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How do you take a picture of a black hole and what have we learned from seeing one for the very first time?
By now I expect you’ve seen this picture many times.
Our first ever actual bona fide photo of a black hole, made by the Event Horizon Telescope
and revealed to the world in a press conference on April 10th.
Since then it’s got plenty of coverage, because … I mean look at it.
It’s a freaking black hole.
It’s black, it’s holey, it’s everything we hoped it would be.
Now that the giddiness has subsided and I personally have stopped spending hours on
end staring at a black spot, we can take a breath and actually look at the real science
here and discuss exactly how a picture like this could be taken and what we can learn
from it.
The beast in question is the supermassive black hole in the center of the M87
elliptical galaxy.
It has an estimated mass of several billion times that of the Sun, which gives it an event
horizon larger than the solar system.
M87 is 53 million light years away, so resolving that black hole is equivalent to resolving
a grain of sand on the beach in LA – if you’re standing in New York.
By comparison, the Hubble Space Telescope would struggle to see a large watermelon over
that distance.
Sound impossible?
No, it’s only almost impossible.
The only hope for getting that sort of resolution is with interferometry – and even then it’s
… still almost impossible.
To understand how the team pulled it off, let’s look at interferometry.
Interferometry is a way of combining the light taken by two or more telescopes separated
by some distance to massively improve their resolution.
We can think of light from a very distant point as coming in a series or plane waves.
A given wavefront will reach one telescope slightly before the other.
So they arrive at a different part of their wave cycle – there’s a phase difference
between them.
And that phase difference itself is different for light coming from different points on
the sky.
If we can measure that phase difference precisely, we can measure the angular separation between
two points to a precision far better than any single telescope.
An interferometer is tuned to objects of a particular size.
It resolves between two points on the sky if the separation between those points results in a relative
phase shift of around one wavecycle.
In other words, the extra distance the wavefronts have to travel to reach the second telescope
should be different for the two different points on the sky, and that difference should be
of order one wavelength for maximum resolution.
That means that an interferometer can resolve points separated by an angle that is the same
as the ratio between the observed wavelength and the separation of the telescopes – also called
the baseline.
The longer the baseline and the shorter the wavelength, the better the resolution.
The ratio between wavelength and baseline is the same as the ratio between the size
of the object you’re looking at to the actual distance through space to that object.
The rule for the resolution of an interferometer is the same as the rule for the resolution
of any telescope – it’s the diffraction limit – the observed wavelength divided
by the diameter of the telescope.
So to observe the event horizon of the M87 black hole you need to resolve one one-millionth
of 1% of one degree on the sky – that’s our grain of sand between New York and LA.
By good fortune, this was only just possible.
If you build an interferometer that spans the planet Earth the wavelength you need in
order to get this resolution is around 1mm, which is around the shortest wavelength radio
light.
Much smaller and we’re in the infrared, which for this sort of very long baseline
interferometry isn’t possible.
Another important factor is that any two telescopes can only measure the angular separation in
one direction on the sky.
You need a minimum of three telescopes to start to form an image.
And the more telescopes over a range of separation is even the better.
The Event horizon Telescope - the EHT - consists of nine radio observatories across the globe,
from the south pole to Greenland - and the number is growing.
The EHT functions as an interferometer by literally matching the identical mm-separated wavefronts
that reach these telescopes separated by thousands of kilometers.
It can do this because the telescopes are synchronized with atomic clocks.
The petabytes of data from all the telescopes is brought together – via sneakernet – hard
disk drives on airplanes – and some pretty insane image processing algorithms bring it
all together to produce the image.
OK, so that’s the way-too-short version of the incredible many-year effort to produce
this image.
Let’s get to the science.
What are we seeing here, and what does it tell us?
So black holes are, of course, black.
We see them by their influence on surrounding space.
In this case it’s the energy released by matter falling into the black hole.
The supermassive black hole at the heart of the M87 galaxy is currently active – it’s
currently surrounded by an accretion disk – a whirlpool of gas heated to millions
Kelvin that’s falling into the black hole.
It’s also blasting out a jet of energetic particles, channeled by the intense magnetic
fields around the black hole.
