Can contradictory things happen at the same time? Does the universe continue about its business when we're not looking at it? These questions have been raised in the context of quantum mechanics ever since the theory was formulated in the 1920s. While most physicists dismissed these issues as "just philosophical", a small minority (inspired by the examples of Louis de Broglie, Albert Einstein and Erwin Schroedinger) continued to question the meaning of the most successful theory of science, and often suffered marginalisation and even ridicule.
It is one thing to apply quantum mechanics to calculate atomic energy levels or the rate at which atoms emit light. But as soon as one asks what is actually happening during an atomic transition, quantum mechanics gives no clear answer. The Copenhagen interpretation, forged by Niels Bohr and Werner Heisenberg, emphasises the subjective experience of "observers" and avoids any description of an objective reality; it talks about the chances of different outcomes occuring in a measurement, but does not say what causes a particular outcome to occur. For decades, students have been taught to avoid asking probing questions. An attitude of "shut-up-and-calculate" has dominated the field. The result is widespread confusion, and a strange unwillingness to ask clear and direct questions. As the late cosmologist Dennis Sciama once put it, whenever the subject of the interpretation of quantum mechanics comes up "the standard of discussion drops to zero".
The publication of John Bell's book Speakable and Unspeakable in Quantum Mechanics in 1987 provided a point of reference for a change in attitude that gained real momentum in the 1990s.
Bell spearheaded a movement to purge physics of some inherently vague notions inherited from the founding fathers of quantum mechanics. For instance the "measurement apparatus" was treated by Bohr and Heisenberg as something fundamentally distinct from the "system being measured": the latter was subject to the laws of quantum mechanics whereas the former was not. But if everything — including our equipment — is made of atoms, how can such a distinction be anything more than an approximation? In reality everything — "system", "apparatus", even human "observers" — should obey the same laws of physics. The clarity of Bell's writings forced many people to confront the uncomfortable fact that quantum mechanics as usually formulated had a problem explaining why we see definite events taking place.
Bell advertised what he saw as two promising avenues to resolve the quantum paradoxes: the theory must be supplemented either with a new random process that selects outcomes (the "dynamical reduction of the state vector") or with extra "hidden variables" whose unknown values select outcomes. Theories of both types have been constructed. Indeed, a correct hidden — variables theory was written down by Louis de Broglie as long ago as 1927, and was shown by David Bohm in 1952 to account completely for quantum phenomena. The de Broglie — Bohm theory gave an objective account of quantum physics; yet, until about 10 years ago, most physicists had not heard of it. Today, many have heard of it, but still very few understand it or work on it. And it is still not taught to students (even though in my experience many students would love to know more about this theory).
One wonders where things will go from here. On the one hand, in the last five years the subject of the interpretation of quantum mechanics has suddenly become more respectable thanks to the rising technology of quantum information and computation, which has shown that something of practical use — novel forms of communication and computation — can emerge from thoughts about the meaning of quantum mechanics. But on the other hand, there is a danger that the problem of the interpretation of quantum mechanics will be pushed aside in the rush to develop "real" technological applications of the peculiarities of quantum phenomena.
The rise of quantum information theory has also generated a widespread feeling that "information" is somehow the basic building block of the universe. But information about what? About information itself? As noted by P.W. Anderson in a recent Edge comment on Seth Lloyd, not only does it seem unjustified to claim that "information" is the basic stuff of the universe: worse, an unfortunate tendency has developed in some quarters to regard the theory of information as the only really fundamental area of reseach. Personally, I find quantum information theory very interesting, and it has without doubt enriched our understanding of the quantum world: but I fear that in the long run its most enthusiastic practitioners may lead us back to the vague subjectivist thinking from which we were only just emerging.