Waves in an Impossible Sea: the 2024 science book of the year

It’s rare that a book comes along that changes the way experts in their field think about the fundamentals, while simultaneously being accessible and informative to those without expert knowledge themselves. That’s why, as 2024 winds down, it’s a no-brainer selection to choose Matt Strassler’s incomparable new book, Waves in an Impossible Sea: How everyday life emerges from the cosmic ocean, as this year’s best science book. (Yes, even in a year where I myself wrote my own book!) Strassler, a world-renowned expert in particle physics in his own right (and longtime science blogger), takes the reader on a whirlwind tour of physics from a conceptual point of view.

Even classic topics such as Galileo’s relativity and Newtonian gravity are presented in novel, accessible ways, with common historical terms like “Newton’s first law” replaced by the more intuitive ones such as the “coasting law.” Strassler takes the reader on similar journeys through other classical topics such as mass, waves, and fields before entering the more unfamiliar world of the quantum, which suddenly seems more accessible than ever standing atop the foundation Strassler lays earlier in the book. By the time we get to the book’s last two sections, including one on the Higgs, it seems almost anticlimactic. As counterintuitive and esoteric as the Higgs seems to many, in Strassler’s hands, it seems as though the Universe wouldn’t make sense without it.

A proton isn’t just three quarks and gluons but a sea of dense particles and antiparticles inside. The more precisely we look at a proton and the greater the energies that we perform deep inelastic scattering experiments at, the more substructure we find inside the proton itself. There appears to be no limit to the density of particles inside, but whether a proton is fundamentally stable or not is an unanswered question.

It’s rare for me — a PhD theoretical physicist myself — to read a physics book and find myself stopping to appreciate and digest something I’ve learned (and taught) dozens of times before, because I’m conceptualizing it in a novel, unfamiliar, and yet profound way. But Strassler’s book took me months to get through in the best possible way: because as straightforward as the writing style was, there were information-dense nuggets that I found thoroughly enjoyable to stop and ruminate on at length, as they helped improve and evolve not only how I understand many aspects of physics for myself, but how I explain them to others, including (very recently) here and here.

Because of how extraordinary this book was, I reached out to Matt and asked him if he’d be willing to do an interview with me, mostly about his book but including some questions that go beyond what a general reader would find inside. He graciously agreed, and so I’m very pleased to present to you a question-and-answer exchange with Prof. Matt Strassler, author of Waves in an Impossible Sea, which is Starts With A Bang’s pick for the best science book of 2024!

The first robust, 5-sigma detection of the Higgs boson was announced a few years ago by both the CMS and ATLAS collaborations. But the Higgs boson doesn’t make a single ‘spike’ in the data, but rather a spread-out bump, due to its inherent uncertainty in mass. Its mass of 125 GeV/c² is a puzzle for theoretical physics, but experimentalists need not worry: it exists, we can create it, and now we can measure and study its properties as well. Direct detection was absolutely necessary in order for us to be able to definitively say that.

Ethan Siegel (ES): It’s now been more than 12 years since the discovery of the Higgs boson was announced: the last fundamental particle to be found in the Standard Model. Although most people have now heard of the Higgs boson, very few understand what it is or what it does. How did this particle, and misconceptions surrounding it, motivate you to write your new book, Waves in an Impossible Sea?

Matt Strassler (MS): It always bothers me when we physicists water down our science too far, so that the result is misleading or even false. I feel that doing so underestimates the intelligence of our readers and listeners. To make matters worse, our bad explanations often contradict our good ones, creating logical inconsistencies that make it impossible for a non-expert to make sense of what we’re saying. There has to be a more intelligent and more honest way to convey the lessons of science.

Descriptions of the Higgs boson and the associated Higgs field offer a perfect illustration. The Higgs field is a crucial part of the universe; I’d put it on the top ten list of the most essential ingredients for life. It’s something we ought to explain well. However, in every article and book I’ve seen on the subject, the description of the Higgs field is flawed. So I took it as a personal challenge to write a book with a correct yet non-technical explanation of the Higgs field and how it works.

