In defense of Supersymmetry
Jacob Aron - a deputy editor at New Scientist - tweeted out this week:
“Supersymmetry is roughly 50 years old, and there is not a single shred of evidence that it is even remotely true. Are there any other theories (not just physics) that have lasted half a century without any justification?”
It was massively uninformed, garbage take. A textbook example of begging the question. Even if we ignore the obvious parroting of his take from a host of retired, deceased or otherwise armchair Physicists - there is a dangerous nuance Aron’s language. Physics is not in the business of being “true”. It never has been. It’s in the business of explaining our observed reality to the best approximation possible.
Modern journalists sometimes tweet divisive things all the time to get a reaction. It's a technique. Most of my colleagues were more restrained in their response. I - unfortunately - was on the tail end of my second dose of the Moderna vaccine. I was not so restrained.
Unfortunately, Aron’s take isn’t uncommon. It often based on a misconception of how Physics works as well as - I suspect - pushback from the semi-celebrity (and associated wealth) afforded to “String Theorists” over the past decades. Some of course silly, others temporarily fashionable, some are borderline delusional, but most simply aimed to do good work.
I thought this week we’d pick apart this tired class of refrain - and its historical consequences for American Science. But first, context.
What is Supersymmetry?
Supersymmetry is a symmetry just like any other. Flat spacetime - the usual backdrop for virtually all physical modeling - possesses supersymmetry. Supersymmetry is an extension of the rotational, temporal and translational symmetries of familiar, flat spacetime. As it turns out, this collection of more traditional symmetries - often called Poincare Symmetry - has a unique, nontrivial extension. Supersymmetry is precisely that unique extension.
Symmetry in general is a powerful tool in physics, as it restricts the type of phenomena that can happen. Some of these restrictions are surprising: light has precisely two polarization modes because of Poincare symmetry. There are precisely three kinds of pion because of Poincare symmetry. Indeed, all of the observed physics of elementary particles organizes itself around Poincare symmetry.
Quantum mechanics brings about a curious twist. Poincare symmetry together with quantum theory provide an explanation for the Pauli Exclusion Principle: that no two electrons may share the same quantum state - and therefore the same physical space (modulo spin). This fact bestows the restriction each electron orbitals of atoms may only be filled by two electrons (two because of spin), which in turn generates all of known chemistry. Virtually all of everyday experience boils down a generic observation:
Fermions (like electrons) are particle that have mass. Bosons (like photons) are particles that communicate between fermions. In practical terms, our capacity to stand on the ground, to sit in a chair, to pick up our phone and interface with the everyday world is an admixture of the Pauli exclusion principle and the electromagnetic force. Both of which are explained - in part - by the Poincare symmetry of spacetime.
The supersymmetry of spacetime is subtler symmetry - or at least, it acts in a subtler way. At face value, supersymmetry implies a symmetry between bosons and fermions. Practically speaking, it implies that for any fermion of mass m, there should also be an associated boson of precisely the same mass. These particles should otherwise have identical interactions - with say, the photon, aka the electromagnetic field. All particles - in other words - have an almost identical super partner particle. The only difference being that one is a boson and one is a fermion.
Supersymmetry and Quantum Mechanics
This symmetry - supersymmetry - is baked in to the generic formulas of quantum field theory.
A historical, “mathematical detail” of quantum field theory was the infinite energy associated to mere existence of a quantum field. The energy associated with a quantum system included a sum of a particle’s mass times an infinite sum of constant “vacuum energy”: terms. Those terms have nothing to do with the calculations we test in experiment, so various textbooks prescribe various ad hoc method for simply ignoring them.
With the discovery of the universe’s late-time accelerated expansion, this vacuum energy now becomes an important ingredient in observational cosmology. We can no longer simply “ignore” these infinites. We now have to explain why the vacuum has SOME energy, but why that energy is NOT infinite. This fact remains an open problem in physics.
Amusingly, the vacuum energy of a fermion of mass m precisely cancels the vacuum energy of a boson of mass m. Precisely. In other words, the vacuum energy of a supersymmetric system is precisely zero, precisely because it requires the existence of an otherwise identical fermion - boson pair.
As it turns out, this kind of precise cancellations happen all the time in physics. As Dick Feynman once observed, a symmetry of antimatter cancels matter exactly in dynamical situations, saving quantum field theory from violations of causality. The symmetry the induces those cancellations - the symmetry that demands the very existence of antimatter - is Poincare Symmetry.
Models of particle physics that respect Supersymmetry have zero vacuum energy. To incorporate the observed the small, positive vacuum energy known as “Dark Energy” requires a slight departure from supersymmetry. In other words - it requires the those fermion/boson pairs to have different masses. And that - in part - is why particle physics has been so bullish on supersymmetry in recent decades: there’s potentially a host of new particles yet to be found. Better still, it suggests a possibly explanation for why supersymmetry should be broken.
The Benefit of the Doubt
Given that we haven’t seen any super partners yet, a best case reinterpretation of Aron’s tweet is:
“Supersymmetry is roughly 50 years old, and there is not a single [observation of a supersymmetric partner particle].”
It might seem like a fair question, until you look at some examples between theoretical ideation and experimental verification:
Pauli’s Neutrino was proposed in 1930 to explain “missing” momentum in particle collisions, but direct detection of such particles not made until 1956. 26 years.
