DonorsChoose Payoff: The Time Machine

Today, we have the first claimant of a donation incentive, from Sarah, who asks:

If you could go back in time to any lab and be there as X discovery was being made, which lab/when/where would you go? I figure this could be spun a couple of ways, either to talk about some really cool science or some really interesting personalities/history of science stuff, or both.

She suggests either the modern discovery of BEC, or the Michelson-Morley experiment for a historical entry. Both of these are excellent choices-- I was on the periphery of a group chasing BEC in 1995, and it was an exciting time, even for people who weren't near the head of the race. And Michelson-Morley is a classic, not to mention the fact that Morley was a Williams grad...

The original Rutherford scattering experiment would be another good historical choice, especially given Rutherford's description of his reaction to it ("It was quite the most incredible event that ever happened to me in my life. It was almost as incredible as if you had fired a 15-inch shell at a piece of tissue paper and it came back and hit you."). I'll go a little off the beaten track and vote for the Davisson-Germer experiment at Bell Labs in the mid-1920's.

As you know, Bob, the Davisson-Germer experiment is one of the first experiments to demonstrate the wave nature of the electron, which is pretty darn cool. That's not the most attractive feature of it, though, at least not from the perspective of an experimentalist. The coolest thing about their experiment is that it only worked because they broke it.

Davisson and Germer were looking at the reflection of electrons off a nickel target (whether they were deliberately trying to prove the de Broglie hypothesis is unclear to me-- different versions have it different ways, and I haven't read a definitive account). They shot electron beams of various energies at a piece of nickel, and measured how many bounced off at different angles.

These experiments needed to take place under vacuum, as the presence of air between the electron gun and the target would screw things up, so they had the whole experiment in an evacuated chamber. One day, though, they broke a glass tube attached to the chamber, and let air in.

Among other things, this oxidized their nickel target, changing the composition of the surface. To correct this, after they repaired the vaccum system, they heated the nickel to drive the oxide off. Unbeknownst to them at the time, they overdid the heating a little, and actually melted the target.

Prior to the vacuum break, they hadn't seen anything all that interesting. After the repair, though, they suddenly saw a huge number of electrons bouncing off the surface at a particular angle, around 54 degrees. This is due to the diffraction of electron waves off the atomic planes in their sample. They hadn't seen it before the break, because their original sample consisted of many randomly oriented crystals, and the diffraction patterns from all the different orientations washed out into a big mess. When they melted the target, though, it cooled back down into a single large crystal, which showed clear diffraction, and the big surprising peak.

Depending on which version of the story you read, either they immediately recognized this as proof of the wave nature of electrons (proposed by de Broglie in 1923), or they said "What the hell?" and didn't know what to make of it for a year or two, until they heard about the de Broglie theory. Either way, it was the first proof of the wave nature of matter, and won Davisson a share of a Nobel Prize.

I think this would be a cool experiment to be around not only because I'd like to know the real story (I've only seen accounts in textbooks), but also because I'd love to see the moment when they first saw the diffraction peak. Dramatic progress in experimental physics usually isn't a "Eureka!" moment, but more often comes from a "What the hell is that?!?" moment. I've had one of those myself, and I think it would be really cool to see one of the biggest "What the hell is that?!?" moments of physics history first-hand.

(The Rutherford scattering thing is attractive for much the same reason. Penzias and Wilson finding the cosmic microwave background would be another good example.)

So there's my time machine moment. If you've got another to suggest, leave it in the comments. If you'd like to ask your own question, make a donation of $50 or more, and forward me the confirmation email, and I'll respond within 24 hours.

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By Bob Hawkins (not verified) on 15 Oct 2008 #permalink