\begingroup$ Yeah, sure. But that's what I said in my own form of words. For fiction, more latitude about what works and doesn't is OK. One trivial point: the word is "accessible". $\endgroup$
    – a4android
    Commented Dec 6, 2017 at 3:16
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Time is relative. It depends where your observer is... If your astronaut travels with the speed of light (impossible btw) than it would be for him near instantly (Well except acceleration and deceleration).

But we on earth would think he travelled 6 billion years into the future. (Since we would age but he wouldn't.)

An observer* who is outside of earth, the exit point, and the travel path would see the astronaut going from earth to the other point in 6 billion years.

But I think you asked for a scenario, where an observer* would see the astronaut on earth and the next second 6 billion light years away. In that case nobody of your audience would think he travelled through time.

*Well it must be an omniscient being, since he would need to be far away and the light needs to travel far.

As far as I understand time and space cannot be separated. That's why physicists talk about spacetime. Also time is relative. It is really confusing. Watch some videos about it and be confused. For example the twin paradox^^

Answer: For your question: No it isn't timetravel. At least not in any sense your audience would understand. But ask in Physics SE or phone a professor at your local university.

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