At the
International Congress of Mathematicians of 1900,
David Hilbert presented his famous list of twenty-three problems considered central for the development of the mathematics of the new century. The sixth of these was
the axiomatization of physical theories. Among the new physical theories of the century the only one which had yet to receive such a treatment by the end of the 1930s was quantum mechanics. QM found itself in a condition of foundational crisis similar to that of set theory at the beginning of the century, facing problems of both philosophical and technical natures. On the one hand, its apparent non-determinism had not been reduced to an explanation of a deterministic form. On the other, there still existed two independent but equivalent heuristic formulations, the so-called
matrix mechanical formulation due to
Werner Heisenberg and the
wave mechanical formulation due to
Erwin Schrödinger, but there was not yet a single, unified satisfactory theoretical formulation.
After having completed the axiomatization of set theory, von Neumann began to confront the axiomatization of QM. He immediately realized, in 1926, that a quantum system could be considered as a point in a so-called
Hilbert space, analogous to the 6N dimension (N is the number of particles, 3 general coordinate and 3 canonical momentum for each) phase space of classical mechanics but with infinitely many dimensions (corresponding to the infinitely many possible states of the system) instead: the traditional physical quantities (e.g. position and momentum) could therefore be represented as particular
linear operators operating in these spaces. The
physics of quantum mechanics was thereby reduced to the
mathematics of the linear Hermitian operators on Hilbert spaces. For example, the famous
uncertainty principle of Heisenberg, according to which the determination of the position of a particle prevents the determination of its momentum and vice versa, is translated into the
non-commutativity of the two corresponding operators. This new mathematical formulation included as special cases the formulations of both Heisenberg and Schrödinger, and culminated in the 1932 classic
The Mathematical Foundations of Quantum Mechanics. However, physicists generally ended up preferring another approach to that of von Neumann (which was considered elegant and satisfactory by mathematicians). This approach was formulated in 1930 by
Paul Dirac.
In any case, von Neumann's abstract treatment permitted him also to confront the foundational issue of determinism vs. non-determinism and in the book he demonstrated a theorem according to which quantum mechanics could not possibly be derived by statistical approximation from a deterministic theory of the type used in classical mechanics. This demonstration contained a conceptual error, but it helped to inaugurate a line of research which, through the work of
John Stuart Bell in 1964 on
Bell's Theorem and the experiments of
Alain Aspect in 1982, demonstrated that quantum physics requires a
notion of reality substantially different from that of classical physics.
In a complementary work of 1936, von Neumann proved (along with
Garrett Birkhoff) that quantum mechanics also requires a
logic substantially different from the classical one. For example, light (photons) cannot pass through two successive filters which are polarized perpendicularly (e.g. one horizontally and the other vertically), and therefore,
a fortiori, it cannot pass if a third filter polarized diagonally is added to the other two, either before or after them in the succession. But if the third filter is added
in between the other two, the photons will indeed pass through. And this experimental fact is translatable into logic as the
non-commutativity of conjunction <math>(A\land B)\ne (B\land A)</math>. It was also demonstrated that the laws of distribution of classical logic, <math>P\lor(Q\land R)=(P\lor Q)\land(P\lor R)</math> and
<math>P\land (Q\lor R)=(P\land Q)\lor(P\land R)</math>, are not valid for quantum theory. The reason for this is that a quantum disjunction, unlike the case for classical disjunction, can be true even when both of the disjuncts are false and this is, in turn, attributable to the fact that it is frequently the case, in quantum mechanics, that a pair of alternatives are semantically determinate, while each of its members are necessarily indeterminate. This latter property can be illustrated by a simple example. Suppose we are dealing with particles (such as electrons) of semi-integral spin (angular momentum) for which there are only two possible values: positive or negative. Then, a principle of indetermination establishes that the spin, relative to two different directions (e.g.
x and
y) results in a pair of incompatible quantities. Suppose that the state
ɸ of a certain electron verifies the proposition "the spin of the electron in the
x direction is positive." By the principle of indeterminacy, the value of the spin in the direction
y will be completely indeterminate for
ɸ. Hence,
ɸ can verify neither the proposition "the spin in the direction of
y is positive" nor
the proposition "the spin in the direction of
y is negative." Nevertheless, the disjunction of the propositions "the spin in the direction of
y is positive or the spin in the direction of
y is negative" must be true for
ɸ.
In the case of distribution, it is therefore possible to have a situation in which
<math>A \land (B\lor C)= A\land 1 = A</math>, while <math>(A\land B)\lor (A\land C)=0\lor 0=0</math>.