How to Make an Apple Pie from Scratch

by Harry Cliff

  Fortunately, the LHC is brilliant at making beauty quarks. Since protons are made of quarks bound together by the gluon fields, when you smack them into each other you tend to get lots of quarks. In a year, the LHC will create billions of beauty quarks and antiquarks inside LHCb, whose design has been specifically honed to study them.

  Since these decays are really, really rare, it took a while for LHCb to collect enough collisions to be able to make precise enough measurements. However, every year, billions more beauty quarks were produced, and more and more these rare decays were spotted. At first, the results all seemed to agree with the standard model fairly well, but as the precision improved, hints of small deviations started to appear.

  The first big clue came in 2014, when a team at LHCb compared how often a beauty quark decays into a strange quark, a muon, and an antimuon to the equivalent decay but swapping the muons for electrons. As far as the forces of the standard model are concerned, the electron and its heavier cousins the muon and the tau are completely identical, the only difference being that the muon is 200 times heavier and the tau a whopping 3,500 times heavier than the electron. The fact that the forces treat these three leptons the same is known as “lepton universality,” and it’s a hard rule of the standard model. Lepton universality means you’d expect the beauty quark to decay to muons just as often as it decays to electrons.

  But that isn’t what the team found.

  Instead, it seemed that the muon decay was only happening 75 percent as often as the electron one, as if beauty particles preferred to decay into electrons. That said, the uncertainty on the measurement was pretty big, about 10 percent, so there was a good chance that it was just a random fluctuation like the bump that had fooled everyone at ATLAS and CMS in 2015. However, a couple of years later another measurement using a separate data sample found a very similar effect. This time, the muon decay was only happening around 69 percent as often as the electron one, and what’s more, the uncertainty was smaller.

  It was at this point that the theory community started paying attention. As signs of new particles faded away at ATLAS and CMS, something seemed to be emerging from the data at LHCb. Further measurements of different beauty decays involving taus (the heaviest copy of the electron) saw similar effects. Meanwhile, thousands of miles away, the BaBar experiment in California and the Belle experiment in Japan had also seen hints of beauty decays breaking the sacred rule of lepton universality. On their own, none of these deviations are significant enough to declare that the standard model had finally been broken, but as more and more anomalies have cropped up a coherent picture could be starting to emerge.

  At the start of spring 2019, I met up with theorist Ben Allanach in his office at the Department of Applied Mathematics and Theoretical Physics. Ben is a supersymmetry expert, having spent his entire career working on various models and helping his experimental colleagues come up with new ways of searching for sparticles at the LHC. However, the slew of negative results from ATLAS and CMS has turned him away from the subject, for now at least.

  “A lot of people got a little depressed, particularly those of us who did supersymmetry for a long time. There were a plethora of different reactions and some people are still going strong, but I think a lot of people have got turned off it.”

  As far as Ben is concerned, the anomalies in beauty decays are where the action is: “They’re the best hope we have at the moment, I definitely think it’s exciting.” The big question on everyone’s minds now is, are these anomalies the real deal? After all, we’ve been fooled by unlucky statistical flukes before. Ben doesn’t think that’s likely in this case: “There are just too many of them for it to be a fluctuation. There’s something going on.” The bigger fear is that these anomalies could be due to some misunderstood effect, either in the theory of how quarks behave or something we’ve gotten wrong in our experiments. While huge care is taken to try to account for every possible effect that might bias your result, these big particle detectors are unbelievably complicated and there’s always a chance that you could miss something.

  “If you were a betting man,” I asked, “where would you put your money?”

  Ben paused for a moment and turned to look out of the window. “Well, you’d really want another experiment to independently confirm it…”

  “But if I pushed you.”

  “I’d take just about evens on it being real new physics—which is really high. It’s the best thing I’ve seen in my career.”

  Since moving away from supersymmetry, Ben has started taking a different approach to solving problems. Instead of working on grand theories based on a single elegant principle that solves lots of problems at once, he’s now listening carefully to what the data are saying and trying to build understanding from the bottom up. So if these anomalies are real, and that’s a big if, what could be causing them?

  “There are basically two camps. Either it’s something called a Z prime—a new force field—or a leptoquark.” These are essentially new quantum fields that are interfering with how the beauty quarks decay. A Z prime would be a force field much like the Z boson of the weak force, except one that breaks lepton universality, for instance by pulling more strongly on muons than electrons. A leptoquark, on the other hand, would be a rather more exotic beast.

  One of the great mysteries of the standard model is why it contains twelve matter particles—six quarks and six leptons—and why they appear in three copies or generations. The electron, up quark, and down quark that make up our apple pie sit in the first generation, with additional, heavier, unstable copies of these particles in the second and third generation. The patterns in these matter particles echo the patterns in the periodic table of the chemical elements that Mendeleev drew up in the nineteenth century. In the case of the chemical elements, those patterns pointed to a deeper structure, which was eventually revealed to be the quantum structure of atoms. Could the matter particles of the standard model be hinting at something similar?

