Where are we now, having covered four-and-a-quarter centuries of physics? At every point, what was the key observation or experiment? What was the problem associated with that? How did it get solved?

In 1585, Stevin performed his leaden ball experiment in Delft. The associated problems were the pronouncements inherited from Aristotle on falling objects. Stevin, a hero in the vanguard of ‘experimental philosophy’, was not impressed (or, in any case, not deterred) by this ancient stuff.

The solution of the problems surrounding falling objects was reached tactically by ‘separation of difficulties’. On one front, Galileo performed his lengthy studies of ‘natural’ motion by means of clever observations and experiments on objects falling freely or rolling along inclined planes. That approach was concluded by Huygens's radical statement on the relativity of motion. From this he derived the rules for collisions and the first mathematical equations in theoretical physics: the centrifugal acceleration, the oscillation period of the pendulum, fall along curved paths such as the famous cycloid, and more.

The other front focused on the origin and description of the acceleration in free fall. Newton rehabilitated the shaky concept of ‘force’ by casting it in the strict mathematical form of ‘universal gravity’ and devised a brilliant formalism for calculating the results of accelerations (a technique we now call differential and integral calculus, invented independently by Leibniz).

The success of this approach was phenomenal, but it left all questions about the origin of forces and the structure of matter unanswered. Matter cannot be stable if it is subject to gravity only, so other ingredients were needed. Chemistry provided some clues through the discovery of chemical elements and the fact that these combine in whole-number ratio. H2O is a chemical compound, where H1.8O would be just a mixture. Research into electromagnetism also offered some hope of understanding the inner forces of matter.

Maxwell's discovery that electromagnetic waves all propagate with the speed of light led to the 1887 Michelson-Morley experiment. This landmark observation showed that the speed of light does not depend on the motion of the emitter or the receiver of the light: the speed of light is invariant.