Acoustic doppler effect at subsonic, sonic and supersonic speeds

[bernhard.rothenstein]

Defining the period measured by a moving observer relative to a stationary source as
T=t(n)-t(n-1) where t(n) and t(n-1) represent the times when two successive wavecrests are received respectively, the theory of the Doppler Effect leads zero and negative periods in the case of sonic and supersonic motions of the observer. . Do you consider that a zero period is associated with the fact that the moving observer receives one and the same wave crest, a negative period being associated with a reversal in the order in which the wave crests are received.
Thanks for your answers

 

[alvaros]

Do you consider that a zero period is associated with the fact that the moving observer receives one and the same wave crest

Yes, like you were surfing the wave.

negative period being associated with a reversal in the order in which the wave crests are received.

Yes, if you move faster than the waves.

What would happen if the wave was Electro-Magnetic and you move at c ?
I know that Einstein thougt about it, when he was a child, but I don't know the answer.

 

[bernhard.rothenstein]

Yes, like you were surfing the wave.


Yes, if you move faster than the waves.

What would happen if the wave was Electro-Magnetic and you move at c ?
I know that Einstein thougt about it, when he was a child, but I don't know the answer.

Thanks. There is a situation when the e.m wave moves through a medium with u<c. In that case we can have the situations "luminal" and "superluminal" with simillar results as in the case of an acoustic wave.
But if the e.m. wave propagates through empty space then nobody can move with v>c and so Einstein's dream is not feasible.