In the end, though, the Higgs field was really a subplot. The heart of the book is its story of fields and of how elementary particles arise from them, and why an elementary particle like an electron is more a rapidly vibrating wave than a tiny dot. Once this is clear, understanding what the Higgs field does and why it’s so important, and what the discovery of the Higgs boson really means, becomes much easier.

When a symmetry is restored (yellow ball at the top), everything is symmetric, and there is no preferred state. When the symmetry is broken at lower energies (blue ball, bottom), the same freedom, of all directions being the same, is no longer present. In the case of the electroweak (or Higgs) symmetry, when it breaks, there’s a spontaneous process that occurs, giving mass to the particles in the Universe.

ES: I have to fess up that, when humanity first discovered the Higgs, I created what I now realize is a “physics phib” as you’d call it, where I likened the Higgs field to a background of rain and particles that acquire mass to otherwise dry sponges that soak up that rain. There are many other such “phibs” when it comes to the Higgs field and the Higgs boson both. What is the danger you see in phibs such as these? Does it harm the public perception of science?

MS: I like your phib better than most, because it avoids misrepresenting what mass is. The ones that compare the Higgs field to a space-filling soup or molasses, and say that the Higgs field gives mass to things by slowing them down, are literally medieval! They violate two of Isaac Newton’s three laws of motion, encouraging us to imagine, falsely, that having mass makes it difficult for an object to move.

All of these phibs (even yours) tend to imply that the Higgs field somehow makes particles bulkier or more sluggish. Yet what is really going on has to do with the energy that particles carry inside them. Moreover, even though we scientists (including me) all say, as shorthand, that “the Higgs field gives particles their masses”, that’s really not true; the Higgs field doesn’t give a particle energy, or anything else. It just changes the amount of energy that’s required for the particle to exist. To see why that’s true, one must first understand that elementary particles are vibrating entities.

More generally, I’m not dogmatic about phibs; some are mostly harmless. Also, when time is short, a phib may be the only option. Still, I feel that a speaker or writer should always make it clear when a statement is a phib, so that listeners will know not to take it too seriously, and will know to dig deeper if they really want the truth. And I think phibs can mostly be avoided in long-form articles, books and videos.

The danger in a phib is that no phib exists in isolation. Presented with a combination of phibs and truths that contradict one another, how can anyone know which ones to believe? It’s inherently confusing, and unfortunately some people, thinking they must not be smart enough to understand what’s going on, blame themselves. Others conclude the reverse: that scientists must have no idea what they are talking about! Still others view scientists as collectively incompetent at explaining things — and there, I fear, they are somewhat correct. None of these conclusions is good for science and its relation to society. After all, science research is funded by industry and by taxes, and thus by non-scientists, who deserve clear, correct and trustworthy explanations of the science that they’ve supported.

So I’ve spent a lot of time developing a coherent set of accurate explanations that are logically consistent with one another. (When you think about it, making a consistent story out of correct explanations is a lot easier than doing so out of incorrect ones!) I hope my book will convince some of my colleagues to adopt them.

This picture shows US Coast Guard ship Cutter Munro departing from the docks in San Diego, CA. So long as this boat is not accelerating, an observer who is below the deck and has no window to the outside cannot tell whether their boat is stationary or in constant motion relative to the outside world.

ES: Your book gives five main background sections — motion, mass, waves, fields, and quantum — before finally reaching the culminating sections on the Higgs and the cosmos. In each of those background sections, you take something that’s a well-known principle (e.g., the principle of relativity) and reframe it in a nontraditional way (e.g., “the coasting law”). What was your thought process behind relating these concepts that are so familiar to physicists in terms that physicists rarely conceive of them?

MS: As a teacher and blogger, I’m profoundly aware of how the words scientists use can get in the way of our explanations. Sometimes a scientific term seems worth keeping, but often it’s misleading, ambiguous, or unnecessarily abstract. Each case, though, involves its own reasoning. I’ll give you a few examples.

For instance, scientists use the term “kinetic energy” for the energy of motion. But what’s wrong, in a book, with simply using “motion-energy”? Doing so removes an unnecessary abstract word, reducing the number of things a novice reader needs to keep track of.

