It's been a month since the Event Horizon team released the spectacular
image of the black hole at M87. Looking at it I cannot help but be impressed
with the amount of effort it took to create this high-resolution image, in
fact the highest resolution image ever created! It's also been a little bit of
an obsession of mine to really understand exactly what it is we are
seeing and what we are not seeing in this image. So let's take a deep look
into the abyss, to the very edge of space and time as we know it.
Welcome back to Launch Pad, I'm Christian Ready, your friendly neighborhood
astronomer. Now that I've finally pulled my head out of the images, the press
releases, the simulations, and the papers that were submitted to the Astrophysical
Journal, I think I can finally talk with you now about what this image is really
showing us because yes, it's an image of a black hole, but there's a lot more
going on in this image then first meets the eye. For starters, this black hole is
located at the center of M87, a giant elliptical galaxy about 53
million light-years away in the constellation Virgo. And M87 is
one of the largest galaxies in our local universe, and may have a mass as great as
200 times the Milky Way. The dominant feature of this image is a five thousand
light-year jet. This jet is accelerating away from the black hole at speeds that
are a sizable fraction of light. Although the jet appears to be pointing toward
the right in this image, we have to remember that it's 53 million light
years away and five thousand light years long.
In reality, the jet is only shifted away from our direct line-of-sight
by about 17 degrees so at this distance it appears foreshortened and pointing
toward the right on the sky. The jet is powered by the supermassive black hole
at the center of the galaxy. The black hole is so large that its event horizon
is larger than our entire solar system. A black hole's event horizon is a purely
mathematical surface. Inside, there is nothing that can move fast enough to
escape the black hole's gravity, and that includes light itself. It's the ultimate
point of no return; any events that take place inside this region are forever
hidden from the rest of the universe. That's why we call it the event horizon.
The black hole's event horizon is defined by the Schwarzschild radius which
in turn is defined by the mass of the black hole multiplied by some constants.
The more massive the black hole, the larger it Schwarzschild radius and its
event horizon. The event horizon represents the minimum distance that
light can escape from it, but only if it's traveling directly away from the
black hole. If light is moving at some sideways direction, its path is bent
until it falls back into the black hole. At 1.5 times a Schwarzschild radius, light
is trapped into an orbit. This is called the photon sphere. But this orbit is
unstable. Light either falls into the black hole or escapes, but even then it
is bent by the black hole's gravity. This effect is called gravitational lensing
if the black hole is rotating, light can orbit at a range of distances from the
event horizon so it can get lensed out to a distance of about 2.5 times the
Schwarzschild radius. The result is a shadow about five times the event
horizon's diameter cast upon a ring of lensed photons. And that means we can never
actually see M87's event horizon or even its photon sphere. In
the best-case scenario, we can only see its shadow and the lensed photon ring. So
where are these photons coming from? Well, there are two sources. The first is from
the accretion disk that surrounds the black hole. Matter spirals around the
black hole until it reaches a critical distance where it can no longer maintain
a stable circular orbit. This innermost stable circular orbit is called ISCO and
represents the point of no return for any matter that gets any closer.
Depending on the black hole spin, the disc cuts off at about three times the
Schwarzschild radius. Any closer and matter spirals in. Matter in this region
is heated to billions of Kelvin and radiates mostly at x-ray wavelengths,
This is not the light that we are seeing in the EHT image. But some of this matter
is accelerated into a vortex which in turn powers the 5000 light-year jet. A
powerful magnetic field forms inside the vortex. Free electrons
spiral around the magnetic field lines emitting synchrotron radiation. These
synchrotron photons are then caught up in the black hole's photon sphere before
being lensed in our direction. These photons have a characteristic wavelength
of around a millimeter which is long enough for them to pass through the
surrounding maelstrom and produce the image that we are seeing. Obviously, the
image appears a little blurry but at the same time it's still the highest
resolution image we've ever taken. To understand why let's remember how hard
it is to actually create an image in the first place.
It all has to do with angular resolution. To quickly recap, a telescope's angular
resolution depends on the wavelength it's observing and its overall diameter.
