Letter: Letters : Classic quantum

From Giancarlo Ghirardi, International Centre for Theoretical Physics

Trieste, Italy

Ian Percival raised objections to John Cramer’s transactional interpretation
of quantum mechanics
(Letters, 2 August, p 49).
I would also like to point out
that this interpretation does not contain a parameter that would locate the
boundary between those events that follow the rules of ordinary quantum theory,
and those which show collapse of some sort. In other words, this interpretation
is silent on the essential problem of identifying the split between quantum and
classical, reversible and irreversible, deterministic and stochastic.

The dynamical reduction models described in Mark Buchanan’s article “Crossing
the quantum frontier”
(New Scientist, 26 April, p 38) deal with this
problem in a satisfactory way. I take issue, however, with Buchanan’s argument
that some features of the Ghirardi-Rimini-Weber (GRW) model of quantum mechanics
make it almost as unpalatable as the standard theory. The model introduces small
changes to Schrödinger’s equation which ensure that quantum superpositions
in ordinary, everyday objects do not persist for long. This eliminates
Schrödinger cat paradoxes, and yet leaves unspoiled the usual accurate
quantum description for electrons, atoms and molecules.

While superpositions in the GRW theory collapse sporadically and
instantaneously, this isn’t necessary. Phillip Pearle has developed a related
continuous theory that achieves the same ends, and Pearle, Rimini, Grassi and
myself have generalised this Continuous Spontaneous Localisation model to
include gravity as a causal element.

But we have also shown that for any continuous theory, there is a
corresponding discrete one having equivalent physical implications. And it is
interesting that John Bell, shortly before his death, said that he preferred the
discrete models. He considered the precise positions and times of the
localisations as fundamental “space-time events” that he thought should form
part of a satisfactory theory.

Since the conceptual status and the achievements of the discrete and
continuous approaches are the same, it is seriously misleading to claim that the
first is as unpalatable as quantum mechanics while the latter is
satisfactory.