Is time travel a science? Or fantasy?
1. Speaking from "Time Machine"
As we all know, the degrees of freedom that humans have achieved so far in space and time are very different. We can move freely in the direction of space, but we cannot control time at will. Time is like a long river. Everything in the world seems to be a floating object in the river, and it can only follow the waves.
The end of reality is often the starting point of fantasy. If time is a long river, can there be ships in this long river? Floating objects can only drift with the waves, but the ships can split the waves. If there are ships in the long river, we can take such ships for time travel, not only to peek into the future, but also to return to the past, and maybe change history.
In science fiction, this imaginary ship is called the "time machine".
The earliest and most famous novel about the time machine is "The Time Machine" by British science fiction writer Wills, published in 1895.
However, Wells was not the first writer to touch on the subject of time travel. Many writers have been involved in this subject before him, including even the American satirist Mark Twain. A Connecticut Yankee in King Arthur's Court (A Connecticut Yankee in King Arthur's Court) is said to be the earliest novel involving reverse time travel.
But in those literary works earlier than Wells, there is no such thing as a time machine that allows people to choose a "destination" (specifically, "destination time"), and it is rarely The mechanism of time travel is only an explanation in the sense of science fiction.
Wells' "Time Machine" is a breakthrough in both aspects, it quickly aroused great interest from readers, and was filmed twice in 1960 and 2002, and the United Kingdom even published for "Time Machine" Commemorative stamps were issued for the 100th anniversary.
When Wells wrote "Time Machine", Einstein's theory of relativity has not been put forward. People's understanding of time and space generally remains on Newton's absolute view of time and space [Note 1] The book Machine surprisingly puts forward the view that time is the fourth dimension, and it echoes dramatically with the relativistic view of space and time that comes ten years later.
Wells sees time as the fourth dimension, with the aim of giving the green light to time travel by analogy between time and space. So does modern physics recognize this green light? This is what this article will discuss.
2. Facing the future and returning to the past
We know that in Newton's absolute view of space and time, time and space are not affected by any material or movement (this is the main meaning of "absolute"). Obviously, in such a view of time and space, time travel does not have a theoretical basis, its existence is just a fantasy.
However, the introduction of the special theory of relativity produced a major change in the concept of time and space. In the special theory of relativity, time and space are no longer absolute concepts, but are closely related to the choice of reference system. In particular, the passage of time will slow down in the motion reference system, which is a well-known time delay effect, and its existence has been confirmed by a large number of physical experiments.
This new result brought by the special theory of relativity opens the first possibility with theoretical basis for time travel: that is, future-oriented time travel becomes possible.
According to the special theory of relativity, if someone wants to travel to the future, the time machine he needs is a spaceship that can run at a high speed close to the speed of light. The farther the future you want to reach, the faster the spacecraft needs to reach.
If he wants to reach the earth after 20,000 years (time on earth) after 20 years of flight (time on the spaceship), all he has to do is let the spacecraft fly at a speed equal to 99.99995% of the speed of light for 10 years, and then Fly back at the same speed. Then 20 years later, when he returned to the earth, the calendar on the earth has turned over a full 20,000 years, and he can see the human society after 20,000 years if he wishes (if the human society still exists at that time) .
It is conceivable that such a traveler from ancient times will be warmly welcomed by future historians and archaeologists.
In fact, not only future historians and archaeologists will very much welcome such a time traveler, why would someone who was contemporaneous with this time traveler hope that he could bring back what he saw in the future world For everyone? Unfortunately, the special theory of relativity opened the door to future-oriented time travel, but failed to provide the same theoretical feasibility for time travel to the past.
If the mathematical framework of the special theory of relativity must be interpreted in a broad sense, then only the speed of light can cause the time series in a certain frame of reference to be reversed.
However, the special theory of relativity itself sets up a barrier of light speed between the sub-light speed and the super-light speed. There is no known physical process that can make the originally sub-light speed moving objects-including people-enter the state of super-light speed motion. Within the theoretical framework, time travellers can reach the future, but cannot return to the past. This is obviously far from our free movement in space.
Moreover, future-oriented time travel does not necessarily require a time machine. It can also achieve the same goal by freezing travelers and thawing them for several years, so if a time machine exists, its truly unique value is not future-oriented Instead of returning to the past.
