In the previous chapters, we have seen how the view on the nature of time has changed in the long years. Until the beginning of the 20th century, people believed in absolute time. In other words, each event can be marked in a unique way by a number called "time", and all good clocks are consistent in measuring the time interval between two events.
However, for observers, no matter how they move, the speed of light is the same. The scientific discovery led to the theory of relativity, which abandoned the concept of absolute time. You cannot mark the time of an event in a unique way. On the contrary, each observer has his own time measurement, which is recorded by his own clock, and the clocks carried by different observers are not necessarily the same.
In this way, time becomes a more personal concept than the observer who measures time. Time is still regarded as a straight track, and you can only go in one direction or the other. What will happen if the track is curved and forked and the train that has been moving forward returns to the station it passes?
In other words, is it possible for anyone to travel to the future or go back to the past? H G wells explored these possibilities in a time machine, just like countless other science fiction writers. Many scientific fantasy concepts, such as submarines flying to the moon, have been scientifically realized. So, what is the prospect of time travel?
It is possible to travel in the future. In other words, the theory of relativity shows that people can make a time machine to make you jump forward in time. You step into the time machine, wait, step out, and find that the time on earth has passed, much longer than the time you have experienced. We don't have the technology to achieve this today, but it's just an engineering problem: we know it can be done. The twin paradox we discussed in chapter 6, using this situation is a way to build this machine. In this way, when you sit in this time machine, it launches, accelerates to near the speed of light, lasts for a period of time (depending on how far you want to travel in time), and then returns. Because according to the theory of relativity, time and space are related, and the time machine is also a spaceship. Don't be surprised. Anyway, as far as you are concerned, in the whole process, the place you stay is only in the time machine. And when you step out, you will find that you have spent more time on the earth than you have spent. You have crossed into the future. But can you go back? Can you create the necessary conditions for traveling against time?
1949, Kurt G?del discovered the new solution of Einstein's equation, that is, the new space-time allowed by general relativity. This is the first sign that the laws of physics may really allow reverse time travel. Many different mathematical models of the universe satisfy Einstein's equation, but this does not mean that they correspond to the universe we live in. For example, their initial or boundary conditions are different. In order to decide whether they conform to our universe, we must check the physical predictions of these models.
Godel is a mathematician. He is famous for proving the incompleteness theorem. This theorem means that it is impossible to prove all the true statements, even if you only prove all the true statements in an obviously boring subject like arithmetic. Just like the uncertainty principle, Godel's incompleteness theorem may be the basic limit of our ability to understand and predict the universe. Godel and Einstein spent their old age at Princeton Institute for Advanced Studies. During that time, he became familiar with general relativity. Godel's spacetime has a strange property: the whole universe is spinning.
[illustration]
Time machine
Two authors are in the time machine.
What do you mean the whole universe is spinning? Rotation means continuity. Doesn't this mean that there is a fixed reference point? In this way, you may ask, "What does it rotate relative to?" The answer is a bit professional, but it basically refers to the rotation of distant objects relative to the direction of small gyroscopes or gyroscopes in the universe. The rotation property of Godel spacetime has an additional mathematical effect. If a person travels a long distance from the earth and then returns, he may have returned to the earth before leaving.
Einstein once thought that general relativity did not allow time travel, but his equation did, which really made him depressed. Although Godel's solution satisfies Einstein's equation, our observations show that our universe does not rotate, at least not obviously, so it does not correspond to the universe we live in. Godel's universe will not expand, but ours will. However, since then, scientists studying Einstein's equations have discovered other time and space where general relativity does allow reverse time travel. However, the microwave background and the observation of the abundance of hydrogen, helium and other elements show that the early universe does not have the curvature that these models allow time travel. If the borderless hypothesis is correct, the same conclusion can be drawn according to the theory. In this way, the question becomes: if the universe does not have the curvature needed for time travel from the beginning, can we distort the local area of time and space enough to make time travel happen later?
