Re: gravity is only a theory

Friar Broccoli wrote:
> Rick Merrill wrote:
>

[snip]

> >
> > including from the Gravity Probe B.
>
> It is intended to measure certain aspects of the curvature of
> space, which I absolutely do not understand, however it is only
> indirectly related to gravity waves.
>

It will direcly test predicted effects of the GR curvature
of spacetime around the earth, produced by the earth's rotating
mass, on the motions of gyroscopes. In Newtonian physics a
gyroscope in free fall would keep its spin direction constant
relative to the fixed stars.

In general relativity there are two known effects due to spacetime
curvature on the spin direction of a gyroscope in free fall
around the earth which cause the direction to change with time.

One is called the geodetic effect, it's due to the curvature
and is fairly large. The other is called frame dragging, it's
very small, and exists because as well as being a large mass,
the earth also rotates. In GR the solution for the spacetime
outside a rotating mass picks up some effects of the rotation,
and this also affects the motion of the gyroscopes.

For a gyroscope in a polar orbit, precession due to the geodetic
effect and due to the Lense-Thirring effect (frame dragging)
are at right angles, so gravity probe B should be able to measure
both.

Gravity probe B is a technological tour de force: they needed
to manufacture nearly perfectly uniformly spherical gyroscopes
and to guarantee that the satellite moves as closely as
possible in free fall, for about 1 year, and many other
problems had to be solved, too.

It provides a test within our solar system which is
sensitive enough to distinguish GR from closely related but
alternative theories of gravitation.

As you say it's only indirectly related to gravity waves,
in that GR does predict gravity waves which have been
indirectly measured.

David

.

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