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Double Slit Experiments I by Robert Nemiroff Michigan Technological University

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Physics X: About This Course Pronounced "Fiziks Ecks" Reviews the coolest concepts in physics Being taught for credit at Michigan Tech o Michigan Tech course PH4999 o Title: Extraordinary Concepts in Physics o Aimed at upper level physics majors o Light on math, heavy on concepts o Anyone anywhere is welcome No textbook required o Wikipedia, web links, and lectures only o Find all the lectures with Google at: "Starship Asterisk" then click on "Physics X" or http://bb.nightskylive.net/asterisk/viewforum.php?f=39

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Double Slit ExperimentDouble Slit Experiment: Importance "I think I can safely say that nobody understands quantum mechanics." - Feynman "Any other situation in quantum mechanics, it turns out, can always be explained by saying 'You remember the case of the experiment with two holes? It's the same thing." - Feynman

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Double Slit ExperimentDouble Slit Experiment: Importance Voted by physicists reading Physics Today magazine as the most interesting experiment ever done in physics. Two Slit Experiment illuminates: Uncertainty principle Interference Entanglement Coherence and decoherence How truly strange QM is

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Double Slit ExperimentDouble Slit Experiment: Basics Most easily done with photons but, amazingly, can be done with any particle. Idealized Geometry: Geometry source slit screen image screen

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Double Slit ExperimentDouble Slit Experiment: Basics The source is a point source and illuminates both slits equally. Therefore, on the average, both slits will see the same mean number of photons. The slits are identical. They have identical width D. The wavelength of the light is λ. The slit screen is completely opaque except for the slits, which are completely transparent.

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Double Slit ExperimentDouble Slit Experiment: Basics Unless otherwise important to the experiment, photons should be considered to have the same wavelength λ. The image screen records the position of photon impacts with arbitrarily high accuracy.

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Double Slit ExperimentDouble Slit Experiment: Basics Practical geometry (almost anyone can set this up): light wavelength: λ width of slits: W Distance between slits: D W ~ a few λ D ~ a few W

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Double Slit ExperimentDouble Slit Experiment: Basic Results Real result of Double Slit Experiment:Double Slit Experiment

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Double Slit Experiment: Basic Results Photons traversing the two slits might interfere o interference pattern created at the image screen o geometric projection of each slit onto the image screen shows no overlap region even in the "plane wave" approximation o therefore, without single slit diffraction, no interference can occur o at any point on the image screen, the projection of one slit's light is usually "brighter" than the other interference minima usually not zero

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Double Slit ExperimentDouble Slit Experiment: Wave-Particle Duality Wave-Particle Duality The results are considered the definitive indication of wave-particle duality: that photons, electrons, etc. can act like BOTH waves and particles. Photons interfere as waves o common example: water waves Photons are recorded at the image screen as particles These are the SAME photons! Examples of things that are both particles and waves: o photons o all fundamental particles o everything

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Double Slit Experiment: One Photon at a Time The usual double-slit experiment is done except now the photon rate is reduced so that, on the average, only one photon passes the slit screen at any time. What happens? 1.No (two-slit) interference pattern appears. 2.A normal (two-slit) interference pattern eventually appears. 3.The image screen remains blank. 4.The experimenter loses interest and begins to watch TV.

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Double Slit Experiment: One Photon at a Time 2. A normal (two-slit) interference pattern eventually appears. Comments: One of the most amazing results in all of physics. Single photons appear to go through both slits. Dirac (paraphrase): "Photons interfere only with themselves." This assumes that nobody knows which slit individual photons went through.

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Double Slit Experiment: Electrons Replace Photons The usual double-slit experiment is done except now electrons replace photons. The electrons have mass but the same wavelength as the photons. What happens? 1.An interference pattern eventually appears. 2.No interference pattern eventually appears. 3.The image screen remains blank. 4.Monsters from the deep devour Cleveland.

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Double Slit Experiment: Electrons Replace Photons 1. A normal interference pattern eventually appears. Comments: Another of the most amazing results in all of physics. Single electrons appear to go through both slits. Dirac: "Electrons interfere only with themselves." This assumes that nobody knows which slit individual electrons went through.

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Double Slit Experiment: One Slit Blocked The usual double-slit experiment is done except now one slit is blocked. What happens? 1.An interference pattern appears. 2.No interference pattern appears. 3.The image screen remains blank. 4.A janitor offers to unblock the closed slit.

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Double Slit Experiment: One Slit Blocked 2. No interference pattern appears. Comments: Photons go through only the open slit. A single slit pattern appears behind this slit. The former existence of the second slit does not affect the result.

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