This path is traveled not just in the physical sciences, but in the life sci-
ences as well; it describes both the atom and the gene.
It also describes the black hole, whose existence was first hypothesized
more than two centuries ago by the English geologist John Michell. In a
paper published by the Royal Society, Michell stated, “If the semi-diame-
ter of a sphere of the same density as the Sun were to exceed that of the
Sun in the proportion of 500 to 1, a body falling from an infinite height
toward it would have acquired at its surface greater velocity than that of
light, and consequently supposing light to be attracted by the same force
in proportion to its vis inertiae (inertial mass), with other bodies, all light
emitted from such a body would be made to return towards it by its own
proper gravity.”^9 The basic idea of a black hole is clearly contained in this
statement: the gravity of the object is so strong that no light can escape
from it.
With the development of Einstein’s theory of relativity, interest in the
concept of a black hole picked up steam. In the 1930s, work was initiated
by the astrophysicist Subrahmanyan Chandrasekhar and continued by
Robert Oppenheimer (among others), who in just a few years would head
the Manhattan Project, which developed the first atomic bomb. They con-
cluded that stars possessing greater than a certain mass would undergo
an unstoppable gravitational collapse and become a black hole. Black
holes thus progressed from hypothetical construct to entities that might
conceivably be observed, either indirectly or directly. Supermassive black
holes, with masses millions of times the mass of the Sun, are now be-
lieved by many physicists to lurk at the core of major galaxies, including
the Milky Way galaxy in which Earth resides. In 2004, astronomers
claimed to have detected a black hole orbiting the supermassive black
hole at the center of the Milky Way galaxy (fortunately, Earth is situated a
comfortable distance away from the center).^10 Although we will never see
a black hole, as John Michell was well aware, the evidence for their exist-
ence is now extremely strong.
What is known about black holes is that they are completely determined
by their mass, their charge, and their spin. These are the only things we
can ever know about a black hole, and so when we see a black hole with a
given mass, charge, and spin (the macrostate), all the gazillions of possible
microstates occurring within the black hole correspond to that single mac-
rostate. Black holes are therefore the ultimate limit of how high the en-
tropy can go. The higher the entropy, the less the information, and a black
hole of a given mass, charge, and spin conveys the least possible informa-
tion about the region of space it occupies. The goings-on in the inside of
the black hole appears to be high on the list of things we cannot know.
196 How Math Explains the World