Marxism and religion; quantum reality. Part II
Les Schaffer
schaffer at SPAMoptonline.net
Wed Jan 3 13:32:11 MST 2001
[ Part II ]
The cat and the quantum wave
Of course, what may really be bothering Norris is less the irreducible
probabilism of quantum phenomena than, on the Copenhagen interpretation,
their observer-dependence. That and the epistemic conception of truth on
which observer-dependence was originally advanced.
These qualities bothered Einstein and other physicists as well. Led by
Einstein and Bohr, the two sides engaged in an extended debate. In the
beginning, this debate relied mostly on thought experiments. The most
famous of these concerns what has come to be called 'Schrodinger's cat'.
Erwin Schrodinger was the one who developed the mathematical wave
function that accurately allows us to predict the probabilities of the
values associated with quantum properties. Schrodinger later developed a
mystical bent, advancing notions not unlike those we are now hearing
from Bhaskar.
At the time, however, Schrodinger was solidly on the side of Einstein.
His cat paradox was meant to reduce the Copenhagen interpretation to
absurdity. In this thought experiment, a cat is placed in a box along
with a cyanide pill. If the cyanide pill is released, the resulting gas
will kill the cat. The cyanide pill, however, is triggered only by a
certain value of a quantum property. According to the Copenhagen
interpretation, the quantum property will not actually have this or any
value until it is observed. Before observation, there will only be a
distribution of probabilities associated with each value.
What Schrodinger wanted to know is whether, prior to observation, the
cat is alive or dead. Certainly, Schrodinger assumed, the cat cannot be
both, and if it cannot be both, then there is something wrong with the
Copenhagen interpretation.
Evidently no one was prepared for the peevishness of the Bohr side. The
first answer was that prior to observation the cat is indeed both alive
and dead in some kind of superimposed states of probability. A more
considered response was to reflect on what counts as an observation.
Einstein reputedly professed disbelief in the ability of a mouse or even
a cat to alter the course of the universe (Wolf 1988). Perhaps, however,
such humble creatures are sufficient to collapse the probability wave
into one or another determinate state of reality. If so, then even
without human observation, the cat is either alive or dead but not both.
The lingering question then is what exactly it takes to collapse a
quantum wave. Where, when, and how does this take place? As these
questions still go unanswered, Norris cites them as weaknesses of the
Copenhagen interpretation.
I agree with Norris that these outstanding questions render the
Copenhagen interpretation somehow incomplete. The questions, however,
might well be answered within the confines of the Copenhagen
interpretation. Thus, not even these unanswered questions show the
Copenhagen interpretation to be fundamentally wrong. As would happen
more than once, from Schrodinger's thought experiment, the Copenhagen
school escaped from what had seemed a manifest absurdity.
Quantum connectedness - the nonlocality of reality
The Einstein-Rosen-Podolsky (EPR) experiment was another thought
experiment designed to show a fatal flaw in the Copenhagen
interpretation. It was an ingenious experiment to which Norris devotes
considerable space. Remember that the Heisenberg uncertainty principle
establishes a minimal uncertainty that always remains whenever we
attempt direct measurement of any two conjugate properties of an
elementary particle. Suppose, however, we measure the properties
indirectly without disturbance?
How might this be done? Suppose two elementary particles, A and B,
collide and go off in different directions. The classical conservation
laws tell us that their properties should be inversely related. Suppose
we precisely measure one property g of particle A. That precludes us
from precisely measuring the conjugate property d of A. However, because
we know that the value of g for B must be the inverse of g for A, we are
also able to identify precisely g for B. Similarly, we can precisely
measure d for B and, from that, infer precisely the value of d for A.
In this way, it seems, we can precisely identify the values of both of
two conjugate properties for two particles without the problem of
measurement disturbance. After all, our two measurements of the distant
particles A and B could disturb each other now only by some mystical
action at a distance. Moreover, since we can wait to make our
measurements until after A and B are light years apart, any action at a
distance would further have to travel at warp speed. Star Trek
notwithstanding, however, warp speed violates Einstein's theory of
relativity, which stipulates that nothing can travel faster than light.
If the conclusion of the EPR experiment is right, then it demonstrates
against the Copenhagen interpretation that the conjugate properties of
the two particles must have precise values even before they are
observed. Once again, Einstein's side seemed to expose a logical flaw in
the Copenhagen interpretation.
Once again, however, Bohr just denied that any of the properties have
specific values until they are measured. It is not enough, the Bohr side
claimed, to avoid measurement disturbance in each of the two particles
taken alone. Instead, the two particles together - along with the
measurement devices - now constitute a unified whole with its own
composite probability wave. Thus, any disturbance anywhere in the system
will still necessarily reverberate throughout the whole.
Bohr's response still seems to possess a lingering problem. The effects
of any such disturbance must still travel, and nothing can travel faster
than light. Thus, if the measurements of A and B are taken far enough
apart, the effect of the disturbances will not have had time to arrive.
