# Why does matter even bother?

1. Mar 23, 2009

### Forestman

I know that GR says that gravity is caused by a curvature in spacetime, but why would matter have to move into the curve, why not just site there on the curve.

2. Mar 23, 2009

### marcus

Forestman, anything the piece of matter does corresponds to some path in spacetime.
What you call sitting still is one possible path (shall we call the path a "world-line"?)

I'm not saying this is how Nature works, only that this is how it works according to GR.
There is an idea of a nice path----analogous to the shortest distance paths, geodesics, on a curved surface.

Curved geometry (in particular curved 4D geometry) can influence where matter goes by deciding things like shortest distance.

When there are forces and interactions and potential hills, then geometry can still play a role. There will be a Lagrangian and an action integral and (in theory) the matter will want to follow a path that optimizes the action. E.g. a least action path. And the geometry is still a key ingredient.

You could call the physical action integral "bother". And then you could ask a new version of your question: "Why does matter choose the spacetime path with the least bother?"
(which indeed might happen to be a sitting-still type of path from somebody's perspective.)
Why does matter optimize? And depending on how you set it up it might maximize instead of minimize, but always some kind of optimum.
And the answer might be that we don't know why matter chooses optimal or least bother paths.

I hope you get several more answers. It is an interesting question. You might look up action principle on Wikipedia.
http://en.wikipedia.org/wiki/Principle_of_least_action

Last edited: Mar 23, 2009
3. Mar 23, 2009

### ZikZak

"Sitting there" at one point in spacetime is equivalent to asking a particle to sit there at a particular point in space and time.

particles don't do that.

4. Mar 23, 2009

### Forestman

So what you guys are saying is that since time is bent as well as space an objects world line would be bent towards a massive body. That makes sense. Thanks.

5. Mar 23, 2009

### A.T.

Similar question:

6. Mar 23, 2009

### Forestman

A.T. thanks for that, what you said makes a lot of sense to me.

Hey TimothyTang, you have an interesting theory and all, but I would need to see some evidence and math for it. There is a lot of evidence for GR though.

7. Mar 23, 2009

### jefswat

My previous post got deleted because I was typing it as the moderator deleted some other ones and I suppose it had nothing to do with the original question anyway. However I think it would be interesting to note in the debate with objects following the "nicest path" in GR that no matter how hard one tries there are certain things that can't be proved by math. I mean that we have to take some things for granted and make a FEW assumptions (they aren't at all outlandish). For instance in basic gemetry there are a few things that have no proof which I can't exactly remember. I think they are a point and 2 other things. All things said it isn't outlandish to make an assumption of what a point is. The point(no pun intended) that I am trying to make is that I believe there is a limit to how far math and physics an go to proving why certain things happen. That isn't to say they can't offer an explaination, but you have to start somewhere and there has to be something that you can't prove that you take for granted based on the evidence you have at hand. Nature doesn't have a limit like that though so we have to make due with what we have to explain it