When this happens – a process metaphorically known as "quantum tunneling" because the escaping particle has to somehow dig its way through an energy barrier that it cannot leap over – the alpha particle escapes and we see radioactivity.Ī similar quantum tunnelling process happens, in reverse, at the centre of our sun, where protons fuse together and release the energy that allows our star to shine. That means there is a small, but non-zero, chance that the particle could, at some point, find itself outside the nucleus, even though it technically does not have enough energy to escape. But, because an alpha particle inside a nucleus has a very well-defined velocity, its position is not so well-defined. Usually these are bound inside the heavy nucleus and would need lots of energy to break the bonds keeping them in place. Alpha particles are two protons and two neutrons emitted by some heavy nuclei, such as uranium-238. Heisenberg's idea can also explain a type of nuclear radiation called alpha decay. In that case, the electron could be moving fast enough to fly out of the atom altogether. This means that the error in measuring its momentum (and, by inference, its velocity) would be enormous. The uncertainty principle explains why this doesn't happen: if an electron got too close to the nucleus, then its position in space would be precisely known and, therefore, the error in measuring its position would be minuscule. By classical logic, we might expect the two opposite charges to attract each other, leading everything to collapse into a ball of particles. Take atoms, for example, where negatively-charged electrons orbit a positively-charged nucleus. The uncertainty principle is at the heart of many things that we observe but cannot explain using classical (non-quantum) physics. 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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