Gravity, Gravitons, and Geodesics.

Glenn

Hi,

Hopefully someone can explain this to me in laymans terms...

If I am understanding what I am reading correctly, gravity the result of the shape of space-time? I don't understand why this is considered a force at all if it is the result of the shape of space-time. Where does the graviton come into play?

Thanks,
Glenn

homology

I'll shoot although hopefully others will provide better responses than mine.

A force causes an acceleration, and the curvature of space-time also causes acceleration. I believe a decent picture is this: image 2-D space, so a flat planar universe and imagine a particle moving along a straight line at constant velocity. Now imagine that at some point on this plane it is not flat, but there is an object there. The usual analogy is of a bowling ball on a rubber sheet. The rubber sheet being our 2-D space. so what the particle comes across this deformation of space its direction changes, an acceleration. So this might be a way of reconciling the label: force with gravity.

The graviton is the hypothetical particle predicted to exist which would be the carrier of the gravitation force (just like the gluon carries the strong force, the photon carries the electromagnetic force and the weak gauge bosons carry the weak force).

But like I said, hopefully someone more knowledgable will add to this.

Kevin

selfAdjoint

The graviton is a theoretical particle predicted by string theory but not yet demonstrated to exist. If it exists and is the cause of gravity it will do away with spacetime curvature and produce gravity the way the photon carries electromagnetism. Then gravity would be a (quantum) force in your sense.

At the present time however, our best theory of gravity is Einstein's 1915 General Theory of Relativity. One of the basic propositions of that theory is the Principle of Equivalence: On a sufficiantly small scale, it is impossible to tell the difference between an imposed force and a gravitational one.

Notice that the shape is the shape of spacetime, not just of space. Therefore a curved geodesic goes through time as well as space, and by curving, causes those travelling along it to experience an acceleration. Anything that produces an acceleration deserves the name force, no?

To homology: Ships that pass in the night

Glenn

Does that mean that the 2D rubber sheet analogy and the graviton are mutually exclusive?

If the graviton was experimentally found, would it do away with the rubber sheet analogy as a means of explaining the behavior of gravity?

Thanks,
Glenn

Nenad

yes, if the graviton is found, General Relativity will be wrong.

sol2

Will comologically realization in the quantum world morphisize to include relaitivity? If you marry gravity and electromagnetism what do you have? A new dimension of thinking?

Further explanations on the graviton? Any other information would be accepted

Nenad

No, because if the graviton is found, then there is no curvature of space time, the reson there is gravity will be because of the graviton. You cant have both explaining the same thing.

sol2

What geometry are you using?

ArmoSkater87

Couldnt you say the graviton cause the curvature of spacetime...lol

dodo

General relativity states that a mass causes a curvature in space-time. Now, ...

Can the reciprocal be correct? Namely, a curvature of space-time (by whichever means) can (I guess) be perceived as gravity, but... would it also seem to "have" mass?

(Forget about if we'd actually SEE something there... I mean if we'd perceive a mass by its effect with its surroundings; f.i., at subatomic sizes, would particles appear to collide with the distortion?)

Of course, the consequence of such an statement would be that space-time curvatures and masses ARE actually one and the same thing. I just wonder.

Can somebody point me to a theory (or think of an experiment) which can distinguish, in the context of your choice, between a mass and a spacetime curvature (regardless of the latter's origin)?

jtolliver

Not really. Any lorentz-invariant interaction that preserves causality can be described in terms of particles. GR satisfies those conditions. Gravitational waves have many properties traditionally associated with particles (If you pretend spacetime is flat and you add gravitational forces to compensate), such as energy-momentum. Using curved space-time(the technique traditionally used in GR), and using flat space-time with an additional force that makes it act exactly as though it were curved(this was the technique used in classical mechanics, except that that force didn't quite make it act exactly as curved space-time) are just different ways of looking at the same thing. The first way is much more convient mathematically, and the second way doesn't really explain why we have this additional force. The graviton's only look like particles when we assume spacetime is flat. If we assume spacetime is curved we see it is really only an effect of the curvature of spacetime, even though it looks exactly like a particle.

