Understanding the nature of the very small is extremely difficult. Few things in the world are more familiar or basic than light. It is somewhat surprising, therefore, that physicists are still struggling to understand the fundamental nature of such a familiar phenomenon.
Isaac Newton (1642-1727) visualised the basic nature of light as a stream of particles. Charles Huygens (1629-1695) developed a wave theory of light but the significance of his work was dimmed by the shadow of Newton's enormous reputation. The wave theory eventually moved centre-stage when the brilliant experiments of Thomas Young (1773-1829) demonstrated the wave-like properties of light.
In Young's experiment, light is directed at a board which contains two small slits. A screen on the far side of the board displays the light which traverses the slits. The light forms a pattern of alternating dark and bright stripes on the screen. This pattern, called an interference pattern, can only be explained by treating light as a wave motion.
Light moves from the light source as a circular wave front (just as waves move outwards in a circle when a pebble is dropped in water). When the light hits the board, it can only pass through the two pin-holes, each of which now transmits a circular wave front of light beyond the board.
Those two spreading wave fronts overlap and interfere with each other before reaching the screen. The bright stripes on the screen represent places where wave crests from one slit meet wave crests from the other and reinforce each other.
The dark stripes represent places where wave crests from one slit meet wave troughs from the other and cancel each other out. By measuring the spacing between the dark and the bright stripes, one can calculate the wavelength of light.
For almost 200 years, Young's experiment was cited as proof that light is a wave. Then, in 1905, Albert Einstein proposed that it was sometimes useful to consider that light acts as a stream of particles (called photons or quanta).
The electron, a sub-atomic particle was discovered in 1897 by JJ Thompson. Not only is it unambiguously a particle but it also carries a negative electric charge. In 1925, Louis de Broglie published a complementary version of Einstein's proposal, claiming that very small bodies like electrons can also act as waves.
In 1925, quantum mechanics, the physics which describes the behaviour of the very small (atoms, sub-atomic particles, photons), was invented. It formally proposed that all atomic and sub-atomic objects have a dual wave/particle nature. Quantum mechanics describes the behaviour of the sub-atomic world very accurately. However, it has proved extremely difficult to understand the basic nature of things, as opposed to describing their behaviour.
Young's experiment can also be carried out using electrons and identical results are obtained to those described using ordinary light. This result is completely counter-intuitive. Electrons are definitely particulate; how can they also behave as waves? If you knocked two holes in a wall and hurled thousands of balls at the wall you would end up with two distinct piles of balls on the other side, not a series of lines of balls.
The electrons must travel as waves in Young's experiment in order to produce the interference pattern, but they leave the electron source as particles and arrive at the screen as particles (you can see the individual dots as they arrive at the phosphorescent screen).
OK, you may say, put a lot of electrons together and they somehow jostle each other into a wave pattern. But, amazingly, if you fire the electrons through the experiment one at a time you still get the same interference pattern.
Things can get stranger still; if you observe the electrons on their flight through the experiment they react in a very odd manner - the interference pattern disappears and you get a distribution of light on the screen equivalent to the distribution of balls thrown at a wall containing two holes. Even stranger, the same thing happens even if you don't watch the electrons, but if you introduce the potential for watching them.
Electrons have a magnetic tag associated with them. Place a device at the left slit which flips the magnetic tags of all the electrons passing through to the "up" direction and place a device at the right slit which flips the magnetic tags of all the electrons passing through to the "down" position. You now have the potential to identify the slit through which every electron which arrives at the screen passed on its journey. This in itself makes the interference pattern disappear, whether or not you avail of the potential to identify the slits.
NOW go one step further. Interpose a device between the slits and the screen to remove the magnetic tags which would identify the slits. The interference pattern reappears. If you incompletely erase the tags the sharpness of the interference pattern is degraded to an extent proportional to the extent of removal of the tagging.
It seems that the basic nature of sub-atomic particles and of light is both (not either/or) wave and particle. The expression of "waviness" or "particulateness" depends on the circumstances. At one extreme, only particulate properties are expressed, at the other only wave properties are expressed, and in between both wave and particle properties are displayed.
The difficulty in visualising the dual particle/wave nature of the very small stems from the fact that we normally don't encounter this duality in the everyday physical world. However, if we think more generally we can easily identify cases where a characteristic can also express itself in the opposite manifestation. For example, one must sometimes be cruel to be kind.
If you find it difficult to understand what I have been flailing at explaining, don't worry. I don't really understand this stuff myself. But we are in good company - that of Albert Einstein, who said: "Fifty years of conscious brooding have brought me no nearer to the answer to the question - what are light quanta? Nowadays every Tom, Dick and Harry thinks he knows it, but he is mistaken."
William Reville is a senior lecturer in biochemistry at UCC.