Same predictions. Different worlds.
No experiment has ever told these apart — every cell below is a philosophical commitment, not a measurement.
Highlight
| Question | Copenhagen | Many Worlds | Pilot Wave | Objective Collapse | QBism | Relational | Consistent Histories |
|---|---|---|---|---|---|---|---|
| Collapse?The apparent jump from many weighted possibilities to one actual result at measurement. Interpretations disagree on whether it is a physical process, a bookkeeping update, or nothing at all. Full definition → | YesThe textbook projection postulate | NoNever | NoNever | YesSpontaneous, physical | DependsBelief update | DependsRelative to observer | NoA calculational tool, not an event |
| Deterministic? | No | YesGlobally | Yes | No | No | No | No |
| Hidden variables?Additional facts, beyond the wave function, that settle what a measurement will find — either outright or as probabilities. Pilot-wave theory adds them (particle positions); Bell showed that no such theory, deterministic or not, can be local and still match quantum mechanics. Full definition → | No | No | YesParticle positions | No | No | No | No |
| Multiple universes? | No | YesBranching worlds | No | No | No | No | NoOne actual history per family |
| Observer dependent? | DependsNeeds a classical side | No | No | No | YesAgent-centred | YesSystem-relative | DependsThe physicist picks the framework |
| Adds new physics? | No | No | DependsGuidance equation | YesTestable, in principle | No | No | No |
| Explicitly nonlocal?Correlation between distant events that no local mechanism can reproduce, as Bell's theorem makes precise — given that settings are chosen freely and each run has a single outcome. Interpretations differ over which of those assumptions to give up, and over whether anything acts at a distance. Full definition → | DependsCorrelations, no story | NoNo action at a distance, supporters argue | YesGuidance is instantaneous | YesCollapse spans entangled pairs | NoUpdating isn't physics | NoLocal, supporters argue; critics disagree | NoLocal, Griffiths argues |
| The open question | What counts as a measurement? | Why the Born-rule probabilities? | Peace with special relativity | No collapse has ever been observed | What must the world be like for the Born rule to hold? | Can two observers' accounts always be reconciled? | Which framework, and why? |
Fair questions
They all reproduce the same predictions — that's the entry fee. Where they differ is in what they add (worlds, particles, collapses) or refuse to add. Experiments score predictions, and on predictions they are tied.
Mostly no — with one serious exception. Objective-collapse models change the equation slightly, so precision experiments genuinely test them, and every null result so far has tightened the screws. Pilot wave is testable too if its quantum-equilibrium assumption ever fails, though no one knows how to arrange that.
Polls at conferences are famously split — and famously small. A 2013 poll of 33 physicists put Copenhagen first on 42%, with information-flavoured views and many worlds behind it and 12% expressing no preference. A poll at a Bohmian meeting the same year put pilot wave first on 63%, which shows how completely the room decides the answer. A century in, this remains an open question — which is rather the point of this site.