I referred to “Newton’s first law” as “the coasting law” because it matters not at all that it’s his first law rather than his seventh. It seemed better to have readers remember the law not by its name but by its meaning: that an isolated object traveling in steady motion will coast at the same speed and direction forever.

A third and very important choice was to replace the word “particle”, in the last third of the book, with the word “wavicle”, a term from the 1920s which is also used these days by some other scientists, including Neil deGrasse Tyson and Frank Wilczek. The problem with calling an electron a “particle” is that it immediately makes you imagine an object in the shape of a tiny dot. But in fact, in quantum field theory (the modern language of particle physics), an electron is never a dot. It is always spread out to a greater or lesser degree, and it is wavelike in various ways, including the crucial fact that it is always vibrating. What makes it “particle-like” is not its shape but its indivisibility — the fact that it travels as an indivisible unit.

Joining the suffix “-icle” to the word “wave” captures both the shape of electrons and the fact that they come in individual units — that you can’t divide an electron into pieces. In this sense, “particle” creates misconceptions about what electrons and their cousins are, whereas “wavicle” opens our minds to the unfamiliar ideas that are needed to understand electrons better.

Of course, all of my choices of this type are judgment calls, and I imagine that a few of my colleagues and more advanced readers may disapprove of some of them. But I hope most readers will find them beneficial, at least within the context of my book.

This side-by-side illustration shows the two types of traveling waves: a plane compression wave, or a longitudinal P-wave at left, alongside a transverse S-wave at right. While P-waves can travel through solids, liquids, and gases, S-waves can only travel through solids.

ES: Because the concept of waves is so central to not only the book but to quantum physics in general, you spend a lot of time discussing three concepts: the medium, the field, and the wave. For familiar waves like sound waves, seismic waves, or water waves, it’s easy to identify all three. But for certain types of wave, we have only the field and the wave, leaving us wondering whether there’s a medium at all. What would you say to someone who who can’t quite wrap their heads around the concept of a field (or a wave) without a medium?

MS: Just to make sure our starting point is clear, let’s take an example. The air of the atmosphere is an extended substance… a medium. Air pressure, a property of the air that can vary over time and place, is a field, one that exists throughout the atmosphere. And finally, sound waves can be thought of in multiple ways. We normally view them as waves in the air (waves of the medium). But we can also view them as waves of air pressure (waves of a field of the medium.)

All the familiar waves you mentioned involve the rippling of a substance… a medium. And all familiar fields, like water pressure or air density, are properties of a medium. However, light waves (including visible light, radio waves, microwaves, etc.) are puzzling. They are certainly waves in fields — the electric and magnetic fields. But are these fields properties of a medium?

Nineteenth century physicists assumed they must be. Calling that medium the “luminiferous aether”, they tried to find signs of it, and failed. Then in 1905 Einstein claimed there is no such medium, and that the electric and magnetic fields just somehow exist on their own. It’s a crazy idea, as though air pressure could make sense in the absence of air. I liken it in the book to the Cheshire Cat’s grin — a grin without the cat.

Incidentally, Einstein changed his mind about this after he invented his theory of gravity in 1915, which led him to view the cosmos as a medium, gravity as due to the curvature of that medium, and (by the 1920s) the luminous aether as a part of this same medium. String theorists often follow this reasoning, implicitly viewing extra dimensions of space as a bizarre medium whose properties may include the electric and magnetic fields, as well as other fields. That’s a tale for another time (see question 9).

Yet even if that’s true, there’s something really weird going on. Einstein’s relativity implies that if light waves or gravitational waves or other cosmic waves have a medium, then that medium must have properties that no ordinary material could have. This is a big topic in the book.

Still, coming back to your question, what if Einstein was right in 1905, and the electric and magnetic fields have no medium — how should we think about that grin without a cat? It’s hard to say. This may be one of those cases where physicists’ ignorance, or the limitations of the human brain, lead us to ask a question with no answer. So to someone who struggles with the concept of a field or wave without a medium, I would say: you’re not alone. Scientists struggle too.

It’s all rather humbling: even though we know a great deal about the fields of nature, we know almost nothing about their origin and meaning. I think it’s just too early to guess how this story of waves, fields and media will turn out in the end.

