Advertisement

Redshift as energy loss?

topic posted Tue, February 10, 2009 - 8:17 PM by  Mike
Share/Save/Bookmark
Ok, through my classes this week in intro to quantum, E&M and general mechanics, I pieced together something interesting.

If the energy of a photon e = hf, and light can be affected by gravity (both things which seem so far to be true), then,

Would a stream of photons leaving an extremely heavy galaxy lose energy due to gravitational potential? Would this not show up as a redshift (from 'expected' frequencies)?

And if this is true, how can we know how much of the observed redshift is due to the expansion of the universe, and how much is due to gravity? Halton Arp has a going theory that there is no expansion of space-time. I learned about it in astro class last term, and we all thought he'd gone off his rocker, but he might be on to something.

Related: if space-time is expanding, and photons are being red-shifted (implying lower energy), where does the energy go? Is it still conserved somehow, or does an expanding universe contradict the conservation of energy laws? Does this bug anyone else?
posted by:
Mike
Canada
Advertisement
  • Re: Redshift as energy loss?

    Tue, February 10, 2009 - 8:18 PM
    related?
    adsabs.harvard.edu/abs/2004...s...7077M

    apparently doppler-shifted photons follow energy conservation laws, but cosmological expansion redshifted photons do not. This bugs me
    • Re: Redshift as energy loss?

      Tue, February 10, 2009 - 11:40 PM
      I read the abstract that you linked to, and it appears to contradict your claim that cosmological redshifted photons do not follow energy conservation laws. However I did not understand the last 3 sentences in the abstract.
      • Re: Redshift as energy loss?

        Wed, February 11, 2009 - 11:31 AM
        One of us is misinterpreting that article. :)

        I gathered that THEY were the ones claiming that cosmological redshifted photons do not follow energy conservation laws. (up until yesterday I'd assumed that everything had to)
        "The mechanism by which energy is conserved with the normal Doppler shift is applied to the cosmological redshift and the energy violation disappears. However, an additional luminosity-dependent recession factor is introduced."

        From the full-text of the paper (available free! follow the link to it) they demonstrate how energy is conserved in Doppler shift phenomena. But their starting point for the cosmological redshift is that under the current models, energy is not conserved. The "luminosity-dependent recession factor" is introduced to force a fit

        Either way, this implies a connection between redshift and energy. All photons leaving galaxies have to climb out of the energy well created by the galaxy. Would this change in energy not cause a redshift? If so, how does this affect the Hubble constant? And if not, why?

        1 out of 3 physics profs has to "get back to me" so far :)
        • Re: Redshift as energy loss?

          Wed, February 11, 2009 - 12:43 PM
          I would assume (for now) that climbing out of the gravity well created by the very galaxy would have some effect on it. I will explain why.

          Recently, I have come across the Faraday Rotation phenomenon (at school, of course), that I haven't learned about before. So, as far as humble I have understood, the medium, (placed in a magnetic field), the initially "chopped"/polarized light is passed through (in our case it was a 650 nm laser through a quvette filled with different solutions, Distilled water and Ferric Chloride solution) has effect on the rotation of light, which depends on the Verdet constant, which in its tern is a function of wavelength.
          So, we had to take measurements of an empty quvette, and a filled with different stuff one. Then, later when calculating the Verdet constant, we had to account for the quvette glass's Verdet as well. Now, since I have just acquainted with the phenomenon, (well, in the process of it), I might be wrong, but so far, I believe that if the edges/glass walls of the quvette are not inside the magnetic field, their presence has little or no effect on the outcome - the calculated Verdet constant of the solution; however, if they are, then, well, it does effect it.
          So, back to the topic, I would assume that the gravity well does have effect on Doppler effect and that it might obey the inverse-square law as well, though I don't know if the latter works at light-speed ranges. If we think of the gravity well effect as of that of the quvette walls, we might see some connection(s).
          Of course, the shift would depend on the strength of the gravitational pull, (hell, black holes suck crap right in and don't ask whether you are a photon or a neutron star), which, in its turn would depend on the mass of the galaxy itself.
          Hubble constant might be effected, however I won't go that far for now - I need some time to think about it. In Astronomy tribe, some year and a half ago I posted an article, (and someone else too, I believe), on corrections to the Hubble constant. Go to my photo album, scroll down to the WMAP photo - there you will find a link to at least one of the articles.
          ...
          • jon
            jon
            offline 1

            Re: Redshift as energy loss?

            Wed, February 11, 2009 - 12:55 PM
            tired light... alternative hypotheticals

            en.wikipedia.org/wiki/Tired_light
            • Re: Redshift as energy loss?

              Wed, February 11, 2009 - 1:14 PM
              From the Wiki-article:

              "Today, tired light is remembered mainly for historical interest, and almost no scientist accepts tired light as a viable explanation for Hubble's Law."

              Well, I do not think in our discussion it is assumed to be about the definition/explanation of the Hubble constant. Merely as an additional "variable" to it of some sort.

              It is the way I have understood it, at least 'till now.

              • Re: Redshift as energy loss?

