LEONARD SUSSKIND has been the Felix Bloch Professor in theoretical physics at Stanford University since 1978. He is a member of the National Academy of Science and the American Academy of Arts and Sciences, recipient of numerous prizes including the science writing prize of the American Institute of Physics for my Scientific American article on black holes. His contributions to physics include the discovery of string theory, the theory of quark confinement, the development of Hamiltonian lattice gauge theory, the theory of scaling violations in deep inelastic electroproduction, the theory of symmetry breaking sometimes known as "Technicolor theory", the first theories of cosmological baryogenisis apart from Sakharov's work which was unknown in the west, the string theory of black hole entropy, the principle of "black hole complementarity", the holographic principle, the matrix description of M-theory, the introduction of holographic entropy bounds in cosmology, the idea of a string theory "landscape".
During the last 40 years or so, scientists have discovered that the existence of intelligent life depends upon a complex and delicate balance of initial conditions given in the Big Bang itself. Scientists once believed that whatever the initial conditions of the universe, eventually intelligent life might evolve. But we now know that our existence is balanced on a knife's edge. The existence of intelligent life depends upon a conspiracy of initial conditions which must be fine-tuned to a degree that is literally incomprehensible and incalculable.
This fine-tuning is of two sorts. First, when the laws of nature are expressed as mathematical equations, you find appearing in them certain constants, like the gravitational constant. These constants are not determined by the laws of nature. The laws of nature are consistent with a wide range of values for these constants. Second, in addition to these constants there are certain arbitrary quantities which are just put in as initial conditions on which the laws of nature operate, for example, the amount of entropy or the balance between matter and anti-matter in the universe. Now all of these constants and quantities fall into an extraordinarily narrow range of life-permitting values. Were these constants or quantities to be altered by a hair's breadth, the life-permitting balance would be destroyed and life would not exist.
For example, the physicist P. C. W. Davies has calculated that a change in the strength of gravity or of the atomic weak force by only one part in 10100 would have prevented a life-permitting universe. The cosmological constant which drives the inflation of the universe and is responsible for the recently discovered acceleration of the universe's expansion is inexplicably fine-tuned to around one part in 10120. Roger Penrose of Oxford University has calculated that the odds of the Big Bang's low entropy condition existing by chance are on the order of one out of 10 10 (123). Penrose comments, "I cannot even recall seeing anything else in physics whose accuracy is known to approach, even remotely, a figure like one part in 1010 (123)."5 And it's not just each constant or quantity which must be exquisitely finely-tuned; their ratios to one another must be also finely-tuned. So improbability is multiplied by improbability by improbability until our minds are reeling in incomprehensible numbers.
