We don't expect these things, we merely observe them. Indeed, the fact that QM is linear and hence time-reversible is violently at odds with everyday experience, so it is emphatically not the case that we "expect" these things. This just turns out to be how they are. The tensor product is merely the most compact description of these observations, kind of like how untyped lambda calculus turns out to be a compact description of universal computation (which is also not a thing that one would a priori expect).
> We don't expect these things, we merely observe them.
Sure, but your original claim was that
> It's the tensor product because there are logically no other possibilities. The tensor product says everything you can possibly say about the interactions of two systems whose states are described by a (possibly infinite) set of numbers and whose interactions correspond to some basic constraints, like being time-reversible.
I merely wanted to point out that your claim sounded quite broad and you need to assume many things about the mathematical structure of QM here (based on established observations of course). So, unless you simply take those for granted, you would have to come up with an explanation for them in order for there to be
> logically no other possibilities
In this case, though (if you take all those observations for granted), your claim becomes almost tautological IMO.
It is a tautology. The heavy lifting is being done by the phrase "whose interactions correspond to some basic constraints". Maybe the word "basic" implies more simplicity than is warranted, though if you actually write down what the constraints are, it's a short list and they are not very complicated. That those constraints lead to the tensor product is tautological. I'm not saying this is a Deep Insight, only that it is the answer to "Why the tensor product?"
BTW, just because something is a tautology doesn't mean it can't lead to deep insights. Darwinian evolution is a tautology too: if you have a variety of self-reproducing systems, then the ones that are better at reproducing will make more copies than those that are worse. Well, duh! That's what "better at reproducing" means! The thing that makes it a Deep Insight is that this tautological observation can actually explain some very complex data. Likewise, that the tensor product is the mathematical construct that describes linear interactions between systems that obey conservation laws is a tautology. What makes that a Deep Insight is not that, but the fact that the resulting relatively simple math makes surprising predictions (entanglement in particular) that turn out to be confirmed by experiment.
