The closer quarks get, the weaker they pull

Lex Fridman opens by asking Frank Wilczek, a theoretical physicist at MIT and a Nobel laureate for the co-discovery of asymptotic freedom, what the most beautiful idea in physics is. Wilczek does not name an equation. He says the beauty is that we can write a compact description of the world that is both precise and, at the level of its operating system, remarkably complete. Physicists get uneasy, he adds, when that description disagrees with observation even at the level of a part in a billion.

Fridman quotes Einstein's line that the most incomprehensible thing about the universe is that it is comprehensible. Wilczek treats that surprise as the deepest result of the seventeenth-century scientific revolution, with Newtonian physics as the shining example: you could aspire to completeness, precision, and a concise account of how the world runs. The miracle has kept getting better. There are smaller miracles too, he says, especially the way high symmetry governs the form of the equations, and the mind-expanding structure of quantum mechanics.

Is comprehensibility a fact or a hope? Wilczek calls it a fact. He points to rising per-capita wealth after the scientific revolution, to instruments that sense tiny vibrations and infer colliding black holes far away, and to laws tested to a part in a billion. Pencil-and-paper abstractions, now also computer calculations, have to agree with magnets, accelerators, and detectors, or physicists get very upset. That agreement, he argues, is evidence that we already understand nature at a profound level.

He is careful about what "the universe is comprehensible" actually means. Dark matter is an observed source of mass that nobody can name. Knowing the basic equations is not the same as solving them: chemistry is a standing reminder that molecular design is, in practice, extremely hard. Much of what we care about is historical, the path from the early universe to people, frogs, and societies, and the fundamental laws only lay a foundation. The laws themselves advertise their limits. They tell you what happens from a starting point, not what the starting point should be, and chaos means tiny differences in initial conditions explode. If you want an apple pie, Wilczek says, you are much better off starting with apples than with quarks.

Wilczek's earlier book *A Beautiful Question* asks whether the world embodies beautiful ideas. His high-level answer runs through symmetry. Plato already imagined atoms built from the five perfectly symmetrical solids, a guess that mathematical symmetry should govern the world. Modern physics has out-Platoed Plato, Wilczek says: the symmetries we now use are far more extensive, and they are the raw material of our working theory of matter.

The same theme shows up in human art, from the tiles of the Alhambra to wallpaper. That is not a coincidence in his telling. Humans evolved to interact with the world advantageously and to learn from it, so we enjoy symmetry because the world has it. Fridman mentions Dostoevsky's claim that beauty will save the world. Wilczek will not go that far, but he does say beauty is a tremendous source of insight. At the shortest distances, among particles whose properties appear only at high energy, everyday life gives you almost nothing, and experiments are hard. You cannot follow a purely Baconian, data-first path. You guess, and the guesses are aesthetic: what would be a nice description consistent with what we know? Then you try it. In many profound cases, it works.

There is a humbler source of symmetry as well. Simple viruses often build protein coats as Platonic solids because they are, as Wilczek puts it, really dumb: they know how to make a pentagon, then another, glued the same way, and a highly symmetrical object appears. Fractal broccoli is the same recipe. The secret is stupidity, doing one simple thing over and over. Brains are not exempt. Starting from a single cell, the same developmental program, with modules switched on and off, repeats until you get the regular, intricate drawings of the neuroanatomist Santiago Ramón y Cajal. Repeated units, moved from place to place, look more or less the same. That is symmetry as a method of construction.

In *Fundamentals: Ten Keys to Reality*, Wilczek stresses that space and time are very big. Take a broad enough view, he jokes, and the history of the universe is simpler than the history of Sweden, because your standards are lower. The quantitative picture is stark. The universe was much hotter, denser, and more uniform about 13.8 billion years ago; that is the Big Bang, and matter has been expanding and cooling ever since. Map those 13.8 billion years linearly onto one calendar year and the Big Bang is January 1 at midnight. Dinosaurs emerge around Christmas. The extinction that let mammals, and eventually humans, inherit the Earth falls on December 30. All of human history is a small part of the last day, and a single lifetime is infinitesimal inside that span.

