Interpretation of "A Brief History of Time"

This book review interprets a milestone in the world's scientific works, Professor Hawking's "A brief history of time," Stephen Hawking.

"A Brief History of Time" is one of the most famous and best-selling popular science books in the world. After its publication in 1988, it appeared on the bestseller list for 237 consecutive weeks, which was four and a half years, setting an unprecedented best-selling record. The book has been translated into more than 40 languages ??and has printed more than 10 million copies. , which means that on average every 700 people in the world own a copy of "A Brief History of Time", so one of Professor Hawking's students jokingly said: "Professor Hawking's popular science book is better than Madonna's book on sex." Easy to sell. "

In recent years, there have been many books called "A Brief History of Everything", such as "A Brief History of Everything", "A Brief History of Humanity", "A Brief History of the Future", etc. Professor Hawking's "A Brief History of Time" pioneered this naming method.

The book "A Brief History of Time" is not very thick, with only more than 200 pages and hundreds of thousands of words, but the content covered in the book is very extensive, from the theory of relativity to quantum mechanics, from the universe From expansion to elementary particles, from black holes to wormholes, Professor Hawking introduced them all. So after reading this book, you will have a deeper understanding of our world and universe.

But to be honest, the book "A Brief History of Time" is not very friendly to ordinary people - it is too difficult. Because Professor Hawking only gave a very brief introduction to many concepts in physics, and some of them were even mentioned briefly. People without a certain foundation of physics knowledge may not understand what Professor Hawking is talking about. An organization has conducted a survey. Many people who have read "A Brief History of Time" have forgotten all the content except the jokes and history mentioned in the first chapter. After reading it, they have not read it. On Kindle, "Time" The average reading progress of "A Brief History of Time" is only 6%, which means that many people bought "A Brief History of Time" but gave up after reading the first chapter.

So I cheered up and tried to explain to you the essence of "A Brief History of Time" and Professor Hawking's contribution to the study of the universe. Because when Professor Hawking commissioned his two Chinese students to translate "A Brief History of Time" into Chinese, he once said: "I hope that one-fifth of the world's population will understand my research."

< p> Professor Hawking believes that his most important academic contribution is the study of singularities and black holes, so in this interpretation, I divide Professor Hawking's important theories into three parts in chronological order - —

First, Professor Hawking and Professor Penrose proved the famous singularity theorem, which is related to the general theory of relativity and the expansion of the universe;

Next, Professor Hawking proposed The boundaryless universe model believes that the universe has no boundaries, no beginning and no end, which is a challenge to God;

Finally, Professor Hawking proved that black holes can also emit radiation, and this radiation is known as It's called Hawking radiation.

Let’s first talk about what the singularity theorem proved by Professor Hawking and Professor Penrose.

Before the twentieth century, people had always assumed that the universe was stationary. Even though Newton's law of universal gravitation had stated that different objects would attract each other, people still invented various methods. , to maintain the idea that the universe is static, even the great god Einstein stumbled on this issue.

Einstein published the general theory of relativity in 1915, proposing that space and time can be curved. According to Einstein’s equations, the universe is not static, but Einstein believed that this must be because of something. Wrong place. So Einstein forcibly added a "cosmological constant" to his equations. In this way, according to the modified equations, the universe became stationary.

But in the 1920s, when scientists were observing other stars, they discovered that many stars had experienced a phenomenon called "red shift," which meant that these stars were moving away from the earth. , originally people thought that the movement of these stars was random, that is to say, some stars are getting farther and farther away from us, but some stars are getting closer and closer to us.

But in 1929, the American astronomer Hubble (the same Hubble who named the Hubble telescope) discovered that the vast majority of stars are moving away from the earth, and these stars are farther away from the earth. The further away they are, the faster they are moving away from Earth.

What does this mean? Let me give you an example:

If you take a balloon, draw a few dots with a watercolor pen on different places on the surface of the balloon, and then blow up the balloon. As the balloon gets bigger and bigger, you draw The distance between those points will become farther and farther, and the farther away the two points are, the faster they will move away from each other.

This balloon is actually equivalent to our universe. The points on the balloon are equivalent to the earth and other stars. Most stars are getting farther and farther away from the earth. This means that we The universe is constantly expanding.

After Einstein learned about this observation result, he discovered that the cosmological constant he forcibly added to the equation was completely redundant. The universe was not static, but indeed expanding, so love Einstein later said: "The cosmological constant was the biggest mistake I made in my life."

After discovering the expansion of the universe, scientists found a new model to describe the expanding universe based on Einstein's general theory of relativity. This is the Friedmann Universe. Model.

