Re: Quantum questions
- From: Oh No <NotI@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>
- Date: Sun, 24 Jun 2007 10:12:18 +0100
follow-ups to sci.physics.foundations
Thus spake Jane Gillett <j.gillett@xxxxxxxxxxxxxxxxx>
Oz,
Thankyou for being willing to answer questions on quantum theory. Sorry if
it seems to you that I ask for help and then go away. I've been trying to
refine my questions so as to keep them as short as possible while still
being meaningful - and found I learned a lot in that examination. However,
I'd be grateful if you could address the issues below.
I do not have a problem with considering an electron as both a wave and a
particle - the conclusion you come to depends on the way you investigate;
that is inevitable and familiar, particularly as in the case of electrons
we are trying to describe things we have no experience of, using concepts
set up in other contexts; it's not surprising that they do not always fit
and can cause inconsistencies. I have no problem in considering each as a
valid theory in the area where it gives predictions which match
experimental data..
It depends on whether one is interested in predicting results, in which
such an approach is fine, or whether one wants a genuine description of
an electron in its own terms, in which case it falls short.
QUESTIONS
1. Work has shown that we are limited in the precision with which we can
measure certain attributes of an electron (Heisenberg) or that we can only
measure probabilities not actual values (Schrodinger); we can only measure
the probability of an electron being in a certain state, we cannot
determine the actual state. OK. No problem with that.
However, and this is my question, AIUI quantum theory states that in
addition to our being unable to determine the state of an electron to
closer than the limits given by Heisenberg, we must consider that the state
is actually undefined - has no value, not simply that we do not know it -
has no value until we attempt to discover it. Why? What is the basis for
that opinion? I cannot see from any of the work quoted that the state is
undefined, only that we do not know it and cannot know it beyond certain
limits.
Also, even if you accept the above, I cannot see that that state of affairs
can actually occur. ISTM that where the state of an electron is a factor
contributing to the state of another body/particle/wave/whatever, then that
dependent entity effectively takes on the role of "investigator" since its
own state cannot be effectively defined without taking into account the
condition of the electron. Since I cannot comprehend an electron existing
without some other body affected by its existence (hence by its state),
unless it is by itself in some remote part of the universe, then I cannot
see that any electron is ever without an "investigator" so will never be in
an "undefined" state. The alternative would be a rippling sequence of
"state definitions" taking place when some human (why human?) investigator
decided to take a look.
Indeed. The term state is confusing, because it appears to suggest
something which belongs to the electron. Actually, it better describes
the interrelationship between the electron and other matter. That is a
thoroughly confusing notion too. I prefer to use "ket". The mathematical
structure of quantum mechanics concerns kets, which are often
misleadingly referred to as states or state vectors. It is better to
think of a ket not as the state of a particle, but as a label for what
we know of the particle (or can know in principle) from measurement.
2. Electron pairing and spin. The suggestion is made (Bohr) that if one
electron has one form of spin, eg spin-up, then there will be, in
existance, a "partner" electron which has spin-down; this is to keep the
universe spin-neutral. It is further suggested that once this "pairing" has
taken place - and the implication is that it is in existence even before
the spin state of one is determined - then the individual electrons retain
that pairing so that even if they are separated spacially, even by long
distances, the spin relationship is maintained. This means that if you
examine an electron (say electron 1) and determine its spin and find that
it is in, say, the up direction, then its partner will <at the exact time
of the examination of electron 1> take on a spin in the down direction.
What does "at the same instance mean in this case? Are they necessarily in
the same space/time frame?
Why should the universe always be spin neutral?
Is there any basis for this spin pairing? Why is it proposed?
Really I think you are asking questions for which answers are not
properly known. One cannot think of spin as something spinning in a
space-time background. In my view spin is intrinsically related to the
fundamental structure of space-time, but there is a great deal about
that relationship still to be learned.
What is the basis for saying this pairing is maintained despite the act of
separation? Why shouldn't the nearest electrons be the ones to take on any
compensatory spin if such is needed? What about all the electrons existing
between the pair?
3. Bell and non-locality. IAGTU that Bell put forward something called the
Inequality Principle and used it to test Bohr's suggestion (2 above) about
pairing.
What is this Inequality Principle? I've looked on the web and haven't found
anything I can understand. Can you put it into an ordinary punter's
language?
He made some assertions about Einstein's "Condition of locality" - what is
this condition? This seems to be concerned with Einstein's opposition to
the pairing suggestion on the basis of faster than light communication.
I understand Clauser and Aspect carried out "experimental verifications" of
Bell's "Inequality". I understand they found "violations" which was the
result needed to imply that Bell's Inequality Theory is correct and that
"nature is non-local" although I gather their conclusions have since been
queried on the grounds of some assumptions they made. I have not found any
information on what they did. Can you tell me anything please?
Unless someone's having me on and the book I'm reading is a spoof, this
non-locality idea seems incredible, exciting, useful even - or frightening!
(What is the potential for you, or someone else, to be able to control
things at a distance - but I guess that's not really being suggested, not
ATM anyway).
But seriously, having an effect which (a) does not change with distance,
(b) acts instantaneously and (c) effectively links up locations without
actually crossing space..... If true, this is really something mindblowing.
Are we truly talking seriously about there being something non-local?
Cheers
Jane
You are talking of entanglement. There is no ability to control things
at a distance. This applies, not in general to electrons, but to pairs
generated by a particular process such that they have opposite spins.
More realistically we study photons. A pair of photons is created from a
spin 0 initial state, such that we have no knowledge of the spin of
either, but the two must sum to zero. The two photons are then detected
at a distance from each other. When the spin of one is determined, the
spin of the other becomes immediately known.
Now, we are free to decide which spin axis to use. Usually we imagine
two experimenters, Alice and Bob. Alice will determine one spin, and Bob
the other. Suppose Alice decides to measure spin up/down, and finds spin
up. Then Bob's photon must be in a spin down state. But suppose Alice
decides to measure spin left. Bob's photon has to be in a spin right
state. Thus Alice's decision on which axis to choose affects the state
of Bob's photon. Alice can actually decide which axis to use after the
production of photons, and her decision affects the state of Bob's at a
distance greater than light speed.
Bob cannot actually tell from the experiments he does, what decision
Alice made. He finds half the photons in a spin up and half spin down,
whichever way Alice arranges her apparatus.
But when Alice and Bob get together later and compare notes, they can
find correlations in the results. The results of Bob's experiments have
been influenced by Alice's decision on how to arrange her apparatus,
although this decision is made after the initial production of the
photons, and at a distance greater than light speed. There is just way
to say so until he knows Alice's results also.
Regards
--
Charles Francis
moderator sci.physics.foundations.
substitute charles for NotI to email
--
Posted via a free Usenet account from http://www.teranews.com
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