How does a black hole distort time?

Distorted gravity giant

Gravity is to blame: this visualization shows how a black hole distorts its own appearance. If we were to look at it from the side, as shown here, we could see the top and bottom of the brightly shining ring of matter around the event horizon at the same time. A kind of Doppler effect also makes the left side with the gases racing towards us brighter than the right.

Black holes exert such an enormous force of gravity that they themselves break through the spacetime structure. For those who exceed their event horizon, there is no way back. This even applies to light, as Albert Einstein postulated. Just outside the event horizon, however, hot gases and star debris race around the black hole at almost the speed of light and emit intense, bright radiation.

What does a black hole look like?

Because these gravitational giants swallow all light, they themselves remain invisible. But they betray themselves through their influence on the light in their environment. Both the radiation from the accretion disk and the light from nearby stars are bent in their surroundings in such a way that it forms a circular ring of light around the event horizon. The dark shadow of the black hole can be seen in the middle - as recently confirmed by the first photo of a supermassive black hole.

In the case of this famous photo, however, the astronomers looked down at the black hole and its accretion disk from above. But what would it look like if you looked at it from the side? This is the question Jeremy Schnittman from NASA's Goddard Space Flight Center and his team asked. With the help of special software, they created this visualization based on astrophysical calculations.

Distorted by gravity

This visualization shows a black hole seen from the side. While a less extreme object such as the ring planet Saturn would resemble a sphere with a narrow ring resembling a line from this perspective, this is different with the black hole. Because its effect of gravity on the light means that its appearance is severely distorted. This will normally reveal hidden areas.

In the picture we see the dark shadow of the black hole in the middle, surrounded by a bright, fine line of light - the so-called photon ring. It consists of light that has circled the event horizon several times, becoming increasingly thin and elongated, before it then escaped and reached our eyes.

Above and below at the same time

Another special feature: we see the accretion disk from above and below at the same time, instead of just from the narrow side. Because their light is also distorted by the black hole and as a result one image of this ring appears bent upwards, a second one bent downwards. The left side of the glowing disk of matter seems to shine brighter than the right. This comes about because the gas in this ring moves almost at the speed of light and relativistic effects take hold. They make the light of the material rushing towards us appear brighter than that of the gas that is flying away.

“Simulations like this help us understand what Einstein meant when he said that gravity distorts space-time,” explains Schnittman. Until recently there were only such visualizations for astrophysicists to understand what was happening. It was only thanks to the Event Horizon telescope that they got their first real look at one of these gravity giants.

Source: NASA / Goddard Space Flight Center

26th September 2019

- Nadja Podbregar