When enormous amounts of gas fall inside
a black hole
the gas glows. A lot! So much so that in
the absence of light pollution you can
see the glow with a naked eye from as
far as 30 billion light years.
Such objects are called quasars. But how do they work?
Why does the gas glow?
Well, the gas particles reach very high velocities due
to the high gravitational acceleration
from the black hole. And high velocities
of gas particles means high friction
between them. Which itself results in the
temperature of the gas to increase. And
by the Wein's displacement law, we know
that the hotter an object is the more it
glows. But there is a catch! The naked eye
cannot understand the difference between
a quasar and a regular star in the sky.
In fact if you don't know what to look
for you can't even distinguish a planet
from a star for yourself. So how do
scientists know for sure that these
stars in the sky are black holes with a
lot of gas falling into them, and not
stars which are stars like the Sun? Well,
I already gave a hint quasars are very
far away the more distant ones are 10, 20
or even 30 billion light-years away. For
reference the radius of the observable
universe is 45 billion light years. We
measure the distance via the redshifted
light they emit. And the reason they are
visible to us from so far away is
because they have very high luminosities.
As high as one trillion times that of the
Sun. There is no way a regular star be so
bright from so far away. The brightest
stars have luminosities at most
1 million that of the Sun. So quasars
have to be supermassive black holes
there's no way around that.
Sometimes it is possible to also find
stellar black holes with this method.
Sometimes a binary system composed by a
star and a black hole will shine in
x-rays. That's because matter from the
star falls inside the black hole heats
up and releases x-rays. Not so bright as
a quasar and not for so long but enough
to be observable from Earth.
These systems are called x-ray binaries.
X-ray binaries can be a star with a
black hole or a star with a neutron star.
But the evidence of black holes don't
stop here. Another proof of black holes
is the motion of stars. We can't see
black holes, which aren't quasars, but we
certainly can see stars. Some stars in
the center of our galaxy are orbiting
something. But that something doesn't
emit light and there are nearby stars
sometimes get really close to it. And by
knowing the mass of the stars and some
orbital mechanics, it turns out that that
something has mass as much as 4 million suns.
Well what kind of object other than
a black hole doesn't emit light and has
mass as much as 4 million Suns. None! It has
to be a black hole and specifically a
supermassive black hole.
Did you know light is affected by gravity?
According to general relativity
the acceleration of a photon due to
gravity is four times the massive object
times the gravitational constant divided
by the distance of the photon and the
object times the speed of light squared.
And this is a very small number. G is
very small, 6.67 * 10^-11
and it is on the
numerator and C squared is very large
9*10^16 and it is on the
denominator so for a photon to be
significantly affected by an object
gravity. 1. the object has to be massive.
and 2. the photon needs to get really
close to it. This is the explanation why
you often see artistic pictures of black
holes like this.
To this day however we have never observed such an effect with
black holes alone. We have only done it
with either galaxies galaxy clusters or
quasars which have black holes in them
but they don't contribute to the total
mass of these objects very much. Dark
matter does. But they are predicted by
the theory so we should see them sooner
or later.
No black hole conversation
would be complete without mentioning
gravitational waves. In September 14 2015
the LIGO team detected gravitational
waves for the first time in human
history. The waves were generated by two
black holes spiraling and eventually
merging. But could the source of these
waves be anything else other than black
holes? like neutron stars or regular
stars? No because the mass of this object
spiraling were 36 in 29 the mass of the
Sun. And there is no way neutron star
could be so massive. The mass of the
heaviest neutron star is just below
three times the mass of the Sun. Let
alone 29. Could they be regular stars then?
Such massive stars do exist. Again no!
Regular stars have significantly larger
volume than
stellar-mass black holes. And if they
spiraled they would merge before they
had the chance to reach high enough angular velocities to generate gravitational
waves detectable by the current LIGO.
This is especially true for 30 solar
mass stars. Which are gigantic. The waves were produced by stellar black holes, period.
To this day we have found three black holes with this method.
But there are plans to directly
observe a black hole. Yes you heard me
right DIRECTLY. How? By using radio
telescopes from of all over the globe
and a little bit of math a technique
which is called very long baseline
interferometry. We can combine images
from multiple telescopes all pointing to
our black hole at the center of our
galaxy and produce an image with better
detail than ever before. This project is
called the event horizon telescope. And
it is going to give us a real picture of
the surroundings of our black hole.
Because we are tired of seeing images
like these all the time which aren't
anything more than just art.
Will that
image give us information we don't know?
Will it expose general relativity to
experimental evidence of inaccuracy?
That remains to be seen!
