@level98 I don't think I am? I have an account on there, but it's just my name. (I’m user 1727)

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.

In 1783, Newton’s 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.

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.

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.

@level98 Thanks!

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 don’t 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

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.

"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."

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?

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

This is a remarkable read about the aftermath of traumatic brain injury.

https://www.nytimes.com/interactive/2023/11/22/opinion/brain-traumatic-injury-writing.html?unlocked_article_code=1.AU0.lRh8.KSkNJ2Pzv0bj&smid=nytcore-android-share

Prints available here:

https://nucleusportland.com/collections/lily-seika-jones-prints

Would decorate my whole house with these watercolors by Lily Seika Jones (rivuletpaper on IG). I’d build more walls to have more places to put them.

@jc00ke Very generous! Saw this just as I was about to order one. Are they keychron?

Can anyone recommend a clickity-clackity mechanical keyboard that is nice to type on, but has a low barrier to entry? (That is, you don’t have to make lots of decisions about keycaps, switches, etc.) Bonus if it does Bluetooth.

Charles Darwin's "On the Origin of Species by Means of Natural Selection" was published #OTD in 1859.

Yesterday I was thankful for his explanation of the process by which certain saurischian dinosaurs became, over millions of years, delicious and small enough to fit on a plate:🦖→🦃.

@cabel “Eugene still hasn’t paid me pack the $157.05 he owes me, but I have a plan.”

Chemist Rachel Fuller Brown was born #OTD in 1898.

She discovered the antifungal antibiotic nystatin while doing research for the NY State Department of Health, hence the name.

Image: Smithsonian Institution

@gregpak Only 1 left at that price.