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From a wikipedia article:

In physics, the law of conservation of energy states that the total energy of an isolated system cannot change—it is said to be conserved over time. Energy can be neither created nor destroyed, but can change form; for instance, chemical energy can be converted to kinetic energy.

What is the reason behind this Law: is there a proof that we can't destroy energy? I mean if we couldn't destroy energy in our universe maybe we can do it in other universes the point is why is it a law and not a theory

Abhimanyu Pallavi Sudhir
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Mark
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    How would you, if at all? :) – mikhailcazi Aug 29 '13 at 11:50
  • I know this is behind the scope of imagination.but the question is how could it be a law without proof? – Mark Aug 29 '13 at 11:55
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    Well, many laws have been stated by just experiments - no proof. Boyle's Law? Charles' Law? etc. All were just experimentally derived. It wasn't proved. Same as this! – mikhailcazi Aug 29 '13 at 12:21
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    Did you know that energy doesn't exist? How can you destroy something that doesn't exist? :) – rook Aug 29 '13 at 13:07
  • Doesn't exist? Really how is this – Mark Aug 29 '13 at 14:47
  • @mikhailcaizi what experiment was used to verify this law? I'm curious. – Mark Aug 29 '13 at 14:48
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    Energy is a concept, a way the scientists describe the world, behind the energy always stand some physical process. Energy is not a substance.. it's like a currency of inter process communications. – rook Aug 29 '13 at 15:54
  • DownVote? what is the problem? – Mark Aug 31 '13 at 12:15
  • @neomhmd: rook was joking. – Abhimanyu Pallavi Sudhir Sep 07 '13 at 07:50
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    @neomhmd: I don't know. The question seems fine. I'll upvote to reverse the downvote. – Abhimanyu Pallavi Sudhir Sep 07 '13 at 07:50
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    Most so-called "scientific laws" or "natural laws" are simple facts. They don't need proof. For example, it's a natural law that if you stand on the Earth, and drop a stone from your outstretched hand, it will fall to the ground. (Gallileo, and then Newton, and then Einstein each improved on that description by providing increasingly sophisticated mathematical expressions of the same basic idea.) If you assert that the stone will not fall, nobody will take you seriously because anybody who is sufficiently skilled in the art can conduct the experiment and see the result for him/herself. – Solomon Slow Jan 27 '16 at 20:54
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    Same goes for conservation of energy: If you claim that the energy of a closed system is not always conserved, then the burden of proof is on you because nobody else has ever been able to perform an experiment that contradicts the law. – Solomon Slow Jan 27 '16 at 20:57

2 Answers2

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The point of defining energy is to obtain a conserved scalar quantity -- as an example of how this works out, have a look at the derivation of the Newtonian formula for kinetic energy, where energy conservation is disguised as "kinetic energy is equal to the heating produced when it's destroyed".

There are plenty of conserved scalar quantities of course -- charge, each component of momentum, whatever -- so the real definition of energy is as the conserved quantity mandated by time-translational invariance through Noether's theorem.

Abhimanyu Pallavi Sudhir
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  • I have read in several different places that Einstein for years didn't know about the first Bianchi identity - this sounds like an urban myth but I never made not of the source at the time. Do you know anything about this? – Selene Routley Aug 29 '13 at 12:20
  • @WetSavannaAnimalakaRodVance: That cannot be . Einstein certainly had a lot of physical intuition. If he knew anything about the fact that the covariant derivative kills the einstein tensor (which he did, as you can see from his 1915 and 1916 papers), he would certainly know about them. And reallyy, the first bianchi identity. How can he not know thatl? – Abhimanyu Pallavi Sudhir Aug 29 '13 at 12:28
  • That;s what I thought: I thought, though that there might be a grain of truth insofar that I could imagine that his derivations might not have called on the identities succinctly but rather could have been more first principles, i.e. deriving what he needed from the identities from first principles. – Selene Routley Aug 29 '13 at 12:34
  • Interesting that you mention general relativity, which is where energy conservation happens to not apply. In a Friedman-Lemaitra-Robertson-Walker Universe, there is not time-like Killing vector (the equivalent of time translation invariance in GR), so energy is not conserved globally (or rather not even definable). – Volker Aug 30 '13 at 08:07
  • @Wolker: That is correct, but the stress-energy=m-omentum tensor is still divergence - less . – Abhimanyu Pallavi Sudhir Aug 30 '13 at 09:20
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    Nice answer, in my opinion, the answer would be nicer if you add statements such as "Because you are made of atoms, and has to obey such laws; therefore you can't destroy energy" :) – Shing Feb 07 '16 at 09:55
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It is much more appropriate to call it a law than a theory. A theory is an explanation, whereas a law is based on repeated observation and/or experimentation. If you are concerned that it isn't or can't be proven then you could call it a hypothesis, assumption, or postulate of conservation of energy. Of those three, postulate would probably be the best answer, as conservation of energy is accepted. However, the conservation of energy has been demonstrated so many times in so many ways, and in so many fields, it probably has more experimental support than many of our other scientific laws.

jdj081
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