Re: Doesn't Time Travel defy Conservation of Mass / Energy?

Date: Wed Aug 1 21:54:23 2001
Posted By: Benjamin Monreal, Grad student, Physics, MIT
Area of science: Astronomy
ID: 996459194.As

Message:

Hello Andrew,

First of all, time travel may be allowed under General Relativity only in certain configurations of black holes and wormholes and rotating universes. It is highly speculative at best, and it is safe to say that most physicists think that it is impossible. I do not believe in it myself, although perhaps this belief is personal rather than scientific.

The types of time travel allowed by General Relativity indeed do not violate conservation of energy. They never involve a particle "winking out" in the present and "reappearing" in the past or future, like time machines in the movies. Instead, usually time travel would involve a long journey through space. Some proposed time-travel scenarios I have heard of:

If the Universe as a whole were rotating (it's not), it might be possible to fly a loop around the whole thing and return before you arrived.
Waiting for the end of the universe, and avoiding a certain type of "big crunch" by a making a certain maneuver, may result in time travel - possibly re-emerging at the beginning of the universe.
Flying a certain looping trajectory around a maximally-rotating cylinder with ten times the mass of the Sun may take you backwards in time.
Various speculative flights into "black holes" and out of "white holes" could result in time travel. (The white hole-black hole connection is probably the same as the "wormholes" Kip Thorne mentions. One end of a wormhole would look just like a black hole; the other end would look like a white hole.)

I should point out right away:

the universe is not rotating, and
very large massive cylinders are probably fundamentally (as well as technologically) impossible to construct.
Wormholes and white holes, even small ones, are not known to actually exist. They are allowed to exist, but it's hard to imagine why or how they would ever have formed to begin with. Certainly none have been observed.

So I can't speculate whether any of these are possible or not. Perhaps they have been disproven already, I don't keep up with the current research in GR. But I am confident that they all conserve energy. Conservation of energy is really built-in to the theory of relativity from the beginning. Any process you can think of, within this theory, will naturally end up conserving energy - no matter how clever or complicated the process is. Time travel (if it exists) would be no exception. For example, imagine a time-travel journey that involved flying into a black hole and emerging from a white hole earlier in time. Flying into the black hole must make the hole heavier: conservation of energy is OK. The arriving time-traveler would then be seen to emerge from a "white hole", and this would make the hole lighter. So, over the whole history of the universe, you can follow this packet of mass around:

in the beginning there's some mass in a white hole;
later on it turns into a time-traveler and his spaceship, who fly around for a while;
still later a time-traveler and a spaceship of the same mass fly into a black hole;
thereafter, the black hole sits there forever.

The original packet of mass-energy is perfectly conserved.

I think that the proposals to "fly around the edge of a rotating universe" or "fly loops around a massive cylinder" end up removing rotational energy (slowing the rotation) of the object, but I really don't understand these proposals in any detail.

I can recommend the fascinating book by Nick Herbert, "Faster than Light: Superluminal Loopholes in Physics", which is where I first learned about all of this stuff. For the real meat on relativity, consider Misner, Thorne, and Wheeler's massive textbook "Gravitation".

Hope this helps,

-Ben

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