Re: What is the propagation speed (if any) of gravitational influence?

Hi Jim,

Thanks for the excellent question.  The short answer is:

The propagation speed of gravity is the speed of light, which is about 
299800 km/sec.

Now for the longer answer:

Light is a form of "electromagnetic radiation".  This means it is an 
oscillation in the electric and magnetic fields.  A simple way to think 
about this is to think about what happens when you wiggle a charged 
particle back and forth.  At some distance away, you can see the electric 
and magnetic fields changing, because of the particle's motion.  This 
change (oscillation) in the fields is called electromagnetic radiation, and 
it travels at about 299800 km/sec.  Your eyes are able to detect  this 
electromagnetic radiation if the fields are wiggling at the right speed 
(frequency), so we call it "visible light" if the frequency's right.  If 
the frequency's slower, we call it infrared light or radio waves, and if 
it's faster, we call the radiation ultraviolet light, X-rays, or Gamma-
Rays, but it's all the same thing, electromagnetic radiation.  The 
situation you described with the electromagnet  results in a brief burst of 
electromagnetic radiation, as the field changes.  This change propagates at 
the speed of light, because it is a form of electromagnetic radiation.

Similarly, you can make "gravitational radiation" by wiggling a particle 
that has mass.  This is like the situation you describe in which a second 
moon suddenly appears in the sky.  Gravitational radiation, although quite 
different from electromagnetic radiation, travels at the same speed.  
Because it takes such a big object to make much of a gravitational force, 
no one has been able to reliably produce and measure gravitationl 
radiation.  Nonetheless, theory predicts that it exists, and we have been 
able to see its effects.  For example, rotating neutron stars slow down 
because of the energy they lose in the form of gravitational radiation 
("gravity waves"), and the rate at which they slow down agrees with the 
theoretical  prediction of how much gravitational radiation they should 
produce.

There are lots of interesting websites about gravity waves where you can 
find much more detail about  this stuff.  Here are a few examples that I 
ran across:


LIGO Fact Sheet

Gravitational Radiation

Gravity Waves

Scientific American Physics Q and A

			-Steve Levin

_______________________________________________
DISCLAIMER:  Just because I work for JPL/NASA/Caltech doesn't mean anything 
I say is in any way official.  This is just me talking, not NASA, JPL, or 
Caltech.