Only if distant realities are nonlocally connected in some weird way can
the Copenhagen interpretation escape the paradox.
It turns out that distant realities are nonlocally connected in some
weird way. One begins to appreciate Fritz Capra's allusion to the 'Tao
of physics' and the argument that the entire universe is one,
interconnected whole.
The demonstration of quantum connectedness begins with Bell's theorem.
John Stewart Bell was a theoretical physicist at Cern who in the 1960s
proved mathematically that if the empirical findings of quantum
mechanics are correct, then reality cannot be local. Instead, there will
always be more correlation between distant particles than can be
accommodated by any assumption of locality.
In contrast with positivists, critical realists do not generally uphold
mathematical proof as a scientific standard in empirical matters. And
indeed Bell's theorem proves reality is nonlocal only if the predictions
- as opposed to the theory - of quantum mechanics are correct. Yet
despite myriad tests, the predictions associated with quantum mechanics
have never been falsified. Instead, quantum mechanics has gone from one
predictive success to another. Quantum facts are among the best
confirmed of any scientific theory we have, and they serve to explain
many puzzles in other domains as well. If social constructionists in the
sociology of science really want to make their case, then they must try
their hand against quantum mechanics. It is not likely they will
succeed.
Explaining nonlocality
One such success has been experimental confirmation of Bell's theorem.
The correlations Bell said would be indicative of nonlocality have now
been empirically observed. Thus, not even Norris disputes the
nonlocality of reality. What Norris continues to dispute is the
Copenhagen interpretation of it. Viewing the Copenhagen interpretation
as dispensing altogether with rock bottom reality, Norris' argument is
that physicists should not just accept the Copenhagen interpretation as
it is but continue searching for more underlying or 'hidden' variables
that can make better sense of quantum findings.
In particular, Norris champions, if not the specific theory itself, then
at least the direction pursued by David Bohm. Originally aligned with
Bohr, Bohm was persuaded by Einstein in the 1950s that an 'ordinary
reality interpretation' of quantum reality is possible. Along with
really existing probability waves, Bohm's model includes also at every
moment really existing particles with definite, observer-independent
property values. According to Bohm, it is only the interactions between
a particle, wave, and measurement device that make quantum properties
appear observer-dependent (Herbert 1985: 48-50).
Norris makes the point that the hegemony of the Copenhagen
interpretation has been such as to give Bohm's account short shrift.
Bohm's account concedes that reality is nonlocal, which has sometimes
been counted against it. Yet, even if we have to surrender locality,
Norris asks, is it not still better to preserve something of reality?
Norris asks a fair question. And some of the alternatives make one
shudder. For example, Norris devotes an entire chapter to David Deutsch.
Deutsch is the most famous of the contemporary defenders of the 'many
worlds' interpretation of quantum mechanics. This interpretation solves
paradoxes like Schrodinger's cat by fracturing the world. With the
uncertainty of each quantum event, reality literally splits. Thus, in
one reality the cat is alive, and in another it is dead. We continue not
to know which world we are in until we look, but the paradox at least is
gone. Copies of ourselves simultaneously exist in parallel universes.
Every time a choice is to be made, reality further divides. Fairly soon,
parallel realities form a tree with infinite branches. In some
realities, we choose one way and in other realities another. In this
way, according to Deutsch, does the many worlds' interpretation also
solve the philosophical problem of free will.
Norris finds Deutsch's many worlds appalling and so do I. As Norris
argues, the theory is too wildly extravagant ontologically for the
problem it is meant to solve. To me, moreover, Deutsch's treatment of
free will entirely destroys responsible choice. Suppose we find
ourselves tempted by a rewarding but immoral choice. If nothing will
prevent the proliferation of untold realities in which we choose
immorally, why not let this be one of them? Let the more principled
copies of ourselves split off to their own doleful reality.
Quantum duality
Yet, as much as I share Norris' repugnance for the many worlds
interpretation, I am less inclined to dismiss it. Norris does not
discuss certain features of quantum reality needed to get the full feel
of the quantum weirdness that motivates both the Copenhagen and the many
worlds interpretations. I refer specifically to the famous one and two
slit experiments that led Bohr to develop the principle of
complementarity.
In a typical arrangement of the one slit experiment, electrons are shot
through a hole to show up on a photographic plate. This is not much
different from what happens in a television set, where a stream of
electrons shows up as a spot.
Suppose, however, we narrow the slit through which the electrons must
pass. Beyond a certain point, the slit size too precisely identifies the
electrons' lateral momentum. According to the Heisenberg uncertainty
principle, position has to give.
Position's give shows up on the photographic plate. The electrons no
longer land in a single point. Instead, they are now distributed in
neat, concentric circles like a bull's eye. This is the interference
pattern we would expect if what passed through the hole were not
particles but waves.
Well, which is it? Are electrons particles or waves? They seem to travel
like waves but to impact like particles. The situation gets murkier. We
may think that the interference pattern reflects multiple electrons
passing through the hole, interfering with each other in the process.
