The essence of the standard model for particle physics was formulated in an article by Steven Weinberg in 1967, but the term “standard model” was apparently first introduced by him in a talk in 1973. Regardless whether it is its name or its formulation that marks its origin, the standard model is by now at least 50 years old. The fact that we still call it the standard model, means that there has not been any improvement for the past 50 years. Otherwise the improvement model would have become a new “standard model.”
(my academic grandfather)
From different perspectives, one can either see this longevity as a good thing or as a bad thing. The fact that it survived this long is a testimony to how well it works. In fact, the only clear shortcoming is that it still contains the neutrinos as massless particles. We know that neutrinos have relatively small but nonzero masses, but there is no successful formulation yet that incorporates masses for the neutrinos into the standard model. Nevertheless, despite this shortcoming, the standard model is still an amazing triumph in the scientific endeavor to understand how the physical world works.
On the other hand, there are several notions about what an improved standard model should include. One obvious thing is the idea that gravity should be included in the standard model as a fourth force. However, if we treat gravity as a force in the same way that the other forces in the standard model are treated, are we not taking a step backward? Didn’t Einstein say that gravity is not a force? It is the result of curve spacetime. Ironically, naive attempts to describe gravity as a force in terms of quantum field theory has led to some insurmountable problems. So, I don’t think it is a good idea. Then of course there was the attempts associated with string theory. Enough said about that.
The more I think about it, the less I am convinced that gravity needs to be quantized. What it comes down to is whether it would be possible to entangle the curvature of spacetime with a superposition of different mass density distributions. From a purely formal point of view, one can treat the stress-energy tensor as a quantum observable. The expectation value for this stress-energy tensor observable of a state representing the superposition of mass density distributions would still give a well-defined “classical” stress-energy tensor in the Einstein field equation. Hence, no entanglement and no need for a theory of quantum gravity. Well, I am not completely sure about it because I haven’t done the actual calculation yet.
Other things that are believed to be missing in the standard model are dark energy and dark matter. There we still have much work to do before we can even start to think about changing the standard model. I am not as knowledgeable about everything associated with these concepts as I would like to be, but I need to be convinced that we really need such exotic explanations. In my view, it could be just complications in the calculations of what is being observed.
So, apart from the neutrino masses, I would not be surprised if the standard model in its current form is pretty much as good as it can get. We may be celebrating many more years … decades … centuries of the standard model.
Flippiefanus 