Jun 15, GianCarlo Ghirardi, former professor of theoretical physics and then emeritus at the physics department of our university, passed away on 1st. GianCarlo Ghirardi. Angelo Bassi and Tullio Weber. University of Trieste, Trieste, Italy. Detlef Dürr. Ludwig-Maximilians University, Munich, Germany. PDF Full. Jun 4, The ICTP community is mourning the sudden passing of long-time colleague GianCarlo Ghirardi. Professor Ghirardi was associated with ICTP.
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Scientists Michael Arbib John S. Dieter Zeh Ernst Zermelo. Ghirardi and his colleagues were motivated by the ggirardi of nonlocality and entanglement that appear in Einstein-Podolsky-Rosen experimentsand especially by the enthusiastic support of John Bell for their work.
Bell’s Theorem and Inequalities pointed the way to the many physical ghlrardi that have confirmed the standard model of quantum mechanics, but Bell and others were never satisfied that standard quantum mechanics could “explain” what is “really” going on in the microscopic world despite the extraordinary accuracy of the theory. Ghirarrdi most famous proposal for a solution to these mysteries was Albert Einstein ‘s original criticism that quantum mechanics was “incomplete” and that ghirradi information was needed to restore his intuition of “local reality.
Ghirardi’s work is focused more on an explanation for the sudden “collapse” of the wave function. Bell had criticized the work of John von Neumannwho 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. Information physics identifies the boundary as the place and moment when irreversible new information enters the universe.
GianCarlo Ghirardi – Citazioni di Google Scholar
Information increases, but entropy increases even more, to satisfy the second law. This happens in the photographic film when silver atoms precipitate in the emulsion. The GRW scheme represents a proposal aimed to overcome the difficulties of quantum mechanics discussed by John Bell in his article “Against Measurement. Obviously these modifications must leave practically unaltered all standard quantum predictions about microsystems. To be more specific, the GRW theory admits that the wavefunction, besides evolving through the standard Hamiltonian dynamics, is subjected, at random times, to spontaneous processes corresponding to localisations in space of the microconstituents of any physical system.
The mean frequency of gincarlo localisations is extremely small, and the localisation width is large on an atomic scale. As a consequence no prediction of standard quantum formalism for microsystems is changed in any appreciable way.
The merit of the model is in the fact that the localisation mechanism is such that its frequency increases as the number of constituents of a composite system increases.
He tried to think of an electron as represented by a wavepacket — a wave-function appreciably different from zero only over a small region in space. The extension of that region he thought giajcarlo as the actual size of the electron — his electron was a bit fuzzy.
But that was wrong.
But however far the wavefunction has extended, the reaction of a detector to an electron remains spotty. Then came the Born interpretation. The wavefunction gives not the density of stuff, but gives rather on squaring its modulus the density of probability.
Probability of what exactly? Why this aversion to ‘being’ and insistence on ‘finding’? The founding fathers were unable to form a clear picture of things on the remote atomic scale. They became giancadlo aware of the intervening apparatus, and of the need for a ‘classical’ base from which to intervene on the quantum system.
And so the shifty split.
The kinematics of the world, in this orthodox picture, is given a wavefunction maybe more than one? The X s are somehow macroscopic. This is not spelled out very explicitly. The dynamics is not very precisely formulated either. Then the x s must not be confined to some vague macroscopic scale, but must extend to all scales. In the picture of de Broglie and Bohm, every particle is attributed a position x t. Then instrument pointers — assemblies of particles have positions, and experiments have results.
Particles are not attributed angular momenta, energies, etc. Peculiar ‘measurement’ results for angular momenta, energies, and so on, emerge as pointer positions in appropriate experimental setups. Considerations of KG [Kurt Gottfried] and vK [Norman van Kampen] type, on the absence FAPP [For All Practical Purposes] of macroscopic interference, take their place here, and an important one, in showing how usually we do not have FAPP to pay attention to the whole world, but only to some subsystem and can simplify the wave-function The position of the particle there was just a feature of the wavepacket, not something in addition.
The Landau—Lifshitz approach can be seen as maintaining this simple non-dual kinematics, but with the wavefunction compact on a macroscopic rather than microscopic scale. We know, they seem to say, that macroscopic pointers have definite positions.
And we think there is nothing but the wavefunction. So the wavefunction must be narrow as regards macroscopic variables. So there must be some kind of ‘collapse’ going on in addition, to enforce macroscopic narrowness.
But actually the idea that an electron in a ground-state hydrogen atom is as big as the atom which is then perfectly spherical is perfectly tolerable — and maybe even attractive.
The idea that a macroscopic pointer can point simultaneously in different directions, or that a cat can have several of its nine lives at the same time, is harder to swallow. And if we have no extra variables X to express macroscopic definiteness, the wavefunction itself must be narrow in macroscopic directions in the configuration space.
This the Landau—Lifshitz collapse brings about. It does so in a rather vague way, at rather vaguely specified times.
