# Black holes

## Notes

- It means all of the predictions we have made about black holes appear to be correct - we never could have said that definitively without an actual image. It means that predictions about how gravity works at this scale are apparently correct. It means we can image things, successfully with a telescope the size of the planet. It means black holes are no longer science fiction, aren't just predictions or expectations but definitely there. It means that general relativity doesn't change even at scales as huge as a super massive black hole. It means that our predictions of it's mass made from observing stellar orbits were pretty much right on.
- The density is what makes it a black hole, not the mass. Assume a perfectly spherically symmetric object. This object is a black hole if its radius is smaller that the Schwarzschild radius r_s corresponding with its mass. In this case the Schwarzschild radius becomes the radius of the event horizon.
- Enough photons in a small enough volume would collapse into a Kugelblitz, a black hole made from radiation, the same way it happens for matter. However the amount of energy (and therefore radiation) required is really high, since it has to be equivalent to an already large mass. Only true if the invarient mass (density) of the collection of photons is sufficiently high. For co-propagating photons, the invarient mass is zero as you could choose your reference frame such that the photons had any energy you like.

## Links

- What Exactly Are Black Holes?
- Why every picture of a black hole is an illustration
- How to Understand the Image of a Black Hole (2019)
- Breakthrough discovery in astronomy: press conference (2019) (HN)
- Astronomers Capture First Image of a Black Hole (2019)
- THAT Black Hole picture ⚫ - Sixty Symbols (2019)
- Zooming into Sagittarius A* black hole (Twitter)
- Why is light affected by black holes? (2020)
- Black Hole Paradoxes Reveal a Fundamental Link Between Energy and Order (2020) (HN)
- Spotting Quantum Black Holes in the Lab (2020)
- SXS - Simulating eXtreme Spacetimes
- What Black Holes Bring to the Galaxy (2020)
- (Relatively) Physically Accurate Black Hole Visualization in Unreal Engine (2020) (HN)
- 40 Years of Black Hole Imaging: Early work 1972-1988, by Jean-Pierre Luminet (2018) (Tweet)
- New Black Holes Offer Physicists a Radical Probe of the Cosmos (2021)
- Douglas Stanford - “New roles for wormholes” (2020) (Reddit)
- Lab-grown black hole behaves just like Stephen Hawking said it would (2021) (Reddit)
- Black Holes, Hawking Radiation and Holography (2021)
- The Doubly Warped World of Binary Black Holes (2021) (Article)
- Supermassive Black Hole Graphics Shader Tutorial (2021)
- How does gravity escape a black hole? (2021)
- A guide to living at a black hole (2021)
- Model of a black hole surrounded by gas glowing under gravity and friction
- Physicists confirm two cases of “elusive” black hole/neutron star mergers (2021) (HN)
- The Largest Black Hole in the Universe - Size Comparison (2021)
- What if Planet 9 is a Primordial Black Hole? (2019) (HN)
- A Dyson sphere around a black hole (2021) (HN)
- The Map of Black Holes | Your Ultimate Guide to Black Holes (2021)
- Event Horizon Telescope Traces Magnetic Fields Around a Black Hole (2021) (HN)
- Black Holes: The Edge of All We Know (2020)
- In 1783, an English rector predicted black holes using classical mechanics (HN)
- Are Fuzzballs the new understanding of Black Holes? (2021) (Reddit)
- Imminent merger of a supermassive black hole binary (2022) (HN)
- Tiny Galaxies Reveal Secrets of Supermassive Black Holes (2022)
- Black hole merger GW190521 may need many more black holes to explain it (2022) (Tweet)
- Visualizing Black Holes with General Relativistic Ray Tracing (2022) (HN)
- Black Holes Shown to Act Like Quantum Particles (2022) (HN)
- Astronomers reveal first image of the black hole at the heart of our galaxy (2022) (Tweet) (HN)
- Can we make a black hole? And if we could, what could we do with it? (2022) (HN)
- Black holes finally proven mathematically stable (2022) (HN)
- WebGL Black Hole (Code)
- Quantum Signatures of Black Hole Mass Superpositions (2022)
- Ask HN: Is it black holes all the way up? (2022)
- Real-time Black Hole Rendering in OpenGL
- Scientists find first observational evidence linking black holes to dark energy (2023) (HN) (Reddit)
- Gravitational Condensate Stars: An Alternative to Black Holes (2023)