# Gravity From Moving Objects

*Most recent answer: 12/20/2012*

Q:

Hi, this may be a dumb question, but is the gravity of a massive object affected in anyway if it is moving versus stationary or even accelerating? Also does a moving object have a different affect on spacetime than a stationary object? For example is there a kind of gravitational "wake" in space caused by a moving object similar to a boat in water?

- Joe (age 32)

Ohio

- Joe (age 32)

Ohio

A:

That's a great question. The answer is basically "yes", but with one quibble about wording.

Whether an object is "moving" or even "accelerating" depends on the choice of reference frame used to describe it. Nevertheless there are aspects of relative motion which do not depend on reference frame choice and which are very much present in gravity.

For our purposes here, let's just pick a center-of-mass based frame and use "velocity" and "acceleration" in the familiar way. The term in the general relativistic equations describing the local distortion of space-time includes not only the energy density ("mass") but also the momentum density, i.e. motion.

One of the most important implications of relativity then is that accelerating masses radiate energy as gravitational waves, just as accelerating charge radiate electromagnetic waves. The rotation of binary pulsars slows down measurably due to the energy lost by this process. (see )

Even when the distribution of mass doesn't change, e.g. when a sphere rotates, the motion gives an effect similar to a gravitational version of a magnetic field. This effect from the Earth's spin has recently been measured by the Gravity Probe B experiment.

We have some more discussion on an old answer:

Mike W.

Whether an object is "moving" or even "accelerating" depends on the choice of reference frame used to describe it. Nevertheless there are aspects of relative motion which do not depend on reference frame choice and which are very much present in gravity.

For our purposes here, let's just pick a center-of-mass based frame and use "velocity" and "acceleration" in the familiar way. The term in the general relativistic equations describing the local distortion of space-time includes not only the energy density ("mass") but also the momentum density, i.e. motion.

One of the most important implications of relativity then is that accelerating masses radiate energy as gravitational waves, just as accelerating charge radiate electromagnetic waves. The rotation of binary pulsars slows down measurably due to the energy lost by this process. (see )

Even when the distribution of mass doesn't change, e.g. when a sphere rotates, the motion gives an effect similar to a gravitational version of a magnetic field. This effect from the Earth's spin has recently been measured by the Gravity Probe B experiment.

We have some more discussion on an old answer:

Mike W.

*(published on 12/20/2012)*