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

The positron, the antiparticle twin of the electron, had been discovered (in cosmic-ray showers) and named (another modern -tron, short for positive electron) within the past decade. It was the first antiparticle, vindicating a prediction of Dirac’s, based on little more than a faith in the loveliness of his equations. According to the Dirac wave equation, the energy of a particle amounted to this:

±√something. Out of that plus-or-minus sign the positron was born. The positive solution was an electron. Dirac boldly resisted the temptation to dismiss the negative solution as a quirk of algebra. Like Wheeler in making his leap toward advanced waves, he fol owed a mirror-image change in sign to its natural conclusion.

Feynman considered the wild suggestion coming through the earpiece of his telephone—that al creation is a slice through the spaghetti path of a single electron—and offered the mildest of the many possible rebuttals. The forward and backward paths did not seem to match up. An embroidery needle pul ing a single thread back and forth through a canvas must go back as many times as it goes forth.

—But, Professor, there aren’t as many positrons as electrons.

—Wel , maybe they are hidden in the protons or something.

Wheeler was stil trying to make the electron the basis of al other particles. Feynman let it pass. The point about positrons, however, reverberated. In his first published paper two years before, on the scattering of cosmic radiation by stars, he had already made this connection, treating antiparticles as ordinary particles fol owing reversed paths. In a Minkowskian universe, why shouldn’t the reversal apply to time as wel as to space?

Mr. X and the Nature of Time

Twenty years later, in 1963, the problem of time having given up none of its mystery, a group of twenty-two physicists, cosmologists, mathematicians, and others sat around a table at Cornel to discuss the matter. Was time a quantity entered in the account books of their equations to mark the amount of before and after ? Or it was an al -

enveloping flow, carrying everything with it like a constant river? In either case, what did it mean to say now? Einstein had worried about this, accepting the unwelcome possibility that the present belongs to our minds alone and that science cannot comprehend it. A philosopher, Adolph Grünbaum, argued that the usual notion of the forward flow of time was merely an il usion, a “pseudoconception.” If it seemed to us as conscious entities that new events kept

“coming into being,” that was merely one of the quirky consequences of the existence of conscious entities

—“organisms

which conceptually register (ideational y represent)” them. Physicists need not worry about it unduly.

When Grünbaum finished his presentation, a participant with a loathing for what he viewed as philosophical and psychological vagueness began a hard cross-examination.

(The published version of the discussion identified this interlocutor only as “Mr. X,” which fooled no one; by now, Feynman hiding behind such a cloak made himself as conspicuous as an American secretary of state quoted as

“a senior official aboard the secretary of state’s plane.”) GRÜNBAUM: I want to say that there is a difference between a conscious thing and an unconscious thing.

X: What is that difference?

GRÜNBAUM: Wel , I don’t have more precise words in which to say this, but I would not be worried if a computer is unemployed. If a human being is unemployed, I would worry about the sorrows which that human being experiences in virtue of conceptualized self-awareness.

X: Are dogs conscious?

GRÜNBAUM: Wel , yes. It is going to be a question of degree. But I wonder whether they have conceptualized awareness.

X: Are cockroaches conscious?

GRÜNBAUM: Wel , I don’t know about the nervous system of the cockroach.

X: Wel , they don’t suffer from unemployment.

It seemed to Feynman that a robust conception of “now”

ought not to depend on murky notions of mentalism. The minds of humans are manifestations of physical law, too, he pointed out. Whatever hidden brain machinery created Grünbaum’s coming into being must have to do with a correlation between events in two regions of space—the one inside the cranium and the other elsewhere “on the space-time diagram.” In theory one should be able to create a feeling of nowness in a sufficiently elaborate machine, said Mr. X.

One’s sense of the now feels subjective, arbitrary, open to differences of definition and interpretation, particularly in the age of relativity. “One can say easily enough that any particular value of t can be taken as now and that would not be wrong, but it does not correspond to experience,” the physicist David Park has said. “If we attend only to what is happening around us and let ourselves live, our attention concentrates itself on one moment of time. Now is when we think what we think and do what we do.” For similar reasons many philosophers wished to banish the concept.

Feynman, staking out a characteristic position in such debates, rejected the idea that human consciousness was special. He and other rigorous scientists, their tolerance broadened by their experience with quantum-mechanical measurement problems, found that they could live with the imprecision—the possibility that the now s of different observers would differ in timing and duration. Technology offered ways of tightening the definition, at least for the sake of argument: less subjectivity arose in the now recorded by a camera shutter or a computing machine.

Wheeler, also present at the Cornel meeting, proposed the

example of a computer on an antiaircraft gun. Its now is the finite interval containing not just the immediate past, the few moments of data coming from the radar tracks, but the immediate future, the flight of the target plane as extrapolated from the data. Our memories, too, blend the immediate past with the anticipation of the soon to be, and a living amalgam of these—not some infinitesimal pointlike instant forever fleeing out of reach—is our now. Wheeler quoted the White Queen’s remark to Alice: “It’s a poor sort of memory that only works backwards.”

The absorber theory of Wheeler and Feynman had by then lost the interest of an increasingly single-minded particle physics, but it held center stage in this eclectic gathering. It had been born of their concern with reversible and irreversible processes, and now it served as common ground for three different approaches to understanding time’s flow, the arrow of time. As particle physicists had passed the absorber theory by, a new generation of cosmologists had taken it up. Their field had begun a transition from mere stargazing astronomy to an enterprise asking the grandest questions about the universe: whence and wherefore. It was beginning to stand out among the modern sciences as an enterprise not ful y scientific, but an amalgam of philosophy, art, faith, and not a little hope. They had so few windows through the murky atmosphere—a few overworked glass contraptions on mountain tops, a few radio antennae—yet they believed they could peer far enough, or guess shrewdly enough, to uncover the origins of space and time. Already their space was not the flat, neutral stuff of their parents’ pre-Einsteinian intuition, but an eerily plastic medium that somehow embodied both time

and gravity. Some of them, but not al , believed that space was expanding at high speed and dragging its contents farther and farther apart, on account of an explosive big bang ten or fifteen bil ion years before. It no longer seemed safe to assume that the universe was the same everywhere, infinite, static, Euclidean, ageless, and homogeneous: world without end, amen. The strongest evidence for an expanding universe was stil , in 1963, Edwin Hubble’s 1929 discovery that other galaxies are streaming away from ours, and that the farther away they are, the faster they seem to be moving. Whether this expansion would continue forever or whether it would reverse itself was—and would remain—an open question.