Re: Gravity and the red shift?

From: John C. Polasek (jpolasek_at_cfl.rr.com)
Date: 01/02/05 

Date: Sun, 02 Jan 2005 02:14:54 GMT

On Wed, 29 Dec 2004 14:45:39 -0600, Tom Roberts <tjroberts@lucent.com>
wrote:

>Jim wrote:
>> I was just thinking about the gravitational lensing that some galaxies
>> can cause, when I wondered what effect a galaxies own gravity would
>> have on its own light.
>
>Light emitted from within a galaxy and then leaving will be redshifted
>due to the difference in "gravitational potential" between point of
>emission and point of leaving. But as it enters our galaxy (to be viewed
>by us), it will be blueshifted accordingly. Whether not not these two
>effects cancel depends on the details.
>
>
>> From what I can understand, gravity would originally slow any light
>> leaving a galaxy, until it escaped any such effects, were it would
>> then speed up, increasing its wavelength, and possibly causing a
>> slight red shift.
>
>When you say "slow any light" you must define what you mean by that. For
>instance, relative to any locally-inertial frame along its path, light
>is never "slowed". Because of the way light behaves, it is FAR better to
>think of it as redshifted or blueshifted, rather than as "slowed".
>
>
>> if any such effect exists, i am curious to find out how physicist
>> manage to differentiate between this and the red shift caused by all
>> the galaxies moving away from us.
>
>For a single light ray the redshifts/blueshifts due to these three
>processes are indistinguishable (and they combine):
> different "gravitational potentials" of source and detector
> cosmological expansion between cource and detector
> local motions of both source and detector
>But on average one can hope that the first and third effects
>approximately either cancel or average out. Indeed, by observing many
>galaxies and correlating with independent measures of their distances,
>the cosmological redshift emerges as a clear result; the other effects
>contribute to the errorbars on the cosmological redshift.
>
>
>zawy@yahoo.com wrote:
>> A red shift in light (due to gravity) leaving a galaxy would be much
>> less than the red shift of light leaving the moon because the density
>> of the moon is much greater than the density of a galaxy.
>
>First you mean the AVERAGE density of a galaxy. But in any case, it is
>not density that matters, it is the "gravitational potential" of the
>source and the "gravitational potential" of the detector that matter.
>These are only loosely correlated with density, and are almost
>independent of average density of a galaxy.
>
>As the light we observe from other galaxies comes from stars, the
>emitting atoms are generally at significantly lower "gravitational
>potential" than the surface of the moon or of the earth, so we observe a
>net redshift from this effect. Note that local motions on the surface of
>the star can exceed this gravitational redshift (e.g. from our sun); the
>local motion also has a significant random component that can smear out
>the emission lines considerably (Doppler effect)....
>
>
>I have put "gravitational potential" in quotes, because it is not really
>applicable in GR, but is approximately valid in Newtonian-like
>situations, like the emission and detection locations of this
>discussion. The correct way to compute redshifts is more complicated
>(and seamlessly includes all 3 effects listed above)....
>
>
>Tom Roberts tjroberts@lucent.com
In order for a galaxy to generate redshift of even z = .1
gravitationally the escape velocity at the location would need to be
1.38x10^8 m/s.The blue shift at earth would be negligible 7x10^-10.
Observed redshifts are surely due to recession velocities, not
gravity.

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