Chapter 33, Problem 20TE

To determine

Difference between a black hole’s photon sphere and event horizon.

Answer to Problem 20TE

Photon sphere is a region around a black hole where photons move in circular orbits, whereas event horizon is a mathematical boundary around a black hole beyond which light cannot escape.

Explanation of Solution

The difference between a black hole’s photon sphere and event horizon are following:

Sr. No.	Photon sphere	Event horizon
1	A region around a black hole where photons move in circular orbits.	A mathematical boundary around a black hole beyond which light cannot escape
2	Light from an object inside the photon sphere can escape.	The escape velocity of an object is greater than the speed of light inside the event horizon.
3	The photon sphere is at a larger distance than the event horizon.	The event horizon is closer to the black hole than the photon sphere.

Table (1)

Conclusion:

Therefore, Photon sphere is a region around a black hole where photons move in circular orbits, whereas event horizon is a mathematical boundary around a black hole beyond which light cannot escape.