Black holes can't even escape light, so why can they still be seen and photographed? Science tells you the truth

A black hole is an extreme source of gravity that can swallow everything around it. All matter goes there without returning, even light is no exception. Therefore, it seems that there is no invisible "hole".

The human eye relies on visible light to observe things. Since black holes don't even emit light, how can we see them? Therefore, it is normal not to be able to see black holes. But the problem is that scientists often say they have discovered a black hole and even taken photos of it, which is a bit mysterious.

In these photos, the black hole is clearly a bright fireball, so why is it said that even light cannot escape? Many netizens are puzzled and find it incredible. Today, let's solve this mystery together.

First, let's talk about the origin of black holes. According to Einstein's general theory of relativity, when a substance shrinks to an extremely small size, a strange phenomenon will occur: the curvature of space-time will become infinite, all matter will collapse toward the core, and a spherical space with infinite curvature will be formed around the core. This spherical space is like a funnel, sucking all the surrounding matter into the infinitesimal singularity of the core.

This is a black hole.

How small does matter have to be to become like this? As early as 1916, the famous astrophysicist Karl Schwarzschild told us that there is a critical point for the volume of mass, and the calculation formula for this critical point is: R=2GM/C^2. Here R represents the Schwarzschild radius, G is the gravitational constant, M is the mass of the object, and C is the speed of light.

People call this critical point radius the Schwarzschild radius.

From this formula, we can see that the Schwarzschild radius of an object is proportional to its mass. So intuitively, how big is the Schwarzschild radius? According to the formula, the mass of the sun is 1.9891*10^30kg and the radius is about 700,000 kilometers, so the Schwarzschild radius is about 3 kilometers; the mass of the earth is about 6*10^24kg and the radius is about 6371 kilometers, so the Schwarzschild radius is about 9 millimeters.

That is to say, if the mass of the sun remains unchanged and shrinks to a radius of less than 3 kilometers, and the mass of the earth remains unchanged and shrinks to a radius of 9 millimeters, they will become a black hole. But how can the sun be shrunk to 3 kilometers and the earth to 9 millimeters without changing their mass?

It can be said that there is no force that can do this at present, so the sun and the earth will never become black holes. According to the current theoretical framework, there are only two possibilities for the formation of black holes: one is the primordial black hole formed during the Big Bang, and the other is the black hole formed by the core collapse after the supernova explosion.

In both of these formation methods, under high density caused by extremely high temperature and pressure, objects are forced to collapse within the Schwarzschild radius of their own mass, thus forming a black hole.

There may be extremely tiny primordial black holes, but since the smaller the black hole, the higher the temperature and the faster it evaporates, these particle-level black holes will be evaporated as soon as they appear. Therefore, they only exist in theory so far, and no evidence of their actual existence has been found.

In the late stage of evolution, a massive star will explode as a supernova, and the iron core remaining in the core will collapse into a black hole under great pressure. Scientific research shows that the mass of the star that forms a black hole must be more than 30 to 40 times that of the sun, in order to leave a black hole with a mass of more than 3 times that of the sun in the core.

Stars with a mass less than 30 times that of the sun will also experience a supernova explosion at the end of their evolution, but what will be left behind will only be a neutron star with a mass between 1.44 and 3 times that of the sun; stars with a mass less than 8 times that of the sun will not experience a supernova explosion, and after the end of their evolution, only a white dwarf with a mass less than 1.44 times that of the sun will be left behind. The corpse left behind after the death of the sun is a white dwarf; red dwarfs with a smaller mass than the sun will eventually slowly burn out, leaving behind a black dwarf.

Since black holes cannot be seen, how can we take photos of them? Black holes absorb all light due to their infinite gravity, and do not emit a single light. The human eye relies on visible light to observe things, so theoretically, black holes cannot be seen by the human eye. But the problem is that black holes can hide their shape, but they cannot hide their power, which is extreme gravity.

This extreme gravity will devour everything around it in an ugly way, which exposes the whereabouts of the black hole.

The edge of a black hole's spherical Schwarzschild radius is called the black hole's event horizon. This place is the critical point between what a black hole can see and what it cannot see. All observable "events" occur outside the event horizon. Going a little deeper, into the Schwarzschild radius, the falling speed is greater than the speed of light and can no longer be observed.

Therefore, what humans "see" is not the "hole" of the black hole itself, but the light emitted around the "hole", the light emitted around the event horizon. In order for a black hole to swallow the surrounding matter into its stomach, it must first pull the surrounding matter to the vicinity of its own event horizon. This process is not completed instantly, but takes a process.

As matter attracted by the black hole's gravity approaches its Schwarzschild radius, it moves faster and faster, forming an accretion disk around the black hole's equator. The linear speed of rotation can reach tens of thousands of kilometers per second. The closer it is to the Schwarzschild radius, the closer it is to the speed of light. If we calculate based on the kinetic energy formula, we will know that the collision energy of these substances is extremely large.

Therefore, the matter captured by the black hole has long been broken into elementary particles before falling into the black hole. The huge energy generated during the collision will burst out in the form of bright visible light and high-energy rays. Since the escape velocity outside the Schwarzschild radius has not yet reached the speed of light, these visible and invisible lights will break free from the black hole's gravity and burst into space.

From there, the light is captured by the human eye and radio telescopes, and the black hole is "seen" and can be photographed.

Black holes that have no matter or very little matter captured around them, have not formed an accretion disk, and cannot emit light or high-energy rays are difficult to detect. However, if there is celestial activity nearby, it is still possible to infer from the abnormal movement of the celestial body that there is an invisible gravitational source nearby, and thus infer the possible existence of a black hole.

There is such a black hole in the HR6819 triple star system 1,120 light years away from us. Astronomers have discovered that the two visible stars in this system are interacting with an invisible gravitational source. After analysis, they believe that there is a stellar black hole without an accretion disk. Therefore, this system is a triple star system consisting of two stars and a black hole. According to observation and calculation, the mass of this black hole is about 4.2 times that of the sun, making it the closest black hole to us so far. (Picture above)

Therefore, a black hole can be seen or observed with a radio telescope only if there is celestial matter around it that is captured and accreted by it, or at least celestial motion that is affected by it. If there is nothing around the black hole, or only a very small amount of particles, no accretion disk, and the celestial body is very far away and cannot be affected by the black hole, the black hole will be silent and cannot be observed.

Theoretically, there are a huge number of black holes in the universe, but in reality, very few have been observed. Observations have found that there is a supermassive black hole at the core of almost every galaxy, and scientists estimate that there may be about 400 billion black holes in the entire observable universe. There may be hundreds of millions of black holes in the Milky Way alone, but only a dozen have been observed so far.

Since black holes are generally very far away from us, it is very difficult for scientists to photograph them. For example, photographing the M87 black hole and the Sagittarius A* black hole at the center of the Milky Way galaxy required the use of a large number of radio telescope resources around the world. Through networking, a radio telescope array as large as the Earth was formed. Hundreds of scientists from various countries worked together, and after several years of photographing, analyzing, and analyzing data, they finally synthesized the two black hole photos.

That’s all for today. Welcome to discuss and thank you for reading.

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