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Is the Universe Leaking Energy? Tamara M. Davis

  1. Jun 11, 2012 #1
    Interesting Article on Cosmology...some good insights, no math, illustrations. Lots for us novices to consider.

    By Tamara M. Davis a Scientific American article]

    Is the Universe Leaking Energy?


    My synopsis:

    Emmy Noether found that conservation of energy relies on time symmetry. The changing shape of the universe due to cosmological expansion means that spacetime is changing....is not symmetric..... and so conservation of energy does not apply. So the universe does not violate the conservation of energy; rather it lies outside that law’s jurisdiction.

    One interesting insight:

    Photons traveling in an expanding universe appear to lose energy via cosmological redshift. What about matter: You find that the de Broglie wavelength of particles increases by exactly the same proportion as a photon’s wavelength does! Thus light and matter seem to behave in exactly the same way when it comes to 'energy loss'.

    Some here may not like so much her description of Doppler shift and implied cosmic microwave background radiation redshift:

    In small enough regions the universe makes a pretty good approximation of flat spacetime. But in flat spacetime there is no gravity and no stretching of waves, and any red-shift must just be a Doppler effect.... so the relative motion of the emitter and observer means that they see photons from different perspectives and not that the photons have lost energy along the way.
    Last edited: Jun 11, 2012
  2. jcsd
  3. Jun 11, 2012 #2
    1. Noether theorem refers to continuous symmetries of the Lagrangian of the theory, the ground state may as well break that symmetry (spontaneous symmetry breaking). Nevertheless, the conservation law associated with it is still valid (along classical trajectories, for the correlation functions--propagators, there are Ward-Takahashi identities);

    2. You are mixing the continuous time-translation with the discrete time-reversal symmetry;

    3. In GR, the stress-energy tensor is coupled to variations of the metric tensor. The gravitational field also carries energy-momentum.
  4. Jun 11, 2012 #3


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    Yes, Davis is saying the putative energy loss of photons due to cosmological redshift is an illusion, which is consistent with the position taken by Bunn and Hogg in http://arxiv.org/abs/0808.1081.
  5. Jun 11, 2012 #4

    And so do you conclude something different than Davis??

    It's possible my attempt at a quick summary misstated Davis' position,,,she does mention continue time translations as a requirement....
  6. Jun 11, 2012 #5
    I don't believe that there really is such a thing as a photon, a little packet traveling around. I think that "photons" are only meaningful when energy and matter interact. Otherwise they have no existence.

    The total energy of the field is conserved. This is all that matters.
  7. Jun 12, 2012 #6


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    Light is transmitted in packets of discrete energy, with the amount given by their frequency (E = hf). We call these photons.
  8. Jun 12, 2012 #7
    Well, I once had a discussion with a group of physicists and most of the thought that photons didn't really exist. I have also read a quotation of the head of the physics department of a major university that he thought the whole idea was bogus. So pardon me if I do not take your statements as authoritative.
  9. Jun 12, 2012 #8
    How do we verify your claims?
  10. Jun 12, 2012 #9


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    But it is the conventional view. Simply taking some physicist's word for it (and, yes, even if he happens to be the department chair) should not constitute a resounding authoritative statement either. Especially if you cannot recall what, exactly, the argument is. It's probably worth while to think it through yourself -- consider the arguments for and against the existence of photons -- before you make such definitive and contrarian statements.
  11. Jun 12, 2012 #10


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    The photoelectric effect (for which Einstein won his Nobel prize) is proof of the fact that light is transmitted in discrete packets of energy:
  12. Jun 13, 2012 #11
    Not so.
    The Davis article concludes very neatly, unambiguously, and correctly, that conservation of cosmological energy is OUTSIDE the theory of 'conservation of energy'. [It is not part of general relativity.] In curved spacetime, velocity, distance, speed, energy do not have the 'classical' appearance of flat spacetime and so 'conservation of energy' loses it's meaning.

    Regarding 'photons don't really exist' are you sure the physicsts didn't mean that you do not have to think of light in free space that way?? Maybe that the wave perpsective is preferable or satisfactory?? I'd agree with that picture. As Chalnoth posted, it sure is tough to maneuver around the photoelectric effect without photons!!
  13. Jun 13, 2012 #12


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    Can a free photon exist in space which will never be absorbed? Specifically, from the CMBR? If you say NO, it cannot exist, then you are agreeing with ImaLooser as to what is conventional.

    So that is my question to you. Can it exist? Because if you say NO, I believe there are testable consequences in an suitably open universe (since a photon headed to deep space would never encounter any matter and therefore cannot be emitted in that direction).

    Or is that crazy talk? :smile:
  14. Jun 13, 2012 #13
    I think they are making a statement about the term "really exist"

    You can argue that atoms are mathematical abstractions that don't "really exist". You can argue that we only sense things though sensory input so the entire external universe doesn't "really exist."

    Care to share the quote?

    If you are making the statement that photons don't exists and neither to electrons, you are making a philosophical statement about the meaning of existence, and that's not an argument I care to get into.

    If you argue that photons don't exist but electrons do, then this makes zero sense.
  15. Jun 13, 2012 #14
    Yes. Photons are particles as much as electrons, and if you can have free electrons in space, you can have free photons.

    You take an electron and a position together and you get two photons. You take two photons and bang them together and you get an electron/positron pair.

    The only difference is that electrons have rest mass, which changes the Doppler equations.
    Last edited: Jun 13, 2012
  16. Jun 13, 2012 #15
    And what about electrons. Do you not believe in those either? Also you have fermions and you have bosons. From a particle physics point of view, there is no difference between "energy" and "matter". They are all particle-waves. The difference that was see is merely because some particles obey Pauli exclusion and some don't.

    One general problem with cosmology is defining "total".
  17. Jun 13, 2012 #16


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    Photons carry momentum and energy, which in turn means that they interact with gravity. So even if the photons are never absorbed, they still affect the motions of other objects through the gravitational interaction.

    While this effect is pretty negligible today, it was the dominant form of gravitation in the very early universe.
  18. Jun 17, 2012 #17
    I agree that is what she says in the article.

    How then does the universe cool? Does an 'illusion' cool it down?
  19. Jun 17, 2012 #18
    The energy loss may be an illusion, but the decrease in density due to objects getting farther apart is not. One still has to correct for the redshift to get the temperature of distant phenomena right (aside from analysing the spectra to determine temperature), but the redshift plays no part in local processes such as cooling gases (unless distant heat sources become important).
  20. Jun 18, 2012 #19


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    Huh? This was the primary form of cooling in the early universe. Sure, it doesn't have much impact today, but that's more an artifact of the temperature of the CMB being so much lower than the temperatures of most astrophysical objects. But the lowering of temperature due to redshift dominated everything early-on.
  21. Jun 18, 2012 #20
    My guess is they either took some interpretation of QM that didn't regard photons as particles (though I can't think of one off the top of my head,) or they were talking an experiment in which light exhibited wavelike properties. In the latter case, they didn't reject the concept of light behaving as particles, just light behaving as particles in that experiment.
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