[extropy-chat] Cramer on Afshar

scerir scerir at libero.it
Sat Nov 13 07:49:32 UTC 2004


[Cramer]
> I predict that a new generation of "Quantum Lawyers" 
> will begin to populate the physics literature with 
> arguments challenging what "is" is and claming
> that the wounded interpretations never said that 
> interference should be completely absent in a quantum 
> which-way measurement.

This seems perfect. Unfortunately the first 'lawyer'
is Afshar. Essentially his experiment is based on
the Greenberger-YaSin-Englert-Zeilinger-etc.
relation V^2 + K^2 = 1, which says that the
Visibility of interference fringes becomes
more and more vanishing when the Knowledge
of the 'which way' the particle went through
becomes more and more definite. In general
there is a smooth (also in experiments eh!)
transition between the particle-like and
the wave-like behaviour. So the orthodoxia
just says that we cannot reveal a 'sharp'
particle-like and a 'sharp' wave-like behaviour
at the same time (for a single quantum).
That is to say we cannot set V=1 and K=1
at the same time, in the above (uncertainty)
relation.

Afshar claims that he was able to reveal
the 'sharp' particle-like and the 'sharp'
wave-like behaviours 'at the same time'
(= in the same experiment). He claims that
in his experiment V=1 and K=1. Unfortunately
in his experiment V<<1, by definition, since
his little wires do not positively show interference
fringes at all.

That said, imo, Afshar experiment *is* great,
because it shows that something unexpected
is going on. It is the possibility of following
the (causal, reasonable) course of quanta
in space and time. And this is exactly what Bohr
thought.

'However, since the discovery of the quantum of action, 
we know that the classical ideal cannot be attained 
in the description of atomic phenomena. In particular, 
any attempt at an ordering in space-time leads to a break
in the causal chain, since such an attempt is bound up 
with an essential exchange of momentum and energy between 
the individuals and the measuring rods and clocks used 
for observation; and just this exchange cannot be
taken into account if the measuring instruments are 
to fulfil their purpose. Conversely, any conclusion, 
based in an unambiguous manner upon the strict conservation 
of energy and momentum, with regard to the dynamical
behaviour of the individual units obviously necessitates 
a complete renunciation of following their course 
in space and time'. 
[Bohr, Atomic Theory and the Description of Nature, pp. 97-8, 
Cambridge University Press, 1934]






More information about the extropy-chat mailing list