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There are a lot of questions and answers on this site about the unification of forces, and all of them univocally say that at high energy levels, all the forces get unified. But none of them answer my question specifically, that is, what is the mechanism that causes forces to unify at high energy levels.

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The idea is that at extremely high temperatures and energies, like at the beginning of the universe, all the forces were one, but as the the temperatures decreased, the forces seperated into the main four forces.

Why are Electromagnetism, Strong & Weak Nuclear and Gravitational Forces different?

I found one answer where knzhou specifically goes into some details:

When we say all the forces were unified, we mean that all of the Standard Model forces were described by a common set of charges, which are intermixed by 24 gauge bosons. These gauge bosons are all identical in the same way that the 8 gluons are identical. In particular, you can't point at some subset of the 24 and say "these are the gluons", or "this one is the photon". They were all completely interchangeable.

What does it mean to say that "the fundamental forces of nature were unified"?

But this does not explain the mechanism how higher energy levels cause the bosons to become identical (interchangeable). Basically here the explanation is just that at high energy levels, the bosons become identical. This could mean that the bosons set of quantum state's become identical, that is, all of them become massless (I assume), their spins becomes the same (though it does not say what spin), and all the other quantum numbers too.

My question could be the description of the mechanism that leads to this quantum state, and how high energy levels cause the bosons quantum numbers to become identical.

And the fact that the bosons are identical, does not explain for example what that unified force is like, is it attractive, or repulsive too (I guess that comes from the spin)?

Question:

  1. What is the mechanism that causes forces to get unified at high energies?
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I think you should look at it the other way around. There is only one force at any temperature. At lower temperatures, symmetry-breaking structures form that make it appear to be several forces. When you raise the temperature, the symmetry-breaking structures disappear again.

As an analogy, on Earth's surface you can distinguish the vertical direction from horizontal directions by the acceleration of gravity, and you can split the horizontal directions into north-south and east-west by using a compass. If the Earth loses its magnetic field, then you can no longer identify special horizontal directions and they become "unified". If the Earth vaporizes and spreads out so you're at a random location in a uniform gas (which would happen at very high temperature, I suppose), then you can't distinguish vertical from horizontal either. The directions themselves didn't change. There wasn't really a process or mechanism by which they unified.

The 24 bosons of SU(5) grand unification are sort of like the three spatial dimensions here. They aren't identical in the absence of symmetry-breaking structures, but they are interchangeable. (Different directions are still different, but any set of coordinate axes is as good as any other.)

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    $\begingroup$ Thank you, I do understand that you make the distinction between identical and interchangeable. It works perfectly in your analogy with dimensions. Though, with particles, I do not think it could (or I do not see), because particles have quantum numbers. Those need to be identical for the particles to be interchangeable, don't they? How can you interchange a boson with another if they have different spins for example? $\endgroup$
    – Árpád Szendrei
    Commented Nov 2, 2020 at 20:13
  • $\begingroup$ @ÁrpádSzendrei All of the bosons have spin 1 and are massless. Aside from that they just have charges associated with their own gauge field, which are interchangeable under the gauge symmetry. $\endgroup$
    – benrg
    Commented Nov 3, 2020 at 0:18
  • $\begingroup$ Thank you, this it very interesting. Do you please have a reference for this (including that they all have spin 1)? I am wondering because then this unified force must be both attractive and repulsive (like EM), since photons do have spin 1. I would like to see how, what mechanism makes the bosons at high energies all change their spin to 1, and what you are saying about the charges being interchangeable under gauge symmetry. $\endgroup$
    – Árpád Szendrei
    Commented Nov 3, 2020 at 1:25
  • $\begingroup$ @ÁrpádSzendrei Sorry, the last comment is wrong; I was only thinking about the GUT when I wrote it. I don't actually know why spin 2 becomes spin 1. The spin-1 gauge forces are like gravity acting on compact extra dimensions, and I suppose the angular momentum ends up evenly split between the big four dimensions and the others. I can't remember ever reading anything where this was addressed explicitly. Interchangeability under the gauge symmetry just means that, e.g., if you take rotation in the xy plane and rotate it 90° in the yz plane, it becomes rotation in the xz plane. $\endgroup$
    – benrg
    Commented Nov 3, 2020 at 18:31

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