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Q & A: Moving radio broadcaster

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Most recent answer: 02/20/2013
Q:
Does a radio wave broadcasted from a moving airplane move faster than broadcasted from a standstill? Is there a finite speed for radio waves and can that speed be slowed or speeded up?
- michael
cincinnati, ohio, usa
A:
No, the wave broadcast from a moving airplane has the same speed, c, as one from an earth based antenna.   This is one of fundamental principles of the Special Theory of Relativity. The frequency, however, will slightly change due the Doppler effect.  Thatís the principle on which police radars work.  A wave reflected off of a moving car back to the police car's receiving antenna has its frequency changed.

LeeH

(published on 10/22/2007)

Follow-Up #1: police radar

Q:
Is this how police radars work? They emit a radio wave at a certain frequency and when it comes back to the radar, the frequency has shifted due to the doppler effect. The faster the car moves (away or towards the radar), the more the frequency shifts ('red' for away, 'blue' for towards). Then a cpu computes how much the frequency has shifted and then... find the car speed. No? Physics are absolutely wonderful!!!
- Anonymous
A:
That's basically it. The one slight change is that the returning signal isn't directly sampled and analyzed but rather first mixed with some of the source radiation. When you mix two high frequency waves at slightly different frequencies, they go in and out of phase with each other. The amplitude of the combined waves oscillates up and down at the difference frequency of the two waves. So that's what gets sampled (at a conveniently low frequency) and analyzed.

Mike W.

(published on 12/10/2011)

Follow-Up #2: extreme Doppler shift?

Q:
So if a radio (or other EM wave source) were moving at the speed of light a stationary observers would see a spectrum of frequencies approaching fx2 to fx.5 as it passes?
- andrew
new york
A:
Andrew- Your question makes some assumptions. There are no observers moving at the speed of light with respect to other observers. All observers must move slower than the speed of light with respect to each other. There are no "stationary" observers, unless you wish to call all  observers stationary. Each is stationary in its own reference frame.

What you can say is what the Doppler shift multiplier of the light frequency is as an emitter approaches you at speed v: 
 ((c+v)/c-v))1/2
or
 departs from you at speed v:
 ((c-v)/c+v))1/2.

Mike W.

(published on 02/20/2013)

Follow-up on this answer.