# How often does mass effect space

1. Jul 12, 2008

### Nacho

I have a question about gravitation, that may be a "duh" question and apparent -- to everybody but me.

Say that a mass effecting the geometry of space/time is the cause of gravitation, per what I understand about what Einstein said. Wouldn't that mean that the mass has to do this on a continuing basis? It seems to me that a one time effect couldn't keep up the long term effects that we see of gravitation.

If that is the case, I have a related question. How often is the effect propogated, and what is the limiting factor of how often/fast it is performed? The simple answer might be "it is done at the speed of light (or hypothetical gravitons)", but I'm not so much asking how fast as how OFTEN. Bringing in quantum actions, I don't know if it could be continuous. Is how often the effect performed somehow related to the HUP?

2. Jul 12, 2008

### Fredrik

Staff Emeritus
Einstein's equation has a mathematical expression representing the geometry of spacetime on the left-hand side and a mathematical expression representing all the energy and interactions in all of spacetime on the right-hand side. Note that it's spacetime, not just space. That's why "once" is enough, or at least would have been enough if it hadn't been for quantum mechanics. Any local change of the stress-energy tensor caused by quantum effects would in theory propagate at the speed of light. The key word here is "any", but of course if the changes are small enough, quantum gravity effects will be important.

3. Jul 12, 2008

### Nacho

I don't understand what you are saying here -- sorry. If "once" is enough per Einstein (and neglecting QM), then take the mass away abruptly. If the presence of the mass is not continually communicated, how does the local space know it is gone (maybe movement of mass VS presence of mass)?

4. Jul 12, 2008

### HallsofIvy

First, the word is "affect", not "effect". Second, the influence of mass is continuous. There is no question of "how often".

5. Jul 12, 2008

### Nacho

Isn't that something that has to be reconciled with QM?

6. Jul 12, 2008

### Fredrik

Staff Emeritus
The right-hand side of the equation specifies everything about the matter in spacetime. When you talk about taking mass away, you're already contradicting GR. The only kind of "change" that's possible in GR is already present in the stress-energy tensor (the right-hand side of the equation).

7. Jul 12, 2008

### Nacho

I just meant remove that mass from that location in space, and move it somewhere else. That area of the space was effected by the presense of the mass, and and if it was a one-time change how that space would know to return to not being effected. I'm not talking about all mass and all locations in space, just a location in space that has an accumulation of mass, such as our sun, and the local gravitational effects caused by it.

8. Jul 12, 2008

### Fredrik

Staff Emeritus
I never thought you meant anything else. I'm just saying that this "move" is either described by the stress-energy tensor, or it isn't. The first alternative corresponds to one spacetime geometry and the second alternative to another geometry. The first of those two geometries describes gravitational waves propagating away from the relevant event. The second doesn't.

9. Jul 13, 2008

### Staff: Mentor

"How often" do your feet apply a force to a bathroom scale when you stand on it? See - the question doesn't really make any sense.