First Lesson
Calculating the escape velocity of corpuscular light from a star five hundred times the diameter of the Sun.
Imagine a world where light itself can fall. In the late 1700s, scientists thought of light as a stream of tiny, solid high-speed bullets. They called these bullets corpuscles. Because these bullets have mass, they must feel the pull of gravity just like a dropped apple.
An English clergyman named John Michell realized this meant light could be trapped. In 1783, he wrote a letter to the Royal Society of London. He proposed that if a star is heavy enough, its gravity will pull its own light back down to its surface before it can escape.
To understand his idea, think about throwing a baseball straight up. If you throw it gently, it rises, slows down, and falls back. If you shoot it out of a cannon, it goes much higher. If you launch it fast enough, it escapes Earth's gravity forever.
This minimum speed is the escape velocity. It depends entirely on how heavy the star is and how tightly its mass is packed. If you squeeze a star into a smaller size, its surface gravity gets stronger, and its escape velocity goes up. What happens when this speed matches the speed of light?
Michell used Newtonian gravity to calculate this limit. He set the kinetic energy of a moving light particle equal to the gravitational pull holding it back. When these two forces balance at the star's surface, the particle stalls. Squeeze the star any further, and the light falls back down.
Let us look at the math Michell used. We want to find the exact size of a star where the escape velocity equals the speed of light, which we write as the letter $c$. If we use our sun's mass, how small must we squeeze it to make it vanish?
R = \frac{2GM}{c^2}Our calculation shows that if you squeeze our giant sun down to a radius of about three kilometers, it becomes a dark star. Its gravity becomes so intense that its escape velocity exceeds the speed of light. It would still be there, but it would be totally invisible to us.
How do you find a star that emits no light? Michell was incredibly clever. He realized that if a dark star had a bright companion star orbiting close to it, we could still see the bright star wiggle. We would see a star dancing with an invisible partner.
If any luminous bodies should happen to be situated in the regions of the other, we might still perhaps discover their existence.— John Michell, Letter to the Royal Society (1783)
This was the very first prediction of what we now call a binary system containing a black hole. Michell's letter was forgotten for over a century because scientists later decided light was a wave, not a particle. Waves, they believed, had no mass and would ignore gravity. We will explore how that view changed later.
Michell, J. (1784). 'On the Means of Discovering the Distance, Magnitude, &c. of the Fixed Stars.' Philosophical Transactions of the Royal Society of London. — This paper contains the very first mathematical description of a body with gravity so strong that light cannot escape.
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