In order to resolve the photon ring at 1.3 millimeters, the
telescope needs to be the size of Earth. EHT achieves an astonishing 25 micro arc
second resolution. That's sharp enough to read the lettering on a dime in Los
Angeles from New York City. M87's photon ring is 42 micro arc second across so
EHT is just able to distinguish it. But the image is a little bit blurred
and that's why the ring appears thicker than it probably is in real life. It
also helps explain why the shadow isn't completely dark; some of the photon
ring's light is being blurred into it. But there's another reason why the image
appears the way it does: some of that light is coming from plasma funneling
its way into the black hole as well as being launched in the jet. Now exactly
what we see depends in part on our viewing angle but the any emission that
originates in front of the black hole proceeds directly toward us while
emission from behind the black hole is lensed around and adds to the
photon ring. The most obvious feature of this image is that the ring appears
asymmetric and that's due to the black hole's spin. Any plasma that is rotating
toward us has its emission Doppler boosted while any emission that is
rotating away from us is dimmed by the exact same effect. But
exactly how is the black hole spinning? To find out the team modeled how the
black hole might appear using a series of general relativistic magneto
hydrodynamic simulations. These simulations model the dynamics of the
magnetic field, fluid flows from the disk and jet, as well as the warped space-time
due to the gravity of the black hole. Since the jet is tilted by about 17
degrees from our point of view, the team was able to use that to constrain the
rotational axis of the black hole. Now all they had to do is work out different
parameters for the mass, the spin direction, the spin rate, electron
temperature, and a host of other parameters. They found that a clockwise
spinning black hole produced a brightening on the ring at the bottom
of the image. Those results were then blurred to match EHT's resolution and
then compared to the actual image. The best match corresponds to a black hole
that is six and a half billion times the mass of our Sun rotating clockwise on
the sky. That means M87's Schwarzschild radius is 127 astronomical units. If our
Sun were at the center of this image, Pluto's orbit would fit well inside this
black hole's event horizon. In fact, Voyager 1 would have flown just beyond
the black hole's event horizon except for the fact that it's unable to because
of the whole speed of light thing. There's also some variability in the rings. The
April 5 and 6 images show the asymmetry oriented toward the seven o'clock
position while the April 10 and 11 images seem to be oriented toward the
six o'clock position. It's not yet clear what is causing this asymmetry but M87
is approximately one light-day across, so that would set a minimum amount of time
for events to transpire across this black hole's event horizon. Now this is
just the first image made by EHT, but they would like to improve it and
there's a couple of ways to do that. Remember, this image is a reconstruction
of several partial images made by telescopes around the planet. Think of it
as a mirror with tiny pieces of glass separated
or a great distance. Each piece of glass captures a portion of the true image. By
adding more telescopes, astronomers will be able to improve the fidelity of the
image - that is, get an image that's that much closer to the truth, and has to rely
less on algorithms to reconstruct the rest. To that end, EHT already added the
Greenland telescope in 2018 and they're in the process of adding the NOEMA
array at the IRAM Observatory in the French Alps, as well as the 12-meter
telescope at Kitt Peak National Observatory in Arizona. Second, EHT plans
to observe at shorter wavelengths, going from one point three millimeter down to
0.87 millimeter. Now that may not seem like a significant reduction in
wavelength, but remember angular resolution is directly proportional to
the wavelength, so even by going down just by a little bit, they'll be able to
improve the resolution of their image by about 30 percent. Another way to improve
the image would be to increase the baseline. That is, increase the space
between the telescopes. Since the EHT telescope is already
the size of Earth. the next logical step would be to deploy radio telescopes in
space. Even having just one radio receiver in low-earth orbit would
dramatically improve the resolution of EHT. And then there's the black hole at
the center of our galaxy, Sagittarius A-star. Sag A* is about 26,000 light
years away, which means it's a whole lot closer than M87. But Sag A* is
also a thousand times less massive, which means it's a thousand times smaller. That
makes our galaxy's black hole a lot more dynamic and harder for EHT to pin down
in a single image. However, the team have data that they acquired on Sag A* in
2017 and again in 2018, and they're presently in the process of teasing out
a real image from all of that data. Now making these images is hard and there's
a lot of work that goes into them. I spoke about that in my previous video
and I invite you to check that out when we're done here,
only please excuse my reference of Sag A*, I had the black hole wrong prior to
their announcement. We are in a new era of astronomy.
We've now gazed into the abyss of a black hole reaching all the way to the
very farthest edge of space and time as we know it. There is much much more to
learn and I look forward to sharing that with you as we find out more. I also want
to take a moment to thank my Patreon supporters for their generous
sponsorship allowing me to create these videos for you. So if you'd like to help
out please head on over to my Patreon page. And if you'd like to join me on
this journey through this amazing universe of ours, well please make sure
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new videos. Until next time, stay curious my friends.
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