Hi,

If there is no universal reference frame (according to Special Relativity)
how can we measure the motion of the earth relative to the Cosmic Background Radiation (CMB) which would seem to act as a universal frame of reference.

I put this question to Stephen Hawking at a lecture but he really did not answer my question.

Any ideas (is there something hidden in the "hard maths" of General Relativity?)

Anybody any ideas?

John

I am sure that the celebrated COBE satellite measured that Doppler Shift in 1984.
Must try and dig it out.

Peter.

I cant say that I really understand the full gist of your question and its for sure that I have no idea how to answer it anyway..... but as I have been floundering around this material myself just recently I want to add my X cent worth.
I am assuming your question has to do with this dipole anisotropy thing in the CMB map. If so from what I gather it has something to do with the choice of a 'local frame of reference'. The CMB + the galaxies (over a large enough volume) form a local frame and it against this frame that the redshift is measured - not against the CMB as such.
Are we on the same wavelength or is it a different question?
It is all rather "interesting".

JMP

Hi,
You can read about the Cosmic Microwave Background here:
www.phy.duke.edu/~kolena/cmb.htm

Imagine if there was no dipole (of 370 km/sec) then the Earth would be stationary in this (apparently) Cosmic Frame of Reference! I don't known what Albert Einstein would thought have of this.

John

The CMB is really just a collection of last scattering photons. One could Lorentz boost to the CMB frame, but the laws of physics would be the same as in any other frame.
Hence nothing singles out the CMB frame as anything special.

You can also calculate Earth's motion relative to Alpha Centauri, but it doesn't imply anything special about that star.

I think the original question was if GR states there is no URF by measuring our motion against the CMB are we not violating GR since CMF forms a URF.

I think there are two parts to the answer :
As Son Goku pointed out we can measure our shift against anything in the COMOS, we observe RED Shift usually and this implies the other objects are moving away from us and this is due to the expansion of the Universe. Also there is well known relationship between Red shift and distance so that the further away the object is the greater the degree of Shift and hence its relative velocity (to us). In fact extrapolating the distance vs. velocity relationship implies that the furthest we can see into the Universe are those objects whose velocity relative to us is approaching the speed of light.

But to continue on the second part of the answer: The CMB is the cooling afterglow of the Big Bang. It is expanding by being dragged along with the expansion of space (I think I read that the expansion is causing the cooling). So the CMB is moving relative to us anyway , and because of its continued expansion it wouldn't represent a UFR.

The dipole and small differences in its spatial distribution density are interesting and have all sorts of implications when considering things like Galactic groups, the Great Attractor and voids in the Universe.

I remember some years ago reading a book by one of the original gurus on CMB, he was also involved in the COBE project. Maybe some one can remember the title of the book?

Anyway I hope all of the above is correct.

If there is no universal reference frame (according to Special Relativity)
how can we measure the motion of the earth relative to the Cosmic Background Radiation (CMB) which would seem to act as a universal frame of reference.

Three comments (off the top of my head) -

I don't see how the CMB can act as a reference frame. Where is its north pole, for instance?

As far as relativity is concerned, being stationary with respect to the CMB is no more special than being stationary with respect to the Earth.

There is a difference between a universal reference frame (which cannot exist), and a reference frame that is universally accessible (which, I think, could exist).

I'm not an astrophysicist, but I think the above three comments are consistent with the science.

Chris.

I remember some years ago reading a book by one of the original gurus on CMB, he was also involved in the COBE project. Maybe some one can remember the title of the book?

Afterglow of creation by Marcus Chown perhaps?

Albert,
Thanks but I think it may have been 'Wrinkles in Time'

By the way for anyone interested the original problem leading to the discovery was excess noise in a radio dish receiver. At the time the problem was thought to originate from pidgeon droppings on the dish. Hence the saying 'S*** Happens!'