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Foundations
by Greg Egan

4: Quantum Mechanics
Copyright ┬й Greg Egan, 1999. All rights reserved.



The first three articles in this series dealt with special and general relativity, the two great
twentieth-century theories of the geometry of spacetime and its relationship with matter
and energy. This article will describe the ideas behind a second, simultaneous revolution
in physics, one that has had even more profound philosophical and technological
consequences: quantum mechanics.


The Birth of Quantum Mechanics

In the second half of the nineteenth century, the Newtonian description of the dynamics
of material objects was supplemented by an equally successful theory encompassing all
of electrostatics, magnetism and optics. The physicist James Clerk Maxwell brought
together a number of disparate laws that had been found to govern quite specific
phenomena тАФ such as the force between two motionless electric charges тАФ into a unified
description of an electromagnetic field. Light, and most other forms of radiation,
were seen to consist of oscillations in this field, or electromagnetic waves. This
confirmation of the wave-like nature of light made sense of many long-standing
observations, including the phenomenon of interference: if you allow light of a single
wavelength to travel through two adjacent narrow slits in a barrier and then recombine on
a screen, it produces patterns of dark and light stripes. Since the difference in the time it
takes for light waves from the two slits to reach the screen varies from place to place, the
waves shift in and out of phase with each other, resulting in varying degrees of
constructive interference (where the contributions to the field from both slits point in the
same direction), and destructive interference (where they point in opposite directions).
 Egan: "Foundations 4"/p.2




Newtonian dynamics and Maxwellian electrodynamics cut a wide swath through
the scientific problems of the day. However, by the end of the nineteenth century a
number of serious discrepancies had been found between experimental results and
predictions based on these two theories. Newtonian physics was soon to be superseded
by special relativity, but the most glaring problems had nothing to do with the motion of
objects at high velocities, so the explanation had to lie in another direction entirely.
One of the biggest puzzles involved the spectrum of radiation emitted by hot
objects: thermal radiation. This is visible to the naked eye when, for example, the
tungsten wire in a light bulb becomes white hot. There's an idealised class of objects for
which this effect is particularly easy to analyse: if an object is a perfect absorber and
emitter of electromagnetic waves across the entire spectrum, its thermal radiation should
depend solely on its temperature, rather than any idiosyncratic properties of the stuff from
which it's made. Physicists call this a black body, since it should appear black to the