Cause of Gravity? - Gravitons or Curvature of SpaceTime?

[Cosmobrain]

Gravitons are today theoretical things and we should discuss them as such...

 

Gravitions are really objects in effective quantum general relativity, that is the closest thing to a quantum theory of gravity we have right now. The idea is, following standard methods of quantum field theory, to linearise the full theory and look at small fluctuations about some fixed configuration understood as the "vacuum" or "background". The classical part works fine here for GR, you construct gravitational waves. Then you apply standard path integral methods to these fluctuations and you see that the resulting theory does not quite work. This approach is the perturbative approach.

 

Technically the theory you end up with is full of infinities and you can't remove these using known methods of quantum field theory - the theory is non-renormalisable.

 

However, one can treat it as an effective theory and not a fundamental one. These infinities do not appear at tree level or one-loop level, so we can calculate scattering amplitudes for quantum general relativity to low orders with no real problems. We can discuss gravitons in this context perfectly consistently, remembering we have an effective theory.

 

So, what about the full theory? There is some evidence that quantum general relativity, or something close to it is asymptotically safe. Without any technical details, this means that the theory may be well defined, but not as a perturbative theory. That is we cannot understand the theory properly in terms of gravitons. It is quite possible that the full quantum theory of gravity is not a theory of gravitons at all!

When you think about it, pretty much everything has gravity. Even light. That means that photons of light can emit gravitons? Without changing its momentum? that doesn't make sense to me

[ajb]

When you think about it, pretty much everything has gravity. Even light. That means that photons of light can emit gravitons?

Energy-momentum is the source of gravity and as such photons can act as sources.

 

Without changing its momentum? that doesn't make sense to me

If you are thinking about some scattering like situation then why would there be no change on momentum?

[MigL]

As AJB has explaned gravitons are a feature of quantum gravity fields.

 

GR on the other hand is NOT a quantum , or any kind of, field theory.

It is a geometric theory of gravity, Unity+.

[I-try]

MIgel.

 

Referring to the bottom line of your post directly above that states GR "is a geometric theory of gravity". Would it be more accurate to state that GR provides a geometric substitute for the concept gravity or gravitation by declaring such concepts as redundant and only appear to exist in reality because matter is compelled to follow geodetic pathways formed by matter's curvature of what is referred to as space-time?

[davidivad]

MIgel.

 

Referring to the bottom line of your post directly above that states GR "is a geometric theory of gravity". Would it be more accurate to state that GR provides a geometric substitute for the concept gravity or gravitation by declaring such concepts as redundant and only appear to exist in reality because matter is compelled to follow geodetic pathways formed by matter's curvature of what is referred to as space-time?

 

some strange things happen when you go from a flat surface to a curved one. consider the reiman curvature tensor.

[ajb]

GR on the other hand is NOT a quantum , or any kind of, field theory.

I would still say that GR is a field theory, just it is different from the others like electrodynamics. The biggest obvious difference is that the theory is not on a fixed background space-time and the gravitational field is essentially the local geometry.

[MigL]

So the geometry is a field ? Now that would be semantics.

 

Wasn't one of the reasons for GR the elimination of "action at a distance" so abhorred by Einstein ?

And to get to a quantum gravity field theory you'd have to quantise the geometry ?

[ajb]

So the geometry is a field ? Now that would be semantics.

Yes, you have a smooth manifold and the geometry, that is the "local shape" is given by tensor or tensor-like objects on that manifold. We can think in terms of a metric, or more generally a connection. (Or other geometric objects that encode the same information)

 

 

 

And to get to a quantum gravity field theory you'd have to quantise the geometry ?

This is something that is not discussed very clearly in the literature in my opinion. Either we really do quantise the underlying manifold that is replace it with some more general notion of a space, or we can quantise the geometry on the manifold. I am not sure how distinct these two are.

[MigL]

What about the other point I brought up about action at a distance?

 

For the classical field model of gravity |( Newtonian ? ), one test mass has to transfer information to another test mass so as to draw it nearer, i.e. gravitate. One can make the argument that sometimes it has to transfer the information superluminally when great distances are involved.

 

For the GR ( also classical ) geometric model, the test mass transfers information to the space-time manifold and 'curves' it beforehand. The second test mass then receives the already existing information from the space-time manifold and adjusts its path accordingly.

 

A subtle difference but certainly non trivial.

[ajb]

What about the other point I brought up about action at a distance?

We can calculate the speed of gravity as the propagation speed of a gravitational wave. Here we think of a gravitational wave as the small change in space-time due to a small change in the source. The speed of these waves is c. Thus we do not have an instantaneous action at a distance like in Newtonian gravity.

For the GR ( also classical ) geometric model, the test mass transfers information to the space-time manifold and 'curves' it beforehand. The second test mass then receives the already existing information from the space-time manifold and adjusts its path accordingly.

This relies on the 4-d "block" space-time picture, in which everything in all time is set. In the 3+1 picture it is not so clear that this interpretation holds.

[MigL]

Thanks a lot AJB, you crushed all my arguments.

 

But seriously, my personal preference is to think of GR as a geometric model as I-try also explained in post #29.

