# What's gravity from the Quantum point of view

1. Aug 27, 2006

### lokofer

What's gravity from the "Quantum" point of view..

-Newton gave the expression of "Gravity" as a force so $$F= \alpha \frac{1}{r^2}$$

- Einstein believed that gravity was due to "bending of space" so we had the Geodesic equation $$\nabla _u u =0$$ and Riemann Tensor $$R_ \mu\nu =0$$

- But was is Gravity from the "Quantum mechanics" point of view?..:grumpy: if in QM there are no forces, and there's no Space-time then what produces gravity at an atomic level?..thanks.

2. Aug 27, 2006

### blackwizard

Im just a noob but from my understanding there is no gravity in QM, the theory just ignores it, and thats been the biggest problem in physics for ages.

Im confused about u sayin there arnt any forces or spacetime in QM. I think your wrong on that point (but then again what do i no!).

Also, from my understanding, GR and QM cudnt b combined because the spacetime distortiones in GM is like all other force fields. So when put in2 QM the are subject to Quantum Uncertainty which causes Quantum jitters.
The problem was that GR equations say spacetime is smooth and QM says its jittery. Apparantly M-theory is uniting them.

Id appreciate been let no what iv got right and worng. My knowledge is based on 1 book

3. Aug 27, 2006

### ZapperZ

Staff Emeritus
The issue here isn't that there's no gravity in QM. If you look at the Schrodinger equation, for instance, there is a "V" term for the potential energy. That is the potential energy of ANYTHING, be it electromagnetic, or even gravity.

If you put gravity in there, under an appropriate boundary conditions, you CAN get "quantized drop". This is what has been done and reported in experiments done on neutrons. So yes, QM can incorportate gravational potential.

The issue that is relevant here is the quantum field theory description of gravity. In quantum field theory, the classical field doesn't exist. What you have is that in any interaction (electromagnetic, strong, weak), there is an exchange of virtual particles that are essentially the "force carrier". In the Standard Model, the exchange particle is hypothesized to be the "graviton".

Incorporating this into the scheme of quantum field theory is what is causing a lot of problems so far. This is why there are all of these difference routes of trying to get there, even a few that are approaching this in a completely different way.

Zz.

4. Aug 29, 2006

### lokofer

- But in GR there is NO "potential" or "Kinetic part" inside the Lagrangian..only matter and Geommetry..the "potential" description of gravity is just a "Newtonian" concept, that's the problem.

- The approach i like most (however is not exact or even useful? ) is take the Hyper-surface and split them into discrete coordinates $$q_i (t) , i=1,2,3,4,5,....,N \rightarrow \infty$$ then Einstein Lagrangian would become $$L= \sqrt (-g) R \rightarrow L(q_{i} (t) , \dot q_{i},t)$$ and from this "finite" Lagrangian you could recover the Hamiltonian and hence the "Energies" of the Quantum system.. to simplify even more you could impose that the "q's" interact via an Harmonic potential plus several NOn-harmonic corrections , the curvature (classical) if the q's are the angles is given by (Gauss-Bonnet Theorem) $$\pi - q_1 (t) -q_2 (t) -q_3 (t) =K(t)dA$$ where dA is the Area of every infinitesimal triangle.

5. Aug 29, 2006

### ZapperZ

Staff Emeritus
So? You ASKED for "gravity" from the "... Quantum mechanics point of view...", did you not? So I asked, what is the problem with using the classical newtonian gravitational potential in the potential term in the Schrodinger Equation? And what's the problem with the neutron experiment? You WERE aware of that experiment, weren't you?

For some odd reason, you're now changing your topic midway and now wanting the "GR" point of view of having no potential. Make up your mind, why don't you?

Zz.

6. Aug 29, 2006

### RandallB

?QM assumes there are no forces - - Wasn’t the Standard Model built primarily with from atomic observations interpreted with the math of QM?
I doubt the Standard Model could have been built without QM; And there is no rule that QM cannot use SR.
Standard Model defines forces are as particle exchanges, (quantum particles or field equivalent). Virtual photons, W’s, Z’s, Gluons w/ ‘color’ account for forces (EM Weak Strong) between a zoo of particles defined by the standard Model . True without using gravity, But if figure out how to account for gravity they have the name “graviton” set aside ready for it, plus Higgs (particle and/or field).
The graviton makes the most obvious difference between GR and QM. Because if you agree gravity is based on graviton particles (w/ Higgs?) what is the point of GR warped space. That is why most see the two theories as incompatibly separated to their separate domains of the extreme small vs. large. Reconciling them by figuring out gravity completely would seem to require at least one of the two theories to be wrong.

So what is a graviton, that answer waits for the next Newton/Einstein.

7. Aug 29, 2006

### pervect

Staff Emeritus
8. Aug 29, 2006

### WhyIsItSo

Pardon my ignorant input here, but I'm fascinated by this thread. Most of the terms are double-dutch to me, but in GM terms, I feel comfortable with my concept of gravity. Forgive my audacity for putting my thoughts forward in such esteemed company, but...

In reasoning about gravity, I considered the centrifuge. We "use" centrifugal force as a convenience, yet it doesn't exist. At first logic dictates it must be there, until we understand that the balancing force to centripetal comes from a tangential source (pun intended).

In a similar way, thinking of gravity as a force is convenient, yet GM suggests (to me in my limited grasp of it) that gravity is not a force at all.

