🚀 go-pugleaf

In article <10910@sun13.scri.fsu.edu> jac@ds8.scri.fsu.edu (Jim Carr) writes:
>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%.  
>

No wonder they call it spin. :-) Seems the notion of spin in a particular
direction is a bit meaningless if you can detect it in all three
directions. Your comment about polarizing sheets has almost persuaded me,
however we're dealing there with _aggregates_. I was wondering whether it
was possible to measure the spin of one single electron three times
consecutively and get "spin up" in three directions. If you did such a
measurement at 0, 120 and 240 degrees and got YES each time, and then
measured it at 180 degrees, would there be a chance (1 + cos 60)/2 (ie the
angle between the last measurement at 240 and 180) of now getting a YES,
or does the past history of a measurement at 0 preclude this? Could you
possibly get a YES at 60 degrees, exactly opposite your YES at 240 degrees?

>Is your brain numb yet? 

Yes.

Dave Griffiths