The essential part, the real novelty in quantum mechanics, I describe by talking to you about a phenomenon called Interference. Assume you have an original state and a final state, and you can get from the original state to the final state in two different ways. For instance, the original state is a particle - an electron - here, the final state is an electron there. In between, I have a screen which stops the particle except that it has two holes in it and the particle can go from the original state to the final state through one of the holes or through the other of the holes. In the interaction of the particle with the screen, allows the particle to move not quite on a straight line. Now, the very peculiar thing in quantum mechanics, the phenomenon called interference, is this: that there are situations where the fact that an electron can go that way or that way will lead to a high probability of the electron arriving. Or, under other conditions, it can lead to the more peculiar result that the fact that it can go this way or the fact that it can go that way, will make it certain that it never arrives here. That the possibilities how the thing can move can cancel each other. That is called interference. And the part of the quantum mechanics which explains the connection between wave descrip- description and particle description is precisely this study of Interference Phenomenon. Now, if I do anything, let an alpha particle be emitted or not be emitted, and I do anything with it I like, I can calculate the consequences. With one special remark; if you make a measurement which defines the position precisely, this measurement does not require an observer. Indeed, it is entirely independent. Something might happen a billion years ago and I can find in the geological remains that this or that has happened, no observer. What happens in a measurement is what is called - excuse me for introducing a new concept, which I will explain - something must happen which is called an Irreversible Process. And I will show you an irreversible process right here. Here is this hopefully empty cup and I drop it. Now, what happens in physics forward can also happen backward. The equations are so constructed that everything that happens one way can happen also the opposite way. So therefore, having stopped this- dropped this cup, I stand here with my hand open and wait for the cup to rise again, not as I do it, lifting it, but of its own accord, redoing the whole thing and landing in my hand. You all know that if that can happen at all you have to have a lot of patience, a patience greatly exceeding the age of the universe which is about as good as saying it never can happen. And the interference phenomenon, a peculiar thing in quantum mechanics, will show up its consequences in whatever else I do with this object. Because from the end state, I can reconstruct the initial state, except if there is an irreversible process. A measurement is not defined by Eugene Wigner knowing about it, or anybody else, it is defined by an irreversible process which does not allow the original state to be reconstructed from the final state. It is in this sense that Heisenberg should be understood. And he's talking of it- about an observer. It's simply justified as a didactic device, as a device to explain things, so people understand more easily what an observer is than to say what an irreversible process is.