<blockquote>
<p>if nothing can pass through the event horizon, how does the energy get out?</p>
</blockquote>
<p>Nothing gets out, but it doesn't <em>need</em> to get out. As I said <a href="https://astronomy.stackexchange.com/a/32677/16685">here</a>:</p>
<blockquote>
<p>The gravitational field of a black hole is sometimes described as a <a href="http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/black_gravity.html" rel="nofollow noreferrer">&quot;fossil field&quot;</a>. All matter &amp; energy falling into the black hole modifies the spacetime curvature as it approaches the event horizon. And once it crosses the event horizon it can no longer change the spacetime curvature outside the horizon, so those curvature changes are preserved (until something else comes along to add its own curvature changes).</p>
</blockquote>
<p>In other words, the energy of a black hole is &quot;stored&quot; in the spacetime curvature <em>outside</em> the event horizon.</p>
<p>A system of two black holes is similar, but more complex. As well as the rest masses of the two black holes, the combined system has gravitational potential energy and kinetic energy. The sum of those energies is known as the <a href="https://en.wikipedia.org/wiki/Specific_orbital_energy" rel="nofollow noreferrer">orbital energy</a>.</p>
<p>A gravitationally bound system has less energy than an unbound system. You have to &quot;pump&quot; energy into a bound system to separate its components. This energy is known as <a href="https://en.wikipedia.org/wiki/Gravitational_binding_energy" rel="nofollow noreferrer">gravitational binding energy</a>.</p>
<p>In Newtonian mechanics, the orbital energy is constant, but in GR the orbital energy gradually dissipates as the system radiates energy via gravitational waves. <a href="https://en.wikipedia.org/wiki/Two-body_problem_in_general_relativity#Gravitational_radiation" rel="nofollow noreferrer">Wikipedia</a> has equations for the loss of energy &amp; angular momentum, and reduction in orbital period, due to gravitational radiation.</p>
<p>As the orbit of the two black holes decays, the black holes gain kinetic energy, which is quite large when the orbital radius is small, but the system also loses considerable energy through gravitational radiation, especially when the orbit is eccentric or the orbital radius is small.</p>

CC BY-SA 4.0
2024-09-13T16:53:16.540Z
2024-09-13T16:53:16.540Z
PM 2Ring
<p>I recently read that GW150914, a black hole merger between black holes of 36 and 29 solar masses, converted around 3.0 solar masses into pure energy. My question is, where this this energy come from? Did it come from the immense gravitational wave energy around the black hole, and so the black hole lost mass as a result, or did it come from something else? Why are black hole mergers so energetic even if nothing can pass through the event horizon, how does the energy get out?</p>

CC BY-SA 4.0
2024-09-12T01:33:16.657Z
Consider the potential energy of a system of two black holes.
PM 2Ring
CC BY-SA 4.0
2024-09-13T05:07:29.273Z
@PM2RING does this potential energy get released as gravitational waves? If so does the mass loose energy as a result?
Astrovis
CC BY-SA 4.0
2024-09-13T13:20:56.213Z
@PM2Ring ahh ok, that helps me visualize it. I'd accept this as an answer if it was posted.
Astrovis
<black-hole><gravitational-waves><collision>
What makes black hole collisions so energetic?
Astrovis
# What makes black hole collisions so energetic?

(Asked by: username_0 on 2024-09-11)

I recently read that GW150914, a black hole merger between black holes of 36 and 29 solar masses, converted around 3.0 solar masses into pure energy. My question is, where this this energy come from? Did it come from the immense gravitational wave energy around the black hole, and so the black hole lost mass as a result, or did it come from something else? Why are black hole mergers so energetic even if nothing can pass through the event horizon, how does the energy get out?

**Tags:** black-hole, gravitational-waves, collision

> **Comments:**
> **username_1**: Consider the potential energy of a system of two black holes.  
> **username_0**: @username_1 does this potential energy get released as gravitational waves? If so does the mass loose energy as a result?  
> **username_0**: @username_1 ahh ok, that helps me visualize it. I'd accept this as an answer if it was posted.  

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## 1 answers

### Answer by username_1 on 2024-09-13. Score: 4 | Accepted answer

> if nothing can pass through the event horizon, how does the energy get out?

Nothing gets out, but it doesn't *need* to get out. As I said [here](https://astronomy.stackexchange.com/a/32677/16685):

> The gravitational field of a black hole is sometimes described as a ["fossil field"](http://math.ucr.edu/home/baez/physics/Relativity/BlackHoles/black_gravity.html). All matter & energy falling into the black hole modifies the spacetime curvature as it approaches the event horizon. And once it crosses the event horizon it can no longer change the spacetime curvature outside the horizon, so those curvature changes are preserved (until something else comes along to add its own curvature changes).

In other words, the energy of a black hole is "stored" in the spacetime curvature *outside* the event horizon.

A system of two black holes is similar, but more complex. As well as the rest masses of the two black holes, the combined system has gravitational potential energy and kinetic energy. The sum of those energies is known as the [orbital energy](https://en.wikipedia.org/wiki/Specific_orbital_energy).

A gravitationally bound system has less energy than an unbound system. You have to "pump" energy into a bound system to separate its components. This energy is known as [gravitational binding energy](https://en.wikipedia.org/wiki/Gravitational_binding_energy).

In Newtonian mechanics, the orbital energy is constant, but in GR the orbital energy gradually dissipates as the system radiates energy via gravitational waves. [Wikipedia](https://en.wikipedia.org/wiki/Two-body_problem_in_general_relativity#Gravitational_radiation) has equations for the loss of energy & angular momentum, and reduction in orbital period, due to gravitational radiation.

As the orbit of the two black holes decays, the black holes gain kinetic energy, which is quite large when the orbital radius is small, but the system also loses considerable energy through gravitational radiation, especially when the orbit is eccentric or the orbital radius is small.


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