Ghirardi and his colleagues were motivated by the mysteries of nonlocality and entanglement that appear in Einstein-Podolsky-Rosen experiments , and especially by the enthusiastic support of John Bell for their work. Bell had criticized the work of John von Neumann , who could not locate the boundary between the quantum system and the classical measuring apparatus. Werner Heisenberg called this a "cut" or "Schnitt" between quantum and classical worlds. Von Neumann said the cut could be located anywhere from the atomic system up to the mind of the conscious observer, injecting an element of subjectivity into the understanding. John Bell called this movable boundary a "shifty split," and illustrated it with a sketch below.

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In , together with his colleagues and friends Alberto Rimini and Tullio Weber, he published a model for the spontaneous collapse of the wavefunction, which is now referred to as the Ghirardi—Rimini—Weber GRW model. His career as theoretical physicist started in nuclear physics, a mainstream line of research in Italy back in the s and s, in the wake of the Fermi school. In Ghirardi and Rimini established a limit on the number of states in a binding potential — referred to as the Ghirardi—Rimini bound — which can now be found in the legendary Reed—Simon volumes of mathematical physics.

In he, together with Luciano Fonda, Alberto Rimini and Tullio Weber, solved the open problem of the exponential decay law of unstable particles. There were and still are two major open problems in the field: the measurement problem and the relation between quantum non-locality and relativity. While it discussed the completeness of quantum theory, it turned out that the paper was actually about non -locality. Then, in the s, John Stuart Bell, fascinated by Bohmian mechanics but worried by its manifest non-local behaviour, uncovered the deepest insight one could have imagined: the non-local character of quantum phenomena, mathematically encoded in terms of inequalities.

Now, EPR states and Bell inequalities form the basis of quantum communication. Quantum non-locality means that the world is not local. But then where does relativity fit in? The problem was not irrelevant at those times, as several papers claimed that quantum theory would permit superluminal signalling. In refuting one of them in , before the paper of William Wootters and Wojciech H. Before that, in , Ghirardi, Rimini and Weber gave the general proof that quantum non-locality cannot be used to send information faster than light.

But it is with the GRW solution to the measurement problem that Ghirardi made his most profound contribution. What is so special about measurements? But if this was possible, there should be no collapse and quantum effects should be all around us.

Back in the s, Bohmian mechanics and the many-world interpretation had been around for a long time. This was not obvious at all because, as we now know from the works of Nicolas Gisin, Joseph Polcinski and others, almost all modifications to the linear structure of quantum dynamics lead to inconsistencies such as faster-than-light signalling.

Only the careful mixing of nonlinear terms with stochastic ones makes a sensible dynamics possible. Research on collapse models dates back to the s and early s, with key contributions from Philip Pearle, Lajos Diosi, Nicolas Gisin and others. In a nutshell, the GRW model predicates that microscopic systems are supposed to evolve quantum mechanically except at random times, when their wavefunction collapses in space according to a well-defined law.

These collapses are rare for microscopic systems. However, an in-built amplification mechanism makes this effect stronger and stronger as the size of the system increases, to the point that macroscopic objects are always so well-localized in space as to behave for all practical purposes like particles moving along well-defined trajectories. The wave nature of quantum systems and the particle nature of classical objects can thus be described within a single mathematical framework.

After more than 30 years from its formulation, the GRW model is still studied and an increasing number of people is facing the challenge of testing it at increasing levels of precision.

This, in turn, is pushing the development of refined tabletop experiments to control the dynamics of massive quantum systems that can probe the quantum-to-classical transition and potentially serve as new quantum sensors. What better way to honour the scientific legacy of Giancarlo Ghirardi?

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## GianCarlo Ghirardi (1935–2018)

In , together with his colleagues and friends Alberto Rimini and Tullio Weber, he published a model for the spontaneous collapse of the wavefunction, which is now referred to as the Ghirardi—Rimini—Weber GRW model. His career as theoretical physicist started in nuclear physics, a mainstream line of research in Italy back in the s and s, in the wake of the Fermi school. In Ghirardi and Rimini established a limit on the number of states in a binding potential — referred to as the Ghirardi—Rimini bound — which can now be found in the legendary Reed—Simon volumes of mathematical physics. In he, together with Luciano Fonda, Alberto Rimini and Tullio Weber, solved the open problem of the exponential decay law of unstable particles. There were and still are two major open problems in the field: the measurement problem and the relation between quantum non-locality and relativity.

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## Гирарди, Джанкарло

Description[ edit ] GRW posits that particles can undergo spontaneous wave-function collapses. For individual particles, these collapses happen probabilistically and will occur at a given average rate, but not with certainty in any given time interval. Groups of particles behave in a statistically predictable way, however. Since experimental physics has never detected an unexpected spontaneous collapse, if GRW collapses do occur, they happen extremely rarely. GC Ghirardi, A.