Philosophical interpretation of classical physics

Philosophical interpretation of classical physics is used here to mean the consideration of the probabilities arising in quantum mechanical experiments from the point of view that quantum mechanics is reality and does not require further philosophical interpretation. This subject is covered in the early chapters of quantum mechanics texts under headings such as "Uncertainty Relations and the Measurement Process".

Historically quantum mechanics is called probabilistic because experiments that involve quantum mechanics always involve probabilities. There are at least two problems with this old way of speaking. One is that it leads people to mistakenly think that these probabilities go on in unmeasured quantum mechanical systems (See reference below). The other is that, when either is possible, it is philosophically unclean to assign the probabilities to the general theory rather than to its approximations.

Heisenberg uncertainty principle says that a particle does not have an exact position and an exact momentum at the same time. The fact that one cannot measure them is a corollary. Though quantum mechanics may not be the ultimate reality, it is much more real and "physical" than classical mechanics and classical electromagnetism. The correspondence principle says that these are approximations to quantum mechanics. If there is a future theory that replaces quantum mechanics, it will have a "super correspondence principle" that it reduces to quantum mechanics in cases such as atoms and chemistry.

The measurement process

In the measurement process, new particles, such as light are brought in to perform the measurement. If, at first, these measuring particles are described quantum mechanically, the description remains deterministic and no probabilities arise. However to get the information into a notebook or (non-quantum) computer, it must be brought to the human scale where maintaining phase coherence is impossible. Because the classical approximation does not conform to the uncertainty principle, it contains information that the quantum system, which does conform, cannot supply. This (non-physical) information is generated randomly. In addition phase information in the quantum description cannot be represented classically, and is lost. Messiah's example is measuring the position of an electron with light. If the light's wave function is not know and included in the system wave function, the predictions are of probabilities, because the light photons exchange unknown amounts of momentum with the electron.

Persistence of the idea that classical physics is primary

Even though we have all grown up after the discovery of quantum mechanics, the orientation that classical physics is primary has persisted for several reasons. As often noted, the basic reason is that classical physics is a refinement of our every day life perceptions, while quantum mechanics could only be discovered with the tools of physics. If one thinks of himself as a wave, he will never get his feet on the ground when he tries to get up in the morning. Even though we grow up hearing the term "quantum mechanics", we don't really see it until we learn differential equations, and by that time we are used to thinking in classical terms. The Copenhagen interpretation was formulated while quantum mechanics was new and no one was used to it, so it describes quantum mechanics in terms of classical physics in a way that is adequate for practical purposes, but it does not go so far as to call quantum mechanics primary and classical physics an approximation to it that contains meaningless information.

Not accepting quantum mechanics was made respectable by Albert Einstein. He bears a relation to quantum mechanics similar to Isaac Newton's relation to the wave theory of light or that of the last great alchemists to chemistry. He helped discover it but never really believed it.

References

Sources

• Albert Messiah, Quantum Mechanics, English translation by G. M. Temmer of Mécanique Quantique, 1966, John Wiley and Sons
• A lecture to his statistical mechanics class at the University of California at Santa Barbara by Dr. Herbert P. Broida [1] (1920-1978) (a well known experimental physicist)

Examples of errors caused by out of date philosophy

• "Physics and the Real World" by Carlos Bustamante, Jan Liphardt, and Felix Ritort, July, 2005