| Re: A Riddle for Relativists. |
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Group: sci.physics.relativity · Group Profile
Author: rbwinnrbwinn Date: Apr 27, 2008 22:32
On Apr 27, 6:16Â pm, Bryan Olson nowhere.org> wrote:> rbwinn wrote:
>> Tom Roberts wrote:
>>> rbwinn wrote:
>>>> The speed of sound is only 1,087 feet per second. �Half of that would
>>>> only be 543.5 feet per second. �That is not fast enough to change the
>>>> frequency of the light enough to worry about.
>>> Sure it is! This is how police radar and laser guns measure the speed of
>>> traffic, and they do so for motions ~100 times smaller. A laboratory
>>> instrument could measure speed thousands of times better than that.
>
>> Radar does not change frequency. Â It is reflected from a target at the
>> same frequency it had before.
>
> Dude, just look it up.
I used to work on radar. A transmitter sends out a signal, a receiver
picks up a signal reflected from a target. Unless the books about
radar the Navy gave us were incorrect, there is no appreciable
difference in frequency of the signals. There is a carrier wave of
constant frequency which is reflected from targets and received by a
radar receiver at the same frequency. The information concerning
targets is in the form of modulation of the carrier wave caused by the
reflection. Doppler radar works the same way. The carrier wave has
the same frequency whether being transmitted or received. What shows
the Doppler effect used to calculate speed are modulations of the
carrier wave.
Robert B. Winn
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