Question about Time Travel

[jedaisoul]

When defining General Relativity, Einstein explicitly included the possibility, in principle, of travel in the time dimension. His words were “So there is nothing for it but to regard all imaginable systems of co-ordinates, on principle, as equally suitable for the description of nature”. Now, I know that nowadays, the scientific interpretation of “time travel” is restricted to different periods of time passing in different frames of reference. But is there an intrinsic reason for interpreting time travel in that way? If not, it would seem purely arbitrary.

[ajb]

In General Relativity "Time Machines" are understood as Closed Time-like Curves or CTCs. They represent paths in which it is possible to travel back into ones own past.

 

It is not immediately clear if the presence of such curves should rule out a space-time as "unphysical". Some people think it is and so restrict their attention to space-times without CTCs.

 

However, may interesting space-times do have CTC's. Maybe GR somehow rules out these space-times.

 

Honestly, I don't think it is clear with in GR if time travel is allowed or not. Look on Spires for the papers of G.M. Shore on the subject.

[Daecon]

Wouldn't time travel screw up the total Universal amount of matter and energy in the Universe at that specific moment, and therefore the whole thing of "energy can't be created or destroyed"?

 

Also, wouldn't this create some sort of cascade or butterfly type effect on entropy?

[jedaisoul]

Honestly, I don't think it is clear with in GR if time travel is allowed or not. Look on Spires for the papers of G.M. Shore on the subject.

I've had a look at Shore's piece on time machines, and although he discards three possible designs as impractical, it does seem that it is theoretically possible for a "time machine" to travel into the past. Only:

1. It cannot travel into it's own past (it's own back cone).

2. It cannot return to it's own present.

 

Conclusion 1. seems convincing from his description, but conclusion 2 seems too heavily dependent on the mechanisms used to achieve the "rerturn" journey. If it is possible to travel to an "alternate" past, surely it should be possible to travel from that "present" to an alternate past, which happens to be in the original back cone? So although it is impossible to achieve in a single step, perhaps a two step process would be feasible? On the other hand, it's quite possible that I've not fully grasped the implications...

 

Anyway, you have convinced me that it is not "clear with in GR if time travel is allowed or not". Can anyone else give reasons why they disagree with that view?

 

Wouldn't time travel screw up the total Universal amount of matter and energy in the Universe at that specific moment, and therefore the whole thing of "energy can't be created or destroyed"?

 

Also, wouldn't this create some sort of cascade or butterfly type effect on entropy?

I'd say probably yes to the first, and not sure to the second, but you'd really have to ask someone who knows a lot more about GR than I do.

[Klaynos]

Wouldn't time travel screw up the total Universal amount of matter and energy in the Universe at that specific moment, and therefore the whole thing of "energy can't be created or destroyed"?

 

Also, wouldn't this create some sort of cascade or butterfly type effect on entropy?

 

Yes, yes it would...

 

And as for entropy, well there's something called the arrow of time, which also really doesn't like time travel, so if you put entropy into the first line next to matter and energy then all's good.

[asprung]

I think it would. I think that only "now" exists as a colection of sucessive "nows" that you cant go back into, and that the future can only be reached when it becomes "now". If two diffrent "nows' existed there would have to be twice the mass and energy.

[jedaisoul]

I think it would. I think that only "now" exists as a colection of sucessive "nows" that you cant go back into, and that the future can only be reached when it becomes "now". If two diffrent "nows' existed there would have to be twice the mass and energy.

I'd agree with you, but, as I understand it, that is not the mainstream view. I'm really seeking a mainstream answer to the question, but thanks for your comments.