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

We must distinguish between two types of irreversibility. A sequence of natural phenomena wil be said to be microscopical y irreversible if the sequence of phenomena reversed in temporal order in every detail could not possibly occur in nature. If the original sequence and the reversed in time one have a vastly different order of probability of occurrence in the macroscopic sense, the phenomena are said to be macroscopical y irreversible… . The present authors believe

that

al

physical

phenomena

are

microscopical y reversible, and that, therefore, al apparently

irreversible

phenomena

are

solely

macroscopical y irreversible.

Even now the principle of reversibility seemed startling and

dangerous, defying as it did the sense of one-way time that Newton had implanted in science. Feynman cal ed his last statement to Wheeler’s attention with a note: “Prof Wheeler,” he wrote—and then self-consciously crossed out

“Prof”—“This is a rather sweeping statement. Perhaps you don’t agree with it. RPF.”

Meanwhile Wheeler was searching the literature, and he found several obscure precedents for their absorber model.

Einstein himself pointed out that H. Tetrode, a German physicist, had published a paper in Zeitschrift für Physik in 1922 proposing that al radiation be considered an interaction between a source and an absorber—no absorber, no radiation. Nor did Tetrode shrink from the tree-fal s-in-the-forest consequences of the idea: The sun would not radiate if it were alone in space and no other bodies could absorb its radiation… . If for example I observed through my telescope yesterday evening that star … 100 light years away, then not only did I know that the light which it al owed to reach my eye was emitted 100 years ago, but also the star or individual atoms of it knew already 100 years ago that I, who then did not even exist, would view it yesterday evening at such and such a time.

For that matter, the invisible reddened whisper of radiation emitted by a distant (and in the twenties, unimagined) quasar not one hundred but ten bil ion years ago—radiation that passed unimpeded for most of the universe’s lifetime until final y it struck a semiconducting receiver at the heart of a giant telescope—this, too, could not have been emitted

without the cooperation of its absorber. Tetrode conceded,

“On the last pages we have let our conjectures go rather far beyond what has mathematical y been proven.” Wheeler found another obscure but provocative remark in the literature, from Gilbert N. Lewis, a physical chemist who happened to have coined the word photon . Lewis, too, worried about the seeming failure of physics to recognize the symmetry between past and future implied by its own fundamental equations, and for him, too, the past-future symmetry suggested a source-absorber symmetry in the process of radiation.

I am going to make the … assumption that an atom never emits light except to another atom… . it is as absurd to think of light emitted by one atom regardless of the existence of a receiving atom as it would be to think of an atom absorbing light without the existence of light to be absorbed. I propose to eliminate the idea of mere emission of light and substitute the idea of transmission , or a process of exchange of energy between two definite atoms… .

Feynman and Wheeler pushed on their theory. They tried to see how far they could broaden its implications. Many of their attempts led nowhere. They worked on the problem of gravity in hopes of reducing it to a similar interaction. They tried to construct a model in which space itself was eliminated: no coordinates and distances, no geometry or dimension; only the interactions themselves would matter.

These were dead ends. As the theory developed, however, one feature gained paramount importance. It proved

possible to compute particle interactions according to a principle of least action.

The approach was precisely the shortcut that Feynman had gone out of his way to disdain in his first theory course at MIT. For a bal arcing through the air, the principle of least action made it possible to sidestep the computation of a trajectory at successive instants of time. Instead one made use of the knowledge that the final path would be the one that minimized action, the difference between the bal ’s kinetic and potential energy. In the absorber theory, because the field was no longer an independent entity, the action of a particle suddenly became a quantity that made sense. It could be calculated directly from the particle’s motion. And once again, as though by magic, particles chose the paths for which the action was smal est. The more Feynman worked with the least-action approach, the more he felt how different was the physical point of view.

Traditional y one always thought in terms of the flow of time, represented by differential equations, which captured a change from instant to instant. Using the principle of least action instead, one developed a bird’s-eye perspective, envisioning a particle’s path as a whole, al time seen at once. “We have, instead,” Feynman said later, “a thing that describes the character of the path throughout al of space and time. The behavior of nature is determined by saying her whole space-time path has a certain character.” In col ege it had seemed too pat a device, too far abstracted from the true physics. Now it seemed extraordinarily beautiful and not so abstract after al . His conception of light was stil in flux—stil not quite a particle, not quite a wave, stil pressing speculatively against the unresolved infinities

of quantum mechanics. The notion had come far since Euclid wrote, as the first postulate of his Optics , “The rays emitted by the eye travel in a straight line.”

The empty space of the physicist’s imagination—the chalkboard on which every motion, every force, every interaction

played

itself

out—had

undergone

a

transformation in less than a generation. A bal pursued a trajectory through the everyday space of three dimensions.

The particles of Feynman’s reckoning forged paths through the four-dimensional space-time so indispensable to the theory of relativity, and through even more abstract spaces whose coordinate axes stood for quantities other than distance and time. In space-time even a motionless particle fol owed a trajectory, a line extending from past to future.

For such a path Minkowski coined the phrase world-line —“an image, so to speak, of the everlasting career of the substantial point, a curve in the world… . The whole universe is seen to resolve itself into similar world-lines.”

Science-fiction writers had already begun to imagine the strange consequences of world-lines twisting back from the future into the past. No novelist was letting his fantasies roam as far as Wheeler was, however. One day he cal ed Feynman on the hal telephone in the Graduate Col ege.

Later Feynman remembered the conversation this way:

—Feynman, I know why al the electrons have the same charge and the same mass.

—Why?

—Because they are al the same electron! Suppose that al the world-lines which we were ordinarily considering before in time and space—instead of

only going up in time were a tremendous knot, and then, when we cut through the knot, by the plane corresponding to a fixed time, we would see many, many world-lines and that would represent many electrons, except for one thing. If in one section this is an ordinary electron world-line, in the section in which it reversed itself and is coming back from the future we have the wrong sign … and therefore, that part of a path would act like a positron.