Olin Lathrop wrote:
> Russell McMahon wrote:
> >>> Russell McMahon wrote:
> >>> But, only about 500 years at earth's (believed**) absolute velocity.
> >
> >> Now that's a good one, "absolute velocity".
> >> Reminds me of "jumbo shrimp" and "sales ethics".
> >
> > Absolutely, my dear Watson.
> >
> > Hence the '(believed)' and the foot note.
> >
> > Hubble may have something to say about it too.
> 
> There is no such thing as absolute velocity.  Velocity is always relative to
> something else, usually a local inertial observer.  Here on earth we tend to
> measure velocity relative to the local part of the planet's surface.  That
> of course has a rather "interesting" velocity relative to the sun, which
> itself has some velocity relative to the center of the galaxy, which has a
> velocity relative to the local group, etc.
> 
> These is no absolute velocity because there is no point that can be defined
> as the center of the universe.  And as you alluded to, Hubble showed that
> the universe on a large scale is currently expanding.  Your velocity
> relative to one point of the universe will therefore be rather different
> relative to another distant point.  Since there is no center of the
> universe, each of these velocities are equally valid.  The reference
> location for many velocity measurements may be implied from context, hiding
> the fact that there is one, but there is always a reference.

Actually, there is (sort of), as measured by the "dipole anisotropy" of the
cosmic microwave background radiation. COBE has more to say about it than
Hubble, though.

   http://www.iop.org/EJ/article/1063-7869/48/1/L06/PHU_48_1_L06.pdf

   Cosmic microwave background and the Aristotelian ideas of motion

   Immediately after the discovery of microwave background
   radiation (the relic radiation) in 1965, Ya B Zeldovich
   pointed out that it can be used for introducing a peculiar,
   universal, distinguished frame of reference [1]. This is the
   frame of reference in which the background radiation is
   isotropic (the so-called co-moving frame of reference). It
   expands together with the Universe and is therefore common
   for all its parts. It has been reliably proven that in this
   frame of reference the Sun moves towards the constellation
   of Leo with a velocity of about 400km/s.

-- Dave Tweed
-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist