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This question already has an answer here:

I know that we see in experiments (physical and thought) that energy is always transformed into something else, but what propels our universe to behave this way? What is happening at small levels that only allows conservation as a possible outcome and not destruction of energy?

Update: Based on a given answer stating that energy is mass, I should probably then expand this question into "Why must mass must always be conserved?"

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marked as duplicate by ACuriousMind, John Rennie, Qmechanic Feb 28 '16 at 8:59

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    $\begingroup$ How do you define energy at the general level if not as that which is conserved through time translations by Noether's theorem? $\endgroup$ – ACuriousMind Feb 27 '16 at 23:51
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    $\begingroup$ @GyroGearloose Mass is the length of the energy-momentum four-vector. The mass of a system is not the sum of the masses of the parts (the length of a sum of vectors isn't of the sum the lengths) and since energy and momentum aren't conserved, neither is mass. So it isn't conserved. $\endgroup$ – Timaeus Feb 27 '16 at 23:53
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    $\begingroup$ Possible duplicates: physics.stackexchange.com/q/19216/2451 , physics.stackexchange.com/q/2690/2451 and links therein. $\endgroup$ – Qmechanic Feb 28 '16 at 0:14
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    $\begingroup$ Mass is a form of energy; mass alone is not conserved. The Noether theorems, derived from analytical mechanics, tell us that conserved quantities come from symmetries: conservation of momentum comes from translational symmetry (homogeneity of space) ; conservation of angular momentum comes rotational symmetry (isotropy of space); and conservation of energy comes from a temporal symmetry (laws of mechanics do not change with time). So that is the deeper meaning. See en.wikipedia.org/wiki/Noether%27s_theorem $\endgroup$ – Peter Diehr Feb 28 '16 at 1:11
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    $\begingroup$ @GyroGearloose If you want a tutorial about what mass is, any book that covers Special Relativity in depth should go into it. If you want a tutorial about the lack of conservation of energy and momentum then any book that covers General Relativity in depth should go into it. The covsriant divergence of the Stress-Energy tensor is zero, but the divergence of the Stress-Energy tensor would give you conservation. But it's worse, energy and momentum don't even exist in General Relativity, only energy density and momentum density fields and there isn't a frame invariant way to add them up. $\endgroup$ – Timaeus Feb 28 '16 at 1:45
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The answer saying that energy was mass was incorrect. And neither is conserved, and neither is even additive.

In General Relativity you have a Stress-Energy energy tensor. It has ten independent components in any frame. And you can try to extract one of them to be the energy density and three others to give you the components of the momentum density. But that decomposition is locally frame dependent. And even if you did that, you only get a density at every point and since a surface of simultaneity depends on a global frame (which don't always exist and aren't unique when they do exist) trying to add up those densities at different points on a surface of "same time" to get a total energy and a total momentum is hopeless generally.

So there isn't an energy of the universe. And there isn't a momentum of the universe. And even if there were, they could be infinite. And even if it when they are finite, then the mass would satisfy $$(mc^2)^2=E^2-(\vec p c)^2$$ and the mass of the universe would not equal the sum of the masses of the parts. And since the energy and momentum would change over time, the mass usually changes over time.

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