Is gravity present at the quantum levels ?

[apolloman]

Gravity isn't yet part of the quantum scenario, as far as I'm aware.

Considering gravity in GR is not a force but is a warping of spacetime, could it be possible for the warping at quantum levels to be so small that it becomes "negligable" ? I.e. at quantum levels, spacetime is never warped, but is always flat.

An inverse and rather crude analogy might be scaling up of rough surfaces - for example, a billiard ball is smooth to our touch but put under a microscope is rather rough. I'm thinking along the apposite line for spacetime; rough (warped) by large scales, but flat at the quantum scale.

[swampyankee]

I suspect that a definitive answer to your question could get somebody a Nobel Prize. Since I've got roughly as much chance of winning the Nobel Prize in Physics as I have of watching the sun rise in the west, about all I can say is that the behavior of gravity at the very smallest scale is one of the driving forces behind string theory (which is still a hypothesis, but I digress) and the various forms of M-theory.

[apolloman]

I'm aware of string theory and M-theory but don't recall gravity as being a driving force for either of them. I know that strong theory is an attempt at a GUT theory which attempts to incorporate gravity etc but could you kindly elaborate further ?

[swampyankee]

You're probably as aware of string theory and M-theory as I am, but I do recall that unifying gravity (from GR) and the other forces (from QM) is one of the driving forces behind the concept. If I understand what you're asking correctly, you're suggesting that at some very small distance, gravity no longer exists, since if it exists it would warp space. I've got no more problem with that than with a bunch of dimensions rolled up too small to see.

[TobiasTheViking]

I'm aware of string theory and M-theory but don't recall gravity as being a driving force for either of them. I know that strong theory is an attempt at a GUT theory which attempts to incorporate gravity etc but could you kindly elaborate further ?

Gravity isn't as such a driving force in either of them. Or rather, it wasn't to begin with.

String theory was, for a long time, a theory for the three other forces (Electro magnetism, the Strong Force and the Weak Force). But the theory had a lot of inconsistencies.

One of the inconsistencies was a particle predicted by the theory that didn't fit (it was massless and not a photon), but after a lot of research they realized that if the string in string theory was a lot smaller than initially assumed, the massless particle would fit with a Graviton. It wasn't till this happened that String Theory(later m-theory) became an attempt at a GUT.

As for gravity being present at the quantum level.

A proper answer would, indeed, result in a Nobel Prize. But i can tell you what the assumptions drawn from string theory(and other theories) are.

As we go back in time, in simulations both on computers and in the various colliders around the world, the electromagnetic force(originally thought of as two forces) and the weak force unite.

As the distance between the particles(We are talking smaller than an atom here) decrease, three of the forces converge in strength. First the weak force's strength increases, and becomes indistinguishable from the electromagnetic force.

You can't tell the forces apart.

Theories also predict that the Strong Force will merge with the ElectroWeak force once the temperature is hot enough. This has not been confirmed in colliders yet.

Do note that by higher temperature you should really think of the particles getting closer together. After the Big Bang the temperature was extremely hot(everything was packed extremely densely), and thus it is assumed that right after the big bang the protons and neutrons were so close together that the three forces were the same force. But as the universe expanded the particles got further apart, and the forces stopped being equal in strength(and in other aspects).

I think everybody expects the unification of the Strong Force to be real, but there is no confirmation yet.

Now, the predictions on Gravity does NOT converge with the ElectroWeak or the Strong Force, though the strength does increase as the distance decrease. But with M-Theory gravity DOES converge with the other forces.

If String Theory/M-Theory is true, then all the four forces are probably only one force at small enough distances(or when it was hot enough, ie, after the big bang).

You have to decide how much you want to rely on String Theory on this issue.

I don't know what Loop Quantum Gravity says on the issue. If someone does i would love to hear.

Note: Gravity is so weak on the quantum level that it can usually be ignored. Only in extreme cases(black holes, big bang) does the gravity increase so much that it must be accounted for in quantum calculations. But sadly no theory has yet been devised that can incorporate gravity at the quantum level. M-Theory might be true, but right now it is just conjecture. This doesn't mean that gravity isn't present on the quantum level, just that no one knows how to account for it as of yet.

[TobiasTheViking]

Gravity isn't yet part of the quantum scenario, as far as I'm aware.

Considering gravity in GR is not a force but is a warping of spacetime, could it be possible for the warping at quantum levels to be so small that it becomes "negligable" ? I.e. at quantum levels, spacetime is never warped, but is always flat.

An inverse and rather crude analogy might be scaling up of rough surfaces - for example, a billiard ball is smooth to our touch but put under a microscope is rather rough. I'm thinking along the apposite line for spacetime; rough (warped) by large scales, but flat at the quantum scale.

