Quantum Mechanics: Determinism or Not?
What we now see developing is a schism among the luminaries of early 20th
century physicists. They were divided into two philosophical camps: those who
supported the probabilistic Copenhagen Interpretation (Bohr, Heisenberg and
others not yet mentioned) and those who supported a deterministic model of
reality (Einstein, Schrödinger, Born and others). It might be surprising to note
that Einstein - the man who turned classical Newtonian physics on its head -
should be so vehemently opposed to this interpretation of Quantum Mechanics,
especially since it appeared to be an inevitable conclusion arising out of his
own theories. But, in my view, Einstein passionately believed that the universe
is structured in a beautiful and logical manner, however strange that logic may
appear to traditional science. As such, nothing (in the physical realm, at
least) was ultimately beyond human comprehension. The Copenhagen Interpretation,
on the other hand, introduced messy and irrational randomness. This was just
intolerable for Einstein and, it seems, for a majority of scientists, engineers
and technicians to this day. Nevertheless, the Copenhagen Interpretation
remains, among quantum physicists, the current orthodoxy even though it has
several challengers.
The crucial difference between the two standpoints was: determinism or not?
Determinism states that every event has a cause, going back to the beginning of
time, i.e. the Big Bang (or whatever the current theory of the beginning happens
to be). Another way of stating it (theoretically) is to say that, if we could
know all of the pre-existing conditions, we could accurately predict everything
that will happen in the future. This, of course, has far reaching implications,
not the least for the concept of human free will and accountability. For
example, if someone goes out one morning and shoots his neighbour, can he be
held ultimately responsible for his action if it was inevitable from the moment
the universe came into being? On the other hand, quantum mechanics introduces
probability at the most fundamental level. Another big philosophical question
now begs: does quantum level probability allow for choice? If, by deciding to
observe an electron, I collapse the probability wave and determine its position
or momentum, does that signify that the universe does indeed allow for my free
will?
Lest anyone be in any doubt about the quantum description of the electron
vis-à-vis its position and/or momentum, we are not talking about a limitation of
the measuring equipment to determine either or both, we are talking about the
electron having no position nor momentum until we make the measurement of one or
the other. This is crucial to the debate above.
Einstein was not about to lie down and quietly concede the point to Bohr and his
Copenhagen confederates. In 1935 he, Boris Podolsky and Nathan Rosen invented a
thought (gedanken) experiment designed to expose the incompleteness of QM. This
became known as the EPR paradox. The measurement problem as described above is
one of the bizarre outcomes of QM but it leads into another, even more bizarre,
consequence and this is what Einstein, Podolsky and Rosen latched on to in
describing the paradox.
The EPR Paradox
Two particles can exist in a state referred to as "
entangled" - that is, they
behave as one physical system. The EPR argument centred around entanglement
together with a particular measurement. In addition to position and momentum,
another property of a particle is that known as its "spin". Using convenient
terms, we could say that the spin is either "up" or "down". So, with the
entangled pair, if we measure the spin of one particle to be "up", we can say
with absolute certainty that the spin of the other will be "down". The paradox
is this: if we allow the pair to fly apart - even light years apart - the
measurement of the spin of one particle should instantly yield certain knowledge
of the spin of the other. As neither particle (according to QM) had a definite
spin direction until the first measurement was taken, how would the remote
particle receive the information determining which spin direction it should
display. Einstein's Special Relativity specifically rules out faster-than-light
travel so how could information travel across light years in an instant?
Einstein called this "spooky action at a distance" and thought it demonstrated
that QM violated causality (a.k.a. determinism), thus rendering it inconsistent.
In physics this spooky action at a distance is called "nonlocality". Einstein
maintained that something must be missing from QM - probably some form of hidden
variable - that would account for the spookiness.
For many years following the EPR paper, nobody tested the argument
experimentally. In 1964, John Stewart Bell - a young physicist from Northern
Ireland - produced a theorem that rejects all models of reality based on
locality. The proof (Bell's theorem) states in order to assume locality, any
model (including the hidden variable variety) must satisfy a mathematical
inequality, known today as Bell's Inequality. In 1982 actual experiments carried
out by Alain Aspect and his team (and others since) appear to prove that Bell's
inequality is violated and left little doubt that nonlocality is a fact of
nature - at least at the sub-microscopic level. Just so that we are sure about
what nonlocality means, let's use a big world analogy: two men are given flags
and told that if one raises his flag, the other must drop his. They are sent a
short distance apart and we ask first man to raise his flag; immediately the
second man drops his. Ok, we say, the second man must have reacted when he saw
the first flag go up. So we send the first man to New York and the other to
Tokyo. We film and accurately time the proceedings and ask the first man to
raise his flag. Immediately the second man drops his. How? Maybe the second man
had a radio and received a signal? But no, we can even rule that out because his
flag dropped before the time it would take for a radio signal - travelling at
the speed of light, of course - could reach him. It is as if the space between
the two men did not exist and the second man knows instantly what the first is
doing. This is a very simplistic analogy, of course, and it must be pointed out
that there is no present evidence of "big world" nonlocality.
