# Geometry vs. Particles

1. Apr 19, 2005

### infidel

First a little background, I had 3 years of college-level physics and calculus, but I haven't studied physics since, just remained an interested amateur, so this may be way naive. But it's been bugging me so I want to ask here...

Einstein said there really is no 'force' of gravity. Mass warps spacetime into curves. Particles (or planets) move in as straight a line as they can in warped spacetime. This results in the appearance of a force.

I get that (I think.) So my question is, if this is the case why are we still looking for gravitons and gravity waves and trying to unify gravity with E/M, the SN and WN forces? Seems to me if gravity just the result of the curved geometry of spacetime near a mass, this is all unnecessary.

I guess I'll also go way out on a limb and ask if we know how mass warps spacetime or if we can only calculate the amount.

Be gentle with me. :shy:

2. Apr 19, 2005

Staff Emeritus
1)The other three forces (strong, weak, and electromagnetic) are carried by bosons. Particle physicists naturally expect that whatever the successes of GR, gravitation should be carried by a boson too.

2) They say that GR cannot be a final theory because it has singularities. This is disingenuous; QED the most accurate of relativistic quantum field theories has singularity problems too. Look up "Landau pole".

3) There are technical reasons that prevent the coupling of quantum theories such as the standard model to GR. Many workers believe that a quantum theory of gravity would remedy this, and provide a single theory covering all four forces.

3. Apr 19, 2005

### Crosson

Actually, the other three forces can be described as being carried by particles. Be careful not to say that they are particulate, and because they are gravity should be too. Particles and geometry have to do with the way we choose to describe the physics. Standard modelers choose particles, relativists choose geometry.

4. Apr 19, 2005

### pervect

Staff Emeritus
Leaving aside the question of unification for the time being ---

Note that you do not need a theory of "gravitons" to expect gravitational waves. I'm not aware of any experiments that are designed to directly detect "gravitons". There are many experiments in progress to detect gravitational waves, which are predicted by the curvature theory as "ripples of curvature".

A hypothetical gravitational wave detector that was so sensitive that it could detect the fact that gravitational waves were quantized would be a direct detection of gravitons. This strikes me as being unlikely to happen anytime in my lifetime.

Going back to the unification question - the need for a unified theory is at present more of a theoretical need than a practical one. We do not have access to environments that are energetic enough to really test theories of quantum gravity. This also means that the differences won't matter much to experiments we can carry out.

The motive which drives the study of quantum gravity is consistency. Rather than an effort to predict "new physics", the goal is to unify GR and quantum mechanics so that we have one consistent theory, rather than two inconsistent ones. We cannot apporach the question experimentally at the current time, but theoretical work can still be done. Having one consistent theory would be a step forward from having two inconsistent ones that don't agree with each other - even if the incosistencies only show up in regions that are not experimentally accessible.