Fundamentals

  The reason we need to bring in the Higgs condensate is that in the world we observe, the W and Z bosons, unlike gluons or photons, have nonvanishing mass. To consummate the analogy and get similarly beautiful equations, we must bring in a medium, to slow them down.

  This medium-based theory of the weak force took shape over the 1960s. Over the 1970s, experimental evidence in its favor began to accumulate, and eventually it got overwhelming. But one big question remained unanswered: What is that all-important, ubiquitous medium—the Higgs condensate—made out of?

  People gave many speculative answers to that question. Some postulated that it is made out of several different particles, and they invoked new forces or even new dimensions of space. But the simplest, most radically conservative possibility was to make it from a single new particle—the Higgs particle. It became important to check whether Nature uses this simplest option.

  How We See It

  If the Higgs condensate is made from just one ingredient, then we can say a lot about that ingredient. Roughly speaking, if the Higgs particle is a chunk of the condensate, the only question is how big a chunk. Thus, all the properties and behaviors of the Higgs particle can be predicted, once you know its mass. This welcome specificity meant that experimenters could plan their Higgs-hunting strategy with quite definite ideas about what they were looking for, and how they would recognize it if they found it.

  In order to “discover the Higgs particle,” you must do two things: You must produce some of them and you must get evidence of their fleeting existence. Both steps are challenging. To produce heavy elementary particles, you must concentrate a lot of energy into a very small volume. This is done at high-energy accelerators, where beams of rapidly moving protons (or other particles*) are made to collide with target materials, or with one another. In the years prior to 2012, Higgs particle searches were mounted with a succession of ever-higher energy concentrations, but they came up empty. We know now, in retrospect, that they simply didn’t bring in enough energy. The Large Hadron Collider, or LHC, finally did.

  The home of the LHC is a circular underground tunnel measuring about twenty-seven kilometers (seventeen miles) around, beneath a rural area straddling France and Switzerland. When the LHC is operating, two narrow beams of protons traverse the tunnel in opposite directions within a pipe that threads it. Moving at nearly the speed of light, the protons make eleven thousand orbits per second.

  At four points the beams cross. Only a small fraction of the protons collide, but this still amounts to nearly a billion collisions per second. All that firepower produces the concentrations of energy it takes to make Higgs particles.

  The next task is to detect them. Enormous, densely instrumented detectors surround the crossing points. One of them, the ATLAS detector, is more than twice as large as the Parthenon. The detectors track the energies, charges, and masses of the particles that emerge from the collisions, as well as their directions of motion. They feed all this information, at the rate of 25 million gigabytes per year, to a worldwide grid that links thousands of supercomputers.

  All that information gathering is necessary because:

  The events are complicated. Typically, ten or more particles stream out from each one.

  Few of the events—less than one in a billion—ever contained Higgs particles.

  Those events that do contain them, don’t contain them for long. The lifetime of a Higgs particle is about 10−22 seconds, or a tenth of a trillionth of a billionth of a second.

  The rare events that briefly contained Higgs particles also contain a lot of other stuff.

  In short, if you’re going to find the Higgs particle, you have to understand and monitor the rest of what’s happening very well, indeed—and you’ve got to latch on to some nearly unmistakable consequence of a Higgs particle’s brief existence. Otherwise you’ll get inundated with false positives.

  The discovery of the Higgs particle was announced on July 4, 2012. The signal was an excess of high-energy photon pairs. Such pairs were predicted to arise from Higgs particle decays, and the excess swamped any other plausible source.* Since then, several other signals, arising out of other ways that Higgs particles can decay, have been detected as well. So far, the rates at which all these signals have occurred agree with theoretical predictions.

  In “seeing” the Higgs particle, we humans expanded our perception. We peered into a behavior that Nature reveals only rarely, and for very short times, and only after vigorous prodding. To perceptive human minds, empty space will never look empty again. Fish Newton, and Peter Higgs, nailed it.

  GRAVITATIONAL WAVES

  Why We Look, and What We Look For

  Let’s recall that John Wheeler, the poet of general relativity, summed it up this way: “Space-time tells matter how to move; matter tells space-time how to bend.” Wheeler’s summary is catchy, but it is misleading, or at least incomplete, without this important addition: Space-time is a form of matter, too.

  Specifically, it is wrong to think that the curvature of space-time is entirely dictated by something else—that is, “matter.” Bending space-time requires energy, and energy causes space-time to bend. In this way, curvature participates in its own creation. Space-time, in short, has a life of its own.

  We’ve heard this song before. The crowning triumph of Faraday’s field concept, and more specifically of the Maxwell equations that express his concept mathematically, was the discovery of electromagnetic waves. In such waves, the electromagnetic field takes on a life of its own. Changing electric fields create changing magnetic fields, which create changing electric fields, and so on, indefinitely. A self-sustaining disturbance in the fields moves through space. If the disturbance repeats at an appropriate wavelength, we will see it as light. We’ve also learned to “see” other wavelengths, using detectors designed for the purpose, such as radio receivers or microwave dishes.

