Long, painstaking work by scientists around the world has produced the first direct image of the event horizon of a black hole, a supermassive monster called M87.
This image confirmed many of our ideas about black holes.
But science didn't stop when photography came along. Scientists have performed calculations based on what they have learned about M87 combined with general relativity to further predict how one day we will see these objects in detail.
Black holes are incredibly gravitationally intense. Not only are they so massive that even the speed of light is too slow to avoid gravitational pull, they also bend the path of light around them, beyond the event horizon.
If a passing photon gets too close, it will be in orbit around the black hole. This creates what is called a 'photon ring' or 'photon sphere', a perfect ring of light that is predicted to surround the black hole along the inner edge of the accretion disk, but outside the event horizon.
It is also known as the most stable inner orbit and you can see it in the image below, created by astrophysicist Jean-Pierre Luminet in 1978.
(Jean-Pierre Luminet)
Models of the black hole's environment suggest that the photonic ring should create a complex substructure made up of endless rings of light – a bit like the effect you see in an endless mirror.
“The image of a black hole actually contains nested series of rings,” explained astrophysicist Michael Johnson of the Harvard-Smithsonian Center for Astrophysics.
'Each successive ring has roughly the same diameter, but becomes increasingly' sharp 'because its light orbits the black hole several times before reaching the observer.'
(Event Horizon Telescope)
In this historic first photo of M87 (above), we see the accretion disk – a luminous orange-gold piece. The black part in the center is the shadow of the black hole. We cannot actually see the photon sphere because the resolution is not high enough to make it out, but it must be positioned along the edge of the black hole's shadow.
If we could see it, then this ring will tell us very important things about a black hole. The size of the ring can tell us the mass, size and speed of a black hole. We can identify them from the accretion disk, but the photon ring would allow us to further constrain the data for more accurate measurements.
“Each ring consists of photons, lensed onto the observer's screen after they have been collected by a photonic shell from anywhere in the universe,” the researchers write in their paper.
Therefore, in an idealized environment without absorption, each ring contains a separate exponentially distorted image of the entire universe, with each subsequent ring capturing the visible universe. Together, the set is similar to film footage, which captures the history of the visible universe as viewed from a black hole. '
So Johnson and his team used simulations to determine whether photon rings could be detected in future observations. They found it could be done, although it wouldn't be easy.
The M87 shot was a feat of ingenuity and collaboration. Telescopes around the world have worked together to create a very long basic interferometer called an event horizon telescope, where precise distances and temporal differences between telescopes in an array can be calculated to glue their observations together. It is – in very, very simple terms – like having one telescope the size of Earth.
“What really surprised us is that the nested rings are almost invisible to the naked eye in images – even in perfect images – but they are strong and clear signals for arrays of telescopes called interferometers,” Johnson said.
The study was published in the journal Science Advances.
Sources: Photo: Photons orbiting a black hole. (Nicole R. Fuller / NSF)
