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

Philosophers cal ed it empirical equivalence , when they began to catch up. The recent history of quantum mechanics had pivoted on the empirical equivalence of Heisenberg’s and Schrödinger’s versions. The empirical equivalence of very different-seeming theories could be demonstrated mathematical y, as Dyson had shown for Feynman’s and Schwinger’s quantum electrodynamics.

Scientists knew, usual y without thinking about it, that empirical y equivalent theories could have different consequences, mathematics and logic notwithstanding.

For Feynman, especial y, the tension between alternative theories served as a creative force, an engine for generating new knowledge. Perhaps more than any living physicist, he had made a specialty of learning what models could be derived from which principles, and what models from each other. To Dyson’s astonishment, he had stood at a blackboard one day in 1948 and interrupted their heady discussions of quantum electrodynamics to show him something different. Sketching quickly, he derived the nineteenth-century Maxwel field equations—the classical understanding of electricity and magnetism—backward from the new quantum mechanics. Einstein had started with the Maxwel equations and then shifted the perspective of

the observer to arrive at his theory of relativity; Feynman went the other way in a fit of ahistorical perversity. He began with a void, no fields or waves, no concept of relativity, not even a notion of light itself, just a single particle obeying quantum mechanics’ odd rules. Before Dyson’s eyes he traveled back mathematical y from the new physics, with its riddles of uncertainty and immeasurability, to the comforting exactitude of the previous century. He showed that Maxwel ’s field equations were not a foundation but a consequence of the new quantum mechanics. Startled and impressed, Dyson urged him to publish. Feynman just laughed and said, “Oh, no, it’s not serious.” As Dyson understood it later, Feynman had been trying to create a new theory “outside the framework of conventional physics.”

His motivation was to discover a new theory, not to reinvent the old one… . His purpose was to explore as widely as possible the universe of particle dynamics.

He wanted to make as few assumptions as he could.

A theorist who can juggle different theories in his mind has a creative advantage, Feynman argued, when it comes time to change the theories. The path-integral formulation of quantum mechanics might be empirical y equivalent to other formulations and yet—given less-than-omniscient human physicists—find more natural-seeming application to realms of science not yet explored. Different theories tended to give a physicist “different ideas for guessing,”

Feynman said. And the century’s history had shown that when even so elegant and pure a theory as Newton’s had to be replaced, slight modifications could not suffice.

To get something that would produce a slightly different result it had to be completely different. In stating a new law you cannot make imperfections on a perfect thing; you have to have another perfect thing.

He understood explanations as a surgeon understands knives. He had a set of practical tests, heuristics, that he applied when reaching a judgment about a new idea in physics: for example, did it explain something unrelated to the original problem. He would chal enge a young theorist: What can you explain that you didn’t set out to explain?

He knew that why? is a question without an end and that our knowledge of things is inextricable from the language we use. The words and analogies from which we build our explanations are culpably linked with the things explained.

Explanans and explanandum are inextricable after al . An interviewer for the British Broadcasting Corporation, Christopher Sykes, once asked him to explain magnets: “If you get hold of two magnets and you push them you can feel this pushing between them… . Now what is it, the feeling between those two magnets?”

“What do you mean, what’s the feeling?” Feynman growled. His hair, swept back in dramatic gray waves, had receded high atop his head, leaving a statue’s high brow above a pair of heavy eyebrows that curled more impishly

than ever. His pale blue shirt was open at the col ar. A pen and eyeglass case rested in his front pocket, as always. Off camera, a defensive note entered the interviewer’s voice.

“Wel , there’s something there, isn’t there? The sensation is that there’s something there when you push these two magnets together.”

“Listen to my question,” Feynman said. “What is the meaning when you say there’s a feeling ? Of course you feel it. Now what is it you want to know?”

“What I want to know is what’s going on between these two bits of metal.”

“The magnets repel each other.”

“But what does that mean ? Or why are they doing that?

O r how are they doing that?” Feynman shifted in his easy chair, and the interviewer added, “I must say I think that’s a perfectly reasonable question to ask.”

“Of course it’s a reasonable—it’s an excel ent question, okay?”

Reluctantly,

Feynman

now

stepped

into

metaphysics. Particle theorists were toying with a

“bootstrap” model, in which no particle lies at a deepest level, but al are interdependent composites. The name bootstrap paid homage to the paradoxical circularity of having to build each fundamental particle from al the others. Feynman, as he now made clear, believed in a kind of bootstrap model of explanation itself.

You see, when you ask why something happens, how does a person answer why something happens?

For example, Aunt Minnie is in the hospital. Why?

Because she went out on the ice and slipped and broke her hip. That satisfies people. But it wouldn’t satisfy someone who came from another planet and knew nothing about things… . When you explain a why , you have to be in some framework that you’ve al owed something to be true. Otherwise you’re perpetually asking why… . You go deeper and deeper in various directions.

Why did she slip on the ice? Wel , ice is slippery.

Everybody knows that—no problem. But you ask why is ice slippery… . And then you’re involved with something, because there aren’t many things as slippery as ice… . A solid that’s so slippery?

Because it is in the case of ice that when you stand on it, they say, momentarily the pressure melts the ice a little bit so that you’ve got an instantaneous water surface on which you’re slipping. Why on ice and not on other things? Because water expands when it freezes. So the pressure tries to undo the expansion and melts it… .

I’m not answering your question, but I’m tel ing you how difficult a why question is. You have to know what it is that you’re permitted to understand … and what it is you’re not.

You’l notice in this example that the more I ask why, it gets interesting after a while. That’s my idea, that the deeper a thing is, the more interesting… .

Now when you ask why two magnets repel, there are

many different levels. It depends whether you’re a student of physics or an ordinary person who doesn’t know anything.

If you don’t know anything at al , about al I can say is that there’s a magnetic force that makes them repel.

And that you’re feeling that force. Wel , you say that’s very strange because I don’t feel a force like that in other circumstances… . You’re not at al disturbed by the fact that when you put your hand on the chair it pushes you back. But we found out by looking at it that that’s the same force… . It turns out that the magnetic and electric force with which I wish to explain these things is the deeper thing that we would start with to explain many other things… .