The DE is a relativistic effect that regards the behaviour of waves, of any nature and in particular of electromagnetic waves, when there is a relative motion between source and observer. In that case, frequencies and wavelengths of any wave process, emitted from the source, experience a change when they are observed and measured by an observer in relative motion with respect to the source. It is manifest that the relative motion presupposes two cases: source at rest and moving observer, moving source and observer at rest. On this effect three main theories give different results:
1. Special Relativity (SR),
2. Postclassic Theory of Ether (PTE),
3. Theory of Reference Frames (TR).
1. With regard to SR, Albert Einstein demonstrated the Doppler Effect only in the event of source at rest and moving observer. Despite numerous trials, he was not able to demontrate the DE in the second case of moving source and observer at rest. It is a consequence of the Second Postulate of SR, that says:
“Every ray of light moves in the coordinate system at rest with the velocity c, independent of the fact that ray of light is emitted from a resting body (source) or by a moving body”.
The second postulate excludes strictly it is possible to have a difference of velocity of wave, between source and observer, when the source is moving and the observer is at rest. The impossibility to demonstrate the DE, in the second case, is not due to Einstein’s inability but it is a consequence of the second postulate. Certainly it is a heavy problem for the Special Relativity.
2. PTE allows to calculate the DE in both cases but it produces two different equations in the two cases. Also it is a heavy problem because the DE is independent of what is moving but depends only on the relative velocity between source and observer.
3. TR is the theory that allows to calculate the DE in both cases: source at rest and moving observer, moving source and observer at rest. The result of the calculation proves there is the same equation for both cases, because the variation of frequency and wavelength does not depend on whom is moving but only on the relative velocity between source and observer.
A more detailed description of the Doppler Effect, in the three cases, is put at disposal of colleagues in the attached file.
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