Again, since time and space are related, you may not be surprised. A question closely related to reverse time travel is whether you can travel faster than light. It is easy to see that time travel means faster than light travel: by traveling backwards at the last stage of the journey, you can complete your whole trip in any short time you want, so you can travel at unlimited speed! However, as we will see, the reverse is also true: if you can travel at infinite speed, you can also travel against time. It is impossible for one to be established and one not to be established.
Science fiction writers are very concerned about the debate about superluminal travel. Their problem is that according to the theory of relativity, if we send a spaceship to the nearest star, proxima centauri, which is about 4 light years away from us, we expect that it will take at least 8 years for travelers to return and tell them their findings. But if we want to explore the center of our galaxy, it will take at least 654.38 million+years to return. If you want to write about intergalactic wars, the prospects are not good! However, the theory of relativity does give us a comfort, and let us follow the idea of the discussion on the paradox of twins in Chapter 6: For space travelers, this journey may be much shorter than those who stay on the earth. However, people who have traveled in space for several years are not very happy when they come back, because you will find that those who have stayed have been dead for thousands of years. So in order to make people interested in the stories of science fiction writers, they must assume that we will find out how to travel faster than light one day. Most of these authors don't seem to realize the fact that if you can travel faster than light, the theory of relativity means that you can travel against time, as the following five lines of limerick say:
The young lady's name is White.
She can travel much faster than the speed of light.
Relatively speaking,
At the beginning of the day,
But it had arrived the night before.
The key point related to this is that relativity not only thinks that there is no unique time measure that all observers agree on, but also thinks that under certain circumstances, observers don't even have to agree on the time sequence of events. Especially, if the two events A and B are so far apart in space that the rocket must travel faster than the speed of light to reach event B, then two observers moving at different speeds will disagree whether event A occurs before event B or event B occurs before event A ... For example, suppose that event A is the end of the 20 12 Olympic Games1000 final and event B is proxima centauri Parliament/KLOC. Suppose that for an observer on the earth, event A occurred before event B, for example, according to the time of the earth, event B occurred one year later, that is, 20 13. Since the Earth and proxima centauri are about 4 light years apart, these two events meet the above criteria: although A occurs before B, you must travel from A to B at a speed faster than the speed of light. Then, for an observer who is far away from the Earth at a speed close to the speed of light on the proxima centauri, the order of events seems to be reversed: Event B occurs before Event A. The observer will say that if you can move faster than the speed of light, you can travel from Event B to Event A. In fact, if you travel really fast,
If wormholes exist, they can provide shortcuts between distant points in space.
There is something wrong with breaking through the light speed barrier. Relativity tells us that the closer the speed of spacecraft is to the speed of light, the greater the rocket power used for acceleration. We have experimental evidence for this, but not the experience of spacecraft, but the experience of elementary particles in particle accelerators such as Fermilab or CERN. We can accelerate particles to 99.99% of the speed of light, but no matter how much power is injected, it is impossible to accelerate them beyond the speed barrier. The situation of the spacecraft is similar: no matter how powerful the rocket is, it is impossible to accelerate faster than the speed of light. Moreover, because we can travel against time only when superluminal travel is possible, it seems to exclude both high-speed space travel and reverse time travel.
However, there may be a way out. You may be able to curl space-time so that there is a shortcut between A and B, and building a wormhole between A and B is one way. As the name implies, a wormhole is a thin tube of time and space, which can connect two almost flat and distant areas. It's kind of like at the bottom of a towering ridge. In order to get to the other side, you usually need to climb a long distance and then get down-but if there is a huge wormhole running horizontally through the rock mass, you don't have to. You can imagine building or discovering a wormhole leading to proxima centauri from near our solar system. Although the Earth and proxima centauri are 20 trillion miles apart in the usual space, the distance through this wormhole may be only a few million miles. If we send the news of the 100-meter race through the wormhole, we can arrive much earlier than the opening of the parliament. However, an observer who flies to Earth later should be able to find another wormhole, so that he can return to Earth from the opening of proxima centauri Parliament before the Games. Therefore, like other possible superluminal travel, wormholes allow people to travel in reverse time.
