Copernican principle

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The Copernican principle is the philosophical statement that no "special" observers should be proposed. The term refers to the paradigm shift away from the Ptolemaic model of the heavens, which placed Earth at the center of the Solar system. Nicolaus Copernicus demonstrated that the motion of the heavens can be explained without the Earth (or anything else) being in the geometric center of the system, so the assumption that we are observing from a special position can be dispensed with.

The philosopher Immanuel Kant used the expression "Copernican revolution" to describe the effect that his critical method would have on contemporary epistemological thinking. The conditions and qualities he ascribed to the subject of knowledge placed man at the centre of all conceptual and empirical experience, and overcame the rationalism-empiricism impasse, characteristic of the 17th and 18th centuries.

The Copernican principle is applied in cosmology, as the acknowledgement that the Universe is generally homogeneous and isotropic over large scales. These principles are accepted not merely as a philosophical statement but as an acknowledgement that a significant, large-scale deviation from homogeneity and isotropy would be statistically unlikely, and that this acknowledgement has been found to be correct in different contexts in prior observations.

In practice, astronomers observe that the Universe has heterogeneous structures up to the scale of galactic superclusters, filaments and great voids, but that the Universe is essentially homogeneous when considered on scales of at least about 200 million parsecs. However, while this is true of space, the Universe is not homogeneous or isotropic over large scales of time, since it has been progressing from extremely different conditions since the Big Bang, and will continue to progress toward extremely different conditions, particularly under the rising influence of dark energy, apparently toward the Big Freeze or the Big Rip. The Universe is homogeneous over time on non-cosmological time scales, but is not isotropic over time beyond the time scales of elementary particle reactions. The emergence of non-isotropic time on larger scales is one of the most fundamental mysteries in contemporary physics.