From far away, stars are tiny points of light.
But up close, stars are massive, seething,
fiery balls of burning gas. This fierce display
does not last forever. Eventually, the nuclear
fusion which powers the star will burn all
its fuel. Gravity then collapses the remaining
matter together. For very large stars, what
happens next is a display of extremes. First,
the star explodes in a supernova, scattering
much of its matter throughout the universe.
For a brief moment, the dying star outshines
its entire galaxy. But once the light fades
and darkness returns, the remaining matter
forms an object so dense that anything that
gets too close will completely disappear from
view. THIS is a black hole…
The idea of a black hole originated hundreds
of years ago. In 1687, Isaac Newton published
his landmark work known as The Principia.
Here he detailed his laws of motion and the
universal law of gravitation. Using a thought
experiment involving a cannon placed on a
very tall mountain, Newton derived the notion
of escape velocity. This is the launch speed
required to break free from the pull of gravity.
In 1783, the English clergyman John Michell
found that a star 500 times larger than our
sun would have an escape velocity greater
than the speed of light. He called these giant
objects “dark stars” because they could
not emit starlight. This idea lay dormant
for more than a century.
Then, in the early 20th century, Albert Einstein
developed two theories of relativity that
changed our view of space and time: the special
theory and the general theory. The special
theory is famous for the equation E=mc2. The
general theory painted a new and different
picture of gravity. According to the general
theory of relativity, matter and energy bend
space and time. Because of this, objects which
travel near a large mass will appear to move
along a curved path because of the bending
in spacetime. We call this effect gravity.
One consequence of this idea is that light
is also affected by gravity. After all, if
spacetime is curved, then everything must
follow along a curved path, including light.
Einstein published his general theory of relativity
in 1915. And while Newton’s theory of gravity
could be expressed using a simple formula,
Einstein’s theory required a set of complex
equations known as the “field equations.”
Only a few months after Einstein’s publication,
the German scientist Karl Schwarzschild found
a surprising solution. According to the field
equations, an extremely dense ball of matter
creates a spherical region in space where
nothing can escape, not even light. A curious
result, but did such things actually exist?
At first, the idea of a black sphere in space
from which nothing could escape was considered
purely a mathematical result, but one which
would not really happen. However, as the decades
passed, our understanding of the lifecycle
of stars grew. It was observed that some dying
stars became pulsars, another exotic object
predicted by theory. This suggested that dark
stars could actually be real as well. These
strange spheres were named “black holes,”
and scientists began the hard work of finding
them, describing them and understanding
how they are created.
But how do you find an object in space that
is completely black? Luckily, because black
holes have a large mass, they also have a
large gravitational field. So while we may
not be able to SEE a black hole, we can observe
its gravity pulling on its neighbors. With
this in mind, astronomers looked for a place
where a visible star and a black hole were
in close proximity to one another. One such
place is binary stars.
A binary star is a system of two stars orbiting
one another. We can spot them in several ways.
You can look for stars that change position
back and forth ever-so-slightly. Alternatively,
if you observe a binary star from the side,
the brightness will change when one star passes
behind the other. So it’s possible that
somewhere in space, there’s a binary star
consisting of a black hole and a visible star.
In fact, such binary systems have been observed!
Astronomers have found stars orbiting an invisible
companion. From the size of the visible star
and its orbit, astronomers calculated the
mass of its invisible neighbor. It fit the
profile of a black hole.
Since we can’t see a black hole, is
there a way to find its size? From Einstein’s
field equations, we know that given the mass
of a black hole, we can determine the size
of the sphere that separates the region of
no escape from the rest of space. The radius
of this sphere is called the Schwarzschild
radius in honor of Karl Schwarzschild. The
surface of the sphere is called the event
horizon. If anything crosses the event horizon,
it’s gone forever — hidden from the rest
of the universe.
This means, once you know the MASS of a black
hole, you can compute its SIZE using a simple
formula. And it’s actually quite easy to
measure the mass of a black hole. Just take
a standard issue space probe and shoot it
into orbit around the black hole. Just like
any other system of orbiting bodies — like
the Earth orbiting the Sun, or the Moon orbiting
the Earth — the size and period of the orbit
will tell you the mass of the black hole.