But in macroscopic systems, as a consequence of the prescribed equationspointers very rapidly point, and cats are very quickly killed or spared. The orthodox approaches, whether the authors think they have made derivations or assumptions, are just fine FAPP — when used with the good taste and discretion picked up from exposure to good examples. At least two roads are open from there towards a precise theory, it seems to me. Both eliminate the shifty split.
The de Broglie—Bohm-type theories retain, exactly, the linear wave equation, and so necessarily add complementary variables to express the non-waviness of the world on the macroscopic scale.
The GRW-type theories have nothing in their kinematics but the wavefunction. It gives the density in a multidimensional configuration space! The big question, in my opinion, ggirardi which, if either, of these two precise pictures can be redeveloped in a Lorentz invariant way.
All historical experience confirms that men might not achieve the possible if they had not, time and time again, reached out for the impossible. The “moment” at which the boundary between quantum ghirxrdi classical worlds occurs ghirrardi the moment that irreversible observable information enters the universe. Although GRW make the wave function collapse, and gincarlo mathematics is “precise,” they still can not predict exactly when the collapse occurs.
It is simply random, with the probability very high in the presence of macroscopic objects. In the information physics solution to the problem of measurementthe timing and location of the Heisenberg “cut” are identified with the interaction between quantum system and classical apparatus that leaves the apparatus in an irreversible stable state providing information to the observer.
GianCarlo Ghirardi on Measurement In his elegantly written and nicely illustrated book, Sneaking a Look at God’s CardsGhirardi starts his discussion of the measurement problem by noting that the principle of superposition of states means that some observables lack a precise expectation value, so we can speak only of the ghiradri of outcomes.
The most giancaarlo example, he says, is the diagonally polarized photon in a linear combination of vertical and horizontal polarization states discussed in the case of Dirac’s Three Polarizers. In our specific case, a macroscopic object “is capable of not having the property of being in some place.
The reader needs to be warned that the problem cannot be avoided, and that it inevitably involves macroscopic systems. In fact, owing to the huge difference of scale between human beings and the microscopic systems we want to study, any attempt to obtain information about them requires a process of amplification that ghirarxi correlates the microscopic properties to situations that are macroscopically perceivable and hence distinguishable to our perception.
The problem is traditionally approached with reference to what is technically known as von Neumann ‘s ideal measurement process, named from the scientist who first formulated it in precise terms.
For this purpose we can refer to experiments of the kind discussed in chapters 3 and ghiratdi, in which photons with definite giaancarlo of polarization were sent into a birefringent crystal, and we enrich the analysis by including the dynamics of the detecting apparatus.
In the discussion of those chapters we simply said that “the detector detects does not detect the arrival of a photon,” insofar as this was the only relevant information for the analysis we were then making. Now, however, we must inquire into the precise sense of this assertion and investigate the physical processes to which reference is made in the cases of concrete experiments ghirafdi out in laboratories.
A reasonable and sufficiently realistic description of what happens as pointed gincarlo in chapter 6 would be the following: It follows that the interaction between microsystem and measuring apparatus should giancarol about a macroscopic change of the apparatus in such a way that, by observing the state of the apparatus after the process, we would be able to obtain the desired information. Instead giancatlo what we did in the foregoing chapters, i.
Schematic representation of a measuring apparatus with the pointer in position 0, which indicates that it is ready to register the arrival of a photon either in the upper shaded region U or in the lower shaded ghiardi L. The zero position of the pointer is the state of the apparatus ghkrardi it has interacted with a photon and can be designated as “the state of the apparatus ready to carry out a measurement.
To take gghirardi the problem it will be convenient to analyze first of all the case in which a vertically polarized photon is sent into a birefringent crystal placed in front of the apparatus. As we know, the photon will be propagated along the ordinary ray and will end up striking the region L of the apparatus. Alternatively, we will consider a horizontally polarized photon that we know will with certainty follow the extraordinary ray and will end up striking region U of the apparatus Fig.
Schematic representation of the effect obtained by activating an ideal measuring apparatus by states having the precise polarizations that the apparatus is set up to reveal. For simplicity we will suppose, with von Neumann, that the photon passes through the apparatus without changing its state. The quantum evolution of giancaelo two states can be represented as follows: We are now prepared to show how the problem of the quantum theory of measurement arises.
First, since the two ghirrdi described above involve physical systems that are completely normal for the theory in question the apparatus is composed of electrons, protons, neutrons, etc.
We now prepare an initial state that is a linear combination of the two initial states considered above: Then, for the initial state we have the following: The argument shows in a simple way how to realize a state of the type considered in the preceding section. In the case when the apparatus is set off by a polarization state that is the superposition of the states that it is programmed tiancarlo register, and under the assumption that the measuring process is governed by the linear laws of the theory, one must conclude that the final state does not correspond to an apparatus with the pointer in a definite position.
The quantum mechanical ambiguity associated with the superposition becomes, as it were, transmitted to the macroscopic system. As discussed in Section