Getting physical possibility straight: what makes an event physically possible?

CRISTIAN LÓPEZ; MANUEL HERRERA

Bratislava

Congreso; East European Network for Philosophy of Science; 2018

There is a broadly-assumed viewpoint in philosophy of science and philosophy of physics supporting the idea that physical possibility is determined by what is possible according to the physical laws that obtain in the actual world. Since, in philosophy, a possible world is a collection of possible events representing possible but alternative stories to the actual world, a physically possible world is any possible world in which natural laws that obtain in the actual world are satisfied.This view allows introducing more rigorously the modal parlance in physics and to differentiate the notion of logical and metaphysical possibility from physical possibility. John Earman, in his famous A Primer on Determinism, has promoted this conventional wisdom on the matter in arguing that an event is physically possible if and only if it is allowed by actual world?s physical laws. Accordingly, an event is said to be physically possible if and only if there is a physically possible world in which such event may occur. One of the strongest reasons for favoring this view is that physical laws play an essential role in scientific explanation and modal generalizations. Physical possibility, theoretical explanation and physical laws seem to go hand-in-hand to the extent that the physical content of a theory is outwardly determined by the set of possible worlds wherein the theory is true, to wit, wherein its physical laws hold.Notwithstanding this widely-held viewpoint, there are relevant contexts in which physical possibility has been understood a bit differently. As well known, certain events or explanations are discarded by the physics community for being physically impossible though keeping with laws of nature. Frequently, factual or material circumstances (as a particular setting or boundary conditions) are invoked for establishing that an event is physically impossible despite conforming to the theory?s laws. In other cases, an event or solution are regarded as physically impossible due to violation of metaphysical principles. For instance, some solutions (that is, models of the universe) of Einstein?s field equation are not seriously taken to be physically possible as they involve closed timelike curves that might violate the principle of causality. Finally, purely mathematical considerations are sometimes also brought up: solutions featuring wormholes or white holes are discarded as physically possible solutions as they are mathematically unstable. Overall, physicists? discourse is pledged of extra-nomic criteria to speak about what is physically possible and what is not. In this presentation we shall show that the conventional viewpoint on physical possibility turns out to be inadequate to sharply capture the notion of physically possible as conceived in scientific practice. As mentioned above, we shall argue that physicists typically invoke various extra-nomic criteria when determining the physically possible, taking examples from general relativity. Even though such criteria are not always clear or explicit in the scientific discourse, the point we want to make is that they matter in something like the way physical laws are generally supposed to matter: they favor certain generalization and forbid others, and they also play an essential role in explanation of phenomena. By involving such criteria, we hope not only to reach a far-reaching, practice-based understanding of physical possibility, but also to get a sharper notion of what is the theoretical content of a physical theory.