# Two particles in a vacuum

1. Nov 25, 2009

### Rubix

I've been thinking about this. I don't know a whole lot about physics so i'm sorry if I ask something stupid. If there are two particles in a vacuum, according to the theory of gravity, the two particles will be attracted to each other and collide. But, how does this one particle know that there is another particle existing next to it? Is that where the so called "graviton" comes into play?

2. Nov 25, 2009

### fatra2

Re: gravity

Hi there,

Before going into the idea of a "graviton", you can understand this behaviour with the field theory. Just imagine that the first particle develops a gravity field (mostly like the magnetic field develop by a magnet) around it. By placing another massive particle in this field develops a force on that particle.

Cheers

3. Nov 25, 2009

### mikeph

Re: gravity

I don't understand this. Say you have two 1kg masses 1ly apart, fixed for >1 year, then released simultaneously to move freely. When mass A moves towards mass B, it cannot know whether mass B has been released until about 1 year after it is released, so shouldn't it's rate of acceleration be less than you would expect?

I have studied electrodynamics and it is very different from electrostatics, but I have never thought about the gravity analogy (despite studying general relativity, where the metrics were still static)

4. Nov 25, 2009

### chris772

Re: gravity

If the two particles have an adequate gravitational field to act on each other, they would move at a calculated speed. As the distance closes between the objects the speed would increase. Newton's Law of Universal Gravitation states that the force between two objects is equal to the gravitational constant multiplied by the product of the two objects masses divided by the distance between them squared.

$$F = G(m1m2 / r^{2})$$
$$m1, m2$$ = masses of objects
$$r$$ = distance between objects
$$G\ =\ 6.673(10)\ \times\ 10^{-11}\ m^{3} kg^{-1} s^{-2}$$

I think wikipedia has a better representation of the formula than I can do on this forum:

To answer your question, the objects are not aware of each other the gravitational properties of them merely react to the laws of physics. So if the two objects have the ability to gravitate they will move toward each other at a calculated velocity.

I'm also new to physics so I probably botched this answer, but it's the best I can do.

5. Nov 25, 2009

### mikeph

Re: gravity

I think I have found some sort of answer, my issue was that the law is not invariant under a special relativity transform. I guess GR fixes this, but I can't quite see how.