# Q & A: Speed of Gravity

Q:
Ok--I am an UI alumini - BSME - 1979. I was at the U of I during March 2002 as my daughter competed in the Illinois State Science Fair (in physics). I found out about the web site while reading the July/August ILLINOIS alumni magazine. Question: What is the speed of gravity? I will pass on this web site to my daughter’s grade school (she is now a high school freshman). With best regards.
- Bill Healy
Wheaton, IL
A:
Bill -

If by the speed of gravity you mean how fast changes in gravity propagate out when the source of the gravity changes, the answer is the speed of light. In other words, if two stars collide 100 light-years away from here, we won't feel any difference in their gravity for 100 years- the same length of time it would take before we could see the collision. For more information, look at the answer .

Perhaps you are thinking instead of the acceleration caused by gravity, not the speed at which gravitational changes travel. Near the surface of the Earth the acceleration caused by gravity is about 9.81 m/s^2. Basically, this means that in one second, a falling object will speed up by 9.81 m/s if other forces such as friction with the air aren't important. Of course both the size of that number and the direction of the acceleration change from place to place.

Tamara and Mike

(published on 10/22/2007)