So true propositions including laws would be false if the world had been different, which he says absolutely cleary.
We should not mix up two different ways of creating ‘possible worlds’:
1. a possible world where there are other laws of physics
2. a possible world with the same laws of physics but other events occurring
You suggest Swartz mixes those two. I am sure he doesn’t. His ‘possible worlds’ are of category 2: the laws of physics (chemistry, biology…) are fixed, but we don’t know them all. By doing their experiments his researchers discover which law describes the movement of the arm. It is determined what movement the research object will do, but the researchers do not know how it is determined. They find the real description by seeing if the research object will raise his arm or not. In this way the description follows the raising of the arm (or not). But not the law of nature itself.
Another point that may create confusion: the meaning of physically possible. You are continuously taking it to mean:
a. According to the laws of physics and the initial conditions of the universe
Doug and I using it as:
b. According to the laws of physics
To extend b. a little: an event is physically possible when there is a set of initial conditions that produce this event according the laws of physics.
You are right when you use definition a. that if I did not want to go to Paris last weekend in this universe, then it is physically impossible. I said that several times. But even in this universe, the sentence “If I had wanted to go to Paris last weekend, I could have gone to Paris” is meaningful and true.
In the first place because of formal logic: the sentence as a whole is still true even if the antecedent is not true, even if it is not true because it describes a physically impossible event. What the sentence means is that (o shit here it comes…) that when I imagine that everything is exactly the same at that moment, that only a tiny change (now I wanted to go to Paris), would suffice for me to have gone to Paris. But in this formal meaning of the sentence, I am not interested in if that would be physically possible or not, i.e. if this event would be possible given how the universe banged.
In the second place there is the conventional meaning of such sentences: everyday people are travelling to Paris, because they want to. It is possible: there are trains, planes, touring cars, and all affordable for normal people like you or me. That means: if I want to, I could go to Paris. So what it means is that in a huge number of very similar cases, people go to Paris, because they want to. Referring to the big bang as condition for people to able to go to Paris is an overly technically interpreted way of looking at such sentences. That’s why I say that you sticking to this technical meaning means you are suffering from the philosophical disease.
But both interpretations make it clear: a going back to exact conditions that took place far before the events happening is in no way needed.