Trying to reason this through, I note that gravity is the only force I know about that I cannot feel unless some force opposes it, and then I only feel it to the magnitutude of the opposing force. When I stand on the ground, I feel the force of it pressing against my feet. But maybe assuming there is a force pushing down on me is like believing in centrifugal force? Perhaps we should say "For every force there is an equal and opposite force or effect".

If I am travelling through space, approaching the Earth but not on a direct collision course, I can imagine I'll be drawn towards the planet. Yet I feel no acceleration. The comfortable notion is that gravity acts as a force on all parts of my body simultaneously, but if it isn't a force? What if I actually experience no acceleration at all; indeed, I would feel none.

Hence, the space-time bending. In truth, I experience no acceleration because none exists. My path is actually a straight line. I explain the contradictory evidence of my eyes by assuming the photons reaching me are not nearly as affected by space-time bending, hence their "straight path" is different than mine. I should imagine that I too am bending space-time since I have a mass, and I wonder how this interacts with the bending created by Earth's mass.

Is my "baby grasp" more or less on track so far?

I understand photons have no mass, but display some behavior of particles. Is this somehow involved in explaining why what I see does not match reality? Light bends a little by gravity, doesn't it? The is the reason a black hole is black, yes?

Ah, is that it? Velocity must have some part to play on the equation of space-time bending, and what will constitute the "straight path" for any given object.

I hope someone more knowledgeable than me can pick up what I'm thinking and point me in the right direction...

9. Aug 29, 2006

### WhyIsItSo

Uh oh! I just found:

Can someone explain that to me in terms of my previous post (ie, very simply )

Last edited by a moderator: Apr 22, 2017
10. Aug 31, 2006

### Chronos

Einstein believed gravity is the background [i.e., the 'aether'] which all other fields are embedded upon, and emergent from. If this idea is correct, it makes sense that gravity cannot be quantized using the methods employed by quantum field theory - at least in my mind. It again however appears relativity gets very weird on atomic/subatomic scales. It appears a greate deal of spooky action at a distance goes on down there. A reasonable compromise seems to be that communication [lightspeed] is instantaneous over sufficiently short distances. Defining what determines the limiting distance appears to be the hard part.

Last edited: Aug 31, 2006
11. Aug 31, 2006

### pervect

Staff Emeritus
Yikes! This is a rather speculative post, to say the least. Note that we already know how to quantize gravity at low energies, that comparing the background to the outdated aether idea is rather misleading, and while you may have some personal speculative theory that communication is instantaneous over short distances, if you want to post it to PF you should have a published paper in mind (not just your own personal idea), esp. being a science advisor who is supposed to help set high standards.

12. Aug 31, 2006

### pervect

Staff Emeritus
You are pretty much on the right track. You might want to take a look at Baez's paper on GR

http://math.ucr.edu/home/baez/einstein/

As far as curvature goes, you might want to remember that it's space-time that's curved, not just space.

You can picture geodesic deviation and curvature by drawing curves in space, but you have to remember that one of the spatial axes represents time. If you draw two lines on a curved surface, and note how they start to diverge, if you also remember that one of the directions on your curved surface represents time, you might get the idea. For keywords, look up geodesic deviation.

13. Aug 31, 2006

### RandallB

To put Chronos comments in perspective it is important to understand that the GR “Aether” is not comparable to the outdated luminous ether of Maxwell’s time. (I separate them as ether and Aether).
His is simply filling a need for GR to have something in space to curve without defining something that can move – at least not that we can see in our 3D’s. At the same time supporting the idea that ‘empty space’ is not so empty – there is something thing there to support Maxwell’s light propagation as well as the warping needed for gravity.

Plus I don’t think GR is as simplistic as:
Space-time may have curves but that is not the stuff of GR. GR uses at least four dimensions and one of them does not represent “time”.
Plus no one of them is comparable to a spatial dimension we know.
It’s these four dimensions that curve and warp (I not sure it matters if the is ‘time’ with that 4D or if it is Newtonian absolute or not); and the result of that is our 3 dimensions with gravity and non-absolute time extending from that 4D background.
From this perspective the fact that we can “already quantize gravity at low energies” is no more complete than Newton, and IMO still leaves QM and GR quite incompatible with each other.

Sorry WhyIsItSo I don’t see how GR can be explained “very simply”.

Last edited: Aug 31, 2006
14. Sep 1, 2006

### WhyIsItSo

"Simply" is a subjective term. In pervect's defense, he was attempting to "pitch his explanation" at a level I would understand.

If that understanding is inaccurate... well, one has to start somewhere, yes?

15. Sep 1, 2006

### Chronos

RandallB has captured the essence of the difference between ether and aether. In the Leiden address, Einstein attempted to illustrate the difference. He did not entirely succeed. Einstein's ether is squishy [e.g., the equivalence principle], whereas Maxwell's aether is rigid.

16. Sep 3, 2006

### JustinLevy

I also heard of this approach used for ions above a metal surface, and they experimentally verified the "average distance" from the surface as dictated by the different eigenfunctions (infinite potential on one side and a constant slope on the other, which I think gives Airy function solutions).

17. Dec 29, 2006

### MeJennifer

Sorry but this triggers my curiosity.
Could you explain exactly what you mean here?

Last edited: Dec 29, 2006