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 radius.

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 angular momentum.

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 is neutralized.

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...