Nick Douglas discussing "Cat in the Box"
with me...
 RT>> Yes, but the point of the experiment is to ask WHEN  do the
 RT>> probabilities collapse into one outcome -  inside the box before we
 RT>> open it or only when we  open it?
 ND> Hmmmmmmmmmmmmmmm... Does it matter? Not that I want to minimize the
 ND> issue.
Well, it does to scientists and those interested in Quantum Mechanics I 
ess.
 ND> I guess that they collapse inside before we open.
That's what common sense would tell you, but that isn't what is happening in 
the Quantum domain. In the quantum domain it is like this: the cat in the box 
is _neither_ alive nor dead (or _both_ alive and dead) until we open the box 
and have a look. Until we open the box we don't know. Once we open the box we 
know what happened. The cat is _either_ alive or dead and not a mixture or an 
absence of both states. However, it is the act of opening the box that 
determines the outcome.
This translates into the quantum world in such experiments where we want to 
find out which direction of "spin" an electron possesses in a particular 
plane. The cat in this case is an electron. We don't know the spin of an 
electron until we measure it but when we do measure it we have no idea what 
its state of spin was before we measured it. Once we know the spin of an 
electron, any further measurements of its spin in the same plane are always 
the same.
For example say we have a device that splits electrons depending on whether 
they spin "up" or "down". We send 1000 electrons through the machine (a 
Stern-Gerlach magnet) and find that 500 were "up" electrons and 500 were 
"down" electrons. We then pass the 500 "up" electrons through another 
"up"/"down" splitting device, and find 500 "up" electrons. Similarly, we pass 
the 500 "down" electrons through another "up"/"down" splitting device and 
find 500 "down" electrons.
Can you see the problem? When we put the original 1000 electrons through the 
first splitting device we got 500 "up" electrons and 500 "down" electrons 
(Remember, we did not know what their spin was before we measured them).
We then took 500 electrons that we measured to be "up" and passed them 
through another splitting device and this time they ALL came out "up" instead 
of 250 "up" and 250 "down" electrons. The same happens with the 500 "down" 
electrons which all come out "down".
So how comes an unmeasured electron has a 50-50 probability of being either 
up or down, but an electron with a measured spin, of say "up", is always 
measured to be 100% "up" thereafter?
 ND> BTW, I must
 ND> admit that I heard about Schroedinger's cat from a Douglas Adams book
 ND> (One of the Dirk Gently books).
Well I haven't read any of Mr. Adams' books so I can't comment. If you are 
interested in it then I can suggest some reading if you so wish?
    Relatif Tuinn
... 2 + 2 = 5 for extremely large values of 2.
--- Spot 1.3a #1413
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