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Re: [Phys-l] Special Relativity



Short answer: The answer to all such questions is:

_Draw the spacetime diagram._

In this case, some events of interest include the reception of the
first word, the reception of the second word, ... the reception of
the last word.

You can save yourself some trouble by using spacetime graph paper.
http://www.av8n.com/physics/spacetime005blue.pdf
http://www.av8n.com/physics/spacetime005blue-only.pdf
http://www.av8n.com/physics/spacetime005red.pdf
http://www.av8n.com/physics/spacetime005red-only.pdf
http://www.av8n.com/physics/spacetime005redblue.pdf

On 02/01/2011 08:00 AM, Edmiston, Mike wrote:
Since all methods of information transmission via E&M waves involves
some encoding/decoding process, I would suggest that compensation for
a Doppler-shifted carrier should be part of the decoding process.
That is, if the recipient is getting "distorted" information, then
the transmission wasn't properly decoded.

I liked the previous answer better: "It depends."

Yes, undistorted decoding is a desirable property of any decoder,
but it is not the only desirable property, or even the always-
predominant one.

Another often-desirable property of any communications protocol is
that the encoding be _instantaneously decodable_. This is important
for any sort of interactive communication, e.g. telephony.

When there is a redshift:

*) If the message is decoded word-by-word, the best you can hope for
is that the individual words will be undistorted ... whereas the
prosody -- i.e. the spacing between words-- will be stretched out
by the redshift.

*) If the message is decoded phoneme-by-phoneme, the timbre of the
voice will be undistorted, but the words themselves will be stretched
out.

*) If the analog waveform of the speech is decoded microsecond-by-
microsecond, the timbre of the voice (along with everything else)
will be distorted.

*) At the opposite extreme, you could encode the complete message onto
a DVD and transmit it by means of a very fast carrier pigeon. The
recipient could play the DVD without any distortion ... but the
communication would be completely non-interactive.

**) In all cases, the point is that the arrival time of the end of
the message will be delayed relative to when the message started
(or would have started, if it had been interactive), so you will
necessarily be left with some time on your hands. How you decide
to dispose of the extra time is up to you.

So ... it depends.

Bottom line: Draw the spacetime diagram.