Three-inch pieces of light

Because of quantum mechanics, the chopping of photons is a fuzzy endeavor.

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January 1, 1927 | Vol. 11 | No. 299

Three-inch pieces of light

A method of cutting off three-inch pieces from a beam of light, like a meat cutter slicing a bologna sausage, though the light moves at 186,000 miles a second, is described by Dr. Ernest O. Lawrence and Dr. J. W. Beams of Yale University.

Though light travels so fast that it can encircle the earth seven times in a second, the two physicists made use of a shutter that turned the light on and off with such rapidity that each “piece” of light was only about three inches in length. Each flash lasted a hundred billionth of a second.

The investigation was undertaken in an endeavor to measure the length of what are called “quanta” of light, for according to modern ideas, light is transmitted as separate pulses, each of which is called a quantum. Physicists have been uncertain as to how long these quanta are, but by some it was believed that they were as much as a yard in length.

These extremely short flashes of light were measured by a very delicate photoelectric cell, which gives off an electric current when illuminated, and they found that so long as the total amount of light reaching the cell was the same, the resulting current was not affected by the length of the individual flashes. One three inches long produced an effect as well as a piece of light many miles or more in length, and this shows, say the investigators, that the individual quanta are less than three inches in length.

UPDATE | March 24, 2012

Photon size a trickier question today

Femtosecond lasers (this one at Imperial College London) emit pulses lasting quadrillionths of a second.

Thanks to the advent of the laser and an optical technique called mode-locking, today researchers can readily generate light pulses in the femto­second range, one ten-thousandth the duration of the briefest beat made by Ernest Lawrence and J.W. Beams in 1927. A handful of teams around the world even have their sights set on an attosecond pulse, which would flash every billionth of a billionth of a second (SN: 3/27/10, p. 16).

But this apparent carving up of light into smaller pieces doesn’t mean that the photon’s size has shrunk, for at least two reasons. The simplest: Laser pulses contain many photons, not just one. And the deeper, more bewildering: The photon is not what it was once thought to be.

When Lawrence and Beams chopped up light, quantum mechanics was in its infancy; the work of Schrödinger, Heisenberg and Dirac had not finished rocking the very foundations of physics. More than eight decades on, researchers see (if not crystal clearly) that until a definite question is asked, and the experimental setup specified, there is no definite answer to how big a photon is. Until it’s measured, a photon is any size.

Nobel laureate Anthony Leggett of the University of Illinois at Urbana-Champaign suggests a specific way to pose the question, one that’s not too far from Lawrence and Beams’ original approach: “What is the extent of the photon wave packet as it is emitted from the atom?” Multiplying the lifetime of the excited atom by the speed of light gives an answer of around a meter.

But the quantum world is one of probabilities. Within that meter, there’s a nearly 100 percent probability of getting the photon. Still, there’s some smaller probability that the photon shows up in Lawrence and Beams’ 3-inch chunk too. —Elizabeth Quill 

Credit: Dept. of Physics, Imperial College London/Photo Researchers, Inc.

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