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If you were to go back in time, how would you account for the extra mater in the Universe? Would not every atom in your body, the oxygen in your lungs and all that you brought with you, already exist as something else?

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  • $\begingroup$ This question does not appear to be about a World you are Building, and would perhaps be more appropriate for Speculative Natural Science. $\endgroup$ – Azuaron Oct 26 '16 at 1:17
  • $\begingroup$ Energy and matter are equivalent. If matter has appeared, some energy disappeared. (E=mc^2) . Makes sense too, the wormhole created would be of negative energy, thus netting you a equilibrated system. $\endgroup$ – Bloc97 Oct 26 '16 at 4:14
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    $\begingroup$ Time travel in the past is simply not possible, it is literally fantasy... magic so why do you even worry about the science ? $\endgroup$ – Charon Oct 26 '16 at 9:21
  • $\begingroup$ So what if they already exist? Many stories do a once-only approach to time travel so you don't get infinite do-overs but that's a plot device, not physics. $\endgroup$ – Loren Pechtel Oct 26 '16 at 23:43
  • $\begingroup$ Why would you need to "account" for it at all? If you're going back in time, that means it's possible for more than one of you to exist at any one time. Why should that be different for individual atoms? $\endgroup$ – Nicol Bolas Oct 27 '16 at 1:38
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This depends on how your time machine works.

Let's imagine past!William built a time machine back in 1970. The time is finally right to test it, so future!William dusts it off and opens the door. On the other side of the door...is his laboratory in 1970. When he walks through the door, he is in 1970; when he walks back through, he's in 2016.

Conservation of energy applies to a closed system, where nothing is entering or leaving. "The Earth" isn't a closed system, since the Sun is constantly adding energy. "The Universe" on the other hand is generally considered closed, since there's no known method for energy to enter or leave it.

But with your time machine turned on, "the Universe at midnight on January 1, 1970" can't be considered a closed system, because now things can move back and forth between 1970 and 2016. You've effectively forced "the Universe in 2016" and "the Universe in 1970" to be part of a larger closed system in which conservation of energy holds.

This is really weird, and runs counter to our intuitions about physics, but is a logical result of the way this hypothetical time machine works. You aren't actually producing energy out of nowhere; anything which comes through the door in 1970 has departed through the door in 2016.

As a side note: equivalent to conservation of energy is the claim that "nothing depends on time" (via Noether's Theorem). In other words, there's no law of physics which gives different results if your clock is five minutes slow. So if conservation of energy doesn't hold for the Universe in a single instant any more, then time symmetry doesn't either, which could have some interesting consequences...including, a convenient way to handwave any other weirdness in your fictional world's physics. ;)

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  • $\begingroup$ Good point about Noether's Theorem. Liked your argument that connecting two points in time expands the universe as a closed system so conservation of mass energy still holds. Although Lee Smolin has suggested local physical laws might not apply to the universe as a whole. In which case, the conservation laws might not apply globally. Plus one for me. Nice answer, Draconis! $\endgroup$ – a4android Oct 27 '16 at 3:34
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Well, atoms don't have serial numbers, so there wouldn't be any physical reason your atoms would be identified as your past self's atoms. Especially since many of your atoms are replaced on a regular basis, along with the air in your lungs.

This is essentially a question of where the soul is, and whether your soul or being can be in two places at once. When asked where the soul is, someone wise said, "The same place the music is in the organ."

So, if your characters travel in time, I would not be worried about that. This isn't actually a time travel paradox. If you do want to make difficulty of concurrent existence a thing, that was a major part of Time Cop (I think that was the one). They had it that if you meet yourself, you are pulled together and your flesh slowly fuses with their , disturbingly and painfully.

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    $\begingroup$ define ''soul'' $\endgroup$ – Charon Oct 26 '16 at 9:22
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    $\begingroup$ With respect, atoms do have serial numbers of a sort, each are unique at the sub atomic level and have lived as individuals for billions of years. $\endgroup$ – William Denis Guest Oct 28 '16 at 1:31
  • $\begingroup$ @WilliamDenisGuest I had understood it that there wouldn't be a physical problem if two atoms of the same sub atomic nature existed. Is there reason to believe it would cause unusual behaviour? Thanks for pointing this out. $\endgroup$ – J. Doe Oct 28 '16 at 1:48
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Whatever else time travel does, it doesn't involve the conservation of matter. Besides the conservation of matter, strictly speaking, only applies to closed systems and there is no reason why this law should apply to the universe as a whole.

It is possible to construct a model where the conservation of matter does have temporal properties. Assume normal matter moving in time has a positive temporal quality, when it is moving backwards in time it now has a negative temporal quality. When you add the two temporal qualities together the nett result is zero.

Once the matter stops moving backwards in time and is now moving forwards in time its temporal quality is gain positive. Adding this to the previous result of zero and the universe is back to the original amount of positive matter in the universe.

This suggests time travel won't have any affect on the conservation of matter. There is no need to account for extra matter in the past, because effectively no extra matter has been added.

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  • $\begingroup$ @HDE226868 Thank you for your explanation. Much appreciated. First time I ran into this same situation here. Sensible advice. $\endgroup$ – a4android Oct 27 '16 at 22:46

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