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 Posted: Fri Apr 13, 2007 10:33 pm
As I was walking home tonight, I got to thinking about that old hypothetical obsession of Sci-Fi authors and modern physicists - Time Travel. Two thoughts came to me:
First, let's say one actually succeeds in going back in time. BUT, since the future hasn't happened yet, what guarantee is there that one will wind up back in the SAME future as the one left (instead of say, one in which Hitler won WW2, or got hit by a stray shell in WW1, etc.)?
Second, and this is the more interesting. Say (for the sake of simplicity) that you've been sent back to the past in the same way as the terminators did in the Terminator movies. The problem as I see it is that the matter that you are composed of already exists in the universe. So, as far the universe is concerned, we are double-counting that matter which means that in essence you are adding to the matter/energy of the universe (let's call that x) new matter/energy (call that y), so that after you've been sent back, the new amount of matter/energy of the universe is x + y, whereas previously it was just x. So it would seem that time travel would appear to violate the conservation laws... right? AFAIK, I've never heard of anyone else realising that....
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Posted: Sun Apr 15, 2007 4:55 pm
I'm going to have to step aside about the conservation laws: some solutions of GR allow for the potential of time-travel, but I do not know how to interpret the implications of the corresponding stress-energy tensor.
The conservation of energy can be found by imposing temporal invariance onto the universe. Modifying the equations to allow time travel would result in different conserved currents.
Though, any time travel must be self-consistent. That is, you cannot go back into the past and change anything. Though, I can be fatalist about things.
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Posted: Sun Apr 15, 2007 11:49 pm
I find that this page has some interesting things to say about logical/historical consistency and time travel, although I don't think it addresses the conservation problem.
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Posted: Tue Apr 17, 2007 6:52 am
A Lost Iguana I'm going to have to step aside about the conservation laws: some solutions of GR allow for the potential of time-travel, but I do not know how to interpret the implications of the corresponding stress-energy tensor. The conservation of energy can be found by imposing temporal invariance onto the universe. Modifying the equations to allow time travel would result in different conserved currents. Though, any time travel must be self-consistent. That is, you cannot go back into the past and change anything. Though, I can be fatalist about things. why cannot you change anything?
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Posted: Tue Apr 17, 2007 1:05 pm
poweroutage why cannot you change anything? Personal preference. It helps avoid all paradoxs. http://en.wikipedia.org/wiki/Novikov_self-consistency_principle
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Posted: Tue Apr 17, 2007 7:11 pm
A Lost Iguana I'm going to have to step aside about the conservation laws: some solutions of GR allow for the potential of time-travel, but I do not know how to interpret the implications of the corresponding stress-energy tensor. The conservation of energy can be found by imposing temporal invariance onto the universe. Modifying the equations to allow time travel would result in different conserved currents. Though, any time travel must be self-consistent. That is, you cannot go back into the past and change anything. Though, I can be fatalist about things. What does the stress-energy tensor have to do with anything? BTW, I've learned about another physicist who is attempting to crack this technological holy grail: http://www.youtube.com/watch?v=oRWwI61so5Qhttp://en.wikipedia.org/wiki/Ronald_MallettOh, and thanks for that page, Layra-chan. I really hate it when movie/TV time travellers travel "only" in time and don't wind up in the vacuum of space....
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Posted: Wed Apr 18, 2007 8:33 am
Cynthia_Rosenweiss What does the stress-energy tensor have to do with anything? The stress-energy tensor contains the information about the energy and momentum in a particular solution of GR; I assume it would be the source of any conservation laws in GR. I do not know if the solutions of GR that allow time travel have peculiar energy situations. I can tell you that in the Lagrangian density approach of QFT, energy conservation comes from the time-invariance of the Lagrangian densities. Similarly, invariance with respect to a translation will lead to linear momentum being conserved. Allowing time travel will affect what we should use to construct the Langrangians and the corresponding conservation laws [off the top of my head, causality in SR is enforced by hand when you second quantise (canonical QFT)].
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Posted: Thu Apr 19, 2007 8:33 pm
The Kerr Black Hole solution to Einstein's equations, which give a slowly rotating black hole, allow for time travel, in that there are closed time-like curves. It also has a region that pumps out energy for nothing.
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