What is The double slit experiment?

Originally devised to determine if light travels as waves or particles. Feynman was quoted as saying:

Any other situation in quantum mechanics, it turns out, can always be explained by saying, "You remember the case of the experiment with the two holes? it's the same thing"

The double slit experiment

If light travels as particles we can imagine particles of light (photons) as bullets fired from a rifle. Imagine a brick wall with two holes in it, each the same size and large enough to fire bullets through, with a second wall behind where the bullets will strike. After firing a few rounds you would expect to see on the second wall two clusters of hits in line with the two holes. This is of course precisely what you get with bullets, so if we get the same result with photons we can say they are particles.

Now imagine that instead of particles, that light travels as a wave, we can replicate that with a water tank. As the wave spreads out from its source it would reach both holes at the same time and each hole would then act as a new source. Waves would then spread out again from each of the holes, exactly in step, or in phase, and as the waves moved forward, spreading as they go, they would eventually interfere with one another. Where both waves are lifting the water surface upward, we get a more pronounced crest; where one wave is trying to create a crest and the other is trying to create a trough the two cancel out and the water level is undisturbed. The effects are called constructive and destructive interference.

If we carried out this procedure with light instead of water, and if light travels as waves, then the pattern on the second wall would appear as an interference pattern of alternate dark and light bands across the wall. Particles, on the other hand, would produce two separate areas of light (where the bullets would hit). This experiment has in fact been carried out many, many times, with the same results every time, and the results are nothing less than amazing.

When the experiment is set up with both slits open, the resulting interference pattern clearly shows that light behaves as a wave. Now if that was all there was to it we could all fold up our tents and go home happy in the knowledge that light travels as a wave; but there is much more to it than that. This is where the word 'weird' can become over-used.

If the experiment is set up to fire individual photons, so that only one photon at a time goes through the set up, we would not expect the same interference pattern to build up; we would surely expect that a single photon would only go through one hole or another, it cannot go through both at the same time and create an interference pattern. So what happens?

If we wait until enough individual photons have passed through to build up a pattern - and this takes millions of photons - we do not get two clusters opposite the two holes, we get the same interference pattern! It is as if each individual photon 'knows' that both holes are open and gives that result. Each individual photon, passing through the set up will place itself on the wall in such a position that when enough have passed through they have collectively built up an interference pattern, when there cannot possibly be any interference!

If we repeat the experiment, this time with only one hole open, the individual photons behave themselves and all cluster round a point on the detector screen behind the open hole, just as you would expect. However, as soon as the second hole is opened they again immediately start to form an interference pattern. An individual photon passing through one of the holes is not only aware of the other hole, but also aware of whether or not it is open!

We could try peeking, to see which hole the photon goes through, and to see if it goes through both holes at once, or if half a photon goes through each hole. When the experiment is carried out, and detectors are placed at the holes to record the passage of electrons through each of the holes, the result is even more bizarre. Imagine an arrangement that records which hole a photon goes through but lets it pass on its way to the detector screen. Now the photons behave like normal, self respecting everyday particles. We always see a photon at one hole or the other, never both at once, and now the pattern that builds up on the detector screen is exactly equivalent to the pattern for bullets, with no trace of interference. As if that was not bad enough, it gets even worse! We do not need place detectors at both holes; we can get the same result by watching just one hole. If a photon passes through a hole that does not have a detector, it not only knows if the other hole is open or not, it knows if the other hole is being observed! If there is no detector at the other hole as well as the one it is passing through, it will produce an interference pattern; otherwise it will act as a particle. When we are watching the holes we can't catch out the photon going through both at once, it will only go through one. When we are not watching it will go through both at the same time! There is no clearer example of the interaction of the observer with the experiment. When we try to look at the spread-out photon wave, it collapses into a definite particle, but when we are not looking it keeps its options open.

What the double slit experiment demonstrates is this: Each photon starts out as a single photon - a particle - and arrives at the detector as a particle, but appears to have gone through both holes at once, interfered with itself, and worked out just where to place itself on the detector to make its own small contribution to the overall interference pattern. This behaviour raises a number of significant problems! Does the photon go through both holes at the same time? How does a photon go through both holes at the same time? How does it know where to place itself on the detector to form part of the overall pattern? Why don't all the photons follow the same path and end up in the same place?

As a possible explanation it could perhaps be said that this is just one more example of the extraordinary nature of light, after all it does have some very unusual properties. Photons have no rest mass for example, a very odd property! Light is also unique in that it always travels at the same speed. However you move, and however the light source moves, when you measure the speed of light you always come up with the same answer. By way of comparison, two cars approaching each other and each having a speed of 30 mph will be approaching each other at a speed of 60 mph. Two light beams, both travelling of course at the speed of light, will be approaching each other at the speed of light, not twice the speed of light. Perhaps the weird behaviour of photons in the experiment is due to the weird nature of light. Unfortunately further experiments have demonstrated that this is not the case. Electrons have been used instead of photons, and they not only have mass, they have an electric charge, and furthermore they move at different speeds depending on circumstances, like normal everyday objects. The double slit experiments still gives the same result using electrons as it does using photons; electrons also alter their behaviour depending on whether or not they are being observed. The experiment has even been performed using atoms, again with the same result, and atoms are large enough to be individually photographed, they are very real solid objects. This odd behaviour of particles is a very real phenomenon.

The double slit experiment is not simply an oddball theory that has no application in the real world. This strange behaviour of particles lies at the very heart of our understanding of the physical properties of the world. Quantum theory is used in many applications, including television and computers, and even explains the nuclear processes taking place inside stars.

One possible explanation for quantum weirdness is a theory concerning the nature of the wave that is passing through the experiment. The key concept of the theory, which forms a central part of the Copenhagen Interpretation, is known as the 'collapse of the wave function'. The theory seeks to explain how an entity such as a photon or an electron, could 'travel as a wave but arrive as a particle'. According to the theory, what is passing through the experiment is not a material wave at all, but is a 'probability wave'. In other words, the particle does not have a definite location, but has a probability of being here or there, or somewhere else entirely. Some locations will be more probable than others, such as the light areas in the interference pattern for example, and some will be less probable, such as in the dark areas. In this theory, an electron that is not being observed does not exist as a particle at all, but has a wave-like property covering the areas of probability where it could be found. Once the electron is observed, the wave function collapses and the electron becomes a particle. This theory rather neatly explains the behaviour of the particles in the double slit experiment. When we are not looking at the particle, the probability wave, of even a single particle, is spread out and will pass through both slits at the same time and arrive at the detector as a wave showing an interference pattern. When we observe the electron by placing detectors at the slits, it is forced into revealing its location which causes the probability wave to collapse into a particle. If the theory is correct, its implications are staggering. What it suggests is that nothing is real until it has been observed!

Nothing is real until it has been observed! This clearly needs thinking about. Are we really saying that in the 'real' world - outside of the laboratory - that until a thing has been observed it doesn't exist? This is precisely what the Copenhagen Interpretation is telling us about reality. This has caused some very well respected cosmologists (Stephen Hawking for one) to worry that this implies that there must actually be something 'outside' the universe to look at the universe as a whole and collapse its overall wave function. John Wheeler puts forward an argument that it is only the presence of conscious observers, in the form of ourselves, that has collapsed the wave function and made the universe exist. If we take this to be true, then the universe only exists because we are looking at it. As this is heading into very deep water I think we will have to leave it there and move on to the next experiment.

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