                Wed, February 11, 2009 - 4:44 PM
                I believe that the red-shift numbers they have been using for distant galaxies, the size of our galaxy & universe, and expanding universe theories, have been cross-referenced with type 1a supernovae, which apparently only give off an exact amount of light, and thus given a certain amount of darkening of that light, a distance marker. This is therefore the new calibrator far past the 200 or so light year limit of parallax, which was the original calibrator of space distances..

                Please correct me guys on my 200 light year limit I've given to parallax. That's just hearsay from a friend who's an astronomer.

                • Re: Redshift as energy loss?

                  Wed, February 11, 2009 - 11:02 PM
                  Thank you Jon. I believe that's the answer I needed to hear (although it's a blow to the ego to discover that my "unanswered" question was first raised 80 years ago :)

                  I'm still skeptical... blame my astro prof from last year, who encouraged us to question the assumptions underlying modern cosmology (how exactly does degeneracy pressure work again?). Type Ia supernovas give us distance (to the supernova), assuming that they all do emit the exact same amount of light. Fair enough

                  But the CMB analysis is something else. It's hard to argue when something fits theory so well (see xkcd.com/54/). The implications are driving people nuts though! See en.wikipedia.org/wiki/Acce...g_universe

                  "Models attempting to explain accelerating expansion include some form of dark energy: cosmological constant, quintessence, or phantom energy, with the latest WMAP data favouring the cosmological constant. The most important property of dark energy is that it has negative pressure which is distributed relatively homogeneously in space."

                  Inventing "phantom energy" to explain an irregularity in a model means its time to re-question some assumptions :)

                  From en.wikipedia.org/wiki/Cosm...background :
                  "The photons that were around at that time have been propagating ever since, though growing fainter and less energetic, since the exact same photons fill a larger and larger universe"
                  ... which sounds suspiciously like "tired light" :)

                  Expanding universe is most definitely in favour now, but there's considerable disagreement about how. Even the value of the hubble parameter is being fought over. Foods for thought anyway
  • Re: Redshift as energy loss?

    Thu, February 12, 2009 - 11:36 AM
    One of many problems with this thinking is: If the redshift was just from gravity, then, galaxies at the same distance with greater mass would show a greater redshift. Yet the redshift we measure in galaxies changes with distance, not mass. We could go on and on, even nearby large stars should show a redshift greater than small stars. Yet we see none of this redshift from mass. There is no evidence to support this idea.
    • Re: Redshift as energy loss?

      Thu, February 12, 2009 - 9:57 PM
      Double thanks :)

      Riddle me this: Why does gravitational lensing work, if light energy (frequency) isn't affected by climbing out of potential energy wells?
      • Re: Redshift as energy loss?

        Thu, February 12, 2009 - 10:44 PM
        Mike wrote:
        "Riddle me this: Why does gravitational lensing work, if light energy (frequency) isn't affected by climbing out of potential energy wells?"

        As a follow-up to your question, here's a Gedanken experiment of my own. Suppose that a Klingon is standing on the surface of a neutron star having 99.999% of the mass needed for black hole formation (if you believe in black holes), sending a Morse-code message with his gamma-ray flashlight, to his buddies on their orbiting spaceship. Would the light be red-shifted?

        Assuming that the bloke and his flashlight survive long enough to send the message in the first place, I think that it would. I can't accept that there would be zero gravitational influence on the light until something magic happens at the threshold of black hole mass.
        • Re: Redshift as energy loss?

          Fri, February 13, 2009 - 5:03 AM
          I think that as long as they are both stationary there will be now shift (either red or blue) at all.
          The shift is due to emission -and- motion (of at least one body (of them)).


          Look up the shift formula - it has the "approaching" and "receding" parameters (in the +/- of both)

          • Re: Redshift as energy loss?

            Fri, February 13, 2009 - 10:38 AM
            "I think that as long as they are both stationary there will be now shift (either red or blue) at all.
            The shift is due to emission -and- motion (of at least one body (of them)).


            Look up the shift formula - it has the "approaching" and "receding" parameters (in the +/- of both) "

            This is only true in special relativity (weak acceleration) but not general relativity.
        • Re: Redshift as energy loss?

          Fri, February 13, 2009 - 10:35 AM
          "Suppose that a Klingon is standing on the surface of a neutron star having 99.999% of the mass needed for black hole formation (if you believe in black holes), sending a Morse-code message with his gamma-ray flashlight, to his buddies on their orbiting spaceship. Would the light be red-shifted?"

          Yes, it would be redshifted into the long radio wavelengths.

          Gravitational redshifting was demonstrated on Earth by bouncing a laser of an orbiting satellite. The experimental results fit very nicely with predictions from general relativity.
    • Re: Redshift as energy loss?

      Fri, February 13, 2009 - 10:31 AM
      "Yet the redshift we measure in galaxies changes with distance, not mass. "

      Incorrect, the redshift is effected by both mass and distance though the effect of a galaxy's mass is much less pronounced (because it is diffuse and thus there is only weak gravitational redshift) then the effects of distance.