Since even light cannot escape from a black hole, why can it be "seen"?

Black holes have always been an incredible existence in the universe, first predicted by Einstein's gravitational field theory. In 1971, scientists discovered the first real black hole, Cygnus X-1; in 2019, scientists used the Earth-caliber Event Horizon Telescope to take the first photo of a black hole, the M87 galaxy black hole 55 million light-years away from us.

Now, the general consensus in the scientific community is that black holes are the inevitable destination of massive celestial bodies after death, the top corpses in the universe, and the most extreme celestial bodies without a doubt. They can devour everything that exists in the universe and are truly invincible.

Black holes are common celestial bodies in the universe. Scientists estimate that there are at least 400 trillion black holes in the observable universe and at least 100 million black holes in the Milky Way. However, because the extreme gravitational field of a black hole prevents even light from escaping, people have not been able to observe them directly so far. Less than 100 black holes have been indirectly observed, including more than 60 in the Milky Way.

A black hole has an event horizon, which is a spherical extreme gravitational field formed around each black hole according to its own mass. Science believes that this is the critical point of time and space. Once any matter or even light enters this boundary, it cannot escape, and no information will be leaked. Therefore, humans cannot directly observe any situation inside a black hole.

However, a black hole will leave three physical quantities outside the event horizon, namely gravity, angular momentum, and electric charge. These three physical quantities will affect the surrounding space-time, so people can indirectly observe black holes. The most typical performance of a black hole is that it is "gluttonous", and it "eats" all the matter that approaches it. The professional term is accretion.

Due to the angular momentum of a black hole, it will cause nearby matter to move and rotate at high speed during accretion. The closer it is to the event horizon, the faster the speed will be, eventually approaching the speed of light. 60 to 90% of the matter will fall into the event horizon and disappear, and the rest of the matter will escape into space in the form of energy flows or matter jets.

This is because when matter outside the event horizon collides at high speed to become elementary particles, it emits strong visible light and high-energy rays, or matter jets. These are all escaping energy and matter. People can see these energies and the event horizons they surround through optical and radio telescopes, and thereby calculate the size and mass of the black hole.

The extremely strong gravity of a black hole will cause disturbances to the celestial bodies near it. When they reach a certain distance, they will be attracted by the black hole and revolve around it. In this way, a strange phenomenon will be seen in the universe: one or several stars revolve around an invisible space. Scientists can judge from this that there may be a black hole there and calculate its mass.

The black holes discovered now are "seen" based on the accretion and gravitational disturbances of black holes. In fact, people have not seen real black holes, but only "think" that black holes exist there through accretion phenomena and gravitational disturbances, including the first black hole photo taken by humans, and the supermassive black holes in the center of the Milky Way and other galaxies.

In the early years, the theory about the formation of black holes was that they were formed by the explosion and collapse of stars with a mass more than 30 times that of the sun when they died. However, this cannot explain the existence of some supermassive black holes, because the black holes formed by the collapse of stars are initially only 3 to 10 solar masses in size. Although this type of black hole will devour nearby interstellar matter, the speed is very slow. It takes hundreds or even hundreds of millions of years to devour a star with the mass of the sun.

The largest supermassive black hole discovered now has reached 100 billion solar masses, such as the SDSS J073739.96+384413.2 black hole, which reaches 104 billion solar masses. This black hole can only swallow about 60 solar masses of celestial matter every year, and the universe is only 13.8 billion years old. No matter how much it relies on "tasting good", a stellar-mass black hole seed can never grow to such a huge size.

Research suggests that there may be two sources for this giant supermassive black hole. One is that it is formed by the direct collapse of giant molecular clouds in the early universe. The initial seed of this black hole has a mass of more than 100,000 times that of the sun. The other is that it is formed by the death and collapse of the first generation of massive stars in the universe, forming dense seed black holes, which then continue to merge and grow rapidly.

Some recent observations have further proved this scientific conjecture:

On November 10, 2023, American scientists announced that they had discovered a black hole in the UHZ1 galaxy 13.2 billion light-years away from Earth using data from the Chandra X-ray Observatory and the Webb Telescope. The mass of this black hole is about 10 million to 100 million times that of the sun, making it the farthest black hole ever discovered. This discovery was published in the international journal Nature Astronomy.

One light-year means the time it takes for light to travel in one year, so humans who rely on light to perceive the world see celestial bodies as they did one year ago, and 13.2 billion light-years means this black hole was what it was 13.2 billion years ago, which is only 470 million years after the Big Bang, based on the age of the universe. It is impossible that such a huge black hole was formed so early by the accretion of a star seed black hole.

Just a few days ago, on February 20, 2024, Chinese and foreign media reported that researchers from the Australian National University published a paper in the journal Nature Astronomy, discovering a black hole 12 billion light-years away from us with a mass 17 billion times that of the sun. This black hole is located at the center of a quasar code-named J0529-4351.

The discovery of this black hole further proves the early formation laws of black holes, but this black hole also exhibits some shocking characteristics: First of all, it is not only the brightest black hole discovered so far, but also the brightest quasar and celestial body in the universe, or the brightest object in the universe, with a brightness of 500 trillion times that of the sun!

Secondly, this black hole has an amazing swallowing capacity. Its accretion disk is a terrifying 7 light years in size, and it can swallow 413 solar masses a year, which means it "eats" more than one solar mass of matter every day! This is the most "eating" and fastest "eating" black hole discovered so far.

The bigger the black hole is, the faster it eats. Even without considering its ability to eat more after it gets bigger, according to the current "eating method", after 12 billion years, it will eat nearly 5 trillion suns! In other words, the current black hole is at least 50,000 times the mass of the sun!

Black holes are a very important existence in the universe, and may ultimately connect the life and death of the universe. Humans and even the universe are inextricably linked to black holes. In the future, human survival and destruction may also be affected by black holes. As exploration and research continue, more strange discoveries about black holes will surely be revealed in the future. Let us wait and see.

This is an original article from Space-Time Communication. Please respect the author’s copyright. Thank you for your understanding and support.

Data link: https://www.newscientist.com/article/2417791-monster-black-hole-powers-the-brightest-known-object-in-the-universe/?utm_source=Live+Audience&utm_campaign=536b448141-briefing-dy-20240220&utm_medium=email&utm_term=0_b27a691814-536b448141-51426400