Speed of Gravity vs Acceleration Due to Gravity

August 8, 2008 by Victor Chabala

In the context of classical theories of gravitation, the speed of gravity refers to the speed at which a gravitational field propagates. This is the speed at which changes in the distribution of energy and momentum result in noticeable changes in the gravitational field which they produce.

In layman’s terms, this means the length of time it takes the effect of the gravity of object A to reach and affect object B.

As we see here, although there is still some debate, the first test of the speed of gravity, which occurred in January of 2003, indicates that the speed of gravity is about the same as the speed of light.

The speed of gravity is NOT the same as acceleration due to gravity, which, for the Earth, is 9.8 meters/second/second ( m/s2) or, in the English system of measurement, 32 ft/s2 (source).

Now, let’s review some real world physics- speed and acceleration are NOT the same thing- in point of fact, as any physics book will tell you, speed and velocity are NOT the same either. 75 miles per hour is speed, 75 miles per hour due north is velocity. Simply put, velocity has direction, speed doesn’t.

Getting back to acceleration and velocity, they are NOT the same either. Physics defines acceleration as a change in velocity over time- this means velocity is a COMPONENT of acceleration.

Now, again, mass does not move mass, force does as per Newton’s First Law of Motion, and since Newton’s Second Law of Motion states that force = mass times acceleration ( F = ma),
Now, getting back to the issue of the speed of gravity, the only way for that to have an effect is for the earth’s gravitational field to suddenly fluctuate due to some sort of gravitational anomaly and a distance between the two objects for said fluctuation to traverse, which would have caused some rather nasty tectonic effects, such as tidal waves, massive earthquakes, etc. I don’t recall any of those occurring on 9/11/01, so clearly, there was no gravitational anomaly- this isn’t Captain Jonathon Archer’s Enterprise cruising through the Expanse.