Problems with light - one part of the whole spectrum of radiation - dominated physics at the beginning of this century. Wrangling over the amount of radiation that could be emitted or absorbed by a given object, Max Planck (1858-1947) realised that the only way to make the sums work out was to state that energy could only exist in 'packets', which he called 'quanta', rather than in a 'stream'. All very well, but Maxwell's beautiful equations described light waves. Now Planck was staring at the improbable fact that light exists as quanta, discrete packets. This 'wave-particle duality' has caused generations of physics undergraduates to wish they'd taken up vocational sheep guidance instead.

One way of demonstrating this unnerving truth involves a beam of light, a barrier with two slits and a light-sensitive screen behind the barrier. When a stream of light is beamed towards the barrier, it travels through the slits making waves that 'interfere' with one another causing bands of lighter and darker areas on the screen. This is what you would see if you dropped two rocks into a pond. Waves propagating outwards make two series of rings, when they meet the rings interfere with each other.

So far so good. The odd thing happens when you turn the 'tap' of the light beam right down so that it is emitting one photon (particle of light) at a time. Intuition says that a random pattern of dots should appear on the screen. Instead, the same light and dark bands build up slowly. The photons appear to behave in concert, as if they 'know’ where each one in front and behind is landing.

Quantum theory has been described as the most profound advance in the history of human thought. Einstein - a founding father - said he spent hundreds more hours thinking about quantum theory than he had about General Relativity but with less success. His theory of gravity called General Relativity describes the actions of large bodies and becomes relevant at high velocities. Quantum theory describes the world of the very small. It becomes relevant for objects of atomic size or smaller.

Apart from the odd aberrant like Democritus of Abdera nobody took the atomic theory of matter seriously until the nineteenth century. Boys will be boys, and it didn't take more than a few decades at the turn of the century before fully grown men were smashing atoms with an even smaller particle called the electron - carrier of electric charge. By 1913 the atom was known to have internal structure. The nucleus, consisting of protons and neutrons, sits in the centre surrounded by a 'cloud' of electrons. The scale of this is not to be contemplated before breakfast. Imagine an electron cloud having roughly the perimeter of Heathrow airport; the nucleus on this scale would be no bigger than a golf ball in the middle of the airport.

In one moment I've seen what has hitherto been
Enveloped in absolute mystery,
And without extra charge I will give you at large
A lesson in Natural History
(From The Hunting of the Snark, Lewis Carroll)

To describe matter at these minute distance scales, an entirely new kind of theory was needed. Starting with Planck in 1900, a succession of achievements by Einstein, Niels Bohr (1885-1962), Werner Heisenberg (1901-76), Louis de Broglie (1875-1960), Erwin Schrodinger (1887-1961) and others led in 1926 to the development of quantum theory - a revolutionary account of the microscopic laws of physics that overturned our most cherished notions of reality. Like 'certainty’ for example. There is an assumption that using powerful magnification, you can determine where a minute object is and what its speed is. Imagine the dismay within the scientific community when Heisenberg discussed his Uncertainty Principle. The principle states that you cannot know precisely both the position and the momentum of an elementary particle at the same time. Worse than that; such a particle does not have both properties (position and momentum) at the same time. The more accurately you measure the position of an object, the less accurately you know its velocity. If this seems crazy to you, take comfort. Niels Bohr is alleged to have said that anyone who is not shocked by quantum theory has not understood it. Needless to say, such a radical change is only of importance for minute objects - the size of molecules or smaller.

The poet only asks to get his head into the heavens. It is the logician who seeks to get the heavens into his head. And it is his head that splits.
(G K Chesterton)