On April 4th, 2017, a privileged group of telescopes on mountains across the
planet switched on at the same time.
For the next week they danced in unison,
collecting radio waves dispatched from the center of our Milky Way galaxy and
from the galaxy m87.
Together they make up the event horizon telescope, a global
project to capture the first ever picture of a black hole.
That's right.
Ever since physicists first conceived of black holes centuries ago, every image of
one from our textbooks and our space agencies, they're all illustrations.
Until now.
We are delighted to be able to report to you today that we have seen
what we thought was unseeable.
For centuries, physicists have theorized that
an object with enough mass and density could trap even light in its
gravitational field, just as you have to travel faster to leave Earth than you do
to leave the Moon, there could be a place where you'd have to travel faster than
the speed of light to escape.
And nothing moves faster than light.
The math from
Einstein's theory of general relativity describes an area completely invisible
to us within a boundary called the "event horizon," and at the center of that black
hole is a singularity, a point of infinite density which is where physics
as we know it breaks down.
They showed up in the math long ago and they kept
reappearing and they sort of persistently would not go away, but
Einstein always thought that there must be some physical mechanism that prevents
stars from collapsing to an infinitely small point, which is actually pretty
reasonable. I mean, because it sounds insane.
Eventually scientists began to
see things that only made sense if black holes were real, like the orbits of these
stars around the centre of the Milky Way galaxy.
You see these stars just slingshotting around an invisible point and a black hole
is the most likely explanation for putting that amount of mass in that
small space, for something that's completely dark.
We can also see the glowing material that spirals around black holes: Friction
heats this matter up tens of millions of degrees and anything that hot emits
X-rays that we can detect with telescopes that orbit above Earth's atmosphere.
This is a pair of galaxies that pass through each other. There are
at least nine suspected black holes here, but you can only see them when you look
at the X-ray layer.
These dots are X-ray sources linked to suspected supermassive
black holes at the center of galaxies three to ten billion light-years away.
And that's just from this small patch of sky.
Some super massive black holes also
feature gigantic jets of particles, seen here in radio wave data from the galaxy
m87, which has a much bigger black hole than the one in the center of the Milky Way.
No other known source of energy could power these things and nothing we
know of besides two black holes colliding could have produced the
gravitational waves we detected in 2015.
Scientists think there are black holes
large and small all over the universe. We can see their fingerprints but we didn't
have the mug shot.
Directly imaging a black hole has been impossible because
they're either too small, too far away, or both.
Sagittarius A*, the black hole
at the center of our galaxy has the mass of four million Suns, but it would fit
inside the orbit of Mercury.
Imaging it from Earth is like taking a picture of a
DVD on the surface of the Moon, with huge clouds of dust and gas in between.
So many things had to go right for this image to exist, so the first thing that
has to happen is there has to be some slice of light that travels all the way
from the edge of the black hole without getting knocked off course or absorbed
by any of the gas or anything in between, and then it also has to make it through
the Earth's atmosphere which a lot of frequencies of light don't.
They landed on a wavelength of 1.3 millimeters at the high frequency end of the radio
spectrum. With that wavelength and with eight observatories across the world, the
event horizon telescope had a chance at seeing a black hole, as long as the
weather cooperated.
You have to have clear weather in all of those places at
a time when the Earth is oriented in such a way that all of
those telescopes can see the black hole simultaneously. They can really only
observe once a year.
There was so much data involved that it had to be flown on
airplanes. They waited six months for the hard drives to arrive from the South Pole,
which closes during winter time.
This multi-telescope method is called
"Very long baseline interferometry," it correlates timestamped data from
distant telescopes to boost the signal and quiet the noise.
Each pairing of
telescopes contributes a piece of the puzzle, but the image doesn't just pop
out after that. They had four groups working for months to generate the image
that best represents the data.
Each group was working individually and like in
isolation from the other groups, working with the same data, to see that each
group came up with the same image or not.
And the result of all that work is this.
The bright parts are the matter and lights swirling around the black hole
and it's brighter on the side that's moving toward us.
And the dark part is
the black hole's shadow, which includes the event horizon plus a region where
light could escape, but doesn't.
The size and shape of the shadow appear to
confirm the theory of general relativity.
Today, general relativity has passed
another crucial test, this one spanning from horizons to the stars.
Humanity's first image of a black hole isn't crisp and beautiful like the illustrations or
the movie Interstellar. It's better.
The picture we see this week is made of
scraps and bits of light that's been traveling across the universe and
collected by these, you know, aluminum dishes on top of mountaintops and then
combined in a supercomputer to make this image.
So that's why it's real.