So where is the way back to the past?
Ten years after the special theory of relativity, Einstein proposed the general theory of relativity. In the general theory of relativity, time and space are not only closely related to the choice of reference system as in the special theory of relativity, but also depend on the distribution and movement of matter.
An important result from this is different from the special theory of relativity: our definition of "future" is no longer absolute, it will be affected by the movement of matter. At different times and in different places, the "future" may point in different directions
This is a wonderful result. It shows that space and time are dragged by the movement of matter in a certain sense like a fluid. Even the direction of time may be changed by dragging.
Since the direction of time can be dragged by the movement of matter, is there any possibility of the distribution and movement of a certain substance, its drag on the time direction is so significant that the future direction is dragged into the past direction, and even different time directions End to end, connected into a closed curve?
If such a closed curve exists, it is undoubtedly a time machine, because the spacecraft moving along this curve is flying normally every moment, feeling the passage of positive time, but its trajectory is not only in space And will return to the starting point in time.
If you take a spaceship to make a 10-year trip along such a curve [Note 2], you will not only return to the place where the spacecraft departed at the end of the trip, but also meet yourself 10 years ago [Note 3]!
Physicists call this wonderful curve "closed time-like curve", which is synonymous with the science fiction term of time machine in general relativity. If there is a closed time-like curve, time travel has a theoretical possibility.
So in general relativity, is there a closed time-like curve? Or to be more precise, is there any material distribution and motion that makes the closed time-like curve possible? Physicists have done a lot of research on this issue.
3. General relativity and time travel
In 1949, the famous logician Gödel discovered a very peculiar solution in the general theory of relativity, describing a whole rotating universe now known as the "Gödel universe".
In this universe, the rotation of matter will have a drag effect on the time direction. The farther away from the center of rotation, the more significant the drag effect. Where it is far enough, the dragging effect is sufficient to form a closed time-like curve.
Therefore, in the Gödel universe, as long as the spacecraft moves along some orbit away from the center of rotation, time travel can be achieved in principle.
Gödel, a logician who used to shake the entire mathematical world with Gödel's incompleteness theorem, used his rotating universe to shake many physicists, including Einstein himself.
It is a pity that the Gödel universe is not compatible with astronomical observations. First, there is no overall rotation in the universe we live in [Note 4]; second, the cosmological constant is negative in the Gödel universe, but the cosmological constant we observed is positive, so the universe we live in Obviously not the Gödel universe.
Not only that, quantitative calculations also show that even if we really live in a Gödel universe, it is difficult to achieve time travel, because the time required to run a week along the closed time-like curve in the Gödel universe and the material density of the universe Regarding the observed density of matter, it takes at least tens of billions of years to run along the closed time-like curve for at least a week. Therefore, the Gödel universe has no practical significance for time travel.
However, although the Gödel universe has no practical significance, its findings indicate that general relativity does allow the existence of closed time-like curves, which is an encouraging result in itself.
Since then, physicists have discovered other solutions that allow closed time-like curves in general relativity.
For example, in 1974, FrankJ, Tipler, a physicist at Tulane University in the United States, studied the space-time outside an infinitely long rotating cylinder [Note 5] and found that as long as the rotation speed is fast enough The dragging effect of the cylinder on the outer space-time is also sufficient to form a closed time-like curve.
As another example, in 1991, astrophysicist Gott of Princeton University discovered that two infinitely long parallel cosmic strings passing each other at a speed close to the speed of light would also form a closed time-like curve around.
Unlike Tepler's artificially introduced rotating cylinder, although there is no clear experimental evidence for the existence of the cosmic string, it is something predicted by many cutting-edge physics theories. Therefore, Gault's results can be regarded as a further step in the theoretical possibilities of the time machine.
However, for the convenience of mathematics, both Tepler and Gott introduced infinitely long material distributions (ie, "infinitely long rotating cylinder" and "infinitely long parallel cosmic strings"), which is obviously impossible in the real world Strictly achieved.
If the distribution of matter is not infinite, can a similar result be obtained? Physicists have also done research on this, but the situation is not optimistic: in 1992, the famous physicist Hawking gave a frustrating result, that is, if the energy density is non-negative everywhere, then try to in any finite space-time area In order to succeed in the efforts to build a time machine, it is necessary to produce what physicists least want to see—the singularity of time and space [Note 6].
