We begin with light, which has for centuries been at the forefront of much thinking about the nature of the universe.
What is light? Answers, or parts of answers, have been given by the
following thinkers, among others: Newton, Huygens, Maxwell, Einstein,
Schrodinger. In the 17th century, Newton and Huygens proposed
radically different ideas about the nature of light. Newton argued
that it consists of tiny particles that speed about; but Huygens' view
was that light is a kind of wave, not unlike a water wave. This
latter view was the accepted one for over two centuries, culminating
in Maxwell's astonishing mathematical demonstration in the 19th century
(i) that electricity and magnetism are two aspects of the same thing:
``electromagnetic waves'' - this built on earlier results of others
such as Faraday - and (ii) that the speed of such waves is
precisely the speed of light, providing dramatic evidence that light
too is an electromagnetic wave. It seemed conclusive in favor of
Huygens, and indeed some physicists at the time stated that physics was
pretty nearly over as an active science since all major questions were
settled: Newton had explained motion and gravity (
etc) and
Maxwell had explained electricity, magnetism, and light.
But it was not to be. The early 20th century found glaring gaps in these explanations. What had seemed like minor difficulties turned out to be major ones, and wholly new conceptual frameworks were in store.
Some of the problems can easily be illustrated: