by Ethan Siegel April 28, 2016 from Forbes Website European Gravitational Observatory, Lionel BRET/EUROLIOS.
Why Does Gravity Move
at the Speed of Light?
This is because as fast as light is  moving at the speed of light  it isn't instantaneous:
But gravitation doesn't necessarily need to be the same way...
It's possible, as Newton's theory predicted, that the gravitational force would be an instantaneous phenomenon, felt by all objects with mass in the Universe across the vast cosmic distances all at once.
NASA/JPLCaltech, for the Cassini mission.
If the Sun were to simply wink out of existence, would the Earth immediately fly off in a straight line, or would it continue orbiting the Sun's location for another 8 minutes and 20 seconds?
If you ask General Relativity, the answer is much closer to the latter, because it isn't mass that determines gravitation, but rather the curvature of space, which is determined by the sum of all the matter and energy in it.
If you were to take the Sun away, space would go from being curved to being flat, but that transformation isn't instantaneous.
Because spacetime is a fabric, that transition would have to occur in some sort of "snapping" motion, which would send very large ripples  i.e., gravitational waves  through the Universe, propagating outward like ripples in a pond...
Sergiu Bacioiu from Romania,
under
c.c.2.0 generic.
Since gravitational waves are massless yet have a finite energy, they must move at the speed of light!
Which means, if you think
about it, that the Earth isn't directly attracted to the Sun's
location in space, but rather to where the Sun was located a little
over 8 minutes ago.
David Champion,
Max
Planck Institute for Radio Astronomy.
The orbits of the planets were so well studied and so precisely recorded for so long (since the late 1500s!) that if gravity simply attracted the planets to the Sun's prior location at the speed of light, the planets' predicted locations would mismatch severely with where they actually were.
It's a stroke of
brilliance to realize that Newton's laws require an instantaneous
speed of gravity to such precision that if that were the only
constraint, the speed of gravity must have been more than
20 billion times faster than the
speed of light...!
The Earth, for example, since it's also moving, kind of "rides" over the ripples traveling through space, coming down in a different spot from where it was lifted up.
It looks like we have two effects going on:
LIGO/T. Pyle, of a model of distorted space
in
the Solar System.
The inexactness of the cancellation is what allows us to determine, observationally, if,
In theory, we know that the speed of gravity should be the same as the speed of light.
But the Sun's force of gravity out here, by us, is far too weak to measure this effect. In fact, it gets really hard to measure, because if something moves at a constant velocity in a constant gravitational field, there's no observable affect at all.
What we'd want, ideally,
is a system that has a massive object moving with a changing
velocity through a changing gravitational field. In other words, we
want a system that consists of a close pair of orbiting, observable
stellar remnants, at least one of which is a neutron star.
The predictions from
Einstein's theory of gravity are incredibly sensitive to the
speed of light, so much so that even from the very first binary
pulsar system discovered in the 1980s,
PSR 1913+16 (or the
HulseTaylor binary), we have
constrained the speed of gravity to be equal to the speed of light
with a measurement error of
only 0.2%!
NASA (L), Max Planck Institute for Radio Astronomy / Michael Kramer,
http://www.mpg.de/7644757/W002_PhysicsAstronomy_048055.pdf
We were able to do another type of indirect measurement in 2002, when a chance coincidence lined up the Earth, Jupiter, and a very strong radio quasar (QSO J0842+1835) all along the same lineofsight!
As Jupiter moved between
Earth and the quasar, the
gravitational bending of Jupiter
allowed us to measure the
speed of gravity, ruling out an
infinite speed and determining that the speed of gravity was between
2.55 × 10^{8} and 3.81 × 10^{8} meterspersecond,
completely consistent with Einstein's predictions.
whose path was gravitationally altered by Jupiter in 2002, allowing an indirect confirmation that the speed of gravity equals the speed of light. Image credit: Fomalont et al. (2000), ApJS 131, 95183
http://www.jive.nl/svlbi/vlbapls/J0842+1835.htm
LIGO just saw the first one, after all! Unfortunately, due to our inability to correctly triangulate the location from which these waves originated, we don't know from which direction the waves were coming.
By calculating the
distance between the two independent detectors (in Washington and
Louisiana) and measuring the difference in the signal arrival time,
we can determine that the speed of gravity is consistent with the
speed of light, but can only place
an absolute constraint that it's
equal to the speed of light within 70%.
at the two detectors in WA and LA, with an uncertain origin to their direction. Image credit: Diego Blas, Mikhail M. Ivanov, Ignacy Sawicki, Sergey Sibiryakov
https://arxiv.org/abs/1602.04188.
The best results, at the present time, tell us that the speed of gravity is between 2.993 × 10^{8} and 3.003 × 10^{8} meters per second, which is an amazing confirmation of General Relativity and a terrible difficulty for alternative theories of gravity that don't reduce to General Relativity! (Sorry, Newton...!)
And now you know not only
what the speed of gravity is, but where to look to figure it out...!
