The Ghirardi–Rimini–Weber theory (GRW; also known as spontaneous collapse theory) is a collapse theory in quantum mechanics. GRW differs from other collapse theories by proposing that wave function collapse happens spontaneously. GRW is an attempt to avoid the measurement problem in quantum mechanics. It was first reported in 1985.
The Ghirardi–Rimini–Weber theory
GRW says that particles can undergo spontaneous wave-function collapses. For individual particles, these collapses happen probabilistically and will occur at a given rate with high probability but not with certainty; groups of particles behave in a statistically regular way, however. Since experimental physics has not already detected an unexpected spontaneous collapse, it can be argued that GRW collapses happen extremely rarely. Giancarlo Ghirardi, Alberto Rimini, and Tullio Weber suggest that the rate of spontaneous collapse for an individual particle is on the order of once every hundred million years.
Justification for GRW
GRW and all collapse theories want to reconcile the mathematics of quantum mechanics, which suggests that subatomic particles exist in a superposition of two or more states, with the measured results, which only ever give us one state. We can easily prepare an electron to have a spin that is mathematically both up and down, for example, but any experimental result will yield either up or down and never a superposition of both states. The orthodox interpretation, or Copenhagen interpretation of quantum mechanics, posits a wave-function collapse every time one measures any feature of a subatomic particle. This would explain why we only get one value when we measure, but it doesn't explain why measurement itself is such a special act. More importantly, the orthodox interpretation doesn't define what counts as "measurement", and there is much dispute on the question. GRW originated as an attempt to get away from the imprecise talk of "measurement" that plagues the orthodox interpretation.
By suggesting that particles spontaneously collapse into stable states, GRW escapes the ideas that measurement is a special act or that some specific part of measuring a subatomic particle causes the particle's wave function to collapse. At the same time, GRW theory is compatible with single-particle experiments that do not observe spontaneous wave-function collapses; this is because spontaneous collapse is posited to be extremely rare. However, since measurement entails quantum entanglement, GRW still describes the observed phenomenon of quantum collapses whenever we measure subatomic particles. This is because the measured particle becomes entangled with the very large number of particles that make up the measuring device. (For any macroscopic measuring device, there are sure to be very many orders of magnitude more than 3x1015 entangled particles, so the likelihood of at least one particle in the entangled system collapsing in any given second is extremely high.)
- Ghirardi, G.C., Rimini, A., and Weber, T. (1985). "A Model for a Unified Quantum Description of Macroscopic and Microscopic Systems". Quantum Probability and Applications, L. Accardi et al. (eds), Springer, Berlin.
- Ghirardi, G.C., Rimini, A., and Weber, T. (1986). "Unified dynamics for microscopic and macroscopic systems". Physical Review D. 34: 470. Bibcode:1986PhRvD..34..470G. doi:10.1103/PhysRevD.34.470.
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