# Unifying gravity with the other four forces

g.lemaitre
if gravity is not a force its just the extent to which space is warped by mass like a bowling ball on a rubber sheet, then is it remotely probable that gravity cannot be unified with the other four forces? we have strong evidence that gravitational waves exist, though I forget what that evidence is, but is it at all remotely probable that gravitational waves exist and gravity is not quantized?

if gravity is not a force its just the extent to which space is warped by mass like a bowling ball on a rubber sheet, then is it remotely probable that gravity cannot be unified with the other four forces? we have strong evidence that gravitational waves exist, though I forget what that evidence is, but is it at all remotely probable that gravitational waves exist and gravity is not quantized?

It depends in fact of what we are trying to unify. I mean: "Why are we so certain that gravity and the three other forces have the same nature?" The latter (the EM, weak and strong forces) are more or less “variations” around the initial Maxwell’s theory (like Debussy would have written around the initial work of Beethoven): despite of distortions, extensions and adds of the theory, this is the same background. These forces exist in presence of the natural gravitation field of the earth. We ask in fact our self if the gravitation field has some common point with EM fields. The discussion proposes one: gravitational waves because of the word (and of the concept behind it) “wave”. It argues that all what is “waving” is carrying energy, thus must be quantized. Consequently the gravitational waves should be quantized too. It sounds logic and, since each of us knows that earthquakes exist (and some of us have eventually survive one of them), we can imagine what an underground gravitational wave could “look like”. The reduction of the common point between EM and gravitational waves to a unique common point: the energy, and the start of a mental abstraction with it, represents an enormous intellectual risk. This way of doing may result in a mathematical construction without any connection with the reality. Intuitively, thinking about a gravitational wave, I would ask myself: what is a wave in general, what is waving? Observing the surface of the river or of the sea, I would immediately get the answer: a collection of molecules... Thinking about EM waves, I would suggest a collection of polarizations but thinking about gravitational waves... and this would lead me to a questioning about the nature of vacuum, space-time and so and ... Since the “Thirring Lense” effect has been more or less proven by the “Gravity Probe B” experiment, we may accept the idea that masses deform the rubber sheet or, as mentioned in the opening text: “we may have strong evidence that gravitational waves exist”. For me, the remaining question is: “What is the real effective nature of what we have called space-time (the rubber)”?

LBloom
To answer the original question, yes its possible gravity cannot be unified with the other forces, although I'd be very hesitant to say it can't be quantized (we don't have evidence for it but a 100 years of success with quantum theory point to quantization along with a host of theoretical thought experiments). The question you're asking is the most fundamental one in physics and no one has the answer.

Yes, classically gravity is described in the language of geometry. However there is another perspective and in fact a consistent formulation of a QFT for a spin two particle will lead to Einsteins equations in the classical limit. So yes the geometrical interpretation came first but then came the more quantum interpretation.

I would also like to state that Gauge theories can similiarly be described be geometrically (at least classically I think...) using some more complicated ideas (fiber bundles and connections thereon). Just because something is classically geometrical doesn't mean it can't be quantized. (QED and QCD are quantum gauge theories)

Finally its likely that spacetime as we know it is an emergent property, it only makes sense at large scales and not at the Planck scale. Don't let classical formulation be an obstacle for a quantum one!

ibysaiyan
if gravity is not a force its just the extent to which space is warped by mass like a bowling ball on a rubber sheet, then is it remotely probable that gravity cannot be unified with the other four forces? we have strong evidence that gravitational waves exist, though I forget what that evidence is, but is it at all remotely probable that gravitational waves exist and gravity is not quantized?

Interesting question posed indeed... I wonder through what medium would the gravitational waves permit or am I thinking this classically as opposed to some other idea.

I understand that in GR gravity is an intrinsic property of space-time (curvature).

P.S Goes to show just how much knowledge I lack.

chill_factor
To answer the original question, yes its possible gravity cannot be unified with the other forces, although I'd be very hesitant to say it can't be quantized (we don't have evidence for it but a 100 years of success with quantum theory point to quantization along with a host of theoretical thought experiments). The question you're asking is the most fundamental one in physics and no one has the answer.

Yes, classically gravity is described in the language of geometry. However there is another perspective and in fact a consistent formulation of a QFT for a spin two particle will lead to Einsteins equations in the classical limit. So yes the geometrical interpretation came first but then came the more quantum interpretation.