  A leptoquark would be a new particle that could decay to both leptons and quarks at the same time, acting as bridge between these two different and apparently unrelated types of matter particles. If such a thing existed, it could be the first part of a jigsaw puzzle that could eventually reveal the ultimate origins of the matter particles that make up our universe.

  That would be a huge deal, arguably the biggest discovery in particle physics since the standard model was first written down. When the anomalies started to build up, Ben and his colleagues began conservatively by just adding an extra quantum field to the standard model to see if you could explain all the anomalies at the same time. Now they are starting on the harder project of trying to figure out whether these new quantum fields fit into a bigger, more elegant structure.

  Despite the LHC finding no evidence of supersymmetry, Ben still thinks that something must explain the fine-tuning of the Higgs field. “It’s like dropping a pencil on a table and it landing exactly nib down and standing upright. The fact that it’s staying up is telling us something.” Amazingly, one of the theories they’ve been studying to explain the beauty anomalies may also do the job that supersymmetry has failed to do, namely to stabilize the Higgs field and prevent the universe from collapsing into an uninhabitable wasteland.

  The explanation for both effects may be that the Higgs boson is not a fundamental particle but a mixture of other new fundamental quantum fields. The reason the Higgs was thought to be so sensitive to fluctuations in the vacuum, like the kite in the hurricane, is that it has zero spin. However, if it’s made of other fields stuck together whose spins just add up to zero, then it would no longer be affected by those nasty vacuum fluctuations. What’s more, the new quantum fields that make up the Higgs might also explain the patterns in the matter particles of the standard model.

  We’re at a turning point in our understanding of the ingredients of the universe, a
moment of anxiety and crisis, excitement and opportunity. No one knows whether these anomalies are real, whether they’ll get stronger or gradually fade away. But whatever happens, nature is speaking to us. Of course, we all hope that these anomalies are real, because if they are, we will have finally peeled back another layer of reality and seen the first signs of what lies beyond the standard model. It would also be great for experimental physicists like me, the beginning of a new era of exploration that could be even more exciting than the heady days of the 1960s and 70s when the basic building blocks of nature were being discovered.

  But if the worst happens and these anomalies melt away, we will nonetheless have learned something profound. If in 2035, when the LHC powers down for good, we have still found nothing apart from the Higgs and the nightmare scenario is realized in its full horror, it may be the crisis needed to trigger a rethink of our approach to fundamental physics. It will be clear that we do not understand something deep about the nature of quantum fields, the vacuum, possibly gravity too. Because if we want to go right back to the moment the universe began, to the “b” of the big bang, we are going to need a complete picture that describes all three together. It is only then, as Carl Sagan put it, that we’ll be able to invent the universe.

  Skip Notes

  *1 You might think that neutrinos could fit the bill, but they’re too light and zip around the cosmos too fast to match the dark matter data.

  *2 This is closely connected to another big problem in fundamental physics—the so-called cosmological constant problem. The energy of these vacuum fluctuations should cause space to expand so rapidly that no stars or galaxies could form. Why this terrifying quantity of vacuum energy didn’t rip the universe to shreds is one of the greatest mysteries in physics.

  *3 Urban legend holds that the ATLAS experiment once set up a group to look at ways of reducing the number of meetings, which only exacerbated the problem when it started holding regular meetings.

  *4 The site is arXiv.org—an online repository where scientific articles are uploaded before they’ve been peer-reviewed or published in a scientific journal.

  *5 Exactly the same thing occurs when a neutron decays into a proton. This happens via the W boson field, even though the W boson particle is more than eighty times heavier than a neutron and therefore way too heavy to be created directly in the decay.

  CHAPTER 13

  Invent the Universe

  It’s time to face facts: we are still a long way from knowing how to make an apple pie from scratch. Although there are lots of promising ideas out there and we are continually learning more from experiments and observations, we don’t yet know how the particles in our apple pie ultimately survived the big bang, and we can’t explain why the Higgs field settled at just the weirdly specific value that makes the existence of atoms possible. We don’t know what dark matter is, and without dark matter’s gravitational influence ordinary matter would never have clumped together in large enough quantities to form galaxies, stars, and planets, and you need planets and stars to grow apples.

  Even putting these mysteries aside, we aren’t sure whether we might be missing other quantum fields beyond the ones in the standard model. We can’t even really explain why our universe contains the quantum fields it does, or whether the quantum fields we do know about might be made of even more fundamental ingredients. And these are just a handful of the questions that we know we don’t have answers to—or, to borrow a phrase from former U.S. secretary of defense Donald Rumsfeld, the “known unknowns.” There are almost certainly a whole load of unknown unknowns as well, questions that are so far beyond our ken that we haven’t even thought to ask them yet. In other words, we have a hell of a lot still to learn.

