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.
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