Джеймс Глик - Genius - The Life and Science of Richard Feynman

Gedankenexperiment was failing, Feynman decided to bring the lawn-sprinkler problem back into the world of matter—stiff metal and wet water. He bent a piece of tubing into an S. He ran a piece of soft rubber hose into it. Now he needed a convenient source of compressed air.

The Palmer Physical Laboratory at Princeton housed a magnificent array of facilities, though not quite up to the standards of MIT. There were four large laboraories and several smal er ones, with a total floor space of more than two acres. Machine shops supplied electrical charging devices, storage batteries, switchboards, chemical equipment, and diffraction gratings. The third floor was devoted to a high-voltage laboratory capable of direct currents at 400,000 volts. A low-temperature laboratory had machinery for liquefying hydrogen. Palmer’s pride, however, was its new cyclotron, built in 1936. Feynman had made a point of wandering over the day after he arrived at Princeton and had tea with the Dean. By comparison, MIT’s even newer cyclotron was an elegant futuristic masterpiece of shiny metal and geometrical y arrayed dials; when MIT

had final y decided to invest in high-energy physics, it had not stinted. Princeton’s gave Feynman a shock. He made his way down into the basement of Palmer, opened the door, and saw wires hanging like cobwebs from the ceiling.

Safety valves for the cooling system were exposed, and water dripped from them. Tools were scattered on tables. It could not have looked less like Princeton. He thought of his wooden-crate laboratory at home in Far Rockaway.

The mystery of the lawn sprinkler. When it sprays water, it spins counterclockwise.But what happens when it is made to suck water in?

Amid the chaos, it seemed reasonable enough for Feynman to borrow the use of an outlet for compressed air.

He attached the rubber tube and pushed the end through a large cork. He lowered his miniature lawn sprinkler through the neck of a giant glass water bottle and sealed the bottle with the cork. Rather than try to suck water from the tube, he was going to pump air into the top of the bottle. That would increase the pressure of the water, which would then flow backward into the S-shaped pipe, up the rubber hose, and out the bottle.

He turned on the air valve. The apparatus gave a slight tremble, and water started to dribble from the cork. More air—the flow of water increased and the rubber tube seemed to shake but not to twist, at least not with any confidence. Feynman opened the valve farther, and the bottle exploded, showering water and glass across the

room. The head of the cyclotron banished Feynman from the laboratory henceforth.

Sobering though Feynman’s experimental failure was, for years to come he and Wheeler both delighted in tel ing the story, and they were both scrupulous about never revealing the answer to the original question. Feynman had worked it out correctly, however. His physical intuition had never been sharper, nor his ability to translate fluently between a palpable sense of the physics and the formal mathematical equations. His experiment had actual y worked, until it exploded. Which way does the lawn sprinkler turn? It does not turn at al . As the nozzles suck water in, they do not pul themselves along, like a rope climber pul ing himself up hand over hand. They have no purchase on the water ahead. And the idea of force exerted as a torque within the curve of the S is beside the point. In the normal version, water sprays forth in organized jets. The action and reaction are straightforward and measurable. The momentum of the water spraying in one direction equals the momentum that spins the nozzle in the opposite direction. But in the inverse case, when water is sucked in, there are no jets. The water is not organized. It enters the nozzle from al directions and therefore applies no force at al .

A development in twentieth-century entertainment technology—the motion picture—incidental y provided an advance in the technology of thought experiments. It was now natural for a scientist, in his mind’s laboratory, to play the film backward . In the case of the lawn sprinkler, reversibility proved to be an il usion. If the flow of the water were visible, a motion picture of an ordinary lawn sprinkler

played backward would look distinctly different from the sucking lawn sprinkler played forward. Filmmakers themselves had been seduced by the new, often comical insights that could be gained by taking a strip of cel uloid and running it backward through the projector. Divers sprang feet first from lakes as a spray of water col apsed into the space left behind. Fires drew smoke from the air and created a trail of new-made paper. Fragmented eggshel s assembled themselves around shuddering chicks.

For Feynman and Wheeler reversibility was becoming a central issue at the level of atomic processes, where spins and forces interacted more abstractly than in a lawn sprinkler. It was wel known that the equations describing the motions and col isions of objects ran equal y wel forward and backward. They were symmetrical with respect to time, at least where just a few objects were concerned.

How embarrassing, therefore, that time seemed so one-way in the real world, where a smal amount of energy could scramble an egg or shatter a dish and where unscrambling and unshattering were beyond the power of science.

“Time’s arrow” was already the catchphrase for this directionality, so evident to common experience, yet so invisible in the equations of physicists. There, in the equations, the road from past to future looked identical to the road from future to past. “There is no signboard to indicate that it is a one-way street,” complained Arthur Eddington. The paradox had been there al along, since Newton at least, but relativity had highlighted it. The mathematician Hermann Minkowski, by visualizing time as a fourth dimension, had begun to reduce past-future to the

status of any pair of directions: left-right, up-down, back-front. The physicist drawing his diagrams obtains a God’s-eye view. In the space-time picture a line representing the path of a particle through time simply exists, past and future visible together. The four-dimensional space-time manifold displays al eternity at once.

The laws of nature are not rules control ing the metamorphosis of what is into what wil be. They are descriptions of patterns that exist, al at once, in the whole tapestry. The picture is hard to reconcile with our everyday sense that time is special. Even the physicist has his memories of the past and his aspirations for the future, and no space-time diagram quite obliterates the difference between them.

Philosophers, in whose province such speculations had usual y belonged, were left with a muddy and senescent set of concepts. The distress of the philosophers of time spil ed into

their

adverbs: sempiternally ,

hypostatically ,

tenselessly , retrodictably . Centuries of speculation and debate had left them unprepared for the physicists’ sudden demolition of the notion of simultaneity (in the relativistic universe it meant nothing to say that two events took place at the same time). With simultaneity gone, sequentiality was foundering, causality was under pressure, and scientists general y felt themselves free to consider temporal possibilities that would have seemed farfetched a generation before.

In the fal of 1940 Feynman returned to the fundamental problem with which he had flirted since his undergraduate days. Could the ugly infinities of quantum theory be eliminated by forbidding the possibility that an electron acts

on itself—by eliminating, in effect, the field? Unfortunately he had meanwhile learned what was wrong with his idea.

The problem was a phenomenon that could only be explained, it seemed, in terms of the action of an electron on itself. When real electrons are pushed, they push back: an accelerating electron drains energy by radiating it away.