Ghirardi-Rimini-Weber theory
Encyclopedia
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. It was first reported in 1985.
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 108 entangled particles, so the likelihood of at least one particle in the entangled system collapsing at any given moment is extremely high.)
Objective collapse theory
Objective collapse theories are an approach to the interpretational problems of quantum mechanics. They are realistic, indeterministic and reject hidden variables...
in quantum mechanics
Quantum mechanics
Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...
. GRW differs from other collapse theories by proposing that wave function collapse happens spontaneously. GRW is an attempt to avoid the measurement problem
Measurement problem
The measurement problem in quantum mechanics is the unresolved problem of how wavefunction collapse occurs. The inability to observe this process directly has given rise to different interpretations of quantum mechanics, and poses a key set of questions that each interpretation must answer...
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. Ghirardi, Rimini, and Weber suggest that the rate of spontaneous collapse for an individual particle is on the order of once every 108 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 superpositionQuantum superposition
Quantum superposition is a fundamental principle of quantum mechanics. It holds that a physical system exists in all its particular, theoretically possible states simultaneously; but, when measured, it gives a result corresponding to only one of the possible configurations.Mathematically, it...
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
Spin quantum number
In atomic physics, the spin quantum number is a quantum number that parameterizes the intrinsic angular momentum of a given particle...
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
Copenhagen interpretation
The Copenhagen interpretation is one of the earliest and most commonly taught interpretations of quantum mechanics. It holds that quantum mechanics does not yield a description of an objective reality but deals only with probabilities of observing, or measuring, various aspects of energy quanta,...
, 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
Copenhagen interpretation
The Copenhagen interpretation is one of the earliest and most commonly taught interpretations of quantum mechanics. It holds that quantum mechanics does not yield a description of an objective reality but deals only with probabilities of observing, or measuring, various aspects of energy quanta,...
.
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
Quantum entanglement
Quantum entanglement occurs when electrons, molecules even as large as "buckyballs", photons, etc., interact physically and then become separated; the type of interaction is such that each resulting member of a pair is properly described by the same quantum mechanical description , which is...
, 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.)
See also
- Objective collapse theoryObjective collapse theoryObjective collapse theories are an approach to the interpretational problems of quantum mechanics. They are realistic, indeterministic and reject hidden variables...
- Wave function collapse
- Measurement problemMeasurement problemThe measurement problem in quantum mechanics is the unresolved problem of how wavefunction collapse occurs. The inability to observe this process directly has given rise to different interpretations of quantum mechanics, and poses a key set of questions that each interpretation must answer...
- Quantum decoherenceQuantum decoherenceIn quantum mechanics, quantum decoherence is the loss of coherence or ordering of the phase angles between the components of a system in a quantum superposition. A consequence of this dephasing leads to classical or probabilistically additive behavior...
- Penrose InterpretationPenrose interpretationThe Penrose interpretation is a prediction of Sir Roger Penrose about the relationship between quantum mechanics and general relativity. Penrose proposes that a quantum state remains in superposition until the difference of space-time curvature attains a significant level...