Unity

A Brief History of Time is a book that I have long heard of, but I have always been "intimidated" by its difficulty and have not started reading it. However, as I deepen my study of physics, I can no longer resist the temptation to challenge this masterpiece and to enrich and improve my understanding of the study of the world and time in modern physics as much as possible.

As an advanced organism, human beings have never stopped exploring the mysteries of the universe for millions of years. From Aristotle in 340 BC to the various astronomical and cosmological theories that are constantly developing today, they all reflect our persistent pursuit of our own origins and the meaning of life. In this book, what impressed me most was what the author mentioned: Any physical theory is temporary, because no matter how many times the experimental results are consistent with a certain theory, you cannot guarantee that the next result will not contradict it. If a contradiction occurs, we can only abandon or revise the theory.

From the first chapter, Hawking raises the ultimate question of the universe to readers: Why are we here? Where do we come from? This is what ancient philosophers and modern scientists have been struggling for for hundreds and thousands of years. In fact, looking at the entire history, this ultimate question is the reason for the existence of human science. For the development of physics, humans have tried their best to develop a unified theory that can summarize all observed phenomena in the universe into a unified system.

In ancient times, philosophers tried to understand the nature of the universe through logical reasoning and thought experiments, and more often through religious descriptions of "explanations". For example, the turtle universe assumes that the world or the universe is carried by a giant turtle. The turtle is often depicted as standing on four elephants, and these elephants are standing on the back of another larger turtle, and so on. If you think about it more deeply, this model is actually a classic example of explaining the ultimate question of the universe, that is, "What is under the world?"

As the book says, the earliest theories describing the universe all involved this idea: “Events and natural phenomena were controlled by spirits with human emotions who acted in a very humanlike and unpredictable manner. These spirits inhabited natural objects, like rivers and

mountains, including celestial bodies, like the sun and moon. They had to be placated and their favors sought in order to ensure the fertility of the soil and the rotation of the seasons.”

However, people gradually realized that as scientific observation and research methods gradually improved and were promoted, more theories that seemed to be closer to the essence of the universe, or theories that could better unify the observed phenomena, were promoted, such as the Copernican system, the Rutherford atom, etc. People began to pay attention to the basic components of the universe we live in, trying to explore the laws of the universe from the perspective of composition. However, until we discovered quarks, the essence of the universe was still unknown. Later, with more explorations of general relativity and quantum mechanics, people tried to combine these two theories that seemed to be closer to the answer to the universe. In them, people could only adjust the value of the gravitational strength and the cosmological constant, but it was not enough to eliminate the mass of infinite particles derived from the mass-energy equation.

When people get stuck in a dilemma, new theories will emerge. Under this "infinite" problem, the "supergravity theory" briefly appeared, which seemed to explain the "infinity" of the universe, but it was also contrary to people's observations. It was not until 1984 that the string theory, which was more acceptable to people, appeared. In this theory, the view that the basic object is a particle was stripped away and turned into an infinitely thin string, and the long-range force in the previous particle theory was described as a connecting pipe.

The search for a unified theory of the universe continues, and it seems that it is impossible to find a theory that applies to all observable phenomena. This leads to several possible ultimate answers to the universe:

1. There is indeed a complete unified theory, and if we are smart enough, we will find it one day.
2. There is no final theory of the universe, only an infinite sequence of theories that describe the universe more and more accurately.
3. There is no theory of the universe: it is impossible to predict events beyond a certain point, and events only occur in a random or arbitrary way.

With the development of quantum mechanics, people have come to realize that everything seems to have a certain degree of uncertainty. If you like, you can attribute this randomness to the interference of God. Yes,

God plays dice.

As for (2.), if it is true, we can expect to find particles that are more basic than quarks and electrons, forming a "box-in-box" sequence. However, gravity can set a limit to this sequence, so there must be an ultimate theory of the universe. But even if we find the true ultimate theory, it can never be proven, and we will never know whether it is correct.

In Newton's time, an educated person could roughly explain the entire human knowledge and operation. But with the development of the times, this is no longer possible. Even if we find a complete system, we are constrained by the uncertainty principle of quantum mechanics and cannot predict things, let alone accurately solve the equations of this system.

Einstein once asked, "How much choice did God have in constructing the universe?" Because if the no-boundary hypothesis is correct, God has no freedom to choose the initial conditions. Yes, most scientists are studying what the universe is, but they don't have a full understanding of why the universe is what it is. From the 18th century to the 19th and 20th centuries, philosophers have changed from discussing questions such as whether the universe has a beginning to "the remaining task of philosophy is only language analysis." Therefore, if we find a unified theory, it should also be available to all people to understand and explore.