The sign of a truly great scientific theory
is by the outcomes it predicts when you run
experiments or perform observations.
And one of the greatest theories ever proposed
was the concept of Relativity, described by
Albert Einstein in the beginning of the 20th
century.
In addition to helping us understand that
light is the ultimate speed limit of the Universe,
Einstein described gravity itself as a warping
of spacetime.
He did more than just provide a bunch of elaborate
new explanations for the Universe, he proposed
a series of tests that could be done to find
out if his theories were correct.
One test, for example, completely explained
why Mercury’s orbit didn’t match the predictions
made by Einstein.
Other predictions could be tested with the
scientific instruments of the day, like measuring
time dilation with fast moving clocks.
Since gravity is actually a distortion of
spacetime, Einstein predicted that massive
objects moving through spacetime should generate
ripples, like waves moving through the ocean.
Just by walking around, you leave a wake of
gravitational waves that compress and expand
space around you.
However, these waves are incredibly tiny.
Only the most energetic events in the entire
Universe can produce waves we can detect.
It took over 100 years to finally be proven
true, the direct detection of gravitational
waves.
In February, 2016, physicists with the Laser
Interferometer Gravitational Wave Observatory,
or LIGO announced the collision of two massive
black holes more than a billion light-years
away.
Any size of black hole can collide.
Plain old stellar mass black holes or supermassive
black holes.
Same process, just on a completely different
scale.
Let’s start with the stellar mass black
holes.
These, of course, form when a star with many
times the mass of our Sun dies in a supernova.
Just like regular stars, these massive stars
can be in binary systems.
Imagine a stellar nebula where a pair of binary
stars form.
But unlike the Sun, each of these are monsters
with many times the mass of the Sun, putting
out thousands of times as much energy.
The two stars will orbit one another for just
a few million years, and then one will detonate
as a supernova.
Now you’ll have a massive star orbiting
a black hole.
And then the second star explodes, and now
you have two black holes orbiting around each
other.
As the black holes zip around one another,
they radiate gravitational waves which causes
their orbit to decay.
This is kind of mind-bending, actually.
The black holes convert their momentum into
gravitational waves.
As their angular momentum decreases, they
spiral inward until they actually collide.
What should be one of the most energetic explosions
in the known Universe is completely dark and
silent, because nothing can escape a black
hole.
No radiation, no light, no particles, no screams,
nothing.
And if you mash two black holes together,
you just get a more massive black hole.
The gravitational waves ripple out from this
momentous collision like waves through the
ocean, and it’s detectable across more than
a billion light-years.
This is exactly what happened earlier this
year with the announcement from LIGO.
This sensitive instrument detected the gravitational
waves generated when two black holes with
30 solar masses collided about 1.3 billion
light-years away.
This wasn’t a one-time event either, they
detected another collision with two other
stellar mass black holes.
Regular stellar mass black holes aren’t
the only ones that can collide.
Supermassive black holes can collide too.
From what we can tell, there’s a supermassive
black hole at the heart of pretty much every
galaxy in the Universe.
The one in the Milky Way is more than 4.1
million times the mass of the Sun, and the
one at the heart of Andromeda is thought to
be 110 to 230 million times the mass of the
Sun.
In a few billion years, the Milky Way and
Andromeda are going to collide, and begin
the process of merging together.
Unless the Milky Way’s black hole gets kicked
off into deep space, the two black holes are
going to end up orbiting one another.
Just with the stellar mass black holes, they’re
going to radiate away angular momentum in
the form of gravitational waves, and spiral
closer and closer together.
Some point, in the distant future, the two
black holes will merge into an even more supermassive
black hole.
The Milky Way and Andromeda will merge into
Milkdromeda, and over the future billions
of years, will continue to gather up new galaxies,
extract their black holes and mashing them
into the collective.
Black holes can absolutely collide.
Einstein predicted the gravitational waves
this would generate, and now LIGO has observed
them for the first time.
As better tools are developed, we should learn
more and more about these extreme events.
What other mysteries in astronomy and physics
fascinate you.
Let me know your suggestions for future episodes.
In our next episode, we talk about the Fermi
Paradox again.
But this time, the Robot Fermi Paradox.
Where are all the alien robots?
Oh, and make sure you stick around for the
blooper.
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