Karl Schwarzshild used Einstein’s theory of General Relativity to predict the existence of black holes in 1916, but they were not observed until 1971. 55 years.
General Relativity also implies the existence of gravitational waves. Henri Poincare himself postulated their existence in 1905 - before Einstein’s theory was complete! Concrete models of gravitational waves in General Relativity were first computed in1916, but indirect, astrophysical evidence for them had to wait until 1993. Direct measurements on Earth weren’t achieved until 2015. 99 years!
Grand Unified Theories : the idea that the strong nuclear force should merge with the weak and electromagnetic forces has been around since at least 1974 - since it was observed the the weak and electromagnetic forces merge at high energy. As yet no observational evidence exists for these GUT models, a 47 year drought, and yet GUT model building persists.
In short, there is sometimes a disconnect between theory and experiment in physics. And sometimes it takes over a century to resolve that discrepancy. There are many theoretical reasons to be worried that we have not yet observed super partners at the LHC, but a timescale of 50 years isn’t one of them.
To say that “not a single shred of evidence exists” for Supersymmetry, is ignorant at best. There is plenty of theoretical evidence for supersymmetry, there are also numerous applications of these ideas to mathematics and - amusingly - condensed matter physics.
At worst, the claim is a reflection of deep, personal politics.
This Complaint is Not New
Aron’s implicit complaint echos earlier instances of resentment towards and amongst theoretical physicists. One of which ended with profound consequence.
Particle physicists have been looking for supersymmetry for a long time. Searches go as far back as the early 1990’s and CERN’s LEPII experiment. It was a critical time for particle physics research. In 1990 the entire subdiscipline of condensed matter physics - the field that gave rise to semiconductors, nanotechnology, superconductors and the like - threatened to divorce itself from the American Physical Society.
Why? APS’ full-throated support of the Superconducing Super Collider (SSC). In 1987, the SSC was a particle physics experiment being built in Texas. Two billion dollars was already spent to dig an underground ring for the collider. A collider with a single beam energy of 20 TeV, which would have dwarfed the LHC’s 7 TeV, would have found the Higgs - and whatever other particles lay in wait between the 14- 40 TeV range of center-of-mass collision energies. All of this would have happened decades ago.
High energy physics research is expensive. There were cost overruns. The scale of experiments required now involve an entirely different kind of research. LHC experiments like CMS and Atlas resemble large, multinational corporations. Costs for the SSC would have been entirely American-borne.
The escalating costs - and any potentially associated cuts to other fields like Condensed Matter - drove may physicists to oppose the project. Some vociferously. Eventually too: the president of the APS.
To date, the loss is incalculable: we simple don’t know if, when or where the next collider will be built. The center of gravity of particle physics, superconducting magnet production and installation, and all sorts of other associated industry, advanced technology and other associated jobs shifted to Europe. Much of that technology transfer - arguably from the very roots of condensed matter theory itself - would have gone directly to industry in the US.
Incidentally, this happened again, with the loss of US support for the nuclear fusion reactor project ITER.
To quote a speech by particle physics and Nobel-Laurent Steven Weinberg celebrating the famous work of an American trio of condensed matter physicists, Bardeen, Cooper and Schrieffer.
"Condensed-matter physics and particle physics are relevant to each other, despite everything I have said…. Sometimes these ideas become transformed in translation, so that they even pick up a renewed value to the field in which they were first conceived. The example that concerns me is an idea that elementary-particle physicists learnt from condensed-matter theory – specifically from the BCS theory. It is the idea of spontaneous symmetry breaking."
The LHC - Europe’s replacement for the SSC - eventually found the Higgs Boson responsible for the spontaneous symmetry breaking of the Electroweak symmetry of the Standard model of Particle Physics. Directly realizing the condensed-matter inspired idea in the realm of particle physics.
In Summary
Supersymmetry is a modeling tool, and a powerful one at that. It’s a symmetry of standard, four dimensional spacetime. To question its existence is akin to questioning the existence of “cosine” or “imaginary numbers”. To discount something as merely “nice maths” to discount all of theoretical physics.
Theoretical physics is applied mathematics. Making predictions is an output, but so is understanding what leads to those predictions. Just as an experimental particle physicists who builds the electronics is just as much of a particle physicist as the one who analyzes the data is just as much of a particle physicists as the one who raises the money is just as much of a particle physicist as the one who manages the computing clusters is just as much of a particle physicist as the one who organizes the experiment.
In a mature Science like Physics, the division of labor between theory and experiment is a natural consequence of specialization. To be sure the “herding” behavior of research in high energy theory has been well documented. But that’s more from a lack of direction, not a lack of imagination.
When Planck’s cosmology data was prepared in 2013, virtually the entire community turned on a dime to make predictions. Unfortunately, no new physics was observed.
There is plenty of evidence that our current understanding of physics is incomplete. To write that off as “only nice maths” would suggest we stop looking because experiments haven’t found anything new.
But that’s not a Scientist’s perspective. As Poincare said in his book Science and Method:
“The scientist does not study nature because it is useful to do so. He studies it because he takes pleasure in it, and he takes pleasure in it because it is beautiful.”
Of course, for the bean counters that have to keep the economy moving, it just so happens that fundamental scientific research also provides the backbone for virtually all of modern industry. Like difficult predictions, this time between cause an effect can be quite long.
