If doppler shift can be caused by gravity

  1. hog

    hog 5

    ... In other words a photon is energy-mass equivalent so it is effected by gravity. If the photon comes from a place of very high gravity, and we see it shifted, does the emitter have to be moving? ( this is a trick question. I'm heading to Mach's Principle :-)

    If high gravity can dopple shift a photon to zero energy, is that an event horizon? ( yeah yeah .. another trick question ).

    If we look at the night sky, and it looks dark. Is that because there are no stars way out there, or are we seeing an event horizon that we call the night sky?

    And if that were true, would the universe really have to be expanding? ( that's my real question )
  2. jcsd
  3. Drakkith

    Staff: Mentor

    The night sky is not dark to those with sensitive enough eyes and the capability to gather lots of light and take long, multiple exposures. In other words, telescopes. It is filled with EM radiation, but most of it is either too dim to see, or the wrong wavelength. The limit in brightness is not because the universe is expanding, although that does have an effect, it is because the universe is a finite age. This creates a temporal event horizon thanks to the finite speed of light, meaning that there is a point where we cannot see any further because there is nothing else to see! We are seeing the earliest point in time that we can see.
  4. Bandersnatch

    Bandersnatch 1,571
    Science Advisor
    Gold Member

    No it doesn't have to be moving. Gravitational redshift is a separate process from doppler shift.(and both are different from cosmological redshift)

    You can't really redshift a photon to zero energy. There's always some energy left, although I suppose it could be infinitely small, and so, undetectable.

    "Aha!" you're probably thinking just now, "I've made the famous theory tremble in its foundations!". But hold your horses.
    Your proposition, although unstated directly in your question, appears to be that in a static, spatially infinite universe, a photon travelling from any direction in the sky has got an infinitely massive universe pulling on it from behind, ergo, it should be infinitely redshifted.
    However, consider that it's got an infitite amount of mass in front of it as well, so it should be at the same time infinitely blueshifted towards the direction of motion. Or, in other words, the effect would cancel itself.
    There's quite a few more problems with it, but this one should be enough to dismiss the idea.
  5. russ_watters

    Staff: Mentor

    The problem with this question is that it is based on a structure to the universe that is not observed. The universe is homogeneous. In other words, everywhere we look, it looks almost exactly the same. So the question is unanswerable because no such "place of very high gravity" exists.
  6. hog

    hog 5

    Yep. You guessed it. But if the universe is infinite, then the doppler shift is being caused by the gravitational lensing of the stars the photon passes on its way in .. and that would be a lot of stars. So the stars in front have no effect .. until the photon gets there.

    I realize that doesn't explain the red shift of the photons "inside" the event horizon, but I'm working on it :-) Could it be that those photons are actually "lensed" from behind the local stars, and we just think they come from our "inside" the horizon stars ?
  7. hog

    hog 5

    Start from here and devide the universe into spherical shells of many light years thickness. The mass in each shell increases by R^3. It doesn't take very many shells to have the last one contain nearly infinite mass. And if the universe is infinite ... the mass is infinite in every direction. I think that almost has to generate an event horizon in reverse .. just like a Black Hole that prevents light from escaping. Infinite stars in every direction gives us Olber's Paradox. I don't think it has been properly explained away by local doppler shifting of photons in the known universe. I think that black sky is an event horizon caused by the Dark Matter that lies beyond it. ( Mach's Principle ).
  8. Drakkith

    Staff: Mentor

    The mass is spread out. The volume of the universe increases just as fast as the mass does. So the "density" of gravity, which probably isn't the correct term, never gets larger. So no event horizon. This is unlike a black hole where the gravity increases as you get closer to the center, resulting in an event horizon forming.

    Olbers Paradox is perfectly explained by the current model of the universe as finite in age and expanding.

    Dark matter does not act like you imagine it does, nor does Mach's Principle.
  9. Drakkith

    Staff: Mentor

    Uh, no. As light passes a source of gravity it is initially blueshifted on approach, and subsequently redshifted an equal amount as it leaves. There is no net shift either way.

    No. And personal theories are not allowed here on PF either as per the rules.
  10. russ_watters

    Staff: Mentor

    The universe is not currently thought to be infinite.
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