[vampster (OP)]

Hi,

I have some questions regarding spacetime curvature and gravitons:

• If spacetime curves, why is there the need for a force particle? Does it not mean that there is no force but objects just go in a "straight" line in the curved spacetime? Why is the graviton necessary?
• Why is it that spacetime curvature depends on the speed of the passing object? On a plain every object takes the same path, regardless of its speed, but in curved spacetime this is not the case: a rock will fall to the ground, but a lightwave does not. If both were to follow the same "straight" line in the curved spacetime would they not both fall to the ground? Or put differentlly: why does a beam of light send in the direction of earths orbit not follow the same path as earth does?

[Bill S]

Hi Vampster,

As I understand it, QM requires particles to mediate forces.  Someone will probably put me right on that, but for the moment I'm sticking with it.

Only if you say, as some experts do, that gravity is not a force, can you dispense with the graviton.  As discussed in various places on TNS, Einstein did not say that his idea replaced the gravitational force with spacetime curvature.  You may be able to answer some of your own questions if you think of gravity as a force that can be described, mathematically, as curvature of spacetime. 

Of course, that will probably leave you with lots of questions about the physical nature of spacetime curvature; but, join the club!  What better place to bring those questions, than here.

[PmbPhy]

Hi,

I have some questions regarding spacetime curvature and gravitons:

I'd like to point out that there is no quantum theory of gravity and as such gravitons are merely theoretical particles which have never been detected.

• If spacetime curves, why is there the need for a force particle?
  

Why shouldn't there be one? If I asked you in the context of Newtonian mechanics "Since tidal gradients exist in certain kinds of fields they why do we need a gravitational force.

Einstein himself used the tern gravitational force in his general relativity (GR) textbook ]The Meaning of Physics. Its on page 82 if you have a copy. Einstein wrote

In analogy with Newton's equations, the first term would be regarded as the expression for inertia, and the second as the expression for the gravitational force.

To see how force is defined in GR see: http://www.newenglandphysics.org/physics_world/gr/grav_force.htm

However if there is a quantum theory of gravity then the purpose of the graviton is the same as the purpose of the photon in electrodynamics, i.e. it carries the force.

Does it not mean that there is no force but objects just go in a "straight" line in the curved spacetime? Why is the graviton necessary?[/li][/list]

There's no such thing as a straight line in curved spacetime. Particles follow what are referred to as "geodesics."

If you're good at math then you'll be able to follow this
See:  http://www.newenglandphysics.org/physics_world/ma/geodesics.htm

Ask where you get lost. Think of a geodesic is a path of extremal (max or min or inflection point). E.g. suppose you're at the equator and you wish to drive to the north pole in the shortest distance. Then hope in your are and drive straight north. That's a geodesic.

I recommended reading the article I wrote on the subject entitled

Einstein's gravitational field which is online at
http://xxx.lanl.gov/abs/physics/0204044

• Why is it that spacetime curvature depends on the speed of the passing object?
  

It doesn't.

On a plain every object takes the same path, regardless of its speed, but in curved spacetime this is not the case: a rock will fall to the ground, but a lightwave does not. I

Yes, it will.

[yor_on]

Well, I don't think of gravity as a 'force'. Although tidal gradients may exert a force upon something (spagettizising you as you fall into a black hole f.e ) it's still about geodesics as far as I get it? So yes, then again, what is a 'force'?