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Suppose, we somehow made a time machine and go back to the past or go in the future and we come back bringing a 10kg object from past or future. Now, in present the mass of universe is increased by 10kg. But, we know the mass of whole universe is constant.

Now, let me know, is this possible to increase the mass of the universe by this method if we have a time machine?

Luffy
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    Shouldn't you ask on Physics SE about laws of physics? This is Philosophy SE. "Whole universe" is the entire spacetime, you are not increasing its mass by moving it around from one spacetime point to another, with or without time machines. – Conifold Sep 01 '21 at 05:35
  • I asked here because I thought it would be off topic there because time machine doesn't exist. – Luffy Sep 09 '21 at 11:40

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Energy is not conserved in General Relativity. The most robust model of time travel, closed timelike curves, only work for time travel after the 'machine' is established, but could allow such a mass to go backwards.

The energies involved in warping spacetime are gigantically greater than that of ordinary matter. In the collisions that cause detectable gravitational waves a significant fraction of the mass of the celestial bodies is lost into the waves - 5% in the GW 2015-09-14 event, and it can be well over 30%. A weight of 10kg would be below a rounding error compared to the scale involved in bending space.

CriglCragl
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  • “Energy is not conserved in General Relativity.” – No, this is too strong a statement. Even your linked source states

    ‘it would be irresponsible of me not to mention that plenty of experts in cosmology or GR would not put it in these terms … In particular, a lot of folks would want to say “energy is conserved in general relativity, it’s just that you have to include the energy of the gravitational field along with the energy of matter and radiation and so on.” …

     – Just Some Old Man Sep 01 '21 at 21:48
  • … There’s nothing incorrect about that way of thinking about it; it’s a choice that one can make or not, as long as you’re clear on what your definitions are.’ The view he ascribes to experts in GR is very common. This view is that energy is always conserved. – Just Some Old Man Sep 01 '21 at 21:48
  • @JustSomeOldMan: "In special cases, yes. In general, it depends on what you mean by 'energy', and what you mean by 'conserved' ." https://math.ucr.edu/home/baez/physics/Relativity/GR/energy_gr.html & what you mean by 'universe', I'd add. – CriglCragl Sep 01 '21 at 21:59