Mark T. Smith
Why I Love Physics

My long love affair with physics began when I was 12 or 13 years old, growing up in a chaotic household in the moist heat of Memphis with three younger brothers constantly at each other’s throats and a jittery mother who sometimes joined in the bedlam and fainted when the shouting reached a certain decibel level, a time when I was first mesmerized and baffled by the opposite sex and further confused by an aunt who drove recklessly in her little MG sports car despite all of our warnings and by other irrational behaviors of friends, teachers, and just about every human being I knew.

At moments, I managed to escape. In a large closet, miraculously bare of the clutter in the rest of the house, I built my own laboratory. I stocked it with test tubes and petri dishes, Bunsen burners, resistors and capacitors, coils of electrical wire of various thicknesses and grades. Among other projects, I began making pendulums by tying a fishing weight to the end of a string. I’d read in Popular Science or some similar magazine that the time for a pendulum to make a complete swing was proportional to the square root of the length of the string. With the help of a stopwatch, I verified this wonderful law. Then, I used it to actually predict the swing time of new pendulums even before I made them. I was amazed and enthralled. Here was air and light and sky. Here, I witnessed firsthand the regularity and order of nature. Beneath the apparent complexity in the physical world, there were simple and dependable rules of behavior. And the rules were beautiful. On a piece of paper, I made a graph of the rule for pendulums. It was a lovely curve. By this age, I knew about mortality and death — my grandfather had died when I was 9. But this law for pendulums seemed immortal. It seemed older than Earth, maybe older than the universe. Was this part of the world? Was this God? This was physics.

Physics is concerned with the primal forces of nature — gravity, electricity and magnetism, the forces that bind particles together within the centers of atoms — and the response of matter to these forces. Physics tries to understand these forces and capture them in mathematical equations, as in the rule for the pendulum.

Physics also is concerned with the nature of time and space, aspects of reality that most of us take for granted. Einstein’s shocking proposal that two identical clocks in motion relative to each other do not tick at the same rate was pure physics.

Physicists want to take apart and take apart until they have reached that which no longer can be divided or split or separated. And then they want to understand that tiny thing completely.

Biologists and chemists work with systems. Physicists work with the elements of systems. Where a biologist might study how potassium atoms enter the outer wall of a nerve cell and start an electrical pulse shuddering through the cell, a physicist would study the forces within an individual potassium atom, the orbits of its electrons and the electricity they generate, and how they affect the electrons in nearby atoms. Physics is the ultimate reductionist science. Physics condenses every physical system to its most essential parts and then tries to fathom and quantify those elemental parts. I once was having lunch with the great chemist Roald Hoffmann — we were at a little Middle Eastern restaurant in Cambridge — and we talked about the difference between his science and mine. Roald was extolling the complexity of chemical systems, the way that different parts interact with each other to produce intricate structures, like the ornate bending and folds of certain large molecules. I, in turn, celebrated the simplicity of physics, its relentless mission to pare down nature to its most fundamental elements. Physicists want to know the very smallest Russian doll. Physicists want to take apart and take apart until they have reached that which no longer can be divided or split or separated. And then they want to understand that tiny thing completely. I loved that purity, that clarity.

I started college at Princeton with the mistaken idea that I wanted to be an engineer. I admire engineers. We need engineers. They make things work. But in my engineering classes, I was taught to memorize formulae and apply them, rather than how to derive those formulae from elemental forces. I was not satisfied. Then, in my sophomore year, a renegade member of the physics faculty named Bill Gerace approached me in the shadows of a lab room and posed the following problem: If you put a frictionless bug on a frictionless clock, starting at the 12 o’clock position, and the bug starts sliding clockwise, at what hour mark does the bug fall off? A well-posed problem. I went back to my dorm room, wrote down the equations to be solved, and came back to Professor Gerace the next morning with the answer. (The angle of fall-off is cos-1(2/3) or about 48 degrees, corresponding to a time of 1:36.) At that point Gerace invited me to join a handful of undergrad physics majors he was quietly mentoring, each of us given our own desk in his sprawling office in the basement of Palmer Laboratory. There was a blackboard, of course. Between our official classes, we members of Gerace’s tiny physics guild taught ourselves relativity, quantum mechanics, and other mysteries and beauties of modern physics. (The most brilliant member of that cadre, Bob Jaffe ’68, has long been a colleague of mine at MIT.)

All of us in that little group loved physics. We felt that we were entering a special and privileged world. We felt we were learning secrets most people didn’t know. And we felt powerful with our new knowledge. We wrote beautiful equations on the blackboard. We discussed mathematical techniques of analysis. We debated the twilight foothills between the known and the unknown — the strange ability of subatomic matter to behave like both particles and waves, the unsolved problem of the nuclear force, the question of whether black holes really existed, the origin of the universe. I have always been philosophically inclined, and one aspect of physics that entranced me in Gerace’s apprentice shop was the implication of physics for the deep and eternal questions. Such as: Do we have free will? Are our actions and thoughts fully predetermined by cause-and-effect relations? Does space go on forever? Is there a smallest nugget of material reality? How did the universe come into being? Why is there something, rather than nothing? (Well, maybe most people don’t ponder this one, but physicists do.)

The answer to the bug-fall-off question was, of course, previously known to science. I experienced a thrill in working on this problem and getting the right answer, but that experience paled beside the thrill of my first original research problem as a graduate student in physics, at work on a problem whose answer was unknown. This was the early 1970s. The first black hole, Cygnus X-1, recently had been discovered, and a new generation of graduate students was rushing off to do thesis research on the behavior of black holes and their surroundings in space. I had set to work on studying (with pencil and paper, mathematics, and computers) the evolution of a disk of gas swirling around a black hole — a situation thought to be common if black holes really existed and also thought to reveal properties of the associated black hole. Despite expectations from other scientists, my calculations showed that such a gaseous disk would be highly chaotic. Instead of remaining quiet, the gas would seethe and flare, sending out sporadic bursts of X-rays. Another theoretical physicist, Doug Eardley, two years my senior, joined me in this discovery and added mathematical smarts that I lacked. I was exhilarated. At a modest level, Doug and I had contributed to scientific knowledge. We had found something true about nature that previously was unknown by human beings.

I have not made any momentous discoveries in my career as a theoretical physicist. But each new research problem has brought a fresh challenge. In each problem, I’ve had the pleasure of confronting a locked house and scheming for weeks and months to find the secret door in, of visualizing the physical situation and then representing it mathematically — with clean and elegant and pristine equations — and then figuring out how to solve those equations. And always the constant admiration of the magnificent cathedrals of modern physics, relativity and quantum mechanics, whose mystery and beauty continue to amaze me and other physicists.

I remember a moment years ago with my 10-year-old daughter when I held a prism in front of the window. She exclaimed at the rainbow of colors cast on the opposite wall. “What made that?” she said with delight.

Alan Lightman, a novelist, essayist, physicist, and educator, is Professor of the Practice of the Humanities at the Massachusetts Institute of Technology. His book Screening Room was named one of the best books of 2015 by The Washington Post.