SeanF

2001-Nov-12, 04:07 PM

Okay, I have a question about the Doppler Effect, specifically in regards to relativistic velocities. Let me try and step through what I'm thinking with very simple examples.

The Doppler Effect is basically a result of the movement of the transmitter (or receiver). If, for example, the transmitter is moving away from me at a velocity equal to one-half the velocity of the signal, the signal would be red-shifted for me by a factor of 1.5, right? Basically, the number of waves the transmitter emits in one second would take 1.5 seconds to all reach me, because of how far the transmitter moved during that second.

However, if we're talking about an EM signal, and the transmitter is traveling at .5c, then we have the time dilation of SR to consider as well, correct? At that speed, the dilation would be at about 0.866, meaning I would see a clock on the transmitter as ticking off 1 second in 1.15 seconds of my own time.

So, that some number of wavefronts that is transmitted in one second for the transmitter would appear to take 1.15 seconds for me.

Including the Doppler effect, it seems to me that those wavefronts would take a total of 1.725 seconds to all reach me (1.15 x 1.5).

So, am I misunderstanding something? Would the measured red-shift on a transmitter moving at .5c be 1.725, or would it still be 1.5?

The Doppler Effect is basically a result of the movement of the transmitter (or receiver). If, for example, the transmitter is moving away from me at a velocity equal to one-half the velocity of the signal, the signal would be red-shifted for me by a factor of 1.5, right? Basically, the number of waves the transmitter emits in one second would take 1.5 seconds to all reach me, because of how far the transmitter moved during that second.

However, if we're talking about an EM signal, and the transmitter is traveling at .5c, then we have the time dilation of SR to consider as well, correct? At that speed, the dilation would be at about 0.866, meaning I would see a clock on the transmitter as ticking off 1 second in 1.15 seconds of my own time.

So, that some number of wavefronts that is transmitted in one second for the transmitter would appear to take 1.15 seconds for me.

Including the Doppler effect, it seems to me that those wavefronts would take a total of 1.725 seconds to all reach me (1.15 x 1.5).

So, am I misunderstanding something? Would the measured red-shift on a transmitter moving at .5c be 1.725, or would it still be 1.5?