The idea of wormholes between different regions of time and space is not an invention of science fiction writers, but its source is very respectable. 1935, Einstein and Nazhen Rosen wrote a paper. In the paper, they point out that general relativity allows what they call a "bridge" to exist, but now it is called a wormhole. The Einstein-Rosen bridge can only last for a short time, and the spacecraft can't cross it: due to the contraction of the wormhole, the spacecraft will hit a singularity. However, it has been suggested that an advanced civilization may keep the wormhole open. In order to achieve this goal, or distort time and space in any other way, so as to travel in time. You can prove that you must have a space-time region with negative curvature, like a saddle surface. Ordinary matter has a positive energy density and gives space-time a positive curvature, just like a sphere. Therefore, in order to curl space-time in a way that allows reverse time travel, people need substances with negative energy density.
What does negative energy density mean? Energy is a bit like money: if your bank balance is positive, you can allocate it in different ways, but according to the classic law that you believed a century ago, you can't overdraw. Therefore, these classical laws have ruled out any possibility of negative energy density, that is, traveling backwards. However, as mentioned in the previous chapters, the quantum law based on the uncertainty principle has surpassed the classical law. Quantum laws are more generous. As long as your total balance is positive, you can overdraw one or two accounts. In other words, quantum theory allows the energy density in some places to be negative, so long as the energy density in other places is positive, it can be compensated and the total energy can be kept positive. Therefore, we have reason to believe that space-time can not only be distorted, but also be bent into what is needed to allow reverse time travel.
According to Fei Enman's summary of history, on the scale of a single particle, to some extent, time reversal does exist. In Fei Enman's method, the motion of particles in time is usually equivalent to the motion of antiparticles in time. Using his mathematical method, people can create a pair of particles/antiparticles together and make them annihilate each other, just like the movement of a single particle in a closed circle in time and space. In order to see this, first imagine this process in a traditional way. At a certain moment, such as time a, a pair of particles/antiparticles are produced, and they all move in chronological order. Then at the later moment b, they interact again and annihilate each other. Before A and after B, neither particle exists. But according to Fei Enman's theory, you can look at it from different angles. A particle is produced at time A, it moves to time B, and then returns to time A at the opposite time. Instead of particles and antiparticles moving together in time, only a single particle moves along a "circle" from A to B and then back. When an object moves in time (from A to B), it is called a particle. However, when an object moves backward (from B to A), it appears as an antiparticle moving forward. Antiparticles can be thought of as particles that travel through the past time.
Therefore, the virtual particle/antiparticle pair can be regarded as a particle moving in the closed loop of time and space.
This kind of time travel can produce an observation effect. For example, suppose that one member of a particle/antiparticle pair (such as antiparticle) falls into a black hole, leaving the other member with no partner to annihilate. This abandoned particle can also fall into the black hole, but it can also escape from the vicinity of the black hole. If so, it looks like a particle emitted from a black hole to a distant observer. However, you can have a different but equivalent visual description of the emission mechanism of black hole radiation. You can think of this member of a pair of particles falling into a black hole (such as antiparticle) as a particle traveling backwards from the black hole. When it reaches the point where the particle/antiparticle pair appears together, it is scattered by the gravitational field of the black hole into particles moving in the positive direction of time and escapes from the black hole. Or conversely, if a particle member of a particle pair falls into a black hole, you can regard it as an antiparticle traveling against time and come out of the black hole. In this way, the radiation of black holes shows that quantum theory allows time to travel to the past time on a microscopic scale.
Therefore, we can ask whether quantum theory allows the following possibilities, and whether we can finally try to build a time machine once science and technology progress. At first glance, it seems that it should be possible. Fei Enman's historical summation hypothesis is aimed at all history. In this way, it should include a period of history, whose time and space are distorted to the extent that it allows travel against time. However, even if the known laws of physics do not seem to rule out time travel, there are other reasons to question whether this is really possible.
