Re: Gravity is the curving of space, so why shpuld there be a graviton?

Hi Eduardo,
  
Gravity is one of the weakest forces around:  if it weren't for the facts
that it works over such large distances and that it is always attractive,
we might not even notice it at all.  Certainly on the scale of atoms and
molecules where quantum mechanical effects are important the effects of
gravity are negligible, and similarly on the scale of stars and galaxies
where gravity becomes dominant we can ignore quantum effects.  So at
present we have the theory of general relativity and the theory of quantum
mechanics, two extremely successful theories that don't generally overlap.

But physicists aren't satisfied with having a theory of gravitation which
is still essentially 'classical' (ie it doesn't consider quantum effects).
One of the biggests quests in modern day physics is to find a 'grand
unified theory' (aka GUT) that joins all types of forces.  This would
include finding a theory of 'quantum gravity', and in this type of theory
the gravitational force would be transmitted by virtual particles called  
'gravitons'.  

For more on gravitation you can check out the answer to a similar MadSci
question 
'Gravity' or look in "Gravity's Fatal Attraction" by Mitch Begelman
and Martin Rees.

Cheers,
  Meghan