Scientists have discovered that in the Friedman universe model, if we push forward in time, the universe will shrink. If we push back to 15 billion years ago, the distance between all galaxies will be It will become 0, as if the entire universe was squeezed tightly into a point.

This point is called a singularity. It is the beginning of space and time in the universe.

The properties of a singular point are very special. It is difficult for us to intuitively imagine what it is, but through mathematical calculations we can know that the volume of a singular point is infinitely small, the degree of curvature is infinite, and the density is infinite. Gravity is also infinite.

There are many infinitesimals and infinitesimals here, but mathematics actually has no way to really handle the calculations of infinitesimals and infinitesimals. This means that although the Friedman universe model is based on the generalized It was proposed by the theory of relativity, but all physical theories, including general relativity, fail at the singularity.

Therefore, Professor Hawking said that the emergence of singularities proves that general relativity is only an incomplete theory. Because it failed at the very beginning of the universe.

Now that we understand what the singularity is, let’s take a look at what contribution Professor Hawking made in this research process? How does he view the singularity?

When the theory of singularity was first proposed, not all physicists agreed with this statement. According to Professor Hawking, Einstein himself did not believe this statement. Stein believes that if we push time forward, different galaxies will not collide with each other, but will be exactly staggered.

But when Professor Hawking read some related research by Professor Penrose, he realized that the singularity theory should be established.

After several years of hard work, Professor Hawking and Professor Penrose used rigorous mathematical methods to prove that if the general theory of relativity is correct, and there are indeed as many stars, galaxies, etc. in the universe as we have observed, matter, then the universe must have been born in a singularity a long time ago. This singularity is the beginning of the universe.

This theory is now called the Penrose-Hawking Singularity Theorem.

Many people were reluctant to accept Professor Hawking’s research conclusion at first because they did not like the idea that the universe had a beginning. Others believed that this conclusion violated scientific determinism and ruined perfection. The general theory of relativity, but there is no way, because Professor Hawking did not just talk nonsense, he was down-to-earth and used mathematical calculations to prove this result. Mathematics cannot lie to people, and people cannot argue with mathematical laws.

So now, whether they like it or not, almost all physicists admit that our universe was indeed born in a singularity.

We say that proving the singularity theorem is one of Professor Hawking's greatest scientific contributions. However, later, Professor Hawking changed his mind and proposed the unbounded universe model, believing that the universe does not need to be born in In the midst of singularity.

So next, let’s take a look at what’s so special about the boundless universe model proposed by Professor Hawking?

As we mentioned earlier, Professor Hawking and Professor Penrose proved the singularity theorem. After that, almost all physicists admitted that our universe was born in a singularity. The universe The moment of birth is called the Big Bang!

The Big Bang theory is currently the most mainstream model describing the origin of the universe. Let’s take a look now at the Big Bang model, a description of the moment of the origin of the universe -

At the moment of the Big Bang, the volume of the universe was 0 and the temperature was infinitely high. After that, the universe began to expand rapidly and the temperature began to rise. Reduced;

One second after the Big Bang, the temperature of the universe dropped to 10 billion degrees, which is probably the highest temperature that can be reached when a hydrogen bomb explodes;

One hundred million degrees after the Big Bang Seconds later, the temperature of the universe continued to drop to 1 billion degrees, and protons and neutrons began to combine together to form atomic nuclei;

After that, the temperature of the universe continued to drop and continued to expand. One million years later, the universe The temperature dropped to several thousand degrees, and atoms began to form;

As the temperature of the universe continued to drop, stars, galaxies and other materials began to form;

Now, 15 billion years later, the universe has It has expanded many, many times and the temperature is already very low.

The current temperature of the universe is only a little higher than absolute zero, but the energy of the Big Bang still remains in various places in the universe, and we can still observe it. Scientists call it the microwave background radiation. , as if it were filled with cosmic background noise.

Although the Big Bang theory sounds mysterious and a bit incredible, it is consistent with our observational evidence today, so it has been widely accepted by physicists.

However, this theory still makes many physicists a little embarrassed. You see, from my previous description, you can feel that the big bang is the singularity of the universe. So, in the universe All matter, even space and time, only appeared after the Big Bang. Any events before the Big Bang have no meaning to us, or do not exist at all. In other words, the space and time of the universe are not infinite. , but has a very special boundary and beginning. This beginning is the Big Bang Singularity.

But the problem is, as we mentioned before, the big bang singularity is a very special existence. Here, all the laws of physics fail, and people simply don’t know how to get from this singularity. Something will emerge from the dot. So, how did the laws that the universe follows now come about? Who sets these rules?

Is it really God?

Because scientists have not yet fully explained this problem, there is only a glimmer of hope for the religious community.