Something you should note is that so far the three forces other than gravity all have Messenger Particles. That is, the force given by ElectromMagentism, the Weak Force, and the Strong Force, can all be explained and calculated as sending particles back and forth.

When two magnets attract each other they do it by the magnetic field they are submerged in(like a gravity field, but with polarization, and other obvious differences), but you can also think of them attracting each other by sending photons back and forth. One magnet sends a "come here" photon to the other magnet, which moves closer. And the other magnet also sends its own messenger particles, of course.

A messenger particle for gravity hasn't been detected yet, but it would be a weird universe if one of the forces doesn't have a messenger particle and the others do.

I am pretty sure(though i would love confirmation on this) that without a messenger particle, gravity can't be united with any quantum field theory at all.

If you think of gravity as messenger particles instead of as a warping in space(or a gravity field), could space still be too flat? If one quantum of gravity(one messenger particle) can be send, then that would mean that space would be warped at least a tiny amount.

[Shaula]

One of the reasons that String theorists say String theory is on the right track is that it predicted a spin-2 boson in addition to the known spin-1 bosons. Gravity must be mediated by such a particle, if it is mediated by a particle and not something fundamentally different from the other forces, (it is a second rank tensor) and when this first popped out of the equations people got very excited.

[Ken G]

What is being said above is all true, and all very much the way these frontier topics are currently framed. Personally, I'm wary of what I call reverse logic-- the idea that if we have a theory that works great when you have 10²⁶ protons getting together to have a gravitational effect, then it will also work for 1 proton. The reason I see that as reverse logic is that the logic of science is to let the universe tell us what it is doing, not to tell the universe. Of course, we do try to unify what the universe is telling us, and we could not have something that 1 proton is doing be inconsistent with that same thing happening 10²⁶ times. But the reasoning cannot be applied safely in the opposite direction, from the 10²⁶ down to the 1. There are times when we seem to get out "ahead" of the universe (such as the unification of the three forces discussed above), but never in the complete absence of experimental data.

I would not be at all surprised if the gravity of 1 proton, if we ever get a handle on it at all, bears little resemblance to the gravity of 10²⁶ of them, just as the trajectory of 1 electron bears little resemblance to the trajectory of 10²⁶ electrons moving together. Nevertheless, one can presume that the gravity of large numbers of particles emerges from what it does for each particle-- but unfortunately we have little experimental access to the latter. Would we have ever come up with quantum mechanics if we could not do experiments on quantum systems? How close to particles need to get before the astonishingly weak force of gravity rises to the level of the other forces and affects the experimental outcomes?

Maybe it's just true that we will be forever relegated to treating gravity as a macroscopic phenomenon, and all the theories we have for it are going to apply to macro objects, not particles. If we ever do find things like particle-mass black holes, and there is some way to tell that, or situations where the force of gravity rises to the level of the other forces it is normally so much weaker than, then we should not be surprised if at that scale all the forces turn out to obey very different physics than either GR or quantum mechanics. Maybe they will, and maybe they won't, and maybe we'll just never know-- that's the great thing about science, it deals in mystery. We can do out best to unify, and our best to verify, but there's no guarantee we will be able to demonstrate that we got it right. Maybe we really are getting out ahead of nature and anticipating her beauty, or maybe the only thing we are getting ahead of is ourselves.

[DrRocket]

Gravity isn't yet part of the quantum scenario, as far as I'm aware.

Considering gravity in GR is not a force but is a warping of spacetime, could it be possible for the warping at quantum levels to be so small that it becomes "negligable" ? I.e. at quantum levels, spacetime is never warped, but is always flat.

An inverse and rather crude analogy might be scaling up of rough surfaces - for example, a billiard ball is smooth to our touch but put under a microscope is rather rough. I'm thinking along the apposite line for spacetime; rough (warped) by large scales, but flat at the quantum scale.

Nobody really understands the role of gravity at very small distance scales. What is really needed is a unification of general relativity and quantum field theories -- a so-called theory of everything. But no such theory currently exists, although there is a great deal of research effort being expended towards the eventual formulation of such a theory.

However, it is thought in many circles that gravity may become more important at extremely small (and extremelysmall here means at something like Planck distances) distance scales. This is because gravity a universally attractive force with basically an inverse square law. Where as the strong and weak forces in the nucleus are not of such a form.

If you can formulate a theory that successfully iintegrates both gravitational and quantum phenomena in a mathematicall consistent package and that accurately predicts what is observed, you will probably get an all-expense paid trip to Stockholm.