|MadSci Network: Physics|
Hello Bryan, Magnetic dipole transitions can come in quite a few different systems and at quite a few different energies. However, perhaps the most famous of magnetic dipole transitions is one of the simplest and also happens to be forbidden. "Forbidden" in this case doesn't mean "never occurs" but rather "very, very seldom occurs." The transition I'm referring to is that found in a hydrogen atom. The transition from the electron & proton spins parallel to antiparallel is not allowed by simple quantum mechanics rules, but it does in fact happen if you include a few additions(or have lots of patience!). The energy for this transition is quite small and corresponds to a 21cm wavelength of radiation. That comes out to around 59millionths of a single electron-volt of energy. The reason it's so famous is that, other than just being a simple system (ie, Hydrogen!), the universe at large is also populated by lots of hydrogen. Much of radio astronomy uses the 21cm wavelength to study the universe. Regions of the sky which may appear dark in visible light, will be very bright with radio waves such as the 21cm ones produced by hydrogen undergoing the magnetic dipole transition. After the photon is emitted, the hydrogen atom will be in a lower energy state than it was before. Without getting energy from somewhere else it will remain in the lower energy state. You can get an idea for it from an analogy with bar magnets. Hold two bar magnets parallel and try to bring them together from the side. Now hold them anti-parallel and do the same thing. Thank you very much for the questions. Magnetic dipole transitions are a bit outside my field of study. It was fun to read upon them again as I have not gotten to think about this subject much since graduate school. best wishes, Michael --------------------------- Michael S. Pierce Materials Science Division Argonne National Laboratory
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