Ghirardi-Rimini-Weber theory

The Ghirardi-Rimini-Weber theory, or GRW, 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.

The Ghirardi-Rimini-Weber Theory

GRW claims that particles 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. Because experimental physics has not detected unexpected spontaneous collapse, it is necessary to posit that GRW collapses happen extremely rarely. Ghirardi, Rimini, and Weber suggest that the rate of spontaneous collapse for an individual particle is on the order of once every 10⁸ years. [cite journal | author = Bell, J.S. | year = 2004 | title = Are there quantum jumps?| journal = Speakable and Unspeakable in Quantum Mechanics | volume = | pages = 201–212 | doi = ]

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 standard model, 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 standard interpretation doesn't define what counts as "measurement" and there is much dispute on the question. [cite book|title=Quantum Mechanics and Experience |last=Albert |first=David |authorlink= |coauthors= |year=1994 |publisher=Harvard University Press |location=Cambridge |isbn=0674741137 |pages=80-111 ] GRW originated as an attempt to get away from the imprecise talk of “measurement” that plagues the standard model.

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 10⁸ entangled particles, so the likelihood of at least one particle in the entangled system collapsing at any given moment is extremely high.)

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