Space tells a similar story. A convenient size for the observable universe is how far light can travel in 13.8 billion years: 13.8 billion light-years. Earth is a fraction of a light-second by comparison. Psychological intuition cannot hold those distances. You need exponential notation and abstract concepts. Wilczek hastens to add that this does not make us small. Looking at those cellular drawings of the brain, you see vast numbers of processing units. He tried to estimate how many thoughts a person can have in a lifetime, a fuzzy question he is proud of having defined fairly precisely, and the answer is billions of meaningful thoughts.

Several independent clocks point to the same order of magnitude. Movies are snapshots; brains integrate a frame, then take the next, and invent continuity afterward. If the flicker is too slow you see the snapshots. The crossover is about 40 per second. Use that as a visual processing rate and you reach several billion thoughts in a life. Skilled piano playing, video games, and speech land within a couple of orders of magnitude of the same unit. Wilczek will not argue 2 billion versus 1.8 billion; any reasonable estimate, he says, sits between a hundred million and a hundred billion. Fridman jokes that 99 percent of those thoughts are food, sex, or the internet. Wilczek's reply is that there is more to life than winning Nobel Prizes.

Once thoughts interact, the way particles do, a combinatorial explosion takes over. Possibilities expand exponentially with the number of things you consider, and they rapidly outstrip those billions. Chess is still beyond complete human comprehension; the best players now lose consistently to computers. Large classes of problems become intractable as they scale. The world is inexhaustible in that sense, which is part of its beauty: simple, powerful laws can compress combinatorial complexity. Wilczek thinks we can find general principles of the operating system that let us control things and ask deep questions, and that we will never have a theory of everything in any meaningful sense, because everything, truly everything, will not sit still.

The common picture of the Big Bang as a bomb exploding in empty space is, Wilczek says, a misconception. As we understand it, the event is space itself. Run the expansion backward and matter everywhere comes together into a very hot, very dense, yet very homogeneous plasma of elementary particles: quarks and antiquarks, gluons, photons, electrons and antielectrons, all of it, far hotter than the surface of the Sun. Take the equations as they stand and the temperature heads toward infinity; then the equations break down into infinity equals infinity. That is a singularity. We do not have a sensible idea of what happened before the Big Bang. Different equations would be needed for the earliest moments, and we do not really know why things started that way, even though a great deal of evidence says they did.

We cannot visit and we cannot run controlled experiments. Most of the record is heavily processed. Subtle techniques and powerful instruments are required to recover what has survived. What they reveal is a period when everything we can look at was much hotter and denser, still obeying the laws of physics as we know them today, in equilibrium plus small quantum fluctuations. Run that forward and, in broad strokes, you get the universe around us. Wilczek is optimistic that we will both measure more and figure out more, including times much earlier than we can observe now.

Gravitational waves are the most tangible prospect, because they interact so weakly with ordinary matter that they send a minimally processed signal from the Big Bang. The signal is faint after traveling a long way, but people are gearing up to look. LIGO, he says, is an awe-inspiring engineering achievement, the most sensitive and precise device on Earth. Early-universe waves would probably have much longer wavelengths than LIGO can sense. A contemplated project would put lasers at solar-system-scale separations, artificial planets or moons, and watch tiny relative motions as a signal of radiation from the Big Bang. Another path is an imprint of those waves on the microwave background, our present window on the earliest universe, though those photons have been processed much more strongly by matter.

A sensor the size of the solar system would be, for Wilczek, a pinnacle artifact of human endeavor. Building it would demand an understanding of motion, of the interference of light, of lasers, of quantum theory, and of timing, knowledge from the very small to the very large, all in the service of curiosity and on a grand scale. The fact that we can design such things and they work is, he adds, an extraordinary demonstration that we really do understand a lot, and that better answers let us ask more ambitious questions.

