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Modern Physics and Syadvada 103
scale show a dual aspect, a particle aspect and a wave aspect. The two aspects which are totally contradictory in every day experience are complementary at the level of atoms. Why so ? because nature is so constituted that experiments which demonstrate the particle aspect and those which demonstrate the wave aspect are mutually incompatible. We can have. only the one set-up or the other, and never the two can be combined or built together into some super-apparatus to demonstrate both the aspects at the same time. We ask : What is it that makes these experiments mutually incompatible ? It arises from the far reaching, and totally unexpected, fact that an act of observation, even an ideal observation supposed to be made with 'perfect' instruments is inevitably accompanied by certain minimum disturbance. The disturbance cannot be eliminated, cannot be analysed or allowed for. It is inherent in the nature of things. It disturbs in an unpredictable way, the state of the system under observation. We cannot even think of an experiment-a thought experiment, as it is called, that can be made free of the concomitant minimum uncertainty. The effect of this inevitable disturbance is altogether negligible for a big object, but for an atomic object the effect is drastic. It drastically modified the state of the system under investigation. (This is technically called the 'reduction of the wave packet'). It is because of this disturbance, an integral feature of an act of observation, that an experiment to study the wave aspect of an atomic system is incompatible with a set-up to study the particle aspect.
We spoke of the wave-particle duality. Consider the usual arrangement for obtaining interference fringes. For the light beam each photon must pass through both the holes (at the same time) to produce interference fringes. This is observed on the plate P. Suppose we wish to find out how a photon can simultaneoulsy go through the two holes. How can this happen ? For this purpose, we determine the momentum of the plate P in the Y-direction. The plate had to be kept rigidly fixed to observe the fringes. But to observe the momentum of the plate, it must be completely free to move in the Ydirection. Further, if we are to be able to decide whether the photon came from the direction of the hole A or from the hole B, the uncertainty in the momentum in the Y-direction of the plate should be small compared to hyo/c.
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