Let others praise ancient times, I am glad I was born in these.
Ovid, Ars Amatoria, III.121–2 (published 2 AD)
Will there ever be an end of physics, and if so, are we approaching it?
I know this seems like a low priority question. An end of physics? What’s next? No more algebra homework? The FDA telling us that we are eating too many vegetables?
Why would I even ask this question? Recently, Quanta Magazine published an article called Contemplating the End of Physics by Robbert Dijkgraaf who is the current director of the Institute for Advanced Studies (IAS) in Princeton, NJ. The IAS is an independent postdoctoral research center known around the world as the past home of scholars such as Albert Einstein among many other names you are certainly familiar with. Today IAS still has a breathtaking list of permanent faculty and a who’s who of visitors. If you would like an interesting history of the IAS, I recommend this book. I do not personally know the current director, Dr. Dijkgraaf, but I do admire his outreach efforts and highly recommend his videos on YouTube of talks given over the years on many topics of interest to physics enthusiasts and experts alike. This article kicked off a discussion in my sphere of scientists and science communicators about the end of physics.
This is not a new concern, and our community has these sorts of discussions from time to time which I believe is quite healthy. In fact, these discussions come fairly regularly and this is not the first time it has happened in my professional lifetime, or even in the history of physics.
In 1894, Albert Michelson (of the Michelson-Morley experiment) thought he was witnessing the end of physics and went as far as to say,
While it is never safe to affirm that the future of Physical Science has no marvels in store even more astonishing than those of the past, it seems probable that most of the grand underlying principles have been firmly established and that further advances are to be sought chiefly in the rigorous application of these principles to all the phenomena which come under our notice. It is here that the science of measurement shows its importance — where quantitative work is more to be desired than qualitative work. An eminent physicist remarked that the future truths of physical science are to be looked for in the sixth place of decimals. (emphases mine)
We do not know for sure who that “eminent physicist” referred to by Michelson is, but the common lore in my circle is that Michelson was referring to William Thomson, also known as Lord Kelvin. This is the conclusion the great Steven Weinberg arrived at in his excellent book, and I have no reason to question his findings.
These are astonishing words from Michelson given that J.J. Thompson would discover the electron in 1897, just three years later. In 1903, Michelson would double down on this idea and write, “The more important fundamental laws and facts of physical science have all been discovered, and these are so firmly established that the possibility of their ever being supplanted in consequence of new discoveries is exceedingly remote.”
To echo a phrase I read from Sabine Hossenfelder, this wasn’t his best work. I doubt anyone of the time could foresee how important electrons and by extension electronics would become to our society from their vantage point. Michelson was also speaking and writing as the quantum revolution was just barely beginning and the radical change in the way physicists would come to view their world was in no way evident to Michelson at that time.
What does this have to do with us? A great deal. But first, we need to define our terms.
In my academic circle, when people are talking about the end of physics, they are talking about the end of particle physics. This is sometimes referred to as the end of “fundamental physics,” which I understand why it is known this way, but is perhaps a little too hard on my non-particle physics colleagues. Particle physics is most directly related to the study of the fundamental laws of physics.
This is a very important distinction because there are many kinds of physics and the concern is certainly not with the end of astronomy, or solid state physics, or atomic physics. All of these subjects are booming right now and from where I sit, I cannot see this coming to an end soon.
So, why do people fear we are nearing the end of physics as we have defined it? The argument goes that we are victims of our own success. Since the end of world war two, there has been a fairly steady stream of discoveries from the world of particle physics. We (collectively) built bigger and bigger machines and proposed more and more encompassing theories. These two sides of the physics coin have historically done a great job at providing each other with certain checks and balances. The history here is wonderful but will have to wait for it’s own post.
The two long term ideas of my professional lifetime was the existence of the Higgs boson and the existence of supersymmetry, which I will have to write about each at length on their own. To look for experimental evidence of each of these ideas, the high-energy community built several particle colliders which culminated in the 2012 announcement by scientists at CERN’s Large Hadron Collider (LHC) that the Higgs boson did exist and that we had measured its mass — a virtual holy grail. A wonderful day for fundamental physics.
An announcement about supersymmetry has yet to come and as it looks right now, maybe it never will. If supersymmetry exists, it seems to be out of reach of our current particle collider and perhaps of any possible collider, no one really knows. If you are thinking, “you can’t win them all,” you are mostly right. But, without supersymmetry, the Standard Model of Particle Physics as it is currently written, is complete and like Alexander we weep because there are no more particles left to discover. We have the rules of the universe, and we don’t exactly like how they fit together, but all the fundamental pieces are there.
This is the end of physics as described. Are we, finally, in the boat described by Michelson all those years ago? Do we just do our best to measure the properties of this last particle to ever be discovered to the proverbial sixth decimal place?
I cannot say that this isn’t a possibility, but I would be very surprised. First, even if the particle content of the SM never grows again in my lifetime, it will most likely be reorganized at a fundamental level for any number of reasons. I would say that we are much closer to the position Michelson was in, not because we can see the end of physics, but because of a coming seismic shift in understanding. Michelson did not know it had already started and I think neither do we, so I cannot point to some little cited paper that will change our world.
The quantum revolution started because of irresolvable inconsistencies in what was known at the time. This is exactly where we are today in many ways. There are things that we know about but cannot fit neatly into our current models and understanding such as dark matter and dark energy. If the solution to dark matter is a particle (or several particles) then the SM is not complete and will require reorganization or extension. If dark matter is not a particle, then we will need to make a fundamental shift in how we understand gravity, at the very least. Perhaps the same change will allow us to understand dark energy, maybe it will require its own fundamental shift.
This is the problem, we do not know if or when this new paradigm will present itself. I can say we are in Michelson’s boat, but no one can really say for how long we will remain there.
By some strange turn of events, the solution to both dark matter and dark energy could turn out to be something very simple. Perhaps someone proposes something that is extraordinarily clever that fixes both these problems in a way that does not require anything new or particularly fundamental (I don’t really have something like this in mind, this is just to play the devil’s advocate). There is still the problem of quantum gravity which could not be more of a fundamental problem and again one that will require a fundamental change in the way we view our universe, just as it did with quantum mechanics.
So. Physics is dead, long live physics! I do not think we are at the end of physics by any reasonable definition, neither fundamental nor the more applied disciplines.