Here we are, Fridman notes, hairless apes with microphones, and Sweden is harder to describe than the cosmos. How did complexity arise from a homogeneous early universe? Wilczek says we understand in broad outline how it *could* happen, not why it happened exactly this way, and that the origins of life and the inevitability of intelligence remain open. Early on the universe was quite homogeneous but not completely. There were fluctuations of about a part in 10,000 in the density of the primordial plasma. Gravity then does what gravity does: denser regions attract more matter, get denser still, and matter clumps.

Once many things clump, the equations get complicated. Physicists know the laws and, to a certain extent, have to wave their hands. Chemistry and radiation say that as matter clumps it can radiate, lose energy, slow down, cool, and form stars and planets. In broad terms there is no mystery; that is what the equations tell you should happen. Applying the laws of individual units to these systems is computationally delicate, and more profoundly the equations are chaotic: tiny differences in the initial state can lead to enormous differences later. At some point fundamental physics says, chemists, biologists, this is your problem.

Complex structure is conceivable given the right temperature and the right stuff. You need chemical bonds that are reasonably stable, so complex structures can exist, and bonds that can still be broken a little with energetic injections from the Sun. Carbon is a piece of luck: it makes backbones and elaborate branchings, which is biochemistry, with both change and a useful degree of stability. Physics can tell you it is conceivable. If you want to know what can really happen you go to chemistry; if you want to know what actually happened you consult the fossil record and biologists. The languages differ and the questions differ. They are not inconsistent. They are complementary.

Fridman offers those early fluctuations as our earliest ancestors. Wilczek agrees, calling it a modern version of the Hindu philosophers' *tat tvam asi*, "that art thou." Ask what those little quantum fluctuations in the early universe were, and they are the seeds out of which complexity, including, plausibly, humans, evolved. You do not need anything else.

Asked about other intelligent civilizations, Wilczek says we do not know, but he has a strong suspicion the answer is yes, based on the one sample we have. Earth formed roughly 4.5 billion years ago with a useful temperature, a long-lived star, and liquid water. Once the planet cooled from a boiling mass into a solid world with water, relics of what we call life appeared very quickly, almost as old as the Earth itself. General conditions for life, he thinks, may be enough to make it happen relatively fast, and those conditions are probably satisfied in many places in the universe, even within our galaxy.

Intelligent life is a different issue. It took a long time and looks contingent. For most of the history of life, things were single-celled; multicellular life rose only about 600 million years ago. Intelligence is a luxury. Many more creatures have big stomachs than big brains, and most have no brains at all in any reasonable sense. Dinosaurs ruled for a long time; some were pretty smart, but at best bird-brained, since birds came from dinosaurs, and it could have stayed that way. Humans are a very recent, very contingent development. Wilczek is much less sure about technological civilizations. It is conceivable to him that we are the only example in the galaxy. He has different opinions on different days of the week. He also worries, in the spirit of humility, that our particular kind of intelligence is not very special, and that there could be many kinds of life.

Fridman has just spoken with Sarah Walker about defining life from a physics perspective. Wilczek thinks "what is life?" is a dangerous question, because the answer cannot be more precise than the question, and the question assumes we already have a definition and a clean natural kind. There are edge cases such as viruses, and some people would even say electrons have consciousness. Fuzzy concepts do not yield precise science. An adjacent question has been more fruitful: what states of matter are possible? Solids, liquids, and gases barely scratch the surface of macroscopically distinct patterns, or phases. New states keep appearing. Liquid crystals are important ingredients of cell membranes. In the last decade or so, physicists have been talking about states with dynamics in time as well as space, time crystals, a subject now flourishing.

