Reinventing the Dark Star
1796: Pierre-Simon Laplace
Ideas travel faster than the winds these days, and are often just as ephemeral. In centuries past, though, a good idea often was forgotten quickly, even when it was recorded in one of the leading scientific journals of the day.
Thus it was with John Michell's introduction of the idea of a "dark star." Although it was published in the proceedings of London's Royal Society, one of the world's leading scientific organizations, it apparently didn't register in the scientific salons of France. In 1796, a French mathematician and scientist proposed the idea again.
Pierre-Simon Laplace, the son of a peasant farmer, was one of the most brilliant men of his day. He had made many important contributions to mathemathics, probability, astronomy, and other fields. He had studied motions of the moon and planets. His studies led to a landmark volume on celestial mechanics.
In 1796, Laplace published another important work, Exposition du Systeme du Monde. Among other points, it contained the idea that the Sun and planets collapsed from a large cloud of gas and dust.
The work also discussed the concept of dark stars.
Like Michell, Laplace combined Newton's law of universal gravitation, the idea that light consists of particles, and the idea that the speed of light is limited. Laplace realized that a minimum speed was required to escape the gravitational pull of a star or planet — the escape velocity. For a body of sufficient mass and density, the escape velocity should be greater than the speed of light. In that case, the body would be invisible to the outside universe — even if the body were a star. Laplace wrote:
There exist in the heavens therefore the dark bodies, as large as and perhaps as numerous as the stars themselves. Rays from a luminous star having the same density as the Earth and a diameter 250 times that of the Sun would not reach us because of its gravitational attraction; it is therefore possible that the largest luminous bodies in the Universe may be invisible for this reason.
The idea seemed reasonable at the time, but it quickly lost favor. Just a few years later, another scientist determined that light behaves like waves instead of particles. (In fact, light acts as both particles and waves, although that was unknown as the time.) Gravity would not trap light waves, scientists thought, so dark stars could not exist. Laplace himself dropped the idea from the third edition of the Exposition.
In the early 20th century, though, the idea of invisible stars was resurrected by the work of Albert Einstein, which set up the theoretical basis for our modern understanding of black holes.