⚛ Quantum time
Quantum mechanics treats time as the stage, not an actor. That works brilliantly — and it's exactly what breaks when quantum rules meet gravity.
Time is a parameter, not something you measure
Position, momentum and energy are observables: there are quantum rules for measuring them. Time isn't. Pauli showed in 1933 that, in the standard theory, there can't be a well-behaved 'time observable' paired with energy. What we measure are clocks — physical things that change. Established
The time–energy uncertainty relation
There is a famous relation ΔE × Δt ≳ ħ/2, but it doesn't mean what position–momentum uncertainty means. Δt is how long a system takes to change noticeably (Mandelstam and Tamm, 1945): a short-lived particle has a spread of energies — which is why unstable particles show up as broad bumps in collider data. Established
Measurement and the arrow
Quantum evolution runs just as well backwards; measurement seems not to. Decoherence explains why interference disappears in practice — quantum information leaks irreversibly into the surroundings, much like entropy growing. Established Whether anything more — a real 'collapse' — happens is a matter of interpretation. Contested
No definite path — even in hindsight
Wheeler's delayed-choice experiment, done with single photons in 2007, shows you can't picture a photon as having taken one definite route before it's measured — even when the measurement is chosen after it set off. No signal goes back in time. Established (the results) · Contested (what they mean).
Time from correlation
Apply quantum rules to the whole universe and time disappears from the equations (the Wheeler–DeWitt equation, hole H1). Page and Wootters showed time can reappear inside a static whole, as the correlation between a clock and everything else — demonstrated in miniature with photons (2014). Established (the formalism) · Contested (that our time works this way).
Even the order of events can be quantum
In the 'quantum switch', two operations happen in a superposition of both orders — A then B and B then A — demonstrated with photons (2015, 2017). No extra time dimension needed: order itself can be quantum. Established (the experiments) · Contested (the interpretation).
See it in the labs
Further exploring
- Quantum Country — Andy Matuschak & Michael Nielsen ↗Free essays on quantum mechanics, with review questions built in so ideas stick.
- Quantum Flytrap Virtual Lab ↗Build your own optical-table experiments with beam splitters and entangled photons.
Sources: Pauli, Handbuch der Physik (1933); Mandelstam & Tamm, J. Phys. USSR 9 (1945); Zurek, Rev. Mod. Phys. 75 (2003); Jacques et al., Science 315 (2007); Page & Wootters, PRD 27 (1983); Moreva et al., PRA 89 (2014); Procopio et al., Nat. Commun. 6 (2015); Rubino et al., Sci. Adv. 3 (2017).