(Image: Hal Bergman/Getty Images)
Some of the world鈥檚 greatest physicists couldn鈥檛 tell you why our leading theory of everything is labelled 鈥淢鈥 Is there an answer?
I ARRIVED at the Harvard Science Center and made my way to the fifth floor. But when I got there, a flood of students and professors came pouring down the hallway. 鈥淚s the Witten talk in there?鈥 I asked a student emerging from the room where the lecture was scheduled to take place. 鈥淭oo many people,鈥 he said. 鈥淲e鈥檙e changing rooms.鈥
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I was here at Harvard, looking for physicist Ed Witten, thanks to a conversation I鈥檇 had 17 years earlier in a Chinese restaurant. My father had leaned across the table and asked, 鈥淗ow would you define nothing?鈥 He was trying to understand how a universe could be born of nothingness, and he enlisted me 鈥 his 15-year-old daughter who was failing mathematics and who had opted out of physics 鈥 in his search to uncover the nature of reality. It was a quest that had us reading everything we could get our hands on, talking our way into physics conferences, driving through the Los Alamos desert, poring over fragile manuscripts in Philadelphia and, most recently, taking an epic road trip down the California coast where we met some of the most brilliant physicists alive.
And so we had arrived at M-theory: the best hope for a theory of everything that would stitch together space, time, matter and forces into a single comprehensible universe. We had learned so much about it 鈥 its radical revision of the nature of space-time, its ability to equate drastically different worlds, and its suspiciously vacant ontology. But there was one thing about M-theory we still didn鈥檛 know: what on earth did the M stand for?
Magical mystery tour
Every book and article I consulted gave the same answer: nobody knows. Perhaps it鈥檚 magic, they said, or mother. Nobel prizewinner Steven Weinberg guessed that it stood for matrix. Another Nobelist, Sheldon Glashow, wondered if it was an upside-down W, for Witten. Even Stephen Hawking wrote, 鈥淣o one seems to know what the M stands for, but it may be 鈥master鈥, 鈥miracle鈥 or 鈥mystery鈥.鈥
How can nobody know? I thought. Witten made it up and the guy鈥檚 still alive. Why doesn鈥檛 someone just ask him?
M-theory was born at another Witten talk, nearly two decades earlier. At the time, physicists had hit a wall with their best hope for uniting quantum mechanics and general relativity: string theory. The theory showed that when you zoom in closely enough on an elementary particle like an electron or a top quark, you find that it is actually a tiny string wiggling around in 10 dimensions. Different particles play like musical notes on a vibrating string. But physicists had five equally consistent but fundamentally different string theories 鈥 and when it comes to theories of everything, no one wants five.
In a seminar room at the University of Southern California in 1995, a 44-year-old Witten stood in the glow of an overhead projector. 鈥淲hat I鈥檓 going to try to convince you of today is that there鈥檚 really only one string theory,鈥 he told the audience. 鈥淭hat these five theories are really different manifestations of the same thing.鈥
Through a maze of mathematics Witten showed that the five theories 鈥 each of which contained different elementary objects (ranging from particles and strings to membranes that exist in many dimensions), different strengths of interaction and different mathematical properties 鈥 were all just different ways of looking at one and the same reality. What was that reality? It remained a mystery, and Witten refused to speculate.
Whatever that ultimate reality was, it had the properties of a funhouse mirror, distorting, reflecting and refracting the world. What looked big from one perspective looked small from another, high energy could look like low energy, 11 dimensions like 10, particles like strings and strings like membranes. No description was more valid than any other. There was no basic ingredient, no invariant truth lurking behind appearances, just reflections of reflections. Physicists called it the second superstring revolution. Witten called it M-theory.
Now, at Harvard, I followed the crowd to the new classroom and tried to snag a seat, but it quickly became apparent that the bigger room wasn鈥檛 big enough. Whoever had organised this thing had forgotten that Witten was a full-blown superstar. Soon we were off again, this time headed to another building with a larger auditorium. I rushed to keep up with the ever-growing crowd as it made its way outside, people pushing and shoving one another in the race to score a seat.
I had never seen physicists get so rowdy. As a group, they aren鈥檛 considered the most aggressive or athletic bunch, but with a Witten lecture on the line, they were ready to sprint and tackle anyone who got in their way. Running alongside them, I noticed a blind man in the midst of the stampede, waving his white stick and trying to keep his footing as the other physicists pushed past. I momentarily slowed down and considered helping him manoeuvre through the crowd. But he would probably rather do it on his own, I told myself, before charging, elbows out and full speed ahead, towards the auditorium door. M is for mayhem.
