da Nature.
e' veramente interessante, sopratutto per quelli che ricostruiscono il
dibattito sulla quantistica solo pensando a Bohr e Einstein.
C'e' anche un lavoro di Cini, non di divulgazione ma scientifico, che
discute il problema del sistema di misurazione come oggetto quantistico.
buona lettura.
Concepts
/Nature/ *437*, 625 (27 September 2005) | doi: 10.1038/437625a
Thinking big
Philip W. Anderson^1
<http://www.nature.com/nature/journal/v437/n7059/full/437625a.html#a1>
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<http://www.nature.com/nature/journal/v437/n7059/full/437625a.html#top>
Abstract
Fritz London's single-minded thinking led him to surpass even Einstein,
as he believed correctly that quantum mechanics was right at all scales,
including the macroscopic.
Fritz London began his career in physics as one of the originators of
quantum theory during 1925−27. His training as a philosopher, before
taking up physics, no doubt enhanced his contribution to the 'copenhagen
interpretation' — the first general attempt to understand the world of
atoms according to quantum mechanics. But London did much more than
create the first theory of the chemical bond, and has not had the
recognition he deserves.
He was among the few pioneers who deliberately chose, once atoms and
molecules were understood, not to focus his research on further
subdividing the atom into its ultimate constituents, but on exploring
how quantum theory could work, and be observed, on the macroscopic scale.
For a few years, London worked at trying to found chemistry on quantum
theory, but in the end was overwhelmed by Linus Pauling's more heuristic
approach; he never published his book on the subject. He then became
intrigued by the twin phenomena of superfluidity and superconductivity,
which, he was convinced, were macroscopic manifestations of quantum
mechanics.
In 1935, London was the first to propose that superfluidity was
Bose−Einstein condensation, and then in the late 1930s, with his brother
Heinz, he developed the first heuristic theory of superconductivity. His
pair of books on these subjects appeared around 1950 and admirably
framed the questions that were soon to be answered — in the one case by
Oliver Penrose, Lars Onsager and Richard Feynman, and in the other by
John Bardeen, Leon Cooper and Robert Schrieffer. But London fell ill in
1950 and died in 1954, so he did not live to see the triumphs of his
intuitions.
He had paid, however, for his unpopular choice of subject matter —
quantum theory on the macroscopic scale — by having to settle for a job
in the pre-war South. This meant being out of mainstream physics, and
may have resulted in him being excluded from the Manhattan bomb project
on which all his early associates worked.
In 1939, in an obscure paper called 'The observation problem in quantum
mechanics', London and Edmond Bauer took on the notorious Bohr−Einstein
debates. This is the earliest paper I know of that expresses the most
common-sense approach to the uncertainty principle and the philosophy of
quantum measurement.
In reading about these debates I have the sensation of being a small boy
who spots not one, but two undressed emperors. Niels Bohr's
'complementarity principle' — that there are two incompatible but
equally correct ways of looking at things — was merely a way of using
his prestige to promulgate a dubious philosophical view that would keep
physicists working with the wonderful apparatus of quantum theory.
Albert Einstein comes off a little better because he at least saw that
what Bohr had to say was philosophically nonsense. But Einstein's
greatest mistake was that he assumed that Bohr was right — that there is
no alternative to complementarity and therefore that quantum mechanics
must be wrong. This was a far greater mistake, as we now know, than the
cosmological constant.
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E. SEGRÉ; VISUAL ARCHIVES/PHYSICS TODAY COLLECTION
Lone thinker: Fritz London took an opposite tack from both Albert
Einstein and Niels Bohr.
At this point London took an opposite tack from either Bohr or Einstein.
He found it difficult to believe Bohr's idea that there was a real
'complementarity' even though he had been an early contributor to that
line of thinking. Instead he took the then radical step of assuming that
quantum mechanics was not wrong, but right at all scales, including the
macroscopic. This explains why London was intrigued by the realization
that in the 'super' forms of matter, he was seeing quantum theory
showing itself on the (relatively) everyday scale.
Taking London's point of view, one immediately begins to realize that
the real problem of quantum measurement is not in understanding the
simple electron that is being measured, but the large and complicated
apparatus used to measure it. This apparatus has all kinds of properties
that are not obvious consequences of quantum mechanics: rigid slits, for
instance, and a photographic plate that darkens irreversibly where an
electron hits it.
These properties are a real intellectual challenge to understand from
first principles; the first thing one realizes is that time, for the
measurer and the photographic plate, has a sign — earlier or later. This
sign is not contained in the quantum theory and has to be the result of
the organizing principles of quantum particles assembled into very large
macroscopic objects. This and the fact that the apparatus has a definite
position in space require that a quantum description of it can only be
given in terms of a superposition of an unimaginably large number of
different quantum states.
The electron interacting with it attaches (entangles) one part of its
wave function to one batch of these states, the other part to a
different batch. And these batches differ in so many ways that they can
never be made to cohere again; they represent two entirely separate
macroscopic histories of the apparatus. The message is that what is
needed is an understanding of the macroscopic world in terms of quantum
mechanics. This is the direction that London chose.
And that brings me to superfluid solids. Moses Chan and his student
Eun-Seong Kim have recently shown that helium (and probably hydrogen),
if solidified below a tenth of a degree Kelvin, flow through their own
crystal lattice like a superfluid. (This has yet to be confirmed, but I
believe it.) This means that a rigid object — the most primitive of our
physical intuitions — is not a system in a simple, single
quantum-mechanical ground state, but only arises as a consequence of
thermal fluctuations.
Thus, Albert Einstein's clocks and rigid measuring rods, which play such
a key role in the theory of relativity, must be not primitive but
derived in a very complex way from the underlying quantum laws of
microscopic physics. At which point I could immodestly take the
opportunity to announce that after all, "more is different!"
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