This series explores an anomaly CERN scientists announced last December at the Large Hadron Collider (LHC), where protons are smashed together very close to the speed of light. My first installment explained how two detectors observed results at odds with predictions of the Standard Model. In the jargon of the field, they found a “diphoton excess at 750 GeV.” (My first piece explains what that means.)
This might be a very big deal. The Standard Model, which has withstood all experimental challenges for forty years, is our best theory of the fundamental particles that make up the matter and forces we know about. If the anomaly holds up, we will have come face to face with the Standard Model’s limitations.
But that’s a big “if.” The results are too preliminary for us to say anything for sure right now. Fortunately, CERN restarted the LHC experiments this month and is expected to make another announcement this summer. The new data may show that the anomaly was just statistical noise, but whatever happens, there is much to be learned from these efforts to probe the edges of our understanding. We may learn something about Nature, or we may learn that the existing theory has survived yet another test. In either case, by following how science gets done you can see why it is so exciting—the process as well as the results.
In the lead up to this summer’s announcement, I will take you through our present understanding of particle physics: the Standard Model, the Higgs boson, and why we suspect there is something beyond the Standard Model for the LHC to find. To do that, I need to give you a way to picture how the Universe works at these incredibly small scales. This second installment lays the foundation by exploring the basic language of particle physics. That language is called quantum field theory, but it is not so much a specific theory as the framework for all our fundamental theories of Nature, both the well tested (quantum electrodynamics and quantum chromodynamics, which are parts of the Standard Model) and the more speculative (supersymmetry and quantum gravity).
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The quantum world is strange. On small scales, particles can act like waves, objects can tunnel straight through solid walls, and the more you know about where something is, the less you know how fast it is going. Thanks to these bizarre phenomena, the word “quantum” is often synonymous with “magic” in films, books, and popular culture. (This attitude sadly abused by many charlatans who wish to wrap themselves in the trappings of science.) It is not magic, of course, but it is genuinely hard to understand. It was the great Danish physicist and Nobel Laureate Niels Bohr who said, “Anyone who is not shocked by quantum theory has not understood it.” There are indeed fundamental questions about the behavior of physics at the smallest scales that we do not have satisfying answers to—yet.
Quantum field theory is the basic language of the most accurate physical theory yet devised.
However, our understanding of the quantum world has grown enormously since Bohr’s time. Indeed, there are important differences between the quantum mechanics developed in the early twentieth century and the quantum field theory I will talk about here. We still use the former in many situations, not only in physics but also engineering: your computer relies on the “magic” of quantum mechanics for its existence. But the latter is a deeper, more fundamental theory, what you get when you require quantum mechanics to abide by the rules of Einstein’s theory of special relativity—the description of what happens when things are moving near the speed of light. Special relativity is the theory that gives us the most famous equation in history, E = mc2, which tells us that mass can be converted to energy and vice versa. As a result, any theory that includes special relativity must be able to deal with the creation or destruction of particles. Quantum mechanics could not accommodate this possibility; quantum field theory can.
When you use the rules of quantum field theory to make calculations, it is truly incredible. It has enabled us to formulate the most precise scientific theory ever devised, capable of making predictions to one part in a trillion. I will try to explain some of those rules, to give a picture of how I think about things and their interactions at these incredibly small scales (and very high speeds). For example, how should you picture what’s going on when a physicist says “at the LHC, we collided two protons traveling close to the speed of light and created a Higgs boson; it lasted for roughly 10−22 seconds before decaying into some photons”?
I will evade the philosophical questions that lurk at the heart of the quantum world: questions about the reality of the wavefunction, hidden variables, or multiverses. In my day-to-day working life, I take the attitude that is sometimes summed up as “shut up and calculate.” The philosophical questions are interesting and important, but I do not need to answer them to carry out my research. From this vantage point, I will give you a sense of how I view the quantum world: how particles move and how they interact with each other. Of course this won’t give you a deep working knowledge of quantum field theory, but my goal is to convey some useful analogies for making sense of what goes on at the LHC.
Fields and Quantum Fields
The fundamental problem we face when we try to visualize quantum behavior is how to describe what things are. Consider electrons, photons, and quarks. Are they particles or waves? We think of particles as pointlike things that have a definite location—small billiard balls, say. Waves, by contrast, ripple out through space and exhibit behavior such as diffraction and interference.
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