Waves And Particles 2

Paige The Oracle made a very interesting posting earlier. I have responded to this post but I would like to take yet another section from the fisrt version of ITLAD. This is to do with wave-particle duality. It is an analogy I thought up that may get across the totally counter-intuitive nature of light.

- Light is shone through a single hole in a barrier. As the light flows out it encounters a second barrier, this time with two holes. The light acts like a wave in that each hole then starts its own wave pattern the other side of the second barrier. Immediately the two waves start to interfere with each other. A screen is set up after the second barrier. When the light hits this screen it shows a pattern of light and dark stripes. These stripes are called interference fringes. They correspond to where the light waves add together (constructive interference) and where the waves cancel each other out (destructive interference).

In this way Thomas Young in 1800 proved that light functions as a wave. However in 1905 Einstein proposed that light was also made up of individual particles. He even gave them a name; photons. As far as he was concerned it was the only way that a particular puzzle for early 20th century science could be explained. When light was shone on a solid object it seemed to ‘kick out’ electrons from the surface. This phenomenon, known as the ‘photoelectric effect’, could not be explained. However, as Einstein said in his 1905 paper, if light is made of particles then each particle hits the surface and knocks out the electrons. This supposition was supported by experimentation and in recognition of this discovery Einstein was awarded the Nobel Prize for physics. One problem solved, many opened. This dual nature of light (wave and particle) was to cause a radical review of our perception of electro-magnetic radiation. If light was also made up of discrete packets how can this be squared with the fact that light is also a wave? We need to re-visit the Young Experiment.

We need to understand what would, or should, happen if photons were sent through the two slits. As an analogy imagine that the two holes were holes in a barn door, the holes being about three times the size of a soccer ball. Now we kick balls against the door. After a few dozen balls are kicked through the holes we stop the exercise. We will then find two piles of balls on the other side of the barn door. We would expect to find the pattern (two piles of footballs) to be exactly the same if, instead of both holes being open at the same time, each hole was open on its own for half the time and then the other hole for the other half. What you would not expect would be a group of balls centred half way between the two holes, right behind the solid door. Footballs, like particles, do not interfere with one another.

Now imagine a single photon being fired at the barrier. The photon, in order to get to the other side, has to go through one or other of the two holes. Recording such a small particle of light needs a super sensitive photographic plate and this is set up at the other side of the barrier.

Each photon, as it arrives, registers on the photographic plate as a single white spot. As thousands, then millions of photons arrive at the plate a pattern begins to emerge. Common sense would lead you to assume that there would be two circles of white light coinciding with the trajectory of each photon through whichever hole it selected, just like our piles of footballs in our barn door analogy. However we are dealing with the quantum world where common sense does not exist. In fact what we do get is the interference pattern again. Now think about this; each particle goes through one hole on its own, however something seems to interfere with it as it goes through to form the unexpected interference pattern. This leaves us with one of only two options, both of which are peculiar in the extreme. The first option is that the photon starts out as a particle, and arrives as a particle, but en route it seems to go through both holes. In doing so it interferes with itself as it comes through the holes, and then works out exactly where to place itself on the photographic plate to ensure that ultimately, and with all its fellow protons, a perfect pattern of light and dark stripes is to be found. The mystery of this first option is how does the photon manage to go through both holes at the same time, and having done that how does it ‘know’ where to place itself on the photographic plate?