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So from what I understand, each fundamental force (with the exception of gravity unless you count the hypothetical graviton) has a force carrier particle that mediates the force. Does that mean if the grand unified theory, where each force unites as one at high energy, is true then there would be a singular force carrier particle that would have had decay into the particles we have today? If so, does that mean the electroweak force has a force carrier particle (I know the $Z$ boson plays a role with the electroweak force theory but I have to admit I'm not quite sure how after reading about it)?

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  • $\begingroup$ We don't know if the final unified theory will be a field theory. We'll find that out when once we discover the true final unified theory ... this might take some time. $\endgroup$ Oct 15 '21 at 22:36
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    $\begingroup$ Yes, the SU(5) unified theory has 24 force carriers, just as electromagnetism has one, and weak forces have three. $\endgroup$ Oct 15 '21 at 23:00
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Firstly, let me notice that a Grand Unified Theory (or GUT, for short) is a hypothetical unification of the Standard Model interaction, but it is not necessary that it corresponds to reality. There are GUT models, but it is not certain as of now that any of them actually describe reality (although they are built so that they recover the Standard Model and many haven't been ruled out yet).

That being said, yes, there are force carrier particles (plural) carrying the interaction of the "grand unified force", just as the Standard Model has carriers for its own interactions. In the Standard Model, we have

Notice that it is not a single carrier per interaction. The appropriate number depends on the details of the interaction. More technically, each carrier is associated to a generator of the gauge group. Since the number of carriers depends on the details, it also follows that the number of carriers on a GUT depends on the specific GUT being considered. As noted in the comments, the SU(5) GUT has 24 carriers. Out of these 24, 12 will eventually be identified as those of the Standard Model (the 12 ones I listed above), while the other 12 are bosons not present in the Standard Model, which lead to new predictions, such as proton decay. Other models have other numbers of carriers. The SO(10) GUT, for example, has 45 (24 of which are the ones from SU(5)).

If there is a correct GUT, the explanation for why we don't see these extra particles at the LHC, for example, is that they are way heavier than the ones we've seen so far. ELectroweak interactions occur at the scale of $10^2$ GeV, while the scale in which we expect grand unification to happen is close to $10^{15}$ GeV, meaning present-day experiments can't gather enough energy to see these sorts of particles.

In short, there are new carriers (plural) in GUTs, in addition to those of the Standard Model. The weak interactions do have carriers (plural), those being the $W^+$, $W^-$, and $Z$ bosons.

As a final remark, it is worth pointing out that GUTs do not attempt to describe gravitation and hence are not attempts at a theory of quantum gravity, much less at a theory of everything. While your post suggests that you are aware of this, I believe it is good to reinforce this point for other readers of this post.

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There are a lot of ideas merged together in this question. First of all a unified field theory does not have to be quantum in nature, which is how the concept of a force carrying particle emerges. It is a working assumption among most physicists (most that I've interacted with) that QM and QFT will not be dethroned. The short answer to your question (appearing in the comments) is, yes. A UFT will have force carrying quanta once the full machinery of QFT is applied.

However, as the other comment points out, we don't know what that will look like. Not only would the interactions need to be described by field theory but that should also emerge from a locally gauged symmetry of the matter fields (spinors) and that might not survive when we finally identify the GUT, and successfully quantize gravity, or replace it with whatever works. Recent research in QG is leading to the concept of space-time and symmetry emerging from some primitive point set of unorganized "events". What we call force carriers and matter particles might emerge from this event space in a way that defies our current understanding. If that happens we might need to completely change how we describe things.

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  • $\begingroup$ And again, why the down vote. At least comment to help improve. $\endgroup$
    – DrDRB
    Jan 8 at 14:54

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