PhilosophersofPhysics

The Philosophy of Physics | Philosophy in Physics | Physics

The core idea in plain words

Physicists tell us how the world behaves: equations that predict where the planet will be, what the detector will click. Philosophers of physics ask the next question: what must the world be like for those equations to work? Is time real or just a bookkeeping device? Does the electron have a position before we look? Is probability out there in nature or only in our heads?

This page is a short guide to the people who asked those questions best — some of them physicists who could not stop philosophising, some philosophers who learned the mathematics properly.

An analogy: the map and the territory

A London Tube map is fantastically useful and wildly wrong. Distances are distorted, rivers vanish, the lines are straight. You can navigate perfectly with it and still have no idea what London looks like.

Physical theories might be like that. Quantum mechanics predicts results to ten decimal places. But whether its wavefunction is a real wave, a list of odds, or a record of our knowledge is exactly the question the map does not answer. The philosophers of physics are the people who walk out of the station and check the streets.

The people

The founders

• Ernst Mach (1838–1916) — physics should talk only about what can be observed. His attack on Newton’s absolute space shaped the young Einstein.
• Henri Poincaré (1854–1912) — conventionalism: some “facts”, like which geometry space has, are partly choices we make for convenience.
• Albert Einstein (1879–1955) — the great realist. He accepted quantum mechanics’ predictions but insisted it was incomplete. The 1935 EPR paper with Podolsky and Rosen is still the hinge of the debate.
• Niels Bohr (1885–1962) — complementarity: wave and particle are mutually exclusive but jointly necessary descriptions. Physics, he said, is about what we can say about nature.
• Werner Heisenberg (1901–1976) — the uncertainty principle, and a later turn to Aristotle’s idea of potentia.

The dissenters

• Hugh Everett III (1930–1982) — take the Schrödinger equation literally and never let the wavefunction collapse. The result is many branching worlds. See The Everett Picture.
• David Bohm (1917–1992) — particles really do have positions, guided by a “pilot wave”. Deterministic, but non-local.
• John Stewart Bell (1928–1990) — proved that no local hidden-variable theory can match quantum predictions. Experiments since (Aspect in 1982; loophole-free tests in 2015; the 2022 Nobel Prize to Aspect, Clauser and Zeilinger) side with quantum mechanics.

The philosophers

• Hans Reichenbach (1891–1953) — the philosophy of space, time and probability; the “common cause” principle.
• Karl Popper (1902–1994) — science advances by falsification, and probabilities can be real propensities.
• Tim Maudlin — the sharpest modern voice on non-locality, and on why we should take the ontology of a theory seriously.
• David Albert — Quantum Mechanics and Experience and Time and Chance; see Physics and Chance for the related debate on the arrow of time.
• Julian Barbour and Carlo Rovelli — physicists arguing that time, as we usually picture it, is not fundamental.
• Sean Carroll — a vocal Everettian and writer on entropy and time.

A worked example: Bell’s inequality in one line

Suppose two particles are created together and flown apart. Alice and Bob each measure along one of two settings. If the outcomes were fixed in advance by local hidden properties, a simple counting argument (the CHSH version) shows the combined correlation $S$ must satisfy

Quantum mechanics predicts, for the best choice of angles,

Experiments get about 2.7–2.8 after real-world losses. So something in the assumption list has to go: locality, or the idea that outcomes are fixed before measurement, or (more exotically) the freedom to choose settings independently. Physics hands us the number; philosophy argues about which assumption to throw away. That is the job in a nutshell.

Common misconceptions

• “Shut up and calculate settles it.” It settles predictions. It doesn’t say what exists.
• “The observer means a conscious mind.” In most interpretations, “observation” is any interaction that leaves a record.
• “Bell proved nature is random.” He proved it isn’t locally determined. Bohm’s theory is deterministic and still matches the data.
• “Philosophers don’t know the maths.” The good ones do — Maudlin and Albert write equations.

Connections

• The Philosophy of Physics — the questions themselves, gathered in one place.
• Quantum Reality and Quantum Mechanics and Everyday Life — book notes on interpretation.
• The Road to Reality — Penrose on the mathematics that underlies it all.
• After Physics — David Albert’s later essays.
• Physics and Narrative — how stories and laws relate.