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Conservation of Energy -- does it ever not apply?

  1. Oct 15, 2015 #1
    Are there situations when conservation of Energy does NOT apply? Thanks.
  2. jcsd
  3. Oct 15, 2015 #2
    When the system is not closed so that energy can be added or subtracted.
  4. Oct 16, 2015 #3
    Are there closed systems to which conservation of E does not apply?
  5. Oct 16, 2015 #4
    No one has discovered such a closed system.
  6. Oct 16, 2015 #5
    Is the Universe considered a closed system?
  7. Oct 16, 2015 #6
    Nobody knows. In some string theories, gravity can leak right out of our universe so no, in other theories the universe is entirely self contained.
  8. Oct 16, 2015 #7


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    I don't know about that but conservation of energy does not apply on cosmological scales.
  9. Oct 16, 2015 #8
    To expand on that, light loses energy more and more (turns red, redshift) as it travels further and further through space. Well, I believe it has something to do with the relative distance and speed of stars, but either way, the energy is lost. Someone please correct me if I'm wrong.
  10. Oct 16, 2015 #9
    Yeah, I think you are wrong on that. The red shift comes from the relative speed of the observed light source. Light does not lose energy as it travels.
  11. Oct 16, 2015 #10
    Ah, my mistake. In that case, what exactly did phind's mean when he said energy conservation doesn't apply on cosmological scales?
  12. Oct 16, 2015 #11
    I'm not sure but he may have been alluding to dark energy causing the acceleration of the expansion of space. However, my layman's understanding is that this is not a violation of the conservation law.
  13. Oct 16, 2015 #12


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    Light DOES lose energy as it moves through the expanding universe. Red shifting represents a loss of energy. But that's not the point here. What IS the point is that lack of energy conservation is because you can't even define energy properly on cosmological scales. This has to do with Noether's theorem and a proper explanation is beyond me.

    Just to be sure I'm clear, light does not lose energy in local systems where energy is conserved, such as a solar system.
  14. Oct 22, 2015 #13


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    Total energy is conserved.

    You can have different types of energy: potential energy, linear kinetic energy, rotational kinetic energy, internal kinetic energy (heat), etc. While total energy is conserved, it can be converted from one type of energy to another - even in a closed system.

    In other words, the bookkeeping for energy can be a nightmare.

    Yes, total energy is conserved in every situation, but the type of energy you're interested in for a given situation often isn't - which can create some unpleasant practical problems in the real world.
  15. Oct 22, 2015 #14


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    And you believe that to be true on cosmological scales?
  16. Oct 22, 2015 #15
    Are you asking if he believes that closed systems on the cosmological scale conserve energy, or are you asking if he believes that the universe itself is closed? I don't think there is any experiment that you could do that would prove that the universe is a closed system, or that it isn't. You can believe that closed systems conserve energy in all situations, but the universe itself is not closed.
  17. Oct 22, 2015 #16


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    I should have been more targeted. I'm asking if anyone believes that energy is conserved across cosmological scales. It is my understanding, as stated in post #12, that you can't even DEFINE energy across cosmological scales.
  18. Oct 22, 2015 #17
    Oh, in post #12, you said that the explanation is beyond you. I took that to mean that you were simply ignorant of the physics, maybe it's not your area of expertise. You're saying the physics governing energy at this scale isn't even defined?
  19. Oct 22, 2015 #18


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    Yes, that is exactly my understanding, as posted by some senior members on this forum, and specifically as explained by Sean Carroll here:


    His statements about it being defined seem ambiguous but his belief that it is not conserved is not. The statements about it not being defined were expressed more strongly on this forum. Sadly, I don't remember by whom.

    EDIT: OK, here's one:

    Last edited: Oct 22, 2015
  20. Oct 23, 2015 #19
    Phinds, given what has been discussed here and on other threads to which have been alluded there seems to be a sentiment of E being 'cancelled out' when we move to the cosmos scale. Is this purely a mathematical 'result' or is it being suggested that E is not merely transferring form and quantity...or becoming net-zero (I.E. in the context of -/+ offsetting E)...but it is actually dissapearing...ceasing to exist in ANY form/frame of reference, dimension, universe?? Being "destroyed"??
    That would seem...unlikely on many levels. Density of M or E diminishes as V increases. But to suggest E disappears...perhaps hasn't merely moved to another (hypothetical) uni/multiverse does not seem logical.

    But again, I still cannot wrap my head around the notion that the 'balloon' won't contract v. burst!

    Also, your discussion of red-shift and loss of E on a cosmos scale ...do you mean that an amount of light, if you will, which has mass, does ultimately lose or better TRANSFER energy to it's surroundings (E.g. thermo transfer) over huge distances? That sounds right. On a smaller scale, when we say there's a blue shift or red shift we are NOT then suggesting that E in that defined system has increased or decreased (respectively)..Rather, it's a discussion of measured E via wave lengths, observer's perspective/ref frame, etc.. Does that make sense?
    Am I thinking about all this appropriately?

    Thanks for your, others' continued insight!

  21. Oct 23, 2015 #20


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    But the energy of photons as they are stretched (red shifted) by the expansion of space DOES just decrease. Hard to get your head around at first but think about this ... where does it go if it goes somewhere? It does not radiate away, it just decreases. The photons start with some amount of energy and when they get to us they have less energy and nothing along their path gained any energy. This is not just math, it's reality.
  22. Oct 23, 2015 #21
    Yah, definitely hard to conceptualize that!
  23. Oct 23, 2015 #22
    But don't we also have no way to know that nothing along the path gained energy? Isn't it possible if not likely that it simply gets converted into something we're unfamiliar with?
  24. Oct 23, 2015 #23
    I concur, NJrunner...why wouldn't this system play by the rules which are well established? Energy being destroyed feels wrong... and ominous on many levels; here's to hoping it's just a matter of needing a new way to measure!
  25. Oct 23, 2015 #24

    Not an expert on the field, but from what I have read your opinion is a popular one but not everyoen agrees and the issue seems still open.
  26. Oct 23, 2015 #25


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    Google found this comment on the subject..


    Not sure I'm qualified to comment on it though.
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