Copenhagen interpretation

In plain terms

Quantum mechanics needs a boundary between the quantum system and the classical apparatus that reads it, and you can ask where the boundary is and what happens at it. The Copenhagen interpretation answers by refusing the question.

The traditional teaching account, associated with Bohr, treats the apparatus as classical by fiat and treats the state's jump on measurement as a primitive fact. More radically, Bohr held that the whole experimental arrangement determines what can meaningfully be said, so asking what the electron was doing between measurements isn't left unanswered but rejected as meaningless.

As a strategy this isn't foolish. It delivered a century of flawless physics, and its refusal to over-describe has aged better than several of its rivals' enthusiasms. But it's a decision not to explain, and everyone who wants an explanation eventually leaves it.

In CSD

The disagreement is narrower than the usual caricature. CSD agrees with Copenhagen that the state isn't a physical object, and it agrees that the boundary between classical and quantum is where the real content lives. The parting of ways comes one step further on, as CSD declines to leave the boundary unanalysed.

Here measurement is an ordinary physical interaction on the substrate that produces a record, and the two rules Copenhagen keeps apart, smooth evolution and abrupt update, are one process seen at two levels of description. The contrast is interpretive and not empirical, as no experiment distinguishes the two positions and none is proposed. This entry states a position, there's no theorem behind it, and the absence of a formal reference is deliberate.

Mathematically

There's no formal content, and the absence is the information, as the corpus contains nothing bearing directly on this position.

That's itself telling, as the disagreement is about whether the measurement boundary requires analysis, and no theorem settles a question of that kind.

Related entries

• Many-Worlds interpretationMany-Worlds keeps the smooth quantum evolution and gives up the single outcome, and CSD makes the opposite choice, one trajectory and one world, as a settled commitment.
• Bohmian mechanicsBohmian mechanics is CSD's nearest neighbour, deterministic, with definite states, and openly non-local, and the dispute between them is over where the probabilities come from.
• QBismQBism is epistemic all the way down, as the quantum state is one agent's betting book and there's nothing underneath, and CSD agrees about the state and insists on the underneath.
• Measurement problemQuantum mechanics contains two incompatible rules of change and no instruction for when to switch between them, and CSD replaces the switch with theorems about what no dynamics can do.
• CollapseIn CSD the quantum state never jumps, and that's a theorem about volume-preserving maps rather than a preference among interpretations.