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How can the expansion of space cause light to loose energy?

  1. Apr 12, 2005 #1
    how can the area or distance between two objects getting larger cause something traveling between them to strech?
     
  2. jcsd
  3. Apr 12, 2005 #2

    selfAdjoint

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    Note that from the light viewpoint it relativistically isn't "travelling between them" it just is with them together timelessly. So if the move apart it, and its wave lengths, must stretch. Notice that this is no longet true for forces carried by massive particles like the weak force. Also the expansion rate is proportional to the length, so each "proton width" hardly expands at all; therefore the confined gluons and the strong force are very little affected.
     
  4. Apr 12, 2005 #3
    If I understand your question correctly: When two things stretch at the same rate and the space between the two objects stretch as well when something passes between them it is subjected to those same forces and it expands too.

    As for the title of your thread (which is a good one; the subject too :smile:) I don't think it is so much as light loses energy. Because energy can neither be created nor destroyed. The light travels through space (it's medium) and as its medium expands the distance between molecules of space is greater which makes it more difficult for the light to travel. Not by much though. Anyway, the energy is more spread out making it seem as though there is less of it, but there really isn't. I'm pretty sure that's how it works. My physics class is studying light and sound waves, so based on that information I'm pretty sure.

    By the way: Welcome to PF! Glad to have you here. :biggrin:
     
  5. Apr 12, 2005 #4
    I wish I could have read Self's post before I posted mine...I was still typing though :redface:.
     
  6. Apr 12, 2005 #5
    Self, so let me see if this is right:the wave length the only portion of the light that would be affected?
     
  7. Apr 12, 2005 #6

    selfAdjoint

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    In the wave representation, what othe portion is there? This turns out to be a wave nature "observation', and the photon picture goes along for the ride, losing momentum and energy acording to Einstein's equation p = hf.
     
  8. Apr 12, 2005 #7
    That makes sense. What part do the gluons you mentioned play in the senario?

    I hope you don't mind me asking so many doltish questions :redface:.
     
  9. Apr 12, 2005 #8
    thanks allot for all of your help
     
  10. Apr 12, 2005 #9
    Your most welcome. Do you have an idea on how gluons play into this?
     
  11. Apr 12, 2005 #10
    im pretty sure that the gluons do not have a cross sectional size that can be affected. and i would just like to bring your attention to your first post. there has been no experimental evedence that light travels on space. as i was just trying to write a paper of a simalar idea. yet a prof of mine told me to look at the experiment done my michelson morley. this is back when they thought aether made up the universe. the would then need to detect a diffrence in the speed of light if the aether or "space" as you call it was moving in some direction. so yes i think what you have to say was a very good idea and one i am almost convinced of i think you should look at this data it might give you a hand with your idea.
     
  12. Apr 13, 2005 #11
    Nope. The individual photons do indeed lose energy as space expands.

    MF
    :smile:
     
  13. Apr 13, 2005 #12

    Chronos

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    I'm not sold on that. Do sound waves lose energy when the source is receeding?
     
  14. Apr 13, 2005 #13
    firsty - sound waves are quite different to photons. Why would you expect them to behave the same way? What makes you think that photons are in any way analagous to sound waves?

    secondly - where photons are concerned the doppler redshift (caused by the relative motions of the source and detector in a given frame of reference) is not the same as the cosmological redshift (caused by the expansion of the reference frame between source and detector).

    MF
    :smile:
     
    Last edited: Apr 13, 2005
  15. Apr 13, 2005 #14

    turbo

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    Light is EM waves propagating through EM fields. When light is redshifted, the frequencies of the light waves are reduced and the effective wavelengths increase.

    As I pointed out in another post recently, Einstein never modeled light as photons, but as EM waves propagating through the aether. He was unable to reconcile the aether with GR, so he claimed that it had NO properties sensible to us. He knew that it had to exist to allow the propagation of light through space, but he couldn't get it to make sense in GR. The Unruh Effect is an example of interaction of matter with the aether (vacuum energy fields, ZPE), and should provide us with an instructive model about the properties of "empty" space and the nature of light.
     
  16. Apr 13, 2005 #15
    I could equally well argue that light is comprised of quantum objects which we call photons, which propagate according to the configuration of mass/energy in their vicinity.

    show me an experiment which you think demonstrates light behaving as a wave and I'll show you an alternative interpretation based on the motion of discrete quantum objects.

    when space expands the energy of individual photons in that space is reduced, this is interpreted as an increase in wavelength of the light (ie redshifting).

    Please can you provide me with the details of this post?

    Einstein was awarded the Nobel prize for explaining the photoelectric effect on the basis of light being composed of discrete quantum objects.

