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Gravity Probe B

Testing Einstein's Universe

Special & General Relativity Questions and Answers

If astronomers can use the cosmic background radiation as a reference frame doesn't that invalidate special relativity?

Yes, because the expansion of the universe is not covered by special relativity, and is a property of the general relativistic treatment of motion which has features not present in special relativity, just as special relativity has features in it that are not compatible with newtonian physics. The cosmic background radiation will represent a fixed frame of reference for any object that is 'at rest' with respect to the expansion of the universe. In other words, if you are 'going with the flow' of the expansion, you will see the background radiation as a perfectly smooth surface. If you are moving with some peculiar velocity relative to the local 'Hubble flow' then this motion will be reflected in the cosmic background as a blue shift ( higher temperature) in the direction you are moving, and a redshift ( cooler temperature) in the direction you are coming from. This reference frame, however, is different for every observer in the universe so it does not qualify as a global frame of reference in the strictly special relativistic sence of 'reference frames'.

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All answers are provided by Dr. Sten Odenwald (Raytheon STX) for the NASA Astronomy Cafe, part of the NASA Education and Public Outreach program.