Physics > Metaphysics

Last time, we saw that in trying to establish the principle "Whatever changes is changed by something else," Feser argued that a potentiality can't be actualized by nothing, because then there would be no way of explaining when the change occurred.

This seems reasonable: the ball melts when it does because of the heat that has been applied to it, the window breaks when it does because the baseball hit it at that moment, and so forth.

But, over the last hundred years, physicists have discovered systems that change from one state to another without any apparent physical "trigger." These systems are described by quantum mechanics.

The simplest such system is the hydrogen atom. It's just an electron bound to a proton. Two particles - that's about as simple as you can get. According to QM, the electron can occupy one of a discrete set of energy levels. The electron can be excited to a higher energy level by absorbing a photon.

When the electron drops from a higher energy level to a lower level, it emits a photon: a quantum of light.



Quantum mechanics describes this process beautifully, but it only predicts the average time the electron will stay in the higher energy level. It doesn't give any clue as to the specific time the electron will drop to the lower level. More precisely, the transition rate (the probability of a transition per unit time) is constant: it doesn't matter how long it has been since the atom was excited, the transition rate stays the same.

Compare this to human lifetimes. There is some average human lifetime. But the probability of death ("transition to the lower state" - ha ha) depends on how long the person has been alive. A 90 year old has a much higher probability of dying in the next year than does a 30 year old.

In contrast, the electron has the same decay rate after one year as after one microsecond.

When you first encounter this, you can't quite wrap your brain around it. Surely there must be some internal mechanism, some kind of clock, that ticks along and finally "goes off," causing the transition!

But no such mechanism has ever been found. QM has had an unexcelled record of accurate predictions, without any need for such a mechanism. Further, we have good reason to suspect that, if there were such a mechanism, then QM would not be accurate in its predictions. (I'll come back to this point in a later post.) So, the absence of violations of QM is evidence that Feser's expectation - that there is always a reason for a change to happen when it does - is just wrong.

Feser seems to have missed out on the last hundred years of physics: he shows no hint that there might be a problem here. (His index includes "quantum mechanics" exactly once, in an unrelated context.) It seems that physics is not, after all, irrelevant to metaphysics.