Is there a state, or a group of states, that corresponds to life? Maybe, Wilczek says, but the answer cannot be more definite than the question. Fridman pushes back: a question can point in a direction, and the answer can become more precise after the fact, once macroscopic patterns are found and someone decides to hang the word "life" on them. Wilczek agrees that is not a disagreement. Science often refines common words into technical terms, as it did with force, energy, and symmetry. If we can identify linked states of matter that carry on self-reproduction, development, and information processing, we might be tempted to classify them as life.

Consciousness, Wilczek says, is a very imprecise word, loaded with connotations you do not want at the start of a scientific analysis. Science often begins with simple cases and works up. The everyday picture, a rich inner life that seems absent from the equations, is a great question, and he says he is gearing up to spend part of coming years on it. He is much more comfortable studying self-awareness, which strips away the mystical aura. In simple form it is contiguous with feedback: a system that looks at its own state and responds is already a kind of self-awareness. More sophisticated versions appear in computers that inspect their internal state, and in recurrent neural nets, which are hard to understand and sit at a frontier.

Clarifying what you have to know, what you have to not know, when you know that you do not know, and when you think you know what you do not, would, he thinks, illuminate consciousness in a way that scratching your chin and talking about qualia never will. Fridman offers a robotics back door: engineer systems that exhibit qualities of consciousness without first understanding how they work, and enter through psychology. Wilczek says they are on the same wavelength. He adds that he is morally certain that recurrent nets, robotic design, and advanced computers will show the value of looking at internal state in a structured, encapsulated way, seeing highly processed and selective information, and making choices without knowing how they are made. There will also be an unconscious. That architecture, he suspects, is essential to efficient information processing, which is why it evolved.

Brains come at a high cost. They are rare in evolution, have high metabolic demands, require an active, warm-blooded lifestyle, and compete with digestion. They have to pay back. Wilczek thinks a lot of the value is in social interaction. Fridman is spending the rest of that day with Boston Dynamics' legged robots, and his passion is human-robot interaction. A display of consciousness looks useful there, and there is a gray area between the display and the thing itself. Whatever consciousness turns out to be, Wilczek thinks addressing it through its use, working up from simple cases and from engineering experience with efficient computation and social interaction, will shed light that abstract musing never would.

Sarah Walker worries that we may never fully understand a system we are inside of, that the observer is also the observed. Wilczek grants limitations on how much information we can process. He also points to silicon friends, to instruments that make up for perceptual deficits, and to using different concepts for different questions. A creature living on a 2D plane can still form the idea of three dimensions; we have even produced four-dimensional spacetime. He does not see a show-stopper. His working example is quantum chromodynamics, QCD, the theory of the strong interaction that he helped discover: how quarks and gluons make protons, neutrons, and related particles, one of the four basic forces as presently understood.

The equations are beautiful and, in special high-energy circumstances at accelerators, thoroughly tested. People try to knock them down and cannot. The situations in which you can calculate their consequences are still very limited. No one has demonstrated analytically, in a way a human can understand, that a theory built on quarks and gluons, which you do not observe freely, actually produces the protons and neutrons you do observe. That is the problem of confinement. Take the same equations to gigantic computers, though, and by God you get the world. Machines can do calculations no human will ever do in the same way. When natural abilities run out, Wilczek prefers workarounds, concepts, instruments, or both, to getting defeatist. He sees no logical contradiction that would halt the process.

He has elevated the habit into a principle, complementarity, following Niels Bohr: you need different ways of thinking even about the same things in order to do justice to their reality. Prescribe drugs for a person, or ask what happens if they move very fast or are exposed to radiation, and you need physical thinking about materials. Predict how they will behave in a social situation and you need psychology. There is nothing wrong with that. In *Fundamentals* complementarity is the culminating chapter, less a fact about the world than an attitude encouraged by the facts. It can be appropriate, useful, and unavoidable to use very different descriptions of the same object, even mutually uninterpretable ones, that are both correct and sources of different insight.