Yet, the same pattern emerges when what passes through the hole is only
one electron at a time. Somehow, even a single electron presents a wave
interference pattern.
Still, each electron is only hitting the plate at a single spot. So how
do the separate electrons know how to land on the plate in such a way
that collectively the hits are patterned like interfering waves?
The Copenhagen interpretation answers in two steps. First, it invokes
the principle of complementarity. Elementary particles behave like waves
when they are not observed and like particles only when they are
measured. According to the Copenhagen interpretation, we cannot do
without these two, paradoxical but complementary, ways of understanding
what is going on. The second step of the answer is that the wave itself
is a wave of probabilities. Although the electrons only hit in one spot
or another, they combine to form the interference pattern we see because
they hit according to invariant probabilities associated with the
travelling wave.
The two slit version of the experiment is even spookier, for then we
have to assume that, before they hit their target, individual electrons
somehow go through each of two distinct holes simultaneously. They can
do this easily enough if they are waves, but once again they impact the
target as point particles. Again, it appears as if elementary particles
exist as probability waves when we are not looking but as definite
particles when we are (Herbert 1985).
Seas of unrealised potentia
I said before that the discourse on quantum mechanics is often as
ambiguous as the phenomena themselves. If I am less antagonistic than
Norris to the Copenhagen interpretation, it is because, to me at least,
the Copenhagen interpretation actually encompasses several subordinate
interpretations that need to be prised apart.
On one interpretation, that most often assailed by Norris, the
Copenhagen interpretation is equivalent to instrumentalism. The
instrumentalist interpretation is content just to describe and predict
quantum behaviour mathematically. It is a positivist approach to science
that rejects any search for underlying reality. I agree with Norris that
the proponents of the Copenhagen interpretation often present this as
their view. I share Norris' dissatisfaction with it.
I would also join Norris in rejecting an epistemic conception of truth
that equates reality with our knowledge of it. Norris convincingly shows
that the proponents - and even the antagonists - of the Copenhagen
interpretation often appeal to such an epistemic understanding of truth.
Again, I have no problem sharing Norris' rejection of this
interpretation.
I see, however, yet another interpretation associated with the
Copenhagen school. This is the interpretation according to which rock
bottom reality just consists of probability waves or distributions.
This interpretation itself divides into two subordinate interpretations.
Although the proponents of the Copenhagen view slip from one to the
other, I would countenance only one.
The distinction between the two has to do with the ontological status of
probability. Probability might be considered entirely subjective or
epistemic. As such, probability is a function only of our knowledge and
not of reality itself. As our knowledge increases, what was initially
only a probability becomes more certain. If we regard the probability
waves of quantum phenomena in this epistemic way, then the Copenhagen
interpretation again reduces to a form of instrumentalism, and I reject
it on realist principles.
In contrast, however, it is also possible to construe at least some
probabilities ontologically. The probabilistic nature of such phenomena
then is not just a reflection of our limited knowledge. It is an
irreducible quality of the reality in question. That reality just is
irreducibly probabilistic.
At least sometimes, the proponents of the Copenhagen interpretation
speak as if this were their view. Although Heisenberg also often speaks
as if the probabilities associated with quantum phenomena are only
epistemic (Herbert 1985: 171-172), at other times he speaks of the
unmeasured quantum world as a sea of unrealized potentia, waiting for a
trigger to actualize them according to invariant probabilities (Herbert
1985: 194-195). Seas of potentia may be an odd and nondeterministic kind
of reality, but they are still a reality.
The dialogue must go on
I certainly do not know if the Copenhagen view is correct even on this,
final interpretation. I do not think, however, that belief in potentia
can fairly be described as irrealist. Not unless we equate realism with
determinism. I agree with Norris that on this view, our understanding of
quantum mechanics cannot be complete. Among other things, it still needs
an account of what counts, and why, as the kind of observation that
causes a probability wave to collapse into a point actuality. Those
sorts of completion, however, are not the same as removing the indelible
uncertainty that lies at the heart of the Copenhagen interpretation.
Whatever my disagreements with Norris, QTFR is not just a stimulating
book but an important one. As we who subscribe to critical realism do
not yet have physicists among us, Norris has courageously performed a
real service by beginning investigation in this area. Those critical
realists who can follow Norris' path definitely should do so. The
dialogue Norris initiates must continue.
Notes
1. Potentia is Latin for power, ability, possibility. - Ed.
References
Alston, William 1996. A Realist Conception of Truth. Ithica: Cornell
University Press.
Bhaskar, Roy 1975. A Realist Theory of Science. Leeds: Leeds Books.
Herbert, Nick 1987. Quantum Reality: Beyond the New Physics. Garden
City: Anchor.
Wolf, Fred Alan 1988. Parallel Universes: The Search for Other Worlds.
New York: Touchstone.
*****
--
Mervyn Hartwig
Editor, Journal of Critical Realism (incorporating 'Alethia'),
International Association for Critical Realism
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