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  1. #1
    waz's Avatar
    waz Guest

    Default red shift: doppler or einstein?

    Sam Wormley wrote:
    I don't think this answers the question.
    This is just 2 equations to describe how a wavelength shift can be related
    to two different causes.

    Question .. how can we distinguish a red shift due to a moving object from a
    red shift due to a gravitational field.

  2. #2
    md's Avatar
    md Guest

    Default red shift: doppler or einstein?

    relativity predicts a red-shift for light traveling "up" in a gravity field.
    doppler predicts a red-shift for light emitted from an object moving away from us.

    How can we distinguish the two? When we measure the red-shift of a distant object, how can we
    conclude that it moves away from us? It might also be that it is not moving away, but it is
    very heavy instead?

    10" LX200GPS-SMT

  3. #3
    Sam Wormley's Avatar
    Sam Wormley Guest

    Default red shift: doppler or einstein?

    md wrote:

    Relativistic Redshift

    Gravitational Redshift

    Doppler Effect

    These three sources of redshift can usually be sorted by the context
    of other data made in the measurement process.

  4. #4
    md's Avatar
    md Guest

    Default red shift: doppler or einstein?

    "Sam Wormley" <> wrote in message news:exPPd.66163$EG1.51167@attbi_s53...

    thanks for the formulae, but they did not really answer my question.

    10" LX200GPS-SMT

  5. #5
    David Nakamoto's Avatar
    David Nakamoto Guest

    Default red shift: doppler or einstein?

    One way is to take the measurements in a binary system. From the spectral type
    of the component stars, an estimate of their distance, and their orbital period,
    all measurable through telescopes and some inferring, you can get the pairs
    motion around each other and through space, at least in the line of sight. From
    this you can eliminate causes of red shift due to motion, eliminate them, and
    uncover other red shift effects.

    This was, in fact, how gravitationally induced red shift was measured for the
    first time, using the white dwarf companion of Sirius, I believe, if not the one
    around Procyon, but I believe it was Sirius. The period, mass, and the pair's
    mutual motion through space are measurable or can be calculated from the
    observed. From this, all red shifts due to motion can be eliminated. Then
    because the companion has a high surface gravity, it can produce a gravitational
    red shift, which was what was left when the other causes were eliminated, and it
    matched what Einstein predicted for the mass of the companion.
    --- Dave
    It don't mean a thing
    unless it has that certain "je ne sais quoi"
    Duke Ellington

    "md" <not given to avoid spam> wrote in message

  6. #6
    John C. Polasek's Avatar
    John C. Polasek Guest

    Default red shift: doppler or einstein?

    On Mon, 14 Feb 2005 05:08:56 GMT, "David Nakamoto"
    <> wrote:

    Gravity can be ruled out pretty much because it is a feeble effect.
    The sun has pretty good gravity 27G but the gravity redshift z = 635/c
    = 0.0000021.
    A galaxy with this shift would have Doppler velocity of 635km/second
    which is very small cosmologically, well, 0.0000021 of c. The distance
    computed using Hubble's constant would be 30,000 LY which is only
    about 1 2 millionth of the radius of the universe (13BLY).
    Mr. Dual Space

    If you have something to say, write an equation.
    If you have nothing to say, write an essay

  7. #7
    md's Avatar
    md Guest

    Default red shift: doppler or einstein?

    "John C. Polasek" <> wrote in message

    it would solve some dark matter issues :-) perhaps we have it all wrong and are those galaxies
    much heavier than we thought ;-)

  8. #8
    Martin Brown's Avatar
    Martin Brown Guest

    Default red shift: doppler or einstein?

    waz wrote:

    In an isolated universe with no other features to observe and only a
    point source to look at you can't. But we live in a more interesting
    universe - galaxies are not point sources and space isn't quite empty.

    The gravitational redshift is so very much weaker than the Doppler or
    cosmological expansion components that you would need galaxies to be
    incredibly massive objects and all of the object would have to be inside
    the deepest part of the potential well - otherwise you would see the
    edges of your putative ultra massive galaxy at a different redshift to
    the centre. This is not observed.

    If galaxies were so immensely massive their gravitational lensing
    effects on even more distant objects would not match what is observed.

    The final nail in the coffin for gravitational redshift as the main
    component is the so called Lyman forest of neutral hydrogen clouds sat
    in between us and very remote quasars and galaxies. It would require
    very contrived physics to place these clouds in just the right positions
    inside a deep gravitational field to mimic what is actually observed and
    they would not be stable.

    It is altogether simpler and much more reasonable that these objects are
    in fact very distant and very luminous. Once Blandford & Znajeck figured
    out how to get ~30% of rest mass energy out of matter dropping into a
    black hole and coupled with relativistic beaming the energy budget for
    these objects is no longer an issue. It was a problem in the 60's when
    hydrogen fusion was the most efficient way of releasing energy known and
    was isotropic (rather like Kelvin's problem with powering the sun by
    burning coal).

    We have even seen the relativistic beams in some objects - eg M87,
    Cygnus A and a host of other less well known bright objects.

    Martin Brown

  9. #9
    Bjoern Feuerbacher's Avatar
    Bjoern Feuerbacher Guest

    Default red shift: doppler or einstein?

    John C. Polasek wrote:

    What makes you think that the radius of the universe is 13 BLY?


  10. #10
    Bjoern Feuerbacher's Avatar
    Bjoern Feuerbacher Guest

    Default red shift: doppler or einstein?

    md wrote:

    Err, that would not solve dark matter issues - that would make them
    *bigger*. We can measure with methods independent of Hubble's law for
    many galaxies how far away they are. Then we can, using the seen
    brightness, estimate how much mass is there. And that visible mass is by
    far not enough to account for the observed red shift. So, if the red
    shift is due to the gravitation of the galaxy, there has to be a *huge*
    amount of non-visible, i.e. dark matter in them!



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