Analysis:
Light does not represent time, but time moves at the speed of light. There is no precise definition of time. Einstein himself later proved that superluminal cannot go back to the past.
How people's views on the nature of time have changed for a long time. Until the beginning of this century, people believed in absolute time. In other words, each event can be marked in a unique way with a number called "time", and all good clocks are consistent when measuring the time interval between two events. However, the discovery that the speed of light is always constant for any moving observer leads to the theory of relativity; In the theory of relativity, people must abandon the idea that there is only one absolute time. On the contrary, each observer carries a clock that records his own time measurement-clocks carried by different observers may not necessarily read the same. In this way, time becomes a more subjective concept for the observer who makes the measurement.
When people try to unify gravity and quantum mechanics, they must introduce the concept of "virtual" time. Virtual time and spatial direction are inseparable. If a person can go north, he can turn his head and go south; Similarly, if a person can move forward in his spare time, then he should be able to turn around and go back. This shows that in virtual time, there can be no important difference between forward and backward. On the other hand, when people look at "real-time" time, as we all know, there is a huge difference between forward and backward directions. Where does the difference between the past and the future come from? Why do we remember the past instead of the future?
The laws of science do not distinguish between the past and the future. More precisely, as mentioned earlier, under the joint action (or symmetry) called C, P and T, the laws of science remain unchanged. (C) refers to replacing particles with antiparticles; P means mirror image, so the left and right are interchanged; T means to reverse the direction of motion of all particles, that is, to reverse the motion. Under all normal circumstances, the scientific laws that constrain the behavior of objects remain unchanged under CP joint symmetry. In other words, for the inhabitants of other planets, if they are our mirror images and are made of antimatter instead of matter, then life will be exactly the same.
If the laws of science remain unchanged under CP joint symmetry and CPT joint symmetry, they must remain unchanged under separated T symmetry. But in the real time of daily life, there is still a big difference between the direction of forward and backward. Imagine a glass of water falling from the table to the floor and being broken. If you record it, you can easily tell whether it is going forward or backward. If you go back, you will see the pieces suddenly get together and leave the floor, and then jump back to the table to form a complete cup. You can conclude that the video is played backwards, because this kind of behavior has never happened in daily life. If this happens, there will be no business in the ceramic industry.
Why have we never seen a broken cup get up, leave the ground and jump back on the table? The usual explanation is that this violates the second law of thermodynamics, which points out that in any closed system, disorder or entropy always increases with time. In other words, it is a form of Murphy's law: things tend to get worse: a complete cup on the desktop is in a highly orderly state, while a broken cup on the floor is in a disorderly state. It is easy for people to change from a cup on the table to a broken cup on the ground later, not the other way around.
The increase of disorder or entropy with time is an example of the so-called time arrow. The arrow of time distinguishes the past from the future and gives time a direction. There are at least three different time arrows: the first is the thermodynamic time arrow, that is, the disorder or entropy increases in this time direction; Then there is the psychological arrow of time, which is the direction in which we feel the passage of time, in which we can remember the past instead of the future; Finally, there is the cosmological time arrow. The universe is expanding, not contracting.
In this chapter, I will demonstrate that the unbounded state of the universe and the weak anthropic principle can explain why all three arrows point in the same direction. In addition, why there must be a clear time arrow. I will prove that the psychological arrow is determined by the thermodynamic arrow, and both arrows must always point in the same direction. If people assume that the universe has no boundary conditions, we will see that there must be clearly defined thermodynamic and cosmological time arrows. But for the whole history of the universe, they don't always point in the same direction. However, I will point out that only when they point in the same direction can there be suitable conditions for the development of intelligent life to ask why the universe is expanding in disorder.
First of all, I want to discuss the thermodynamic time arrow. The fact that disordered states are always much more than ordered states is the reason why the second law of thermodynamics exists. For example, consider a box of puzzles. There is only one arrangement that can combine these small pieces of paper into a complete picture. On the other hand, there are a lot of arrangements when small pieces of paper are out of order and can't be put together into a painting.
Suppose a system starts from one of these ordered states. With the passage of time, this system will evolve according to the laws of science, and its state will change. Later, because there are many disordered States, the possibility of being in disordered state is greater than that of being in ordered state. In this way, if a system obeys a highly ordered initial condition, the degree of disorder will increase with time.
Suppose the pieces of paper in the jigsaw puzzle box start from an orderly combination and can be arranged into a picture. If you shake the box, these pieces of paper will adopt other combinations, which may be out of order, and cannot form a suitable picture, because there are too many out of order combinations. Some pieces of paper may form some pictures, but the more you shake the box, the more likely it is that these pieces of paper will separate, and these pieces of paper will be in a completely chaotic state, in which they cannot form any kind of pictures. In this way, if the paper starts from the initial condition of a highly ordered state, the disorder degree of the paper is likely to increase with time.
