![]() On one side we could write "delta-E" to represent our uncertainty in the value of the energy of a particle, and on the other side of the balloon write "delta-t" which would stands for our uncertainty in the time the particle had that energy. ![]() One can illustrate the basic results of the uncertainty principle with a not-quite-filled balloon. So we have, in quantum physics, what are called "complimentary pairs." (If you'd really like to impress your friends, you can also call them "non-commuting observables.") This relativistic version states that as one gets to know the energy of an elementary particle very well, one cannot at the same time know (i.e., measure) very accurately at what time it actually had that energy. Another way of stating this principle, using relativity in the formulation, turns out to be that one gets another version of the uncertainty principle. The Heisenberg Uncertainty Principle basically stated that if one starts to know the change in the momentum of an elementary particle very well (that is usually, what the change in a particle's velocity is) then one begins to lose knowledge of the change in the position of the particle, that is, where the particle is actually located. We can picture a baseball thrown at us at 100 miles per hour having a similar effect as a bat being thrown at us at ten miles per hour they would both have about the same momentum although they have quite different masses. It is classically defined as the mass of a particle multiplied by its velocity. Momentum is a fundamental concept in physics. ![]() This classical approach is that if one looks at an elementary particle using light to see it, the very act of hitting the particle with light (even just one photon) should knock it out of the way so that one can no longer tell where the particle actually is located - just that it is no longer where it was. However, the uncertainty principle is a fundamental property of quantum physics initially discovered through somewhat classical reasoning - a classically based logic that is still used by many physics teachers to explain the uncertainty principle today. So the term "uncertainty principle" may strike us as something akin to the terms "jumbo shrimp" or "guest host" in the sense of juxtaposing opposites. ![]() In scientific circles we are perhaps used to thinking of the word "principle" as "order", "certainty", or "a law of the universe". In this article we shall examine another feature of quantum physics that places fundamental constraints on what can actually be measured, a basic property first discovered by Werner Heisenberg, the simplest form known as the "Heisenberg Uncertainty Principle." In the first article, we discussed the double-slit experiment and how a quantum particle of light (a photon) can be thought of as a wave of probability until it is actually detected. ![]()
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