## Gravity, Gravitons, and Geodesics.

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

 PhysOrg.com physics news on PhysOrg.com >> Promising doped zirconia>> New X-ray method shows how frog embryos could help thwart disease>> Bringing life into focus
 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

Recognitions:
Gold Member
Staff Emeritus
 Quote by 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?
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

## Gravity, Gravitons, and Geodesics.

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

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

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

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

 Quote by sol2 Will comologically realization in the quantum world morphisize to include relaitivity?
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.

 Quote by 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.
What geometry are you using?

 Can the graviton live below Planck length? Because we have reached a limit does not remove the nature of the gravitational forces. Supergravity has made its appearance for us in this dynamical relation?

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

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

 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.

 Quote by 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.
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.

 Quote by Glenn If I am understanding what I am reading correctly, gravity the result of the shape of space-time?
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
 It will be seen from these reflections that in pursuing the general theory of relativity we shall be led to a theory of gravitation, since we are able to "produce" a gravitational field merely by changing the system of co-ordinates.
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
 Question You keep talking about “curvature” of spacetime. What is curvature? AnswerThe word curvature is an analogy, a visual way of extending ideas about three dimensional space to the four dimensions of spacetime. Travelers detect curvature—in both three and four dimensions—by the gradual increase or decrease of the “distance” between “straight lines” that are initially parallel. In three space dimensions, the actual paths in space converge or diverge. Think of two travelers who start near one another at the equator of Earth and march “straight north.” Neither traveler deviates to the right or to the left, yet as they continue northward they discover that the distance between them decreases, finally reaching zero as they arrive at the north pole. They can use this deviation to describe the curved spherical surface on which they travel. Similarly, in four-dimensional spacetime, travelers detect the deviation from parallelism of nearby worldlines of free particles, each of which follows an ideally straight spacetime path, often called a geodesic. This curvature can be measured by the travelers and varies from place to place in spacetime.
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: ...
 Where does the graviton come into play?
It plays a role in GR similar to the role in EM. It facilitates the interaction.
 Quote by homology A force causes an acceleration, and the curvature of space-time also causes acceleration.
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.

 Quote by selfAdjoint The graviton is a theoretical particle predicted by string theory but not yet demonstrated to exist.
It doesn't come from string theory. It comes from quantum gravity/quantum field theory.
 If it exists and is the cause of gravity it will do away with spacetime curvature ...
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.
 Quote by Nenad yes, if the graviton is found, General Relativity will be wrong. .... 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.
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.
 Quote by ArmoSkater87 Couldnt you say the graviton cause the curvature of spacetime...lol
Absolutely. That is 100% correct.
 Quote by 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.
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

 Quote by Nenad gravity isnt really a force.
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

Blog Entries: 47
Recognitions:
Gold Member
Homework Help
Quote by pmb_phy
 Quote by Nenad gravity isnt really a force.
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
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.)
 From Wald, p 67: "The basic framework of the theory of general relativity arises from considering the opposite possibility: that we cannot in principle--even by complicated procedures-- construct inertial observers in the sense of special relativity and measure the gravitational force. This is accomplished by the following bold hypothesis: The spacetime metric is not flat, as was assumed in special relativity. The world lines of freely falling bodies in a gravitational field are simply the geodesics of the (curved) spacetime metric. In this way, the "background observers" (geodesics of the space-time metric) automatically coincide with what was previously viewed as motion in a gravitational force field. As a result we have no meaningful way of describing gravity as a force field; rather, we are forced to view gravity as an aspect of spacetime structure. Although absolute gravitational force has no meaning, the relative gravitational force (i.e., tidal force) between two nearby points still has meaning and can be measured by observing the relative acceleration of two freely falling bodies. This relative acceleration is directly related to the curvature of spacetime by the geodesic deviation equation (3.3.18). How does this viewpoint of general relativity that there is no such thing as gravitational force square with the well known "fact" that there is a gravitational force field at the surface of the Earth of 980 cm s-2. --[snip]-- We could use the time translation symmetry of this example to define a preferred set of background observers. We then could define the gravitational force field of the Earth to be minus the acceleration a body must undergo in order to remain stationary. Thus, in this case a well defined meaning can be assigned to gravity as a force field. However, in the absence of time translation symmetry---e.g., in a case where there are several massive bodies in relative motion---there exists no natural set of curves whose comparison with geodesics could be used to define gravitational force."

[QUOTE=robphy]Let's not be hasty here.
In general relativity, gravity is NOT a force [in the general case].
Sure it is.
 From Wald, p 67: (I boldfaced the key statements.)
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

 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.