I am currently in the process of reading an enlightening short book entitled Quantum Chance by Nicolas Gisin, an authority on the foundations of quantum mechanics. You can actually download it for free here if your institution has a Springer publishing subscription. The book stands out as being accessible to the educated lay reader without sacrificing much in the form of profundity.
The main topics of this book are the implications of Bell’s theorem. Prior to Bell’s paper (pdf!), a possible view of quantum mechanics was that it predicted the statistical distribution of many events. However, back then, it was plausible that there existed an underlying theory, one we had yet to discover, that was described by a set of “hidden variables”. The idea was that these hidden variables would allow us to calculate the trajectory of a single particle deterministically, but that we just didn’t know the equations obeyed by the hidden variables. Quantum mechanics was just an approximate theory allowing us to calculate probability distributions of many events.
Behind the hidden variable theory was a philosophical stance called realism, strongly espoused by Einstein. Simply stated, realism is the belief that reality exists independent of the observers. This is counter to the orthodox view of quantum mechanics, which was emphasized by Bohr. The orthodox view is that the measurement of quantum systems causes a “wave function collapse” and that observables have no meaning until they are measured. The implication of the orthodox view is that reality is in the eyes of the observer and does not exist independent of the observer. There are other interpretations of quantum mechanics out there as well, but it is my understanding that these were the two prevailing views before Bell worked out his theorem.
Even though Bohr found it quite easy to give up the notion of realism, I find it quite difficult to abandon. In the very least, one should be able to describe the mechanism giving rise to “wave function collapse” if this indeed even occurs. Regardless, Bell’s theorem, when it was published in 1964 (pdf!), showed that local realism was untenable in quantum mechanics.
What does this mean? Well, I’ve described what realism means, so let me now take on locality, which is implicit in the hidden variables idea in the way Einstein originally conceived of it. According to Wikipedia “the principle of locality states that an object is only directly influenced by its immediate surroundings”. This sounds quite vague, but Bell was able to show in a rigorous mathematical sense that if Bell’s inequality was violated, that an event on one side of the universe can instantaneously affect another event on the other side of the universe. Stunningly, experiments suggest that quantum mechanics does indeed appear to violate Bell’s inequality.
For a realist (and for adherents to most other interpretations of quantum mechanics), Bell’s theorem then suggests that the universe is inherently nonlocal. This notion of nonlocality, the idea that two things are somehow connected over vast empty space on an instantaneous time scale, bothered both Einstein and Newton greatly. Newton, whose theory of gravity is also nonlocal said:
It is inconceivable that inanimate Matter should, without the Mediation of something else, which is not material, operate upon, and affect other matter without mutual Contact…That Gravity should be innate, inherent and essential to Matter, so that one body may act upon another at a distance thro’ a Vacuum, without the Mediation of any thing else, by and through which their Action and Force may be conveyed from one to another, is to me so great an Absurdity that I believe no Man who has in philosophical Matters a competent Faculty of thinking can ever fall into it. Gravity must be caused by an Agent acting constantly according to certain laws; but whether this Agent be material or immaterial, I have left to the Consideration of my readers.
I suspect that the solution to the nonlocality problem in quantum mechanics may end up needing a large conceptual overhaul. It is going to take a work of great insight to preserve locality, if it indeed can at all be preserved in some contrived way. Whatever the solution to this problem, I hope that I am alive to see it. I won’t be betting on it though.