# What is the problem with gravity?

1. Mar 20, 2012

### eiyaz

For the longest time educators had repeatedly taught me of the problem with gravity. We have four fundamental forces electromagnetism, strong interaction, weak interaction, and gravitation.

We are able to combine three of these forces but not gravity hence the ongoing development of quantum gravity.

Yet according to general relativity, gravity is NOT a force, but a fictitious force no different from centrifugal force according to wikipedia.

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

If gravity is not real and is not mediated by a particle what is their to combine? Do we have any evidence that gravity is "real" and is mediated by a particle? If not what is the problem?

What am I missing here?

2. Mar 20, 2012

### mathman

3. Mar 20, 2012

### eiyaz

Not only does that not address the question (I've read that article already) it is also extremely condescending to link wikipedia without quoting the paragraph which answers my question. If you don't understand the question you do not have to be a troll about it.

My questions is if gravity is a fictitious force what issues arise at the quantum level? I may need a more basic explanation. Another issue is the fact that we are able to combine 3 fundamental interactions up to the moments after the big bang, but not gravity. Why do we need to combine gravity with the other three if gravity is not a real force?

In fact if someone can answer just this question:

Why do we need to combine gravity with the other three if gravity is not a real force?

That would be great. I am not able to find the answer on wikipedia so please quote the paragraph if you are linking wikipedia.

Last edited: Mar 20, 2012
4. Mar 20, 2012

### pervect

Staff Emeritus
We have indirect evidence of the existence of gravitational waves - though no direct evidence, yet.

Graitational waves are expected to carry energy, and our detection of them is via the energy loss they cause. (We don't see them direclty yet, but we see the drain in energy as the orbits of the binary pulsars decay, and the amount of decay is what we predict with GR).

So gravity may not be as "fictitious" as you seem to think it is from wikipedia, unless you think "fictitious" things can carry energy. In which case your defintion of "fictitous" is a bit different than mine :-).

5. Mar 20, 2012

### Mark M

Hi eiyaz,

During the 20th century, physicists had combined classical quantum with Special Relativity into quantum electrodynamics. In QED, particle interactions occur in a Minkowski Space, a flat spacetime with no gravitational effects.

This worked fine until it was time to combine general relativity with quantum field theory. In quantum field theory, particles interactions are analysed with a Feynman Diagram:

So, to analyze gravity, it had to be quantized - reduced to particles, gravitons. So the problem comes when you realize gravitons also have a gravitational field (due to their momentum/energy) , so essentially, "gravity gravitates".

So, you get a graviton curving spacetime, further producing gravitons to transmit gravity, then producing more gravitons, and so on.

This leads to a Feynman diagram with many loops, which results in an ultraviolet divergence, a mathematical error that is beyond repair.

6. Mar 20, 2012

### nitsuj

If I see what you're getting at eiyaz, I am with you.

Here is my confusion.

I misunderstand what a "force carrier" is, because I find it odd gravity could fall (pun) under the same term.

I prefer to imagine gravity as a property of space & not a "member" of the force mediating particles.

That's just the way measurements work I guess.

7. Mar 20, 2012

### marcus

Perhaps it is best to think of gravitation as the dynamic geometry of the world. The ability of geometry to interact with matter. "Matter tells geometry how to curve, geometry tells matter how to flow."

So UNIFICATION (of forces) is the wrong idea--the wrong goal for a research program--in that case.

But unification of GR and QM does not need to mean a "unification of forces" as force. It can simply mean to formulate geometry as a quantum field theory---to attain a quantum geometry that is not based on any prior fixed classical geometry. A fully dynamical quantum theory of geometry (interacting with matter) ON WHICH matter fields can be defined.

This is pretty much what the quantum relativists who are working out various approaches to QG are doing.

Yes, but the "ongoing development" you mention is not a unification-of-forces program in any naive sense. One can artificially require LQG to cough up gravitons by pinning it down to approximately flat geometry, but the graviton is not fundamental to the theory. It is formulated at the fundamental level without such mathematical devices.

There is no contradiction. Quantum relativists work on LQG, which does not assume gravity is fundamentally mediated by a particle defined on a fixed prior geometry. They are working out the quantum geometry of the space upon which the matter fields are defined.

"If gravity is not real...?" Well what is real is geometry. And since gravity=geometry, therefore gravity is real. We have to understand space and time. The final question quoted seems vacuous to me. No need to ask it. We don't have to worry about the naive conception of gravity, being mediated, being combined... etc.

