At that time, the name of "black hole" had not yet arisen, and it was called "Schwarzschild singularity", which was quite different from today's reputation. Einstein, Eddington and other Daniel of general relativity regard it as a false continuation of the "dark star" more than 200 years ago.
From the speculation of 1783 to the debate of 19 16 to 1960, physicists are actually discussing whether there is this problem in black holes.
In the heated discussion, the black hole gradually established its cosmic coffee place. Even though it has encountered too many misunderstandings in these hundreds of years, it still gracefully enters the field of vision of physicists today and is more and more deeply rooted in people's hearts.
What are our misunderstandings about black holes since we first met? How do we look at it now?
1783, an Englishman fired a shot into the sky on an imaginary planet, and the speed of the shell leaving the hall was 300,000 km/s. ...
This man is John Lin Kewei, a British natural philosopher. He boldly combined the popular theory of light particles with Newton's law of gravity at that time and made a thought experiment of light shell.
At that time, people already knew that although we were all bound by the gravity of the earth, as long as the speed was high enough, we could get rid of it. The minimum initial velocity that can get rid of this bondage is called "escape velocity". On the surface of the earth, this speed is11.2km/s.
On the other hand, if the speed does not reach the escape speed, the object will be pulled down by gravity.
Michel proved by Newton's law of gravity that the square of the escape velocity of a celestial body is directly proportional to its mass and inversely proportional to its radius. The smaller the constant mass radius, the greater the escape velocity of celestial bodies.
If the radius of a planet can be compressed, the escape speed can exceed 300,000 km/s, which means that the planet makes it impossible for light to escape. On such a planet, Michelle's light shell will never fly into space.
Take the earth as an example, as long as it is compressed to a radius of only 1/3 inches, the size of a chocolate bean will produce this effect.
Can such a high-density planet exist? Michelle thinks it's possible. He even thinks that there are a lot of invisible planets in the night sky and calls them "dark stars". This is the earliest and most primitive concept of black hole.
1783165438+1On October 27th, Michelle reported the prediction about dark stars to the Royal Society. It was 13 years later that Pierre-Simon Laplace, a French natural philosopher, made the same prediction in the 1 version of his masterpiece Cosmic System Theory.
However, in 1808, Thomas Young discovered the phenomenon of double-slit interference of light, which made the balance of optical "wave-particle struggle" tend to the wave theory proposed by Christiann Huygens. Newton's theory of light particles has been replaced by the theory of light fluctuation.
Light particles that are affected by gravity like shells become light waves that seem unaffected by gravity (people didn't know what effect gravity would have on light waves at that time). Probably for this reason, Laplace's theory of cosmic system has deleted the description of dark stars since the third edition. The concept of dark star is silent, and no one cares.
It was not until 100 years later that Einstein balanced the theoretical balance of optics, ended the "wave-particle dispute" of light and developed the "wave-particle duality" of light.
1915438+01This month, the general theory of relativity was born, which allowed physicists to re-establish the cognition of gravity to light, but this time the concept of "curvature of spacetime" was used. Gravitation is the curvature of space-time intuitive feeling, and light and all objects must make "short-line" motion in space-time without external force.
The so-called "short line" can be said to be the real shortest path in time and space, while the daily "straight line" is more of a sensory definition.
Less than a year after the publication of general relativity. 19 16, the dark star predictions of Michelle and Laplace were presented in a more bizarre way in physics by a German artillery captain: karl schwarzschild.
At that time, Schwarzschild, who was still curled up in the trenches of the Russian front, abandoned the complicated problem of celestial rotation in a concise and effective way, and calculated the curvature of spacetime inside and outside any non-rotating spherical celestial body according to the field equation of general relativity, and obtained the exact solution describing the black hole.
Taking the speed of light as the escape speed, any celestial body has a schwarzschild radius, which corresponds to the critical perimeter of the dark star calculated by Michel and Laplace. However, thanks to the concept of "curvature of spacetime", the curl of space means that light cannot escape, and the curl of time also means that time flows slowly (time expansion effect).
However, Einstein disagreed with the idea that "celestial bodies will collapse into singularities after being compressed to schwarzschild radius".
