13    … Metaphysics

Clauser was not happy with the result of his experiment. He had been thinking that his gadget would yield the opposite conclusion. Indeed, he once told me that he had been so sure of things that he was willing to place a bet that quantum mechanics would turn out to be wrong. Two-to-one odds were the best that a colleague would give him, and he accepted them. Unfortunately—or fortunately, if you prefer—Clauser lost the bet. He mailed off a two-dollar bill to his colleague: so far as he knows, that colleague still has it up on his office wall.

After all, Clauser had wanted to discredit quantum theory.

I was convinced that quantum mechanics had to be wrong. … I kept saying, “Well, we did the experiment, what could be wrong?” Obviously we got the “wrong” result. I had no choice but to report what we saw—You know, here’s the result. But it contradicts what I believed in my gut has to be true. The result, I didn’t expect. I hoped we would overthrow quantum mechanics.1

But it was not just a matter of youthful rebelliousness. It was not just a matter of wanting to overthrow a cherished theory. It was also a matter of having to figure out what his experiment was telling us.

Quantum mechanics predicts the impossible—we have known that for decades. But what Clauser’s and Aspect’s and all the other metaphysical experiments are telling us that the real world accomplishes the impossible. And how can that be? What have we learned from all the experiments testing Bell’s Theorem? Nature violates Bell’s restriction: what does this astonishing result tell us?

The question is hard to answer in any simple way, and there is no agreement among workers in the field. We are still sorting it all out.

One possible conclusion to be drawn is that there are no hidden variables. There is no real physical situation, no actual state of affairs. If this is so, it means that my Great Predictor is silent for a reason: that there is simply nothing to be said more than what he does say. It also means that quantum mechanics is not a half-theory at all, but a full theory, a theory perfectly suited to the strange new world on which we have inadvertently stumbled.

This conclusion is not airtight, for it rests on an assumption—the so-called “locality” assumption—which I will discuss later. But for now, let us ask what it could possibly mean.

It means that particles in the quantum realm do not possess certain properties. But I would argue that properties are essential for our thinking. They are built into the very way our minds work. How could we think of an electron spinning but not spinning in a definite direction? How might we imagine a particle with not zero speed, not with this speed or that speed, but with no particular speed … but nevertheless produced at a certain moment at a certain place, and detected a certain amount of time later a certain distance away? How could we hold in our mind’s eye the image of a thing without a location? It all seems like a very violation of logic.

It is not like forgetting where you parked the car. You think that the car might be on this street or that street—but imbedded in this way of thinking is the conviction that the car is somewhere, at a place that exists but that you do not happen to recall. This is different. This would be a car without the property of location. Without location until you find the car, at which point its whereabouts become entirely real.

To say that hidden variables do not exist is to call into question the very meaning of what we mean by a thing. For surely, things have properties, and these properties have consequences. Consider:

In many ways, electrons seem quite ordinary. An electron can be produced—by an electron gun, say—at a certain place and time. It can be detected—by the screen on a TV set, say—at another place and time. An electron has a perfectly definite mass and electric charge and magnitude of spin.

All this makes us think of an electron as being a thing in the ordinary sense of the term: sort of a tiny pebble. But nobody has ever seen an electron—their presence is only inferred indirectly—and maybe we are a little hasty in treating them so cavalierly. For consider good news. Good news can be produced at a certain time and place, it can be detected by a person at some other time and place, it has an effect on that person, and it travels quite rapidly. Nevertheless, nobody would think of it as a thing. Maybe an electron is more like news, and not so much like a pebble.

But it is only certain properties that the electron does not possess. The particle has a perfectly definite mass and charge, for instance. Furthermore, it is not only electrons that we are speaking of here: photons, neutrons, atoms … every denizen of the microworld partakes of the same enigmatic quantum nature.

There are times when I think that what we really need is a new terminology. We speak of particles as things. We say that something left the electron gun and arrived at the detector. But when we speak in these terms, we are naturally led to ask all sorts of questions about these particles. Why can’t we see them, for instance? And what shape are they—spheres, cubes, or perhaps shaped like some Chinese ideogram? What is their color? Are they cheerful or gloomy, sweet smelling or acrid? Carrying on in this way, we can be easily led astray—led astray by the set of unconscious associations that the word “thing” raises within our minds. Our language forces on us a certain way of thinking, a way that apparently we must be careful to resist.

Every time a quantum particle enters a detector, the detector responds by doing something. Maybe it tells us that the particle’s spin is along some direction. Maybe it tells us that the particle is over there. Maybe it tells us that the particle is zipping along at such-and-such a velocity. But what do these responses mean? A measurement is supposed to reveal a property of the thing studied—a thermometer tells us the temperature of the air, a barometer its pressure. These are not matters of opinion, not matters of taste, but facts: real properties of a real world. But if there is anything we have learned so far, it is that the microworld is different. We used to think that a particle has a spin that points in some direction, that it is at some place, and that it is moving with some speed. And we used to think that the detector has merely found out these properties. But now … well, now we had better be careful.

Because if things do not have these properties, then what has a measurement told us?

Quantum theory has an answer to this question. The answer is that a measurement does not reveal a property of the microworld: rather, the measurement creates that property. Prior to the measurement the electron spin had no particular direction: after the measurement it does.

That is a gigantic shift in thinking. Do you like that shift? Is it congenial to you? Read what the brilliant physicist E. T. Jaynes has to say about it:

From this, it is pretty clear why present quantum theory not only does not use—it does not even dare to mention—the notion of a “real physical situation.” Defenders of the theory say that this notion is philosophically naïve, a throwback to outmoded ways of thinking, and that recognition of this constitutes deep new wisdom about the nature of human knowledge. I say that it constitutes a violent irrationality, that somewhere in this theory the distinction between reality and our knowledge of reality has become lost, and the result has more the character of medieval necromancy than of science.2

The ideal of science is that we are investigating a real situation that exists independently of us. But if we are not … then what are we scientists doing?

Notes