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

"Wave function having collapsed," did I hear you say?  I'm not sure
how much that picture is to blame for your puzzlement.  In any event,
Penrose' account as you have described it seems correct.  If you
call the 3 axes A, B, and C, one can measure angular momentum first
along A, then B, then C, and get "yes" each time.  It's worth noting
though that if you then measure angular momentum along axis A again,
you need not get the answer "yes" again.  In fact you will get "yes"
only with a 25% chance.  

Penrose is extremely idiosyncratic in this book, but here he seems
right.

I don't know if the experiment has been done but it seems easy
enough.