Conference decwet::physics

    I think that the experiment might be a little difficult to reproduce in
    a home lab, part of the experiment involves the measurement of spin
    values of individual electrons 

    Sorry, that should have been photons. The experiment consisted of
    changing polariser angles while photons were in flight and verifying
    that measuring the spin of one of a pair of photons which had been
    initially a single quantum system affected the spin of the other of the
    pair in a time which was less than light would have required to
    traverse the distance between the photons.
    
                            Dennis 

    Dennis,
    	Tx, do you know where this is written up? I've yet to find the
    experiment details.
    
    /Dean

    Dean,
    
       sorry, I've only seen a general description (can't remember where)
    which describes the experiment only to the level of detail in .2. I
    believe that the pairs of photons were produced by the decay of a
    particle with zero angular momentum, giving a known spin state for the
    system. I guess that the setting of the polarisers and subsequent
    measurements were then synchronised to some master clock source to give
    the necessary time discrimination.
      As a possible source for further information you could try the WWW if
    you have access to it. Try searching on Alain Aspect, Bell's Inequality
    or just plain old QM.
    
                            Dennis 

>                     <<< Note 419.2 by WARFUT::CHEETHAMD >>>
>                        -< What I realy meant was..... >-
>
>    Sorry, that should have been photons. The experiment consisted of
>    changing polariser angles while photons were in flight and verifying
>    that measuring the spin of one of a pair of photons which had been
>    initially a single quantum system affected the spin of the other of the
>    pair in a time which was less than light would have required to
>    traverse the distance between the photons.

Is this related to the "photon-free" photography that was written up 
in Scientific American recently?

- tom]

    re .5
    
       No, it was an experiment carried out by Alain Aspect and his team
    from the Institute of Atomic Optics in France (Paris I think) aimed at
    demonstrating the non-locality of Quantum Mechanical relationships. As
    I stated in .2 my understanding of the experimentis that pairs of photons 
    with total zero angular momentum were produced and allowed to separate. A
    measurement of spin was decided upon and carried out upon 1 photon by 
    variable polarisers after the photons had seperated, with measurements
    then made to verify that the other photon of the pair had assumed a
    conjugate value in a period less than would be taken by a light speed
    signal to pass between the particles. Not entirely sure about this but
    I believe that a statistical distribution referred to as Bell's
    Inequality was used to verify the result. (anyone care to
    confirm/deny/laugh themselves sick at this)?
    
                                 Dennis

Re .4
>      As a possible source for further information you could try the WWW if
>    you have access to it. Try searching on Alain Aspect, Bell's Inequality
>    or just plain old QM.

Unless I'm thinking of the wrong theory, try looking up Einstein, Rosen & 
Podolsky (sp?), who originaly theorised the existence of this effect.

		Ian G.

    re .7
    
>Unless I'm thinking of the wrong theory, try looking up Einstein, Rosen & 
>Podolsky (sp?), who originaly theorised the existence of this effect.
    
    Not sure how related it is to the Aspect experiment but I believe the
    order of those names is usually Einstein, Podolsky, Rosen (EPR). I
    don't care but a computer search might.

    re .7
    
      EPR actually proposed this as a paradoxical thought experiment which,
    due to the perceived requirement for super luminal signaling, cast
    doubt on the correctness of QM. I don't think that Einstein was
    particularly enamoured of QM.
    
                            Dennis

There is a pretty good description of the Aspect experiment in a book
published last year in the United States, called Where Does the Weirdness
Go?  Sorry, I don't remember the author's name.  It's a very nice popular
view of quantum paradox and quantum weirdness, with some of the latest
thinking on the measurement problem.  I recommend it.

Steve

    I guess I'm not even sure what the mystery is yet. I'm new to physics,
picked up a couple of books several months back and got hooked.  When I read
about double slit type experiments, I imagine a WBZ radio signal through two
slits and would expect it to interfere. Since a photon seems to be a "energy
level" to kick an electron, the energy in a radio wave is too small. but if I
were in a ship heading near light speed towards WBZ and the two slits, I'd
expect a combination of the fields from the slits to tick up as photons in a
distribution.  One book, Gribbin I think, tells of experiments where a photon
fires through a crystal, and splits into two photons, one horizontally
polarized, the other vertical.  Looking at one you can determine the other,
but what is the difference tossing a coin on a glass floor.  One man above,
the other below, if one or the other looks at the coin, they can immediately
tell what the other man will see, regardless of distance. Is the aspect
experiment supposed to determine whether looking at or not changes
the result on the other side?  eg with odds of tosses with two coins..

Coin A heads, Coin B heads 25 %
Coin A tails, Coin B tails 25 %
Coin A heads, Coin B tails 25 %
Coin A tails, Coin B heads 25 %
Coin A NOT equal to Coin B 50 %

	I must be missing something!
/Dean
    

     The Aspect experiment is supposed to demonstrate that the two particles
    remain part of a single quantum system although seperated spatially.
    This is demonstrated by showing that observing a property of 1
    particle, in the case of tha Aspect experiment spin, has an instantaneous 
    effect ( or at least an effect propagated at superluminal speeds ) on
    the corresponding property of the other particle of the pair. 
     My understanding of the 2 slit experiment is that placing a detector
    capable of detecting particles (usually photons although I believe that
    the experiment has been done with electrons) at 1 slit (i.e. making an
    observation) causes the collapse of the particle wave functions and
    hence the disappearance of the fringe pattern.
    
