Time Travel: Is It a Meaningless Concept?

[MikeeMiracle]

TL;DR Summary

Is time travel a meaningless concept?

Assumptions:

There is no "absolute" time, time is all relative.

Taking the twins paradox as an example, both twins measure a different proper time when they re-unite due to each twin taking paths of differing lengths through spacetime.

Conclusion:

Time Travel itself is a completely meaningless concept, it could only exist if absolute time existed. There are just different events, two different observers take different spacetime paths between those events and thus measure a different proper time between them.

Is this an accurate conclusion? Is time travel a meaningless concept?

Thanks in advance

 

[DaveC426913]

Summary:: Is time travel a meaningless concept?

Assumptions:

There is no "absolute" time, time is all relative.

Taking the twins paradox as an example, both twins measure a different proper time when they re-unite due to each twin taking paths of differing lengths through spacetime.

Conclusion:

Time Travel itself is a completely meaningless concept, it could only exist if absolute time existed. There are just different events, two different observers take different spacetime paths between those events and thus measure a different proper time between them.

Is this an accurate conclusion? Is time travel a meaningless concept?

Thanks in advance

If you substituted the word "distance*" for "time" would your premises and conclusion still make sense?

Two twins take different routes to the cottage.

Both twins measure a different distance when they re-unite due to each twin taking paths of differing lengths through spacetime.

Distance Travel itself is a completely meaningless concept, it could only exist if absolute distance existed.

 

[PeterDonis]

Time Travel itself is a completely meaningless concept

I think you need to be clearer about exactly what you mean by "Time Travel".

 

[Drakkith]

Time Travel itself is a completely meaningless concept, it could only exist if absolute time existed.

Something doesn't have to exist or be possible to have meaning. Arriving back to my house after a trip to the store but before I ever left isn't possible, but it is a meaningful concept. Everyone knows what this means to some degree and it can easily be defined more rigorously using concepts from SR and GR.

 

[MikeeMiracle]

I think you need to be clearer about exactly what you mean by "Time Travel".

This is a very good point...which is making me thing long and hard about how to express this.

If an event can be in the past for one observer and in the future for another observer then each observers pasts and futures are all relative to themselves. This is what drew me to the conclusion of time travel being meaningless if it is all relative to the observer.

Having considered the question further however, no matter what the spacetime separation of the observers, given events sufficiently far the past and future, some events will always be in the past for both observers and some future event will always appear in the future of both observers, no matter where those observers reside.

Thus I guess I have just answered my own question, time travel is possible as there is a universal definition of past and present given events sufficiently far away.

I guess if the observers are in non-causaly connected parts of the universe the question also becomes meaningless.

I feel sufficiently knocked back towards the valley of dispair from the slope of enlightenment :)

 

[PeterDonis]

If an event can be in the past for one observer and in the future for another observer

Now you need to be clear about "past" and "future" and "observer".

In pre-relativity physics, we could divide up spacetime into those two categories: past and future, divided by an instant called the "present".

In relativity, however, this is no longer the case. Now spacetime has three regions, not two. There is the past--your past light cone--the future--your future light cone--and a third region that doesn't have a well-known name, although I like Roger Penrose's "elsewhere"--the region containing all the events that are spacelike separated from you.

That brings us to the third term, "observer". In your post, you appeared to implicitly be using that term to refer to one particular event on an observer's worldline. Usually in relativity, the "observer" just means the entire worldline, but if we talk about the "past" or "future" or "elsewhere" for that observer, we do have to pick a particular event on the observer's worldline. So you were implicitly using the right definition, but it's always a good idea to be explicitly aware of what definitions you are using for important terms.

Given the above definitions, you should be able to see that, while we can always choose particular events on two observers' worldlines such that some other event is in the past for one and the future for the other, this can never cause problems. But see further comments below.

time travel is possible as there is a universal definition of past and present given events sufficiently far away.

