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:


https://www.ligo.org


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
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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
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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
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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
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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
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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