I would also like to state that Gauge theories can similiarly be described be geometrically (at least classically I think...) using some more complicated ideas (fiber bundles and connections thereon). Just because something is classically geometrical doesn't mean it can't be quantized. (QED and QCD are quantum gauge theories)

Finally its likely that spacetime as we know it is an emergent property, it only makes sense at large scales and not at the Planck scale. Don't let classical formulation be an obstacle for a quantum one!

A single graviton cannot be detected, ever, even in principle. It can literally *never* be verified by experiment.

A single graviton cannot be detected, ever, even in principle. It can literally *never* be verified by experiment.

This is believed true, but not that significant. It is not that different from QCD predicting that free quarks don't exist (but it is not nearly as ironclad as this prediction of QCD). What would be a problem is if there were never any testable consequences of quantum gravity. It is premature to worry about that (esp. as the various research programs, including string, are making predictions that are in principle testable, despite none of the theories being complete).

Naty1
...then is it remotely probable that gravity cannot be unified with the other four forces?

If by that you mean 'we may never find the unifying mathematics', then sure, it IS remotely possible but the ingenuity of scientists and mathematics suggests otherwise...

Because it is believed that early in the universe a transition occurred from a highly unstable and very energetic environment where everything was the 'same'...that is 'unified'...to the one we observe today with different particles [mass], different types of energy, different forces, where things APPEAR unique and distinct, scientists believe they can find the 'missing linke.

Rough analogy: Look at an ape, look at humans: who would think they were 98% the 'same'...that is have almost the same genes??

We'd like to know if electrons, photons, and the separate forces we observe are a chance occurence of this breakdown or one that is highly likely to be repeated in every such transition.

Let me clarify that there is a distinction between unifying the forces and having a theory of gravity consistent with the quantum nature of reality. There is near universal agreement that the latter is necessary. As to unification, I do not believe there is really strong evidence that that the 4 forces must be unified. The SM does not unify even three of them, and grand unification of electroweak+strong is not experimentally verified, nor is proved (IMO) that any way to repair the high energy inconsistencies in the SM must involve unification.

I would phrase it: QG is necessary; unification of the 4 forces is aesthetically pleasing, and there are strong hints that it is likely true of our universe.

friend
The symmetry of the SM is U(1)SU(1)SU(2) that defines the differences between all the particles and forces. And this symmetry is independent of the curvature of the background spacetime and is also independent of the energy of the particles since this symmetry is internal, as I understand it. So if these symmetries don't change that define the particles, then how can all the particles and forces become one when they are always distinguished by this symmetry? Or if the particle defining symmetry is independent of the background metric, then how can particles be united with gravity/geometry?

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The symmetry of the SM is U(1)SU(1)SU(2) that defines the differences between all the particles and forces. And this symmetry is independent of the curvature of the background spacetime and is also independent of the energy of the particles since this symmetry is internal, as I understand it. So if these symmetries don't change that define the particles, then how can all the particles and forces become one when they are always distinguished by this symmetry? Or if the particle defining symmetry is independent of the background metric, then how can particles be united with gravity/geometry?

The problem is (but perhaps am I wrong in believing that) that we make experiments concerning the SM in a context (the LHC, on the surface of the earth) where the intensity of gravity is not so important as, say, in the vicinity of a hughe black hole. I would phrase it: we experiment the SM when gravitation is a negligeable parameter of the discussion. Or: the SM is true when gravitation = 0. We are in the same situation than the research was at the beginning of the 20th century after the first formulation of the theory of relativity and before the formulation of its generalized version. Don't you think so?

The symmetry of the SM is U(1)SU(1)SU(2) that defines the differences between all the particles and forces. And this symmetry is independent of the curvature of the background spacetime and is also independent of the energy of the particles since this symmetry is internal, as I understand it. So if these symmetries don't change that define the particles, then how can all the particles and forces become one when they are always distinguished by this symmetry? Or if the particle defining symmetry is independent of the background metric, then how can particles be united with gravity/geometry?

Symmetry breaking.

friend
Symmetry breaking.

Is there any real proof of the process of symmetry breaking, as opposed to, say, a slow increase in the effects of a symmetry having more and more effect? Is there any evidence of the actual break, even if we don't know what exactly the symmetry was that broke?

Symmetry breaking.

Do you mean "symmetry breaking" as a precizion and in opposition with SU(3) x SU(2) x U(1) which is the symmetry gauge group for the invariance of the Lagrangian density function of the Standard Model?