Re: The time it takes to emit one photon
- From: "nightlight" <nightlight@xxxxxxxxxxxxxx>
- Date: Fri, 12 Aug 2005 08:43:47 +0000 (UTC)
> My point was that QM can predict (the probabilities of outcomes of)
> measurements.
It does that, too. But QM was already quite useful well before
probabilities entered the theory (via Born postulate). Probabilistic
view of Psi, introduced originally into QM as an erroneous footnote and
its later errata by Born, is useful mostly for scattering applications
of QM. For chemistry, molecular & atomic bound states, structures and
spectra, solid state,... it doesn't do anything that Schrodinger's or
Fermi's view of |Psi|^2 as density of matter/charge doesn't do more
clearly (e.g. via Hartree-Fock self-consistent matter fields).
Basically, the direct product of single particle Hilbert spaces (the
centeral formal element of QM Measurement "Theory", entanglement and
other QM magical aspects), is useful formal & computational method for
systems of few distinct (types of) particles, primarily in scattering
setups. The pedagogical literature makes it, unfortunatelly, the
centerpiece of the QM (although not of QFT).
Barut has shown how this multiparticle product space formalism, which
is the formal basis of all the QM magic claims ("predictions"), is
nothing more but a linearized approximation (obtained through the
under-constrained/weaker variation of the action, the Barut's ansatz cf
[3], which yields simultaneously the statistical aspects and the
linearized form for the QM formalism of composite systems) of the
interacting classical Maxwell field with the classical
Dirac/Schrodinger matter field. No new physics is added by the
composite system QM formalism (or by the second quantization, which is
equivalent to the infinite product case with anti/symmetrization). The
QM magic (entanglement, Bell nonlocality, QC... etc) is based solely on
the non-physical (thus non-existent in nature) artifacts of that
particular linearization approximation copmutational scheme, which
became dominant through historical accidents, but which is otherwise
arbitrary (this becomes evident when one recognizes it in its more
general form develped independenly in mathematics as the Carleman
linearization).
> However QM cannot say a single thing about what happens to
> the system while measurements are not done.
Of course it does say quite a bit. The dynamical evolution equations
yielding interference effects or the energy spectra for stationary
states, molecular and atomic structures, etc. These phenomena are all
computed without any reference to the "measurement" or any need of the
measurment "theory" postulates.
In relation to the measurement, QM does have to answer how does the
combined system 'aparatus1' plus 'object1' evolve (through the QM
unitary evolution of a composite system). It has to show how the
"measurement1" by the 'aparatus1' of the 'object1' is reflected in the
formalism when the composite system 'object1' + 'aparatus1' is treated
as a single 'object2'. That is precisely how von Neumann arrived to the
conclusion that the unitary evolution of the composite system is
inadequate and that it requires an additional form of non-unitary
evolution, which he introduced via his projection postulate (and
attributed to the consciousness of the observer).
Therefore, the self-consistency and the requirement of the independence
of the results on the (arbitrarily chosen) boundary between the
'object' and 'aparatus' carry implications which were first followed
through, as far as they can go, by von Neumann.
Your arguments so far are the standard "pedagogical" song and dance
(with all its capricious dictums and taboos, slippery twists and turns,
pulled out of thin air), the usual little initiation rite played to the
perplexed physics students to entrance them just enough so they can
"shut up and calculate." My masters thesis was on the topic of QM
measurement problem, and after hundreds of papers, preprints,
monographs, discussions with my professors ... etc, I was more
perplexed than before I started. It wasn't until some years later,
after leaving academia to work in industry, where I got a chance to see
how the quantum optics experiments were really done (unlike the magic
tricks shown to students e.g. see [1] for a recent shameless example),
that I realized that what I thought all along were the basic
experimental facts which needed to be explained by QM (such as double
slit or beam splitter anticorrelation "mystery", or Bell's inequality
violations) weren't the facts at all. They turned out to be simple and
entirely non-mysterious experimental phenomena that a 19th century
physicist would have no problem with, but merely wrapped in a peculiar
Quantum Optical jargon, in which the common terms such as "counts" and
"correlations" are quietly redefined (in references going back,
ultimately all to the Glauber's 1964 Les Houches Lectures [2], the
master document of the Quantum Optics) so that the QO-counts, which
they coincidentally also call just "counts", can be negative (via the
mostly unmentioned standard QO subtractions), where the
QO-correlations, which they also call "correlations", don't correlate
any events at all (Glauber's functions), where the QO-probabilities and
QO-information, which they also call "probabilities" and "information"
can be negative, etc.
------------------------------------------------------------
1. J.J. Thorn, M.S. Neel, V.W. Donato, G.S. Bergreen, R.E. Davies, M.
Beck
"Observing the quantum behavior of light in an undergraduate
laboratory"
Am. J. Phys., Vol. 72, No. 9, 1210-1219 (2004).
Preprint:
http://marcus.whitman.edu/~beckmk/QM/grangier/Thorn_ajp.pdf
Experiment Home Page: http://marcus.whitman.edu/~beckmk/QM/
2. R. J. Glauber, "Optical coherence and photon statistics"
in Quantum Optics and Electronics, ed. C. de Witt-Morett,
A. Blandin, and C. Cohen-Tannoudji.
Gordon and Breach, New York, 1965, pp. 63-185.
3. See also a recent Physics Forum thread with more detailed
discussion of [1] & [2] and additional references:
http://www.physicsforums.com/showthread.php?t=71297
For a discussion of the implication of Barut's self-field ED for
QM measurement theory see:
http://www.physicsforums.com/showpost.php?p=540794&postcount=100
http://www.physicsforums.com/showpost.php?p=541484&postcount=114
http://www.physicsforums.com/showpost.php?p=541708&postcount=118
.
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