reply to post by sirnex
I'm not sure if your mental block is comprehension or preconceived notions but I don’t really care…
"The measurement effect. With sublime understatement, this phenomenon is referred to as "the measurement effect." When we measure (or detect, or
see, or quantify, or determine, or otherwise gain knowledge of) something at the quantum level, the very act of measurement will have an effect on the
thing itself. To all intents and purposes, the act of a sentient being in seeking a measurement will cause the thing to have a property which can be
measured, and thereby produce a definite property that can be measured.
Since around 1927, the standard quantum mechanical explanation for the difference between results in the double slit experiments particularly, and
for the measurement effect generally, is that in one set of experiments, we know (or more precisely, we can in principle know)[2a] which slit the
electron went through; and in the other set of experiments, we don't know (i.e., we cannot know even in principle)[2a] which slit the electron went
through. This conclusion is one facet of the "Copenhagen interpretation" of quantum mechanics (so named because it was developed by Niels Bohr's
institute located in Copenhagen, Denmark), which represents the closest thing to a consensus among physicists for the last seventy years or so.
The difference is whether we know. The difference is whether we choose to have the information available.
If we demand to know which slit the particle went through, then a particle must appear at one slit or the other so that we will have an answer to our
question; and so our curiosity has caused there to be a particle at one of the slits, and now there is a particle; and if there is a particle at one
slit or the other, it must obey the rules for particle motion, and so it does.
Conversely, if we do not demand to know which slit the particle went through, no particle need appear at either slit; and so we have not caused there
to be any particle, and now there is no particle; and if there is no particle at either slit, the system remains free to roam the universe in whatever
form seems most pleasing to itself.
And all of this is determined at the time we demand the knowledge, not at the time we institute any mechanical processes for obtaining the
"These situations are principally derived from the phenomenon known as "delayed choice," proposed by John Wheeler, see Quantum Theory and
Measurement, J.A. Wheeler and W.H. Zurek, eds. Princeton Univ. Press (1983). The QM predictions have been experimentally realized and
At least one quantum eraser double slit experiment has been accomplished with electrons. I. Neder, et al., "Entanglement, Dephasing, and Phase
Recovery via Cross-Correlation Measurements of Electrons," Phys. Rev. Lett. 98, 036803 (2007-Jan-19), e-print at
This is said to be the electron equivalent of "ghost interference" as reported by D.
Strekalov, et al., "Observation of two-photon 'ghost' interference and diffraction," Phys. Rev. Lett. 74, 3600 (1995), which is nicely described
in P. Chingangbam, et al., "Two particle ghost interference demystified," e-print at arxiv.org...
also Qureshi T., et al., "Quantum Eraser Using Spin-1/2 Particles" (2005), e-print at arxiv.org...
The experimental realizations of delayed choice of which I am aware have all been accomplished with photons. This is mainly due to the ingenious
scheme developed by Marlan O. Scully which employs entangled pairs of photons to accomplish which-path measurement 1) with no interaction; and 2) with
the ability to manipulate the which-path information. M.O. Scully and K. Drühl, Phys. Rev. A 25, 2208 (1982). For pedagogical purposes, I have
continued to use electrons as the quantum units under discussion. With recent successes in achieving entanglement among atoms, it seems likely that
these experiments may be repeated in the near future with various quantum units of matter (if this has not already been achieved). QM being what it
is, there is no question but that the results will be consistent with those obtained with photons.
Summary of the basic idea of delayed choice experiments is available at
For an elegant delayed choice experiment, see Yoon-Ho Kim, et al., "A Delayed Choice Quantum Eraser", Phys.Rev.Lett. 84 (2000) 1-5
Commentary on this experiment is available at
Another excellent experiment is reported at G. Scarcelli, Y. Zhou and Y. Shih (Dep't of Physics, Univ. of Maryland). "Random Delayed-Choice Quantum
Eraser via Two-Photon Imaging." (Dec. 22, 2005) arxiv.org...
Recently, a group advised by Alain Aspect reported a successful delayed-choice experiment with single photons and direct intervention by the
experimenter. V. Jacques, et al., Experimental realization of Wheeler's delayed-choice GedankenExperiment (Oct. 31, 2006), e-print
Paul Kwiat and Rachel Hillmer describe "A Do-It-Yourself Quantum Eraser" in the May 2007 edition of Scientific American . All you need is a laser
pointer and some polarizing filters, and they even tell you where to get the polarizing filters. If I were still in high school, this would be my