Its most rigorous form is a theorem, if quantum mechanics is correct. An electron's primary description is its wave function, which can predict where it will be found or how it is moving, but you process the wave function one way for position and another for momentum, and those ways are mathematically incompatible. Wilczek's image is a stone you can sculpt into the Venus de Milo or into David, but not both. Heisenberg's uncertainty principle is the physical correlate: you cannot have position and momentum determined at once. Einstein was uncomfortable with complementarity; the Bohr-Einstein debates, in a broad sense, were about whether this feature of quantum mechanics is permanent. So far quantum mechanics keeps winning, and even the Einstein-Podolsky-Rosen challenge turned into a confirmation.

Outside quantum mechanics, complementarity is less a theorem than a way of thinking that, Wilczek says, avoids unnecessary quarreling. Free will and determinism are his favorite pair. He thinks it is a fundamental truth that free will is both an illusion and not. The whole hierarchy of the world looks weird in that particular way, which is humbling: we may never be on the sturdy ground humans like. Physics keeps teaching humility, from Copernicus, Earth is not the center, to Darwin, humans are not the pinnacle of creation, to the apparent result that mind emerges from matter without special life forces or souls. He finds complementarity liberating. He recalls Dr. Johnson, hearing a preacher say "I accept the universe," answering that he damn well better. Accepting the universe is mind-expanding. Demanding only one way to think is limiting.

Tied to self-awareness, the picture he expects is a God's-eye view that sees everything going on in a brain or a computer, which can be deterministic, and the self's view, aware of only a very small part, which experiences free will. He is pretty sure that is how it will work out. Free will is a concept he definitely experiences, and he trusts other people do too; it is essential in psychology and law. At the same time he thinks mind emerges from matter, and that there is an alternative description of matter that, up to subtleties of quantum mechanics he does not think are relevant here, really is deterministic.

Fridman then asks the Platonic question: what is the smallest thing in the universe? As far as we know, Wilczek says, the fundamental particles in our most successful description are points. They have no internal structure. Operationally they obey equations for singular concentrations of energy, momentum, and angular momentum localized at individual points. That mathematical structure is only partly revealed in the world, because reading a precise position out of a wave function takes a lot of energy, an idealization of applying an infinite amount. At the mathematical level, we still build the world out of point particles. Do they exist? Yes. Quarks exist, electrons exist, photons exist.

Existence, precisely, means the equations contain reproducible entities that exist in vast numbers, with definite mass, spin, and a few other properties. We call those entities electrons, and the physical world is full of exemplars. Electrons can be isolated and shown to be identical, which is why chemistry works. Photons, the units of light, can now be studied one by one. Quarks and gluons are harder: they do not appear as particles you can isolate. A proton is, roughly, three quarks and a lot of gluons. At high energies they can propagate relatively freely for a short time, leave tracks, and then get recaptured into protons and mesons. Predicting that behavior is what Wilczek's Nobel Prize was for. Gluons cannot be bottled like electrons, but they can be used theoretically as building blocks and liberated briefly at accelerators.

Asymptotic freedom is, to Fridman, a weird inversion of everyday forces: the closer you are, the stronger the effect, except for quarks, where the strong interaction weakens until they act like free particles. A proper description, Wilczek warns, has to bring in quantum mechanics and relativity and more patience than a podcast easily supplies. Relativity matters because short distances mean large momenta, and large momenta mean large energy, so short-distance behavior and high-energy behavior are tied together. Quantum mechanics says that to analyze short distances you need probes carrying a lot of momentum, which is why you cannot determine position and momentum at once. There is also the vacuum's habit of spontaneously making quarks and gluons for short times, fluctuating into and out of existence.

Everyday life does know forces that increase with distance and go flabby at short range: rubber bands. What is hard is seeing how that could be a *fundamental* force consistent with everything else we know. Asymptotic freedom says there is a very particular kind of force, involving gluons with very special properties, that does this. When Wilczek, David Gross, and David Politzer did the work, experiments already hinted that quarks and gluons behaved that way, but there were no equations. They found the equations, showed they were basically unique, and produced a complete theory of the strong interaction: quantum chromodynamics. Quarks and gluons interact very weakly when close, which, through relativity, is the same as interacting very weakly at high energies, where the simplicity of the fundamental interaction is revealed.