Professor Hawking recalled that in 1981, the Pope invited many scientists and held a cosmology conference. After the scientists' speeches, the Pope said that various histories after the Big Bang, various You can all study the laws, but you scientists should not study the moment of the Big Bang, because that was the moment when all things were born, and that was God’s business.

Professor Hawking also gave a speech at this conference. The theme of the speech was the boundless universe model. After hearing the Pope’s words, Professor Hawking muttered in his heart, “It seems that the Pope didn’t understand. What I’m talking about!”

Because at this time, the boundaryless universe model proposed by Professor Hawking means that the universe has no beginning and no moment of birth, so there is no place for God at all. .

Why did Professor Hawking say this?

Let’s take a look at what the boundless universe model is all about.

When we mentioned the singularity theorem and the Big Bang model before, we always had a premise, that is - the general theory of relativity is correct.

But this premise may not actually be tenable. This does not mean that general relativity is wrong, but that it is an incomplete theory that is not combined with quantum mechanics.

In the twentieth century, the two most important discoveries in physics are also the two most basic theories in physics today, which are the theory of relativity and quantum theory.

Generally speaking, relativity deals with the macroscopic realm, such as the movement of galaxies, while quantum mechanics deals with the microscopic realm, such as particle motion. When studying general theories, these two sets of The system is like water in a well but not in the river.

But Professor Hawking believes that when studying the Big Bang Singularity, it is necessary to combine the theory of relativity and quantum mechanics. He believes that the Singularity Theorem has already explained that in the Big Bang Singularity, In extreme cases, general relativity can no longer describe the universe well. At this time, quantum effects must be taken into account.

On the basis of general relativity, there are only two possibilities for the universe, either it has an infinite time, or it has a beginning like a big bang singularity. However, if quantum mechanics is introduced, A new possibility will appear, that is, a finite and unbounded universe.

Finite and unbounded means that the space and time of the universe are finite, but have no boundaries. Does it sound a little dizzy? Still a bit contradictory? Let me give you an example and you will understand it easily:

Let’s use the earth as an analogy. Our earth is a finite planet. We know its diameter and size, but the earth There is no boundary or beginning. If you keep walking on the earth, you will never reach the boundary of the earth, let alone fall outside the earth. Therefore, the earth is a finite and boundless planet.

Professor Hawking’s boundaryless universe model is a four-dimensional universe in which space and time are intertwined. In this universe, space and time are like the surface of the earth, which is limited in scope, but there is no A boundary is formed, and no singularity is formed. Every point in space and time is just like a certain point on the earth. There is nothing special. The laws of science are applicable at any point in space and time, and failure and collapse will not occur.

In the unbounded universe model, the universe does not have a special singularity, and there is no need for God to specify a special law. According to Professor Hawking, this universe is completely self-sufficient and does not It will be affected by any external things, and there is no moment of creation and disappearance. In this way, this universe does not need God to create it.

It should be noted that Professor Hawking himself has repeatedly emphasized that the boundaryless universe model is currently just an idea, which is equivalent to his own opinion. The current mainstream universe model is still the big bang model.

Of course, the boundaryless universe model is not just talk. It is an attempt to combine the theory of relativity and quantum mechanics, and it is also a scientific theory that can be tested.

The previous two theories are all related to the Big Bang and the singularity. In addition to this field, Professor Hawking's other most important academic contribution is the study of black holes. So let’s take a look at what contribution Professor Hawking made to the study of black holes. Why did he say that black holes are not that black?

Let’s first take a look at what a black hole is.

This starts from the principle of stars. We all know that matter will generate gravitational force, and the greater the mass of the object, the greater the gravitational force. Stars like the sun, It has a very strong gravitational pull. A star will have a tendency to collapse inward under the influence of its own gravity. So if this is the case, why is the sun still so big after so many years? Why didn't it collapse due to internal gravity?

This is because, inside the sun, there is a supporting force that can balance the gravity. The temperature inside the sun is very high, and hydrogen atoms with atomic number 1 will combine if they collide violently. Together, they form helium atoms with atomic number 2. During this process, huge energy (nuclear fusion) will be released, which is equivalent to countless hydrogen bombs inside the sun, constantly exploding, which will provide the sun with a The repulsive force of support, in this way, when the gravity and support force of the sun balance each other, the star can remain stable, so the sun has not collapsed so far.

Then the problem arises. The internal support of the star is supported by these "nuclear raw materials", but these raw materials will burn out sooner or later. Once burned out, the star will Under the influence of gravity, it continues to collapse. What will happen at this time?