We see that jet extending 5000 light years outside the galaxy.
To understand exactly where the light we're seeing comes from let’s look more closely at the regions
around the black hole.
The event horizon itself is the point where even outward-pointing light can’t escape
the black hole.
For a non-rotating black hole the size of the event horizon is called the Schwarzschild
radius, and it’s proportional to the mass of the black hole.
But we’re not seeing the event horizon here – that’s somewhat smaller.
What we’re seeing here is the black hole shadow inside the bright ring of the photon
sphere.
The photon sphere is where gravity is so strong that light itself can orbit the black hole.
That orbiting light will eventually leave the photon sphere– either falling into the
black hole or escaping outwards.
We only see light that escapes directly towards us, so the photon sphere looks like a photon
ring.
For a non-rotating black hole the sphere should be at 1.5 times the Schwarzschild radius.
If the black hole is rotating then you can get photon orbits over a range of distances.
Measuring the radius of the photon sphere potentially gives you both black hole mass
and spin.
There are two main sources of light feeding the photon sphere.
First there’s the inner part of the accretion disk.
That disk terminates at a few times the Schwarzschild radius – again depending on black hole spin.
That’s the innermost stable circular orbit – the ISCO – and it’s the closest you
can get to the black hole and still orbit in a stable way.
Any closer and anything besides light will quickly spiral into the black hole.
But the accretion disk is not the source of light in this image.
We also expect this crazy, 5000-light year-long jet to be launched here, from material
swept off the accretion disk that is whipped into a near-light-speed vortex before being blasted
through the galaxy above.
Remember that the EHT observes radio light with a wavelength of around a millimeter.
1.3 mm to be precise.
That wavelength should be dominated by synchrotron radiation, not from the thermal radiation
of the accretion disk.
Synchrotron results from electrons spiraling in magnetic fields.
So that synchrotron light shines from the jet vortex, it gets trapped briefly in the photon
sphere, and then makes its way to us.
Let’s take a look at the image again.
The ring is the photon sphere, blurred due to the fact that this observation is incredibly
difficult and the EHT is “only” the size of the Earth.
In principle this could be a very narrow ring.
The asymmetry is probably due to relativistic beaming – brightness massively amplified
when the material is moving in the same direction as the emitted light.
In this case, light from the magnetized plasma vortex is beamed on the side of the black
hole where it’s moving towards us – or at least forwards before the light gets deflected
in the photon sphere.
On the opposite side of the black hole the emission is dimmed by the same effect.
The team simulated this whole process with a magneto-hydrodynamic simulation that weaves
in all of the physics of fluid flow and magnetic fields, in this case with the addition of
the warped spacetime of a black hole using Einstein’s theory of general relativity.
Finally, they simulated the blurring of the image due to the whole observational process.
The team simulated a wide range of parameters like black hole mass and spin rate, while
they were able to nail down the rotational axis rotation based on the jet that we see leaving the galaxy.
With that constraint they could find the configuration that fit the actual image.
They found that monster has a mass over 6 billion solar masses, with an event horizon that's about
1/5 the size of that ring– so that’s VERY big, even by supermassive black hole standards.
The black hole is indeed spinning – almost as fast as it can spin.
So the plasma vortex should be rotating in the same direction as the black hole.
The asymmetry in the ring tells us the rotational direction of the vortex, so combine
that with the axis direction from the jet and we have the rotation direction of the
black hole – roughly clockwise from our perspective, with an axis pretty close to our line
of sight.
One cool fact is that, just like those gravitational wave signals from a couple of years ago, the
black hole looks just like we predict based on Einstein’s general theory of relativity.
Eerily so, in fact, given that general relativity was derived over 100 years ago.
Back then when black holes emerged from Karl Schwarzschild’s solution to the Einstein equations,
they seemed like a deep abstraction – a prediction that seemed too outlandish to be
real, and certainly too difficult to observe to ever verify.
Since then, evidence has mounted and few scientists doubt their existence.
But this actual picture given to us by the Event Horizon Telescope and its brilliant
team – finally hits us with visceral reality of the black hole, Einstein’s wildest prediction
and the strangest object in all of space time.
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