Notice that "present" did not appear on my list of terms above. Also notice that my definitions of "past" and "future" (and "elsewhere") above were not "universal". They are different for every single event in spacetime. What prevents there from being problems because of this is that spacetime as a whole has a causal structure such that any two events will have a well-defined relationship in exactly one of the three categories--event B will either be in the past of event A, the future of event A, or the "elsewhere" of event A.

Notice, finally, that "time travel" also did not appear on my list of terms above. And I still don't understand what you mean by it. As far as I can see, given the other terms and there definitions, there is no useful additional term that "time travel" could correspond to, so I'm not sure why it's an issue for you to begin with.

 

[PeterDonis]

What prevents there from being problems because of this is that spacetime as a whole has a causal structure

Although this is a "B" level thread, I think it's worth keeping in mind that in General Relativity, there are mathematical solutions of the Einstein Field Equation that do not have all the nice causal properties I was implicitly assuming (which the flat Minkowski spacetime of SR of course does have). Most physicists believe that solutions that don't have those properties are not physically reasonable, but a full discussion of that would go way beyond "B" level (and if anyone is interested in it it should be done in a separate thread, probably at "A" level).

 

[MikeeMiracle]

Notice, finally, that "time travel" also did not appear on my list of terms above. And I still don't understand what you mean by it. As far as I can see, given the other terms and there definitions, there is no useful additional term that "time travel" could correspond to, so I'm not sure why it's an issue for you to begin with.

Thank for your all your comments, as ever your definitions are far better than my own. They are however along the lines of my original thoughts which prompted me to create this post to begin with.

Referencing the quote above, I guess you could say the "definition" of "time travel" i am referring to would be the pop-sci definition you find amongst the general public and portrayed in various films.

The struggle of responders to determine or define "what i mean" for me just re-enforces my original conclusion. That the concept used by the general public itself has no useful meaning as all past and future events are all relative to the person recording those events.

I am in various "science" Facebook groups, for amusement if you like at their misconceptions and they like to talk about "traveling to the future" but as I was lying in bed last night it occurred to me that this "travelling to the future" is not so easily defined. That thought came to me in no small part to what I have learned from reading this forum.

 

[jbriggs444]

this "travelling to the future" is not so easily defined.

I went to sleep last night and traveled to this morning.

Even though we might quibble about exactly how far I time traveled while asleep (lacking a preferred coordinate system), there is universal agreement that my awakening was in the future light cone of my going to sleep.

Useful time travel to the future (to me) would involve the traveler experiencing less subjective time elapsed on a journey than the proper time elapsed on a (hopefully unique) geodesic between the start and end events. One approach for this is "suspended animation". Another is to use an accelerated trajectory, as in the twin paradox.

 

[DaveC426913]

I went to sleep last night and traveled to this morning.
Useful time travel to the future (to me) would involve...

Useful time travel to me would involve being able to come back to now once I'm done.

 

[Nugatory]

Useful time travel to me would involve being able to come back to now once I'm done.

That is, of course, backwards time travel from the future.

An interesting consequence of the non-Euclidean geometry of spacetime is that given any timelike worldline between two events, we can always find a shorter timelike worldline between them - that is, forward time travel in the science-fiction sense is possible. It's backwards time travel, which is to say a timelike worldline from an event to somewhere in the past light cone, that is likely impossible in any physically plausible spacetime.

 

[PeroK]

Useful time travel to me would involve being able to come back to now once I'm done.

You mean like Groundhog Day? You get to start again with the knowledge of your previous attempt to make something of your life.

Or, simply to relive your life again, unaware of how often you've been through the eternal cycle?

 

[PeterDonis]

I guess you could say the "definition" of "time travel" i am referring to would be the pop-sci definition you find amongst the general public and portrayed in various films.

That doesn't help, because (a) I don't know what that is, and (b) pop science and films aren't valid sources.

That the concept used by the general public itself has no useful meaning

In which case there's no point in discussing it here. This forum is for discussing physics, not pseudo-physics from pop science and films that has no useful meaning.

Thread closed.