Robert McNees
The Reverend John Michell read a paper before the Royal Society #OTD in 1783 that included the first prediction of what, given the understanding of gravity at the time, you might call a Black Hole.
https://royalsocietypublishing.org/doi/pdf/10.1098/rstl.1784.0008
Robert McNees
What Michell predicted was the existence of bodies with an escape velocity greater than the speed of light. It's fair to call that a "Newtonian Black Hole."
It's not quite the same thing as a black hole in general relativity, where crossing the horizon means it's impossible to move in any way that takes you back outside.
So how is Michell's object different?
Robert McNees
"Escape velocity" is the speed needed at takeoff for a rocket (or any projectile) to escape the gravitational pull of whatever it is taking off from, without any further acceleration.
In 1783 the speed of light was known to be finite. Michell pointed out that if an object was dense enough, the escape velocity would exceed the speed of light and therefore "all light emitted from such a body would be made to return towards it, by its own proper gravity."
Robert McNees
This sounds like a black hole, right? Escape velocity greater than the speed limit built into our Universe?
Not quite. Escape velocity assumes no further acceleration. Newtonian physics would allow a rocket with enough fuel to escape Michell's object, it would just need to maintain sufficient acceleration for long enough.
But the black holes that arise in general relativity are an entirely different phenomenon.
K. P. Badertscher
@mcnees Does any other correspondence from this time traveler still exist?
Level 98
@mcnees It's great to see a thread about this because it's something that's often skimmed over, and/or is unclear, in pop-sci articles.
I've explained it to people in person (and mentioned it in brief social media replies) a fair bit... but I always like how you put your threads together, and they have far greater reach than any I post! ๐
Robert McNees
In general relativity, the causal structure of spacetime traps things behind a horizon. No rocket can overcome that.
Think of general relativity as laying out rules for how things are allowed to move. The rules in a black hole spacetime simply donโt allow trajectories that pass behind the horizon of a black hole and then make it back outside. Once something crosses over, the only allowed motions take it further in.
I recently posted an illustrated thread about this!
https://mastodon.social/@mcnees/111410664694892538
Robert McNees
@level98 Thanks!
Robert McNees
So a powerful rocket could escape Michell's object, but it couldn't escape a black hole in general relativity.
Once those light cones tilt over, you're out of luck. Your future lies between two contracting null surfaces that converge at a moment of singularity.
If it's any consolation, you get spaghettified (torn apart by powerful tidal forces) before that.
Robert McNees
But even though Michell envisioned something a bit different than the phenomenon we understand in general relativity, this is still a momentous event in the history of black holes.
His paper predates Laplace's 1796 prediction of "invisible bodies" by 13 years, and Schwarzschild's black hole solution of general relativity by 132 years.
Robert McNees
In 1783, Newtonโs law of gravitation was newer than general relativity is today.
Michell surveyed the heavens, extrapolated the properties of the objects he observed, and realized that the Universe (as he understood it) allowed exotic and fantastic gravitational phenomena.
In that sense, he had a lot in common with modern physicists and astronomers.
Robert McNees
There is a great, short essay by Gary Gibbons about Michell's prediction, why it was forgotten for so long, and some of his other contributions to physics.
Did you know that Michell designed the torsion balance experiment used by Cavendish to measure the density of the Earth?
Gary's essay appeared in New Scientist in 1979. It appears they've removed it from the web, but here's a copy Gary posted on his old website.
Robert McNees
In Henry Cavendish's famous paper "Experiments to determine the Density of the Earth," he opens by basically saying "Look, this was all Michell's idea, he just wasn't able to complete it before he passed away. I'm finishing what he started.โ
Robert McNees
Again, here's that thread from a few weeks ago about falling past a horizon, why you can't get back out, and a wormhole connecting two regions that are both โoutside the black hole.โ
Santa Claes ๐ธ๐ช๐ญ๐ฐ๐
@mcnees ๐คฏ mind blown that there was ever such a time. of course there was, but still.
Robert McNees
@DrHyde An interesting tidbit is that the "state of the art" at the time of Michell's work, would have probably been Bradley's measurement of c via aberration of light from distant stars. His value was off the currently accepted value by only about 0.4%!
David Cantrell ๐
@mcnees Oooooh! I knew that c being finite was known even before the Michelson-Morley experiment, but I didn't realise just how long ago it was measured. Thanks for sending me down that little rabbit-hole!
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