  So we don’t yet know how to make an apple pie from scratch, but perhaps an even bigger question is, will we ever be able to find out? Over the course of this book, we’ve seen how thousands of women and men, chemists, physicists, and astronomers, experimenters and theorists, technicians and machine builders, engineers and computer scientists, working together over hundreds of years, have gradually dismantled matter into its most basic components and tracked their origins out into the cosmos, through the hearts of dying stars and eventually all the way back to a trillionth of a second after the big bang. The fact that we can tell so much of this story is one of humankind’s greatest achievements. The question is, how much further does this story go, and can we ever really discover a complete description of how the universe began?

  To try to make this question a little more concrete, let’s first consider what the ultimate apple pie recipe would need to be like in order to qualify as starting “from scratch.” To explain where the matter in our apple pie ultimately came from, we would need a theory that could describe what happened at time zero, the moment the universe began, or as Carl Sagan put it, we need a theory that invents the universe.

  Modern fundamental physics stands on two theoretical pillars: quantum field theory describing the microworld of atoms and particles; and general relativity, the theory of the force of gravity, which sculpts the universe at vast scales. While dazzlingly successful in their own domains—and to be clear no experiment or observation is in conflict with either theory—it is clear that both will fail us as we approach the moment of the big bang.

  The reason is actually pretty simple when you come down to it: quantum field theory ignores gravity and general relativity ignores quantum mechanics. This is absolutely fine for almost every situation that either theory might normally be asked to explain. On the one hand, since gravity is a trillion trillion trillion times weaker than electromagnetism, when you’re doing experiments down at the level of particles, the gravitational forces are utterly negligible compared to the much more powerful influence of the three quantum forces. On the other hand, if you’re an astrophysicist or cosmologist working at the scale of stars, galaxies, or the entire universe, then (except in one very important case that we’ll get to shortly) there’s no need to bother about piddly little quantum effects down at the subatomic level.

  However, at the moment of the big bang, the whole cosmos was subatomic. Literally everything—energy and fields, space and time—was compressed into an infinitesimally tiny point, far, far smaller than an atom. Under these unimaginably extreme conditions both gravity and quantum mechanics would have ruled the universe together. To describe this first moment, particle physics and cosmology, quantum field theory and general relativity, must merge into a unified quantum theory of gravity.

  Finding a quantum theory of gravity has been regarded as the holy grail of theoretical physics for almost a century. Generations of physicists have worked at the problem, and while several potential candidate theories have been found—string theory, loop quantum gravity, causal dynamical triangulations, and asymptotically safe gravity, to name a few—no one knows whether any of them actually describe the real world.

  Nonetheless, if we could find such a theory, we would at least have the language needed to describe the moment the universe began. But in its most ambitious form, the ultimate recipe would go even further than this. Not only would it be a quantum theory of gravity and be able to describe the birth of the universe, it would also explain why the universe contains the basic set of ingredients it does and why they are the way they are. It would explain, for example, why there are six quarks and six leptons, why they have the masses and charges they do, and why there are three quantum forces and why they are as strong as they are. It would explain the strength of the Higgs field, what dark matter is, and how matter got made in the big bang. In other words, it would be what physicists often call a “theory of everything.”

  This is the kind of superambitious ultimate theory that Steven Weinberg, one of the architects of the standard model, described in his book Dreams of a Final Theory, published in 1992. Weinberg’s vision was of a theory founded on a principle of such beauty and power that
it would account for all the apparently arbitrary features of the quantum world without the need to put anything in by hand. This theory would be unique, elegant, and so rigid that any attempts to modify it would make the whole thing collapse. It would in some sense be inevitable, a final explanation that needs no further explaining.

  Now that is an incredibly high bar to reach. Nonetheless, when Weinberg wrote Dreams in the early 1990s, there was a sense among some theorists that such a theory was starting to reveal itself—or as Weinberg put it, “We think we are beginning to catch glimpses of the outlines of a final theory.”

  Weinberg was referring to string theory, which for the past forty years has been by far and away the most popular approach to finding a theory of quantum gravity, and which seemed like it might also be the unique theory of everything.

  THE ULTIMATE THEORY

  On a leafy suburban street on the outskirts of Princeton, New Jersey, stands what appears from the road to be a relatively modest white clapboard house with a small neatly kept garden out front. A painted wooden sign leaning against the steps to the porch warns in large weary letters that this is a “Private Residence,” in what I suspect is a futile attempt to keep curious tourists from peering in through the windows.

  This is the house where Albert Einstein lived for the final twenty years of his life, having left Germany for the last time in 1932 to escape persecution by the Nazis. Visitors to 112 Mercer Street would usually find the aging, wild-haired Einstein in his study, dressed comfortably in a soft sweater and surrounded by papers covered in algebraic symbols. George Gamow, who was an occasional caller in the late 1940s, recalled glimpsing these papers during their conversations, and though Einstein seemed as sharp as ever, he would never bring up what he was working on.