While a string on a guitar has a fundamental frequency that it vibrates at, producing the “tone” illustration at right, you can also “pin down” nodes on the string to cause it to vibrate at higher frequencies: the overtones. The first three overtones illustrate pinning nodes down at one-half, one-third, and one-quarter of the original string length, respectively.

ES: You wrote a lovely Haiku in your book that you entitle “Einstein’s Haiku.” It goes, quite simply:
E equals f h,
And E equals m c squared;
From these seeds, the world.
If we remember that h and c are simply constants, then we find that m, the mass of a particle, is proportional to f, or frequency. This is going to sound very bizarre to someone who hasn’t read your book. Conceptually, how can mass, which we typically think of as a property intrinsic to a quantum particle, be related to a wave-like property like frequency?

MS: The relation certainly seems bizarre, but let me see if I can sketch it quickly. The more familiar part is this: for any object that travels as a unit, including an elementary particle, its mass, as we learn from Einstein’s E=mc² formula, reveals the amount of energy stored within it.

Here it’s important that the word “particle” is misleading, as I mentioned [a couple of questions earlier]. Rather than a little dot, an electron or other elementary particle is a little vibrating entity — an indivisible wave, sometimes called a “wavicle”. Simply because it vibrates, it has energy. And that energy, thanks to the haiku’s second line, is the source of its mass. 

But then quantum physics has its say, telling us something even more amazing and counter-intuitive: that the energy E stored inside of a wavicle is proportional to the frequency f with which it vibrates. The formula that states this is E = f h, the first line of the haiku.

And so the mass of an elementary “wavicle”, a vibrating entity, is proportional to the energy that it carries inside it, which in turn is proportional to the frequency of its vibration.

I realize this ultra-brief sketch of the book’s first two hundred pages is hard to follow. Indeed, one of the main reasons I wrote the book was to lay this story out carefully and clearly. The haiku’s role was to sum that story up in a memorable form. I’m pleased that you liked it!

This reconstruction of particle tracks shows a candidate Higgs event in the ATLAS detector at the Large Hadron Collider at CERN. Note how even with the clear signatures and transverse tracks, there is a shower of other particles; this is due to the fact that protons are composite particles, and due to the fact that dozens of proton-proton collisions occur with every bunch crossing. At higher energies, discoveries that don’t appear at lower energies become possible. Modern particle detectors are like a layer-cake, with the ability to track the particle debris in order to reconstruct what happened as close to the collision point as possible.

ES: Some of the quanta we know of, like the top quark and the Higgs boson, live for an incredibly short period of time. Although you don’t address it when you bring up the concept of quantum uncertainty, there’s also an important uncertainty relation between energy and time. If I create 1000 electrons and measure their masses, I’ll get the same answer every time. What is it telling us, and are there any lessons that are wave-related, that if we create 1000 top quarks or 1000 Higgs bosons, we won’t measure them to all have the same mass?

MS: As with many concepts in particle physics, we actually are familiar with a very similar effect in the context of musical instruments.

If you strike a bell, you’ll hear a clear, ringing tone — biiiiiiiing! — with a definite frequency, a definite musical note. Every time you strike it, its vibrations will have the same frequency, giving the same note. The same is true of the universe; if you strike it in just the right way and create several stationary electrons, each electron will always vibrate at the same frequency, and thus will have the same mass as every other.

But if you were to muffle the bell, perhaps holding it lightly with your hand or covering it with a blanket, the effect of striking it would be different. You’d hear more of a “clunk”; its musical tone would be less distinct, and the sound of the bell would die out very quickly. This is no accident, as these two facts are closely related. For any vibrating object, if you “damp” the vibration as you did for the bell, the vibration will die away quickly and its frequency of vibration will become less definite.

Exactly the same idea (and the same math!) explains how things work for top quarks or other short-lived particles. A top quark is short lived because its vibration is damped by the weak nuclear force and the Higgs force, which, together with quantum physics, cause it to decay away, transformed it into a W boson and a bottom quark. The damping also causes the top quark to have an imprecisely defined frequency, and thus a precise measurement of its mass won’t give exactly the same answer every time.

