15 October 2008

True tales of physics class

My recent post about approximations has me thinking of a couple of funny stories from when I was in college.

Well, funny on the scale of physics stories ...

The coolest class I took in my major was Physics 10. It was only one day a week, only worth one unit. Each class session would feature a different prof talking about something they were doing in their current research. One week, a particle physicist told a story about hanging out with one of the marine biology professors over lunch.

“How do dolphins make those high-pitched sounds they use for echolocation?”

“We don't really know. They don't have anything like our vocal cords.”

“Really? That's interesting. I'm not surprised that vocal cords won't do it; I guess you'd need some kind of specially adapted mechanism, since it takes so much more energy to make a high-pitched sound than a low-pitched sound.”

“It does? Living things which conserve energy have an evolutionary advantage, so I wonder why the echolocation chirps are so high-pitched.”

“That's easy. Do you know the frequency of the chirps?”

“It depends on the species. The one I'm working with now chirps at about 30,000 Hertz.”

At this point in the story, the prof said to the class. “I happen to know that the speed of sound in seawater is about 1500 meters per second. Who knows what I said to the marine biologist next?”

There were forty students in the room, and every hand went up. (When I tell this story to other physics students in recovery they always laugh at that part, because they know the answer, too.) The prof pointed to someone at random, who said, “The smallest fish that species of dolphin eats is five centimeters long.”

“Right!” said the professor. “The marine biologist was very impressed.” Everyone in class laughed.

Secret physicist humour. (I'll reveal the secret after my next story.)

In my upper-division mechanics class, the prof would often propose a problem off the top of his head and then work on it at the board, soliciting comments from the peanut gallery as he went along. It was a tough class, but fun: we were usually all hanging on by our fingernails as the problem unfolded on the board.

One day I found myself in the weeds: I couldn't follow the problem at all. Looking around, I could see that I wasn't alone. The prof was still rolling and I was paying close attention, trying to find the thread again. Was he integrating across the angle a couple of steps back? Why?

I was feeling embarrassed to be so baffled when the prof suddenly stopped cold, staring at the board. Seconds ticked by. Nobody said anything. He took a step back from the board.

He stroked his chin.

He looked back around at the students; we all shrugged.

He looked back at the board.

He took another step back.

Without looking away from the board, he said, “You know that thing, where you're working, it all turns into a sea of Greek letters, and you realize you've forgotten what the problem was?” Chuckles all around. Oh yeah, we knew that thing.

He turned to face us. He had a spooked expression, half-feigned but half-serious. “You can get lost in there.” Students were nodding. “We have a name for those people, who get obsessed with the symbols and never ... come ... back. We call them ... mathematicians.”

For folks curious about the first story: if you divide the speed of sound in a medium by the frequency of a particular sound, you get that sound's wavelength. (1500 m/s) / 30,000 Hz = 5 cm. An object smaller than the wavelength of a sound won't interfere with the sound very much, just as the skinny pilings under a pier don't interrupt the waves coming in, so echolocation wouldn't work on a smaller fish.

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