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

His most important contribution to the understanding of the disaster came in the area of risk and probability. He showed that the space agency and its contractors—

although the essence of their decision making lay in weighing uncertainties—had ignored statistical science altogether and had used a shockingly vague style of risk assessment. The commission’s official findings could do no better than quote Feynman’s comment during the hearings that the decision making became

a kind of Russian roulette… . [The shuttle] flies [with

O-ring erosion] and nothing happens. Then it is suggested, therefore, that the risk is no longer so high for the next flights. We can lower our standards a little bit because we got away with it last time… . You got away with it, but it shouldn’t be done over and over again like that.

Science has tools for such problems. NASA was not using them. A scattering of data points—for the depth of erosion in O-rings, for example—tended to be reduced to simplistic, linear rules of thumb. Yet the physical phenomenon, a hot jet of gas carving channels in rubber, was highly nonlinear, as Feynman noted. The way to assess a scattered range of data was through probability distributions, not single numbers. “It has to be understood as a probabilistic and confusing, complicated situation,” he said. “It is a question of increasing and decreasing probabilities … rather than did it work or didn’t it work.”

On the crucial question of the effect of temperature on Oring safety, NASA had made an obvious statistical blunder.

Seven flights had shown evidence of damage. The most damage had occurred on the coldest flight—at a stil -mild 53 degrees Fahrenheit—but no general correlation could be seen between temperature and damage. Serious damage had occurred at 75 degrees, for example.

The error was to ignore the flights on which no damage had occurred, on the basis that they were irrelevant. When these were plotted—seventeen flights at temperatures from

66 to 81 degrees—the effect of temperature suddenly stood out plainly. Damage was strongly associated with cold. It was as if, to weigh the proposition that California cities tend to be in the westernmost United States, someone made a map of California—omitting the non-California cities that would make the tendency apparent. A team of statisticians formed by the National Research Council to fol ow up the commission report analyzed the same data and estimated a “gambling probability” of 14

percent for a catastrophic O-ring failure at a temperature of 31 degrees.

Feynman discovered that some engineers had a relatively realistic view of the probabilities involved—

guessing that a disaster might occur on one flight in two hundred, for example. Yet managers had adopted fantastic estimates on the order of one in a hundred thousand. They were fooling themselves, he said. They cobbled together such numbers by multiplying absurd guesses—that the chance of a turbine pipe bursting was one in ten mil ion, for example.

He concluded his personal report by saying, “For a successful technology, reality must take precedence over public relations, for nature cannot be fooled.” He joined his fel ow commissioners for a ceremony at the White House Rose Garden. Then he returned home, as he now knew, to die.

EPILOGUE

God forbid that we should give out a dream of our own imagination for a pattern of the world.

— Francis Bacon

Nothing is certain. Werner Heisenberg wrote this message in

the

twentieth

century’s

consciousness.

The

mathematician Kurt Gödel fol owed with a famous proof that no logical system can ever be consistent and complete. The possibilities of true knowledge seemed to fade.

Heisenberg formulated his uncertainty principle narrowly: A particle cannot have both a definite place and a definite momentum. Stil , philosophers took note. The implications seemed to cover a broader territory than the atom and its interior. Yet Feynman scorned philosophers (“rather than embarrass them, we shal just cal them ‘cocktail-party philosophers’”) who overinterpreted the laws of physics by saying, for example,

“That al is relative is a consequence of Einstein, and it has profound influences on our ideas.” In addition, they say, “It has been demonstrated in physics that phenomena depend upon your frame of reference.” We hear that a great deal, but it is difficult

to find out what it means… . After al , that things depend upon one’s point of view is so simple an idea that it certainly cannot have been necessary to go to al the trouble of the physical relativity theory in order to discover it.

Einstein’s relativity did not speak to human values. Those were, or were not, relative for reasons unrelated to the physics of objects moving at near-light speed. Borrowing metaphors from the technical sciences could be a dangerous practice. Did the uncertainty principle impose its inevitable fuzziness on any description of nature?

Perhaps. But Feynman parted company with many of his col eagues. They looked to quantum uncertainty for an explanation of the many kinds of unpredictability that arise in the everyday, human-scale world: unpredictability in the weather, or indeterminacy in human behavior. Perhaps, some speculated, quantum unpredictability was the microscopic loophole through which free wil and human consciousness entered the universe.

Stephen Hawking, typical y, wrote: “The uncertainty principle signaled an end to Laplace’s dream of a theory of science, a model of the universe that would be completely deterministic… . Quantum mechanics therefore introduces an unavoidable element of unpredictability or randomness into science.” Feynman’s view was different. Even in the 1960s he anticipated the understanding that would emerge in the modern study of chaotic phenomena: that unpredictability was already a feature of the classical world.

He believed that a universe without a quantum uncertainty principle would behave—on the scales of planetary storm systems and human brains—just as erratical y and freely as our own.

It is usual y thought that this indeterminacy, that we cannot predict the future, is a quantum-mechanical thing, and this is said to explain the behavior of the mind, feelings of free wil , etc. But if the world were classical—if the laws of mechanics were classical—it is not quite obvious that the mind would not feel more or less the same.

Why? Because tiny errors, tiny gaps in our knowledge, are amplified by the interactions of complex systems until they reach large scales.

If water fal s over a dam, it splashes. If we stand nearby, every now and then a drop wil land on our nose. This appears to be completely random… . The tiniest irregularities are magnified in fal ing, so that we get complete randomness… .

Speaking more precisely, given an arbitrary accuracy, no matter how precise, one can find a time long enough that we cannot make predictions valid for that long a time. Now the point is that this length of time is not very large… . It turns out that in only a very, very tiny time we lose al our information… . We can no longer predict what is going to happen! It is therefore not fair to say that from the apparent freedom and

indeterminacy of the human mind, we should have realized that classical “deterministic” physics could not ever hope to understand it, and to welcome quantum mechanics as a release from a “completely mechanistic” universe.

This discrepancy in beliefs—this subtle disagreement with the more standard viewpoint of physicists like Hawking

—was no quibble. It formed a fulcrum on which turned, as the century neared its close, an essential disagreement about the achievements and the future of physics.