![]() Either way, your observation of either position or momentum will be inaccurate and, more important, the act of observation affects the particle being observed. Or else, given that quantum particles often move so fast, the electron may no longer be in the place it was when the photon originally bounced off it. But chances are that the photon will impart some momentum to the electron as it hits it and change the path of the particle you are trying to measure. You might similarly bounce a photon off it and then hope to detect that photon with an instrument. Seeing a subatomic particle, such as an electron, is not so simple. Each photon on that path carries with it some information about the surface it has bounced from, at the speed of light. You can read these words because particles of light, photons, have bounced off the screen or paper and reached your eyes. One way to think about the uncertainty principle is as an extension of how we see and measure things in the everyday world. Planck's constant is an important number in quantum theory, a way to measure the granularity of the world at its smallest scales and it has the value 6.626 x 10 -34 joule seconds. This is equal to Planck's constant (usually written as h) divided by 2π. Multiplying together the errors in the measurements of these values (the errors are represented by the triangle symbol in front of each property, the Greek letter "delta") has to give a number greater than or equal to half of a constant called "h-bar". The more accurately we know one of these values, the less accurately we know the other. The uncertainty principle says that we cannot measure the position (x) and the momentum (p) of a particle with absolute precision. In one of his regular letters to a colleague, Wolfgang Pauli, he presented the inklings of an idea that has since became a fundamental part of the quantum description of the world. ![]() In fleshing out this radical worldview, Heisenberg discovered a problem in the way that the basic physical properties of a particle in a quantum system could be measured. Among its many counter-intuitive ideas, quantum theory proposed that energy was not continuous but instead came in discrete packets (quanta) and that light could be described as both a wave and a stream of these Heisenberg was working through the implications of quantum theory, a strange new way of explaining how atoms behaved that had been developed by physicists, including Niels Bohr, Paul Dirac and Erwin Schrödinger, over the previous decade. The more familiar form of the equation came a few years later when he had further refined his thoughts in subsequent lectures and papers. ![]() An early incarnation of the uncertainty principle appeared in a 1927 paper by Heisenberg, a German physicist who was working at Niels Bohr's institute in Copenhagen at the time, titled " On the Perceptual Content of Quantum Theoretical Kinematics and Mechanics".
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