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Physics, Astronomy & Cosmology / Re: How can the frequency of a clock in relative motion appear to decrease?
« on: 05/12/2016 20:04:28 »Timey is right. SR can't explain Doppler shift.
This is completely and absolutely wrong.
Consider the following animation. You have a source, emitting light waves outward spherically at c relative to itself and relative to two red and blue dots. Note how the waves hit the red and blue dots at the same frequency as they are emitted by the source.
Now imagine that the source has a relative velocity with respect to the other dots.
As each wave or part of a wave is emitted, it still expands outward at c from the point of emission as measured by the red and Blue dots as per SR and the invariant nature of light speed. But the source is moving away from the Red Dot and towards the blue dot. The center of each expanding sphere is shifted, Though as you can see still remain spherical. As a result each successive wave has a slightly shorter distance to travel to reach the Blue dot and a slightly longer distance to reach the red dot. Thus the wave peaks are spaced both closer together in time and distance for the Blue dot than for the Red. The blue dot measures a Doppler shift towards a higher part of the spectrum and the red a Doppler shift to a lower part of the spectrum. The red dot sees a red shift and the blue dot see and blue shift. The only other factor in SR is time dilation. According to the Red and Blue dots, the source is actually emitting at a slower frequency than that measured by the source itself. This adds an additional factor to what the red and Blue dots see. This results in the Relativistic Doppler shift equation: Fo Sqrt((1+v/c)(1-v/c))fs
where fo is the observerd frequency at the receiver, fs the frequency measured by the source and v the relative velocity between source and receiver (positive when they are approaching each other). So SR does give an explanation for Doppler shift, And not only that, it gives us a formula for Doppler shift that should be correct if SR holds.
This formula actually was tested in a solving a practical problem. There was a communication problem between a surface probe and its orbiting counter part with a mission to one of the outer planet's moons. It was caused by a frequency difference in the communication protocol. They solved the problem by adjusting the orbiter's velocity with respect to the surface probe so that Doppler shift compensated for the Difference and it worked. They used the Relativistic Doppler shift formula to do this. If this SR based equation had been incorrect, they would not have gotten the results they did. (For example Classical Doppler shift depends on the Motion of the receiver and senders with respect to the medium carrying the signal. If this had been the case, they would have had to account for the motion of the Moon and its planet etc, in order to get the right solution to their problem and it would keep changing depending upon the Moon's position in its orbit. As it was, using the Relativistic Doppler shift formula, all they had to deal with was the relative motion between orbiter and surface probe.