In April of 2019, scientists unveiled the
first image ever taken of a black hole.
The photo of the supermassive black hole M87*
was possible thanks to the Event Horizon Telescope,
or EHT.
Comprised of eight radio telescopes around
the world working together, the EHT is effectively
a telescope the size of the planet.
But M87* may have layers that are yet to be
seen, and taking a snapshot may mean extending
the EHT’s footprint beyond that of Earth.
As you probably know, black holes get their
name because they’re so massive that light
that crosses the event horizon can’t escape
their gravitational pull.
They are, by their nature, invisible, so technically
we didn’t really take a picture of a black
hole but rather its shadow.
We can, however, see what’s outside the
event horizon.
In fact that area can be very bright, as matter
being pulled into the black hole rubs together,
gets superheated and gives off electromagnetic
radiation.
The hottest part of that so-called accretion
disk is represented by the orange halo in
the famous photo of M87*.
Knowing the size of the black hole’s shadow
against the accretion disk can help determine
the mass of the black hole, and currently
the EHT is accurate to within about 10%.
But between the accretion disk and the black
hole’s shadow, Einstein’s theory of General
Relativity predicts there is another layer,
called the photon ring.
These photons passed the black hole at just
the right distance and inclination to orbit
it before escaping and reaching an observer.
Really the photon ring consists of an infinite
stack of subrings, each ring getting sharper
and fainter.
Photons that have orbited the black hole more
belong to higher numbered subrings, so the
first subring is made up of photons that have
made at least one half an orbit of the black
hole.
Subring 2 is made up of photons that have
made at least one full orbit before escaping,
and so on and so forth.
Each successive ring is made up of photons
that were shot into an exponentially narrower
window.
Imaging these rings would be yet another feather
in Einstein’s “I was right about General
Relativity” cap, which at this point must
look like a peacock.
It would be the first time we’ve seen space
bent so much that it curves light
into complete loops.
But they would also give us more useful information
about the black hole, providing a better estimate
of its mass and telling us information about
its spin as well.
So, now the question is, how do we go about
detecting the photon ring?
If we’ve already made a telescope the size
of the whole Earth, what’s next?
Well, making one bigger than Earth.
Obviously.
It was once thought that meaningfully upping
the EHT’s power would require putting many
instruments in orbit.
But astronomers were surprised by how strong
and clear the signals from M87*’s subrings
were.
They have since proposed that the EHT’s
resolution could be dramatically increased
with just one instrument, even one that’s
piggybacking on another mission.
And the farther away that instrument is, the
more powerful the telescope becomes.
One placed in low earth orbit could detect
the photon ring’s first subring.
If it was on the moon, detecting the second
subring would be possible.
Over a vast enough distance, the EHT’s resolving
power could be increased a hundredfold.
My nerd heart is already all aflutter imagining
the pictures we could take with a hundred
times the power of the telescope that first
photographed M87* in 2019.
But we may have to wait a bit longer.
Even if everything goes well, the EHT may
not get a space based component for another
decade.
Once that’s in orbit we’ll have our chance
to confirm another prediction of general relativity,
and see light that walked on the very edge
of a black hole.
And this time, nobody better say it’s blurry.
While the EHT may not be expanding into space
for a while, there are plans underway to add
more earth-based telescopes to its arsenal. 
M87* is also shooting out gigantic jets of
matter at almost light speed and we’ve taken
some stunning pictures of those too.
For more on them check out my video here.
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and I’ll see you next time on Seeker.