There are several possibilities for the final destination of a star. If the mass of the star is relatively small, and it collapses to a certain extent, its gravity will eventually balance with the repulsion between the internal particles, and it will eventually become Neutron star or white dwarf, but if the star is huge, larger than half the sun, then any repulsion between its internal particles will not be enough to offset the gravity, and the star will continue to collapse and become extremely small. , an extremely massive black hole.

After understanding the above formation principles of black holes, the properties of black holes can be easily understood. The biggest characteristic of black holes is that they have extremely strong gravity. As long as any material, including light, enters a certain critical area, it will It will never be possible to escape from the black hole. If someone gets close to the black hole, the huge gravity will quickly stretch and tear him apart.

From the outside, if even light cannot escape, then of course the black hole is completely black, which is why the black hole gets its name. But the problem is that the property of black holes that attracts everything conflicts with the second law of thermodynamics. According to the second law of thermodynamics, something like a black hole should have a temperature. As long as there is a temperature, it should emit radiation and particles. Black holes should be no exception. But as mentioned before, even light cannot escape the gravity of a black hole. Why? Is it possible that particles can escape from a black hole?

In fact, almost all physicists, including Professor Hawking, were confused by this question. However, using quantum theory, Professor Hawking later successfully answered this question.

Professor Hawking discovered that black holes do emit particles, but these particles do not escape from the black hole, but emerge from the empty space at the edge of the black hole.

To understand Professor Hawking’s statement, the most important thing is to understand the Heisenberg Uncertainty Principle in quantum theory.

The Heisenberg Uncertainty Principle says that when we measure particles, gravitational fields, electromagnetic fields, etc., we will find that if we measure one of the physical quantities more accurately, then The more inaccurate the measurement of another physical quantity, for example, the more precise you measure the speed of a particle, the fuzzy your understanding of the particle's position will be. It is impossible for humans to grasp the precise position and precise speed of a particle at the same time.

According to the Heisenberg Uncertainty Principle, even an empty space that seems to contain nothing is actually undulating on a microscopic level. Think about it, if there is a space that is completely empty, it means that the intensity of the gravitational field or the electromagnetic field is zero, and the rate of change is also zero. These two physical quantities can be accurately grasped at the same time. This is obviously a violation of , the Heisenberg Uncertainty Principle, is impossible.

So what is going on in the empty space? It's very magical. In the empty space, pairs of particles are actually produced all the time. In order to maintain energy conservation, some of these particles have positive energy and some have negative energy. They collide together. will be annihilated at the same time.

At this point, we can take a look at what Professor Hawking’s explanation means. Professor Hawking proposed that although the edge of a black hole seems to be empty, it is actually producing pairs of particles all the time. The particles with negative energy will be sucked into the black hole, but there are still some Particles with positive energy can escape from the edge of a black hole and go elsewhere. So when we stand outside the black hole and look at it, it looks like the black hole is constantly emitting these particles outward. This is black hole radiation, also called Hawking radiation.

It is precisely because of the existence of this radiation that Professor Hawking said that black holes are not completely black.

Hawking radiation is a very valuable theory in physics, because the birth of a black hole is calculated using general relativity, and the radiation of a black hole is explained using quantum mechanics, that is to say , Hawking radiation is an attempt to combine general relativity, quantum mechanics and thermodynamics, so although Hawking radiation has not been observed, it still has high theoretical value.

At this point, we have finished talking about the essence of "A Brief History of Time", which is Professor Hawking's contribution to the study of the universe.

Let’s review it as a whole -

The first point we talked about is the Penrose-Hawking singularity theory. A singularity is a point with infinitely small volume, infinite curvature, density and gravity. The two people used rigorous mathematical methods to prove that if the general theory of relativity is correct and there is indeed as much matter in the universe as we observe, then The universe must have been born in a singularity;

The second point we talked about is the unbounded universe model proposed by Professor Hawking. In the current mainstream Big Bang model, the universe has a starting point. It is the Big Bang Singularity, but at the moment of the Big Bang, all the laws of physics failed, so scientists still can’t explain why the universe must be the way it is now. Some people say that that was the moment when God created the world. So it cannot be explained by science. Different from the Big Bang model, Professor Hawking introduced quantum mechanics in his boundaryless universe model, believing that the universe has no boundaries, so there are no singularities, no beginning and no end. In this way, there is no need for God to come. The universe was created;

The third point we talked about is about Professor Hawking’s research on black holes, which is Hawking radiation. Professor Hawking proposed that black holes are not completely black. They also emit radiation. According to the uncertainty principle, the void at the edge of a black hole is actually not completely empty, but instead continuously produces pairs of particles. Among them Some particles are absorbed by the black hole, but some particles escape from the edge of the black hole. This is equivalent to the black hole emitting particles outward. This phenomenon is Hawking radiation.