Eight Experiments

Physics has traditionally been characterized as the science of matter in motion. Rough as this characterization is, it illuminates the standing of physics with respect to all other empirical sciences. Whatever else the objects of the various empirical sciences are, they are all instances of matter in motion. Every biological system, every economic system, every psychological system, every astronomical system is also matter in motion and so falls under the purview of physics. But not every physical system is biological or economic or psychological or astronomical. This is not to argue that these other empirical sciences reduce to physics, or that the other sciences do not provide an understanding of systems that is distinct from a purely physical account of them. Still, physics aspires to a sort of universality that is unique among empirical sciences and holds, in that sense, a foundational position among them.

The phrase "matter in motion" presents two targets for further analysis: "matter" and "motion." Present physics elucidates the "motion" of an object as its trajectory through space-time. A precise understanding of just what this is requires a precise account of the structure of space-time. The physical account of space-time structure has changed through the ages, and at present the best theory is the General Theory of Relativity. The nature of space-time itself and the geometrical structure of space-time is the topic of the companion volume to this one: Philosophy of Physics: Space and Time. The present volume addresses the question: What is matter? The best theory of matter presently available is quantum theory. Our main task is to understand just what quantum theory claims about the nature of the material constituents of the world.

As straightforward as this sounds, we must first confront a great paradox about modern physics. The two pillars on which modern physics rests are the General Theory of Relativity and quantum theory, but the status of these two theoretical systems is completely different. General Relativity is, in its own terms, completely clear and precise. It presents a novel account of space-time structure that takes some application and effort to completely grasp, but what the theory says is unambiguous. The more one works with it, the clearer it becomes, and there are no great debates among General Relativists about how to understand it. (The only bit of unclarity occurs exactly where one has to represent the distribution of matter in the theory, using the stress-energy tensor. Einstein remarked that that part of his theory is "low grade wood," while the part describing the space-time structure itself is "fine marble.") In contrast, no consensus at all exists among physicists about how to understand quantum theory. There just is no precise, exact physical theory called "quantum theory" to be presented in these pages. Instead, there is raging controversy.

How can that be? After all, dozens and dozens of textbooks of quantum theory have been published, and thousands of physics students learn quantum theory every year. Some predictions of quantum theory have been subjected to the most exacting and rigorous tests in human history and have passed them. The whole microelectronics industry depends on quantum-mechanical calculations. How can the manifest and overwhelming empirical success of quantum theory be reconciled with complete uncertainty about what the theory claims about the nature of matter?

What is presented in the average physics textbook, what students learn and researchers use, turns out not to be a precise physical theory at all. It is rather a very effective and accurate recipe for making certain sorts of predictions. What physics students learn is how to use the recipe. For all practical purposes, when designing microchips and predicting the outcomes of experiments, this ability suffices. But if a physics student happens to be unsatisfied with just learning these mathematical techniques for making predictions and asks instead what the theory claims about the physical world, she or he is likely to be met with a canonical response: Shut up and calculate!

What about the recipe? Is it, at least, perfectly precise? It is not. John Stewart Bell pressed just this complaint:

A preliminary account of these notions was entitled 'Quantum field theory without observers, or observables, or measurements, or systems, or apparatus, or wavefunction collapse, or anything like that'. That could suggest to some that the issue in question is a philosophical one. But I insist that my concern is strictly professional. I think that conventional formulations of quantum theory, and of quantum field theory in particular, are unprofessionally vague and ambiguous. Professional theoretical physicists ought to be able to do better.

Bell's complaint is that the predictive recipe found in textbooks uses such terms as "observer" and "measurement" and "apparatus" that are not completely precise and clear. This complaint about quantum theory does not originate with Bell: Einstein famously asked whether a mouse could bring about drastic changes in the universe just by looking at it. Einstein's point was that some formulations of quantum theory seek to associate a particular sudden change in the physical state of the universe ("collapse of the wavefunction") with acts of observation. If this is to count as a precise physical theory, then one needs a precise physical characterization of an observation. As Bell put it: "Was the wavefunction of the world waiting to jump for thousands of millions of years until a single-celled living creature appeared? Or did it have to wait a little longer, for a better qualified system ... with a Ph.D.?"

These imprecisions in the formulation of the quantum recipe do not have noticeable practical effects when it comes to making predictions. Physicists know well enough when a certain laboratory operation is to count as an observation, and what it is an observation of. Quantum theory predicts the outcomes of these experiments with stunning accuracy. But if one's main interest is in the nature of the physical world rather than the pragmatics of generating predictions, this ability is of no solace. For the recipe simply does not contain any univocal account of the world itself. To illustrate this, the standard recipe does use a mathematical operation that can be called "collapse of the wavefunction." But if one asks whether that mathematical operation corresponds to a real physical change in the world itself, the recipe does not say. And practicing physicists do not agree on the answer. Some will refuse to hazard an opinion about it.

Bell's complaint might seem incredible. If the problems with quantum theory are not "merely philosophical" but rather consist of the theory being unprofessionally vague and ambiguous as physics, why don't the physics textbooks mention this? Much of the problem has been papered over by a misleading choice of...