Why is There Gravity?

Why is there gavity?
- Kevin (age 10)
Aberdeen,NJ,USA

Good question.  I wish I knew, but that's the way it is.  Scientists can explain lots of  facts and effects of gravity and know why the earth attracts an apple and why and how it falls, but we do not know why gravity exists.  This is one of the great mysteries of life.   There are additional forces other than gravity that affect our lives, such a electric and magnetic forces as well as the forces that keep the nucleus of the atom together.   We can study them and explain lots of facts and consequences BUT we don't know why they exist.   Why don't you become a scientist and figure it out for us.

LeeH

(published on 02/04/2010)

Why that's an easy one. Those forces exist because otherwise the universe wouldn't exist! Like evolution finds the best set of features that optimize life forms, at the begining of the big bang the universe found the set of physical laws we now know because these laws were the ones that would optimize it. But rather taking millions of years to do so, it took an single instant. Don't you think so?
- Anonymous

What you are describing has a fancy name called "Anthropic Principle". 
See:     
I personally don't subscribe to it but some well known scientists do.

LeeH

The usual modern version of this is not that any universe tried to be optimal, but that the physical processes generated many (perhaps infinitely many) 'universes'. Ones that aren't so optimal don't have any inhabitants to wonder why. They may, however, be much more common.

Mike W.

(published on 02/07/2010)

I'm no expert on the topic of gravity, but I've wondered this since stumbling upon the problem of quantitatively identifying some *thing* responsible for its force on mass: what premise(s) in physics disproves the idea that gravity is a collective force of interaction between subatomic particles? Consider the possibility of a shared property between some or all subatomic particles that when combined and not put under the "microscope", an *emergent* property (gravity) is formed. This question may have an answer that is too obvious for me to see or there may already be a stance that explored this question, of which I am not aware. In the unfortunate case, maybe both. Thanks in advance.
- Anonymous
Canada

It's a little hard to comment on that thought, because it's a bit ill-defined so far. The gravity we see has a simple form suggestive of a fundamental law. It could, however, be only an "emergent" law applicable on large distance scales, based on some deeper more fundamental theory. Many people suspect that. In most versions of that idea "large distance scales" means larger than about 10^-35 meters, or what in most contexts we'd call extremely small. A proton, for example, is huge on that scale.

One idea about how gravity might emerge from such physics is presented here:
http://arxiv.org/abs/1001.0785.
It's way over my head.

There are also some ideas about multidimensional spaces in which gravity only takes on its familiar form on distances larger than the separation of different 3-spatial-dimension spaces in some other dimension. Experimental tests show that our ordinary laws work down to the shortest distances tested- a few microns, the last I'd heard. (see e.g. http://www.slac.stanford.edu/econf/C0507252/papers/T032.PDF)

Mike W.

(published on 05/18/2011)