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Does Black Hole Have Temperature? Is it cold or hot?

The bow tie nebula, 5000 light-years away, is the coldest place in the universe. Of course, the premises are "observed" and "naturally occurring." Some people say: Shouldn't the temperature of the black hole be lower?

For black holes, there is no particularly direct way to observe them except through celestial motion, radiation in the surrounding environment, and taking a few blurred photos. So whether the temperature of the black hole itself is cold or hot, we have no way or condition to actually measure it. Black hole temperatures are strictly theoretical predictions, not the coldest "observed."

Others wonder,"Don't black holes always have jets and high-energy rays and so on, so they should be hot? "

Another said,"We can't observe the interior of a black hole. It doesn't make any sense to say whether it's cold or hot. "

First, the high-energy rays of the black hole clearly do not originate from within, but from something outside the black hole's visual interface, such as an accretion disk. When we say "black hole temperature," we do not mean the true temperature inside the black hole, but an equivalent temperature derived from radiation.

Black holes can't even escape light, so how can there be radiation? The story begins in the 1970s...

According to the classical physics of general relativity, it was widely believed that black holes should be completely black, after all, light cannot escape. And for black holes, there is no information other than mass, angular momentum, and charge. There are no elementary particles, let alone atoms or molecules. This raises the question: if an object falls into a black hole, then all the information about what it was made of and how it was made is gone, in other words, the original entropy disappears. If the universe is thought of as an isolated system, wouldn't this mean that entropy decreases and not increases, violating the second law of thermodynamics?

In 1971, Hawking first proposed the "black hole area theorem," proving that the surface area of a black hole horizon can only increase and not decrease. That is, two small black holes can merge into a large black hole, but a large black hole cannot split into small black holes. This seemingly ordinary conclusion was realized by a young man the next year.

In 1972, Jacob Bekenstein was twenty-five years old and studying for a PhD at Princeton University under the guidance of John Wheeler, who had named the black hole. Bekenstein thought to himself when he learned of Hawking's theory that black holes only increase in size and not decrease in size: "Only increase and not decrease? I remember a physical quantity that also says it only increases and does not decrease…"Yes, Bekenstein thought of entropy in thermodynamics.

By some scaling, we can take the surface area of the event horizon of a black hole as a surrogate measure of the entropy of the black hole. Black holes are not entropy free, they just don't have the entropy we think they do. Bekenstein then codified this idea into a paper, formally proposing the concept of "black hole entropy." A whole new field of research was opened up-black hole thermodynamics (BHT), which also had a profound impact on quantum gravity and holography.

Jacob Bekenstein Since black holes have entropy, the second law of thermodynamics has been preserved, but a new problem has arisen: since black holes have entropy, according to the third law of thermodynamics, black holes should also have temperature, and if there is temperature, there will be thermal radiation, which obviously contradicts the black hole "only in and out" characteristic.

Hawking was the first to question it. He believed that black holes were called "black" holes because they did not emit any radiation, so the entropy of black holes should not be the same as entropy in thermodynamics.

But only two years later Hawking changed his mind. Because he studied the problem carefully later, when he introduced quantum field theory into the original general theory of relativity, he found that black holes that could not enter could really radiate energy outward! Black holes are not "black" but "gray"! After this, people called this radiation "Hawking radiation."

According to the traditional definition of temperature, black holes should have no temperature, just like vacuum, because there are no particles at all, let alone how violent they are. But Hawking radiation is very similar to traditional thermal radiation, so black holes should have a "temperature."

But this raises another problem: the entropy of the object falling into the black hole disappears into thin air. Now it does not really disappear, but becomes part of the event horizon in some form. But since black holes have temperatures and thermal radiation, one day they will evaporate. Since thermal radiation carries no information in the traditional sense, the entropy of the object falling into the black hole is now literally eliminated from the universe, thus violating the law of conservation of information. You know, conservation of information, like conservation of energy, is one of the most fundamental laws of nature. So with the birth of Hawking radiation, another thorny problem appears in front of people-black hole information paradox.

See, scientific development is like this, often "press the gourd to pick up the ladle." The black hole information paradox can be said to have troubled Hawking throughout the latter half of his life, and until today this problem has not been completely solved.

The black hole information paradox goes back to hawking radiation. How does energy come out of a black hole that light can't escape? Quantum fluctuations in a vacuum are one of the easiest explanations to understand.

Friends who have seen quantum series should be familiar with it. Quantum fluctuation is simply that a pair of virtual particles will randomly appear in vacuum and then annihilate quickly. But there is a case where this process can be interrupted unexpectedly, when fluctuations appear near the horizon of the black hole.

When two virtual particles, one inside and one outside the visual interface, are separated by Yin and Yang, they cannot annihilate each other as usual. The virtual particle inside is swallowed by the black hole, while the outer particle escapes. Those particles that escape are regarded as particles radiating outward from the black hole.

Although black holes can theoretically emit heat in this way, this radiation is extremely weak, even much weaker than the cosmic microwave background radiation, so black holes are very cold, if at all. Moreover, the temperature of a black hole is inversely proportional to its mass. The larger the black hole, the lower the temperature.

For a black hole with ten solar masses, its temperature is only about 6nK, or 10−9K, which is only about a billionth of a degree above absolute zero. The silver-centered black hole, which has a mass of 4 million times that of the Sun, has a magnitude of 10−14K. For supermassive black holes of greater mass, it can even reach 10−18K.

Since the escaping particles are real particles with positive mass, those swallowed by the black hole are considered to have negative mass according to conservation of mass. As a result, the black hole loses mass by devouring negative-mass particles, and it also gets hotter and hotter, so evaporation becomes faster and faster.

However, the black holes discovered so far are at least several times the mass of the sun star black holes, their Hawking radiation temperature is much lower than the cosmic microwave background radiation temperature, so for a long time, these black holes can not lose mass through Hawking radiation, after all, the ambient temperature is higher than its own. Unless they are tiny black holes with a radius of only 0.1 mm and the mass of the moon, only black holes smaller than this mass will have Hawking radiation temperatures greater than the current background radiation temperature and may begin to evaporate themselves.

However, as the universe continues to expand, the temperature of the background radiation will eventually fall below the Hawking radiation temperature of massive black holes. At that time, these black holes in the universe will begin to "evaporate" one by one, but this evaporation process is also unimaginably long. After about a gugal year (10100 years), these black holes eventually evaporate to particle size and disappear into space with a bang.

This article comes from Weixin Official Accounts: Linvo Says Universe (ID: linvo001), Author: Linvo

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