The singularity of space-time is not new to those who study general relativity. It has a series of headaches, such as the divergence of the density of matter, the divergence of curvature of space-time, etc. [Note 7]. Although no one knows for sure how time and space singularities will affect time travel, this effect is likely to be more or less formidable.
Hawking's result is undoubtedly bad news for the construction of time machines, but careful readers may have noticed that there is a limitation in this result, that is, "the energy density is not negative everywhere." This condition seems to be very reasonable, but when we introduced the wormhole, we already mentioned that the existence of negative energy substances is not only theoretically possible, but has also been confirmed by experiments.
Since negative-energy substances can exist, Hawking's results (to be precise, the conclusion) can be avoided.
The research in this area was actually carried out long before Hawking ’s results appeared-of course not to avoid Hawking ’s results that did not appear at the time: California Institute of Technology physicist Kip Thorne and student Morris (Mike Morris) and others published a study on "traversable wormhole" in 1988 and found that wormholes are not only channels for space travel, but can also be used as tools for time travel-as long as The wormhole's entrance and exit can be transformed into a time machine with proper motion at a speed close to the speed of light [Note 8].
Because wormholes contain negative energy substances, their time machine can avoid Hawking's results and do not lead to space-time singularities (in this sense, negative energy substances are really "positive energy").
This research by Thorn and others linked the two most fascinating concepts of science fiction—wormholes and time machines—together with “a thousand pets”, and soon became a time machine for construction Popular solution.
However, although Thorne et al. ’S wormhole time machine could avoid Hawking ’s results, they immediately encountered another thorny problem. That is, once the wormhole became a time machine, any tiny quantum It is possible for Luo to return to the past through that kind of wormhole and superimpose itself.
This superposition process can be repeated infinitely many times in zero time, and the resulting self-excited effect is enough to completely destroy the time machine in an instant! This effect not only endangered the "wormhole time machine" of Thorne and others, but also threatened other types of time machines.
In 1992, Hawking simply put forward the famous Chronology Protection Conjecture, thinking that the laws of nature would not allow the construction of time machines. But so far, this is only a hypothesis, and Hawking's argument is not impeccable. Physicists who are optimistic about the theoretical feasibility of time machines have successively proposed some models to break through Hawking's blockade of time machines. Discussions in this area are still continuing.
Comment
1. From 1892 to 1895, the Dutch physicist Hendrik Lorentz and others had made some assumptions different from the absolute view of space and time when studying electromagnetic theory, but these assumptions did not become the mainstream, but were later Replaced by the theory of relativity.
2. "10 years" here refers to the time on the spacecraft.
3. In fact, not only will you see yourself 10 years ago at the end of the trip, but you will also see yourself triumphant 10 years later when you set off. If you do n’t see anything when you leave, it means that an accident must occur during the journey, preventing you from returning to the beginning of the journey. In this case, you should probably cancel the trip!
4. Of course, this means that the overall rotation of the universe is not found within the existing observation accuracy. In addition, some readers may ask: What is the overall rotation of the universe? What is this rotation relative to? This type of problem can be seen as the view of the Austrian philosopher Ernst Mach that rotation must be relative and in the same vein. However, although Einstein himself once admired Mach, the general theory of relativity did not strictly follow Mach's philosophical views.
5. Tepler was not the first physicist to study this time and space. As early as 1937, the Dutch physicist Willem Jacobvan Stockum had studied this time and space, but there was no elephant Let's analyze its causal characteristics like that.
6. Exactly, many physicists have obtained similar results, and Hawking's is only one of them.
7. The strict definition of singularity itself is a very difficult subject in general theory of relativity. What is described here is only the characteristics of a certain type of singularity. For a more detailed description, please refer to the singularity part from singularity to wormhole.
8. Specifically, the simplest movement required to make a wormhole a time machine is one that causes the distance in the outer space between the two exits of the wormhole to change rapidly, but the length of the wormhole itself does not change. It is not easy to produce this kind of movement, but it can be done in principle. For a more detailed introduction to "wormhole time machine", please refer to the section from wormhole to time machine from singularity to wormhole.
4. Time travel and causal paradox
In addition to discussing the theoretical feasibility of the time machine, there is another very important aspect, that is, if the time machine exists, what can we do with it?
