# 'Observer in double slit experiment' publications available?

I've just found Dr Quantum video sample where double split experiment is presented conducted out by researchers. Are there any papers published in peer reviewed journals on that experiment to read in detail?

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The question isn't very clear. It should stand on its own rather than requiring that we watch a video in order to know what it's asking about. –  Ben Crowell Aug 9 '11 at 14:24
Judging by your comments to the other questions, what I think you're trying to ask is: "Are there any papers published in peer reviewed journals demonstrating the destruction of the interference pattern in a double-slit experiment when an observer measures which slit the particle passes through?" If that's your question (when phrased that way, nobody needs to watch the YouTube clip) I try to answer it below. –  Anonymous Coward Aug 9 '11 at 17:59

Yes. A recent publication looks at the wave-particle duality of large particles (buckminsterfullerene, $\mathrm{C}_{60}$ (actually more of a many-slit experiment)), however the original diffraction experiments with electrons and such were done in the 1920s-1930s and may be hard to access.

The authors talk about facile ways to collapse the interference by observation, but only actually present the diffraction pattern (or lack thereof) with and without the grating. The paper is quite detailed.

Be careful, however. Whilst the wave-particle duality of electrons is well-characterised, the movie that Dr. Quantum is from pushes a lot of untenable silliness.

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many thanks for your helpful link and answer, I'm getting intrested in physics but do not have strong background and might be tempted to pseudo science. I will try to read the paper. Does it mean there was not collapse of interference by observation experiment exactly as reported in the movie? –  Chesnokov Yuriy Aug 9 '11 at 9:31
@ Chesnokov Yuriy - The clip makes things sound a lot more mysterious than they actually are. Observation is a type of measurement, and measurement involves interaction. On macroscopic scales, interactions such as light bouncing off something into your eye are negligible. On the scale of quantum mechanics, however, any meaningful measurement on a superposition of states induces collapse. To give a concrete example based on the paper, this could be something as simple as the $\mathrm{C}_{60}$ molecule interacting with a photon in an interferometer observing the particle stream. –  Richard Terrett Aug 9 '11 at 10:55
To summarize and elaborate on Richard's explanation: the interference pattern gets washed out by the interaction with light. Whether or not a concious observer sees the light makes no difference. –  Dan Aug 9 '11 at 20:56
@Dan, I see, then the observer interaction in the movie is bogus –  Chesnokov Yuriy Aug 12 '11 at 8:37
It is true that anything that could extract "which way" information will wipe out an interference pattern, but yes, everything else about Dr. Quantum and What The Bleep Do We Know is bogus. There is something which is called the "observer effect" in quantum mechanics, but its nothing like the description in the video. It basically states that there's no way to make certain measurements of a system and leave it completely undisturbed. –  Dan Aug 12 '11 at 9:31

In his 1924 dissertation, de Broglie argued that matter particles should have a wavelength of $\lambda = h/p$, where $p$ is the momentum of the particle. The first confirmation of the diffraction formed by such matter waves was observed in the Davisson-Germer experiment: C. Davisson, L.H. Germer. Phys. Rev. 30 (1927) 705. Independently, G.P. Thomson (son of J.J. Thomson, discoverer of the electron) and A. Reid found similar effects: Nature 119 (1927) 890.

As to the specific experiment your video references, the first proper electron double-slit experiment was performed by Claus Jönsson, in Zeitschrift für Physik 161 (1961) 454. A partial translation of this appeared in AJP 42 (1974) 4. If you don't have access to AJP, don't despair--there's a pdf floating on the interwebs.

And to echo to Richard Terrett's warning about Dr. Quantum in general: while there might be some genuine information here and there, overall What the Bleep was a thinly veiled advertisement for a pseudoscience-driven scam.

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+1 for the comprehensive answer, especially the Jönsson paper. The Thomson and Reid paper may however be inaccessible even to people with nature site licenses, as it is in my case. I erroneously stated that the old papers were slit experiments and have corrected my answer based on you pointing this out. –  Richard Terrett Aug 9 '11 at 8:17
thank you very much for your answer, I was eager to know if the experiment with observer present collapsing wave behaviour to particle actually took place and if it was published in any journal? –  Chesnokov Yuriy Aug 9 '11 at 9:07
Yes, though observer is a bit of a loaded term. Any device that records which-way information will remove the interference pattern... this experiment is usually done with photons rather than matter (as in the video), but fundamentally the effect is the same. For a very interesting variation, look up 'quantum eraser' experiments. –  Stan Liou Aug 12 '11 at 8:37

If you want a paper that explicitly shows the loss of interference due to a "which slit" measurement, the following work by Dave Pritchard's group at MIT (using sodium atoms) is one very nice example.

Journal reference: Physical Review Letters vol. 75 pg. 3783 Nov 1995

There are a few minor errors in the linked Dr. Quantum video (the electrons should still exhibit single-slit diffraction when the slit is measured), but apparently there are much worse parts of that movie than the selected clip, as the other commenters point out.

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thank you very much for the link –  Chesnokov Yuriy Aug 9 '11 at 21:11