Joao:
> I have heard that Schrodinger tried to revive his cat 
> that way and found out that it only works nine out 
> of ten times...

Ah! Now I understand why Einstein and then also Bell 
changed a bit that 'paradox'. In Einstein terms 
|dead> & |alive> are replaced, if I remember well,
by |ink spot on a paper> & |no ink spot on a paper>.
Bell was more audacious and build a paradox
with |cat which is fat> & |cat which is hungry>!

---

Among the old QM paradoxes there is something
little known, by Janossy and Nagy, which imo
is very ... 

(I do not remember if I've already reported here 
this issue, my memory is very very short)

So, there are also weird two-slit esperiments in which 
a single light beam is divided by a (random) shutter in such 
a way that the two slits are **never** open **simultaneously**.
Nevetheless we get the usual interference pattern (I say
interference, not diffraction).

Consider a diaphragm, with two slits, slit 1 and
slit 2. Each of these slits can be opened, or closed,
by a shutter connected with a separate counter.
A weak alpha-particle emitter is placed between
the two counters. Imagine that, in the beginning
of the experiment, both slits are closed.
If an alpha-particle strikes one of the counters,
the slit connected with this counter is opened,
and the counters cease to operate, and a light-source
is turned on, in front of the diaphragm, and this
light-source illuminate a photographic plate placed
behind the diaphragm. Following qm rules, we can write
         psi = 1/sqrt2 (psi_1 + psi_2)
where psi_1 is the wavefunction describing the system
when the slit 1 is open (psi_2 when the slit 2 is open).
Thus, from the theory, we get the usual interference
pattern, on the photographic plate behind the diaphragm.
But if we keep our eyes opened, and we observe which slit
is open (slit 1, or slit 2) then, in accordance with the
'complementarity' principle, and the 'projection' postulate,
a reduction takes place, and no interference pattern
appears on the plate.

The above interesting 'gedanken' experiment is due to 
L. Janossy, and K. Nagy, [Annalen der Physik, 17, (1956), 
115-121]. Btw, Janossy and Nagy thought it was possible
to perform such an experiment, but they also thought
it was impossible to get that interference.

After useful considerations by Leonard Mandel [J. Opt.
Soc. Amer., 49, (1959), 931] at last R.M. Sillitto and 
Catherine Wykes [Physics Letters, 39-A-4, (1972), 333] 
performed the Janossy and Nagy experiment and found a marvelous
interference when just one photon was present in their
interferometer, at a time, and when their electro-optic
(not random) shutter was switched several times during the 
time-travel of each photon.

In terms of photons (particles) the condition for interference
is that the two paths lead to the same cell of phase space,
so that the path of each photon is intrinsically indeterminate
(the usual 'welcher weg' issue).

Of course, the shutter (random or not random) must be switched in 
a time which is less than the uncertainty in the time arrival of 
the photon.



 



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