Seeing a Black Hole with a Planet-Sized Telescope | STELLAR
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Imagine for a moment that I'm a beam of light far outside the visible range, traveling
from a star in a distant part of the universe.
If you want to see where I came from, you'll need two things: a series of radio telescopes
like the Submillimeter Array behind me, and one of the most accurate clocks in the world.
I've traveled a long way to get here, so when I arrive on Earth I'm an extremely
faint signal. To see me you'll need to point multiple antennas at the same point in the
sky – antennas like these. But my faint signal will arrive at each telescope at ever
so slightly different points in time. Using our incredibly precise clock, we can synchronize
these signals and combine those faint images to make a much more vivid Joe.
And if you point enough radio telescopes at the exact same spot, and you have access to
a supercomputer, well, you can even see something we once thought was unseeable...a black hole.
On April 10, 2019, astrophysicists from the Event Horizon Telescope collaboration electrified
the world with the first ever image of a black hole.
This astronomical donut smothered in orange frosting is a supermassive black hole weighing
about 6.5 billion times the mass of our sun, at the center of the M87 galaxy, about 55
million light years away in the constellation Virgo.
The orange ring that we see are photons, produced by hot swirling gas orbiting around the edge
of the black hole. The inner edge of that ring is the event horizon, a precipice of no return.
The EHT is not just one telescope. It's many telescopes, including these here, working
together as a larger telescope, and it let us see something we used to think was impossible.
The idea of dark, massive objects in space, dense enough to capture light itself had been
hinted at by John Michell as far back as the 18th century. Objects that came to be called
“dark stars”. But the first modern hints about black holes arrived as an abstract mathematical
idea inside Einstein's theory of general relativity.
Over the 20th century scientists were looking for black holes, but how do you observe the
absence of light?
Creating an image of a light swallowing cosmic abyss is not unlike tuning in to hear your
favorite song on the radio. Except imagine this is your receiver.
Black Holes may not produce any light waves in the visible spectrum, but those hot clouds
of swirling gas at their edge produce light in other parts of the electromagnetic spectrum
that we can detect. But luckily, space is mostly transparent to radio waves, so that's
what the EHT team chose to look for.
But that gave astronomers one more problem to solve. Radio waves have very long wavelengths,
and the longer the wavelength of light you use, the more difficult it is to produce a
sharp image. Not to mention, as massive as black holes are, its very tiny in the sky.
From our vantage point on Earth, seeing M87* in the sky is like trying to see a bagel on
the moon.
The solution? A telescope the size of the Earth.
Geoff, it's so nice to meet you.
It's really great to have you here, Joe.
You have got to tell me how you used those things to take that awesome picture.
We are using with the Event Horizon Telescope, effectively, a telescope that has a resolution
a thousand times better than the Hubble Space Telescope.
The way you get finer and finer detail, better angular resolution out of telescopes
is you build a bigger diameter aperture.
So what we do is take telescopes that are located around the world, all radio
and we connect them together and we use them to build a single telescope
It's a mirror the size of the whole planet, but most of the mirror is missing.
In order to make that mirror, we have to have the clocks, between the telescopes, carefully aligned.
We can get our clock aligned to more than a picosecond.
A normal stopwatch goes out to like two places, maybe, and you're going
like eleven places beyond that
Exactly!
Over several nights in April 2017, EHT pointed telescopes at 8 different locations around
the world at the black hole, including the eight antennas here at the Submillimeter Array
on Mauna Kea. To act as one, they synchronized their observations using an extremely precise
clock at each site. This clock, called a hydrogen maser, can keep time to within a billionth
of a second.
Then by combining all the data in a supercomputer, they created the first ever radio image of
a black hole.
We're in a very big science room!
What happens in here?
So it's a big supercomputer, specially designed for the purpose of
combining the signals together from all of our different telescopes
and also, to take the combined signal, and format it in a way that we can use for the Event Horizon Telescope.
This is where you saved the black hole
Exactly, light stopped right here.
At the other end of this lab is where the clock signals come in, so the hydrogen maser
is in the bunker that's underneath here
in its concrete shell, and it sends up its reference tone
10 megahertz signal. And that signal gets distributed to all the different clocks that are used throughout the system.
We're interfering waves together, that's what interferometry means
and so if those waves move back and forth a little bit
when you interfere them they destructively interfere and you lose your signal.
We use GPS to timestamp it and then we use the hydrogen maser to make sure that on
the shortest time scales, everything's aligned just fine.
This is where our part of the EHT data comes in
and we record the light and it gets stopped forever.
But this isn't the end of our story. EHT is now trying to take a picture of the
supermassive black hole at the center of our very own galaxy
in the constellation Sagittarius.
Thanks to generations of scientists, we're long past using just our eyes to see
the universe.
With each new discovery, one mystery ends only to reveal ever greater mysteries
for new scientists to uncover, and to keep us all looking out at the stars in wonder.
If you thought a planet-sized telescope was big, just wait until you find out how big the universe is.
Check out Matt O'Dowd from Space Time on the next episode.
Thank you to Draper and their Hack the Moon initiative for supporting PBS Digital Studios.
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