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I The law of conservation of energy is wrong?

  1. Jun 1, 2016 #1
    e few days ago i talked with my teacher about the energy in the universe being constant. but we were completely confused when we came to the concept of:
    "because of the universe expansion everything moves away from eachother. and the same goes for the wavelenghts in light. because of the way electro-magnetic energy is being calculated. the energy increases with the wavelenght but isn't converted in anything."
    is this right or not
    if not where was our mistake.
     
  2. jcsd
  3. Jun 1, 2016 #2
    what is even the formula of the enrgy of a phonton
    E=hλ
    or
    E=hf
     
  4. Jun 1, 2016 #3

    mathman

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    Longer wavelength is lower energy.
     
  5. Jun 1, 2016 #4

    jbriggs444

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    One problem with saying that the amount of energy in the universe is the same from one instant in time to the next is with defining the notion of one instant in time.

    In classical Newtonian mechanics the notion of "at the same time" is taken for granted and conservation of energy works. In special relativity, the notion of "at the same time" depends on the coordinate system you choose but as long as you choose an inertial frame of reference, conservation of energy still works. But with general relativity and curved space time, there is no such thing as a globally inertial reference frame. The notion of "at the same" time becomes a matter of pure convention. Locally one can choose to use an inertial frame so that locally, conservation of energy still works. But globally, it does not.

    This in addition to the problem with even properly defining "energy" at cosmic scales.
     
  6. Jun 1, 2016 #5
    Well conventionally 'seconds' which can be measured by atomic clocks,
    Ideally though we need to know if there is a quantized Planck time, either theoretically or in fact.
    I won't be placing a bet on it.
     
  7. Jun 1, 2016 #6

    jbriggs444

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    Simultaneity has nothing to do with units of time.
     
  8. Jun 1, 2016 #7
    Fair enough, does that imply that in GR, simultaneity is not defined?
     
  9. Jun 1, 2016 #8

    jbriggs444

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    Yes
     
  10. Jun 1, 2016 #9
    The energy decreases with wavelength. But this still has the same problem. In fact, it is accepted that the radiation density of the universe has decreased more rapidly than the matter density of the universe because of this effect. Thermal energy is also reduced.
    General relativity complicates things, but I think if you treat the universe as relatively smooth (using a fluid approximation), it is just a matter of defining an appropriate gravitational potential energy which accounts for the loss in radiation and thermal energy. Maybe this sounds like cheating, but the point is that if you contract the universe, you should be able to get back all the radiation energy that you "lost" by expanding the universe.
     
  11. Jun 1, 2016 #10
    Yes too, but buses mostly do turn on time though (within acceptable limits of uncertainty)
     
  12. Jun 2, 2016 #11
    Check your units, E=hλ doesn't give the correct units for energy.
     
  13. Jun 2, 2016 #12

    jbriggs444

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    Indeed. The fact that GR does not prescribe a particular simultaneity convention does not preclude us from picking one that works locally.
     
  14. Jun 7, 2016 #13
    thanks everyone that helped a lot
    :dademyday:
     
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