Thursday, May 27, 2010

Foundations 4: Quantum Mechanics - Greg Egan

he 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

n 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 — such as the force between two motionless electric charges — 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).

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 Newtonian physics was soon to be superseded by special relativity, but the most glaring problems had nothing to do with the motion of objects at high velocities, so the explanation had to lie in another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."

5 out of 5

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