Quantum theory is the stuff of dreams and of nightmares. More precisely, physicists dream about using it to find the secrets of the universe and have nightmares about actually having to apply it to the world around them. But what exactly is this earth-shattering theory?

Many people have heard about Schrodinger's cat and Heisenberg's Uncertainty Principle, but the ideas that go into these seemingly revolutionary ways of looking at things are both very simple and very complicated. This is, of course, one of the hallmarks of quantum theory and even of modern physics. Something that looks simple from one place looks incredibly complex from somewhere else, but more on that later...

Basic Concepts

As with all good scientific theories, quantum theory consists of seemingly ordinary observations that totally revolutionise the way we look at the world around us. At the beginning of the twentieth century, physicists thought that they were very close to knowing for certain how the world worked, as most of the physics which had dogged theorists for so long (electricity and magnetism, optics, mechanics) appeared to be almost, if not completely, solved. How wrong they were...

Physicists at this time believed that the universe could be described by a finite set of rules which governed everything from the birth of the stars to how a ball falls under gravity (actually, these two processes are remarkably similar - but that's another story!). It was believed that light travelled in continuous waves, like ripples on a pond when a stone is thrown in. However, we now know that light travels as both a wave and a particle. This concept is one of the most fundamental in quantum physics and gives rise to the name of the theory as it is believed that each light particle, or quanta, has a certain discrete amount of energy.

Wave-Particle Duality

Young's Slit Experiment

Evidence for the wave-particle duality of light comes from several sources, the most notable of which are the photoelectric effect, of which more later, and Thomas Young's slit experiments. What Young did was to do what every physics student must do at one time or another and mess about with sending light waves through a piece of material which had been cunningly prepared beforehand by the simple expedient of making two slits in it, thus producing two light sources. What he got was a nice diffraction pattern on a screen in front of the card. The diffraction pattern is composed of brighter and darker areas due to the light waves interfering with each other, which 'proves' that light is a wave.

The Photoelectric Effect

The photoelectric effect requires a simple explanation of its puzzling attibutes. When light strikes a surface it transfers energy to the electrons in the material which, if you've got your apparatus set up correctly, will cause a current to flow. From this the number of electrons liberated by the additional energy provided by the light can be calculated. It can be inferred that if light is a wave then the amount of electrons leaving the material should rise in proportion with the intensity of the light, i.e. how powerful the light souce is. But wait! Researchers into the effect quickly found that no matter how intense the light source, the number of electrons liberated was exactly the same for a given material. "So what?", you might say. Well, this result was very important as it baffled physicists for years, and it might be argued (using a particularly perverse logic) that any result which baffles physicists for years has to be one worth a few paragraphs here and there. And the physicists may very well have stayed baffled had it not been for the timely intervention of Albert Einstein.

Einstein's solution to the photoelectric effect earned him his Nobel prize (that's right, he didn't get it for relativity!) and was the beginning of quantum theory. Basically, all he said was, "What if light consists of particles with a certain amount of energy?", only not quite like that, and in German. But in that simple phrase was the solution - the reason being that it made perfect sense. Remember that electrons are (for the moment!) single particles and so there are only a certain number of them that can receive energy at any one time. Therefore since each light particle contains a certain amount of energy, and only one can hit an electron at once, it doesn't matter if you're using a torch or a floodlight - the same number of electrons will be liberated from the material.

So light behaves in one instance like a wave, and in another like a particle. This is not an intuitive result - how many people sit staring at light bulbs contemplating the nature of the stuff currently blinding them? It is, as we shall see, a very important one.

Electrons and Matter Waves

J. J. Thomson discovered the electron at the end of the 19th century and showed that they were a fundamental constituent of matter. Since light seems to have wave and particle properties it seems natural to assume that matter does too, and certainly Louis de Broglie seemed to think so (he submitted the idea in his doctoral thesis). And the only thing known to prove that something is a wave is the aforementioned diffraction pattern.

C.J. Davisson and L. H. Germer accidentally discovered this as they were working on electron scattering. They found that electrons too produced a diffraction pattern almost identical to that of light. From this and other results, de Broglie's ideas were proved to be correct and the world of quantum physics was a step closer to reality.

Back to the Slits

It seems we have now come full circle - we are back to Young's double slit experiment. Experiments have been done using just one electron at a time to prove the 'matter wave' theory de Broglie so enthusiastcally postulated. And the results? Even when just one electron is let through the slits at one time, a detection pattern that looks uncannily like the diffraction pattern for a ray of light is discovered after a few electrons have gone through one after the other (obviously the results improve the more electrons go through). This has also been shown when one photon of light is allowed through the slits at a time.

It appears then that the light and electrons leave their sources as a particle, interact with the slits as a wave and end up again as a particle. What is wrong with this in quantum theory? Nothing! This was the first inkling physicists had that not only was the world as yet unsolved, it was also unsolvable.