JV. JARED VOGLER

Physics & Possibility

Hugh Everett: Quantum Measurement Without Collapse

Also called: relative-state formulation

  • Open scientific debate
  • Formal theory
  • Interpretation under debate

Hugh Everett proposed describing quantum measurement without a "collapse", the sudden jump to a single result. In his account, the measuring device and the observer are described by the same quantum physics as everything else. Later many-worlds views built a picture of branching worlds on his work, and experts still argue about how to read it.

What does branching actually mean in physics, and where does my personal metaphor begin?

Why it attracts me

Hugh Everett asked a question that sounds simple. If quantum physics describes everything, why should it stop applying when a physicist reads a dial? The textbooks of his day answered with a special rule for measurement. Everett, then a graduate student at Princeton, tried to do without it. I am drawn to that move. It is first-principles thinking (Reasoning From the Ground Up): remove an extra assumption and see what the rest of the theory says.

The idea

Quantum physics has a smooth rule for how a system's state changes over time. Textbooks add a second rule: when a measurement happens, the state suddenly collapses to one result. Everett's 1957 paper kept only the smooth rule and applied it to the measuring device and the observer as well. What comes out is a combined state in which each possible result is paired with a record of that result. Relative to the record "spin up", the particle is up. Relative to the record "spin down", it is down. Nothing jumps. That is what he meant by relative states.

Later physicists, most famously Bryce DeWitt, described this as the world splitting into many worlds (The Many-Worlds Reading of Quantum Physics). How literally to take those worlds is still argued over.

What I think (and don't know)

I think Everett's idea is serious physics, not science fiction. I do not know whether it is right. It faces hard questions, above all how probabilities fit a picture in which every result occurs (The Rule Behind Quantum Probabilities). Other readings of the same theory are still live (Comparing Views of What Quantum Physics Means). I hold it as a strong possibility, not a fact.

Where it connects

Everett is one of the two thinkers behind a family name inspired by Everett and Nash (A Family Name Inspired by Everett and Nash). The other is John Nash (John Nash: When No One Gains by Switching), whose equilibrium describes a situation where no player gains by changing strategy alone, given what the others are doing. The formal links between their work are limited. One historical link is real, though. Everett also published on game theory, then left academic physics for military operations research, including methods for dividing limited resources (Finding the Best Plan Within Limits).

An example

Imagine a detector that checks whether an electron's spin points up or down and prints the answer. In the collapse picture, one answer is printed and the other possibility vanishes. In Everett's picture, the full state holds both: an up electron with a printout reading "up", and a down electron with a printout reading "down". Each record agrees with its own electron. In Everett's account even a person who reads the printout is just another physical system inside that state. No mind has to be added to make a measurement happen, which is the same point What Observing Means in Quantum Physics makes about observers.

Questions I am still carrying

  • How can I keep the image of branching (Mapping the Futures a Choice Opens) without implying that my decisions split the universe?
  • If every result happens, what does it mean that some are more likely?
  • What evidence or argument would change my mind about Everett's picture?

What this does not establish

Everett's proposal is one debated reading of quantum physics. It does not show that people's choices create new worlds, that consciousness steers outcomes, or that it connects formally to Nash's game theory.

Questions I'm still exploring

  • If every result happens relative to some record, what does it mean to say one result was likely?
  • Is branching a real feature of the world, or a useful way of describing the mathematics?
  • What did Everett himself mean, as opposed to what later readings added?

Sources and further reading