Is There Really No Grand Unified Theory and Unification of the Three Forces?

[kodama]

<<Mentor note: Split from https://www.physicsforums.com/threads/can-we-calculate-the-coupling-constants.855036/ >>

is it possible there is no GUT and no unification of the 3 forces?

 

[Orodruin]

is it possible there is no GUT and no unification of the 3 forces?

Of course this is possible. Nobody has made experiments at the GUT scale (or would-be GUT scale) and so we cannot say with certainty how physics at that scale behaves. All we can say is how our current physics would behave if extrapolated to a certain scale, but without actually performing experiments, there is no way of being certain that this is actually the case.

 

[ohwilleke]

is it possible there is no GUT and no unification of the 3 forces?

In the narrow sense, surely. Nothing critical to the existence of the universe requires gauge coupling constant unification, although it would be awesome if it existed, and there doesn't have to be a GUT in the narrow sense of the term that refers to a single Lie group that corresponds to all particles that exist and none that don't and from which the interactions of fundamental physics can be inferred.

In the broader sense of the term "GUT" to refer to some deeper theory that explains from first principles why the Standard Model has the particles and physical constants that it does, however, I think it is hard to deny that almost everyone in the field thinks that this deeper theory to explain the Standard Model exists, even if we may never master it. There is too much of a semblance of order in the structure of the Standard Model for it itself to be the fundamental laws of Nature without further logic mandating it, even if everything that is implied by the deeper theory can be summed up in the Standard Model plus some sort of quantum gravity theory.

 

[kodama]

with GUT there are additional higgs. any prospect LHC will find them, perhaps 750 GEV?

 

[Orodruin]

The scalars related to the GUT scale will not be found at 750 GeV. A 750 GeV scalar would be more likely to be related to various SUSY models or other two-higgs doublet models. Of course, those can be embedded within a GUT, but in itself I would not take the 750 GeV scalar (if confirmed) as a direct sign of grand unification.

 

[kodama]

so what would be scalar massses for non-SUSY GUT's. does MSSM presuppose a GUT like SO(10)?

 

[Orodruin]

Scalar masses related to the GUT scale (SUSY as well as non-SUSY) would be at some $10^{15}$ GeV or so. A SUSY model does not necessarily have an underlying GUT, but many GUT models build upon having SUSY, although it is generally not a requirement.

 

[kodama]

do you have any preference for GUT, such as SO(10)?

 

[Orodruin]

Yes, I would prefer there to be something unexpected at a lower scale. It would make physics much more interesting than an assumed high energy GUT which will not be tested in my lifetime.

 

[kodama]

Yes, I would prefer there to be something unexpected at a lower scale. It would make physics much more interesting than an assumed high energy GUT which will not be tested in my lifetime.

are there any examples of serious proposals, or are you thinking SUSY/MSSM?

 

[Orodruin]

There are several ideas floating around, but one of those would be equally boring. The interesting part comes if there is a mystery in need of a new type of explanation.

 

[kodama]

i know proton lifetime has ruled out SU(5), what are the prospects for SO(10)?

 

[ohwilleke]

with GUT there are additional higgs. any prospect LHC will find them, perhaps 750 GEV?

The 750 GeV bump at the LHC, if it is real (and honestly, I am increasingly inclined to think that it is just a fluke or experimental error despite the fact that this experimental data point has already garnered 750 preprints), is very un-Higgs-like, because while it has the characteristic diphoton decay of a Higgs boson, it does not have any of the other associated decays which would be visible at the LHC if it were Higgs-like.

 

[kodama]

Scalar masses related to the GUT scale (SUSY as well as non-SUSY) would be at some $10^{15}$ GeV or so. A SUSY model does not necessarily have an underlying GUT, but many GUT models build upon having SUSY, although it is generally not a requirement.

do those GUT higgs like scalars also have a hierarchy problem ?

 

[Orodruin]

do those GUT higgs like scalars also have a hierarchy problem ?

They very likely would if you found them at 750 GeV.