    MF
     
  17. Apr 13, 2005 #16

    marcus

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    Hi misskitty. I think your confusion can be traced back to this misconception. You are assuming a simple form of energy conservation that does not apply in cosmology.

    In cosmology you cannot assume that "energy can neither be created nor destroyed."

    Indeed over 99 percent of the original energy in the cosmic microwave background has been destroyed by the expansion of space since the time the CMB was emitted.

    the exact figure is more than 99 percent---according to the best current estimate by cosmologists (z = 1100) it is the fraction 1100/1101.

    It is customary to give highschool physics students, and also in some places freshman college physics students, incorrect ideas and then correct things for those who stick around as sophomores and juniors and become physics majors. Correct information, it is thought, would confuse the beginning students and slow down their learning basic laws like the conventional Energy Conservation law which DOES WORK FINE if you apply it to physics homework problems where it is appropriate. Engineer and chem majors are never going to need to know situations in which conservation does not apply. Only physics majors ever get told the straight dope.

    If your teacher is a highschool teacher then even tho he or she might be a wonderful teacher they may well not know this.

    Some people have ideas about "where does the lost energy go?" but there is no straightforward answer. As far as the light knows, over 99 percent of its energy has been destroyed. I am NOT talking about "thinning out" which happens too (fewer photons per square kilometer) but about the loss of over 99 percent of an individual photon's energy due to spatial expansion.
     
  18. Apr 13, 2005 #17
    Plenty more discussion on this subject in :
    Where did the energy in the CMB go to?

    MF
    :smile:
     
  19. Apr 13, 2005 #18

    marcus

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    hi MF, thanks for the link :smile:
    I was thinking there should be some cosmology FAQ on the web that handles the limitations of energy conservation law in Gen Rel. I just dont happen to have a link. Perhaps the Usenet sci.physics.research FAQ, or Ned Wright's UCLA cosmology site, or something at John Baez site? I think I should be equipped with a link to some standard authority about this, but I dont know one, and it keeps coming up.
     
  20. Apr 13, 2005 #19

    turbo

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    I respect your acceptance of the particle/wave duality. I suggest to you that you might look at this duality as an interesting convergence of classical and quantum physics. Classical physics tells us (quite appropriately) that energy travels as waves propagating through fields. Quantum physics tells us that EM waves collapse and can act as if they are point sources, and the likelihood that a point will be expressed at any location is a function of the sum of the magnitudes of the probabilities involved. Feynman gave some great lectures on this at a non-technical level. you'll need a few spare hours to view these, but they are priceless!!

    http://www.vega.org.uk/series/lectures/feynman/

    This tendency of waves to collapse to a point-like function and the way that atoms on the receiving end must exhibit quantized (fixed-energy) reactions have made us comfortable with the concept that light actually travels in quantum-energy packets. This is not true. I think that light exists as waves propagating through EM fields, and that the waves decrease in energy as they propagate through the field, and that our detectors can only respond in quantum steps, leading us to think (erroneously) that light can only impinge upon our receptors with fixed energy levels and in point sources.

    I can't locate the post right now, but here is the lecture that Einstein gave in 1920, after struggling for years to get rid of the aether. He finally reconciled himself with the aether because it is essential for EM wave propogation. Because he couldn't accept the idea that the aether represented a reference frame against which proper motion might be sensed, he claimed that it had NO properties other than the ability to propogate EM waves. This was probably not the right assumption, as demonstrated by the Unruh Effect. The assumption did not accord with the Machian view of space, either, although that is a bit more theoretical, modeling rotation and proper motion as if they had to be measured relative to the reference frames of every piece of matter in all the universe. I think Mach's aether failed only because he did not have an idea of the enormity of space, and neglected to treat the aether as a local reference fram.

    http://www.tu-harburg.de/rzt/rzt/it/Ether.html

    Yes, but Einstein had a great deal of difficulty accepting the partical/wave duality of light that many seem to credit to his work.

    Einstein is often cited as the man who killed the aether and established the particle nature of photons, but this is in direct opposition to the views he expressed in his later published works. (see link above)

    Hubble is often cited as the man who proved the reality of the Big Bang (cosmological expansion extrapolated back to a point source) although he publicly resisted and /or seriously doubted that explanation until the end of his life.

    These guys were pretty serious scientists. It might be time to rein in the cosmologists riding on their work and ask them to explain how their extrapolations and assumptions can be justified. Just a thought.
     
  21. Apr 13, 2005 #20

    Chronos

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    This is my favorite myth buster episode regarding tired light:
    http://www.astro.ucla.edu/~wright/tiredlit.htm
    Please consider that cosmologically redshifted photons are also time dilated. Photons from a z=2 supernova are only half as energetic as those from a z=1 supernova - but you receive twice as many of them due to time dilation effects. I don't see any missing energy.
     
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