The question is, if people can travel against time, why don't people from the future come back and tell us how to do it? Now that we are in the initial stage of development, there may be good reasons to think that it is unwise for us to share the secrets of time travel. Unless human nature is completely changed, it is inconceivable for a visitor from the future to reveal the secret rashly. Of course, some people will claim that the discovery of UFOs is evidence of the visit of aliens or future people. In view of the huge distance of other stars, if aliens arrive here within a reasonable time, they need to travel faster than light, so the two possibilities are actually equal. There are no visitors from the future, which can be explained in the following way, because we have observed the past and found that it does not allow the future travel to return to the kind of curl needed, so the past is fixed. On the other hand, the future is unknown and open, so it is likely to have the required curvature. This means that any time travel is limited to the future. At this moment, Captain Kirk and the starship Enterprise have no chance to come.
This may explain why there are no tourists from the future in today's world. However, if it is possible to go back to the past and change history, we still can't avoid another question: then, why don't we have a hard time with history? For example, suppose someone went back to the past and gave the secret of the atomic bomb to the Nazis, or you went back to the past and killed your great-great-grandfather before he got the child. There are many versions of this paradox, but it is basically equivalent: if we have the freedom to change the past, we will encounter contradictions.
There seem to be two possible ways to solve the paradox caused by time travel. The first method can be called collaborative history method. It means that even though space-time is curled up and traveling backwards, what happens in space-time must be the coordinated solution of the laws of physics. In other words, according to this view, you can't go back to the past in time unless history shows that you have reached the past and didn't kill your great-great grandfather or take any action, which conflicts with the history that reached your present situation. Besides, when you go back to the past, you can't change the recorded history, you just follow it. According to this view, the past and the future are doomed: you have no free will to do whatever you want.
Of course, people can say that free will is illusory anyway. If there is a complete set of physical theory that restricts everything, it seems to determine your actions. However, for such a complex organism as human beings, it is impossible to calculate its restriction and decision mode, and there are also some randomness caused by quantum mechanical effects. So there is a view that we say that human beings have free will because we can't predict human behavior in the future. However, if a person sets off in a rocket spaceship and returns before that, we can predict his future behavior, because it is part of recording history. Therefore, in that case, time travelers have no free will in any sense.
Other possible ways to solve the paradox of time travel can be called choosing historical hypothesis. The idea is that when time travelers go back to the past, they enter another history that is different from recording history, so that they can move freely without being bound by their original history. Steven Allan Spielberg likes the idea in the movie Back to the Future: Marty McFrye can go back in time and make his parents' love history more complete.
It sounds that choosing historical hypothesis is similar to richard feynman's method of expressing quantum theory as historical summation, as described in Chapter 9. This means that the universe not only has an independent history; It has every possible history, and every history has its own probability. However, there seems to be an important difference between Fei Enman's vision and his choice history. In Fei Enman's summary, every kind of history is composed of complete time and space and everything in it, and time and space can be curled to the point where it is possible to travel to the past by rocket. But if rockets want to stay in the same time and space, and thus stay in the same history, history must be consistent. In this way, Fei Enman's thought of summing up history seems to support the idea of coordinating history, rather than the idea of choosing history.
These problems can be avoided if we accept the speculation that we can call it timing defense. In other words, the laws of physics are the same as preventing macroscopic objects from transmitting information to the past. This guess has not been confirmed, but there is reason to believe it is true. The reason is that when space-time is distorted and may travel too far, the calculation by using quantum theory shows that the particle/antiparticle pair that keeps moving around the closed loop can generate enough energy density, endow space-time with positive curvature, and offset the distortion that allows time travel. Because it is not clear whether this is the case, the possibility of time travel is still up in the air. But don't bet on it, your opponent may have the unfair advantage of knowing the future.