Impossibility of time travel due to energy conservation?

Question

I just watched the movie Terminator Genisys and started pondering time travel.

While pontificating and stretching my limited remembrance of AP Physics, science documentaries, and YouTube videos I remembered the laws of Conservation of Energy and Mass.

Would that not make time travel impossible? You can't send something elsewhere in time as you would be destroying energy and/or mass, by removing it from the current time. And it would be impossible to arrive elsewhere in time, as you would be creating energy and/or mass.

I know this is theoretical and a real answer can't be found till someone invents a time machine. I'm just curious as to what people think of that.

Answer 1

Conservation of energy/mass is the result of a symmetry called time shift symmetry, and if this symmetry is broken energy/mass will no longer be conserved. It is far from obvious that time shift symmetry would be preserved if closed timelike curves were possible, so you can't use conservation of energy as an argument that time travel is impossible.

Answer 2

Considering 'space-time', rather than space and time, time is essentially just a 4th axis along which we can move (generally at a rate of one second per second in the positive direction); the other three being the familiar x, y and z

Sending something backwards through time (if possible) would not involve destroying energy in the present and creating it in the past, but moving it from one place to another. It would be a continuous process, like walking from one spot to another. It would not be a cessation of existence in one time and a reappearance in another, like classical teleportation in space, which is impossible to the best of my knowledge (though I'm curious as to whether it would look like that to an observer).

Conservation of energy doesn't prohibit movement in time any more than it prohibits movement in space.

Entropy probably does prohibit time travel, but that's a different question entirely.

Answer 3

Most time machines seem to require having exotic matter which has negative energy density, so perhaps you can send back some negative energy and some positive energy.

Creating mass isn't an issue, mass isn't conserved and really that's because the mass of a system isn't the sum of the masses of the parts.

And there are other options. For instance, if you use a wormhole to go back in time (wormhole connects the here-now to a there-then) then the wormhole might appear to be a certain size before you come out and then appear smaller when you step out of it, thus it could have a limit to what comes back through it. That could actually be reassuring to imagine a limit to what is coming from the future.

And to the people watching you go in, they might see the wormhole get larger.

Another option is that it be like teleportation where really it is about changing the stuff at on replace to be like the stuff at another place, so you might be sending back information that changes the form if the energy that is already here.