<p>As soon as you start talking about &quot;forces&quot; you aren't doing the General Relativity formulation of gravity.  And you can't understand black holes without General Relativity.</p>
<p>A layman's understanding might be &quot;mass attracts mass&quot;. You are right that black holes show that there must be more to gravity than this.</p>
<p>The Newtonian formula  <span class="math-container">$F=m_1m_2/r^2$</span> is a useful approximation for masses and weak gravitational fields.  It is no good for massless particles and it is no good for strong gravitational fields.</p>
<p>Near a black hole, this formula is simply wrong. It doesn't clearly predict what happens to massless particles like photons.</p>
<p>However General Relativity can predict how light will bend when it passes through a strong gravitational field.  We say that the world line of light is a &quot;null geodesic&quot; in four dimensional spacetime  See <a href="https://physics.stackexchange.com/questions/23058/why-is-light-described-by-a-null-geodesic">Why is light described by a null geodesic</a></p>
<p>Light can't escape a black hole, because all the null geodesics in a black hole end on the singularity.</p>

CC BY-SA 4.0
2021-12-09T18:30:11.573Z
2021-12-09T18:50:47.327Z
2021-12-09T18:50:47.327Z
James K
<p>In General Relativity - which is what is required to describe a black hole - gravity is <em>NOT</em> a force.</p>
<p>Therefore the problem you have is in your second sentence. &quot;Mass attracts mass&quot; is a statement of Newton's universal law of gravitation, which indeed doesn't apply to photons and doesn't adequately predict the behaviour of light paths near massive objects - even if you fudge the issue by saying that the mass in question doesn't mean &quot;rest mass&quot;.</p>
<p>The fact that photons are massless does not stop them following curved geodesics in spacetime.</p>

CC BY-SA 4.0
2021-12-09T18:32:01.340Z
2021-12-09T18:32:01.340Z
ProfRob
<p>I understand light will follow the curved space that the BH is causing due to its mass. I also understand that mass attracts other mass but then photons are massless.</p>
<p>So 0(photon) x m2 (the BH) is equal to zero.  So how is zero force preventing the photon from escaping.</p>
<p>I am not a science denier, I'm just a layman asking questions because I am clearly missing something.</p>
<p>The layman understanding of gravity is &quot;mass attracts mass&quot; but aren't photons and black holes showing us more than this.</p>
<p>The photon is only following the curvature of space but we could say that everything follows the curvature.  Wouldn't &quot;mass curve spacetime and &quot;EVERYTHING&quot; follows the curve&quot; be more accurate than saying &quot;mass attracts mass.&quot; Do we need a second mass, doesn't <span class="math-container">$\frac{m_1m_2}{r2}$</span> equal zero when one of the &quot;objects&quot; is massless?</p>
<p>&quot;Mass attracts mass&quot; obviously works but isn't it only half the of what's happening?I'm struggling to even explain the question.</p>
<p>Sorry for the long drawn out question, I needed to make clear my lack of understanding so the answer doesn't cause more confusion.</p>
<p>Thanks for your patience, and all jokes aside, feel free to answer like a 10 year old because I'm confused.</p>

CC BY-SA 4.0
2021-12-09T14:52:15.893Z
Does this help, https://astronomy.stackexchange.com/questions/1375/does-gravity-slow-or-speed-light-up ? The accepted answer there has a lot of details.
Craig Constantine
CC BY-SA 4.0
2021-12-09T18:53:32.907Z
BTW, gravity isn't just caused by mass, it's caused by the combined effects of [stress, energy, and momentum](https://en.wikipedia.org/wiki/Stress%E2%80%93energy_tensor).
PM 2Ring
<black-hole><gravity><mass><newtonian-gravity>
How is light unable to escape a black hole if photons are massless?
Nuffsed81
# How is light unable to escape a black hole if photons are massless?

(Asked by: username_0 on 2021-12-09)

I understand light will follow the curved space that the BH is causing due to its mass. I also understand that mass attracts other mass but then photons are massless.

So 0(photon) x m2 (the BH) is equal to zero. So how is zero force preventing the photon from escaping.

I am not a science denier, I'm just a layman asking questions because I am clearly missing something.

The layman understanding of gravity is "mass attracts mass" but aren't photons and black holes showing us more than this.

The photon is only following the curvature of space but we could say that everything follows the curvature. Wouldn't "mass curve spacetime and "EVERYTHING" follows the curve" be more accurate than saying "mass attracts mass." Do we need a second mass, doesn't $\frac{m\_1m\_2}{r2}$ equal zero when one of the "objects" is massless?

"Mass attracts mass" obviously works but isn't it only half the of what's happening?I'm struggling to even explain the question.

Sorry for the long drawn out question, I needed to make clear my lack of understanding so the answer doesn't cause more confusion.

Thanks for your patience, and all jokes aside, feel free to answer like a 10 year old because I'm confused.

**Tags:** black-hole, gravity, mass, newtonian-gravity

> **Comments:**
> **username_1**: Does this help, https://astronomy.stackexchange.com/questions/1375/does-gravity-slow-or-speed-light-up ? The accepted answer there has a lot of details.  
> **username_2**: BTW, gravity isn't just caused by mass, it's caused by the combined effects of [stress, energy, and momentum](https://en.wikipedia.org/wiki/Stress%E2%80%93energy_tensor).  

---

## 2 answers

### Answer by username_3 on 2021-12-09. Score: 3

As soon as you start talking about "forces" you aren't doing the General Relativity formulation of gravity. And you can't understand black holes without General Relativity.

A layman's understanding might be "mass attracts mass". You are right that black holes show that there must be more to gravity than this.

The Newtonian formula $F=m\_1m\_2/r^2$ is a useful approximation for masses and weak gravitational fields. It is no good for massless particles and it is no good for strong gravitational fields.

Near a black hole, this formula is simply wrong. It doesn't clearly predict what happens to massless particles like photons.

However General Relativity can predict how light will bend when it passes through a strong gravitational field. We say that the world line of light is a "null geodesic" in four dimensional spacetime See [Why is light described by a null geodesic](https://physics.stackexchange.com/questions/23058/why-is-light-described-by-a-null-geodesic)

Light can't escape a black hole, because all the null geodesics in a black hole end on the singularity.


---

### Answer by username_4 on 2021-12-09. Score: 1

In General Relativity - which is what is required to describe a black hole - gravity is *NOT* a force.

Therefore the problem you have is in your second sentence. "Mass attracts mass" is a statement of Newton's universal law of gravitation, which indeed doesn't apply to photons and doesn't adequately predict the behaviour of light paths near massive objects - even if you fudge the issue by saying that the mass in question doesn't mean "rest mass".

The fact that photons are massless does not stop them following curved geodesics in spacetime.


---

