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

  • Thread starter nag555
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In summary, a black-hole is expected to absorb its own black body radiation due to its intense gravitational field, but any radiation that is directed outward can escape. However, Hawking radiation is different and not like black body radiation. As an object approaches the event horizon, its escape velocity must approach the speed of light.
  • #1
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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  • #2
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.
 
  • #3
in that case, any person who jumps on Earth radially outward should also escape gravity!
 
  • #5
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.
 
  • #6
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.
 
  • #7
thank you for your response...it is clear now..
 

1. What is a black hole and how does it form?

A black hole is a region in space where the gravitational pull is so strong that nothing, not even light, can escape from it. It forms when a massive star collapses in on itself, creating a singularity - a point of infinite density and zero volume.

2. Why can't black holes absorb all types of radiation?

Black holes can only absorb certain types of radiation, specifically electromagnetic radiation, such as light. This is because black holes have a boundary called the event horizon, which is the point of no return. Anything that crosses the event horizon is pulled into the black hole, including light. However, other forms of radiation, such as gravitational waves, are not affected by the event horizon and can escape from a black hole.

3. How does black body radiation relate to black holes?

Black body radiation is the thermal radiation emitted by an object based on its temperature. Black holes emit black body radiation due to the particles near the event horizon being heated by the intense gravitational pull. However, black holes have a maximum temperature, known as the Hawking temperature, and cannot emit radiation above this temperature.

4. Why can't black holes absorb all matter?

Black holes can only absorb matter that crosses the event horizon. Any matter that is outside of the event horizon will not be pulled into the black hole. In addition, black holes can only absorb matter that is within its gravitational reach, so objects that are too far away will not be affected.

5. Can black holes eventually dissipate or disappear?

Yes, black holes can eventually dissipate through a process called Hawking radiation. According to the laws of quantum mechanics, particles and antiparticles are constantly being created and destroyed in pairs near the event horizon. In some cases, one particle may be pulled into the black hole while the other escapes. This results in a net loss of energy for the black hole, causing it to eventually dissipate. However, this process is extremely slow and can take billions of years for a black hole to fully disappear.

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