How To Recreate An Image Of A Black Hole


In 2017, scientists launched an ambitious
experiment to photograph a black hole for the first time,
A network of radio dishes across four different continents joined together to form a giant,
planet-sized observatory, known as the Event Horizon Telescope. It set its sights on two targets: Sagittarius
A*, the supermassive black hole in the heart of our Milky Way, and an even bigger one lurking
in Messier 87. With a globe sized telescope, the EHT should
have enough resolution to directly observe the shadow of a black hole’s event horizon. It’s been over a year, so where’s the
photo? To get some answers, we went to MIT Haystack
Observatory, one of the two hubs responsible for processing all of the observing data. A lot of people wonder, why haven’t you released
an image yet? There are three reasons for that. One is that it took a long time for the data
to get here.At each telescope, we record 64 gigabits per second of data…We record…throughout
the course of a night. So we’re talking about several petabytes of
data. For the data volumes we’re talking about,
the Internet is insufficient to transport the data. nothing beats loading up hard drives onto
an airplane and shipping them. And shipping hard drives from telescopes in
Chile and the South Pole to data correlators just took months. The second reason is because we’re being very
careful on the calibration. After the data were shipped to Haystack, the
team here…started to correlate the data. Once the data were correlated, we handed them
off to the calibration and error analysis team…to find fringes and figure out the
amplitude calibration. For the first time, we had incredibly sensitive
detections on all of our baselines. And that means that we could actually start
to see all the little bumps and wiggles that were instrumental in nature rather than from
the source. I can show some example here: one of our target,
the Sagittarius A star. Everything is rotating around 30 minutes,
which is really fast.The Event Horizon Telescope is designed to actually catch things happening
on very short timescales. It takes a lot of time to check what’s happening. For example, what’s the instrumentation effect? What’s the actual minor effect, or side effect? And the third reason is we’re just being extra
careful. We have internal reviews all the time where
one team goes ahead and works with the data. And then people who weren’t involved in that…can
look at the results and say, “Well did you think about this? Did you try this?” At this stage of the project, the team has
a solid enough data set to start putting together images. The telescope coverage itself…doesn’t cover
all of the planet so when we actually want to make images, there is an infinite number
of possible images can fit our data set. We need to find what is the most likely images
for these sources. It’s like a detective work. We actually have four different teams of people
making images. And they’re working blindly. So one team makes an image, a second team
makes an image. And then at the end we compare them and say,
are these consistent? But that comes with a whole new set of obstacles. For our Milky Way black hole, Sagittarius
A star, there are two challenges. What we are expecting to see is the structure,
or the plasma flow around the black hole is dynamically changing. We are supposed to take a movie of these black
holes. That pose some challenge for the image reconstruction. Another challenge is that because Sagittarius
A star is at the center of our galaxy…we are looking …. through the many spiral arms
of our galaxy. That will slightly blur the image of Sagittarius
A star. So we also need to mitigate a kind of scattering
effect from the interstellar plasma in front of us on the way to Sagittarius A star. If we move on another source, M-87, it also
has a completely different challenge… because M-87 is not in our galaxy, there are some
uncertainties. Before we explain what those are, remember
when it comes to imaging a black hole, scientists can’t observe it directly. They’re looking at the surrounding matter
and light that’s being pulled by gravity around the black hole’s event horizon. You’ve likely heard that phrase before,
it’s the ultimate boundary, the place where….” But underneath that classic saying is a geometric
parameter that underpins how the EHT maps black holes in space time. It’s called the Schwarzschild radius. Back in 1916, German astrophysicist Karl Schwarzschild
was inspired by Einstein’s theory of relativity, and in his free time fighting as a soldier
in World War I, he came up with this formula to calculate the radius: Where M is the mass,
g is the universal constant of gravitation, and c is the speed
of light. The Schwarzschild radius is the distance from
the center of the black hole to the event horizon, that point where light can’t escape. This model works for non-rotating black holes. But the field has advanced since Schwarzschild,
with the Kerr metric, which takes black hole spin into account. Ultimately, the Schwarzschild radius is key,
because it helps physicist unlock the size of a black hole. Apparent size of the black hole is determined
by distance to M-87 and how far the black hole is and how heavy the black hole is, because
the size of black hole is proportional to the mass of the black hole. We have a very good measurement for the distance,
but still there is a factor of two uncertainties for the mass of M-87. The size of the shadow is determined by the
black hole’s mass. The bigger the mass, the larger the shadow. But if the mass is a question mark, that could
pose a challenge. If we cannot get the shadow, may be suggesting
that the current way to measure the mass of the supermassive black hole in many galaxies
could be somewhat wrong and we need to do recalibration. While the teams are still working on images,
they have some guesses as to what the finals might look like. for Sagittarius A star, the
expected image is very different from, for instance, the famous image of “Interstellar.” We will see it’s like a crescent image; one
side is brighter and there is a shadow and the other side is very faint. For M-87, we are expected to see both accretion
flow falling into the black hole and also the jet, which is the plasma flow escaping
from the black hole. Those images are still preliminary, and they
haven’t been released yet, but we expect to have a release of the papers and the images
in the late winter or early spring of 2019. Black holes are basic constituents of our
universe, and these monsters are the anchor point in virtually every galaxy. Understanding their formation and dynamics
could be the ultimate test of Einstein’s theory of relativity, which makes some key
predictions. First that the gravity of a black hole curves
space time and draws everything towards it. And secondly that the shadow of the event
horizon cast by the accretion disk should be mostly circular. Depending on what the final image looks like,
we might need to update the law of the cosmos. Everyone believes something is actually falling
down the black hole and something is escaping the black hole, but we still don’t have any
images. We are now in a very exciting moment that
we can actually test what people thought about a hundred years ago. My take is that Einstein was probably right. Every time we’ve tried to test relativity,
relativity passes the test. the image of the black hole could change our
conception of relativity, or it could confirm it. So you predict that you will see a circular
shadow, and the size of that shadow is predictable from the distance to the source and the mass
of the black hole. We could always get unlucky and not see a
shadow at all. But I think that the preliminary data tells
us there’s something there to be seen. Want more science documentaries, check out
this one right here. Don’t forget to subscribe, and keep coming
back to Seeker for more videos.

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