🚀 go-pugleaf

In article <1992Sep27.185539.594@prim> prim!dave@germany.eu.net (Dave Griffiths) writes:
>I've just been reading Roger Penrose's explanation in the Emperor's New Mind
>of the EPR experiment. In this he claims that it is possible to measure the
>spin of an electron in three different directions at 120 degrees to each
>other and obtain the result YES, the spin of the electron is aligned in
>that direction for each measurement. Surely this is incorrect? He has already

Nope.  It is correct. 

>shown that if you measure the spin in one direction and get the answer YES,
>then there is a probability of (1 + cos 120)/2 = 1/4 that a measurement at
>120 degrees to this will also give the answer YES. Fine. But surely, with 
>the wave function having collapsed, it's not possible for three measurements 
>at 120 degrees to each other _all_ to give YES?

Why not?  The wavefunction was "collapsed" in the original direction 
before you ran it through the first 120-degree analyzer.  The chance of 
getting three YES answers from the system described is just 1/4 * 1/4 * 
1/4 = 1/64 = 1.56%.  

The answer is identical to the answer you get for three Polaroid sheets 
laid at 120-degree angles on top of an sheet that defines the original 
polarization.  Surely you have seen (or done, find some friends with 
Polaroid sunglasses) the experiment where you have crossed (90 degree) 
polarizers and then insert one between them at 45 degrees and see it 
go from black to (dim) light.  Same thing with electrons. 

>If it is possible, has such a result ever been experimentally confirmed?
>
>Please say no.

Sorry, the answer is yes.  I know people who do this sort of thing all 
the time when doing experiments with polarized beams.  

Is your brain numb yet? 

--
J. A. Carr                                    |  "The New Frontier of which I  
jac@gw.scri.fsu.edu                           |  speak is not a set of promises
Florida State University  B-186               |  -- it is a set of challenges."
Supercomputer Computations Research Institute |   John F. Kennedy (15 July 60)