The argument so far: Feser wants to establish that "Whatever changes is changed by something else," a crucial premise (as we will see later) for Aquinas's First Way of proving the existence of God. To do so, he has relied on the idea that when a change occurs there is always an explanation for why it occurred at that particular time. But the study of quantum systems gives us strong reason to doubt that there is always an explanation for why a change occurs at a particular time.
Now, there's another reason to doubt "Whatever changes is changed by something else." It's Newton's First Law of Motion, aka the law of inertia: Whatever is in motion will stay in motion, unless acted on by an outside force. So here is another kind of change (a change of position) that doesn't require an outside changer.
(In fact, the First Way is traditionally known as the argument to a First Mover: "Whatever moves is moved by something else." Feser says that "move," in the Aristotelian-Aquinean sense, is more properly translated as "change." I assume he is correct in this.)
Feser doesn't address the issue with quantum mechanics, but he does address the issue of inertia, or "local motion." He gives three reasons (p.102) inertia is irrelevant to Aquinas's argument.
1. Aquinas means change in general, and not just actual movement. The argument applies to changes of temperature, of state, etc. Thus, "...even if we were to grant that the local motion of an object needn't be accounted for by reference to something outside it, there would still be other kinds of motion to which Aquinas's argument would apply."
But to say this is to get rid of "Whatever changes is changed by something else," and replace it with "Some things that are changed are changed by something else," or perhaps, "Every change of type X is caused by something else." Feser doesn't tell us what the replacement premise is. But, as we will see, none of these possible replacements is sufficient for the First Mover argument to go through.
2. "Second, whether or not an object's transition from place to place would itself require an explanation in terms of something outside it, its acquisition or loss of momentum would require such and explanation, and thus lead us once again to an Unmoved Mover."
Here the same objections apply as in (1.).
3. "Third, the operation of Newton's first law is itself something that needs to be explained...." If it is just in the nature of things that they continue to move, then we need an explanation of why they have that nature. This, Feser says, leads us to the Second Way.
But if it is just in the nature of things to change without any requirement of an external changer (as in local motion or quantum processes), then we have completely abandoned the premise "Whatever changes is changed by something else." If all sorts of things can change without requiring an external changer, then there is no way to argue to a First Changer.
I'll show this in detail next time. But for now, notice that Feser's (3.) doesn't, in fact, show that the inertia problem is irrelevant to the First Way. (3.) may lead to some other argument for God, but it doesn't resolve the issue: that local motion is a change without any external changer.