Most particle half-lives are very short, microseconds or far less. The reason for the long half-life of neutrons is that they decay through the weak nuclear force which is weak.https://en.wikipedia.org/wiki/Free_neutron_decay …
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There is actually a bit of a puzzle what the real lifetime is, since different measurement methods have produced non-overlapping results. https://pdg.lbl.gov/2018/listings/rpp2018-list-n.pdf … https://arxiv.org/abs/1411.3687v1 pic.twitter.com/DBMHqnP7mV
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Lots of interesting issues in how to do this kind of experiment, that usually involves beams of ultra-cold neutrons.https://www.youtube.com/watch?v=UZMWmRTbHt0 …
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Why doesn't neutrons in nuclei decay? The particles in a nucleus are organised in "shells" of different energy levels and quantum numbers. http://hyperphysics.phy-astr.gsu.edu/hbase/Nuclear/shell.html …
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For a neutron to decay the resulting proton needs to have an available state with lower energy. But in stable nuclei all the possible lower energy states are filled. Hence the neutrons have to stay neutrons.
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This shows that the environment can affect half-lives. People have looked for effects of temperature, pressure and even solar distance on radioactivity but there doesn't seem to matter. It is the *close* environment that matters. https://oaktrust.library.tamu.edu/handle/1969.1/148338 …
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Some isotopes decay by electron capture: an electron is gobbled up by a proton, becoming a neutron. If you completely ionize the atom this stops. In other isotopes the effect can go the other way, shortening half-lives from 42 billion years to 32.9. https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.77.5190 …
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During the Big Bang conditions were so hot that neutrons and protons interconverted to a near 50:50 mix, but as things cooled down free neutrons turned into protons that eventually became hydrogen atoms. Only the neutrons caught in deuterium and lithium survived.
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Replying to @anderssandberg
I still find it amazing that such an energetically favorable reaction as fusion barely happened in the early universe, leaving it all to happen slowly inside suns.
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Replying to @tlbtlbtlb @anderssandberg
Most universes should have gone FOOM! and been no more than a lightcone of gamma ray photons expanding into nothingness.
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it is hard to avoid anthropic reasoning; the parameters are fine tuned to allow enough complexity to support consciousness, at least here :D
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