Re: Clock Properties

David wrote:
In Einstein's paper On The Electrodynamics of Moving Bodies he states
that pendulum clocks don't behave the same way as other clocks.

This depends on what one means by a "pendulum clock". In common usage, a pendulum clock consists of a box in which a pendulum swings, plus a mechanism to count those swings. Unfortunately, this is not the complete clock -- it depends inherently on the earth's gravitation for its operation, including its tick rate. In freefall such a pendulum clock does not work at all, and on top of a mountain it ticks more slowly than at sea level, and in an airplane its tick rate varies wildly as the plane maneuvers.

To be considered as a clock in the sense physicists use the word, one must include the pendulum's relationship with the gravitational source. This makes it impractical to use for most investigations of clock tick rate.


I don't understand why physicists believe all clocks except pendulum
clocks share the same properties in terms of running at different
rates in different inertial frames.

This is not mere "belief", this is an experimental observation. So only theories that predict such behavior are valid.


What property does every other
entity have measures time have that pendulum clocks do not and vice
versa.

All the other clocks that we normally think of (in this context, anyway) are self contained, and their tick rate is not directly dependent on any external quantity; that is NOT true for the pendulum "clock".


Tom Roberts
.

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