Black Hole & Black Body Radiation: Why Can't It Absorb?

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Discussion Overview

The discussion centers around the question of why a black hole cannot absorb its own black body radiation, exploring concepts related to black body radiation, gravitational effects, and the nature of black holes.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant questions the ability of a black hole to absorb its own black body radiation, suggesting that its intense gravitational field should prevent any emission.
  • Another participant argues that a black hole does absorb much of its own radiation, but radiation directed outward can escape.
  • A subsequent post draws an analogy to a person jumping radially outward from Earth, questioning whether they would also escape gravity under similar conditions.
  • Further clarification is provided that radiation emitted inside the event horizon cannot escape, while radiation emitted outside can escape if directed appropriately.
  • Another participant explains that the escape velocity near the event horizon approaches the speed of light, which complicates the analogy to jumping from Earth.
  • A later reply expresses gratitude for the clarification, indicating some understanding has been achieved.

Areas of Agreement / Disagreement

Participants express differing views on the nature of black hole radiation absorption and the implications of gravitational effects, indicating that multiple competing views remain and the discussion is not fully resolved.

Contextual Notes

There are unresolved assumptions regarding the nature of black body radiation in the context of black holes and the specifics of escape velocity related to gravitational fields.

nag555
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why can't a black-hole absorb its own black body radiation? (Black body radiation is also and electro magnetic field like light and so a black-hole with its intense gravitational field is expected not to emit even blackbody radiation also!)
 
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Hi nag555, welcome to physicsforums!
A black-hole does absorb a lot of its own radiation; basically all of it that isn't directed exactly (radially) outward----but the radiation that is directed outward, is able to escape.
 
in that case, any person who jumps on Earth radially outward should also escape gravity!
 
nag555 said:
why can't a black-hole absorb its own black body radiation? (Black body radiation is also and electro magnetic field like light and so a black-hole with its intense gravitational field is expected not to emit even blackbody radiation also!)

Any radiation released inside the event horizon will not escape. Radiation released outside it by in-falling material can escape if it is emitted in the right direction. Hawking radiation is different and is not like black body radiation. See the link in the previous post for information on it.
 
nag555 said:
in that case, any person who jumps on Earth radially outward should also escape gravity!
Why? People can't jump as fast as light...

The closer to an event horizon an object is, the faster it needs to move (for our purposes, in the radial direction) to escape. As the object approaches the event horizon, the escape (radial) velocity approaches the speed of light.
 
thank you for your response...it is clear now..
 

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