Last edited: Mar 20, 2012
8. Mar 20, 2012

### Mark M

Marcus,

If you believe that gravity can be described purely geometrically, then how do you believe Quantum Field Theory works in high energies in small enough distances? For example, a field with sufficiently high energy will contribute to $$T\mu \nu$$ and curve spacetime by
$$G\mu \nu =8\pi GT\mu \nu$$
Since elementary particle interactions are shown in Feynman diagrams, does this not require a quantized gravity particle to properly calculate? (Sorry if LQG provides a better way of doing this, I'm not to familiar with it beyond the basics.)

Also, the semi-classical Einstein equations
$$G=\frac{8\pi G}{c^{4}}\left \langle T\mu \nu \right \rangle\psi$$
imply egienstates of the matter fields in question. This would require an egienstate of the gravitational field, which I'm not exactly sure sure can be done without quantization.

Also, there is the issue that everything else in the universe in quantized. Why not gravity?

9. Mar 21, 2012

### eiyaz

Thanks for great response it cleared somethings up for me!!

My definition of fictitious is any force not directly mediated by a particle. Does the energy have to come from a particle? Could mass causing curvatures in space time result in the wave?

Is this definition a result of a mathematical contingency that forms when attempting to quantized gravity? Or does it suggest that the graviton is in fact "real"?

Isn't this what string theory has been trying to do for the last 30 years?

Also I follow Peter Woit's blog Not Even Wrong, who hates string theory. He states that LQG is not a ToE and in fact string theory is the only attempt at ToE (possibly failed attempt), but if gravity is not a fundamental force wouldn't LQG + SM be fairly complete? If three of the four fundamental forces have particles that mediate it (gluon, W' and Z' bosons, and photon), would it be wrong to say that gravitation is not a fundamental force and merely a property of space-time?

What is the general consensus among the physics community regarding gravity? Do most physicists believe that gravity is mediated by the massless graviton? Or do most believe in quantized GR with no particle?

Last edited: Mar 21, 2012
10. Mar 21, 2012

### Mark M

Yes, that's the idea. Like a boat making ripples in water, a large enough body (such as a supernova explosion) should produce ripples in spacetime.

It suggests the graviton would absolutely be a real particle - the messenger particle of gravity. It's essentially what "tells" matter to follow curvatures in spacetime, just as the photon communicates the electromagnetic force. It has not yet been detected, but is theorized to have no mass, no charge, and a spin of 2. In string theory, it also has the distinction of being represented by a closed string, rather than the open strings of everything else.

Yes, but String theory's unification comes as a bonus, it's original attraction was to explain the graviton.

11. Mar 21, 2012

### marcus

Thanks for responding, Mark. I rarely check stuff here because don't want to get spread thin. Enough to do with Beyond forum and Cosmo forum.

I think you followed what I was saying quite understandingly so you can anticipate how I would reply:

"Everything else in the universe in quantized. Why not geometry?"

Since gravity is geometry, your argument helps to confirm my suspicion that geometry has to be quantized in order to make further fundamental progress in understanding the interaction between geometry and matter.

About your equations, the whole QFT business is build on the assumption of flat space. Fortunately the gravity coupling is small! Gravity is weak. So this works very well. Flat space approximation only works when effect of particles on geometry can be ignored. Whole perturbative approach based on this.

As long there are only a few high energy particles the geometric backreaction can be ignored. Approximate flatness. But what about conditions slightly before and after the start of expansion? With extremely high energy density. That is where quantum geometry comes into its own. Removing the initial singularity and understanding what was going on then. Just my two cents. I just watch the current research on this and try to follow the most significant lines of investigation. Maybe we should take this over to Beyond forum or cosmology if we feel like pursuing it further.

Last edited: Mar 21, 2012
12. Mar 22, 2012

### Mark M

Thanks for responding!

Yes, I agree it will eventually turn out that spacetime is discrete. I guess this comes from my prejudice against infetisimal quantities, as they make no sense in reality. Considering no energy or matter can exist at a sub-planckian distance or quantity, it's rather pointless to suggest an infinite expanse as you get smaller and smaller. Further, the Uncertainty Principle makes it so that at such short distances, spacetime would become so dramatically curved it loses meaning, which essentially leaves you with discrete units. It seems far more logical to suppose spacetime is in fact quantized. As Mark Kac said:

"Be wise, discretize!"

Though, I still do not feel this can be used against the existence of the graviton. I feel strongly that it exists. For example, Maxwell formulated the equations of electromagnetism under the assumption that EM waves were continuous. Of course, this didn't have any effect on his equations, and they were extraordinarily accurate. But, after QFT, we know that a messenger particle, the photon, is needed to transmit the force, to "tell" matter when to respond to other EM fields.

I feel the same situation applies to spacetime - something must "tell" (for lack of a better word) matter how to follow curves in spacetime.

Sure, this discussion probably fits better in Beyond than in an SR/GR sub-thread.