  In a similar way, Einstein’s curvature field, which encodes gravity, can also support self-sustaining disturbances. These are called gravitational waves. In gravitational waves, bending of space-time in some directions causes bending in others.

  The equations for gravitational waves very much resemble those that govern electromagnetic waves—with different interpretations of the symbols, of course.* The kinds of sources that trigger the waves are different: Moving electric charges radiate electromagnetic waves, while moving masses radiate gravitational waves.

  Despite their qualitative resemblance, there is a big quantitative difference between electromagnetic and gravitational waves. This quantitative difference arises because, according to general relativity, space-time is extremely stiff. Because of this stiffness, even rapid motions involving large amounts of mass produce only tiny wiggles in space-time. That is both good news and bad news.

  The good news is that when we detect gravitational waves, they bear messages from some of the wildest, most interesting events in the universe, in which big things go whipping about. Gravitational waves give us a new way to perceive the universe that is especially attuned to such events.

  The Laser Interferometer Gravitational-Wave Observatory, or LIGO, was designed with a few spectacular sources in mind. These include blasts from the moments when systems of two black holes, or two neutron stars, or one of each, that have been orbiting each other at last spiral in and merge. As they lose energy to gravitational radiation, the orbits of those systems decay. The decay is slow and gradual until the last few moments, when things move especially fast. It is only then that detectable bursts of radiation are produced.

  The bad news is that gravitational waves are difficult to detect.

  How We See It

  The basic concept that eventually matured into LIGO was contained in a paper published by Rainer Weiss in 1967. To reach the sensitivity necessary to detect gravitational waves, many technological innovations were required. The first successful observat
ion of gravitational waves came almost fifty years later. Weiss, together with Kip Thorne and Barry Barish, received the Nobel Prize in 2017 for their work on LIGO.

  To envision how LIGO detected gravitational waves, imagine three objects at the vertices of a big (imaginary) L. To keep things simple, let’s assume that they’re floating in space. As a gravitational wave passes, space itself is distorted, so that the distances among the three objects change with time. If we have a way to compare the lengths of the L’s arms, we can look for that effect. It provides a signal for gravitational waves.

  Some rough calculations, however, give discouraging estimates for the size of the effect. The fractional change in the lengths is 10−21, or a part in a billion of a trillionth. This seemed, to most physicists, impossible to detect. But Rainer Weiss and friends brought in new ideas and tricks. They used mirrors for their reference objects. They kept the mirrors far apart* and bounced light beams back and forth many times across each arm. This repeated passage of light in effect magnified the lengths of the arms. A standard technique—interferometry—allows you to compare the lengths of light paths to within a fraction of a wavelength. Putting it all together, the tiny ratio of light’s wavelength to the magnified arm length can get you to 10−21.

  These tricks build you a detector that is exquisitely sensitive to the relative motions of the mirrors. The next challenge is to separate motion caused by gravitational waves from all the other things that might change the inter-mirror distances.

  There are lots of things to worry about, of course. The planning documents and discovery papers of the LIGO group go into great depth and detail about the precautions they take and the consistency checks they perform. Here I will mention only one of the most serious. Vibrations of the Earth on which the experiment rests, due to anything from low-grade earthquakes to bad weather to passing trucks, are unavoidable. To suppress the effects of such vibrations, the mirrors are suspended on a quadruple pendulum and stabilized by active feedback. These are marvels of engineering, which take the art of shock absorption and noise cancellation to new levels.

  On the other side of the ledger, vibrations due to gravitational waves are predicted to have some special characteristics, which aid in positive identification. The most basic is that they must excite two separate detectors, in two locations, with matching but offset patterns of motion that are consistent with a disturbance that travels at the speed of light. In more detail, the theory of black hole and neutron star mergers predicts how the vibrations should look, as a function of time, if they are caused by gravitational waves from those sources.

  The first successful detection of gravitational waves took place on September 18, 2015. This detection matched predictions for the burst of radiation from a merger of two black holes, with masses roughly twenty to thirty times that of our Sun, about 1.3 billion light-years away.

  Since then about fifty more events have been detected. An especially interesting one occurred on August 17, 2017. This matched predictions for the merger of two neutron stars. Alerted to this event, astronomers also observed it in several parts of the electromagnetic spectrum, including a gamma ray burst and a lingering visible afterglow. This inaugurated a new kind of “multi-messenger” astronomy, which promises to enrich our perception of strange, faraway events.

