Springyness in space

1. Aug 15, 2013

Ionjump

Is there springyness in space? If there is mass, and the mass is suddenly moved away, does the space quickly move back to it's original state or does it take some finite time?

I'm thinking that there is not yet an answer to this question because a springyness would show itself as waves in the gravity wave experiments.

If it is the case that we don't know if there is springyness, why is it not possible to set up a small scale experiment to test this?

2. Aug 15, 2013

CompuChip

I'm not sure what you mean exactly by "springyness".
Moving an object does affect the spacetime around it, and in certain cases it can lead to a propagating phenomenon called "gravitational waves". At least, in theory and so far supported by indirect evidence only.

In any case we are pretty certain that any effect would propagate at most at c, so any non-local effect will take a finite amount of time.

3. Aug 15, 2013

Ionjump

Thanks Compuchip

By "springyness" I mean to ask if space can behave like a spring. For example, imagine a volume of pure space with no matter in it. The space is of the same density everywhere. Now, lets squeeze a small volume of space such that it is more dense and the area around it is less dense. Now we release the system and allow it to reach equilibrium. Does the hight density space rush back into the low density space? Does this create a reverberation or oscillation of low and high density space?

4. Aug 16, 2013

Bill_K

Interesting idea, but no, space does not have a "springiness", or even a "density". As CompuChip mentions, the closest we come to this is a gravitational wave, which causes distances to increase and decrease as it goes by. These distance variations show up as variations in the metric. I suppose one could regard it as space being "compressed" by the wave, but the nature of a gravitational wave is quite different from an elastic one.

In an elastic medium, any influence that changes the density, such as an external pressure, causes energy to be stored in the medium. This energy will then produce a rebound when the influence is released. Yes, too, in the linearized theory of gravity one can talk about the energy that is carried by a gravitational wave, but it is not associated with changes in the "density" (metric). Rather it arises from the derivatives of the metric. That is, a gravitational wave carries energy only because of the way its metric varies from point to point.

5. Aug 16, 2013

CompuChip

Sorry for the off-topic; but Bill, I was wondering if your sig happens to be a Douglas Adams quote.

6. Aug 16, 2013

Bill_K

Bingo, it's from the last page of "Life, the Universe and Everything".

7. Aug 16, 2013

Ionjump

Thanks Bill

Why are gravitational waves only predicted for large things like rotating star and black hole systems? Why can't we see gravitational waves from smaller things like atoms or protons? Wouldn't a vibrating proton create a gravitational wave as well?

8. Aug 16, 2013

Staff: Mentor

They're still (predicted to be) there, just too weak to detect.

9. Aug 17, 2013

CompuChip

Just like with sound waves and light waves, the intensity also diminishes with the square of the distance from the source. So even these star-based events that emit relatively large gravitational waves will hardly be detectable by the time they reach us. Also note that we have a lot of "background", by the simple fact that we live on big floating and resonating plates of rock, the movements of which we need to filter out (so no boots allowed near the detector ;)).

I guess that if you consider the task of detecting someone loudly shouting your name 100 km from here while you are sitting here on a busy city street with people mumbling around you, you just about get the right idea. For a more precise explanation, I suggest googling around for "gravitational wave detector". I'd quote the wikipedia page but I'm always a bit suspicious about its accuracy when it comes to cutting edge subjects like these.