Concept
Measurement problem
Overview
In quantum mechanics, the measurement problem is the problem of definite outcomes: quantum systems have superpositions but quantum measurements only give one definite result.
The wave function in quantum mechanics evolves deterministically according to the Schrödinger equation as a linear superposition of different states. However, actual measurements always find the physical system in a definite state. Any future evolution of the wave function is based on the state the system was discovered to be in when the measurement was made, meaning that the measurement "did something" to the system that is not obviously a consequence of Schrödinger evolution. The measurement problem concerns what that "something" is, how a superposition of many possible values becomes a single measured value.
To express matters differently (paraphrasing Steven Weinberg), the Schrödinger equation determines the wave function at any later time. If observers and their measuring apparatus are themselves described by a deterministic wave function, why can we not predict precise results for measurements, but only probabilities? As a general question: How can one establish a correspondence between quantum reality and classical reality?
Overview adapted from Wikipedia (CC BY-SA 4.0; see the article history for authors).
Possible passages
Found by search and not yet checked by a person; they may only mention the idea in passing.
The knowledge of the elementary fact enables us to put the problem in an equation. Nothing remains but to deduce from this by combination the complex fact that can be observed and verified. This is what is called _integration_, and is the business of the mathematician. It may be asked why, in physical sciences, generalization so readily takes the mathematical form. The reason is now easy to see. It is not only…
Henri Poincaré, The Foundations of Science
States what Measurement problem is.
So that when the scale is moved over the line by any continuous series of shiftings of one kind, there are two points which no numbers on the scale can ever reach, except the numbers fixed there. This pair of points, thus unattainable in measurement, is called the Absolute. These two points may be distinct and real, or they may coincide, or they may be both imaginary.
Charles S. Peirce, Chance, Love, and Logic
States what Measurement problem is.
This is hinted at even in language, when the ascertainment of quantitative features and relations is called measuring. We measure, _e.g._ the length of different chords that have been put into a state of vibration, with an eye to the qualitative difference of the tones caused by their vibration, corresponding to this difference of length. Similarly, in chemistry, we try to ascertain the quantity of the matters…
Georg Wilhelm Friedrich Hegel, The Logic of Hegel
States what Measurement problem is.
Philosophers who discussed Measurement problem
Related ideas
- Falsifiability
- Physics World
- Quantum decoherence
- Niels Bohr
- Quantum mechanics
- Werner Heisenberg
- Stanford Encyclopedia of Philosophy
- Born rule
- Nature (journal)
- Oxford University Press
- De Broglie–Bohm theory
- Copenhagen interpretation
- Quantum superposition
- Many-worlds interpretation
- Hugh Everett III
- Ghirardi–Rimini–Weber theory
- Observer (quantum physics)
- Penrose interpretation
Place in the tradition
Part of Interpretations of quantum mechanics.