Anthropic Principle

The success of science in understanding the macroscopic, microscopic and cosmological worlds has led to the strong belief that it is possible to form a fully scientific explanation of any feature of the Universe. However, in the past 20 years our understanding of physics and biology has noted a peculiar specialness to our Universe, a specialness with regard to the existence of intelligent life. This sends up warning signs from the Copernican Principle, the idea that no scientific theory should invoke a special place or aspect to humans.

All the laws of Nature have particular constants associated with them, the gravitational constant, the speed of light, the electric charge, the mass of the electron, Planck’s constant from quantum mechanics. Some are derived from physical laws (the speed of light, for example, comes from Maxwell’s equations). However, for most, their values are arbitrary. The laws would still operate if the constants had different values, although the resulting interactions would be radically different.


gravitational constant: Determines strength of gravity. If lower than stars would have insufficient pressure to overcome the Coulomb barrier to start thermonuclear fusion (i.e. stars would not shine). If higher, stars burn too fast, use up fuel before life has a chance to evolve.
strong force coupling constant: Holds particles together in the nucleus of atoms. If weaker than multi-proton particles would not hold together, hydrogen would be the only element in the Universe. If stronger, all elements lighter than iron would be rare. Also radioactive decay would be less, which heats the core of the Earth.

electromagnetic coupling constant: Determines strength of electromagnetic force that couples electrons to the nucleus. If less, than no electrons would be held in orbit. If stronger, electrons will not bond with other atoms. Either way, no molecules.

All the above constants are critical to the formation of the basic building blocks of life. And, the range of possible values for these constants is very narrow, only about 1 to 5% for the combination of constants. Outside this range, and life (in particular, intelligent life) would be impossible.constants_DNA

It is therefore possible to imagine whole different kinds of universes with different constants, all equally valid within the laws of Nature. For example, a universe with a lower gravitational constant would have a weaker force of gravity, where stars and planets might not form. Or a universe with a high strong force which would inhibit thermonuclear fusion, which would make the luminosity of stars be much lower, a darker universe, and life would have to evolve without sunlight. Why don’t those Universes exist? Why does our Universe, with its special value exist rather than another? Is there something fundamental to our physics that makes the present values for physical constants expected?

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