# The assumption that gravity is the same as the 3 quantum forces

Id just first like to say that Im only a third year physics major so I might be wrong, misinformed, mistaken, or any combination of the three about any of this.

I dont understand why people are trying to unify the four fundamental forces. electromagnetic, weak, and strong have already been shown to be the same force, but from what Ive read gravity hasnt been. Isnt it possible that gravity is just a different type of force than the quantum forces?

For example, in my head at least, gravity can be explained by an accelerating universe and inertia. Say the universe is a hypersphere expanding at an accelerating rate. Anything with mass has inertia, and therefore accelerates slower than the surface of the hypersphere. It would be like accelerating a rubber sheet in space with a marble on it. The marble gradually creates a larger and larger "dent" in the sheet. Anything with mass would cause a curvature in spacetime, because of its inertia.

The other effect of an accelerating universe would be to cause an inward force on all massive objects. This would explain why massive objects attract each other, because they are both forced inward on a curve.

If this "theory" were the case, gravity would simply be different from the other 3 forces. There would be no particle exchange, and it would just be a fundamentally different force.

It seems that with gravity the amount of force something feels on something else depends on the mess yes? So more mass=more charge=more "gravity" I don't see why gravity has to be it's own force. Can't it be a by product of all the charges out there?

Demystifier
Gold Member
Isnt it possible that gravity is just a different type of force than the quantum forces?
It certainly is different, but the question is: how much different?
Two extreme positions are
1. It only differs by the value of spin (2 for gravity and 1 for other forces)
and
2. Gravity is not a force at all.

LostInSpaceTime: the equivalence principle equates gravitational mass and inertia mass. No such principle (experimentally accurate ones at least) relates the other charges.

Michael879: the "quantum forces" as you've called them are quantised versions of gauge theories, which are a generalisation of electromagnetism. It is possible to write general relativity in a form that is formally very similar to the gauge forces. Thus there is a belief that they are not so different. Of course, it could just be that the mathematical machinery employed are so general that any plausible theory can be massaged to look alike.

In any case, it's dangerous to think of the gauge bosons that are part of the standard model as actually having a real existence. As with all maths, they are idealised versions of what goes on. It's not actually possible to measure a "one photon state" (in its Fock space) as QED suggests; in reality we end up measuring something which is not very different, but enough to bring into question the reality of the number states in the standard model. It's very much like saying that perfect insulators have a real existence.

When people say that they want to unify the forces, they mean that they want a theory that can calculate some numbers which involve all of them. For example, as far as I'm aware, strong force and electroweak are not "unified" in any sense, except for our ability to calculate some pretty convincing numbers. On the other hand, electromagnetism and the weak force are indeed quite unified in the standard model, where the difference between them arises out of a spontaneous symmetry breaking. "Grand unified theories" were popular for a while, which sought to do the same for the strong force; unfortunately, they didn't really pan out. So the unification of gravity with the rest is really just wanting to not get logical contradictions, rather than looking for what's underneath.

Disclaimer: I've given up reading and trying to understand the views and opinions on how gravity (or alternatively the world) works which are expressed by strangers in an internet forum. So I'll not comment on that part of your post. I hope the little rest still helps you a bit:
Electromagnetic, weak, and strong have already been shown to be the same force, ...
That is, to my knowledge, not true. There are models proposing this to be true, so-called grand unifying theories (GUTs). But those models are not backed up by any experimental result. But they are also not excluded by experimental results, their validity is simply an open question.
What is true, however, is that the interactions in the Standard Model (SM) of particle physics all interactions (gravitational interaction is not part of the SM) are described by the same mathematical mechanism (local gauge groups). What you know as electromagnetic, weak and strong force are effective interactions that more or less straightforwardly derive from this mechanism.

I dont understand why people are trying to unify the four fundamental forces. Isnt it possible that gravity is just a different type of force than the quantum forces?
That is absolutely possible. However, there are indications that there might be a common mechanism. Furthermore, from the standpoint of physics, it is almost necessary to look for common mechanisms:

- What is physics? It is the attempt to quantitatively describe/predict the maximum amount of phenomena/experimental outcomes with a minimum set of rules. A great success of Newtonian gravity is that it describes both, the motion of planets and why your books stand on the shelf instead of floating around your room, with the same mechanism - gravity. That might seem trivial to you because you're used to it but it's imho quite a big deal. You could as well just develop different effective theories for every scenario (planets orbit the sun, books stand on shelves). But that's not in line with the definition of physics I gave above (note that it's not THE definition of physics, but it seems a good one to me).

- As a rough approximate rule: Things on the scale of an atom and smaller are better described by quantum theories. General Relativity is not a quantum theory. As soon as energy densities are large (and fluctuations on that scale are, too), it seems to cry for a quantum theory of gravity.

- I've previously mentioned that interactions in the SM are descibed by local gauge groups. The SM has another gauge group which is technically slightly different: The gauge group of spacetime symmetry is a so-called global gauge group. When gauge groups are "promoted" (that's the term for the change) from global to local, they necessitate the existance of a "gauge field" which causes interactions between particles. It is intruiging to assume that promoting the spacetime symmetries to a local gauge group will create a gauge field that mediates gravitational interaction between particles - the graviton field. There are technical difficulties in this seemingly straightforward approach, but some effective theories in fact go this way.

Ok, to sum these relatively random comments up: On the one hand, there is no reason to assume that gravity and the other interactions relate to a common mechanism other than aesthetic reasons. On the other hand, at some point there is no other reasons than aesthetic ones (save for practical ones perhaps - creating lookup tables for every possible process can become a bit tedious :uhh:) for doing physics research at all.

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It's not actually possible to measure a "one photon state" (in its Fock space) as QED suggests

Why ? The operator $$N = a^{\dag}a$$ is commutating with the hamiltonian ? Or it is not true for QED ?

Why ? The operator $$N = a^{\dag}a$$ is commutating with the hamiltonian ? Or it is not true for QED ?

The Fock states are never in principle measureable, even if they are "observables" formally, because they require a global, infinite detector. In an experiment, your detector measures the presence of a particle in some spacetime region. Asymptotically, far away from the interaction, the particles are almost free and are well approximated by Fock states. That is why I said that they are like "perfect" insulators -- a neat approximation, very useful, characterises an important aspect of the physics, but short of a real existence.

Why ? The operator $$N = a^{\dag}a$$ is commutating with the hamiltonian ? Or it is not true for QED ?

Your operator N corresponds to the observable "the number of particles". In QED (and in real life) the number of particles can change with time, and operator N is not commuting with the full interacting Hamiltonian of QED.

Eugene.

Ok thanks ! I didn't know if N was or not commutating with the full interacting hamiltonian.