(mellow music)
- [Voiceover] Black holes are extremely hard to see.
They are objects so extreme
and their effects on their surroundings so unequivocal,
virtually nothing escapes their grasp.
When anything gets close enough to a black hole
and passes across its event horizon,
there is no known way to escape its capture.
Stars, planets, dust, gas, even entire galaxies
can be devoured by these quiet, deadly celestial traps.
Until very recently we didn't know for certain
that they even existed.
Black holes were theorized as an extreme limit
in Einstein's Theory of General Relativity,
but not until the detection of gravitational waves resulting
from a black hole merger were they actually proven to exist.
The primary way astronomers see a black hole
is by the observing the effect it has on things around it.
The earliest black holes were detected
by observing powerful jets emitting
by in-falling material from a nearby unlucky star.
These black holes were the size
of stars roughly 30 to 100 times the mass of our sun.
Later, we saw powerful gamma ray jets
from the violently active centers
of galaxies created as entire stars fell victim
to super massive black holes millions of times
larger than the sun.
Since black holes are so dark and emit no light
or radiation of their own,
we can only really see them when they are near enough
to something we can see.
Stars, nebuli, interstellar gas and dust
all become very bright just before they fall in.
They're beacons of radiation becoming
the celestial tombstones of its victim.
The number of black holes that we know about
come to us through these indirect methods
of interacting and destroying those things we can see.
But that's not all of the black holes that are out there,
there are many more lurking in the void all by themselves;
quiet and completely dark.
Sometimes if we're lucky we can find a lone black hole
as it passes in front of a star
or galaxy in our line of sight.
The light from the distant background body is bent around
the boundary of the infinite gravitational pull of
the black hole in an effect known as gravitational lensing.
Useful for detecting primarily stellar size black holes,
these effects are very hard to see and timing is important.
We must have telescopes pointed in their direction
as they are passing in front of the background object.
We must be looking in the right place at the right time.
Otherwise, we miss it entirely.
Still, astronomers have found a few black holes
using this method.
There are so many, many more black holes
we don't know about.
We need more ways to find them.
Luckily, in September 2015
the Laser Interferometer Gravitational-Wave Observatory,
or LIGO, introduced a new window to the universe
that helps us see naked black holes in a brand new way;
gravitational waves.
Predicted by Einstein 100 years earlier,
they had never been detected.
Their signal was so faint that there
was no known technology available to show them to us
until now.
Gravitational waves are generated
when two extremely massive objects come together,
objects like black holes.
This computer simulation shows the warping
of space and time around two colliding black holes.
This event was actually observed by LIGO
on September 14th, 2015, by seeing
the gravitational waves created in the encounter.
The colored surface is the space
of our universe as viewed
from a hypothetical flat higher dimensional universe
in which our own universe is embedded.
Our universe looks like a warped two-dimensional sheet
because one of its three dimensions has been removed.
Around each black hole space bends downward
in a steep funnel shape.
A warping produced by the black holes huge mass.
Near the black holes the colors depict
the rate at which time flows.
In the green regions outside the holes
time flows at its normal rate.
In the yellow region it is slowed by 20 or 30%.
In the red regions time is slowed extremely.
Far from the holes the blue and purple bands
depict outgoing gravitational waves produced
by the black holes orbital movement and collision.
Our universe's space as seen
from this hypothetical higher dimensional universe
is dragged into motion by the orbital movement
of the black holes, and by their gravity,
as well as their spins.
This motion of space is depicted by silver arrows
and it causes the plane of the orbit to precess gradually.
The bottom portion of the simulation shows
the waveform or wave shape
of the emitted gravitational waves.
The gravitational waves carry a white energy
causing the black holes to spiral inward and collide.
Here space-time is unimaginably distorted.
The shapes of space and time oscillate briefly, but wildly
and then settle down into the quiescent state
of a merged black hole.
We see the gravitational waves
of the collision propagating out into the universe.
This collision and wild oscillation constitute
a storm in the fabric of space-time.
A very brief but enormously, powerful storm.
During the storm the power output in gravitational waves
is far greater than the luminosity
of all the stars in our observable universe put together.
In other words, this collision of two black holes,
each the size of a large city on Earth,
is the most powerful explosion
that astronomers have ever seen.
Aside from the universe's birth in the big bang.
Our new ability to detect
and see gravitational waves promises
to increase the census of known black holes in the universe.
We can now see black holes as they collide
with one another something we could not do before.
With this discovery gravitational waves proves definitively
that black holes do indeed exist,
and we need not invoke some strange mathematics
to explain the extraordinary energy burst we see
at the centers of galaxies and some nearby stars.
That black holes exist there is no longer any doubt,
but an important question remains,
where are they?