Still, the average over many measurements is stable. That average is what scientists define as “the top quark’s mass.”

The Standard Model of particle physics accounts for three of the four conventional forces (excepting gravity), the full suite of discovered particles, and all of their interactions. There is a fifth force, the Higgs force, that exists as well within the context of the Standard Model. Whether there are additional particles and/or interactions that are discoverable with colliders we can build on Earth is debatable, but many puzzles still remain unanswered.

ES: In physics, we often talk about the four fundamental forces: the electromagnetic force, the force of gravity, the strong nuclear force, and the weak nuclear force. Physicists often state that, at some very early epoch in our cosmic history, the electromagnetic force and the weak force were unified into just one force: the electroweak force. Yet in your book, you talk about the five fundamental forces, and mention the Higgs force as the fifth. Have physicists been overlooking this force for generations? Why do you say that the Higgs is its own fundamental force where others don’t?

MS: Well, as preamble, we should be a little cautious about the electromagnetic and the weak force; they’re not really unified into one force. Instead, what really happens is that they are reorganized into the two electroweak forces: the weak isospin force and the hypercharge force.

About the Higgs force; no, it hasn’t been overlooked. For instance, when I was a graduate student at the Stanford Linear Accelerator Center (SLAC) in the early 1990s, BJ Bjorken, a great physicist who recently passed away, gave a whole lecture to the SLAC community about the Higgs force. (Bjorken, along with Feynman, established the theoretical framework that led to the experimental discovery of quarks.)

But there are a few reasons that the Higgs force has been downplayed. First, it wasn’t clear until the 2010s, when the Higgs boson was discovered and studied at the Large Hadron Collider, that the Higgs field really exists and is probably an elementary field. Before that, we weren’t so confident that the Higgs force is as fundamental as electromagnetism. Second, although the Higgs force follows logically from the existence of the Higgs field, and some of its effects are observed (see the prior question), it hasn’t yet been directly measured as a simple pull between two particles. It will be some time before that measurement is possible. Third, while the famous four forces you listed are based on elegant math that ensures they have many special properties, the Higgs force is pedestrian by comparison. That’s why mathematically-inclined physicists often disregard it.

Now that we’ve seen the data from the Large Hadron Collider, though, I see no reason to treat the Higgs force differently from the others. For instance, if you measured the total force between two electrons separated by a tenth of a proton’s diameter, you’d find it has four components: the strongest is electromagnetism and the weakest is gravity, with the Higgs force and the weak nuclear force in between. They’re all on equal footing.

This to-scale diagram shows the relative masses of the quarks and leptons, with neutrinos being the lightest particles and the top quark being the heaviest. No explanation, within the Standard Model alone, can account for these mass values.

ES: The most surprising lesson I found in your book was the statement that it’s wrong to think of all the quanta of the Standard Model as “massless” before the Higgs field gave them mass. If you could revert our Universe to a state where the Higgs field were “switched off,” as it was in the very early Universe, what would you observe if you were to examine, say, an electron under those conditions? What properties would you expect would differ about the electron from one in our modern-day Universe? Anything at all?

MS: Ah, this is a subtle point. Let’s set the early universe and cosmic history aside for a moment, and just consider: what would happen to the electron’s mass if somehow today, by magic, the Higgs field were switched off? Well, as long as the Higgs field still exists in the universe, it can combine with the strong nuclear force to give electrons a very tiny mass. Here’s why.

Switching on the Higgs field has a big effect on the cosmic environment, one which changes how elementary particles vibrate and thus what their masses are. But it turns out the strong nuclear force also changes the cosmic environment, to a much lesser degree, at the same moment that it causes neutrons and protons to form. Experts refer to this change in the environment as the “switching on of a quark/anti-quark condensate”. It’s like a mini-Higgs field of its own.

If our usual Higgs field didn’t exist at all, then this wouldn’t matter. Electrons don’t feel the strong nuclear force, so a quark/anti-quark condensate has no direct effect upon them.

But the Higgs field interacts both with quarks and with electrons (which is why, when it is switched on, it gives mass to both of them.) Consequently, even when the Higgs field is switched off, it acts as an intermediary between electrons and quarks, indirectly causing them to interact just a little bit. And this indirect effect, when the quark/anti-quark condensate switches on, causes electrons to pick up a tiny bit of mass.

