If instead of water waves, you use light waves and capture – say on a photographic plate – the pattern emerging from the two sources and moving to the right, you get something like the picture on the left. A picture similar to this led Thomas Young, the British physician/physicist to conclude in 1803 that light was a wave. Then came Einstein in 1905 and showed that light is composed of particles, photons. This complicated things, because our experience with particles is completely different. After all, if you set up two holes and send bullets through them and capture the bullets on a wooden screen, there would be only two regions of high concentration of bullets … nothing even remotely resembling the interference pattern shown on the left!
So, how do photons behave? Do they create an interference pattern as they do when you send a lot of them at once in the form of a wave, or do they form an image consisting of two blobs as bullets do? The answer resides in the probabilistic nature of the quantum theory. The location on which a single photon lands on the photographic plate is completely random, but the probability of landing on that location is predicted by quantum theory. (For example, the theory predicts that there are certain regions on the photographic plate at which the probability is zero. Thus no photon will ever land there.) This is similar to a coin toss where we know that the probability is 50% for head ans 50% for tail, but we can’t predict which way the coin goes. What happens is that, although you can’t predict the location of each individual photon on the photographic plate, if you wait long enough, so that a large number of photons is collected by the photographic plate, the interference pattern – which is the pattern of probability predicted by the quantum theory – emerges. Again this similar to the outcome of coin tossing: If you toss a large number of coins, you’ll get almost half head and half tails.