Testing The Copernican Principle

Testing The Copernican Principle

In the sixteenth century, Nicolas Copernicus revived an idea originating with early Greek philosophers that the Sun, instead of the Earth, resided at the center of the solar system. Subsequent observations by Galileo and others validated the Copernican, or heliocentric, view (for a brief introduction to the conflict between Galileo and the Roman Catholic Church, look here.) In the early 1920s, Harlow Shapley correctly argued that the sun did not reside at the center of the Milky Way Galaxy (MWG). Shortly afterward, Edwin Hubble demonstrated that other galaxies similar to the Milky Way existed and that the MWG did not reside at the center of the universe.

Known as the Copernican Principle (or Cosmological Principle), it remains an uncontroversial statement that Earth occupies no special location in the universe—not the center, not near the edge, nor any other geometrically interesting point. However, many people extend this concept further, positing that there is nothing unique or special about Earth or the life inhabiting it. Scientists refer to this extension as the Principle of Mediocrity.

While the Copernican Principle is largely a philosophical statement, a large body of evidence supports its validity. Even so, scientists continue to put the principle to the test. An article posted to arXiv.org outlines a new test to see if the region of space to about 3 billion light-years exhibits any special or unique characteristics. One possible explanation for the current cosmological data (mainly the cosmic microwave background radiation, the large-scale structure of galaxies, and the Hubble diagram derived from Type Ia supernova measurements) is that the Sun resides at the center of a large void where the density inside is less than that outside the void.

Rather than focus on the details of the particular test outlined, I want to highlight how willingly and eagerly astronomers and cosmologists seek to test bedrock principles. While the tests explained in the paper rule out any voids with large density contrasts, the authors describe even more-detailed tests to discern any uniqueness of Earth’s location. A previous TNRTB further demonstrates this willingness to test foundational principles.

In contrast, as my colleague Fuz Rana points out, scientific disciplines related to the origin and development of life don’t seem as willing to test their fundamental assumptions and principles. Since testing is fundamental to scientific advance, RTB appreciates a willingness to test across all disciplines.