So it turns out that having a switched-off Higgs field is different from having no Higgs field at all! These kinds of subtle details are part of what makes particle physics both tricky and a lot of fun.

For the early universe, though, this was presumably academic. As the extremely hot universe cooled, the Higgs field switched on first, giving electrons their current mass well before the quark/anti-quark condensate switched on. Consequently, there was never actually a time where electrons only had a tiny mass from the quark/anti-quark condensate. And so this question, while fascinating, is not relevant historically, as far as we know.

Gravitational waves propagate in one direction, alternately expanding and compressing space in mutually perpendicular directions, defined by the gravitational wave’s polarization. Gravitational waves themselves, in a quantum theory of gravity, should be made of individual quanta of the gravitational field: gravitons. While they might spread out evenly across space, they require no medium to travel through, and their amplitude is the key quantity for detectors, not their energy.

ES: There was a point in the book where you brought up a few possibilities about light and gravity: that gravity has a medium (the fabric of space) and light doesn’t, that they both have a medium to travel through and we just don’t know what the “luminiferous aether” actually is, or that there is no medium at all for either of them. And yet, as I read through the rest of your book, it became clear to me that you favor the “both have a medium” possibility, and don’t like the “there is no medium” one as much. Is this an accurate reading of your viewpoint, and if so, would you share with us why you lean that way?

MS: Well, not exactly accurate, as I’d view either possibility as reasonable. I emphasized the point of view that “both have a medium” in the book because I think it is by far the easiest to explain, both to non-experts and to physics students. By contrast, “neither has a medium” is a more speculative perspective, for which the conceptual groundwork is less complete and much more confusing. Explaining it would have taken several additional chapters. More generally, I wanted to avoid speculative ideas in this book.

The idea that both may have a medium — more precisely, that gravity and electromagnetism are properties of a single medium — is an old idea from the 1920s, due to Kaluza and Klein, who were thinking about extra dimensions of space. If gravity reflects the curvature of obvious aspects of space, then perhaps electromagnetism (whose ripples are light waves) might reflect the curvature of less obvious aspects of space, such as extra dimensions that are too small for us to see or detect with current technology. Einstein was very fond of this viewpoint, as I alluded in my answer to your fourth question. It’s a relatively easy notion to visualize, both intuitively and in math, and it’s a common idea in string theory that even non-experts have often heard or read about.  

The possibility that “neither has a medium” is a much more extreme idea, because it asserts that space isn’t a fundamental phenomenon, and is instead a convenient illusion that “emerges” from something more fundamental. This is hard even for physicists to think about. We do have some concrete examples, though. These ideas have a prehistory going back to the 1970s, but they entered the mainstream in 1997, when Juan Maldacena realized that certain physical systems seem to be describable either as a field/particle theory in three spatial dimensions (i.e. four space-time dimensions) or as a string theory — and thus a theory with gravity — in nine spatial dimensions (ten space-time dimensions).

This is almost as crazy as it sounds. From the point of view of someone expecting to think of a physical system using the field theory viewpoint, the fact that this system can also be described using six additional dimensions of space that were not put in at the start, and that this new description has quantum gravity (and strings too), which the field theory description does not have, is mind-blowing. From the point of view of someone who thought they were doing quantum gravity and string theory in ten space-time dimensions, the fact that six of the dimensions and the gravity could be dispensed with altogether, allowing a much more concise description of the system, is equally mind-blowing.

I would rank Maldacena’s discovery as the most important contribution to theoretical physics that string theory has made since 1985. It showed us that the grand empty space of the cosmos, and the gravity which holds us to the Earth, might simply be convenient fictions — fictions that evolution has led us to trust because, well, they are extremely convenient. Today, billions of years after the Big Bang, describing the cosmos as a large three-dimensional space is by far the best way to make sense of it. But perhaps, at the time of the Big Bang or whatever was the universe’s origin, such a viewpoint is useless, as pointless as trying to describe social trends using only individual psychology. Maybe there’s a different description of the universe, one yet to be discovered, which has neither gravity nor space, or has some other form of space with its own gravity very different from the one we take for granted. If we’re ever to understand the universe’s birth, perhaps we’ll first need to find this otherworldly perspective.

