Copyright © 2004 Krishna Myneni

Quantum theory is the crown jewel of modern science. It is the basis of much of modern technology, such as the semiconductor p-n junction, of which digital computers are the most important result. Quantum theory also led to the development of nuclear reactors (and nuclear weapons), lasers, and magnetic resonance imaging (MRI). In basic science, quantum theory is the foundation for the study of fundamental particles, nuclei, atoms, molecules, solids, light, and the interaction of these systems. The chemistry of simple molecules can be quantitatively predicted using quantum theory, and the behavior of more complicated systems, consisting of hundreds of atoms, can be understood qualitatively by applying the theory. Quantum theory explains many observations for which classical physics (physics as it existed before relativity theory and quantum theory) is an utter failure at providing either qualitative or quantitative explanations.

In the early 20th century, quantum theory began humbly as a somewhat
contrived way of explaining the colors of light emitted by heated
materials. In the span of thirty years or so (see George Gamow's book,
**Thirty Years That Shook Physics**), the theory was made mathematically
rigorous and achieved the enormous successes stated above.
And yet, even in the 21st century, many who have studied it
in-depth, are disturbed by this eminently successful theory of the
way the world works at its very roots. There is indeed something very
strange and troubling about what the quantum theory appears to
imply. To cut to the heart of the problem, quantum theory
seems to say that the physical properties which we can measure
for the basic particles, of which everything is made, don't really
exist. That is, a given property of an electron, such as its position,
velocity, energy, etc. is *not physically real* until that property is
*measured*. For example, an electron does not have a position
until we ask it, by performing a measurement, to tell us its
position. The electron then obliges and gives us its position
through the reading of our instrument. Quantum theory can tell
us, under a carefully prepared experiment, what the
*probabilities* are for different results. By applying the
theory, we can predict the probability that the electron will tell
us, through the instrument, that it is located in a particular region in space.
But, in general, it can tell us nothing more than these probabilities.
Quantum theory *cannot* tell us exactly what result we would
obtain for our measurement! Quantum theory predicts probabilities of
outcomes for measurements, not the actual results! It is this
distinction with classical physics that sowed the seeds of
the physicists' angst.

Prior to John Bell and his famous theorem of 1964, most physicists
would have dismissed the following question:

There is no point to this question they would say, for we cannot know until we make the measurement. Even though the patriarch of quantum theory, Neils Bohr, insisted that the electron did not really have a particular position before we measured it, many physicists at the time thought there was no need for Bohr's rather hard stance on this subject, or even thought that Bohr must be wrong, as did Einstein. But to most it didn't matter whether Bohr was right or not. After all, what's the harm in picturing the electron, a tiny dark point, moving silently and unsuspectingly in space before being violently jolted by its collision with the recording instrument, but always having a definite position nonetheless? Many physicists took a guilt-ridden, secret comfort in such a mental picture. But to any student who asked if the electron had a definite position prior to the measurement, the physicist kept a stiff upper lip and gave the usual response that the question was meaningless, because it was unknowable. One student in particular, John Bell, did not like this response from his professors, and set out to see for himself whether or not Bohr's stance on the reality of physical properties was testable. Years later, as a working particle physicist, he found out something remarkable. Most physicists were not immediately aware of Bell's results, but gradually became aware of them, and were stunned!