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Why This Black Hole Photo Is Such A Big Deal

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.
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