Saturday, November 27, 2004

Variation of the Standard Two-Pin-hole "welcher-Weg" Optics Experiment






Results of different measurements of the Ft value in beta decay
George R. Welch setting up an optics experiment with graduate student Sophia Ilina
Dr. Scully and Dr. Townes, the inventor of the maser and laser (University of California, Berkeley), during the dedication of the Charles H. Townes Reading room
Assistant research scientist Daya Rathnayaka with a UHV deposition chamber


Tomorrow at 4 PM, physicist Shahriar S. Afshar, a Visiting Scientist at Harvard University's Physics Department will give a talk entitled Violation of Bohr's principle of complementarity in an optical "which-way" experiment at Texas A&M University.

Afshar has done a variation of the standard two-pin-hole "welcher-Weg" optics experiment, in which he demonstrates that wave interference is present even when one is determining through which pinhole a photon passes. This result is in direct contradiction to Neils Bohr's Principle of Complementarity, which would require in the quantum world that when one is measuring particle properties [formerly read "measuring quantum properties" -KC], all wave interference phenomena must vanish. Afshar's trick is to find the location of the minimum points of wave interference, place one or more wires at these minimum points, and observe how much light is intercepted when one is determining the pinhole through which the photons passed.

It has been widely accepted that the rival interpretations of quantum mechanics, e.g., the Copenhagen Interpretation, the Many-Worlds Interpretation, and my father John Cramer's Transactional Interpretation, cannot be distinguished or falsified by experiment, because the experimental predictions come from the formalism that all such interpretations describe. However, the Afshar Experiment demonstrates in an interaction-free way that there is a loophole in this logic: if the interpretation is inconsistent with the formalism, then it can be falsified. In particular, the Afshar Experiment falsifies the Copenhagen Interpretation, which requires the absence of interference in a particle-type measurement. It also falsifies the Many-Worlds Interpretation which tells us to expect no interference between "worlds" that are physically distinguishable, e.g., that correspond to the photon's measured passage through one pinhole or the other [the word "measured" added 4/28. -KC].

The Transactional Interpretation, on the other hand, has no problem in explaining that Afshar results. "Offer waves" from the source pass through both pinholes and interfere, creating a condition in which no transactions to the wires can form. Therefore, no photons are intercepted by wires, as Afshar observes. The quantum formalism makes the same predictions.

On this basis, it appears that two of the major interpretations of quantum mechanics have been falsified and should be relegated to the waste basket of physics history. The Transactional Interpretation, which involves a forward/back in time handshake, is one of the few (perhaps the only) interpretation(s) left standing after the Afshar test.

Yay for the home team!

(See also the Power Point presentation for my dad's Hal Clement memorial lecture at Boskone; Google also has an html cache of the Powerpoint presentation.)

LET ME KNOW if anyone reading this attends Afshar's talk or attended a similar one he gave at Harvard recently.

TRACKBACKS: My MT trackbacks don't work. One of these days I'll figure out why. But this entry has received really a lot, so here is a link to Technorati's listing of links to this post. Boingboing also blogged it, but the post has already slid off their main page.

SEE UPDATE, 5/10/04.


1 comment:

Lumo said...

I am in close touch with Afshar right now. Debunking of his interpretation of that experiment is here:

http://motls.blogspot.com/2004/11/violation-of-complementarity.html

All the best
Lubos