                             Dennis
                                   

    re .11

    I wouldn't pretend really to understand QM but hopefully this gives
    some indication of where the mystery is...
    
    A key point regarding the Aspect experiment is that spin is a quantum
    variable rather than, for want of a better term, a macro variable.
    
    If spin were like, e.g. charge, then one could posit that the two
    particles _when created_ had "opposite" values for some variable and
    hence it is not surprising that when measured they are correlated,
    regardless of how far apart the measurements are made.
    
    That is, a macro variable has a definite value, regardless of how or
    whether you measure it while a quantum variable exists as a
    superposition of multiple possible values until you actually measure it
    (after which it has a definite value but that value is not predictable
    in advance except in a probabilistic sense).
    
    The thing about spin is that the measured value depends on the nature
    of the measurement made, something that can in principle be decided
    after the particles have been created and are too far apart for light
    speed communication to take place.
    
    Even under these experimental conditions though the two particles show
    a correlation in measurements that is not the expected value unless
    QM or something like it is in operation. The actual mathematics of this
    escape me.

    re .13. Is there a definite distinction between macro and quantum
    variables or are all variables quantum in nature, with "macro" behaviour
    being the result of observations. Does charge (for instance) have a 
    conjugate variable in the same way as position and momentum or values
    of spin in different co-ordinates are conjugates?. Anyone care to
    enlighten us?
    
                          Dennis

No, charge is not the complement of any other variable.  In a sense, it is
not a variable, because the only known charges in nature are � 1 electron,
� 2/3 electron, and � 1/3 electron.

Steve

  When I first read about the dual slit, I drew a diagram and assigned
time points t1, t2, t3, etc. t1 photon at slits, t2 at screen, t3 at
a distant point and then tried to explain the particle wave duality
to myself with colored pens as I moved the photon through.  It made
sense to me, only if the whole process was a transaction. in otherwords
where the photon lands is as important as where it is emitted. I discarded
that idea as it forces reverse time, until I read Gribbins book that feynman
had the same idea to explain radiation resistance, then forming the idea of 
retarded waves and advanced waves.  I must say I am comfortable with this
concept.  I'm chewing through Einsteins own book Relativity, and if I disregard
time and put myself at the space of each event on my diagram, s1, s2, s3 etc.
it makes sense.  eg if I see a interference pattern at s2, I know I've landed
a photon here and that I will go through two slits (backwards in time)

  An analogy (in normal "time") could be made with a transmission line zip 
cord. Attach a generator to one end t1, a bulb to the other t2 and you have a 
transaction.  You can also attach a bulb to one end t1, then the generator to
the other end t2 and have a transaction.  The event propagates down the line in
either case, charges are opposite (as opposed to time in the above advanced
wave) if the wires were insulated and Litz woven you'd have a hard time
measuring. if you did separate one wire, and measure with a clumsy clamp meter
of large inductance, you'd put the light out (collapse the wave function). I
think I'm getting goofy with this analogy!! 

  So I can visualize how a single quantum state over a large space/time but 
what is it that you could "touch" to see it in action rather than just the 
after effect - because the system is soooo fragile - I dunno!  

/Dean
    

    re .14,.15
    
    I was probably wrong to use charge as my example of a variable that
    doesn't behave like spin because it seems to have added to the
    confusion rather than removing some. Some clarification...
    
    1. I have never read that one cannot get quantum states involving
    charge. I don't know whether it is possible or not. The question is:
    Can one design an experiment to distinguish between a) the two
    particles are created with definite and opposite charge and b) the two
    particles both are in a quantum state involving both being positive and
    negative but are correlated in such a way that whenever one measures
    the charge of each particle (when "far" apart) they measure as having
    opposite charge.
    
    2. Spin *is* quantised, just as charge is and so its magnitude can in
    principle be known *exactly*.
    
    3. The thing about spin that charge does not exhibit, is a concept like
    "orientation". Those double quotes should be noted carefully. Quantum
    spin acts as if there is a measurable spin axis direction but noone (I
    think) believes that anything is actually spinning. Charge has
    magnitude and sign whereas spin has magnitude and "orientation".
    
    The funny thing is that the apparent axis direction depends on what you
    measure. This is where the quantum weirdness slips in because one can
    alter the measuring experiment in real time so that light speed
    communication could not ensure that the spins are correlated in the way
    that the Aspect experiment observed.
    
    I suspect that macro world objects do not exhibit quantum spin type
    properties because the object involves the combination of so many
    zillions of uncorrelated spins of the individual particles.
    
    I believe that more generally the dividing line between the quantum
    world and the macro world is the subject of active theorising,
    experimentation and speculation. Translation: Noone knows.

    re .*
    
    I just wanted to clarify some confusion that crept in via some early
    notes and propagated (in isolation (-:) throughout this topic...
    
    The Aspect experiment involved measuring photon polarisation. Polarisation
    is not the same as spin.
    
    Both spin and polarisation have quantum wierdness in that the result of
    measuring depends on what you measure. However the actual mathematics
    of photon polarisation is slightly more complex than, say, electron
    spin. It's something to do with an electron being a spin half particle
    while a photon is a spin one particle (which brings in an extra square
    root) but I wouldn't pretend to understand why. The result is that it
    is easier to explain the _concept_ of the Aspect experiment using a pair
    of correlated spin half particles. I can only assume that there are
    practical reasons why Aspect chose to use photons and polarisation.