Re: Relatve Movement Between Receding Galaxies?

From: Franz Heymann (notfranz.heymann_at_btopenworld.com)
Date: 02/17/05 

Date: Thu, 17 Feb 2005 06:57:16 +0000 (UTC)

"Brett Aubrey" <brett.aubrey@shaw.ca> wrote in message
news:y_UQd.411941$8l.207030@pd7tw1no...
> "Sam Wormley" <swormley1@mchsi.com> wrote in message
> news:vOUQd.5455$kS6.2643@attbi_s52...
> > Brett Aubrey wrote:
> > > I'm unclear whether this is just a question of semantics, but am
curious
> if
> > > it is reasonable to call the increase in distance over time
between
> distant
> > > galaxies "relative movement"? I would have thought it an
obvious YES,
> > > but am told it's a definite NO. I can expand on this, if need
be. TIA.
> > > Regards, Brett.
> >
> > See: http://www.astro.ucla.edu/~wright/nocenter.html
>
> Thanks. I've seen this before. I'm assuming that since it mentions
> relative velocities, that this is virtually synonymous with
"relative
> movement" and the answer to my question is then YES(?) Regards,
Brett.

Your assertion is good enough for practical purposes if the speeds
involved are negligibly small in comparison with that of light. But,
more correctly, the quantity you quoted is the separation velocity *as
measured by you*. The velocity of B relative to A is the velocity of
B which would be measured by A. It may be obtained from the quantity
you quoted by using the relativistic formula for thr composition of
velocities. 

-- 
Franz
When all else fails, read the instructions

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