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[Phys-L] Re: "moving clock runs slower" (not)



At 10:16 -0700 9/9/05, John Mallinckrodt wrote:

Indeed, a moving clock can "appear" to
run fast. Consider, for instance, a clock that is approaching you at
a velocity close to that of light. This clock will "appear" to run
fast, but if you correct for the finite velocity of light (no
relativity required here) you will determine that the clock is
"really" running slow.

This is exactly the principle that is used in the simple-minded
presentation of the twin paradox that uses a "clock" on the
spacecraft that flashes at specified intervals, which are, of course
received on earth at different intervals, depending on whether the
spacecraft is outbound or inbound. One doesn't need relativity to see
that, while the earth receives the same number of flashes as the
spacecraft emits, the elapsed time of the receipt of all the flashes
is greater than the elapsed time measured on the spacecraft. The
resolution, of course, is done by pointing out that the situation
between earth and the spacecraft is not perfectly symmetrical, since
the spacecraft changes reference frames between being outbound and
being inbound. But the measured intervals between received pulses is
not due to relativity (mostly) but to the Doppler effect.

Having done this calculation several times, both in class and out, I
know that it can be tricky. A small misstep at most any point in the
process can lead to hopelessly confusing results.

Hugh
--

Hugh Haskell
<mailto:haskell@ncssm.edu>
<mailto:hhaskell@mindspring.com>

(919) 467-7610

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