<p>Gravity is a force, and it need not have a "speed"<sup>1</sup>.</p>

<p>A gravitational body sets up a gravitational field around itself. Note that by the time the light wave approaches the body, <em>this field has already been set up</em>. The gravitational force does not need to "reach out" and "catch up to the light" &mdash; it's already there.</p>

<p>In fact, it's not even really a force (though we can treat it as one to an approximation). Gravity bends the fabric of spacetime around it, messing with the meaning of a "straight line". From different reference frames, different lines appear straight. To the light wave, the inspiraling path seems "straight" and thus it follows that path. To an external observer, the light wave is not going straight.</p>

<p><sup>1. It does, but that's the speed at which changes to the field propagate, not the speed at which it "catches up" with other things. Changes in the gravitational field propagate at lightspeed.</sup></p>

CC BY-SA 3.0
2013-09-25T04:49:01.333Z
2013-09-25T04:54:50.397Z
2013-09-25T04:54:50.397Z
Manishearth
<p>Well, I have no astronomical background, so in layman's terms, </p>

<blockquote>
  <p>If nothing travels at the speed of light, except light, how can a black hole also pull light into itself?</p>
</blockquote>

<p>I believe OP is confused over how a gravity wave can capture light if both can travel at most at the speed C. </p>

<p>It does not work quite that way. Say you move a massive BH from position X to position Y and an observer is at 1 LightYear away from Y. Then, the observer will not feel the gravitation effect of BH until 1 year because that's how much it will take a gravity waves to reach to observer. That is, its changes in gravitational field "Delta" propagate at the speed of light.</p>

<p>As others have mentioned, gravity bends/shapes the spacetime fabric, so after one year, at observer, spacetime is bend according to the gravity of BH at Y. Thus, any light that passes near to the observer will bend as it follows the spacetime. Plus, the curvature of space BH create in their vicinity is so severe that space is wrapped completely around itself and hence light can not escape.</p>

<p>Refer to <a href="http://clarkplanetarium.org/cosmic-quiz-why-cant-light-escape-a-black-hole/" rel="nofollow noreferrer">this</a> for simple explanation.</p>

CC BY-SA 3.0
2016-09-07T10:16:13.043Z
Please edit this so it is an answer to the question or post it as a comment.
adrianmcmenamin
CC BY-SA 3.0
2016-09-07T06:18:01.047Z
2016-11-09T16:45:51.750Z
2016-11-09T16:45:51.750Z
Ankit
<p>Is it because it has super massive gravity and energy? Or is it some sort of an anti-light component in it because it used to be a star?</p>

CC BY-SA 3.0
2016-11-08T19:35:38.380Z
Exact question Answered here http://www.phys.vt.edu/~jhs/faq/blackholes.html#q4
ProfRob
CC BY-SA 3.0
2013-09-25T00:41:55.900Z
Why do you say "If nothing travels at the speed of light, except light"? Any massless particle and it's associate field travels at c. It is also the speed of gravity. I think the question is based on an incorrect premise.
Jeremy
<black-hole>
If nothing travels at the speed of light, except light, how can a black hole also pull light into itself?
Timtech
# If nothing travels at the speed of light, except light, how can a black hole also pull light into itself?

(Asked by: username_0 on 2013-09-25)

Is it because it has super massive gravity and energy? Or is it some sort of an anti-light component in it because it used to be a star?

**Tags:** black-hole

> **Comments:**
> **username_2**: Why do you say "If nothing travels at the speed of light, except light"? Any massless particle and it's associate field travels at c. It is also the speed of gravity. I think the question is based on an incorrect premise.  
> **username_1**: Exact question Answered here http://www.phys.vt.edu/~jhs/faq/blackholes.html#q4  

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## 2 answers

### Answer by username_3 on 2013-09-25. Score: 12

Gravity is a force, and it need not have a "speed"1.

A gravitational body sets up a gravitational field around itself. Note that by the time the light wave approaches the body, *this field has already been set up*. The gravitational force does not need to "reach out" and "catch up to the light" — it's already there.

In fact, it's not even really a force (though we can treat it as one to an approximation). Gravity bends the fabric of spacetime around it, messing with the meaning of a "straight line". From different reference frames, different lines appear straight. To the light wave, the inspiraling path seems "straight" and thus it follows that path. To an external observer, the light wave is not going straight.

1. It does, but that's the speed at which changes to the field propagate, not the speed at which it "catches up" with other things. Changes in the gravitational field propagate at lightspeed.


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### Answer by username_4 on 2016-09-07. Score: -1

Well, I have no astronomical background, so in layman's terms,

> If nothing travels at the speed of light, except light, how can a black hole also pull light into itself?

I believe OP is confused over how a gravity wave can capture light if both can travel at most at the speed C.

It does not work quite that way. Say you move a massive BH from position X to position Y and an observer is at 1 LightYear away from Y. Then, the observer will not feel the gravitation effect of BH until 1 year because that's how much it will take a gravity waves to reach to observer. That is, its changes in gravitational field "Delta" propagate at the speed of light.

As others have mentioned, gravity bends/shapes the spacetime fabric, so after one year, at observer, spacetime is bend according to the gravity of BH at Y. Thus, any light that passes near to the observer will bend as it follows the spacetime. Plus, the curvature of space BH create in their vicinity is so severe that space is wrapped completely around itself and hence light can not escape.

Refer to [this](http://clarkplanetarium.org/cosmic-quiz-why-cant-light-escape-a-black-hole/) for simple explanation.

> **Comments:**
> **username_5**: Please edit this so it is an answer to the question or post it as a comment.  

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