Is Backwards Time Travel Possible or Just a Paradox?

[PhysicsBlock]

Hi - Physics is not something I studied in school and to be fair to myself, not doing so saved me from failing the class...

Over the years, my 16 y/o son and I have watched Nova and we do some light reading on Quarks, String Theory etc. My son seems to understand things that I do not and from time to time, he shares his thoughts on "far out" things with me. The reason for my post is what my son said earlier today:

"backwards time travel is impossible because the matter you are made up of now existed in the past as a different form, so going back in time would create a time paradox"

I browsed through the forum and probably don't know where to look, but where would you suggest he look or study to further support the above or otherwise.

Thanks!

 

[Whovian]

I don't know how that makes backwards time travel impossible, but one paradox that could arise from it would be the "grandfather paradox."

 

[genericusrnme]

If there's something preventing time travel I doubt it's anything like that, I don't see any reason why your matter would even interact with its older self.
There are paraxodes that can arise like as Whovian said, the grandfather paradox but the possibility of these, in my opinion, is not enough to justify the statement that time travel is possible.
I think the delicate and large nature of humans would be the biggest problem in time travel really.

 

[bobc2]

Hi - Physics is not something I studied in school and to be fair to myself, not doing so saved me from failing the class...

Over the years, my 16 y/o son and I have watched Nova and we do some light reading on Quarks, String Theory etc. My son seems to understand things that I do not and from time to time, he shares his thoughts on "far out" things with me. The reason for my post is what my son said earlier today:

"backwards time travel is impossible because the matter you are made up of now existed in the past as a different form, so going back in time would create a time paradox"

I browsed through the forum and probably don't know where to look, but where would you suggest he look or study to further support the above or otherwise.

Thanks!

PhysicsBlock, I think when it comes to time travel it is particularly important to first pin down the interpretation and universe model for special relativity. Then, carry out a time travel scenario within the context of that model. For example, one might come up with two different concepts of time travel, depending on whether you assume a block universe or whether you assume a Lorentz Ether Theory model (or perhaps one prefers some other concept).

Further, for example, should you try out a time travel scenario using a block universe model, the implications for time travel would depend on additional assumptions (model details) having to do with consciousness and the characterization of space and time at a fundamental level.

To the extent that one is not willing to accept that a special relativity interpretation can be known (an interpretation suggesting a fundamental mechanism or model explaining special relativity), it would not be possible to pose a serious time travel scenario. Of course one could catalogue a number of representative time travel scenarios, each one consistent with a different propsed model.

An example of some fundamental questions that would have totally different answers, depending on the selected special relativity model, are: "When someone goes back in time, what is it that is actually doing the traveling?" "Is 3-dimensional material traveling in time? What is the fundamental interpretation of the world line of one who travels in time?" "What is the role of consciousness?" "What is meant by reference to a 4th dimension (a mathematical construct, a dimension associated with something physical, etc.)?" "What is the root cause of causality?"

 

[GeorgeDishman]

Hi - Physics is not something I studied in school and to be fair to myself, not doing so saved me from failing the class...

Over the years, my 16 y/o son and I have watched Nova and we do some light reading on Quarks, String Theory etc. My son seems to understand things that I do not and from time to time, he shares his thoughts on "far out" things with me. The reason for my post is what my son said earlier today:

"backwards time travel is impossible because the matter you are made up of now existed in the past as a different form, so going back in time would create a time paradox"

I browsed through the forum and probably don't know where to look, but where would you suggest he look or study to further support the above or otherwise.

Thanks!

I'm just guessing but it sounds as though he is relating an argument sometimes made: if you could go back in time, there would be two of you after the moment when you arrived which would violate conservation of energy.

 

[bobc2]

I'm just guessing but it sounds as though he is relating an argument sometimes made: if you could go back in time, there would be two of you after the moment when you arrived which would violate conservation of energy.

Good point. How to deal with the problem in that case would be particularly dependent on the special relativity model selected.

 

[genericusrnme]

Good point. How to deal with the problem in that case would be particularly dependent on the special relativity model selected.

Nope, conservation of energy requires time to be homogenous, if there is backwards time travel then time isn't really homogenous and so energy doesn't have to be conserved.

 

[George Jones]

Good point. How to deal with the problem in that case would be particularly dependent on the special relativity model selected.

It is more appropriate to consider this in the context of general relativity, of which special relativity is, well, a special case.

 

[bobc2]

Nope, conservation of energy requires time to be homogenous, if there is backwards time travel then time isn't really homogenous and so energy doesn't have to be conserved.

