According to Buddhist Madhyamaka philosophy, all functioning things exhibit 'subtle impermanence' - perpetual activity and restless change even when they appear to be superficially stable.
This can be understood in terms of quantum physics as interactions with the quantum vacuum. In quantum theory, even totally empty space is in a state of seething impermanence, with spontaneously occurring random energy fluctuations and rapid temporary appearances and disappearances of 'virtual' particles.
"In quantum physics, a quantum vacuum fluctuation (or quantum fluctuation or vacuum fluctuation) is the temporary change in the amount of energy in a point in space, arising from Werner Heisenberg's uncertainty principle." http://en.wikipedia.org/wiki/Quantum_fluctuation
These random energy fluctuations interact with any matter in their vicinity, temporarily raising the energy level of atomic nuclei. So the atoms that compose the world we inhabit, and everything built from them, are in a perpetual process of energy transition.
Usually, these temporary energy perturbations have no effect, with the energized atoms rapidly returning to lower energy states. However, if the nucleus is unstable, and only requires a relatively small input of energy to break it up, a quantum fluctuation can cause the atom to 'spontaneously' undergo radioactive decay. This has been likened to a small disturbance to an unstable snowfield triggering an avalanche:
"Radioactive decay is the process by which an atomic nucleus of an unstable atom loses energy by emitting ionizing particles (ionizing radiation). There are many different types of radioactive decay (see table below). A decay, or loss of energy, results when an atom with one type of nucleus, called the parent radionuclide, transforms to an atom with a nucleus in a different state, or to a different nucleus containing different numbers of protons and neutrons. Either of these products is named the daughter nuclide. In some decays the parent and daughter are different chemical elements, and thus the decay process results in nuclear transmutation (creation of an atom of a new element).
The first decay processes to be discovered were alpha decay, beta decay, and gamma decay. Alpha decay occurs when the nucleus ejects an alpha particle (helium nucleus). This is the most common process of emitting nucleons, but in rarer types of decays, nuclei can eject protons, or specific nuclei of other elements (in the process called cluster decay). Beta decay occurs when the nucleus emits an electron or positron and a type of neutrino, in a process that changes a proton to a neutron or vice versa. The nucleus may capture an orbiting electron, converting a proton into an neutron (electron capture). All of these processes result in nuclear transmutation.
By contrast, there exist radioactive decay processes that do not result in transmutation. The energy of an excited nucleus may be emitted as a gamma ray in gamma decay, or used to eject an orbital electron by interaction with the excited nucleus in a process called internal conversion. Radioisotopes occasionally emit neutrons, and this results in a change in an element from one isotope to another.
One type of radioactive decay results in products which are not defined, but appear in a range of "pieces" of the original nucleus. This decay is called spontaneous fission. This decay happens when a large unstable nucleus spontaneously splits into two (and occasionally three) smaller daughter nuclei, and usually emits gamma rays, neutrons, or other particles as a consequence.
Radioactive decay is a stochastic (i.e., random) process at the level of single atoms, in that, according to quantum theory, it is impossible to predict when a particular atom will decay. However, the chance that a given atom will decay is constant over time. For a large number of atoms, the decay rate for the collection is computable from the measured decay constants of the nuclides (or equivalently from the half-lives)...
a collapse (a decay event) requires a specific activation energy. For a snow avalanche,
this energy comes as a disturbance from outside the system, although such disturbances can
be arbitrarily small. In the case of an excited atomic nucleus, the arbitrarily small
disturbance comes from quantum vacuum fluctuations. A radioactive nucleus (or any excited
system in quantum mechanics) is unstable, and can, thus, spontaneously stabilize to a
less-excited system. The resulting transformation alters the structure of the nucleus and
results in the emission of either a photon or a high-velocity particle that has mass (such
as an electron, alpha particle, or other type). http://en.wikipedia.org/wiki/Radioactive_decay
Causality and Randomness
It is sometimes said that radioactive
decay, and other 'random' quantum events, are uncaused. According to the
quantum vacuum fluctuation explanation, this is strictly speaking untrue. It is the
subtle impermanence of the quantum vacuum that energizes the unstable
Small energy fluctuations occur relatively frequently, large fluctuations occur infrequently. Thus nuclei requiring only a small energy input to break them up (e.g. Hydrogen-7) will be more unstable, and have shorter half lives, than those needing a large energy input (e.g. Tellurium-128)
However, the underlying vacuum fluctuations do seem to be truly random and 'uncaused'.