The world at the quantum level, that is at very small scales and very short time intervals, is very different from the kind of world we are used to. The quantum effects only affect very small particles and time instances. In our physical world what we call the laws of nature are an averaging of quantum effects. One illustration of the peculiar nature of the quantum world is found in what is called the ‘double-slit experiment’.

The double-slit experiment

In the double-slit experiment a beam of light is passed from a projector through a card with two adjacent slits and onto a screen.

If light was made up as a wave we would see a striped appearance due to what is called the interference effect.

This is because the light waves that pass through one slit interfere with light passing through the other slit. In this case, light waves are similar to water waves in that they are made up of a movement that goes up and down. If two waves mix together they ‘interfere’ with each other. If the waves are ‘in phase’ (that is, the two waves are moving up and down at the same time) the movement is added together to produce one giant wave. If the two waves are ‘out of phase’ (that is, when one wave is reaching a peak the other is reaching a trough) they cancel each other out and the result is no wave at all.

In the case of the slit experiment, the light travels a slightly different distance from each slit and depending where it hits the screen determines whether the wave is in phase or out of phase at that point, so we get bands of darker and lighter areas.

This would appear to indicate that light is definitely made up as a wave. If we were to consider light as made up of particles then we would get an effect similar to firing marbles through the slits. The marbles can’t interfere with anything in the way that waves do, so there should be two lines where the particles (or marbles) have passed through the slits and hit the wall.

However, light consists of photons, a photon is a unit of light and a beam of light is a stream of photons. So the double-slit experiment can be repeated with a small variation. Instead of sending a continuous beam of light through each slit we send one photon at a time. There should be no interference pattern as the photon has nothing to interfere with. It should be like firing the marbles. However, when this experiment was performed the scientists did observe an interference pattern.

In a refinement of the experiment a detector was set up to determine which of the two slits the photon passed through. If the photon passed through the left slit it couldn’t interfere with anything in the right slit and vice-versa. In this case when the experiment was repeated there was no interference pattern. It’s as if the act of observing the experiment changed the result. When they didn’t observe which slit the photon was going through it acted like a wave. When they did observe it, it acted like a particle.

One way that scientists have attempted to understand this is to consider that the wave is a probability wave. It simply shows all the possible ways that the particle could get from the emitter to the wall. If we see it as a wave, we see all the possibilities. If we see it as a particle we have ‘collapsed the wave’, that is we have taken one of the possible routes and that is now fixed.

As a metaphor, imagine that we have to make a decision and we decide to toss a coin. Heads we do one thing, tails we do another. So we toss the coin but instead of looking at the result we catch the coin and keep it covered. Whilst it is covered both possibilities are open to us but as soon as we look we have ‘sealed’ the result. The decision is made. The difference between the coin toss and a particle is that the particle is ruled by laws of quantum physics. Even before we’ve seen the state of the coin we know it is either heads or tails. In the case of the particle it is neither one nor the other until we collapse the wave.

Heisenberg’s Uncertainty Principle

To understand this better we need to understand a theory proposed by Werner Heisenberg which is usually referred to as Heisenberg’s Uncertainty Principle. This says, in short, that the position and speed (or direction) of a particle cannot both be predicted with certainty. The greater the position is established, the more unknown is the speed or direction, and visa-versa.

Imagine a snooker table. Above it we have mounted a camera. We hit a ball and it travels in a particular direction and at a particular speed. As it travels it bounces off the sides of the table. We take a snapshot of the ball at a fast shutter speed, say 1–5000th of a second. We take another snapshot at a very low shutter speed, say 5 seconds. In the first picture we see a very clear image of where the ball is, but we don’t know in what direction or how fast it is traveling. In the second picture we get a very fuzzy picture of where the ball is but we can see clearly in what direction it is traveling and we can calculate how fast it travels. There is a tradeoff. The more clearly we can determine the position (by using a faster shutter speed) the less clearly we can determine the speed and direction, and visa-versa. The problem arises because position is determined at a fixed point in time and speed and direction are calculated over a period of time. This is a similitude for Heisenberg’s Uncertainty Principle.

We could consider that when we take the picture with the very fast shutter speed we see a particle and when we see it with the slow shutter speed we see a wave. The more we see it as a particle, the less we see it as a wave.

We now take two snapshots of the moving ball, one at 1 second after hitting the ball and the other 2 seconds later. We know for certain the ball was at position A at the time of the first picture and was at position B at the time of the second picture. Did the ball go through position C between A and B? The quick answer is, we don’t know. Not for certain anyway. We know that if the ball traveled over the path shown it probably did but it may have taken a different route. Going back to the double-slit experiment, we can see that whether we observe the particle at a certain position, that is going through one of the slits, determines whether we see the interference pattern or not. If we observe it then there is no interference pattern, if we don’t there is.

When observing moving balls we can make deductions based on our understanding of how balls behave but when we are looking at small particles these basic laws of behaviour do not apply. We could say that in going from point A to point B the particle traveled over every possible path between the two points. There are probable and less probable paths but can only say whether it passed though point C with a certain probability.

Erwin Schrödinger, a contemporary of Heisenberg, objected to this idea and posed the following thought experiment:

A sealed box has a cat inside it with a container of poisonous gas (this is just a thought experiment, no real cat has ever been harmed doing this). This is shown in the figure. A radioactive substance emits particles. If a particle takes one path it will trigger a lever and a hammer will break a container which will release a gas from a cylinder. If the particle takes any other path the lever will not be triggered.

A different example would be to put a radioactive element in the box with the cat. The element is such that in any twelve-hour period it has a fifty-fifty chance of sending off a particle that will trigger the Geiger counter. So we set this up and leave it for twelve hours. When we come back is the cat alive or dead?

This is an extension of the double slit experiment. In our example we could say that if the particle goes through the left slit the cat is alive and if it goes through the right slit the cat is dead. Schrödinger’s argument was that surely the cat knows whether it is alive or dead?

I’ll look at this and the relationship with mind in another article.

By Philip Braham on August 1, 2018

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