Imagine throwing a ball up into the air - the harder you throw it, the faster it travels,
and the higher it goes before falling back down.
If you threw it hard enough, it would actually fly away from the Earth, like a rocket!
But what if the Earth was condensed to the volume of a marble?
No matter how hard you threw your ball, it would always fall back down to you - it could
never escape!
This marble-sized Earth has essentially become a black hole -- a region of space with a gravitational
field so strong that nothing -- not even light -- can escape.
Let’s go back to when the Earth was at its normal volume.
The speed that an object must be traveling at when it leaves the surface of the Earth
is called the Earth’s “escape velocity.”
The escape velocity of the Earth depends on the gravitational pull that the Earth exerts
on the object trying to “escape.”
Gravity is an attractive force between objects with mass, and the gravitational force between
two objects depends on their mass and the distance between their centers.
The greater their masses and the shorter the distance between each object, the greater
gravitational pull one exerts on the other and therefore, the higher the escape velocity
must be.
Since the mass of the Earth is many times larger than the objects we launch from it,
the mass of the object is considered insignificant, and scientists only consider the Earth’s
mass when determining how fast an object (like a rocket, or your ball) has to go to escape
the Earth.
In such an estimation, the Earth’s escape velocity is about 38,617 kilometers per hour
— roughly 50 times faster than the average speed of an airplane!
Now let’s revisit our marble-sized Earth.
In this scenario, the Earth is smaller in diameter but unchanged in mass.
So, the escape velocity will be higher, since an object on the Earth’s surface would be
much closer to its center.
If we were to continue squishing the Earth, the escape velocity would approach the speed
of light!
Nothing can move faster than the speed of light, so your ball -- or any object for that
matter -- could never achieve a high enough velocity to escape the compressed Earth.
And this is how the Earth would become a black hole.
A hundred years ago, the existence of black holes was speculation.
Scientists had developed numerous theories that used black holes as tools for understanding
the fundamental laws of the universe - from explaining the beginning of time to the movement
of planets.
In this way, scientists had mathematical proof that black holes must exist, but little physical
evidence to support the theory - after all, how can you see something from which no light
escapes?
Astrophysicists theorised that they could instead try to capture the “silhouette of
a black hole.”
Since black holes exert an extremely strong gravitational pull, any nearby matter is rapidly
drawn towards them, creating an accretion disk that spirals inwards.
The accretion disk exists just outside of a black hole’s event horizon, which can
be thought of as “the point of no return” -- anything that passes the event horizon
will be sucked in by the intense gravity of the black hole.
Due to frictional heating, the fast-moving matter in the accretion disk glows, emitting
light in the X-ray spectrum.
This light is bent by the gravity of the black hole, creating a glowing ring that we can
see.
In 2019, scientists did just that -- they observed a ring of light emanating from an
accretion disk.
This visually confirmed the existence of black holes for the first time in history!
Using an array of 8 ground-based telescopes across our planet, astrophysicists captured
images of a massive black hole at the center of Messier 87, a galaxy 55 million light-years
from Earth.
This major breakthrough served to support decades-old scientific theories about black
holes, and even some of the stories that science fiction authors had written about them!
With increasingly complex technology that allows us to observe the depths of our universe
at higher resolution, scientists increase our understanding of cosmic phenomena and
find new questions to ask, ranging from the potential to harvest energy from a black hole’s
accretion disk, to discovering plasma “jets” -- streams of energy and hot matter that spew
from the center of black holes.
With a gravitational force so immense, it’s no wonder that scientists can’t escape the
irresistible mysteries hidden deep within black holes!