Perhaps the easiest way to see the motivation for, and the meaning of, the concept I call “macroscopic realism” is to start by discussing what it is not. Of course we are all realists in the context of our everyday lives—how could we not be? Chairs are on this side of the table or the other, meters read 5 amps or 10 amps or some other definite number, particle counters either click or do not, and so on. The difficulty is, of course, that if we take quantum mechanics to be a complete theory of the physical world, then...

Anthony Leggett’s studies of quantum effects in macroscopic systems have stimulated the investigation of fascinating phenomena and will almost certainly lead to the discovery of others. Since the contemplation of phenomena with pleasure is an essential part of the enterprise of natural philosophy, we philosophers thank him for introducing us to macroscopic tunneling of magnetic flux trapped in a SQUID ring, for the prospect of seeing the resonance effect that he calls “macroscopic quantum coherence,” and for his discussions of numerous related effects in Josephson junctions, magnetic grains, and macromolecules.

In addition, Leggett’s program is extraordinarily valuable for deepening our...

Consider quantum theory without the collapse postulate. No experiment has ever disconfirmed its statistical predictions. So one might reasonably bet that no experiment ever will. If not, every possible state of anything in the universe, from the spin states of electrons to the states of consciousness of sentient beings, must evolve in time according to Schrödinger’s equation. A theoretician’s paradise … except for that nagging problem of how to make sense of what the theory says will happen when we try to confirm its statistical predictions.

The problem is worth rehearsing. Supposemis a device that reliably records whether...

There is a serious and unresolved quantum measurement problem. Some, like Ghirardi, Rimini, and Weber (1986), try to solve it by modifying quantum mechanics. If successful, such attempts would result in a theory, distinct from but closely related to quantum mechanics, that is no longer subject to a measurement problem. But the conceptual problem—that of reconciling quantum mechanics itself with the possibility of the very measurements we take to warrant its acceptance—would remain. That problem may be insolvable. But it is my belief that recent developments, some formal, others conceptual, promise hope for a solution.

So-called “modal” interpretations...

Since its introduction several years ago, Wojciech Zurek’s “existential interpretation” (Zurek 1993a, b) has incited passionate debate among physicists and philosophers. Most philosophers remain puzzled about his interpretation. But at the conference on “Decoherence and Modal Interpretations” in Minneapolis in 1995, we held a discussion in which Zurek clarified his interpretational stance. He was responding to my “commentary” about his paper, in which I displayed and discussed a bunch of interpretational “boxes.” After arguing that Zurek’s interpretation must fit into one of the boxes, I asked him which one. Zurek’s answer, and the ensuing discussion, helped to clear up some...

Most versions of the modal interpretation of quantum mechanics are based on van Fraassen’s (1973) intuition that the uncollapsed reduced state of a quantum-mechanical system, which is generally a mixed state, describes in the first place not the actual properties but the possible properties of the system.

According to van Fraassen (1973; 1991, chapter9), the projections that appear in any decomposition of the reduced state of the system all represent possible properties. In most of the later versions of the modal interpretation, a specific decomposition of the reduced state is privileged, namely, the spectral (orthogonal, diagonal) decomposition. These versions are...

Without a sound policy it is tricky business to assign definite values to quantum-mechanical magnitudes. As Kochen and Specker (1967) have shown, a value assignment to all magnitudes of a quantum system can lead to contradictions. The modal interpretation of quantum theory assigns definite values to a limited set of magnitudes without contradictions. It is my aim to expand this limited set as much as possible.

Take a systemαwith a state represented by a density operatorW, and let the projections {P_j}_jbe the eigenprojections ofW. Then, in the modal interpretation in the version¹ by Vermaas and...

The two leading ideas of the version of the modal interpretation that is our starting point here are

1. Quantum mechanics should be interpreted in terms ofpropertiespossessed by physical systems, so that the “measurement problem” no longer occurs (measurement results are treated as properties possessed by a measuring device);

2. The properties assigned to a system should be definable from the quantum-mechanical state—no additional structure should be introduced (in this sense the interpretation is not a “hidden-variables interpretation”). (See Dieks 1994, 1995; Vermaas and Dieks 1995.)

In order to implement the idea that physical properties should be attributed, without...

There are by now many children and grandchildren of van Fraassen’s (1972) modal interpretation of quantum mechanics.¹ These interpretations share the same central idea: quantum-mechanical systems can possess properties even if the state of the system does not confer probability 1 on that property. The usual slogan is that modal interpretations “deny the eigenstate-eigenvalue link,” which means that a system can possess the valueafor the observableAwithout being in the eigenstate |a〉 (whereA|a〉 =a|a〉). In the terms that will be used here, a system can possess the property corresponding to the projection operator |a〉〈a| (=...

The problem of quantum measurement has been with us since the foundations of the theory were laid in the mid 1920s. It has generated much discussion, with little resolution of the questions raised. I will argue in this chapter that this situation has arisen in part because of the confusion brought about by giving two very different concepts the same name, with the expected result that the valid questions related to the two concepts become entangled. It furthermore has led to a restriction on the types of measurements considered within the theory. I am not going to propose any radical...