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Re: Weight



Ah, but the astronauts DO have a measurement--they are moving in a circle
and therefore must be accelerating towards the center of that circular
path. A force must be the source of that acceleration, and that force is
their 'weight'.

Not necessarily, if all the windows of the passenger cabin are shut, they
have know way of knowing that they are moving in a circle. Hence, why the
comment about local measurements.

You should have a caveat here about the limitations of this statement. The
experiment I proposed (taken from an article in "Science" by Arrhenius and
Alfven called "Apples in a Spaceship") will reveal the existence of tidal
forces and will give some detailed information about the orbit, all without
opening the windows. The statement Joel makes above is strictly true only
locally, that is for a spaceship of size zero, but it is a very important
point which should not be minimized in importance. Understanding of the
principle of equivalence is important to understanding the physics. The POE
seems to be one of those fundamental truths; it has never been falsified,
and there have been some impressive tests done (e.g. the Pound-Rebka
experiment). As John has pointed out GR itself is not nearly so well tested
and I acknowledge that. The POE, however, tells us a lot if only we are
willing to open our minds and cast out well intended garbage we learned
a long time ago (like the "fact" that centrifugal force is somehow unreal).

Leigh