This year marks the 50th anniversary of one of the most portentous events in the history of science: the creation of laser. Like many a transformative development, it was met initially with thunderous public indifference, although there were a few mutterings about “death rays”. A number of techno-pundits regarded the upstart gizmo as basically a glorified parlour trick, a “solution looking for a problem”, as Charles Townes, who won the Nobel Prize for pioneering the idea, later wrote.
Half a century later, lasers check out our groceries, read and write CDs and DVDs, guide commercial aircraft, enable eye surgery and dental repairs, target weapons, provide worldwide communications, survey the planet, print documents, cut fabric for clothing and metal for tools, make powerful pointers for PowerPoint slides and are now poised to ignite nuclear fusion, among scores of uses.
Who knew? Certainly not Albert Einstein, who had predicted the laser effect way back in 1917. By then, physicists understood that virtually all the light you see is produced by a process called spontaneous emission. Zap a few atoms with the right amount of energy including energy from light itself and their electrons will absorb the energy and jump up to excited levels, the original “quantum leap”.
But they won’t stay there. That’s because, as the parent of any teenager can tell you, it is the natural tendency of things in this universe to preferentially seek the lowest energy condition, which is why water always flows downhill, shoelaces never re-tie themselves and your check is still in the mail. So the excited electrons soon drop back to lower levels; in the process, they spontaneously shed the surplus energy in the form of photons, the smallest individual units, or quanta, of light. The size of the drop determines the wavelength of the emitted photon. That’s how light emerges from a flickering campfire, the surface of the sun, the bulb in a lamp or the screen of your TV.
By the mid-1950s, scientists had identified several excellent materials and had recognised that putting a mirror on each side of the laser medium would drastically increase the output, reflecting the photons back and forth, and producing more stimulus and more emissions on each transit. If one of the mirrors was partially transparent, a stream of photons would emerge from that end the now familiar laser beam. Finally in May 1960, Theodore Maiman, a physicist at Hughes Research Laboratories, constructed the first laser that emitted light in the visible range.
Today there are dozens of designs exploiting all three of the distinctive properties of laser light. The narrowly defined wavelength allows a laser scanner at the grocery store to bounce its beam off a bar code and read the result when the store lights are on. Indeed, the outputs of different lasers are so sharply differentiated that you can run signals from a bunch of them through the same fiber optic cable simultaneously and still separate them easily at the end. The unidirectional tightness of laser light makes it ideal for surveying because, unlike a flashlight beam, it doesn’t diverge much over distances. Even over really long distances: Laser light from the surface of the Earth, bouncing off a reflector placed on the lunar surface by Apollo astronauts, has revealed that the moon is receding from our planet by about an inch and a half a year.
Finally, the beam’s coherence makes it stunningly powerful. A laser drawing a couple of kilowatts (slightly more than your home hair dryer) can cut through an inch of carbon steel. Out in California, researchers at the Department of Energy's National Ignition Facility are about to concentrate 192 laser beams totaling 500 trillion watts on a capsule of hydrogen, the size of a pencil eraser. If it works, the power of the lasers will shove the hydrogen atoms together hard enough to ignite nuclear fusion, creating a microscopic star and with it, some people believe, the prospect of limitless energy for society using the same energy source that fires up the sun.
One of these days. Perhaps.
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