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Although there are already atleast four questions on it (most of them closed as off-topic), asking the same since none of the answers made much sense to me and did not seem off-topic to me (unless time-travelling is considered off-topic w.r.t physics - although given the energy-mass criterion, I believe it to be still be a valid one for physics).

The question is (in layman's terms) - how is it possible that the law of conservation of energy-mass will hold, if its possible to travel through time ( either to future or to the past)?

Since it would open up the possibility of increasing the mass of the universe from an unknown but finite quantity to effectively an infinite one (independent of whether one travels forward into future or back to the past), time-travel is something the laws of the universe, collectively, would 'conspire/colloborate' so ensure that it never occurs!

Conversely assuming that the law of conservation holds then time travel would not be possible. Any theory that talks about time-travel would effectively be just that, a theory/hypothesis with a trivial proof against it - the proof being the law-of-conservation.

Other links that talk about the same topic are below:

PS: Assuming I am still wrong, if there is any read-up that I could do which explains in layman's terms why, that would be great.

Qmechanic
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Ravindra HV
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    As stated in other answers, energy is conserved only in the case of time translational symmetry. Look for videos on Noether's theorem. – rghome Dec 15 '19 at 18:17
  • Thanks. But I still have the same question. On wikipedia, Nether's theorem links to time translation symmetry, which in turn links to time crystal, which in turn states that time crystal's do not break the law of thermodynamics. I did not quite understand time-translation-symmetry. But in any case it does to appear to imply time-travel. So from what I understood, the question still holds. – Ravindra HV Dec 15 '19 at 18:39
  • I don’t see any problem with the conservation of energy if you travel backward in time in a continuous way, similar to how we travel forward in time. The problem is when you discontinuously jump forward or backward. – G. Smith Dec 15 '19 at 18:48
  • @rghome - Correction. It does not to appear to imply time-travel. So from what I understood, the question still holds. – Ravindra HV Dec 15 '19 at 22:32
  • @G.Smith Can you say what is travelling in continuous way? In any case the rationale would be that once a person travels back in time then effectively the total mass in the system would correspondingly increase (even if its small as compared to planetary masses). So time travel would be expected to break few rules. – Ravindra HV Dec 15 '19 at 22:40
  • Assuming our current age of universe is about 10,000 years and that it universe started with 100kg of mass (big-bang), we should be able to use time travel for example to 2nd year after the birth of universe, increase the mass in the universe of our current time (10000) to 110 kg. While it would immediately mean that between 2nd and upto 10000 th year of the universe, the mass of the universe had been 90kg (all along) the point would still be that we've effectively been able to circumvent the first law of thermodynamics (energy cannot be created or destroyed). – Ravindra HV Dec 15 '19 at 22:58
  • An alternate thought would be that, in theory, if we go back in time and we're able to transfer mass/energy from past, if its possible to do that more than 50% (assuming newton's 3rd law will not be a limitation), then we should be able to transfer 99.999% of the mass from the past into our current time. While we may be able to double the mass of the universe of our current time, it would simultaneously imply that we should have never existed in the first place - since 99.9% of the mass in the previous timeline had been removed ! – Ravindra HV Dec 15 '19 at 23:04
  • Hence the conclusion (in the question) that time-travel would be something that would effectively be restricted by the laws of the universe. – Ravindra HV Dec 15 '19 at 23:04
  • Going forward continuously in time means going from $t$ to $t+dt$ at each step. Going backward continuously means going from $t$ to $t-dt$. Jumping discontinuously means going from $t_1$ to $t_2$ (either earlier or later) without going through any times in between. – G. Smith Dec 15 '19 at 23:06
  • Noether’s Theorem doesn’t care whether a system evolves forward or backward in time. Energy is conserved either way if there is time translation symmetry. – G. Smith Dec 15 '19 at 23:11
  • @G.Smith - So what is it that you see would prevent one from transferring 100% of the mass of the universe (theoretically) from some point in the past to the present? Effectively we should not have existed at all since we would have gone to the past and moved all of its mass to the present, leaving no mass (and as such no chance for life to form) in between? – Ravindra HV Dec 15 '19 at 23:14
  • Sorry, I think that you are now far outside mainstream physics and I’m not going to discuss what I consider a nonsensical scenario. In addition, I am seeing a message that comments are not for extended discussions, so I’m done. – G. Smith Dec 15 '19 at 23:19
  • remember that travel to the past doe snot exist –  Dec 16 '19 at 14:54

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