Lect21 - Lecture 21: Lasers, Schrdingers Cat, Atoms,...

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Lecture 21, p 1 Lecture 21: Lasers, Schrödinger’s Cat, Atoms, Molecules, Solids, etc. Review and Examples
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Lecture 21, p 2 A radioactive nucleus can decay, emitting an alpha particle. This is a quantum mechanical process. The alpha particle is detected with a Geiger counter, which releases a hammer, which breaks a bottle, which releases cyanide, which kills a cat. Suppose we wait until there is a 50:50 chance that the nucleus has decayed. Is the cat alive or dead? Schrödinger’s Cat: How seriously do we take superpositions? c We now know that we can put a quantum object into a superposition of states. c But if quantum mechanics is completely correct, shouldn’t macroscopic objects end up in a superposition state too? c This puzzle is best exemplified by the famous “Schrödinger’s cat” paradox:
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Lecture 21, p 3 Where does it end?!? c it doesn’t end (“wavefunction of the universe”) c there is some change in physics (quantum b classical) c “many-worlds” interpretation… In any event, the correlations to the rest of the system cause decoherence and the appearance of “collapse”. Schrödinger’s Cat Did you ever perceive yourself as being in a superposition? (probably not …) This paradox leads to some dispute over the applicability of QM to large objects. The experiments are difficult, but so far there is no evidence of a “size limit”. According to QM, because we can’t know ( i.e. , predict), until we look inside the box, the cat is in a superposition * of being both alive and dead! { } cat 1 (alive) (dead) 2 ψ Ψ = + * More correctly, we, the atom, and the cat become “entangled”. And in fact, according to QM, when we look, we are put into a quantum superposition * of having seen a live and a dead cat!!
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Lecture 21, p 4
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Lecture 21, p 5 FYI: Recent Physics Milestone! There has been a race over the past ~20 years to put a ~macroscopic
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This note was uploaded on 01/19/2012 for the course PHYS 214 taught by Professor Staff during the Fall '08 term at University of Illinois, Urbana Champaign.

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Lect21 - Lecture 21: Lasers, Schrdingers Cat, Atoms,...

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