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.
Thanks for watching, be sure to subscribe and I’ll see you next time on Seeker.