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Re: electromagnetic literacy



Regarding Marc Kossover's question:

Howdy-

I was directed to this site
<http://www.metaresearch.org/cosmology/speed_of_gravity.asp>
by an online magazine.

The author makes several assertions about the speed of gravity,

He is widely considered a crank.

arguing primarily that it's effects are instantly transmitted. He
claims certain astronomical observations of pulsars as evidence.

I was under the impression that the effects of gravity propagate at
c,

This is what all metric theories of gravity predict including General
Relativity.

although no experiments had yet shown this.

A little while ago a measurement of the effect of Jupiter's gravity
on a background pulsar's signal was reported, and in that report it
was claimed that the measurement indicated that the speed of
gravitation was c within experimental error of about 10%. Since that
time, however, I understand that the claim that the measurement
actually tested the speed of gravitation was called into question.

In particular, I believe that
the work at LIGO and other large interferometers was trying to test
this hypothesis.

I think they are mostly trying to detect gravitational waves (so far
without success). I don't know how they could measure the speed of
those waves, though.

Am I confused? What is the state of our knowledge?

Theoretically, you are correct that the speed of gravitational
interactions is locally c (at least in the linearized weak field
limit). I don't think that this prediction of theory has yet been
adequately tested. But I doubt that Tom Van Flandern's claim of an
infinite propagation speed for gravitational interactions is
observationally viable. It most certainly is theoretically ruled
out by all the reasonable theories of gravity.

Marc "Zeke" Kossover
The Hockaday School

David Bowman