# Does the mass of a spinning object twist spacetime?

1. Apr 6, 2015

### quincy harman

Does the mass of a spinning object twist space time? If so is this why all natural satellites orbit in the same direction as the spin of the said object?

2. Apr 6, 2015

### A.T.

Not sure if that's what you mean by "twisting":

http://en.wikipedia.org/wiki/Frame-dragging

This has to do with how solar systems are created:

3. Apr 6, 2015

4. Apr 6, 2015

### A.T.

Angular momentum is conserved. If it has it now, than it had it back then.

5. Apr 6, 2015

### quincy harman

where does it come from?

6. Apr 6, 2015

### A.T.

From probability. Zero total angular momentum is one among infinitely many other possible values.

7. Apr 6, 2015

### quincy harman

I don't understand. It's more likely to have some angular momentum than none? But it still had to come from somewhere right?

8. Apr 6, 2015

### PAllen

No it doesn't. A conserved quantity can never "come from somewhere", else it is not conserved. (There is more to the issue, but first I want to push on this first - if it's conserved, not only doesn't it need to come from somewhere, it can't come from somewhere.)

9. Apr 6, 2015

### quincy harman

So it is a form of potential or stored energy? and the direction is random? You got me interested. Tell me more! :D

Last edited: Apr 6, 2015
10. Apr 6, 2015

### PAllen

One point is that if there is one isolated gas cloud in the universe, the total angular momentum of any future state (e.g. planetary system) will be the same as the initial angular momentum. Then, the angular momentum is just an initial condition, no different than the initial mass (would you wonder so much where that came from?).

The 'more to it' comment is that if you were to imagine a cloud that happened to have zero total angular momentum, that ultimately split, the probability that each would have zero angular momentum is zero. It would be analogous to two billiard balls colliding such that they hit dead center and ricocheted with zero spin. Instead, each would have nonzero angular momentum (such that the total of both is zero), and each would form planetary systems with non-zero angular momentum.

Thus, whatever you consider about initial conditions, the collapse of clouds to bodies or systems has basically zero chance producing non-spinning bodies.

11. Apr 6, 2015

### quincy harman

So it's essentially just a consequence of inertia and gravity? How do the up and down motions cancel out over time?

Last edited: Apr 6, 2015
12. Apr 6, 2015

### Staff: Mentor

The frame dragging effect is far too small to matter for situations like this.

13. Apr 7, 2015

### A.T.

Inelastic collisions.

14. Apr 7, 2015

### quincy harman

I don't understand how this cancels out the up and down motions over time.

15. Apr 7, 2015

### Ibix

If there is no total up and down movement, then you have a situation like two balls moving towards each other - each is moving, but the center of mass is not moving. What happens when the balls collide? Perfectly elastic ones bounce off each other, possibly even keeping the same speeds in the opposite directions, and leaving the center of mass still not moving. Generally speaking, though, lumps of rock aren't particularly elastic, and will generally either stick together or shatter - and in either case some of the kinetic energy is lost (or, more precisely, converted to some other form - mostly heat). Any inelasticity in collisions steals a bit of kinetic energy, so the mean speed will tend slowly downwards.