However, if God decides that the universe must end in a highly orderly state no matter what state it starts from, then the universe may be in a disorderly state in the early days. This means that the degree of disorder will decrease over time. You will see the broken cup rise and jump back to the table. However, anyone who observes cups lives in a universe where disorder decreases with time, and I will come to the conclusion that such people will have a backward psychological time arrow. In other words, they will remember future events, not past events. When the cup is broken, they will remember what it looks like on the table; But when it is on the table, they won't remember that it is on the ground.
Because we don't know the details of the brain's work, it is quite difficult to discuss human memory. However, we do know how the computer's memory works. So, I will discuss the psychological time arrow of computer. I think it is reasonable to assume that computers and humans kill two birds with one stone. If not, people may crash the stock exchange because they have a computer to remember next year's price.
Generally speaking, a computer's memory is a device that contains elements that can exist in either of two states, and an abacus is a simple example. In its simplest form, it consists of many iron bars; There is a rosary on each iron bar, which can stay in one of two positions. Before the computer memory is stored, its memory is disordered, and the rosary has two possible states. Abacus beads are scattered on the iron bars of the abacus in disorder. After the memory interacts with the system to be memorized, it must be in one state or another according to the state of the system (each abacus bead will be located on the left or right side of the iron bar. ) In this way, memory changes from disorder to order. But in order to ensure that the memory is in the correct state, it needs to use a certain amount of energy (such as moving abacus beads or turning on the power of the computer). This energy dissipates in the form of heat, thus increasing the disorder of the universe. It can be proved that this disorderly increment is always greater than the ordered increment of memory itself. In this way, the heat emitted by the cooling fan of the computer shows that when the computer records an item in the memory, the total amount of disorder in the universe is still increasing. The direction of computer memory in the past is consistent with the direction of increasing disorder.
Therefore, our subjective sense of time direction or psychological time arrow is determined by the thermodynamic time arrow in our mind. Just like computers, we must remember things in the order of increasing entropy. This almost turns the laws of thermodynamics into boring things. Disorder increases with time, because we measure time in the direction of disorder increase. Bet on this and you win.
But why must there be a thermodynamic time arrow? Or in other words, why is the universe in a highly ordered state at the end of what we call past time? Why hasn't it been completely out of order? After all, it seems more likely. And why does the disorderly time direction increase in the same direction as the expansion of the universe?
In the classical theory of general relativity, because all the known laws of science failed at the singularity of the Big Bang, it is impossible to predict how the universe began. The universe can start from a very stable and orderly state. This will lead to the well-defined thermodynamic and cosmological time arrows we observe. However, it can also reasonably start from a very undulating disorder. In that case, the universe is already in a state of complete disorder, so the degree of disorder will not increase with time. Or remain the same, and then there is no clearly defined thermodynamic time arrow; Or it will decrease, then the thermodynamic time arrow will be opposite to the cosmological time arrow. These possibilities are inconsistent with what we have observed. However, as we can see, the classical general relativity predicted its own collapse. At that time, the curvature of space became larger, and the quantum gravity effect became more important. When the classical theory could not describe the universe well, people must use the quantum gravity theory to understand how the universe began.
As we saw in the last chapter, in the theory of quantum gravity, in order to specify the state of the universe, people still have to explain how the possible history of the universe is represented in the past space-time boundary. Only when these histories meet the borderless conditions can people avoid the difficulty of having to describe what we don't know and can't know: they are limited in scale, but there are no boundaries, edges or singularities. In this case, the beginning of time will be a regular and smooth point in time and space, and the universe will begin to expand in a very smooth and orderly state. It can't be completely uniform, otherwise it will violate the uncertainty principle of quantum theory. There must be small fluctuations in density and particle velocity, but the boundaryless condition means that these fluctuations are as small as possible under the condition of conforming to the uncertainty principle.
At the beginning of the universe, there was an exponential or "skyrocketing" period, during which its scale increased by a very large multiple. When expanding, the density fluctuation is always small at first, but then it begins to get bigger. In the area where the density is slightly higher than the average, the gravitational effect of the extra mass slows down the expansion speed. Eventually, such regions stopped expanding and collapsed to form galaxies, stars and humans like us. At first, the universe was in a stable and orderly state, and over time it evolved into a disorderly state of wave ups and downs. This explains the existence of thermodynamic time arrow.
What would happen if the universe stopped expanding and began to contract? Will the thermodynamic arrow, in turn, start to reduce the disorder with time? This leaves all kinds of possibilities of scientific fantasy for people who live from the expansion period to the contraction period. Will they see the broken pieces of the cup get up, leave the floor and jump back on the table? Will they remember tomorrow's price and get rich in the stock market? Since the universe will not begin to shrink until at least10 billion years later, it seems a bit pedantic to worry about what will happen then. But there is a faster way to know what will happen in the future, and that is to jump into a black hole. The process of star collapse forming a black hole is quite similar to the later stage of the whole universe collapse; In this way, if the disorder of the contraction phase in the universe decreases, it can be expected that it will also decrease in the black hole. Therefore, an astronaut who falls into a black hole can remember the direction of the ball on roulette before betting to win money. Unfortunately, it won't be long before he becomes spaghetti. He can't let us know that the arrow of thermodynamics is reversed, and he can't even deposit the money he won in the bank because he is trapped behind the horizon of the black hole. )
At first, I believed that when the universe collapsed, disorder would decrease. This is because I think the universe must return to a stable and orderly state when it gets smaller. This shows that the contraction period is only the time reversal of the expansion period. People in contraction will live in a retrogressive way: they will die before birth and become younger as the universe shrinks.
