What happens after shooting the horizon of a black hole? Can we see the photon ring?

In 20 19, the horizon telescope (EHT) provided us with the first direct image of a black hole. On the one hand, the images it produces are quite unremarkable. It's just a circular blur of light around the dark center area. On the other hand, the subtle features of the image contain a lot of information about the size and rotation of the black hole.

Black holes themselves do not emit light. Any light that passes through its event horizon will be captured forever. The halo we see in the EHT image of M87* is caused by the background radio glow of gas and dust around the black hole. Some light is very close to the black hole and is guided to us by the gravitational lens. Light can pass through a black hole and reach our nearest limit. This is the so-called photon ring.

If we can observe the black hole perfectly, the photon ring will be a thin bright line. Some light from the photon ring is scattered before it reaches us, and the resolution limit of EHT will produce the blurred image we see. But the next generation EHT will have higher resolution and can capture images in a shorter time. This can not only provide detailed images of M87*, but also provide detailed images of supermassive black holes in our galaxy.

One thing ngEHT can reveal is multilayer lens light. Most of the light we see around a black hole comes from the photon ring. That is, the intense lens light that eats black holes. But some light will form a complete cycle around the black hole before we move forward, while a small amount of light will form multiple cycles. Each type of photon path will produce a different halo layer around the black hole. If we can separate these layers, we can better understand the nature of gravity near the black hole.

In addition, as a recent paper on arXiv shows, ngEHT can also help us detect the event horizon itself of black holes. The dark center region of M87* image is not the region of the horizon. This is just the shadow of the black hole caused by the photon ring. But in the central area, there should be internal shadows. The shadow of the event horizon. As recent work shows, this inner shadow will not be a simple circle. Its shape depends on the size and rotation of the black hole.

The gravitational field near the black hole is very strong, which will not only distort the glow of the photon ring, but also distort the shadow of the event horizon. Therefore, although the event horizon is truly spherical, our view of the event horizon may be distorted by the gravity of the black hole. In this latest work, the team showed us how to observe photon rings and internal shadows at the same time. By comparing the two, we will learn more about the dynamics of black holes, including how black holes capture light and matter.

As time goes on, we may finally be able to see the shadow of gravity itself.