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Perhaps you are fascicled from the particle wave duality or from Heisenberg's uncertainty relation, but this what you will learn here will make the other things less important for you. The experiment begins very simple. You need a light source, a wall with two holes and a screen. On side of the wall there is the light source and on the other side there is the screen. When light passes the wall we can see an interference sample on the screen. The maxima are not behind the holes on the screen, but there is one maximum between the two holes on the screen, otherwise it would not be an interference sample. On the right and on the left of this maximum there are dark areas and then again bright areas, but these bright areas are not as bright as the maximum in the middle. Then we have got two dark areas again and so on. This result should not wonder us, because this are waves and because some waves have got a longer way from the light source to the screen than other waves some waves strengthen each other and other waves extinguish each other. When two wave combs clash then they strengthen each other and when a wave comb and a wave valley clash then they extinguish each other. When one hole is closed the maximum is behind the opened hole. Now we will replace the light source through an electron source and we will make the experiment again. This time we get the same interference sample when both holes are opened. This proofs the wave character of the electrons. But it is important that light or electrons cannot be a wave and a particle at the same time. Now it becomes interesting, we do not let many electrons throught the wall, but only one after the other. When one electron passes the wall it cannot handycap himself and because it can only go throught one of the holes it would be logical that the maxima are behind the two holes. But when we wait until many electrons have passed the wall we saw an interference sample again. When we repeat this experiment and we close one hole the maximum is behind the open hole. It seams that electron knows wheather both holes are opened or only one. When we try to measure throught which hole an electron goes we get two maxima behind the two holes. So it is wrong to say that the electron goes throught one of these two holes, because we can say that it goes throught both holes or we can also say that it goes throught not hole, both answers are correct. The consequence is that nothing is real until an observer saw it. We do not know a reciprocal action between the electron, the observer and the instruments, but there must exist a reciprocal action. An electron has got many possibilities and because of our oberservation the electron must choose one of the possible ways. So when it goes throught one of the holes it is logical that the maxima are behind the holes. It is called collapse of the wave function and every particle has got a probability wave. This means that never can be sure where a particle is, we can only say where the most probable place is. A human being has got also a probability wave which we can find in the whole universe, but her strongest point is there where we are. But there is everytime a very little probability that you can find yourself on Mars for example or somewhere else, but this probability is so little that you need not be afraid. When we know throught measurements where this person is then his wave function collapses, because we know his exact position. As long as we observe something it is real and when we do not observe it it is not real any longer. There is another illustration which is called Schrodinger's cat. It is a thought experiment. We give a cat into a box with a radioactive material and a bottle of poison. Because we never know when an atom decays a radioactive material is very good for this experiment. The probability is very improtant for quantum physics. The box must be closed. When an atom of the radioactive material decays the bottle will be broken and the cat will die. But as long as we do not look after the cat if she is alive or dead, then we can say that she is alive and dead or not alive and not dead, both answers are correct. But in this situation we could not never say that she is alive or dead. I hope that these both experiments could tell you something about nature. So when you believe all this, which is not total sure until today, your conception of the world has changed I think.
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