Philosophy of Physics

What it asks

Quantum mechanics is the most accurately tested theory ever produced. It is also a set of rules for calculating what will be observed, and the rules do not say what is happening when nobody observes.

That is not a gap ordinary science fills by more experiment, because every interpretation makes the same predictions. Deciding between them requires argument about what a physical theory is FOR and what would count as understanding one - which is philosophy, conducted by people who have to know the mathematics.

The same problem recurs elsewhere in physics. Statistical mechanics explains the arrow of time by an assumption about initial conditions nobody can justify. General relativity leaves it unclear whether spacetime points are things.

The recurring problems

The measurement problem. The equations evolve a system smoothly into a superposition; measurements always find one outcome. Either the equations are incomplete, or something makes them fail on measurement, or all outcomes occur. Every interpretation is a choice among those.

Nonlocality. Bell showed in 1964 that no theory in which distant events are independent and outcomes are fixed in advance can reproduce quantum predictions. Experiment has confirmed the quantum predictions repeatedly. Something everyone assumed is false, and which thing is still argued.

The arrow of time. The fundamental laws work equally well in both directions and the world plainly does not. The standard explanation invokes an extraordinarily low-entropy early universe, which relocates the puzzle rather than removing it.

What the mathematics refers to. A wave function is defined on a space of many dimensions. Whether that is a description of reality, a bookkeeping device, or a state of knowledge is exactly what is in dispute.

How it developed

Before the twentieth century the questions were part of natural philosophy and metaphysics - Newton against Leibniz on absolute space is the model case.

Relativity and quantum mechanics changed everything by making foundational questions unavoidable for working physicists. The Bohr-Einstein exchanges were a philosophical dispute conducted by two physicists, and neither treated it as a distraction.

For a period the profession discouraged such questions, on the grounds that the theory worked and asking what it meant produced no papers. Bell's theorem in 1964 ended that, by turning an apparently metaphysical question into one with an experimental answer.

Since then the field has become genuinely technical and genuinely philosophical at once. Everett's many-worlds proposal, dismissed for two decades, is now a mainstream option; Bohmian mechanics shows that a deterministic account is possible, contradicting what was widely taught to be proved impossible; and collapse theories make predictions that differ, and are being tested.

The positions within it

Copenhagen and its descendants, many-worlds, Bohmian mechanics, objective collapse theories and QBism divide over quantum mechanics. Substantivalism and relationism divide over spacetime. Scientific realism and instrumentalism run underneath all of it. All are in the Schools section.

Where it stands now

This is the healthiest interface between philosophy and a science: the papers are read by physicists and the philosophers can do the mathematics.

Quantum gravity is where the action is, and it is philosophically loaded, since several leading approaches suggest spacetime is not fundamental. Quantum information has also reframed the older debates by making the question what an observer can know rather than what exists.