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Heisenberg’s Uncertainty Principle I previously examined some of the ides in quantum mechanics and how they affect our concept of reality. Here I want to examine another aspect of this which is known as "Heisenberg’s Uncertainty Principle". Heisenberg’s Uncertainty Principle 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, and visa-versa. The figure shows a snooker table. Above it we have mounted a camera. We hit a ball and it travels in 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 trade-off. The more clearly we can determine the position the less clearly we can determine the speed and visa-versa. The problem is that position is determined at a fixed moment in time and speed and direction are calculated over a period of time. This is an analogy for Heisenberg’s Uncertainty Principle. 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 path 1 it probably did but it may have taken a different route. The question is ‘does it matter’. Going back to the double-slit experiment, we can see that whether we observe the particle at a certain position determines whether we see the interference pattern or not. 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 behavior do not apply. In fact we could say that in going from point A to point B that 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. As an illustration, Schrodinger 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. If the particle goes through path 1 it will hit the plate and the detector will release the gas from the cylinder. If the particle takes any other route the plate will not be hit. A more realistic example would be put a radioactive element in the box with the cat. The element is such that in any12 hour period it has a 50-50 chance of sending off a particle that will trigger the Geiger counter. So we set this up and leave it for 12 hours. When we come back is the cat alive or dead? Well, according to Heisenberg’s Uncertainty Principle, not only do we not know until we look, but also the cat is neither alive nor dead until we look. The reality is not ‘sealed’ (the wave is not collapsed, to use the technical term) until it is established by a conscious mind. There are two themes that I will return to later. One is the relationship between the ‘real’ world and the conscious mind, and the other is the concept of probability.