From: gast@next.ben-fh.tuwien.ac.at (Gast)
Subject: Re: Relative uncertainty principle?
Organization: Technical University of Vienna
Distribution: sci
Date: Fri, 31 Jul 1992 08:50:41 GMT

In article <ARI.HUTTUNEN.92Jul31031010@saha.hut.fi> Ari.Huttunen@hut.fi  
(Ari Huttunen) writes:

[ shortened ]

> Consider a two-slit experiment (like the one shown in the July issue
> of Scientific American that I have lying on the table). If you  
observe/know
> the photon all the way from the light source, through one of the slits  
and
> to the detector, you know with exactness that the photon is indeed a  
particle,
> but the wave-personality has become uncertain. Thus you cannot detect  
any
> interference pattern. [ ... ]

Well that's the first experiment: You have a detector at at least one of  
the slits and one at the screen.

> [ ... ] If you
> wait longer and observe only the effect of photons hitting the detector,
> you will notice an interference pattern. You now know the photon is a  
wave,
> but you have lost knowledge of its particle-quality, thus you don't know
> which slit it went through. (At the right of the picture.) If you  
observe
> the particles hitting the detector but not which slit they went through,  
you
> will have some knowledge of the particle-quality (you observe distinct
> particles) and some knowledge of the wave-quality (you can calculate the
> interference pattern by observing very many of the distinct particles.)

That's the second experiment: you remove the dtetector(s) from the slit(s)  
and measure only the photons on the screen.

Untill now, everything is clear: There are two different kinds of  
experiments which _cannot_be_performed_at_the_same_time_. The  
'personality' of the photon (see later) is dependent on the kind of the  
experiment.

> But why did I write 'relative' in the subject?

Now the mess starts:

> Think about how the laboratory is observing the photon (as a wave). The  
two
> slits are uncertain through which of them the individual photon went  
through.
> Now, think what the photon observes.
> The photon knows exactly where and when it goes.
  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

.. Oups: Quite an intelligent kind of a photon, isn't it?
The key point is that the photon _does_not_know_ its way. As a matter of  
fact there are experiments (with neutrons) that show that  
_single_particles_ can interfere with themselves (-> Ref 1). This  
confusing explaination becomes clearer if you talk about waves instead of  
particles and then you are back to the dualism.

> It is the laboratory that is affected by uncertainty. The photon 
> would see the two slits so vaguely that they would seem to be only one  
slit, 
> through which the photon goes.

Good idea. Yet, how do you explain the following phenomenas:

1) The interference if you don't measure which slit is passed
2) The dependence of the outcome on whether you measure the passed slit or  
not.
3) What about two small slits in a large distance?

> Both the wave-quality of the photon and the location in space of the  
slits
> are large scale phenomenon, the particle-quality of the photon and its
> exact path through the slits are small scale phenomenon. You cannot  
measure
> both small and large scale events exactly.

Wave-qualtity means: momentum defined - position undefined
particle-quality means: position defined - mementum undefined

This is the explaination to

> What if we say:
>	"It is impossible to know simultaneously and with exactness
>	 both the personality of the photon as a wave and as a particle."

Ref 1: At last some comment on the neutron-experiment performed by Rauch,  
Zeilinger at the Atominstitut of Vienna, Austria:

Neutrons are sendt through an interferometer with such a low flux that  
only one after the othe could pass it. Yet there were interferences  
observed (the spin was measured).

Remark: The dependence of the outcome on the kind of experiment leads to  
some difficulties with special relativity:
Suppose you seperate two wavefronts of one particle in space rather far  
from each other like in the two-slit-experiment. Then you randomnly decide  
wheter you perform a Type 1 or a Type 2 experiment as described above. You  
can arrange the detector (which is turned on and off randomnly)  on one  
slit so that the other slit won't know in time (with c as the  
signal-velocity) which kind of experiment is performed. Yet it _has_ to  
know because of the outcome of the experiment depends on the type.

So much fun with this

Harry
--
+-------------------------------+-------------------------------+
! Harald Lakatha                !    Please add to 'subject':   ! 
! gast@next.ben-fh.tuwien.ac.at !  'An Harald - nicht loeschen' !
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