A student named Maaneli decided to defend his favorite theory, the Bohmian model of quantum phenomena.
Update: See also Anti-quantum zeal for a sociological discussion of these issues.This picture, originally pioneered by Louis de Broglie in the late 1920s under the name "pilot wave theory", was promoted and extended by David Bohm in the 1950s. Because Bohm was a holy Marxist while de Broglie was just a sinful aristocrat, the term "Bohmian mechanics" for de Broglie's theory has been used for decades and I will mostly follow this convention, too. ;-)
In this approach, the wave function is an actual wave, a "pilot wave", analogous to the electromagnetic waves. Besides these classical degrees of freedom, there are additional classical degrees of freedom, the positions and velocities of the particles.
It is not surprising in any way that the new, Bohmian equation for "X(t)" can be written down: it is clearly always possible to rewrite the Schrödinger equation as one real equation for the squared absolute value (probability density) and one for the phase (resembling the classical Hamilton-Jacobi equation). And it is always possible to interpret the first equation as a Liouville equation and derive the equation for "X(t)" that it would follow from. There's no "sign of the heavens" here.
That's a tragic tumor, a seed of self-destruction, inserted into the very heart of the pilot wave theory but the Bohmists must obviously try to transform this tumor into a virtue. While Bell would say that the spin and similar observables are "contextual", Shelly Goldstein et al. write:
... referred to as contextuality, but we believe that this terminology, while quite appropriate, somehow fails to convey with sufficient force the rather definitive character of what it entails: Properties which are merely contextual are not properties at all; they do not exist, and their failure to do so is in the strongest sense possible.How nicely radical these people are in writing the very opposite of the actual truth :-) - which says that the ontological reality and the degree of existence of the spin is, in the strongest possible sense, equivalent to the ontological reality of the position or any other observable. And even the previous sentence fails to convey the key point that those who fail to recognize the equal philosophical status of all observables shouldn't have passed their introductory quantum mechanics courses. :-)
In order to celebrate the Martin Luther King Jr Day, I will dedicate the rest of the text to a fight against the segregation of observables. :-) So my statement is very modest - that observables can't be segregated into the "real" primitive ones and the "fictitious" contextual ones - a fact that trivially rules out all theories (such as the Bohmian ones) that are forced to do so. There are many arguments to see why I am right:
One doesn't have to consider a quantum computer. A simple polarizer is a gadget that can measure the x/y linear polarization of a photon - its form of an internal, discrete, spin-like degree of freedom - without changing the location of the photon (which would be pretty hard to do). You can do similar things with the electron, too. In all these cases, the Bohmian theories have no chance to explain why the photon is sometimes absorbed and sometimes it is not. The Stern-Gerlach device is not the only method to feel the spin!