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Bismuth No Longer the Heaviest Stable Element
forii writes "Bismuth-209 was commonly thought to be the heaviest stable element. But now Physicists have discovered that Bi-209 actually is unstable and decays with a halflife of 2*10^19 years. This means that the average 8oz (237ml) bottle of Pepto-Bismol contains one decay event every 36 hours or so."
In the article, it mentions that people actually have predicted this decay using theory. The nucleus is not completely understood, but the theory of basic decay phenomena is pretty complete.
Any time you talk about the quanta of physics, you need to use quantum mechanics. The quanta are of course the so-called fundamental particles, including the proton, neutron and electron.
The nucleus is held together by the strong force. This force must be very strong to keep the protons, whose like charge repels one another, very close together. The strong force only pulls over very short distances: if some nucleons get far enough, their electromagnetic repulsion will continue to push each other apart and they will be separated permanently.
However, the particles in the nucleus don't have enough energy to get over the hump, so nuclei are stable. This is where quantum mechanics applies. Even if the hump is very tall, the nonlocality of quantum mechanics means that some particles can escape if the hump isn't very wide. Because they have a probabilistic spread in space, some of them can creep to the other side. When they get lucky like this, a nuclear decay occurs. The details of the nucleus determine how high the barrier and how wide the hump, both of which affect the probability of tunneling.
In stable nuclei, particles are prohibited from escaping. In this case, it's not that the hump is too high, but that it's asymmetrical. If the nuclear force is strong enough compared to the energy of the nucleons, it can dig a deep well for the particles. In this case, having some possibility of getting past the hump doesn't really help: the area on the other side of the hump is prohibited regardless.
One way to think of this process is to say that quantum mechanics would allow you to borrow the energy you need to jump over a fence as long as you fell back down on the other side, no matter how tall the fence.
But you can't keep the borrowed energy, so you could never jump to the top of a roof, even if it were no taller than the wall you just jumped over.