History Newsletter CENTER FOR HISTORY OF PHYSICS&NIELS BOHR LIBRARY & ARCHIVES Vol. 42, No. 1 • Summer 2010 Bright Ideas: From Concept to Hardware in the First Lasers Adapted by Dwight E. Neuenschwander, technology and circumstances to catch absorbing a photon whose energy with permission, from Bright Idea: The up with Einstein’s vision. matches the energy difference be- First Lasers, an online exhibit of the tween the two levels. Third, Ludwig Center for History of Physics and Niels Einstein’s 1917 paper depended on Boltzmann’s statistical mechanics gave Bohr Library & Archives at the American four facts that were already well known us an expression for the probability Institute of Physics, hereafter called to physicists, but which Einstein put that an atom resides in a state of a “the Exhibit.”[1] http://www.aip.org/ certain energy when it’s part of history/exhibits/laser/. matter in thermal equilibrium at a given temperature. Fourth, Max Almost everyone living in a Planck’s statistical physics gave us an technological society today owns or expression for the energy distribution uses a laser. Compact disc players, in a gas of photons. Einstein’s 1917 supermarket checkout scanners, laser paper put these four pieces together. printers, and laser pointers are among the applications we encounter daily. Meanwhile, scientists and engineers Some specialized laser applications pushed radio techniques to ever include cauterizing scalpels in surgery, shorter wavelengths. In the 1930s industrial cutters and drills, surveying, some hoped they were on the artificial guide stars for astronomical verge of creating a “death ray” (H.G. observatories, and seismology. Wells’ 1898 novel War of the Worlds, wherein the invading Martians were This year, 2010, we celebrate the fiftieth armed with dreadful death rays anniversary of the invention of the laser. that obliterated everything they hit, If you ask people at random, “When became well known in the US about were the principles first conceptualized this time). That turned out (happily) that make lasers possible?”, many to be unworkable, but the effort led guess some date around 1960. That’s to something better—radar—thanks correctf i you mean the construction together in an original way. First, the to the invention of the magnetron of a working laser. But the concept of electrons in atoms exist in discrete (which later was scaled down to build “stimulated emission” that makes lasers states with quantized energy levels. microwave ovens). By 1940, as World War possible was first articulated by Einstein Second, electrons make transitions II began, these ingenious radar devices backn i 1917![2] It took four decades for between these states by emitting or (Continued on page 2) In this issue... Bright Ideas: From Concept to Hardware in the First Lasers ............ 1 The Array of Contemporary American Physicists is Online .......................................... 14 HoPE is Alive and Well ........................................................... 5 Building the Community of Historians of Physics ........................ 15 History on Display ................................................................ 6 Documentation Preserved ..................................................... 17 History of Physics in Spain: CEHIC Group Builds Ground for Historical Research ................................ 7 Recent Publications of Interest ............................................... 23 Old Friends and Book Donations to the NBL&A ........................... 8 Outreach at the CHP and NBL&A ............................................ 25 500+ Oral Histories Now Online ............................................... 9 Friends of the Center for History of Physics ............................... 26 How Scientific Societies Support the Histories of their Sciences ..... 10 Cover Photo: A Federation of History-Minded Scientists and Theodore Maiman looking at a ruby cylinder, the heart of his first laser Science-Minded Historians and Archivists .......................... 11 experiments, 1960. Credit: image courtesy of HRL Laboratories, LLC. AIP Member Societies: The American Physical Society • The Optical Society of America • The Acoustical Society of America • The Society of Rheology • The American Association of Physics Teachers American Crystallographic Association • American Astronomical Society • American Association of Physicists in Medicine • AVS The Science and Technology Society • American Geophysical Union (Bright Ideas, continued from page 1) make a cluster of atoms vibrate in revealing of Princeton worked toward the same could generate rays with wavelengths of waysa ( technique called microwave goal along a different path. Neither a centimeter or less. They were swiftly spectroscopy). Radar