Gentlefolk,
Ditto to most of what Henry and Ian said, though there is still some
legitimate physics being done on non-geometrical interpretations of the
general relativity equations in efforts to create a self-consistent
mathematical theory embracing both relativity and quantum mechanics. Don't
look for the results of oddball theories of gravity to provide competition
for rocketry in the access to space business anytime soon.
For those who might find themselves in a discussion with a Van Flandern
fan:
The field of every quantum charge and every quantum of mass in the universe
extends to the limits of the universe (if any) and always has. In that sense
the "speed of gravity" is timeless and, in effect, infinite. However,
changes in the distribution of mass and/or charge are what create waves and
for local physics to be constant regardless of relative motion, those waves
must propagate at the speed of light.
Van Flandern's main problem apparently is/was (I understand he's
reconsidering the issue now) that wave mechanics doesn't produce significant
(i.e. detectable) aberration effects in the "near field," a longstanding
electrodynamics result which applies to any inverse square force. "Near
field" is roughly "within one wavelength." For a planetary gravitational
oscillator such as Earth, with a period of one year, the "near field" is thus
one light year! One would not expect to see aberration effects he cited from
planetary motions on the scale of the solar system.
Mercury provides a strong confirmation of the speed of gravity in one of
the oldest experimental tests of general relativity on the books--the
equations that predict the advance of the perihelion of an object in an
eccentric orbit contain an algebraic expression that includes the "speed of
gravity." In a very reader-accessible article in Sky and Telescope in
October 1987, Virginia Trimble fixes the speed of gravity "...experimentally
to within about five percent..." from measured precession of Mercury's
perihelion.
An even more elegant measurement is made by observing the binary pulsar
PSR 1913+16. A pulsar is a rotating neutron star which forms a natural
ultra-precise clock. If the pulsar orbits another star, we can measure the
orbital period and eccentricity by watching the Doppler shift of these radio
pulses.
General relativity predicts that orbiting bodies will lose energy by
gravitational radiation. This "gravitational damping" is a direct result of
the finite velocity of gravitation (or "retarded potential," in physicist's
language). For ordinary orbiting stars, this effect is too small to be
noticed--even contact binaries would take about 40 billion years to lose
their orbital energy by gravitational radiation.
But for PSR 1913+16, conditions are extreme enough for gravitational
damping to be measured. The fact that gravitational damping is measured _at
all_ demonstrates that the propagation speed of gravity is _not_ infinite;
The actual measurement confirms that this velocity is equal to the speed of
light to within an accuracy of better than 1%. Ironically, Joseph Taylor
and Russell Hulse shared the Nobel Prize in physics for their use of the
binary pulsar to investigate general relativity in 1993--the same year that
Van Flandern's book was published.
Sorry to miss the last meeting. Car, weather, and Christmas
procrastination issues combined to make me too late to try to get up there.
--Best, Gerald
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