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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.

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    $\begingroup$ Possible duplicate : physics.stackexchange.com/q/7823 & links therein. $\endgroup$
    – user36790
    Commented Aug 18, 2015 at 4:28
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    $\begingroup$ @Keltari edit your question to make it clear why it's not answered by the earlier one, then. While you're at it, I suggest removing "Thoughts?" because that makes your question seem open-ended (which is a bad thing). $\endgroup$
    – David Z
    Commented Aug 18, 2015 at 4:42
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    $\begingroup$ There's a very simple solution that while sending a person into the past, the machine could simple absorb an equal amount of matter and pull it into the future - equal amounts of mass-energy in and out. There's other problems with time travel, but I don't (think) conservation of energy is a big deal breaker. $\endgroup$
    – userLTK
    Commented Aug 18, 2015 at 5:44
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    $\begingroup$ @userLTK That's worthy of recording as an answer. $\endgroup$
    – Selene Routley
    Commented Aug 18, 2015 at 7:40
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    $\begingroup$ Possible duplicate: physics.stackexchange.com/q/151428 $\endgroup$
    – GnoveltyGnome
    Commented Aug 18, 2015 at 14:02

3 Answers 3

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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.

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  • $\begingroup$ Excellent question, and excellent answer. $\endgroup$
    – pela
    Commented Aug 18, 2015 at 8:18
  • $\begingroup$ My biggest problem with standard time travel is still, what would your position be when arriving at your destination? One second from now, Earth has moved hundreds of kilometres through space, so calculating the spatial movement necessary to end up in the desired position would require extreme precision. An error on the 14th decimal would put you inside Earth or in outer space. $\endgroup$
    – pela
    Commented Aug 18, 2015 at 8:24
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    $\begingroup$ @pela: any even remotely sensible approach to time travel requires following a closed timelike trajectory. This is just a path through spacetime, so at any point you would be able to see where you were going and make sure you didn't miss your destination. When you talk about missing Earth I'd guess you're thinking of time travel as some form of discontinuous jump, but no time travel mechanism we know of works like that. $\endgroup$
    – John Rennie
    Commented Aug 18, 2015 at 8:57
  • $\begingroup$ @pela I've essayed on that on Worldbuilding SE. It's important for more knowledgeable readers that Hard SF keep up. $\endgroup$
    – JDługosz
    Commented Aug 18, 2015 at 9:21
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    $\begingroup$ @JohnRennie Yeah, on the other hand it seems that the Terminator movies use time travel mechanisms that we don't know of :) $\endgroup$
    – Hagen von Eitzen
    Commented Aug 18, 2015 at 14:39
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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.

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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.

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