Thread by @rayh

In 1915, a hospitalised Schwarzschild passed the time by solving Einstein's newly published field equations while the war raged nearby. His solution predicted a region of infinite density that would prevent light from escaping - what we now call a black hole.

Unfortunately, he passed away just 6 months later.

arxiv.org/abs/1503.02172

Coincidently, his surname translates as "black shield"

How do we know black holes are real?

At first, the idea of observing something from which light cannot escape seems counter-intuitive, but we *can* observe their influence on the environment around them:

The gravitational influence on orbiting bodies can be observed, such as the star S2 orbiting the Milky Way's own supermassive blackhole: Sagittarius A*

mpg.de/14692117/detec…

Supermassive blackholes are common in galactic centres and, when active, they shine as some of the brightest objects in the sky. We can observe them through emissions from their accretion disks and relativistic jets.

imagine.gsfc.nasa.gov/science/object…

The collision of two blackholes would also cause ripples in the fabric of spacetime itself.

arxiv.org/abs/1602.03837

It would take around a century after Schwarzschild's prediction before we would be able to observe supermassive blackholes colliding using gravitational waves news.mit.edu/2016/ligo-firs…

And only a few years after that, a Earth-spanning telescope took the first photo of a black hole's event horizon
eventhorizontelescope.org/press-release-…

In the not too distant future, projects such as LISA will allow us to observe more of the gravitational wave spectrum from space

lisa.nasa.gov

...and using a network of millisecond pulsars, we can effectively build a galaxy wide GW detector (which, incidentally, could also be used as a kind of galactic GPS).

ipta4gw.org

Black holes are fascinating because (along with objects like neutron stars) they inhabit the fault line in physics; between our models of the large (General Relativity) and the small (Quantum Mechanics)

We now know that black holes are very real, very weird, pretty common and totally awesome.

A great example of this is how LIGO's sensitivity is fundamentally limited by the quantum vacuum:

ligo.caltech.edu/news/ligo20200…