# Q & A: speed of gravity

Q:
I have been reading several of your answers, and i noticed in one off them you said that the speed of gravity is far greater than the speed of light(the time that it takes for a gravitaional cause to have an effect) however i have found from other reliable areas that the speed of gravity is actually faster by a factor of 2.53x10^10 times, meaning the speed of gravitational effect is 6 x 10^18 meters per second, these are from sources such as Van Flandern and Barry Setterfield
- Jamie Ferguson (age 25)
netherlands
A:
I'm a lttle puzzled by your question. Gravity is a disturbance in space-time which propagates at exactly the same speed as light. Did we say something different somewhere?

Mike W.

(published on 10/22/2007)

## Follow-Up #1: gravity on the move

Q:
Regarding the speed of gravity. How does gravity actually propagate from one body to another? How does it know that it has to propagate? If it travels at the speed of light then doesn't it have to know something ahead of time about where it has to go, when it has to go, how large the body is that it is going to be gravitationally connected to, the distance that the target body is? What if the sender is transmitting a gravitational connector to a large body that is a million light years in distance (which will take a million years to reach the target if gravity travels at the speed of light), and finds that when the signal gets there, the body is much bigger (possibly merged with another), it is gone -- moved some light years away or blew up? I know that these are a lot of questions, but just one more -- how does gravity know that it should travel (at the speed of light) from body A to body B -- why not body B to body A? Thank you for your time on this
- John Tomassoni (age 83)
Annapolis, MD, USA
A:
The equations (General Relativity) that describe the way gravity waves propagate are local, just like many other familiar wave equations. Think of the propagation of light and other electromagnetic waves. They don't require knowledge of what they'll hit, they just follow their own local rules. You can picture that with water waves too. Each little bit of water just pushes and pulls on its neighbors, without any planned destination. And of course the waves do travel both ways.

Mike W.

(published on 05/16/2013)

## Follow-Up #2: Waves in matter and space

Q:
I know there are different types of waves in matter, for example P waves, S waves, etc. But do these correlate to waves in space? For example I know of two types of waves through space: gravitational waves and light waves, which both travel at precisely the same speed. Are there any more types of waves through space?
- Luke S. (age 15)
Wauwatosa, Wisconsin, United States
A:
The P waves and S waves in matter usually refer to the two main types of seismic waves in the earth's crust.   They are essentially sound waves and can be caused by an earthquake or by ocean waves crashing onto the shore.   P or primary waves are compressional whereas S waves are transverse shear waves.   The velocity of these waves varies from 3 to 8 km/sec depending on the type of rock the waves are passing through.