Charles Townes: It happened that I was in Washington, D.C., and it’s almost a sort of a fairy story tale — just what a novelist would write about a discovery. I wake up early in the morning, and Arthur Schawlow, who was a colleague of mine, was in the same room. And I thought, well, I wouldn’t wake him up, so I’d go outside, and I went out to the park. The azaleas were out, and a nice bright sun in the early morning, and it was just a beautiful time, and I sat down on a bench. But what was on my mind was that we had a meeting coming up of a group of scientists and engineers who’d been trying to find ways of producing short waves. And I had been trying to do that myself for about five years. I’d tried a lot of different techniques. Some of them worked, but not terribly well. And so here I was, I was beginning to puzzle over how could we get anywhere on this. What would we do that day at the meeting? How could we get anywhere on this problem? Why was it we hadn’t succeeded? So I went over the things that wouldn’t work, why they wouldn’t work. And I recognized, well, if it’s ever going to work, we’re going to have to use molecules. Because molecules already made by nature, very small, they resonate at these high frequencies or short wavelengths, we just somehow have to use those. But of course I’d thought about that before too. And concluded from what’s known as the second law of thermodynamics, that if you have a batch of molecules and you heat them up, yes, they will radiate, they will produce these waves, but they won’t produce very much, because you heat them up enough so they begin to produce a lot, and then the molecules fall apart. So I dismissed that before, and it wouldn’t work. But this time, I thought, well, if it’s ever going to work, it has to work that way. You’ve got to get molecules, but yet it has this problem of the second law of thermodynamics. And it suddenly occurred to me, now wait a minute. One doesn’t have to obey the second law of thermodynamics. That’s when all the molecules are interacting and exchanging energy and so on. We can keep the molecules from interacting, so we can have some molecules with a lot of energy, other molecules with not so much energy, throw away those, and then we’ve got a collection of molecules with high energy only. And now we use what was Einstein’s idea, that always occurs if you have molecules or atoms with excess energy. If a wave comes along that resonates with them, sort of tickles the molecules and resonates with them, they will give up their energy to the wave, and the wave then passes by and picks up some energy. That’s called stimulated emission. The emission of radiation by stimulation of the wave that is coming by. So we have this collection of molecules, all of which have energy, then we can get energy from them by this stimulated emission of radiation. And that was the source of the word “microwave amplification by stimulated emission of radiation.” The next problem was how to get such a collection of molecules. What immediately occurred to me was using molecular beams. That’s a technique that was common at Columbia University. I was quite familiar with it. I hadn’t used it myself but I was very familiar with it, and the idea was to send the beam of molecules, in a vacuum like this, and you put on an electric field, which can pull some of them out of the way, and the rest of them go on this way. And so I could — I knew there was a way of pulling away the molecules that had very low energy, keeping the ones with high energy, then letting them come into a resonator. A resonator — metal resonator — where the waves could bounce back and forth, and build up strength as they rob the molecules of energy. A resonator I was familiar with from Bell Labs experience, radar. I was familiar with molecular beams from Columbia University. The particular way of getting lots of molecules, I figured out how many we’d have to have, and they were quite a few. How do we get that many? Ah, yes. Just the month before there had been a German scientist who had come to Columbia University, given a talk about a special way of selecting molecules in a molecular beam. It gave lots of intensity. Very much more intensity than other people had. That way would work, would give us enough. And I could quickly calculate, yes, it looks like it’s very likely to work. One can’t be sure until you make it work. But I thought it was a very good chance. And of course it was an exciting moment for me to realize that was really the right way to do it.