Number twelve in a series of 'coincidences' that allow intelligent life to exist in the universe.
From Barrow and Tipler's THE COSMIC ANTHROPIC PRINCIPLE.
There is one additional aspect of physics that is essential to the existence and stability of atomic systems--QUANTIZATION. In 1913 Niels Bohr proposed the radical evision of the naive atomic models that imagined the electrons to orbit a nucleus in the manner of a mini solar system. The quantization principle he used restricted the energy of orbital electrons to certain discrete values; multiples of a universal energy quantum fixed by Planck's costant. In the non-quantum atom, electrons can possess all possible nergies. They can reside in any orbital radius so long as their velocity is sufficient to establish an equilibrium between centrifugal and Coulomb forces. All atoms would be different under these circumstances, and, worse still, the continuous buffeting of electorns by photons and other particles would cause a steady chance in electron orbit (and hence chemistry). The quantum principle avoids this: if on electron is added to a proton there is only one orbital radius available to it in quantum theory and consequently all hydrogen atoms are identifical. This could not be the case in a non-quantum theory. Likewise, tiny enviormental perturbations do not upset the structure of the atom because an entire quantum of energy must be added before the electron orbital is altered. Thus, despite its traditional reputation as the harbringer of chance and inderterminism, quantum theory is the basis for the fidelity and large-scale stability of Nature."
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