Photons captured by the immense gravity around the black hole form an intricate structure of light rings between the accretion disk and the event horizon boundary.
People were disappointed with the first direct image of a black hole, released last year. As if it were not enough to capture the image of an object that was considered impossible to see, 55 million light-years away, some were bothered because the image was blurred.
However, if the image had a better resolution, researchers estimate that we would see an impressive substructure around the event horizon: an infinite series of rings of light, gradually sharper, caused by extreme gravitational curvature.
“The image of a black hole actually contains a series of scrambled rings,” explains Michael Johnson, of the Harvard Smithsonian Center for Astrophysics. ” 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 (Event Horizon Telescope) image, we’ve caught just a glimpse of the full complexity that should emerge in the image of any black hole.”
With their immense gravitational pull, black holes capture all photons that cross their event horizon, making their center dark – surrounded by the light generated by the hot gas that forms the accretion disk. The “photon rings” are inside the inner edge of the accretion disk, but outside the event horizon, and are formed by the light trapped in orbit by the strong gravity near the black hole.
Mathematical models suggest that the photon ring must create a complex substructure that consists of infinite rings of light – like the effect you see when you place one mirror in front of another. In the EHT image of the M87, we can see the accretion disk (the yellow and orange part) and the shadow of the black hole (the dark center). We cannot see the photon rings, as they are very thin and the resolution is not high enough.
But if we could see it, that photon ring would be the fingerprint of the black hole – its size and shape encode the black hole’s mass and rotation. With the images generated by the EHT, black hole researchers have a new tool to study these structures.
“Black hole physics has always had profound theoretical implications, but now it has also become an experimental science,” says Alex Lupsasca of the Harvard Society of Fellows. “As a theorist, I am pleased to finally collect real data on these objects that we have been thinking about abstractly for so long.”
To image the M87, telescopes around the world worked together to create a single Earth-sized telescope. “Although capturing images of black holes normally requires many distributed telescopes, photon rings can be studied using only two telescopes that are very far apart. Adding a space telescope to the Event Horizon Telescope would be sufficient,” says Johnson.
This additional telescope would have to go a little further than Earth’s low orbit. A good position would be to place a telescope on the moon. To further improve the resolution, a third party could go even further, beyond the moon, into a stable position between the sun and the Earth. None of this is impracticable, considering that NASA is planning a manned mission to the Moon.