# Why is there gravity?

1. Sep 16, 2006

### priya_india

hey! can anyone tell me the reason for gravitation?
it may seem a very simple but i hav not been able to find it anywhare.and i think noone in the world knows it.

2. Sep 16, 2006

### Staff: Mentor

Nope, no one knows why there is gravity. Maybe whoever or whatever created the universe thought it would be cool for matter to stick together.

Seriously, though, there are different kinds of "why" explanations, also I guess English might not be your native language. Perhaps you should explain in more detail what sort of answer you are looking for.

The usual answer that you'll probably get from physicists is that matter and energy cause spacetime to curve, and that causes the "natural" paths of objects to come together as if they were attracted by a force. That's the explanation provided by Einstein's theory of general relativity. About that, you should ask in the relativity forum, the next one down from this one.

Last edited: Sep 16, 2006
3. Sep 17, 2006

### Thrice

Yup. The presence of matter curves spacetime in such a way as travelling through time now causes you to move (accelerate) through space.

4. Sep 17, 2006

### michael879

you can imagine this "curve" of space-time better if you think of a 2d universe. If you stick a bowling ball on a trampoline, it curves the surface of the trampoline so that if you stick a marble on it, it will be "attracted" to the bowling ball and may even orbit it once or twice.

5. Sep 17, 2006

### notknowing

I know it : see http://home.online.no/~avannieu/darkmatter/
and look at the first article (on quantum gravity).

6. Sep 18, 2006

### gijeqkeij

Nope, the "why" questions can't be answered in physics. We know "how": space-time curvature generate what we feel as gravity.

gijeqkeij

Universe principles are so simple that it's almost impossible for us to understand them

7. Sep 18, 2006

### michael879

if you view the universe as an expanding hypersphere you can explain gravity by saying that the more massive an object is the slower it travels through time. it would be like blowing up a balloon but sticking your finger in the way. With the universing expanding faster than massive objects, curves are formed like gravity. This isnt exactly complete though, youd still have to accept some unexplained force that makes massive objects age slower. Maybe the fact that the universe's expansion is accelerating (not sure if thats right) explains that though (i.e. F = ma).

8. Sep 18, 2006

### gijeqkeij

Nope, the "why" questions can't be answered in physics. We know "how": space-time curvature generate what we feel as gravity.

gijeqkeij

Universe principles are so simple that it's almost impossible for us to understand them

9. Sep 18, 2006

### michael879

umm whyd you say the exact same thing twice?

10. Sep 18, 2006

### masudr

gijeqkeij,

I know it's not possible to have LaTeX, or even any semblance of formatting, for usernames, but one can in posts. Perhaps it might be better to answer OP's questions with better formatting for equations.

Gravitational field equations are:

$$G_{ij} = kT_{ij}$$

There's only one equation there, I hear you say. So why did I use the plural "equations" above? Well, $i,j=1,2,3,4$, so there are 16 equations.

There's lots more about this equation, and what it actually means (and what k is), in all kinds of books and websites. So I won't comment here. In fact, I'm not even sure who I've really addressed this to. I know it says gijeqkeij at the top, but he/she is presumably familiar with all this already. In any case, I've typed it, so I'll post it.

11. Sep 24, 2006

### unscientific

Theoratical physicists have suggested string theory. Just type in " String Theory " in google and discover the fascinating world about it! :D