Black Holes
Event Horizon
The point of no return — boundary of a black hole, defined by escape velocity = c
An event horizon is a boundary in spacetime beyond which events cannot affect an outside observer — the surface of a black hole. Defined by escape velocity equaling speed of light. For a non-rotating black hole (Schwarzschild): r_s = 2GM/c². For Earth's mass: 9 mm. For Sun's: 3 km. For supermassive Sgr A* (4 million M_sun): 12 million km. Anything crossing inward cannot return. Light, signals, particles all trapped inside.
- Schwarzschild radiusr_s = 2GM/c²
- Sun if BHr_s = 3 km
- Earth if BHr_s = 9 mm
- Sgr A* horizon~12 million km (huge SMBH)
- Predicted byKarl Schwarzschild (1916) from GR
- First imagedM87* (2019), Sgr A* (2022) by EHT
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Why event horizon matters
- Defines BH. Boundary where light can't escape.
- GR test. Imaging confirms Einstein's predictions.
- Information paradox. Major theoretical question.
- Hawking radiation. Quantum effects at horizon.
- Cosmic censorship. Singularities should be hidden behind horizon.
- Astrophysics. Accretion disks form just outside.
- EHT science. Direct imaging of structure.
Common misconceptions
- Crossing horizon hurts. Locally smooth; no detection.
- Horizon is hard surface. Coordinate surface; nothing material there.
- You see horizon as edge. Difficult — extreme gravitational lensing.
- Time stops at horizon. External view; locally no.
- Horizon is black. Imaging shows complex structure from accretion.
- Once inside, you escape. No path leads out.
Frequently asked questions
What defines the event horizon?
Surface where escape velocity = c (speed of light). For non-rotating black hole (Schwarzschild metric): r_s = 2GM/c². Inside this radius, all paths lead inward to singularity. Light cones tilt past local horizon — there's no future direction that goes outward. Mathematical definition; physical interpretation as surface.
Is the event horizon physical?
Not a material surface. It's a coordinate surface — point in spacetime where geometry changes character. Crossing it is unremarkable locally (you wouldn't notice). But: external observer sees you slow asymptotically and become infinitely redshifted — appearing to "freeze" at horizon.
What happens crossing it?
Locally, smooth transition. Time progresses normally for you. After crossing, you're doomed — singularity inevitable. Tidal forces (spaghettification) at center; for stellar BH, tidal forces become extreme well before reaching horizon. Supermassive BH: gentle entrance, then long fall.
How was the event horizon imaged?
Event Horizon Telescope (EHT) — VLBI network across Earth — observes radio waves from accretion disks around BHs. Disk silhouetted against background. The "shadow" reflects the geometry: photons that pass close to horizon are captured. Result: dark center surrounded by bright ring — confirmed for M87* (2019), Sgr A* (2022).
How does rotation change things?
Kerr (rotating) black hole has more complex horizon structure. Event horizon plus ergosphere (region where space rotates with BH; can extract energy). Charged black holes (Reissner-Nordström) similar. Astrophysical BHs are mostly Kerr — described by mass and spin only.
Can we communicate from inside?
No. Photons emitted from inside cannot reach observers outside. All paths lead to singularity. Information appears lost — black hole information paradox: where does information go? Hawking radiation might preserve it, but mechanism unclear. Major problem in theoretical physics.
What's at the singularity?
According to GR, point of infinite density and curvature. But: quantum effects must dominate at Planck scale. Modern thinking: actual quantum singularity may not exist; quantum gravity theory needed. String theory, loop quantum gravity attempt to describe.