Notices by Robert McNees (mcnees@mastodon.social), page 2

Helen Sawyer’s husband got a job at an observatory in Victoria, BC. Officially, she worked there as his "volunteer assistant."

It was there that she began her work on variable stars in clusters. The detailed catalogs that she started compiling during this period (and first published in 1939) are still in use today.

https://www.astro.utoronto.ca/~cclement/readhistory.html

Second, astronomer Annie Jump Cannon visited Mount Holyoke the following year, and made a big impression on Sawyer.

After graduating, Sawyer moved to Harvard to work with Cannon and Harlow Shapley on globular clusters. Radcliffe awarded her PhD (1931), because Harvard didn't award science graduate degrees to women.

https://mastodon.social/@mcnees/109496538128937972

Humans first set foot on the Moon #OTD in 1969.

Like any serious code review, this one includes a “can we blame DEI” tangent with a weird right wing grifter.

It’s fine, I’m sure they’ve got things under control. We can trust [checks notes] … Microsoft? Wait what

Maybe putting all the infrastructure of modern society online was a bad idea.

Aerodynamics engineer and mathematician Irmgard Flügge-Lotz was born #OTD in 1903.

She advanced the understanding of aerodynamic pressure on wings and turbine blades, pioneered the theory of discontinuous control systems, and was the first woman named full professor of engineering at Stanford.

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.

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

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.

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?

By 1977, scientists knew that relativity would cause orbiting and ground-based clocks to beat out of sync.

Two competing effects are at work. The satellite moves rapidly in its orbit, so time dilation makes it tick more slowly compared to a ground-based clock. But clocks higher up in Earth's gravitational field should tick more rapidly than Earth-bound clocks.

Special relativity is trying to slow the satellite clock down relative to the ground, general relativity says it should tick faster.

Friends, here is a fun little physics story about satellites and relativity. It’s a day late for an #OTD, but please indulge me.

The Navigation Technology Satellite 2 (NTS-2) was launched into orbit on June 23, 1977, an early step in establishing the GPS NAVSTAR network. It was the first satellite to carry a Cesium atomic clock into orbit! 🧵

Image: Institute of Navigation

Calculating the fractional frequency shift of the satellite clock compared to the ground clock is a homework problem in GR textbooks.

Orbiting at 4.2 Earth radii, SR slows it down by ~ 250 parts in 10¹². The GR effect speeds it up by ~ 697 parts in 10¹².

General relativity produces the larger effect, and the orbiting clocks runs ahead of the earthbound clock by around 446 parts in 10¹². That’s an extra 38,500 nanoseconds per day.

This is a lot for GPS, which requires really precise timing.

Sunday morning is a pretty good time to take a look through NASA’s Project Apollo archive on Flickr.

https://www.flickr.com/photos/projectapolloarchive/albums

This is a totally safe thing to do. You definitely won't look up two hours from now and ask where your morning went.

Images: NASA

Apollo 11 launched #OTD in 1969, carrying Neil Armstrong, Michael Collins, and Buzz Aldrin to the Moon.

It was one of the most remarkable scientific collaborations in history, taking humans from Earth to the surface of another world, then returning them safely home.

Images: NASA

The team that worked out the Apollo 11 trajectories from Earth to the Moon included Katherine Johnson. She also performed calculations for how the Apollo Lunar Module would rejoin the Command and Service Module back in orbit

Image: NASA