Event Horizon vs Photon Sphere vs Singularity
Quick definitions (TL;DR)
- Event horizon: a boundary in spacetime beyond which signals cannot escape to distant observers (in the classical GR picture).
- Photon sphere: a region where light can orbit in unstable circular paths (idealized models).
- Singularity: a breakdown of the classical description (curvature/density blow‑ups in the model); not “a place you can see” from outside.
Event horizon (what it means)
The event horizon is not a physical shell. It’s a causal boundary: inside it, “future‑directed” paths can’t lead back out to infinity. That’s why it’s meaningful for escape and communication.
- Related: Schwarzschild radius explained
- Chapter link: Event horizon context (Chapter 4)
Photon sphere (why it matters for images)
The photon sphere matters because rays passing near it can loop around the black hole before escaping. That behavior is one reason black hole images show “photon rings” and repeated structure.
- Chapter link: Photon sphere (Chapter 4)
- Related: Gravitational lensing explained
Singularity (what it is, and what it isn’t)
In many classical solutions, the singularity is a region where curvature becomes unbounded and the classical equations stop being reliable. It’s best thought of as a sign that the model is incomplete in that regime (quantum gravity is expected to matter).
Importantly: you don’t see a singularity directly from outside the event horizon in the standard classical picture.
- Chapter link: Singularities (Chapter 4)
- Try: Black hole embedding diagram (intuitive geometry visualization)
FAQ
- Is the photon sphere the same as the event horizon? No. The photon sphere is outside the horizon in the simplest models.
- Is the singularity the same as the event horizon? No. The horizon is a boundary; the singularity is a breakdown region deeper in.
- Do all black holes have a photon sphere? Many idealized solutions do; details can change for rotating/charged cases.