鈥淭he crowd was ready to sprint and tackle anyone who got in its way鈥
I body-checked my way into a seat, and Witten鈥檚 lecture finally began. He is unnervingly tall and broad-shouldered, with a large rectangular head that had inspired my father to refer to him as 鈥淓d the Head鈥. He is widely considered the smartest man alive. But despite his looming presence and intellect, Witten spoke in a soft, whispery, high-pitched voice, an incongruity that lends him an air of other-worldliness. M is for mother ship?
Actually, this was the second time I had seen Witten. The first was many years earlier, in 2003, when the held a meeting in Philadelphia. I had scored myself a press pass and my father had snuck in to join me. We were taking an escalator down to the ground floor when we noticed Witten standing a few steps ahead of us. We watched as he stepped off the escalator and up to a revolving door. He attempted to push the door clockwise, and when it didn鈥檛 budge, he inexplicably continued pushing it for several seconds before deciding to turn around and try pushing it the other way. My father and I had looked at each other. We were thinking the same thing: the smartest man alive is just like the rest of us.
Now I watched as he scribbled incomprehensible equations on the blackboard, his slacks covered in chalk. Everyone seemed thoroughly engaged. But I had no idea what Witten was talking about. It wasn鈥檛 just the subtleties of his argument that were lost on me, nor the mathematics that threw me off 鈥 I couldn鈥檛 even figure out what the topic was. M is for mediocre.
When the talk ended, I approached Witten, introduced myself, and asked if he might have some time to talk with me while he was in town. He paused. What seemed like minutes passed. He paused for so long that I began to wonder if I should just walk away, and I was about to when he said, 鈥淚 can do it at my hotel, the Inn at Harvard, tomorrow after I鈥檝e finished my breakfast at 8.30 am.鈥 I nodded, and before I could say another word, he turned to talk with someone else.
The following morning, I made my way into the Inn鈥檚 dining room, which sat beneath a vaulted glass atrium, bathed in the early morning glow. I spotted Witten and awkwardly joined him at his table, seriously hoping that he remembered the 10-second conversation we had had the day before and that he didn鈥檛 think I was some weirdo off the street hoping to polish off his yogurt.
It was more than a little intimidating, talking to Witten in person. It was obvious he had no interest in small talk and that I should jump right in with the questions. 鈥淲hat on earth were you talking about yesterday?鈥 seemed like a bad way to start. But what was a good opener? What exactly does one say to the smartest man alive? M is for mute.
鈥淲hat are you working on these days?鈥 It was the best I could come up with.
鈥淚鈥檓 working on an application of physics ideas to math, particularly to understand knots better,鈥 he said in that surreal whisper.
Knots? Was that what yesterday鈥檚 lecture had been about?
We discussed knots for a bit before moving on. I was eager to get his perspective on M-theory鈥檚 elusive ontology. M is for missing.
鈥淚nitially, when people talked about string theory, they said OK, point particles are really just strings,鈥 I said. 鈥淭hen with the second revolution, with M-theory, we found there were not just strings but 鈥榖ranes鈥 of every dimension. And now we see that strings are equivalent to particles again in certain situations. Is there a fundamental entity that everything is made of?鈥
M-theory, Witten said, would contradict that idea, because in each description a different ingredient of the theory is fundamental and other things are derived. 鈥淭here are fundamental ideas rather than fundamental physical objects,鈥 he said.
Fundamental ideas rather than fundamental physical objects. M is for mind-dependence?
My mind flashed back to the revolving door. 鈥淲hen you are doing everyday things 鈥 say, going to the grocery store or the dry cleaner鈥檚 鈥 are you thinking in 11 dimensions?鈥 I asked. It seemed like a plausible explanation.
鈥淪ometimes I鈥檓 able to continue thinking that way in my daily life and come up with key ideas while I鈥檓 doing an errand or something,鈥 he replied. 鈥淭wo of my key ideas came while I was riding on an airplane.鈥
I smiled. Then I asked the question.
鈥淲hat does the M in M-theory stand for?鈥
鈥淚 didn鈥檛 mean to confuse everyone with that,鈥 he said. 鈥淭he way I described it at the time was that the M stood for magic, mystery or membrane, according to taste. But I thought my colleagues would understand that it was really for membrane. Unfortunately, it got people confused.鈥
So there it was. M is for membrane. Magical mystery solved. All that confusion just because nobody got the joke. To be fair, they probably hadn鈥檛 heard Witten crack a joke before. But still.
鈥淎ll that confusion just because nobody got a joke made two decades earlier鈥
This article appeared in print under the headline 鈥淢 is for鈥︹