This concept is attractive because it shows that there is a beautiful symmetry between the expansion stage and the contraction stage. However, people can't ignore other ideas about the universe and just adopt this one. The question is: Is it implied by the boundlessness condition or is it incompatible with this condition? As I said, at first I thought that borderless conditions actually meant that disorder would decrease during contraction. I was misled in part because of the analogy with the surface of the earth. If people think that the beginning of the universe corresponds to the North Pole, then the end of the universe should be similar to its beginning, just as the South Pole is similar to the North Pole. However, the north and south poles correspond to the beginning and end of the universe in virtual time. There will be a big difference between the beginning and the end of real time. I was also misled by my research on a simple universe model, in which the collapse stage seems to be the time inversion of the expansion stage. However, a colleague of mine, Don Page of Pennsylvania State University, pointed out that the boundlessness condition does not require that the contraction phase must be the time inversion of the expansion phase. Raymond Laffler, one of my students, further discovered that in a slightly complicated model, the collapse and expansion of the universe are very different. I realized that I made a mistake: the unbounded condition means that when the phase contracts, disorder continues to increase. When the universe begins to shrink or is in a black hole, the time arrows of thermodynamics and psychology will not reverse.
What should you do when you find yourself making such a mistake? Some people never admit that they are wrong, but continue to look for new and often inconsistent arguments to defend themselves-just like Eddington did when he opposed the black hole theory. Others first claim that they have never really supported an incorrect view, and if they do, it is only to show that it is not harmonious. In my opinion, if you admit that you are wrong in your publication, it will be much better and cause less confusion. Einstein is a good example. He introduced the cosmological constant when trying to build a static cosmological model, which he called the biggest mistake in his life.
Back to the arrow of time, the remaining question is; Why do we observe that the arrows of thermodynamics and cosmology point in the same direction? Or in other words, why is the time direction of disorderly increase exactly the time direction of cosmic expansion? If people believe that the universe expands first and then contracts, as the borderless hypothesis seems to imply, then why should we be in the expansion stage instead of the contraction stage becomes a problem.
People can answer this question according to the weak anthropic principle. The conditions of contraction phase are not suitable for the existence of intelligent human beings, but it is they who can ask why the time direction of disorderly increase is the same as that of cosmic expansion. The explosion of the universe predicted by the boundlessness hypothesis means that the universe must expand very close to the critical rate needed to avoid collapse, so that it will not collapse for a long time. By then, all the stars will burn out, and the protons and neutrons in them may decay into light particles and radiation. The universe will be in almost complete disorder, and there will be no strong thermodynamic time arrow. Now that the universe is in a state of almost complete disorder, the degree of disorder will not increase much. However, a powerful thermodynamic arrow is necessary for the behavior of intelligent life. In order to survive, human beings must consume ordered energy-food and convert it into disordered energy-heat, so intelligent life cannot exist in the contraction stage of the universe. This explains why we observe that the time arrows of thermodynamics and cosmology point in the same direction. It is not the expansion of the universe that leads to the increase of disorder, but the borderless conditions that lead to the increase of disorder. Only in the expansion stage will there be conditions suitable for the existence of intelligent life.
In a word, scientific laws can't tell the time direction of forward and backward. However, there are at least three time arrows that distinguish the past from the future. They are thermodynamic arrows, which are the time direction of disorderly increase; Psychological arrow, that is, in this time direction, we can remember the past instead of the future; There is also the arrow of the universe, which is the direction in which the universe expands rather than contracts. I pointed out that psychological arrow should be essentially the same as thermodynamic arrow. The boundlessness hypothesis of the universe predicts a clearly defined thermodynamic time arrow, because the universe must start from a stable and orderly state. And we can see that the consistency of thermodynamic arrow and cosmological arrow is due to the fact that intelligent life can only exist in the expansion stage. Contraction phase is not suitable for its existence because there is no strong thermodynamic time arrow.
Humans understand the progress of the universe by building a small and orderly corner in the universe with increasing disorder. If you remember every word in this book, your memory will record about 2 million units of information-the order in your mind will increase by about 2 million units. However, when you read this book, you will at least convert the ordered energy in the form of food 1 kcal into disorderly energy in the form of heat released into the surrounding air through convection and sweat. This increases the disorder of the universe by about 2 billion units, or about 1 100 million times that of your mind, if you remember everything in this book. In the next chapter, I try to add some order to our minds and explain how people combine some of the theories I have described to form a complete unified theory, which will apply to everything in the universe.