What does quantum mechanics say reality is?
Every outcome in its own world, a real collapse, hidden variables, or no picture at all?
Many worlds
The wave function never collapses. Every outcome of every quantum event occurs, in branches that no longer interact.
- Best argument
- It is the simplest version of the theory: just the Schrödinger equation, taken seriously, with no special rule for measurement. The appearance of collapse is explained by decoherence, the well-understood process by which superpositions involving large systems rapidly stop interfering with one another. Advocates like David Deutsch and Sean Carroll argue that the extra worlds are not an extra assumption but a consequence of the theory everyone already uses.
- Strongest objection
- Probability is the hardest problem. If every outcome happens, what does it mean that one outcome has a 90% chance? Something happens in every branch with certainty. Critics also find the ontology extravagant: an unimaginable number of unobservable worlds.
- The reply
- Everettians answer that probability can be understood as the rational weight a person should give each branch before measurement, and David Wallace and others have derived the standard quantum probabilities from decision theory. As for extravagance, they say the theory is economical in laws, which is the economy that matters.
- Defended by
- Hugh Everett III, David Deutsch, Sean Carroll, David Wallace
Collapse is real
Superpositions really do collapse to single outcomes. The theory needs a precise physical account of when and how.
- Best argument
- Objective collapse theories, such as the GRW model, modify the Schrödinger equation so that superpositions spontaneously collapse, very rarely for single particles but almost instantly for large objects. This gives one world, real probabilities, and makes testable predictions that differ slightly from standard quantum mechanics.
- Strongest objection
- The modifications are added by hand, with new constants of nature chosen to fit. So far, no experiment has detected the predicted deviations.
- Defended by
- Giancarlo Ghirardi, Roger Penrose
Hidden variables (pilot wave)
Particles always have definite positions. They are guided by a wave, and the wave function never collapses, but only one outcome is real.
- Best argument
- In Bohmian mechanics, there is one world, determinism is restored, and the measurement problem disappears because particles always have positions; we just do not know them. The theory reproduces all the predictions of standard quantum mechanics.
- Strongest objection
- The theory is explicitly nonlocal: the motion of a particle depends instantaneously on the positions of others, however far away. Extending it to relativistic quantum field theory has proved difficult.
- Defended by
- David Bohm, John Bell
Do not ask
Quantum mechanics is a tool for predicting measurement results, not a description of hidden reality. Questions about what 'really' happens are outside physics.
- Best argument
- This is the spirit of the Copenhagen interpretation associated with Niels Bohr and of later views like QBism. Physics is about what we can observe and predict. The theory works perfectly at that job, and asking for a picture beyond it is asking physics to do metaphysics.
- Strongest objection
- Many find this unsatisfying: the success of the theory demands an explanation, and a theory that refuses to say what the world is like may be incomplete. Einstein's dissatisfaction with this stance drove decades of foundational work.
- Defended by
- Niels Bohr, Werner Heisenberg, Christopher Fuchs
Where you stand
Which interpretation of quantum mechanics is closest to the truth?
In surveys
Choose one to see how people answer.
For the full background, variations and open questions, read the entry: Are there other versions of you?.