A black hole is a puzzling thing! It is a singularity in spacetime (see our "Roger Penrose" feature in Tangente 198, 2021) defined by its mass and spin. So what should we properly call these objects? Jean-Marc Lévy-Leblond advocates reviving the terms hidden stars and dark stars. The term was coined by Laplace in his Exposition du système du monde:
"A luminous star, with the same density as Earth and a diameter 250 times that of the Sun, would, because of its gravitational pull, allow none of its rays to reach us. It is therefore possible that, for this reason, the largest luminous bodies in the universe may be invisible."
Yet, however brilliant Laplace's intuition and daring may have been, these "luminous stars" do not correspond to the same concept.
The Danish astronomer Ole Roemer (1644–1710) showed that the speed of light is finite by studying eclipses of Jupiter's moons. His British contemporary Isaac Newton postulated the corpuscular nature of light (see the article "The Brilliant Intuition of the Existence of Black Holes" ). Light "particles" were therefore thought to have mass. Laplace inferred that these "particles of light" could not reach escape velocity once their star had reached a certain mass; such a star would be a "dark star."
Laplace removed the passage above from the third edition of his Exposition du système du monde, published in 1808.
