Mediocrity principle
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The mediocrity principle is the notion in the philosophy of science that there is nothing special about Earth, and by implication the human race. It is a Copernican principle, used either as a heuristic about Earth's position or a philosophical statement about the place of humanity.
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[edit] The Earth is an unexceptional planet
The traditional formulation of the Copernican mediocrity principle is usually played out in the following way: Ancients once thought that the Earth was at the center of the solar system, but Copernicus proposed that the Sun was at the center. This heliocentric view was confirmed a hundred years later by Galileo, who demonstrated with a telescope that Jupiter's moons orbited Jupiter and that Venus must orbit the Sun. In the 1930s, RJ Trumpler found that the solar system was not at the center of the Milky Way Galaxy (as Jacobus Kapteyn claimed), but 56% of the way out to the rim of the galaxy's core. In the mid-twentieth century, George Gamow (et al.) showed that although it appears that our Galaxy is at the center of an expanding universe (in accordance with Hubble's law), every point in space experiences the same phenomenon. And, at the end of the twentieth century, Geoff Marcy and colleagues discovered that extrasolar planets are quite common, putting to rest the idea that the Sun is unusual in having planets. In short, Copernican mediocrity is a series of astronomical findings that the Earth is a relatively ordinary planet orbiting a relatively ordinary star in a relatively ordinary galaxy which is one of countless others in a giant universe, possibly within an infinite multiverse.
[edit] Critics of the 'ordinary earth' mediocrity principle
In arguing that human's planet, evolution, civilization, and technology are unexceptional SETI advocates invoke the mediocrity principle as a strong reason (via prior probability) to expect abundant extraterrestrial signals. For instance, Carl Sagan used the principle to argue that "there might be one million civilizations in the Milky Way.” The failure to find such signals or evidence is taken by some as a disconfirmation of the mediocrity principle. (The lack of contact is interpreted more often as a scarcity of human-like intelligence than a scarcity of Earth-like planets, but a scarcity of either could be considered refutation of the mediocrity principle, depending on whether the principle is applied strictly to the planet or, more loosely, to its inhabitants.)
Denying the mediocrity principle is very similar to affirming the Rare Earth hypothesis; for example, Gonzalez and Richards (2004) present the case for Earth's uniqueness, in their book The Privileged Planet. They claim:
- "Not that we are at the 'center' of the universe, but rather at the best location for complex life to flourish and to observe what is beyond us." [1]
They argue that not only is the cosmos as a whole finely tuned for life, but within it, the Earth's peculiarities make it an extremely special 'Pale Blue Dot'. Rare Earth made similar claims in 2000. These attacks on the principle are based on the following planetary advantages, all alleged to be uncommon & essential for life:
- Earth occupies the perfect orbit around a non-binary, metal-rich star (the Sun) with stable radiation over an ideal frequency spectrum.
- Earth is a silicate rock with the prerequisite mass, plate tectonics, and iron core to protect developing life.
- Jupiter and the other large outer planets shield the Earth from asteroids without destabilizing its orbit.
- Earth has the perfect amount of water for a long-term active hydrosphere.
- The Moon is anomalously massive, creating large oceanic tides, and stabilizing the Earth's axial tilt. According to Jacques Laskar's calculations this critical feature is otherwise impossible to achieve.
- The Earth's location within the galaxy is rare and important: "Not in the center of the galaxy, not in a globular cluster, not near an active gamma ray source, not in a multiple-star system, or near a pulsar, or near stars too small, too large, or soon to go supernova." (Rare Earth page 282).
Advances in detection of extrasolar planets may validate or disprove many of these hypotheses in the near future.
[edit] As a philosophical statement
There is a stronger, philosophical version of the mediocrity principle. This associates the Renaissance with greater openness to radical ideas. The belief is that the Roman Catholic dogma of the day, with regards to the place of Earth in the cosmos, was that if God made man in God's image and that this was God's most perfect creation, then there was only one logical place to put this most perfect creation—at the center of the Universe. Therefore, Copernicus's suggestion that Earth was not the center of the entire Universe, implied the theological conclusion that man was not God's most perfect creation. Although this is a popular interpretation of history and of man's position in the cosmos, it is not historically accurate. Medieval theologians, most vividly illustrated in Dante's Divine Comedy viewed the heavens as perfect, and Earth (and humans) as the dregs (rather than the pinnacle) of creation. Thus it seemed that Copernicus was actually promoting rather than demoting the Earth by removing it from the "basement", and the paradigm shift was of a very different character.
[edit] See also
- The Rare Earth hypothesis is the antithesis of the Mediocrity principle
- Anthropic principle
- Cosmic pluralism
- Geocentric cosmology of Aristotle and Ptolemy (see: Ptolemaic system)
- Plenitude principle
- Uniformity principle
- Drake equation
[edit] References
- Gonzalez, Richards, The Privileged Planet: How Our Place in the Cosmos is Designed for Discovery 2004, Regnery Publishing, ISBN 0-89526-065-4
- Peter Ward and Donald Brownlee. Rare Earth: Why Complex Life is Uncommon in the Universe. Copernicus Books. January 2000. ISBN 0-387-98701-0
[edit] External links
- Goodwin, Gribbin, and Hendry's 1997 Hubble Parameter measurement relying on the mediocrity principle The authors call this the 'Principle of Terrestrial Mediocrity' even though the assumption they make is that the Milky Way Galaxy is typical (rather than Earth). This term was coined by Alexander Vilenkin (1995)
