Last April, using telescopes scattered across the surface of the Earth, astronomers captured the first image of a black hole. The achievement was possible thanks to combining all the observatories into a single one the size of Earth, called the Event Horizon Telescope (EHT).
The incredible image is also an incredible test for our physical theories and new calculations described in Science Advances suggest that observatories may soon be able to take an even sharper image of a black hole. What they need to focus on is the photon ring.
Most of the image that we saw is produced by photons that were simply deflected by the black hole, but some of them got close enough around the black hole to orbit it a few times. These form the photon ring. The closer they got, the more times they orbited it. The more they orbit, the sharper the view that can be obtained.
"The image of a black hole actually contains a nested series of rings," lead author Michael Johnson of the Center for Astrophysics Harvard and Smithsonian said in a statement. "Each successive ring has about the same diameter but becomes increasingly sharper because its light orbited the black hole more times before reaching the observer. With the current EHT image, we've caught just a glimpse of the full complexity that should emerge in the image of any black hole."
Obtaining a clear image of the photon ring allows for more than just having a sharp image of a black hole. It opens up the way to more precise estimations of a black hole mass, size, and spin. Luckily, it's not a distant possibility; the team had observational astronomers, theoretical physicists, and astrophysicists, who confirmed that observing the photon rings is not farfetched.
"Bringing together experts from different fields enabled us to really connect a theoretical understanding of the photon ring to what is possible with observation," added co-author George Wong, a physics graduate student at the University of Illinois at Urbana-Champaign who designed the software for simulating this. "What started as classic pencil-and-paper calculations prompted us to push our simulations to new limits."
To capture the photon rings we need a slightly bigger observatory than the Event Horizon Telescope. You might be wondering how can we go bigger, given that the EHT is the size of our planet. The solution is as simple as it is ambitious. Add a space-based radio telescope; that would do it.
The EHT has been given funding for upgrades and a new telescope, but due to the coronavirus pandemic, all planned observations for 2020 have been canceled as too many telescopes around the world have had to close. But maybe sometime soon these calculations will be the base of an actual new image of a black hole.