Quantum Crosswords: My TED@NYC Talk

The following is the approximate text of the talk I gave at TED@NYC last night. Approximate, because I’m somewhat prone to ad-libbing when speaking, and may have changed a few things here and there. I don’t really know, because I’m scheduling this post on Tuesday morning, before the actual event, using the draft text I’ve been rehearsing with. But this will give you something to read while I drive back home from The City, and I can provide a more detailed recap later.

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I’m going to tell you the most amazing thing I know, which is this: everything in the universe, from light, to electrons, to atoms— behaves like both a particle and a wave at the same time. The cool quantum stuff I teach my dog— Schrödinger’s cat God playing dice, spooky action at a distance—all starts from the fact that everything in the universe has both particle and wave nature.

But when we look around us, we see waves in water and particles of rock, and they’re nothing alike. So, what bizarre thought process could make physicists think they’re the same? The answer is surprisingly familiar. We were led to the dual nature of the universe through the same process you follow to solve a crossword puzzle.

I don’t mean dictionaries full of words sorted by length. I’m talking about the puzzle as a whole—those theme clues that run all the way across the grid, and contain a five-word phrase or a dreadful pun. You can’t look those up, and you won’t guess them. Instead, you piece them together a letter at a time from the simpler clues that cross them. If all of those other crossing words fit together in a satisfying way, you can be confident that you’ve also got the right the theme answer.

The theme clue that led to the duality of everything is “How does light interact with matter?” The first person to seriously suggest the dual nature of light was Albert Einstein in 1905, picking up an earlier idea from Max Planck. Planck had explained the light emitted by hot objects using a desperate trick: he said that hot objects emit light only in discrete chunks, units of energy that depend on the frequency of the light.

Planck was never happy with this, but Einstein ran with it. He applied Planck’s quantum idea to the light itself, and said that light, which everyone knew was a wave, was also a stream of photons, each carrying a discrete amount of energy. Einstein himself called this the only truly revolutionary thing he did—but it worked brilliantly to explain how light knocks electrons out of metals. Even people who hated the idea had to admit that he had a point.

That’s three letters toward the theme clue about light and matter. The next step came from Ernest Rutherford, who bounced alpha particles off gold and showed that the mass of an atom is concentrated in a tiny nucleus. The cartoon atom you learn in grade school, with electrons orbiting the nucleus like a tiny solar system—that’s Rutherford’s.

Rutherford’s atom has one problem: it can’t work. Classical physics says that electrons whipping around in circles emit light—we use this all the time to generate x-rays and radio waves. Rutherford’s atom should spit out a huge burst of x-rays in the brief instant before the electron spirals down to crash into the nucleus.

But Niels Bohr, working with Rutherford, pointed out that atoms manifestly exist, so maybe the rules need to change. Bohr proposed that in certain special orbits, electrons sit happily, not emitting light. An atom absorbs or emits light only when an electron moves between orbits, and the frequency of the light depends on the energy difference through Planck’s rule.

Bohr’s quantum atom fixes Rutherford’s problem, and also explains the specific frequencies of light emitted by atoms. Each element has its own set of allowed orbits, leading to its own unique set of frequencies. But again, there’s a tiny difficulty: there’s no reason those special orbits should be special. The final piece came from Louis de Broglie, who brought the whole thing full circle. If light, which everyone knew was a wave, behaves like a particle, he suggested that an electron, which everyone knew was a particle, should behave like a wave. And if electrons are waves, then you can easily explain Bohr’s rule for picking the allowed orbits.

It’s a bizarre idea, but it’s also the inevitable result of putting clues together. And once you have the idea of electrons as waves, you can look for just that. By the late 1920’s scientists in the US and UK had seen electrons behaving like waves. These days, we have a beautiful demonstration of this, sending one electron at a time into a barrier with slits cut into it. Each electron is detected at a specific time in a specific place, like a particle, but as we add more and more and more electrons, all the individual electrons trace out a pattern characteristic of waves. Richard Feynman famously said that this illustrates the “central mystery” of quantum physics. Once you know that particles behave like waves and vice versa, everything else falls into place, bit by bit, like letters on a crossword grid.

I tell this story not just because the physics is awesome, but because it illustrates the nature of science. One of the frustrating things about being a scientist is that people are terrified of the subject. From outside, science seems alien, beyond the capabilities of ordinary people. “I could never understand that,” they say, “My brain just doesn’t work like that.”

This is flattering to the vanity of nerds like me, but it’s just not true. Science isn’t esoteric knowledge, it’s a process for figuring the world out. You look at the world, you think of why it might work that way, you test your theory with experiments and observations, and you tell everyone the result of the test.

This is a process you use every day, often just for fun. It’s what you do when you work a crossword puzzle: you look at a clue, think of an answer, and test to see if your guess fits with all the others. And if it does, you write it in ink, and leave it where people can see it.

That process of scientific thinking is used by every human who’s ever lived. It lets you piece together the answers to questions you can’t get right away. Sometimes those turn out to be dreadful puns fitting a puzzle’s theme. And sometimes, they turn out to be quantum mechanics.

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Notes: Yes, I know that this compresses and simplifies the history quite a bit. They only gave me six minutes to work with, so some compression was required. Earlier drafts included more information (and mentioned Marsden and Geiger, who actually did Rutherford’s experiment), but they had to be cut out for time.

It also implicitly pushes a very particular interpretation of quantum mechanics, emphasizing wave-particle duality as the root of everything. This is more or less justifiable in a historical sense, though it’s not the one and only way of getting to modern QM. A lot of people these days take a more information-centered approach– at one of the things I did at the Perimeter Institute, I heard Dan Gottesman claim that you can get all of quantum mechanics by starting with the no-cloning theorem. I’m not trying to deny or disparage those approaches– again, I had six minutes. And the point isn’t really quantum per se, anyway.

The crossword graphic was made with the help of the Armored Penguin crossword maker. The final version you see above was put together by me in PowerPoint.

This is based very loosely on a chapter of the book-in-progress. Two chapters, actually, because Rutherford’s discovery of the nucleus gets its own chapter. I didn’t actually re-read those before writing this, though, so while the argument is similar, the order of points is different.

When and if a good video of the talk becomes available, you can be sure I’ll share it here.