Now, as it happens, this rule is of major importance in the creation of all the elements and resulted in the Periodic Table. Pauli's Exclusion Principle states that no two identical fermions (so that's quarks as well as leptons) can occupy the same quantum state. Electrons, therefore, have two possible quantum states, known as spin up and spin down. I know that's crazy, but that's quantum physics for you.Īt the end of the previous section, the electron on the rotating platform, when it ended up pointing the other way after a 360 degree turn, has changed to a different quantum state. In the quantum domain, particles with spin 1/2, after being rotated similarly, would end up pointing in the other direction. If it turned through 360 degrees, you'd be back looking in the direction you started at, wouldn't you? That's in our familiar world. To try an give you some idea of what "spin 1/2" is, imagine standing on a platform that could rotate. Just to make things more difficult, they still kept the expression "spin". The phenomenon was just considered to be a property of the electron that just was. ![]() ![]() This is why the "spin" idea had to be abandoned. Trouble was, to produce a magnetic field such as the strength they measured, the particle would have to be spinning faster than the speed of light - and, of course, this is not possible. This is a characteristic of a spinning object that has an electric charge. If "spin" doesn't actually mean "spin" in the case of leptons, then why call it that? Well, scientists discovered that one of the most well-known leptons - the electron - possessed a magnetic field (called, rather quaintly, a "magnetic moment"). Enjoying it so far? The angular momentum is a quantum state. This is distinct from the orbital angular momentum, which is the rate at which it may be spinning. It also represents the intrinsic angular momentum of the particle. Whereas a spinning object in our familiar world can be speeded up or slowed down, particle spin is constant and nothing, but nothing can change it. To accurately explain this attribute of fermions, which include leptons, it would require both me and you to have a pretty advanced understanding of physics, so this next bit will be greatly simplified.įirstly, spin is an intrinsic property of a particle. No, of course, this is the quantum domain, so it's bound to be weird. When I tell you that "spin", with regard to a subatomic particle, is a quantum attribute, you'll probably come to the depressing realisation that it won't be the same, or as straightforward, as a spinning top, a merry-go-round or a planet - things we can witness in our familiar world. Perhaps I should try and elaborate on what these things mean, before we go any further. These elementary particles are "spin 1/2" and are subject to the Pauli Exclusion Principle. Each subsequent generation's pair has greater mass than the previous one. Like quarks, there are six different types and come in pairs, representing three generations. Leptons are a group of particles that come under the blanket term of fermions, together with quarks.
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