equipment left tried to build a device. In Moscow, pressed into service to detect enemy over from World War II was reworked to A.M. Prokhorov and N.G. Basov were airplanes. provide the radiation. Many of the world’s thinking in the same direction, and they top physicists were thinking about ways built a maser in 1955. After WWII, physicists had reason to to study systems of molecules by bathing boast that radar had played a crucial role them with this radiation. Who Invented the Laser? in winning the war, and the atomic bomb Physicists had been working for had promptly ended it. What might the Charles Townes of Columbia University generations toward controlling ever physicists create next? As the Cold had studied molecular physics in the shorter wavelengths. After radio (meters) War got underway, the US government 1930s, and during the war had worked and radar/microwave (centimeters, then poured ever larger funds into basic on radar as an electronics engineer. The millimeters), the logical next step would and applied research.[3] Scenting not Office of Naval Research pressed him be infrared waves. Masers had been only military but civilian applications, and other physicists to put their heads modestly useful, more for scientific corporations and entrepreneurs heaped together and invent a way to make research than for military or industrial their own money on the pile. Industrial powerful beams of radiation at ever applications. Only a few scientists thought and university laboratories proliferated. shorter wavelengths. In 1951 he found an infrared maser might be important It was from this fertile soil that the laser a solution. Under the right conditions and pondered how to make one. would grow. —say, inside a resonating cavity like the ones used to generate radar waves Townes thought about the problems The Maser: First Step to the Laser —the right collection of molecules intensively. One day in 1957, studying Alreadyn i the 1930s scientists could might generate radiation on their the equations for amplifying radiation, have built a laser. They had the optical own. He was applying an engineer’s he realized that it would be easier to techniques and theoretical knowledge insights to a physicist’s atomic systems. make it happen with very short waves —but nothing pushed these together. Townes gave the problem to Herbert than with infrared waves. He could leap The push came around 1950 from an Zeiger, a postdoctoral student, and across the infrared region to the long- unexpected direction. Short-wavelength James. P Gordon, a graduate student. familiar techniques for manipulating radio waves, called microwaves, could By 1954 they had the device working. ordinary light. Townes talked it over Townes called it a with his colleague, friend and brother- MASER, for “Microwave in-law Arthur Schawlow. Schawlow Amplification by found the key—put the atoms you want Stimulated Emission of to stimulate in a long, narrow cavity Radiation”. [4] with mirrors at each end. The waves would shuttle back and forth inside so Townes had predicted that there would be more chances for a remarkable and use- stimulating atoms to radiate. One of the ful property for the mirrors would be only partly silvered radiation from the de- so that some of the rays could leak vice: it would be at a out. This arrangement (the Fabry-Pérot single frequency, as etalon) was familiar to generations of pure as a note from a optics researchers. tuning fork. And so it was. The high degree of The same arrangement meanwhile order in such radiation occurred to Gordon Gould, a graduate woulde giv the maser, student at Columbia University who had and later the laser, im- discussed the problem with Townes. portant practical uses. For his thesis research, Gould had already been working with “pumping” Townes was not alone atoms to higher energy states so they in his line of thought. would emit light. As Gould elaborated Joseph Weber of the his ideas and speculated about all the University of Maryland things you could do with a concentrated expressed similar ideas beam of light, he realized that he was Charles Townes and J. P. Gordon standing with the second ammo- independently in 1952. onto something far beyond the much- nia beam MASER at Columbia University, 1955. Credit: AIP Emilio Segrè Visual Archives, Physics Today Collection. And Robert H. Dicke discussed “infrared maser.” In his 2 History Newsletter | Summer 2010 www.aip.org/history notebook he confidently named the yet- medium. He settled to-be-invented device a LASER (for Light on a combination of Amplification by Stimulated Emission helium and neon in of Radiation). Gould, Schawlow and a long glass tube. An Townes now understood how to build electric discharge a laser—in principle. To actually build through