Entropy

I don't really consider gravity a true force. Why? I don't know, to me I see gravity as "bending" an object's path than actually exerting a force on it. I guess you could techniquely say that about any other force, but oh well.

Nenad

gravity isnt really a force. Out of the 4 elementary forces, it is a million million million million times weaker than any of the other 4. The only reason it is soo strong in our world is because of the amount of particles that are together at the same time pulling on us.

pmb_phy

No. That is not a clear interpretation of gravity/GR/spacetime. It is not the shape of spacetime which dictates the presence/presence of a gravitational field. Its the choice of a frame of reference which dictates the presence of a gravitational field. If you take a look at The Foundation of the General Theory of Relativity, Albert Einstein, Annalen der Physik, 49, 1916 then you'll see the following statement by Einstein
I don't understand why this is considered a force at all if it is the result of the shape of space-time.
[quote]
To get a better understanding of what spacetime curvature is please see
http://www.eftaylor.com/pub/chapter2.pdf
Scroll down to page 2-3 and read this
That is a description of the physics. The [b]cause[/e] is the gravitational force. Or in Einstein's own words, from The Meaning of Relativity, Albert Einstein, page 85
[quote]
The gravitational field transfers energy and momentum to the "matter," in that it exerts forces upon it and gives it energy: ...
It plays a role in GR similar to the role in EM. It facilitates the interaction.
It is not the curvature of spacetime that causes gravitational acceleration. Spacetime curvature causes two particles moving under to have a relative acceleration between them, i.e. spacetime curvature causes tidal accelerations. But you can have a gravitational force in the absense of spacetime curvature.




It doesn't come from string theory. It comes from quantum gravity/quantum field theory.
That is incorrect. The graviton is responsible for producing gravitational forces, not just tidal forces. Therefore one should aslo be able to detect gravitons in a gravitational field even when the spacetime is flat. The graviton, if it exists, will have a relative existance in that sense.
That is very much incorrect. You can most certainly have both. If the graviton is detected then we will have the mechanism behind GR. Gravitons are certainly not inconsistent with general relativity.
Absolutely. That is 100% correct.
Gravity is not like other forces like the electric of magnetic forces. The gravitational force is an inertial force where the others aren't.

See - http://www.geocities.com/physics_wor...tial_force.htm

Sure it is. For the definition of gravitational force in general relativity please see any decent GR text or Einstein's papers and books. Or see
http://www.geocities.com/physics_wor...grav_force.htm

Pete

robphy

Let's not be hasty here.
In general relativity, gravity is NOT a force [in the general case].

From Wald, p 67: (I boldfaced the key statements.)

pmb_phy

[QUOTE=robphy]Let's not be hasty here.
In general relativity, gravity is NOT a force [in the general case].
Sure it is.
Yes. I've read that claim and disagree with it. His justification is rather poor in my opinion. Wald has his own personal opinion on that point but he disagrees with everyone else that I've read on this point. I've never seen anyone else make that claim and I can see no justification for it, regarless of his explaination for his claim. However that is probably because his definition is different than everone elses, including Einstein's. Wald seems to think that the negative of the weight vector is what is called the gravitational force. That is not how the gravitational force is defined in all other material on the matter and its is not how I defined it in that web site. I am not the person who created the term "gravitational force" and I'm not the person who defined it in general relativity - that was done by Einstein. However I follow the definition given by authors such as Moller and Mould since I think they make the most sense.

Pete

kurious

Gravity can be a force if it is caused by gravitons.All the gravitons have to do is physically curve space-time:If space-time has mass and is made from particles
(spacetime could be quantized at the Planck length) then gravitons could physically curve those particles into different configurations.That way Einstein's theory can still be correct too.A spin 2 particle is needed for gravity to make it an attractive force,
and high in force-carrier density near large bodies such as stars -these considerations come from quantum mechanics.