While appreciating the curvature of spacetime calculated by Schwarzschild, Einstein did not think that there was a "Schwarzschild singularity" in nature. After all, there is no celestial body that does not rotate. Coupled with his ignorance of star collapse, Einstein arbitrarily denied this rational attribute of general relativity.
1939, Einstein even published an article on the calculation of general relativity to explain why the "Schwarzschild singularity" could not exist in nature.
He assumed a moving particle swarm attracted by gravity, and then proved by calculation that the gravity on the sphere will increase as the collection gets closer and closer, and the particles moving on the sphere have to speed up in order to produce enough centrifugal force.
But when this group is less than 1.5 times the critical perimeter, the gravity will become very large, and the particles on the surface will exceed the speed of light. Therefore, the particle swarm cannot be less than the critical value 1.5 times.
Even Einstein calculated the internal pressure of celestial bodies and came to the conclusion that when the circumference of a celestial body is compressed to 1. 125 times of the critical circumference, the pressure in the center will become infinite, but infinite pressure cannot exist. Therefore, a celestial body cannot be less than 1. 1.25 times the critical perimeter.
Einstein's calculation is correct, but his understanding is wrong. This is because physicists at that time had a tendentious concept: if a celestial body can exist, it must balance internal and external forces. However, the fact is that internal forces can be abandoned.
In this battle to understand black holes, Einstein's intuition that helped him gain insight into gravity hindered his insight into black holes. It can be seen that the correct result sometimes does not necessarily lead to the correct answer.
From the 1920s to 1950s, physicists' research on "Schwarzschild Singularity" actually focused on only one question: Is this kind of object allowed in nature?
It was not until the end of 1960s that the mathematician Kerr calculated the exact solution of the rotating black hole, and the astronomical community made further discoveries in the observation of black holes, and the evidence supporting the existence of black holes began to overwhelm all doubts. 1967, American physicist john archibald wheeler officially named it "black hole". Most physicists begin to face black holes seriously.
Before the 1960s, people mainly used general relativity to study the space-time structure of black holes. The main achievements of black hole physics research in this era belong to the classical theory of black holes.
For example 1967, Werner israel proved the hairless theorem, which stipulated that the horizon must be completely smooth. Based on this theorem, we can also deduce that black holes are only determined by three physical quantities: mass, angular momentum and charge, and advance to the "San Mao theorem".
And in 197 1, Hawking proved the "area theorem" of black holes, that is, the area of the event horizon of black holes never decreases in time sequence. This means that black holes can only merge and never split. At that time, Hawking also proved that the temperature of the black hole was absolute zero according to the classical theory, but this was later falsified by himself.
After 1960s, black holes began to be studied in a new direction of thermodynamics.
Inspired by the concept of black hole entropy of Israeli physicist Jacob Bekenstein, Hawking put forward "Hawking radiation" in 1974, that is, due to vacuum fluctuation, virtual particle pairs generated near the black hole may be separated by the horizon, one virtual particle falls into the black hole, the other successfully escapes, and then becomes a real particle.
To a distant observer, it is like a black hole radiating. Moreover, due to the different space-time structures inside and outside the black hole, the falling particles are mostly negative particles, so the black hole will lose mass due to Hawking radiation. Radiation also means that black holes have temperature.
A black hole with five times the mass of the sun has a theoretical temperature of about 10-7K, and it takes 10 62 years to disappear without eating or drinking. The temperature of a black hole is inversely proportional to its mass, so the smaller the mass, the stronger the radiation, the higher the temperature and the shorter the life span.
The emergence of Hawking radiation can be said to have opened up the research in the quantum field of black holes. A black hole will evaporate, which means that the information it eats will disappear one day, which is not allowed by quantum mechanics. In order to counter the paradox of black hole information, complementary principle and holographic principle appeared, which led to the paradox of black hole fire wall.
So far, how to deal with these paradoxes is still a mystery.
In a word, the specific characteristics of black holes born in general relativity need to be described by quantum mechanics, and Einstein has always questioned quantum mechanics, which may be the reason why he resisted black holes in the first place.
However, for this reason, physicists are more and more fascinated by black holes, because in the field of black hole research, physicists seem to have found the possibility of combining general relativity and quantum mechanics, the two greatest achievements of physics in the 20th century.
In order to touch the physical holy grail of "the principle of everything" (that is, a single theory explains all physical phenomena), it is a crucial step to deeply understand black holes.