At the time the clues were subtle. Twenty years later, at what were then high-energy accelerators, you could almost see the short tracks. Indirect clues had been enough to make the behavior a prediction rather than a postdiction. Early on, big conferences had sessions on testing QCD. Nowadays the same comparisons, far more precise, are called calculating backgrounds: people take the theory for granted and hunt for deviations, which would be the new discoveries. The cutting edge becomes the foundation; the foundation becomes boring.

Quarks and gluons have, broadly, the same relation to nuclear physics that electrons and photons have to atoms and molecules. The dynamical fun in chemistry is electrons moving and exchanging photons; nuclei supply positive charge and most of the mass and, being heavy, hardly move. Protons and neutrons are like atoms made of quark clouds held together by gluons, and residual forces between them make nuclei the way residual forces between atoms make molecules. QCD is a precise, highly symmetrical theory in the same family as electrodynamics. Wilczek likes to say QCD is QED on steroids: eight gluons instead of one photon, three color charges instead of one electric charge. Asymptotic freedom is a blessing for cosmology, because things get simpler at high energy. In the early universe, nuclei dissolved into quarks and gluons moving fast and interacting in relatively simple ways. That opened the early universe to scientific calculation. The universe, he says, was born free.

C is charge conjugation, swapping every particle for its antiparticle. P is parity, a mirror universe. In the mid-twentieth century both turned out not to be exact; the right experiments could tell the mirror from the real thing, and the exceptions taught a great deal about neutrinos and the weak interaction. T is time reversal, running the movie backward. There are very good reasons to think CPT together is an accurate symmetry, on the same level as relativity and quantum mechanics, which makes CP equivalent to T. What is observed is that T is not quite accurate either. At the level of basic interactions, a movie of atoms and quarks would look almost the same run backward, but not exactly. You would need subtle high-energy experiments to see the difference. Jim Cronin and Val Fitch discovered that in the mid-1960s.

For centuries the precise laws had this almost embarrassing extra property: they look the same run backward, and life is not like that. Understanding how grossly time-asymmetric behavior in large objects emerges from nearly reversible equations is an old problem in statistical mechanics and thermodynamics, and exploring the boundary, including exceptions such as time crystals, is now an exciting frontier. Almost true is more disturbing than exactly true. An exact, beautiful regularity can be accepted as a fundamental law. Approximately true but not exactly is weird. Relativity, quantum mechanics, and gauge symmetry so constrain the possible interactions that there are basically only two ways to violate time-reversal symmetry. One of them occurs. It explains Cronin-Fitch and later experiments, has made successful predictions, and might be connected to the matter-antimatter asymmetry of the present universe. The other kind does not occur. That leftover puzzle is the strong CP problem.

The promising proposal, a kind of theory of evolution, is that the strength of the missing interaction is not a constant of nature but a field, something that depends on space and time. Done in a nice, symmetrical way suggested by the theory we already have, the field evolves from the early universe and settles to a value that makes the laws very nearly symmetric under time reversal. It might appear as a constant, but it is a field that evolved. Examined in detail, it has not quite settled to exactly zero. The material that fills space is so rigid that even small leftover motion involves lots of energy, and energy in a moving field means particles. Those particles are axions.

Calculate how much energy sits in the residual oscillations of this axion gas filling the universe, and you get just the right amount, with the right properties, to be the dark matter astronomers want. The theory was not designed for that; when you work out the equations, that is what you get, which Wilczek takes as a good sign. He was a voice in the wilderness for a long time; the idea is now popular, maybe even dominant. There have been false alarms. He does not find any claim of detection convincing. Axions interact extremely weakly, so you have to invent new kinds of antennas, and if it were easy it would already have been done. He thinks there is good hope of reaching the required sensitivity in coming years, or maybe decades, and of testing whether this story of dark matter is right.

