Anthropic principle: the view that the laws of nature must be reconcilable with the existence of natural scientists.

At first this sounds like a truism. However, the anthropic principle is supposed to explain certain properties of the laws of nature. It particularly concerns the values of physical constants, the number and type of basic particles, and the number of spatial dimensions. For these values are tailored to the fact of our existence. Even slight deviations from the numerical values of the physical constants in our universe would rule out all more complex entities including, of course, all possible forms of life. A proton is conceived as being 1836 times heavier than an electron precisely because a world with other mass ratios could not contain any observer to measure the relevant physical values.

Our Tailor-Made Universe

The current standard model of physics -- the theory describing the behavior of physical matter -- does not provide any alternative explanation for these perfectly tailor-made values of physical constants. To be sure, it is possible that the values may be derived independently from some future string theory. However, this would merely shift the problem of the tailor-made universe to another level. In 1973, the cosmologist Brandon Carter offered an alternative theory based on his distinction between two anthropic principles.

According to Carter, the weak anthropic principle holds that our place in the universe is privileged in that its properties are consistent with our existence as observers. But there are or may be other places, times, or universes that do not conform to this principle. Thus, the weak anthropic principle implies that our world does not need to be the only one that currently exists, has existed in the past, or will exist in the future. According to the weak principle, there are or have been other possible worlds (multiverses) with different physical constants from ours. Most of these other universes are deserted and vacant. From this perspective, our tailor-made physical constants are not an extremely unlikely coincidence, but rather a natural consequence of the existence of a multitude of possible worlds. They are even a necessary consequence, if we assume there to be an infinite number of possible worlds.

The strong anthropic principle, by contrast, goes a step further. It claims that the universe must of necessity meet the requirements for the existence of observers. This claim has two readings. It can be understood to mean, first, that there must be a better physical theory, such as string theory, which is able to explain the values of physical constants in a "natural" manner (thus, the strong anthropic principle contradicts the standard model). The second possible interpretation is that God has somehow calibrated the physical constants just so that intelligent life could be created. According to both readings, the assumption of other possible worlds is either ruled out or unnecessary.

A Whiff of Mysticism

Unfortunately, the anthropic principles are not really helpful in our attempt to understand the laws of nature and the universe. Though they teach us that in our world the mass ratio of protons and electrons must be no less and no more than 1836, we are still in need of a physical theory that would exlain why that is what the ratio in fact is in terms of a set of even more fundamental laws. Nonetheless, such principles are popular with many scientists because they add a metaphysical or even mystical touch to their research.

Unsurprisingly, the anthropic principles have also led to a number of controversies, have been hijacked by various religious and esoteric groups, and have inspired books that can only be described as eccentric. Supporters of creationism have presented the strong anthropic principle as proof of God's interference with nature. Many scientists, however, have taken delight in stimulating further controversies just for fun by publishing additional anthropic principles of a more or less arbitrary character. John Wheeler, for example, developed a so-called participatory anthropic principle: Observers are necessary to bring the universe into being.

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