If you don’t have a space probe handy, then
compute the mass and orbit of a companion
star and use that to find the Schwarzschild
Black holes come in many sizes. If it was
made from a dying star, then we call it a
“stellar mass” black hole, because its
mass is in the same range as stars. But we
can go bigger – much bigger. And to do so,
we are going to visit the center of a galaxy.
Galaxies can contain billions and billions
of stars, all orbiting a central point. Scientists
now believe that in the center of most galaxies
lives a black hole which we call a “supermassive
black hole,” because of its tremendous mass.
The size can vary from hundreds of thousands
to even billions of solar masses. For example,
at the center of our own Milky Way galaxy
is a supermassive black hole with a mass 4
million times that of our sun.
Black holes have another property we can measure
– their spin. Just like the planets, stars
rotate. And different stars spin at different
speeds. Imagine we can adjust the size of
this star but keep the mass constant. If we
increase the radius, the spinning slows down…
If we decrease the size, the spinning speeds
up. But while the rotational speed can vary,
the angular momentum never changes – it remains
constant. Even if the star were to collapse
into a black hole, it would still have angular
momentum. We could measure this by firing
two probes into opposite orbits close to the
black hole. Because of their angular momentum,
black holes create a spinning current in spacetime.
The probe orbiting along with the current
will travel faster than the one fighting it,
and by measuring the difference in their orbital
periods we can compute the black hole’s
This spacetime current is so extreme it creates
a region called the ergosphere where nothing,
including light, can overcome it. Inside the
ergosphere, nothing can stand still. Everything
inside this region is dragged along by the
spinning spacetime. The event horizon fits
inside the ergosphere, and they touch at the
poles. So in one sense, black holes are like
whirlpools of spacetime. Once inside the ergosphere,
you are caught by the current. And after you
cross the event horizon, you disappear.
One final property of black holes we can measure
is electric charge. While most of the matter
we encounter in our day-to-day lives is uncharged,
a black hole may have a net positive or negative
charge. This can easily be measured by seeing
how hard the black hole pulls on a magnet.
But charged black holes are not expected to
exist in nature. This is because the universe
is teeming with charged particles, so a charged
black hole would simply attract oppositely
charged particles until the overall charge
There are 3 fundamental properties of a black
hole we can measure – mass, angular momentum,
and electric charge. It is believed that once
you know these three values, you can completely
describe the black hole. This result is humorously
known as the “no hair theorem,” since
other than these 3 properties, black holes
have no distinguishing characteristics. It’s
not a blonde, brunette, or a redhead.
We now have a good idea of a black hole from
the outside, but what does it look like on
the inside? Unfortunately we can’t send
a probe inside to take a look. Once any instrument
crosses the event horizon, it’s gone. But!
Don’t forget we have Einstein’s field equations.
If these correctly describe spacetime
outside the black hole, then we can use them
to predict what’s going on inside as well.
To solve the field equations, scientists considered
two separate cases: rotating black holes,
and non-rotating black holes. Non-rotating
black holes are simpler and were the first
to be understood. In this case, all the matter
inside the black hole collapses to a single
point in the center, called a singularity.
At this point, spacetime is infinitely warped.
Rotating black holes have a different interior.
In this case, the mass inside a black hole
will continue to collapse, but because of
the rotation it will coalesce into a circle,
not a point. This circle has no thickness
and is called a ring singularity.
Black hole research continues to this day.
Scientists are actively investigating the
possibility that black holes appeared right
after the big bang, and the idea that black
holes can create bridges called wormholes
connecting distant points of our universe.
We know a great deal about black holes, but
there are many mysteries still to be solved.
It’s a little known fact that all YouTube
videos are stored in a special fabric called
playtime. When you watch a video, it sends
ripples of energy throughout playtime. And
when you subscribe to a channel, it creates
a teeny, tiny black hole. So if you like
Black Holes, then you know what to do…