Crystals are orderly arrangements of atoms in space. Cool many materials gently and they form spontaneously, ending with less symmetry than the equations that produced them: the basic laws are homogeneous under small translations, and a crystal's atoms sit in particular places. Time crystals are the same idea in time, orderly behavior that certain states of matter arrange themselves into if you treat them gently and let them do what they want, repeating indefinitely. There can be time liquids, and space-time crystals whose pattern repeats only if each step of time is paired with a step in space. Structure in time looks a lot like motion, and indefinite repetition looks a lot like perpetual motion. Thermodynamics, Wilczek notes, is not a fundamental law; it is deduced under limited circumstances. Time crystals are an edge case. They are not a free lunch. The motion is already what the system wants to do, the lowest-energy configuration, so you cannot extract spare energy from it. Intervene and you change it only by injecting energy.

A narrower "theory of everything," unifying general relativity and quantum field theory, would be aesthetically pleasing. Useful? Probably not, at least in the foreseeable future. It might help us understand black holes, which is not useful either. The questions existing theory cannot answer concern very small black holes, where quantum effects and Hawking radiation matter. For astronomical black holes the predicted radiation is so tiny it is unobservable, overwhelmed by other effects, a prediction that has never been tested. Existing theory is perfectly adequate for technology and for almost all astrophysics and cosmology, with the notable exception of the extremely early universe. Fridman cites Eric Weinstein's hope that a source code for reality would give shortcuts for becoming a multi-planetary species. Wilczek cannot say it will not happen, but he can say that the huge steps of the 1970s and early 1980s, QCD, a better understanding of the weak interaction and of quantum mechanics, had minimal impact on rocket design or any technology. Accessing more esoteric physics takes exotic conditions, expensive accelerators, and few particles. He thinks it is a pipe dream.

Space exploration, for him, is more a problem of biology and information than of new laws. Human bodies are not well adapted to space; even Mars would be extremely difficult, expensive, and unstable. If bringing civilization off Earth can mean sending minds we can converse with, actuators we can manipulate, and sensors that send feedback, that is far more realistic, and he thinks it is the long-term future. Hauling bodies around is silly. Cyborgs, human brains or brain-like cells in shells adapted to space, and remote outposts we can communicate with, those he thinks will happen. Physics in the larger sense, analyzing complex situations, new states of matter, clever instruments, has a lot to contribute. Looking for another quark, or even axions, or a so-called theory of everything, does not have much to do with it.

His advice to a young person is, first, to read *Fundamentals*. Then three complementary habits. Cast a wide net: look around for what is promising and what catches your imagination, and do not confuse those; as an undergraduate he intended to study how mind emerges from matter and decided the timing was not ripe for the mathematical thinking he enjoys. Read history and the masters, Einstein in the original, Feynman's lectures, Darwin, Galileo, and biographies that bring great minds down to earth, including Bertrand Russell and H. G. Wells. Get the basics under control as soon as possible: calculus, multivariable calculus, complex variables, group theory, computing, and for experimenters electronics, optics, and instruments, until they are a native language. On mortality he rejects the word afraid. He thinks in the machine-learning trade-off of exploration versus exploitation, and at fifty, in an office under Freeman Dyson, he was congratulated on being liberated because no one expects much of a fifty-year-old theorist. He no longer has to track the latest technicalities of particle physics or string theory; he explores toward machine learning and exploits what he already knows, including antennas to detect axions. The meaning of the unearned gift is interacting with people he loves, family, a wide circle of friends, and institutions that might survive him. It is a positive feedback loop he did not earn and does not understand, except that he has a dopamine system and is happy to use it. All of that, he notes, started from little fluctuations shortly after the Big Bang. Conveying the mind expansion he has enjoyed by interacting with physical reality is one of his missions in life.