[PG (OP)]

I have an astronomy/physics question I can't find an answer to. If you were in a space ship and measured the wavelength of the universal background radiation all around you, or if you measured the red shift of the stars all around you, wouldn't the observed directional doppler shiftt give you an absolute reference system for your true speed? I understand that the information would only apply to your navigation, and not the physics of the atoms in your space ship. If you saw another similar space ship appearing to come towards you, couldn't you trade such navigation information with them and then both of you would know who was moving and who was not? If this works, it would mean that not everything is relative. If it doesn't work, why not?

[NakedScientist]

I'm stumped on this one. Can anyone help ? Where's Dickie, he's a physicist ?

[Dickie]

Measuring your position and velocity against the stars around you will give a good idea of what your spaceship is doing with respect to the stars. But who's to say what the stars you're looking at are doing?

It would be much easier if you could just drop a lead into the ether, and see what your speed is compared to stationary space, but the ether doesn't exist, and any inertial frame of reference is as good as another. You may be looking at the stars as a measure of your speed, but the stars are just as entitled to look at you to work out theirs.

[PG (OP)]

As a clarification of my question. If you either look at all of the stars (or at least a good statistical sample), or to a good sampling the background microwave radiation, then you are measureing your velocicty (within the precision of your sampling) of the entire visible universe. At that point, it wouldn't be that important if the entire visible universe was moving someplace. It would be a suffienctly objective inertial reference for navigational purposes.

The other question is, if you can do it, how come you cann't do it from the earth itself?