The Dog Particle

Bibliologue / by Chad Orzel /

Chad Orzel has spent much of his teaching career explaining quantum mechanics. In his book, How to Teach Physics to Your Dog, he takes on a new breed of student.

How to Teach Physics to Your Dog happened by accident. I wrote a talking-physics-with-the-dog blog post at Uncertain Principles in 2007. That post was picked up by BoingBoing, and read by 50,000 people. One of them was an agent, who suggested I should turn the concept into a book.

My initial reaction was skeptical, but after some thought, I realized that dogs and quantum mechanics are a great fit. Anyone who has a dog knows that dogs would readily accept all sorts of weirdness—for example, my dog Emmy constantly expects treats to materialize in the middle of the kitchen, something that I would find quite alarming.

Dogs see the world as a constant source of wonder, and that’s the appropriate stance to take when approaching quantum physics. Quantum mechanics predicts all sorts of unusual things—particles that are also waves, “virtual particles” created from empty space—but if you put aside human preconceptions about how the world should work, it’s an absolutely fascinating subject.

The quantum rules that govern our universe seem utterly bizarre, but in fact, they’ve been tested to astonishing precision and confirmed in every detail. Take Bose-Einstein condensates (BECs), a unique phase of matter near absolute zero. The concepts behind BECs can be tricky for humans, but as you can see in the following conversation, Emmy’s quick to accept them, and find ways to use them to advance her interests.

The best way to learn how the world really works, it would seem, is to think like a dog.

I’m standing in the kitchen, sipping tea and watching snow blowing across the back yard. It’s cold enough that the digital thermometer has stopped working, which puts it in the single digits Fahrenheit. I’m not looking forward to walking the dog in this.

“Pretty cold, dude,” she says.

“Yeah,” I say. “It’s cold, all right.”

“You better let me outside,” she says, tail wagging. “I’m gonna catch a whole bunch of bunnies!”

“A whole bunch? How do you figure?”

“Well, it’s so cold that they’ll all be together. You know, like one of those Bozo condensates.”

“Bozo condensate?” It’s too early in the morning for this.

“You know. When you get stuff cold enough, all the atoms in it suddenly condense into a single quantum state. A Bozo condensate. Since it’s so cold, all the bunnies will condense together. Then when I catch one, I’ll catch them all. Bunnies galore!” Her whole rear end is wagging.

“Okay, first of all, it’s ‘Bose-Einstein condensate,’ after the Indian physicist Satyendra Nath Bose, who came up with the idea and sent it to Albert Einstein, who then worked out more of the details.”


“More importantly, though, it’s not possible to Bose-condense bunnies.”

“Sure it is. It happens when things get close to zero, and it’s, what, four degrees outside? That’s close to zero.”

“The temperature needs to be close to zero Kelvin, not Fahrenheit. That’s -495 degrees Fahrenheit, or -273 Celsius. The backyard is nowhere near cold enough.”

“But physics humans make BECs all the time, don’t they? I read it on the internet.”

“Sure. It doesn’t just happen, though. We use lasers and magnetic fields to cool a gas of atoms to within a few billionths of a degree of absolute zero, under carefully controlled conditions inside ultra-high vacuum chambers. It took something like seventy years from the Bose-Einstein prediction to the first observation of a BEC in a gas.”

“Why so long?”

“Well, it’s really hard to do. You need the quantum wavelength of the particles you’‘re working with to be comparable to the distance between them, so you need a low temperature and a high density. But not too high, or the atoms will just clump together and make a solid.””

“Wait, isn’t that the same thing?”

“No, not at all. Solids are held together by chemical bonds between atoms. The atoms in a Bose-Einstein condensate aren’t chemically bound, they’re just all in the same wavefunction because they prefer to be together with other atoms. It’s kind of like making a pack of dogs.”

“A pack of dogs?”

“Sure. If you have a bunch of dogs in a big space, they’ll just wander around individually, occasionally bumping into one another, like the atoms in a dilute gas. If you bring enough of them close together, though, they’ll realize that there are a bunch of other dogs there, and that they’d be happier being in a pack. Then they’ll all move together in a group, like the atoms in a Bose-Einstein condensate.”

“And if the density gets too high, you’ll have a disaster!”

“Ummm… Sure, I guess.”

“Yeah, ’cause if you pack too many dogs too closely together, they’ll all start fighting with each other, and it’ll be a big messy lump, not a nice coherent pack.”

“Yeah, that’s a nice analogy.” I scratch behind her ears, and she rubs against my leg.

“So, how cold would it need to be to condense bunnies?”

“Well… A bunch of one-kilogram bunnies separated by ten centimeters or so would need to be at a temperature of about 10-42 Kelvin. Give or take a bit.”

“Oh. That’s really cold.”


“You know, if you think about it, compared to that, it’s positively balmy outside.” I look down, and she’s wagging her tail, obviously pleased with herself. “So, taking me for a walk isn’t really that bad, is it?”

I sigh, and finish my tea. “Fine. Let me get my coat, and we can go for a walk…”

Physicists are forever dividing the world into two classes of things, but no division is as important as that between the two classes of quantum particles, fermions and bosons. The different properties of these two groups are responsible not only for exotic phenomena like Bose-Einstein condensation, but also for all of chemistry.

The protons, neutrons, and electrons making up ordinary matter are all fermions. Groups of fermions are subject to the “Pauli exclusion principle,” which says there can never be two fermions of the same type in the same quantum state. Putting multiple electrons in a single container is like putting unfriendly dogs into the same kennel—the first ones go in quietly, but later additions are forced into higher-energy states involving much growling and barking.

Pauli exclusion is responsible for chemistry: as we add electrons to an atom, they fill up the available energy levels, forcing later electrons to occupy higher-energy states and giving different elements different chemical properties. If not for Pauli exclusion, the complex chemistry needed for life would be impossible.

The fundamental particles that transmit forces are bosons, like the photons that carry the electromagnetic force, or the Higgs boson sought at the Large Hadron Collider, which gives other particles mass. An even number of fermions stuck together will behave as a collective boson, though, so many atoms are composite bosons. Unlike fermions, multiple bosons can occupy a single quantum state—in fact, bosons are happiest when grouped together, like a litter of puppies snuggled up in a single basket.

Bose-Einstein condensation is the most interesting consequence of the collective behavior of bosons. A large collection of bosons at high temperatures will occupy many different energy states, but at a temperature close to absolute zero, all of the particles will “condense” into the lowest-energy state available.

This uniformity gives rise to some well-known effects. A laser can be thought of as a BEC of photons, and BEC is involved in both superfluidity, where liquid helium-4 will flow without resistance, and superconductivity, where pairs of electrons carry electrical current without resistance. A pure BEC in a gas of atoms, however, was not observed until 1995, when Eric Cornell and Carl Wieman produced the first gaseous BEC in rubidium at the University of Colorado’s JILA institute. Wolfgang Ketterle’s group at MIT quickly followed that feat with a sodium-based BEC.

Since then, research groups around the world have used atomic BECs for everything from studying superfluidity, to precision measurements using atom interferometry, to creating a sonic analog of a black hole, a region in a BEC from which sound waves cannot escape.

BECs are a fantastic tool for studying fundamental physics, and will keep physicists busy for many years to come. What they represent for the field of bunny-catching is another matter entirely.

Chad Orzel is the author of How to Teach Physics to Your Dog (available now). He and Emmy blog at Uncertain Principles.

Originally published January 12, 2010

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