# Question about gravity on a moving object

1. Jan 28, 2013

### Low-Q

I have a question about gravity on a moving object.

If a spheric heavy object with uniform mass is travelling at high velocity through space, will that object increase its gravity in front of it and decrease its gravity behind it, while gravity on the sides remains the same?

Vidar

2. Jan 28, 2013

### Staff: Mentor

I'm not sure what you mean when you say high velocity like 90% of light speed?

It seems you're referring to the Alcubierre Warp Drive:

http://en.wikipedia.org/wiki/Alcubierre_drive

If so then this warping of space is due to exotic matter (which we've never seen and dont even know if it exists) and not simply by movement of an object through spacetime.

And lastly there's frame-dragging:

http://en.wikipedia.org/wiki/Frame_dragging

which may be what you're thinking.

3. Jan 28, 2013

### Low-Q

I don't know if my question is related to the links you gave me. Say the earth is travelling extremely fast in one direction. Will a scale show my mass as different depending on where I am located on the earth?

4. Jan 28, 2013

### Staff: Mentor

With respect to what?

You should see no difference in any experiment that studies objects on the earth (and that are moving along with it), using equipment on the earth. You may see differences in the apparent behavior of non-Earthly objects: other planets, stars, etc.

5. Jan 28, 2013

### xAxis

No, the force does'nt depend on the motion of a sphere. Its gravitational field only depends on its mass. The gravitational force will be same in all four directions you mentioned.

6. Jan 28, 2013

### K^2

There are three ways to answer this question.

1) Classical Mechanics only. There is no change. Gravity depends on mass only and propagates instantly.

2) Linearized Gravity. It's going to behave similar to a moving charge. The gravitational field will remain the same, but the body will also generate gravitomagnetic field, which will interact with other moving objects. Obviously, this is frame-dependent as pointed out earlier.

3) General Relativity. The object's gravity changes, and it produces frame-dragging effect. In many ways, it is similar to the above, in that it is frame-dependent and that it will interact in interesting ways to other moving objects. For a general mass configuration, it's an extremely complicated problem, but for a spherically symmetric body, the result can be obtained by changing the frame of Schwarzschild metric to arrive at the exact effect of the moving body.

Note that all of the strange effects in cases 2 and 3 happen primarily because of the choice of reference frame. If you pick one in which the massive object in question is static and the observer is moving, you can describe all of it in much simpler ways.