Re: Uncertainty principle

[Dennis Krause <kraused@WABASH.EDU>]

> Hi all-
> 	This deals with two different unrelated topics.
> 1.  Anyone teaching QM at any level should see the News and Views article
> by Peter Knight, "Where the Weirdness Comes from" in Nature for 3 Sept
> 1998, p. 12.  Knight discusses an experimental determination that
> Heisenberg's explanation of measurement disturbances is incorrect.  That
> is, in the two slit experiment, it is not the momentum kick from
> observing which path was followed by a particle that destroys the
> interference pattern.  The correct view is given by Schroedinger's
> entangled state approach.
> ...
>
> 					Regards,
> 						Jack

	I'm glad Jack raised this note since there seems to be some
controversy over the interpretation of these experiments.  Daniel Walls at
the University of  Auckland argues in favor of conventional Heisenberg
interpretation for the destruction of the interference pattern in the
double slit pattern.  For example, from the abstract of a recent paper
[Phys. Rev. A, Vol. 56, 55 (1997)], we find:

	A ``which-path'' (welcher Weg) measurement necessarily destroys the

	fringes in a double-slit interference experiment. We show that in all
	instances one may attribute this destruction to a disturbance of the
	particle's momentum by an amount equal to at least pi (h-dash-bar)/2d,
	where d is the slit separation, in accordance with the uncertainty
	principle. However, this momentum transfer need not be local; that is,
	it need not act at either of the slits through which the particle

	passes. For well-known welcher Weg measurements such as Einstein's
	recoiling slit and Feynman's light microscope, the disturbance can be
	understood in terms of random classical momentum kicks that act
	locally. In some recent proposals, including that by Scully,
Englert,
	and Walther [Nature (London) 351, 111 (1991)], the momentum
transfer is
	of a peculiarly quantum, nonlocal nature. In this paper we introduce a
	formalism based on the Wigner function, as this describes both the
local
	and nonlocal momentum transfer caused by any welcher Weg
	measurement. We show that for some examples, such as that of Scully,
	Englert, and Walther, there is no momentum disturbance at the slits
even
	though the nonlocal momentum disturbance is sufficient to destroy the
	interference pattern. Finally, we discuss the experimental
signatures of
	nonlocal versus local momentum transfer and demonstrate a strong
	similarity to the nonlocality of the Aharonov-Bohm effect.

I've wondered about this for some time after hearing about it, but haven't
had time to pursue it.    Does the system in this new experiment really
receive a momentum kick  or is it too small to account for the effect as
claimed by the authors?  Does the Heisenberg interpretation really fail
here?


				Dennis



					Dennis

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