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Mass/Energy Creation and Conservation

  1. Jul 16, 2013 #1

    Nugso

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    Hello everyone. First of all, I should like to apologize if the question I'm going to ask looks so ridiculous. As far as I know, energy can't be destroyed or created. It only changes forms. So far so good, but how in the beggining did it occur? How was it created? Where/How did the first energy come?


    My second question is about the conservation of mass. Is mass really conserved? In closed systems? I don't think it really is converted. ( Speaking in terms of the today's matter explanation) Basically, what I've learnt in high school is that it's conserved.( Like in C + O2 -> CO2) Is mass conserved in that? Or by saying mass is conserved, are we talking about mass-energy is conserved? ( Electron + Positron -> Photon)
     
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  3. Jul 16, 2013 #2

    DrDu

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    While energy is conserved in both relativistic and non-relativistic mechanics, mass is only conserved in non-relativistic mechanics. This includes chemical transformations as the speed of the valence electrons is low compared with the speed of light, but not electron positron anihilation.
     
  4. Jul 16, 2013 #3

    Nugso

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    Thanks, DrDu. So in non-relativistic mechanics such as closed or open systems, mass are conserved.

    By the by, where/how did the first energy come from? We say it cannot be destroyed or created, but in fact at some point when the universe was created( Big Bang), it also came into play, but how?
     
  5. Jul 16, 2013 #4
    Drumroll! You have unlocked the next level in the game. Welcome to open questions in cosmology. There are different hypotheses (note: hypotheses) beyond the Big Bang - you can have a look at this section as an introduction.

    Like DrDu mentioned, mass is not conserved in annihilation and pair production (but energy and momentum is conserved).

    No, it's not ridiculous, these are good questions IMO. But if I could answer what happened at (or, hypothetically, before) the Big Bang, you should award me a Nobel Prize :smile:.
     
    Last edited: Jul 16, 2013
  6. Jul 16, 2013 #5

    Nugso

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    Thanks for the link DennisN! Though I don't think I'll ever be able to answer those questions, they're really worth reading and thinking about.





    So, the law of conservation of mass is like the laws of Newtons. They're correct to some extent.


    Who knows, maybe you'll one day win!

    Thanks for the answer by the way. :biggrin:
     
  7. Jul 16, 2013 #6

    WannabeNewton

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    There is no notion of global energy conservation for expanding cosmological models (e.g. Friedmann models) in general relativity. In order to even define a physically meaningful notion of the total energy of a space-time you need asymptotic flatness, which the Friedmann models don't have. Energy is only conserved locally in general.
     
  8. Jul 16, 2013 #7

    Nugso

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    Hi WannaBeNewton, thanks for the answer, but what did you mean by "conserved locally in general"?
     
  9. Jul 16, 2013 #8

    WannabeNewton

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    What I mean is that we always have a differential conservation of energy (to put it loosely, one that holds in arbitrarily small regions) even for expanding cosmological models but only a special class of space-times (of which expanding cosmological models do not belong to) have a physically meaningful notion of total energy and global energy conservation (global in the sense that it includes everything-it is an integral relationship over all of space).
     
  10. Jul 16, 2013 #9

    Nugso

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    Oh, okay! Thanks for the explanation.
     
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