I’ve lately been finding it really difficult to get myself interested in alleged metaphysical issues stemming from assuming that a certain physical theory applies to the entire universe. Many of the ‘philosophical’ problems of quantum mechanics, for example, are of this ilk, as are many in algebraic quantum field theory and general relativity.
The standard practice in philosophy of physics is to use a so-called fundamental theory for your metaphysical inferences. Effective theories are for the most part ignored. The intuition is that the ‘fundamental theory’ describes ‘what is really out there’ while effective theories are somehow more phenomenological, or derivative of the fundamental theory (and hence need not be considered in addition to the fundamental theory).
A question arose in class today as to why is it that people who worry about entanglement in quantum mechanics typically worry about it in relation to special relativity rather than general relativity. The standard answer is that gravity is an effect that is negligible in the entanglement experiments we are considering, so we do not have to worry about what a quantum theory of gravity would have to say about the issue.
That got me wondering about how far someone could use that answer and still maintain that it is useful to figure out what the metaphysics of our world is by supposing that quantum mechanics applies to the entire universe. My worry is this. By using the ‘gravity is negligible’ reason, one is admitting that quantum mechanics is really just another effective theory — it has a limited domain of application. If so, then either
- One thinks that in general it is legitimate to derive metaphysical conclusions using effective theories, or
- One thinks that there is something special about quantum mechanics as an effective theory, which allows one to derive metaphysical conclusions from it, as opposed to other effective theories that are typically ignored (e.g. effective field theories).
If one goes with 1., then it seems to me that the right way to do scientifically-informed metaphysics is to take the various theories we have as each being informative about their respective domains of applicability. This has the implication that we should not be applying quantum mechanics to the entire universe and taking the metaphysical implications of that seriously. For it is classical theories that are most effective at large size scales, not quantum mechanics.
As for 2., I am still struggling to imagine what could be special about quantum mechanics that licenses us to treat it in a different way from other effective theories. One possible reason is that one thinks that the most important aspects of quantum mechanics will still persist in a ‘final theory’ which applies to the entire universe. But whatever these preserved aspects are, it’s not clear to me that they are the same aspects as those that lead to the traditional philosophical problems in quantum mechanics. It might be that the mathematics of the final theory is such that the problems with locality and whatnot that manifest themselves in quantum mechanics are somehow dodged. One can apply the same consideration to other issues in philosophy of physics. Maybe underdetermination in general relativity won’t actually translate to an underdetermination problem in the final theory.
In addition, if you look at the history of physics, it doesn’t seem to me that the aspects of older theories that are preserved in newer theories are those which tend to preserve philosophical problems in the older theories. It doesn’t seem to me as though any of the aspects of classical physics that are preserved in quantum mechanics are those that are philosophically problematic for either theory. In other words, the robust aspects of physical theories often aren’t those that lead to traditional philosophical problems.
Someone help me out here. I seem to be missing out on a lot of fun by being so pessimistic about this enterprise of reading metaphysics off ‘fundamental’ theories.