At the time the future guy shows up in the past, there will be an energy conservation issue, depending on the special relativity model assumed.

 

[bobc2]

It is more appropriate to consider this in the context of general relativity, of which special relativity is, well, a special case.

A special relativity model still needs to be specified in order to anticipate the time travel scenario implications.

 

[Whovian]

A special relativity model still needs to be specified in order to anticipate the time travel scenario implications.

Yes ...

but specifying a General Relativity model directly specifies a Special Relativity model.

 

[conquest]

Hi,

I think the most profound reason against backwards time travel would be that it could break causality. So suddenly effect might precede cause, i.e. you get a response letter to a letter you haven't sent yet.

Any book on special relativity might help understanding the reasoning behind this. I had a really nice one written by Einstein but someone borrowed it and never gave it back :( and I don't remember the title. It was written for people without a heavy physics education though.

The way I interpret your sons argument is in the sense that you might wonder if I were to travel back in time, I should travel back along my own world line and inhabit the space I was in before. So this might be solved if you also become younger in the process or something. Anyway energy conservation shouldn't be a problem since the energy that pops up in the past goes away in the future so we might just swap it betweem times and end up with the same amount of energy/enthalpy or whatever.

 

[bobc2]

Yes ...

but specifying a General Relativity model directly specifies a Special Relativity model.

Of course. And naturally the world-line for the time traveling observer will be described in the context of a general relativity model. But, it doesn't hurt to emphasize some of the significant implications following directly from a selected interpretation of special relativity.

But, again, your point is well taken. After all it's the general relativity universe model (that includes a particular interpretation of special relativity theory) in which the travel scenario is carried out.

 

[bobc2]

Hi,

I think the most profound reason against backwards time travel would be that it could break causality. So suddenly effect might precede cause, i.e. you get a response letter to a letter you haven't sent yet...

This kind of consideration makes it all the more imperitive that you specify your special relativity model before embarking on the analysis. Different interpretations of special relativity (different models) can have quite different implications about causality.

 

[drbanner]

How do we know time travel backwards is not possible? Wouldn't someone have already come back by now? Perhaps alternate reality's/parallel universes would exist anyway if something was changed in the past, so there would be no way to change anything from the future the time traveler had come from anyway, because that reality is now gone. Any change in the past would just create an alternate reality for your future completely different from the one you had come from not really accomplishing anything other than changing the time travelers own reality. Trying to change past events is redundant. What If you gained a betting book like the 'Back to the Future' movie? After the first bet everything else would be different from that point forward in your new reality. That old timeline would be erased, and perhaps even some of the teams/athletes that competed in that other reality. It would be worthless and could not be trusted. How much could one person affect things over time? I think it would shock us all!

Time is sequential. Let’s get over it. The Singularity is as close as it gets, which is theoretically where time stops, or at least Planck Time 10−43 seconds.

Perhaps some will discover how to send a quark back a millisecond? What good would that do us anyway? Time travel backwards is a dead end theory in my opinion.

 

[bcrowell]

"backwards time travel is impossible because the matter you are made up of now existed in the past as a different form, so going back in time would create a time paradox"

I browsed through the forum and probably don't know where to look, but where would you suggest he look or study to further support the above or otherwise.

The magic term that you want to google on is "closed timelike curve" or "CTC." Also "chronology protection conjecture." A good popular-level books that discuss this kind of thing is Thorne, Black Holes and Time Warps, ch. 14. This article http://plato.stanford.edu/entries/time-travel-phys/ also has at least some material that should be intelligible to a non-physicist.

There short answer is that there is absolutely nothing in general relativity that forbids the existence of CTCs (backward time travel), but there are also strong reasons to believe that they don't exist in the actual universe we live in and that creating them through any natural or atificial process is impossible due to the need for exotic forms of matter that don't actually exist. There is definitely no simple, known, valid argument along the lines your son is seeking, because if there were, the chronology protection conjecture would not be an open problem, which it is.

I'm just guessing but it sounds as though he is relating an argument sometimes made: if you could go back in time, there would be two of you after the moment when you arrived which would violate conservation of energy.

This argument doesn't actually work. First off, there is no global law of conservation of mass-energy in general relativity that applies to all spacetimes. We have a FAQ about this: https://www.physicsforums.com/showthread.php?t=506985 You can get energy conservation laws in some specific spacetimes, but even in these spacetimes, the argument doesn't necessarily carry through: https://www.physicsforums.com/showthread.php?p=819700#post819700