# The Naked Scientists Forum

### Author Topic: why can't light escape a black hole.  (Read 1817 times)

#### thebrain13

• Sr. Member
• Posts: 442
##### why can't light escape a black hole.
« on: 16/06/2012 00:56:44 »
They say that a black hole occurs when the escape velocity exceeds light speed. My question is, why does the escape velocity apply to light when it's velocity is a constant? What does an escape velocity mean to an object that can't slow down?

#### Pmb

• Neilep Level Member
• Posts: 1838
• Physicist
##### Re: why can't light escape a black hole.
« Reply #1 on: 16/06/2012 10:08:28 »
I don't understand how the constancy of the speed of light is a problem in this case. Can you clarify? Thanks.

#### thebrain13

• Sr. Member
• Posts: 442
##### Re: why can't light escape a black hole.
« Reply #2 on: 16/06/2012 16:34:58 »
my point is light is not like a ball that you throw upwards, then it slows down and falls back to the ground. It always travels at the speed of light. Are we suggesting that when a photon leaves a body with sufficient gravity it travels upwards for a bit, and then slows down and falls back to the body? Cause that's what it means when an object is under the escape velocity.

Light doesn't travel slower when it exits a body with larger gravity, so how can gravity be responsible for stopping light in its' tracks?

#### Guthers

• Jr. Member
• Posts: 36
##### Re: why can't light escape a black hole.
« Reply #3 on: 18/06/2012 22:42:39 »
my point is light is not like a ball that you throw upwards, then it slows down and falls back to the ground. It always travels at the speed of light. Are we suggesting that when a photon leaves a body with sufficient gravity it travels upwards for a bit, and then slows down and falls back to the body? Cause that's what it means when an object is under the escape velocity.

Light doesn't travel slower when it exits a body with larger gravity, so how can gravity be responsible for stopping light in its' tracks?

Correct, the light doesn't slow down at all, which is a common misconception. However, if you watch from a distance it might appear to.

The light gets infinitely red-shifted by the gravitational field, and therefore has no energy left by the time it gets to the event horizon.

General relativity, bloody hell!

#### CZARCAR

• Hero Member
• Posts: 686
##### Re: why can't light escape a black hole.
« Reply #4 on: 18/06/2012 23:13:43 »
sounds like massless fotons affected by gravity?

#### CZARCAR

• Hero Member
• Posts: 686
##### Re: why can't light escape a black hole.
« Reply #5 on: 18/06/2012 23:19:23 »
ok & maybe=G affects the time/space so it results in?

#### thebrain13

• Sr. Member
• Posts: 442
##### Re: why can't light escape a black hole.
« Reply #6 on: 18/06/2012 23:26:52 »
Okay, quick question though. What happens to the energy that a photon used to have when it is redshifted? Where does it go and in what form?

#### CZARCAR

• Hero Member
• Posts: 686
##### Re: why can't light escape a black hole.
« Reply #7 on: 18/06/2012 23:32:44 »
gravity?

#### The Naked Scientists Forum

##### Re: why can't light escape a black hole.
« Reply #7 on: 18/06/2012 23:32:44 »