Postulates of Relativity and The Cosmic Background - Question

I am an interested amateur without any significant academic background in physics. I have just read Einstein's 1905 paper on Special Relativity and I have a question. First I need to provide my understanding, in case I have misunderstood the basics.

The Theory of Relativity is based on two seemingly contradictory postulates: 1) The validity of the Galilean/Newtonian principle of relativity and 2) The constancy of the speed of light in space.

The priniciple of relativity, as I understand it, states that inertial frames of reference are indistinguishable as far as the laws of physics are concerned. Any experiment conducted in one inertial frame will produce the same results if the experiment is precisely duplicated in any other inertial frame. If you measure a mass in an unaccelerated spaceship going 10 km/s relative to the earth, you will measure the same mass in an unaccelerated ship going 1000 km/s relative to the earth.

I have read about the Cosmic Background Radiation left over from shortly after the Big Bang. This radiation consists of an almost perfectly uniform distribution of photons, neutrinos and gravitons. The average temperature (and thus frequency) of this radiation is independent of direction. No matter where you look, to a very high degree of precision, you will measure exactly the same average frequencies.

OK, if I have misunderstood the facts please let me know and ignore the rest.

Let's say I build a spaceship and put six sensitive directional radio receivers in it (they could be neutrino detectors or graviton detectors instead). Port and starboard, fore and aft, yaw and keel. X, Y and Z. Now let's say I measure the temperature of the background radiation with all six receivers and I find that the temperature is the same in all six directions.

What happens if I fire the thrusters?

If I understand the Doppler effect properly, the receiver in the front end of the ship will detect blueshifted frequencies and will measure a higher temperature, the aft receiver will measure a redshifted background radiation and a correspondingly lower temperature, and the other four receivers will be unaffected.

By comparing the red and blue shifts of the six transmitters as the ship moves in different inertial frames, it should be possible to easily determine the absolute motion of the ship relative to the background, shouldn't it? From inside the ship.

Once again, if I have screwed up, don't bother with the rest.

This means that inertial frames are NOT equivalent. They can be distinguished from each other by the Doppler effect against the Cosmic Background. If this is true, than the first postulate of Relativity is false. The Galilean/Newtonian Relativity Principle does not apply anywhere in the universe. It is an invalid abstraction.

PLEASE tell me where I'm wrong.

I have been up for hours thinking about it.


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