The Standard Model particles and their supersymmetric counterparts. Slightly under 50% of these particles have been discovered, and just over 50% have never shown a trace that they exist. Supersymmetry is an idea that hopes to improve on the Standard Model, but it has yet to achieve the all-important step for supplanting the prevailing scientific theory: having its new predictions borne out by experiment.

ES: You point out very clearly that there’s no experimental or observational evidence at all for any of the grand ideas of the late 20th century that take us beyond the Standard Model: extra dimensions, supersymmetry, grand unification, string theory, etc. Many had hoped that the LHC would turn up something along these lines: a dark matter candidate, a supersymmetric particle, a second (or more) Higgs boson, or even extra, unexplained “missing energy” from certain events. Yet when I asked a room full of highly esteemed particle physicists this past March, “how many of you are certain, and would be willing to bet your entire career on it, that supersymmetry exists in nature,” I was shocked to see that about 75% of people’s hands went up. Can I ask you why you think this is still the prevailing position, whether you think it’s justified, and what you think it would take to shift that mindset?

MS: Well, I’m shocked by that percentage too — especially with the “bet your career” stipulation.  I wouldn’t have bet my career, or a month’s salary for that matter, even before the LHC started. While I’d expect the percentage to depend strongly on which class of physicists dominate in the room — US physicists vs. European, older generation vs. younger, string theorists vs. particle physicists, experimenters vs. theorists, etc. — I’d not have expected more than 50% in any of those categories.  

The most famous ideas that you cited don’t seem to have panned out. But personally, I’d still place decent odds that there are unknown particles, ones you didn’t mention, hiding in the LHC’s vast data sets. Certain vocal scientists are far too quick to dismiss that possibility.

As for supersymmetry (“SUSY”) specifically, why does it remain popular? There are several separate reasons to think SUSY might play a role in nature. At best, the LHC has eliminated only one of them: oversimplifying, it’s the idea that SUSY might explain why the masses of the known particles are so small relative to the  “Planck scale” — the mass scale at which gravity would be as strong as the other types of forces. (It remains possible that SUSY is part of the explanation, but if so, some other idea would have to play a role, too.)

Among other reasons that some scientists like SUSY is that it makes it easier to understand why the strengths of all the types of forces seem to become so similar near the Planck scale. It also fills a gap: we know of elementary fields with spin 0, 1/2, 1, and 2, and SUSY would add a special field with spin 3/2.  More technically, SUSY is the only type of symmetry that is consistent with general relativity (Einstein’s theory of gravity) but has not been seen in nature — and so,  from some points of view, it would be puzzling if it were entirely absent.

Then there’s something profound suggested by string theory: it may be that quantum gravity requires SUSY in order for there to be a smooth, long-lived universe. (I’m not betting my career on string theory either! Yet it’s a useful tool, in that it helps us understand what is and isn’t possible concerning gravity.) Personally I consider this argument for SUSY to be the most compelling, but hardly convincing.

These other motivations for SUSY do not require any LHC discoveries; they are based on other considerations. Any individual scientist will weigh these motivations differently, and some might conclude that SUSY is still too good to be false.

As for what could shift the mindset, I think someone would have to show that quantum gravity does not require SUSY… by demonstrating that a non-supersymmetric quantum theory of gravity, or some other type of theory of gravity without SUSY, is mathematically and physically consistent. So far the only completely convincing candidate for quantum gravity is string theory, where SUSY, at least in a hidden form, seems to be essential.

Yet maybe it isn’t essential even there, and we’re just limited by our incomplete mathematical knowledge.  The math of SUSY is relatively easy — another reason that SUSY is popular and useful — whereas the math of non-supersymmetric theories remains poorly understood.  It’s clear that our ignorance about such theories is profound, and that there’s much left for future generations to learn.

Matt Strassler is the author of Waves in an Impossible Sea: How everyday life emerges from the cosmic ocean, which has been selected as Starts With A Bang’s best science book of 2024.