Text: Quantum mechanics is generally regarded as the physical theory which is our best candidate yet for a universal and fundamental description of the physical world. The conceptual framework employed by this theory differs drastically from that of classical physics. Indeed, the transition from classical to quantum physics marks a genuine revolution in our understanding of the physical world. One striking aspect of the difference between classical and quantum physics is that whereas classical mechanics presupposes that one can assign exact simultaneous values to the position and momentum of a particle, quantum mechanics denies this possibility. Instead, according to quantum mechanics, the more precisely the position of a particle is given, the less precisely one can say what its momentum is. This is (a simplistic and preliminary formulation of) the quantum mechanical uncertainty principle. This principle played an important role in many discussions on the philosophical implications of quantum mechanics and on the consistency of the interpretation endorsed by the founding fathers Heisenberg and Bohr, the so-called Copenhagen interpretation. This, of course, should not suggest that the uncertainty principle is the only aspect in which classical and quantum physics differ conceptually. In particular the implications of quantum mechanics for notions such as (non)-locality, entanglement and identity play no less havoc with classical intuitions. 1. The Heisenberg uncertainty principle, formulated by the German scientist Werner Heisenberg, states that in the world of subatomic particles, the very act of observing alters the reality being observed, and therefore, in that world of subatomic particles, one can never measure all properties exactly. 2. The ³uncertainty² in the uncertainty principle cannot be done away with by better observation techniques; rather, it is part of the nature of reality itself. 3. The uncertainty principle does not apply to the world of ordinary objects, since in that world, the effect of observation on the reality observed is so small as to be negligible. 1. In 1927, the young German physicist Werner Heisenberg was working at the Danish physicist Niels Bohr¹s research institute in Copenhagen, Denmark. The two scientists worked together on theoretical investigations into quantum theory and the nature of physics. When Bohr was away on vacation, Heisenberg had an insight into the limits of physical knowledge: The act of observing alters the reality being observed. 2. Heisenberg's reasoning went like this: To measure the properties (position and momentum) of a particle such as an electron, one must use light, or radiation, as a measuring device. But the energy in the radiation affects the electron being observed. If you adjust the light beam to accurately measure position, the energy of the light beam will change the momentum of the electron; if you adjust the light beam to measure the momentum of the electron, the energy of the light beam will move the electron, throwing off its position. 3. To simplify further, explain that to determine the position of an electron, we shine light on it. The light strikes the electron and causes it to move a very tiny amount. This means that the ³image² of the electron (if we could see it) is slightly blurred, like a photograph in which a person moved while the picture was being taken. We can define probable locations for the electron but cannot locate it exactly. 4. Clarify one more concept before moving on: Uncertainty is not due to any fault on the part of the observer; rather, it is part of the nature of reality. 5. Let students know that the Heisenberg uncertainty principle applies only to the subatomic world, not to the ordinary world of macroscopic objects. It is only in the quantum mechanical world that in order to make a measurement, one must disturb the system. In other words, in order for us to know something is there, we must bump into it; in order to locate an electron, the electron must encounter a photon. In the ordinary world, this does not hold true. For complete discussion on UC see html page in Science Research Folder

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