#### Black Holes

What happens when two black holes collide?

Anonymous

0

What happens when two black holes collide?

Qalaxia Knowlege Bot

0

I found an answer from www.scientificamerican.com

Supermassive **Black Holes Collide** in Galactic Merger Grand Finales ...

Nov 7, 2018 **...** That research suggested that **black holes** at the cores of **colliding** ... these
**colliding** galaxies hide the final stage of **two** merging galactic nuclei.

For more information, see Supermassive **Black Holes Collide** in Galactic Merger Grand Finales ...

S. Prem Kumar

7

Excellent question .. and a very topical one too ! You have no doubt heard about the much celebrated detection of **gravitational wave** signals by the **LIGO **collaboration:

Gravitational waves are ripples in spacetime. If you imagine spacetime as a rubber-sheet which bends and curves, sufficiently violent events involving really massive objects, generate ripples in this fabric which travel at the speed of light and can be detected by us as the ripple deforms spacetime while passing through us.

Detailed properties of gravitational waves follow from **Einstein**'s equations of **general relativity **which correctly describes all gravitational phenomena in our universe.

**What does this have to do with colliding black holes** ? I'm going to make this a bit long, but hopefully not too boring, because there are so many interesting bits in here.

__In a nutshell, when black holes collide they can give off lots of energy and they do this in the form of ____gravitational waves____.__

If the black holes approach each other "off centre" (like pool balls set to miss each other), they attract and start spiralling in towards each other and whilst doing so give huge amounts of energy (you'll have to read to the end if you want to know just how much !) and __ when they finally merge they produce a __** new black hole**, more massive then either of the two that went into the merger.

You'd be perfectly right to ask why** must **they produce another black hole ?

**************************************

But we're getting ahead of ourselves...

# First, what** IS **a black hole? It's something that's formed by bringing together lots

of matter together (like in a supermassive star) whose gravitational force is so

strong that the matter collapses in on itself and curves the spacetime ("rubber sheet")

around it so heavily that even light can't escape. The resulting **black hole** is

featureless, characterised only by its** "horizon"**. Nothing from inside a black hole

can leave the horizon, not even light.

# The "horizon" of a black hole is spherical, and a bit oblate if the original star was

spinning around its axis.

# Black holes look the same no matter what went into making them. You could

make a black hole out of trillions of mangoes or from a Hydrogen-rich star, and

you wouldn't be able to make out by looking at the black hole, what went into making it.

So we say **black holes scramble information.**

# Because of this scrambling property, black holes have a property that's called **Entropy **in thermodynamics !

In fact, black holes give out a teeny bit of quantum radiation just like any regular hot object -- something first shown

in a beautiful quantum calculation by Stephen Hawking.

# Hawking and Bekenstein also showed that the **Entropy **of a black hole is given by the **Area of the Horizon...** and** ...**wait for it... the Area = 4\ \pi\ \left(Radius\right)^2 .

For black holes, the horizon radius is proportional to the Mass of the Black Hole, so **Entropy **of the Black Hole is proportional to the **square of the mass**

**# **Hawking and Penrose proved a beautiful theorem which says that if two black holes with horizon areas A_1 and A_2 merge to form a new black hole, then the horizon area A_{new} **must satisfy:**

A_{new\ }>\ A_1\ +\ A_2

which is actually the black hole version of the** Second Law of Thermodynamics, **which says that the entropy of an isolated system must increase.

In terms of the masses M_1 , M_2 this means that:

\left(M_{new}\ \right)^2\ \ \ \ >\ \ \left(M_1\right)^2\ \ \ +\ \ \left(M_2\right)^2

# Conservation of mass-energy by Einstein's formula E\ =\ M\ c^2 also tells us that M_{new} must be smaller than M_1+\ M_2, with the deificit energy radiated off in gravitational waves !

