232 ERWIN SCHRÖDINGER
wall and escape (an effect now
known as quantum tunneling). The
probability predictions of the wave
equation matched the unpredictable
nature of the radioactive decay.
Uncertainty principle
The great debate that shaped the
development of quantum physics
during the middle years of the 20th
century (and remains essentially
unresolved today) surrounded what
the wave function actually meant
for reality. In an echo of the Planck/
Einstein debate two decades
previously, de Broglie saw his and
Schrödinger’s equations as mere
mathematical tools for describing
movement: for de Broglie, the
electron was still essentially a
particle—just one that had a wave
property governing its motion and
location. For Schrödinger, however,
the wave equation was far more
fundamental—it described the
way in which the properties of the
electron were physically “smeared
out” across space. Opposition to
Schrödinger’s approach inspired
Werner Heisenberg to develop
another of the century’s great
ideas—the uncertainty principle
(pp.234–35). This was a realization
that the wave function meant that a
particle can never be “localized” to a
point in space and at the same time
have a defined wavelength. The
more accurately a particle’s position
was pinned down, for example,
the harder its momentum was to
measure. Thus, particles defined by
a quantum wave function existed
in a general state of uncertainty.The road to Copenhagen
Measuring the properties of a
quantum system always revealed
the particle to be in one location,
rather than in its wavelike smear.
On the scale of classical physics
and everyday life, most situations
involved definite measurements
and definite outcomes, rather than
myriad overlapping possibilities.
The challenge of reconciling
quantum uncertainty with reality
is called the measurement problem,
and various approaches to it
have been put forward, known
as interpretations.
The most famous of these is
the Copenhagen interpretation,
devised by Niels Bohr and Werner
Heisenberg in 1927. This states
simply that it is the very interaction
between the quantum system and
a large-scale, external observer orapparatus (subject to the classical
laws of physics) that causes the
wave function to “collapse” and
a definite outcome to arise. This
interpretation is perhaps the most
widely (though not universally)
accepted, and appears to be
borne out by experiments such
as electron diffraction and the
double-slit experiment for light
waves. It is possible to devise
an experiment that reveals the
wavelike aspects of light or
electrons, but impossible to record
the properties of individual
particles in the same apparatus.
However, while the Copenhagen
interpretation seems reasonable
when dealing with small-scale
systems such as particles, its
implication that nothing is
determined until it is measured
troubled many physicists. Einstein
famously commented that “God
does not throw dice,” while
Schrödinger devised a thought
experiment to illustrate what he
viewed as a ridiculous situation.God knows I am no friend
of probability theory, I have
hated it from the first moment
when our dear friend
Max Born gave it birth.
Erwin SchrödingerDane Niels Bohr (left) collaborated
with Werner Heisenberg, to formulate
the Copenhagen interpretation of
Schrödinger’s wave function.