Damascene Philosophy

QuestionsXIII

Are there other versions of you?

On one serious reading of quantum mechanics, every possible outcome happens, each in its own branch of reality, with its own copy of you.

Reading time
4 min
Difficulty
Deep
Branch
Reality
First posed
Hugh Everett III, 1957; named 'many worlds' by Bryce DeWitt, 1970

The question

Quantum mechanics is the most accurate theory ever devised. It also says something baffling. Before measurement, a particle can be in a superposition, a combination of possibilities, such as being both here and there. When we look, we always see just one result.

Why? The standard textbook answer says that measurement 'collapses' the superposition into a single outcome. But the theory never says what counts as a measurement, why it should behave differently from every other physical process, or where the line falls between the quantum world and the observer. This is the measurement problem.

In 1957 a Princeton graduate student, Hugh Everett III, proposed a radical solution: there is no collapse. The equation that governs quantum systems, the Schrödinger equation, applies to everything, always, including you. When you measure a particle in superposition, you become entangled with it, and the whole system, you included, goes into superposition. Each branch contains a version of you seeing one definite result. Every outcome happens; each is experienced in its own world.

Why it matters

If Everett is right, reality is incomparably larger than it looks, and copies of you are, at this moment, living out every quantum-possible variation of your life. That raises questions about identity, probability and even ethics: should you care about your other branches?

The question also matters for physics itself. The interpretation of quantum mechanics is unsettled a century after the theory was formulated, and the answer bears on what reality is made of and whether it is deterministic.

Key terms

  • SuperpositionIn quantum mechanics, a state that combines several possible states at once, such as a particle described as being in many places, until it is measured.
  • Measurement problemThe puzzle in quantum mechanics of why measurements yield single definite results when the theory's equations describe a superposition of all possible results.
  • Wave functionThe mathematical object in quantum mechanics that describes a system and gives the probabilities of the results of measuring it.
  • DecoherenceThe process by which a quantum system's interaction with its environment makes interference between its possibilities practically undetectable, so that it appears to behave classically.
  • DeterminismThe thesis that the state of the world at any moment, together with the laws of nature, fixes everything that happens afterwards.

The positions

Position I

Many worlds

The wave function never collapses. Every outcome of every quantum event occurs, in branches that no longer interact.

Position II

Collapse is real

Superpositions really do collapse to single outcomes. The theory needs a precise physical account of when and how.

Position III

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.

Position IV

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.

Arguments and objections

Many worlds

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.

Collapse is real

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.

Hidden variables (pilot wave)

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.

Do not ask

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.

Who argued what

  • Many worlds Hugh Everett III, David Deutsch, Sean Carroll, David Wallace
  • Collapse is real Giancarlo Ghirardi, Roger Penrose
  • Hidden variables (pilot wave) David Bohm, John Bell
  • Do not ask Niels Bohr, Werner Heisenberg, Christopher Fuchs

Where you stand

Which interpretation of quantum mechanics is closest to the truth?

Choose one to see how people answer.

Variations

Quantum suicide

Imagine a gun rigged to fire if a quantum event goes one way and click harmlessly if it goes the other. If many worlds is true, in some branches you always survive. From your point of view, would you seem immortal? Most Everettians say this reasoning is a mistake: you should care about the branches in which you die, because the people who love you are in them.

Schrödinger's cat

Erwin Schrödinger designed his famous thought experiment in 1935 to show that applying quantum superposition to everyday objects led to absurdity: a cat both alive and dead in a sealed box. On many worlds, the absurdity disappears: there is a branch with a live cat and a branch with a dead one, each with an observer who sees one.

Go deeper

Is this the same as the cosmological multiverse?

No. The 'multiverse' of cosmic inflation refers to vast regions of space, perhaps with different physical constants, far beyond our observable universe. The Everettian many worlds are branches of a single quantum state, occupying the same space and time. Both ideas may be true, false, or neither, independently.

Related ideas

Question

Is the future already fixed?

If the laws of physics plus the state of the universe settle everything that happens, then everything you will ever do was settled before you were born.

Thought experiment

Are we living in a simulation?

If advanced civilisations can run vast numbers of simulated worlds, simulated minds would vastly outnumber real ones. So which are you more likely to be?

Thought experiment

Are you a Boltzmann brain?

Given enough time, random fluctuations could assemble a single brain with your memories out of chaos. In some cosmologies, such brains vastly outnumber real observers.

Question

Why is there something rather than nothing?

Nothing at all would have been simpler. No universe, no laws, no questions. Yet here everything is.

Question

Are you the same person you were at eight?

Almost every cell, belief and memory has changed. Something makes that child you. The question is what.

Further reading and sources

Open questions

  • If copies of you live out every possible choice, does any choice of yours matter?
  • Should you care about what happens to the other versions of you?
  • Can a theory be true if its central claim can never be directly observed?

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