  THE FUTURE OF PERCEPTION

  Distributed Sensoria

  listen: there’s a hell

  of a good universe next door; let’s go

  —e. e. cummings

  The “phantom hand” illusion is a startling experience. In it, you hide your right hand behind a partition and look at a fake rubber hand near it. A friend taps and strokes both your unseen real hand and its visible facsimile in a random but synchronous way. After a brief interval—typically less than a minute—you will experience the taps and strokes as originating from the rubber hand rather than your own. Diane Rogers-Ramachandran and Vilayanur Ramachandran, pioneers in the study of this and related illusions, called attention to its profound implications:

  All of us go through life making certain assumptions about our existence. . . . But one premise that seems to be beyond question is that you are anchored in your body. Yet given a few seconds of the right kind of stimulation, even this axiomatic foundation of your being is temporarily forsaken.

  A few years ago, for an hour or so, I was in two places at once. I was sitting at home in Cambridge, Massachusetts, and at the same time attending a conference in Gothenburg, Sweden. I got that way through a full-body version of the phantom hand illusion. I saw and heard the world through the “eyes” and “ears” of a robot whose gaze and attention I controlled remotely, using a joystick. I could also “walk around” and talk with people, while they saw my facial expressions displayed on a screen that formed part of robotic me. I gave a short talk, pacing the stage and picking up on the audience’s reactions, joined in a panel discussion, and mingled at coffee breaks.

  At first, as I was learning how to navigate the system, I was acutely aware of the artificiality of the situation. But after a half hour or so, as the mechanics became second nature and no longer required conscious direction, I felt as if I really were in Gothenburg. Yet I remained aware, at the back of my mind, that I was also in Cambridge, sitting in front of a computer screen. My consciousness had expanded—my robot had extended my self.

  The system I was using was crude. No one would mistake the ProBeam platform for a human body, any more than they’d mistake a rubber hand for flesh and blood. Yet it led me to a compelling experience. In the future, more richly endowed platforms and, at the other end, more immersive virtual reality feedback will support sensoria that are widely distributed in space yet deeply integrated within our minds.

  Quantum Perception and Self-Perception

  I think I can safely say that nobody understands quantum mechanics.

  —Richard Feynman

  I consider that I understand an equation when I can predict the properties of its solutions, without actually solving it.

  —Paul Dirac

  Natural human perception is a poor fit to quantum mechanics. In the quantum world, many possible arrangements and behaviors coexist. If you look, you’ll see just one of them—and you can’t tell in advance which one. No single set of perceptions (that is, observations) can do full justice to the state of a quantum system.*

  The crowning achievement of natural human perception, by contrast, is to give us a representation of the world in terms of objects with more or less predictable properties occupying more or less definite positions in three-dimensional space. That’s very useful information for navigating everyday life, and we extract it effortlessly. But fundamental understanding reveals that there’s plenty more to see, and quantum mechanics takes it to another level.

  Fortunately, there are ways, as yet little explored, that we can retrofit the quantum world to human perception. If we can compute an interesting state—say, the state of the quarks and gluons in a proton, or of the electrons and nuclei in a molecule, or of the qubits in a quantum computer—then we can also compute how our observations of those things would have turned out, as many times as we like, as if we had made them. Then we can present the results as “normal” perceptions, on many displays, all presented in parallel. In this way, physicists, chemists, and tourists could immerse themselves in the quantum world, and maybe finally come to understand it.

  Know thyself.

  —Inscription at the temple of Apollo, Delphi

  An oddly parallel issue arises in our self-perception. Many things are happening simultaneously within our brains, but our natural consciousness only allows us to attend to one at a time, and much is hidden from it altogether. You can switch attention from one working module to another, but it’s difficult and unnatural to focus simultaneously on more than one.* As our ability to monitor and interpret brain states improve, it will be possible to present our inner selves to ou
r perceiving self through our visual system, on displays, bypassing the filter of natural consciousness. More will come through, and less will be hidden. People will come to know themselves, and perhaps others, in new ways, and more deeply.

  9

  MYSTERIES REMAIN

  The most beautiful thing we can experience is the mysterious. It is the source of all true art and science. He to whom the emotion is a stranger, who can no longer pause to wonder and stand wrapped in awe, is as good as dead—his eyes are closed.

  —Albert Einstein

  Although we understand a lot about how the world works, there are still big mysteries. These three great questions came up earlier:

  What triggered the big bang? Could it happen again?

  Are there meaningful patterns hidden in the apparent sprawl of fundamental particles and forces?

  How, concretely, does mind emerge from matter? (Or does it?)

  Here we will focus our exploration on two big mysteries that are more sharply focused. They are at the cutting edge of research aimed at deepening our fundamental understanding of the physical world. The first mystery surrounds a strange feature of fundamental laws. They work almost, but not quite exactly, the same if you run time backward. The second mystery emerged from a confounding discovery. Astronomers have encountered, in a variety of situations, what appear to be gravitational forces that have no visible cause. Their observations, on the face of it, seem to indicate the existence of a “dark side,” consisting of two new forms of matter, “dark matter” and “dark energy,” which have somehow escaped previous notice, despite providing most of the mass in the universe.