# Let's get to the staggering, mind-blowing numbers now ! The first black hole merger event that LIGO detected in 2016 involved 2 black holes, one with mass 36 times the sun's mass (!) and the other with 29 times the solar mass, which spiralled and merged in 0.2 seconds(!!) to produce a new black hole with mass 62 times the sun's mass. Let's calculate the power output (energy released per unit second):

\left(29+36-62\ \right)\ \times2\times10^{30}\ kg\ \times\ \frac{\left(3\times10^8\ \frac{m}{s}\right)^2}{0.2\ s}

which is 2.7\times10^{48\ }Watts !!! To get a sense of how mind-bogglingly big this power output is, consider the power output of a typical star like the sun = 10^{26}\ Watts. Total number of stars in the observable universe is about 10^{21}. So the power output from the black hole merger exceeds that of the entire universe !!!

# You know what.. I forgot to say why the merger **must** produce a black hole ...it's simple.. the second law of thermodynamics says that the new object must have an entropy bigger than the sum of the original entropies. Only black holes can carry entropy, so the final object must be a black hole with bigger entropy than the sum of the original entropies.

Mahesh Godavarti

0

Beautiful!

Qalaxia QA Bot

1

I found an answer from www.quora.com

What **happens when two black holes** are about to **collide**? - Quora

Before I answer this, we should cover some ground about **black holes**. **Black**
**Holes** are simply put; just celestial bodies with such a high mass confined within
a ...

For more information, see What **happens when two black holes** are about to **collide**? - Quora

Qalaxia QA Bot

1

I found an answer from www.quora.com

Can matter or energy escape when **two black holes collide**? - Quora

What **happens** during the merger is some of the energy that is contained in the
marginally ... Can **two black holes** just **collide** at their combined escape velocity?

For more information, see Can matter or energy escape when **two black holes collide**? - Quora

Qalaxia QA Bot

1

I found an answer from astronomy.stackexchange.com

milky way - What **happens when two black holes collide** ...

There are many hypothesis saying that there will a **collision** between the milky
way and Andromeda galaxy...so what **happens when two black** ...

For more information, see milky way - What **happens when two black holes collide** ...

Qalaxia QA Bot

0

I found an answer from physics.stackexchange.com

general relativity - **Collision** of a **black hole** & a white **hole** - Physics ...

This can be interpreted as a **black hole** in an universe where second law of ...
result of the **collision** between **two black holes**" so the answer is what I said
above.

For more information, see general relativity - **Collision** of a **black hole** & a white **hole** - Physics ...

Qalaxia Knowlege Bot

0

I found an answer from en.wikipedia.org

Binary **black hole** - Wikipedia

A binary **black hole** (BBH) is a system consisting of **two black holes** in close orbit
around each ... When **two** galaxies **collide**, the supermassive **black holes** at their
centers do not ... The question of how this **happens** is the "final parsec problem".

For more information, see Binary **black hole** - Wikipedia

Qalaxia Knowlege Bot

0

I found an answer from apod.nasa.gov

APOD: 2015 October 20 - When **Black Holes Collide**

Oct 20, 2015 **...** Explanation: What **happens when two black holes collide**? This extreme scenario
likely **occurs** in the centers of some merging galaxies and ...

For more information, see APOD: 2015 October 20 - When **Black Holes Collide**

Qalaxia Knowlege Bot

0

I found an answer from www.scientificamerican.com

The Beauty of **Black Hole Collisions** - Scientific American

How can the ripples from a **collision** of **two black holes** help us understand the
universe? Nergis ... Now imagine **two black holes** coming closer, orbiting each
other and ending that dance in a big **collision**. ... How do we make this **happen**?

For more information, see The Beauty of **Black Hole Collisions** - Scientific American

Qalaxia Knowlege Bot

-1

I found an answer from www.nasa.gov

Astronomers Unveil Growing **Black Holes** in **Colliding** Galaxies | NASA

These images reveal the final stage of unions between pairs of galactic nuclei in
the messy cores of **colliding** galaxies.

For more information, see Astronomers Unveil Growing **Black Holes** in **Colliding** Galaxies | NASA

Qalaxia Knowlege Bot

-1

I found an answer from en.wikipedia.org

First observation of gravitational waves - Wikipedia

The first direct observation of gravitational waves was made on 14 September
2015 and was ... It was also the first observation of a binary **black hole** merger,
demonstrating ... File:Warped Space and Time Around **Colliding Black Holes** (
Courtesy ... of the merger of **two** compact objects such as neutron stars or **black**
**holes**.

For more information, see First observation of gravitational waves - Wikipedia