Exploring the Far Frontier

Half buried under a thick shell of earth and concrete in Cambridge, Mass., a great ring-shaped machine went into operation last week, humming softly while green lines measuring its power drifted across the face of an oscilloscope. Called the Cambridge Electron Accelerator, the machine cost $12 million (paid by the Atomic Energy Commission), is 236 ft. in diameter, and consumes enough electricity at full power to operate 40 medium-sized TV stations. Its practical use is nil. It will never freshen sea water, cure cancer, or solve any other specific problem of applied science. But in the hands of Harvard and M.I.T. scientists, it will probe far beyond the frontier of present physical knowledge. No one knows what waits to be found in this dark region, but physicists are sure it is packed with wonderful secrets. Full knowledge of why energy sometimes "condenses" to form matter, for example, would probably lift human civilization as much as the discovery of electricity.

Scientists have long used high-energy protons (fundamental particles that form the nuclei of hydrogen atoms) as tools to explore the secret innards of matter. Two enormous accelerators, one at Brookhaven National Laboratory, Long Island, the other near Geneva, Switzerland, spew out protons with 30 billion electron-volts of energy. Yet in some ways protons are clumsy tools for basic research; for many subtle experiments, electrons (much lighter negative particles of electricity) are better. But electrons are so much more difficult to handle that scientists have never been able to give them really high energy. The Cambridge accelerator is designed to lick that problem.

Round & Round. The scientists shoot bursts of electrons into the accelerator at close to 186,000 miles per second, which is the speed of light, ultimate speed limit in the universe. Pushing them harder and harder does not make electrons go much faster. Instead they get heavier, turning energy into mass according to Einstein's famous equation: E = mc^2. In the Cambridge accelerator, the electrons get moving at 99.9999996% of the speed of light, and have enough energy to weigh 12,000 times as much as when they were at rest.

Growing these fattened electrons is no easy job. They are shot into the accelerator's vacuum-ring in bunches of about 100 billion, already moving at close to the speed of light and carrying 25 million electron-volts of energy. If left to their own devices, they would move in straight lines, soon hitting the ring's outside wall. But the ring is surrounded by magnets whose power can be varied accurately. When each bunch of electrons enters, the magnetism is just strong enough to make them move in a circle, keeping away from the ring's walls. Round and round they go, picking up energy from 16 electrically charged "cavities" arranged around the ring. The added energy makes them heavier and harder to deflect, so each time they make the circuit the magnets must grow stronger to hold them on course.

Another difficulty is the electrons' habit of losing much of the energy that is stuffed into them. When electrons move in a magnetic field, they turn some of their energy into "synchrotron radiation" that shoots off like mud slinging off a wheel. The more energy they have, the more they radiate away. When they have been fattened to about 1 billion electron-volts (or 1 BEV, as physicists call it), they begin to radiate visible light. At 2 BEV, they radiate the more powerful ultraviolet rays. At 4 BEV, they radiate X rays, losing several million electron-volts of energy in one trip around the ring. A time will come when no amount of energy stuffed into the electrons can exceed the energy they lose. The top practical figure is about 6 BEV, which M.I.T. Professor M. Stanley Livingston, chief designer of the Cambridge accelerator, thinks will be reached within a few months.

Probing the Unknown. Dr. Livingston, who collaborated as a graduate student with Nobel Prizewinner Ernest Lawrence to invent the first cyclotron, in 1930, points out that while the Cambridge electron accelerator does not approach the energy of the 30-BEV proton accelerator at Brookhaven, it has important special talents. Since its electron projectiles are very small compared with protons, they can be used to explore the unknown inner structure of both protons and neutrons. They generate beams of enormously powerful 6-BEV X rays, and these in turn can be used to explore matter. The same big X rays, which are particles of a sort themselves, can also be transformed into other material particles, perhaps into kinds that physicists have never imagined.

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