Role of the Observer

Astralos

In the double slit experiment, there is interference between the fired particles when both slits are opened. Yet, when an observer checks to see what's actually going on by looking at the slits as the particles pass through them, the interference disappears. In other words, the particles act like particles when they are observed, yet they act like waves when they are not observed. Why does this happen, and what does it mean? Certainly, particles don't have a consciousness with which to deduce, "Oh snap, they're watching me--I better go straight through and not make a scene."

Matterwave

Actually, the act of observation itself is not necessary to destroy the interference pattern. The possibility of observation is enough. If you shined a light between the screen and the slits, WITHOUT actually observing to see which hole each electron went through, there would be no interference pattern.

Astralos

Quote by Matterwave

Actually, the act of observation itself is not necessary to destroy the interference pattern. The possibility of observation is enough. If you shined a light between the screen and the slits, WITHOUT actually observing to see which hole each electron went through, there would be no interference pattern.

Does that not suggest that the method of observation (in this case, the light) is responsible for the discrepancy, NOT the act of observing?

billschnieder

http://www.cbc.ca/m/rich/technology/...steinberg.html

"We are all just thrilled to be able to see, in some sense, what a photon does as it goes through an interferometer, something all of our textbooks and professors had always told us was impossible," Aephraim Steinberg, a physicist at the University of Toronto's Centre for Quantum Information and Quantum Control, said in a statement.

The results were published Thursday in Science.

EDIT: more details:

Science, 3 June 2011:
Vol. 332 no. 6034 pp. 1170-1173
DOI: 10.1126/science.1202218

Observing the Average Trajectories of Single Photons in a Two-Slit Interferometer

Sacha Kocsis1,2,*,
Boris Braverman1,*,
Sylvain Ravets3,*,
Martin J. Stevens4,
Richard P. Mirin4,
L. Krister Shalm1,5, and
Aephraim M. Steinberg1,†

Abstract

A consequence of the quantum mechanical uncertainty principle is that one may not discuss the path or “trajectory” that a quantum particle takes, because any measurement of position irrevocably disturbs the momentum, and vice versa. Using weak measurements, however, it is possible to operationally define a set of trajectories for an ensemble of quantum particles. We sent single photons emitted by a quantum dot through a double-slit interferometer and reconstructed these trajectories by performing a weak measurement of the photon momentum, postselected according to the result of a strong measurement of photon position in a series of planes. The results provide an observationally grounded description of the propagation of subensembles of quantum particles in a two-slit interferometer.

Matterwave

It suggests that really interactions are the culprit, in fact, the decoherence theory has made quite a bit of headway into this matter; however, the wave function collapse is still somewhat interpretation based, so I can't say for sure.

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Matterwave	Actually, the act of observation itself is not necessary to destroy the interference pattern. The possibility of observation is enough. If you shined a light between the screen and the slits, WITHOUT actually observing to see which hole each electron went through, there would be no interference pattern.