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Posted: 02/22/2013 5:00 pm
*Former NASA researcher;
computational scientist; emeritus professor of mathematics, James
Madison University; author, 'Reason and Wonder'*

In three previous posts we've discussed assumptions that science once
embraced, later to discard as invalid or unnecessary. Thus far we've
dispatched with absolute time and space (

Part 2), determinism (

Part 3), and dualism (

Part 4). Today we examine

*the principle of local causes*.

*****
Einstein, who deposed Newton, grew intellectually stodgy in old age.
With respect to quantum mechanics (QM), he was positively reactionary. A
die-hard determinist, Einstein rejected the statistical implications of
quantum theory. In a letter to his friend and fellow physicist Max
Born, Einstein confided: "Quantum mechanics is very impressive. But an
inner voice tells me that it is not yet the real McCoy. The theory
produces a good deal but hardly brings us closer to the secret of the
Old One."

By 1935, Einstein believed he had found the Achilles heel of QM. With
two coworkers, he pitched a heroic attempt to expose its fatal flaw by
posing what became known as the "EPR paradox."

Strictly speaking, paradoxes are unresolved; not so for EPR. Einstein lost, and QM won. However, in 1935, when

**E**instein, Boris

**P**odolsky, and Nathan

**R**osen posed their fiendishly clever

*gedanken*
experiment, a betting person might have put odds on Einstein's team. QM
must be incomplete, Einstein felt instinctively. The probabilistic
appearances of quantum events -- such as the spontaneous decay of a
radioactive element, for example -- were mirages, he surmised, the
result not of any propensity of the natural world toward statistics but
rather of our incomplete knowledge of nature. Einstein hypothesized the
existence of "hidden variables" of which the current theory remained
ignorant. If the hidden variables were exposed to the light of day, he
argued, the apparent statistical predilection of nature would evaporate.

*****
As the originator of the theory of relativity, Einstein held the
principle of local causes to be inviolate. According to special
relativity, the velocity of light

*in vacuo*, denoted

*c*,
plays the role of a universal speed limit, faster than which travel is
forbidden by physical impossibility. Nothing -- neither matter nor
information -- should be able to rove from point A to point B faster
than a beam of light can make the transit.

To illustrate, suppose we humans unwisely break the atmospheric
test-ban treaty, detonating a nuclear weapon. Aliens presumably will be
unaware of the ominous event until photons from the flash on earth reach
their distant planet. Most physicists, Einstein included, regarded as
absurd the thought that an event at one point in space could

*instantaneously* effect an outcome in a distant region.

QM, Einstein inferred, violated the locality principle. Sporting the
title "Can Quantum-Mechanical Descriptions of Physical Reality Be
Considered Complete?," the EPR paper considered simple, two-particle
quantum systems. Quanta -- whether electrons, positrons, or photons --
involve a property called spin. It is a helpful analogy to think of such
particles as spinning about an axis like a top. Spin, a form of angular
momentum, is defined by an axis of rotation and an angular velocity.
For subatomic particles, spin is quantized: spin magnitudes must be
whole or half multiples of Planck's constant

*h*. For photons of
light, spin is synonymous with polarization. Photons carry spin
magnitudes of plus or minus 1. No other values are permitted by nature.

Now consider the simplest aggregate of quantum particles: a
two-particle system consisting of one spin-up (+1) photon and one
spin-down (−1) photon. The net spin momentum is thereby zero, the sum of
the individual spins. By conservation of angular momentum, the net spin
of the system must remain zero for all time.

And now things get interesting. As a quantum of angular momentum,
spin is subject to Heisenberg's uncertainty principle, with two enormous
implications. First, until measurement, a particle's spin exists only
as a probability, not as a reality. Second, the act of measuring spin

*determines* its actual value.

Now to Einstein's punch line. Consider the creation of a two-particle
system of net spin zero, comprised of two photons, each traveling
horizontally at velocity

*c *in opposite directions. Suppose
further that the two photons are now separated by an enormous expanse of
space. Suppose finally that we measure the spin of one photon, which
fixes its spin. According to quantum theory, the other must
simultaneously register a spin value exactly opposite that of its twin
in order to preserve angular momentum. But how does the distant twin
know

*instantly* what spin value to assume?

Einstein believed that he held QM by the soft parts. He closed by springing the trap:

"One can escape from this conclusion [that quantum theory is
incomplete] only by either assuming the measurement of [particle 1
telepathically] changes the real situation at [particle 2] or by denying
independent real situations as such to things which are spatially
separated from each other. Both alternatives appear to me entirely
unacceptable."

Einstein euphemistically termed such mysterious (and presumably
impossible) "telepathic" or "superluminal" communication by the colorful
phrase "spooky action at a distance." And so the EPR paradox rested for
30 years, a full-fledged, unresolved, perplexing conundrum about the
nature of reality.

*****
Then, in 1964, John Stewart Bell, an Irish physicist associated with
CERN (the Euroean Center for Nuclear Research), proved a theorem
directly related to EPR. Bell's theorem is extraordinary for several
reasons. Foremost, it's a

*theorem*, not a

*theory*.
Theorems, which are mathematical, rest on the solid foundation of formal
logic and carry 100 percent certitude. In particular, Bell's theorem
established a mathematical inequality by which to experimentally test
whether the principle of local causes is valid or the statistical
predictions of QM are valid. However, there had to be a winner because
the two outcomes are mutually exclusive.

In the decade following the publication of Bell's theorem, numerous
physicists reformulated and sharpened the argument. By the early 1970s,
experimentalists had posed a version involving photon polarization that
could be tested in the laboratory. In 1972, Stuart Freedman and John
Clauser of Lawrence Laboratory performed the long-awaited experiment to
resolve the EPR paradox. By generalizing Bell's inequality, they
directly tested Einstein's assertion that hidden variables could
preserve local causes. The results were unequivocal: "Our data, in
agreement with quantum mechanics, ... provid[e] strong evidence against
local hidden-variable theories." Spooky action at a distance prevailed.

*****
The modern term for spooky action is quantum entanglement. The term
suggests that quantum particles entangled "at birth" remained entangled
forever despite the intervening distance. One is reminded of the psychic
connections widely reported by human twins. Once connected, always
connected, it seems.

In 1975, shortly after the first experimental test of Bell's theorem,
a prescient physicist, Henry Stapp, went out on a limb to write in a
governmental report: "Bell's theorem is the most profound discovery of
science."

At the time, it is doubtful that many concurred. Today, Stapp's words
ring true. Quantum entanglement, now well established, is quickly
making its way into a variety of applications, from cryptography to
quantum computing. Bell's theorem may be the Cullinan diamond of
physics' many gems. By revealing quantum entanglement, Bell's theorem
exposes subtle and mysterious interconnections that may lie outside the
universe's spacetime fabric.

Had Einstein been buried rather than cremated, he would be turning in his grave.

*(This essay was adapted from Chapter 11 of *Reason and Wonder.

*)*
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