I realise one could consider the geometry of space-time a field, but think that would be stretching it. Just my opinion since I don't have to do the math like you do.

[davidivad]

my prediction of the future.

science has reduced itself to "whack a mole."

praise god...

burn everything down in his name!

[A.T.]

What is accepted physics? That there are no gravity acting upon us that holds us to Earth? I don't see where you are getting this conclusion because gravity is a force by accepted physics. I do have an understanding of General Relativity.

 

I think his point is that in General Relativity gravity is not an interaction force (real force) like in Newtons theory, but an inertial force (fictitious force).

http://en.wikipedia.org/wiki/Fictitious_force

 

What is "holding us down" is our inertia, or the tendency to follow a geodesic path in space-time. It is the same thing that holds an astronaut against the rear wall of an accelerating rocket:

http://en.wikipedia.org/wiki/Equivalence_principle

 

One can describe those effects in non-inertial frames by inertial forces, or distorted coordinates like shown in the clip below. Note that in Einstein's model there is no "force of gravity" because this representation uses only (real) interaction forces :

 

[ajb]

But seriously, my personal preference is to think of GR as a geometric model as I-try also explained in post #29.

GR is certainly a geometric theory, but then so are classical gauge theories like electromagnetism, when you formulate it properly.

[MigL]

I agree AJB.

But I consider geometry a dimensional arrangement, of the three spatial plus time for GR.

We can consider the classic EM field. Being conserved and long range means we can assign a value to this field at every point in space-time such as V so that every point is described by (XYZTV). GR then gives us a new metric and curvature as per Kaluza-Klein theory. We note however that to get to QED, it is not the geometry of the EM field which is quantised but the actual field

Loop Quantum Gravity attempts the same thing with GR. It does not quantise the geometry, rather it removes space-time from its background position and moves it foreward ( no background ) as the equivalent to the field and then quantises it ( with certain nuances ). In effect space-time is the field.

[ajb]

We note however that to get to QED, it is not the geometry of the EM field which is quantised but the actual field.

No, but is some sense we are quantising the geometry of a U(1) principal bundle if we think of connections as describing the geometry.

 

Loop Quantum Gravity attempts the same thing with GR. It does not quantise the geometry, rather it removes space-time from its background position and moves it foreward ( no background ) as the equivalent to the field and then quantises it ( with certain nuances ). In effect space-time is the field.

I think we still have a differentiable manifold behind all this and quantise the geometry in terms of the Ashtekar variables. Being background independent usually means that it does not depend on any fixed geometry, but have only a smooth manifold structure. In quantum gravity we should also allow for topological changes in this manifold, but I am not sure how much that is incorporated in loop quantum gravity. I am not an expert in this field.

[michel123456]

I remember Swansont having stated somewhere that "the curvature of spacetime is not the cause of gravity, the curvature of spacetime is gravity".

Hoping to remember correctly.

[Cosmobrain]

The Graviton is just a frustrated attempt to include gravity in the Standard Model

[MigL]

As always AJB, thanks for your insight and an interesting discussion.

[hypervalent_iodine]

!

Moderator Note

idontknowwhyijustknow,

 

Please stop derailing this thread. I have split all of your posts here into a new thread, found here. Confine your posts to there.

[Bird11dog]

Is it possible that gravity might be an artifact of time dilation?

[swansont]

Is it possible that gravity might be an artifact of time dilation?

Time dilation can be from motion, without gravity.

 

Even if you restrict this to gravitational, the dilation depends on the potential, not simply on g.

[Bird11dog]

Of coarse your right. Motion does cause time dilation but I am not referring to why time dilation occurs I am referring to the possibility that time dilation is the mechanism by which we see not only gravity but also inertia. I will try to give a metaphor experiment if you will that describes this mechanism.

Suppose we had a kiddie train track 100 m long. We place a student school desk that has track wheels attached to it on the track. We attach a digital timer to the desk and of course a drive mechanism that can maintain a constant velocity. On the track we place two switches that are 30 m apart. The first turns the digital timer on and the second turns it off. We put a student in the desk and tell him how far apart the switches are and the total mass of his system. We ask the student to calculate his momentum at the end of the 30 m run. The desk will have a constant velocity of 2 m/s. At the end of the run he gives us a value for his momentum. We move the desk back to the starting point in order to do another run. Unbeknownst to the student I can remotely control the timer. I reduce the rate of the timer by 10% and start the run again. At the end of the run we ask the student which run had the greatest momentum and of course he says the second.Isn't this situation somewhat like a rock sitting on the surface of the earth. The atoms of the rock are vibrating around and colliding with other atoms fields and the atoms clocks are speeding up and slowing down ever so slightly so that the collective momentum of the atoms points to the center of the earth?

[jajrussel]

What I mean is that a free falling object, i.e. in a gravitational field, such as Timo's cup, has no forces acting on it according to GR. The ground, or a table, stops this free fall by exerting a force on the cup. That's not semantics.

I am just reading this thread today, so my question is a little late in the day.

 

My understanding is that the cup resting on the table is in equilibrium. Wouldn't this mean that no force is acting on the cup? Isn't it an imbalance of force that causes a cup to fall?

[Mordred]

the table exerts an equal force upon the cup as the downward force so the net force is zero.