I’m rooting around in my bag for a pen, and pull out a laser pointer by mistake. Since I’d really prefer not to be grading, I flip it on and shine it on the floor next to the spot where Emmy is half-dozing. She immediately leaps up (she’s pretty spry for a dog of 12…), and pounces on it. Or tries to, as I flick the spot across the room.
“Get the dot! Get the dot! Getthedotgetthedotgetthedot!” she mutters as I lead her on a lively chase around the room. After a few minutes, I click the laser off, and put it down. Emmy comes over, panting, and I scratch behind her ears.
“That was fun, eh, girl?” I say.
“Important, too!” she says brightly.
“I’m helping fix the weather!”
“Ummm… How does chasing my laser pointer spot around the room help fix the weather?”
“Well, you remember how cold it was on our walk this morning, right? When I got that nasty squeaky snow between my toes, and you had to rub it out? And you said ‘I am so sick of this stupid winter…’ Only, you know, with some extra bad words mixed in.”
“Well, you talk about all these physicists who do laser cooling, and how cold they make stuff, and it’s super cold outside, so I figure it must be all those lasers. And if I can take a few of them out, maybe it will warm up enough for the bunnies to come back out in the yard so I can chase them!” She wags her tail proudly.
I sigh. “OK, I don’t quite know where to start with this… First of all, the amount of stuff you can cool with lasers is really tiny.”
“Sure, but even a small amount of stuff makes a difference if it’s cold enough. I mean, when you drop an ice cube into my water dish, that cools off all the water. And these laser things are a whole lot colder than an ice cube, right?”
“Well, yeah– an ice cube is usually right around freezing, or 273 Kelvin, while laser-cooled atoms are at a temperature of a few millionths of a Kelvin. But a really big laser-cooled sample might run to a hundred billion atoms.”
“And that’s a lot!”
“Not if you’re talking about atoms. An ice cube contains, let’s see… probably about 1023 molecules of water. That’s a trillion times as many atoms as a really big laser-cooled sample.”
“Oh. But there are lots of people doing laser cooling these days, so maybe all of them working together…”
“No. There aren’t a trillion people in the world, let alone a trillion research groups doing laser cooling.”
“And anyway, laser cooled samples are always contained in ultra-high vacuum chambers. You remember the big metal chamber from the time we visited my lab for that photo shoot, right? They’re inside that, not out where they could cool anything else.”
“Why is that?”
“Well, because laser cooling only works for very specific atoms and molecules. You need to have a laser that’s tuned close to but a little bit below one of the frequencies of light that the atoms like to absorb. Then atoms moving toward the laser will see it Doppler shifted–”
“Eeeeeeee-ooooowwwwwww!” Emmy makes a race car noise. I kind of wish I had never used that example with her.
“Yes, exactly. The atoms moving toward the laser see the frequency shifted up, closer to what they want to absorb. And when they absorb a photon from the laser, they get a kick in the direction it was headed, which makes them slow down.”
“Like bouncing a little ball off a big ball!”
“And then you get to chase both balls!”
“Try to focus, please. Atoms moving toward the laser slow down, but atoms moving away see the laser shift even farther from their natural resonance frequency, and so aren’t affected at all. Which is why you can use laser beams to slow specific atoms. But it only works for those atoms, which is why we do laser cooling experiments in the middle of ultra-high-vacuum chambers, so they don’t collide with other atoms and heat back up.”
“OK, but couldn’t you just, like, keep cooling them down? So, you know, when the other atoms from the air hit the laser-cooled atoms, they get a little bit colder, and if you keep doing that, eventually the air gets cold even though it doesn’t interact with the lasers?”
“That’s a good idea,” I say, and she wags her tail. I scratch her favorite spot just behind her ears. “That’s a real thing that people do, called ‘sympathetic cooling,’ and it’s a big part of some experiments. Sometimes, you can’t effectively cool one type of atoms with convenient lasers, so instead you mix those together with another kind of atom that you can laser cool, and cool your target atoms indirectly.”
“Which lets you use laser cooling to control the weather!”
“Um, no. Even sympathetic cooling experiments are done under vacuum. It’s the same problem as with the ice cube– the number of atoms you can effectively laser cool is too small to affect the vastly greater number of atoms in air.”
“Well, what if you got a really big laser?”
“You could cool more atoms with more laser power, but you’re not going to get a trillion times bigger that way. And, anyway, if you tried to do that, the laser power supply would generate so much waste heat that you’d end up making the weather warmer, not colder.”
“Oh, right. The second law of thermodynamics.”
“Exactly.” (I know better than to ask how she knows about that…)
“Um, yeah. Anyway, laser cooling is not responsible for the frigid weather, so you’re not actually doing important work by chasing the laser-pointer spot around the room.”
“OK, maybe it’s not fixing the weather, but it is important.”
“Well, you’re getting a blog post out of this, right?” She looks insufferably smug.
“You’re a very clever dog,” I say.
“So can I chase the spot some more?”
“After I type this up, sure.”
(If you’re new here, and enjoy this, let me note that I have two books full of talking-dog physics: How to Teach [Quantum] Physics to Your Dog and How to Teach Relativity to Your Dog; there are also more talking-dog physics blog posts. I’ve also got a new book, albeit not with Emmy, about how you think like a scientist without even realizing it, which isn’t directly relevant to this post, but is awesome in its own right…)