At first glance, this does not seem to be a problem. Since you can do time travel, you should naturally do what you want after you reach the destination time.
What can be done-as long as it does not violate the laws of physics. But think about it, things are not so simple. For example, if a time traveler returns to his birth, can he stop his parents from meeting him? This does not seem to violate any laws of physics.
For example, if a time traveler shoots at a person who will become his father before his parents meet, the bullet seems to be able to hit the target without violating any laws of physics, causing fatal injury. But if that action is successful, we will immediately fall into the so-called "causality paradox".
Because if the time traveler ’s parents did not know each other because of his obstruction, then there would be no one in the world; and how could he return to the past and stop his parents ’acquaintance without him in the world?
Paradoxes like this consider the innumerable time travel time, they all originate from the possible damage caused by time travel to causal timing.
How to solve this kind of paradox? In science fiction or movies, the solution is often through various coincidences. For example, when the aforementioned "Time Machine" of Wells was filmed in 2002, it may be to explain the protagonist ’s motivation to build a time machine. The director added the protagonist ’s victim and he tried to return Plots saved in the past. In that episode, the protagonist tried his best, but always lost his way. His lover would always die in one way or another.
Obviously, the same technique can also be used to prevent time travelers from preventing their parents from getting acquainted. Missed the opportunity [Note 1].
This way of solving the paradox is jokingly called "Banana Peel Mechanism" by some physicists. Under the "banana peel mechanism", time travellers seem to be able to act freely, but whenever their actions are to cause causal errors, they will always be disturbed by some seemingly accidental factors, causing the actions to fail.
This "banana peel mechanism" is suitable for writing dramatic storylines. But from a physics point of view, it is difficult to imagine that the laws of physics need to be solved in such a bizarre way [Note 2]. What ’s more, the banana peel mechanism also has a fatal weakness, that is, it often only focuses on guaranteeing one or two core events-such as the death of the protagonist ’s lover in the film "Time Machine", or the time traveler ’s parents in the example we give. The acquaintance-the occurrence will not be changed by time travel, but can not take into account other events.
For example, the death of the protagonist's lover in the film "Time Machine" in different ways will leave different reports in the local newspaper; in our example, the time traveler's fall in hospital will also leave a corresponding record in the local hospital. These events are not prominent for a particular story, but they are as important as the core events from the perspective of maintaining causal timing or history.
In fact, there are inextricable connections between various events in nature, and any seemingly small changes may gradually evolve into major events through this connection. Readers who are familiar with the Butterfly Effect in chaos theory should not be unfamiliar [Note 3]
In addition to the banana peel mechanism, another point of view can be seen in some science fiction stories, which is to abandon the law of causality to a certain extent to expand the freedom of movement of time travellers. In this view, history can be changed almost at will, and the results of the change can affect many things in the real world.
The science fiction movie "Frequency" embodies this view. In the film, although the protagonist did not directly travel in time, he had the ability to indirectly change history by establishing contact with his father who died 30 years ago. In the film, every change in historical events will directly change the real world 30 years later. For example, due to changes in historical events that led to the accidental death of the protagonist ’s mother, 30 years later, the protagonist ’s mother ’s photos will suddenly disappear from the frame. Obviously, this view is almost equivalent to giving up the known laws of physics, which is more bizarre than the banana peel mechanism that tries to protect reality.Based on the existing laws of physics, can we understand or avoid the causal paradox that may be caused by time travel?
On this issue, physicists have not formed a consensus. We are here to introduce the reader to two main points of view.
The first view is that time and space are the complete identification of physical events, so once time and space are determined at the same time, the physical events are completely determined.
In this sense, if we compare time to a long river, it is actually a solidified long river, and its cross-section—corresponding to the totality of all physical events at a certain moment—is fixed, as The same as movie film.
According to this view, there can only be one version of history. If the time traveler can go back to the past, the only possibility is that he originally existed in the past.
In other words, the time traveler cannot make any changes to history. He is not even a bystander of history, because he was originally part of history.
This view is undoubtedly disappointing for those who love time travel, because if everything is immutable, then time travel will lose its most important value.
Fortunately, the second view of time travel is much more open. This view comes from a peculiar interpretation of quantum mechanics proposed by the American physicist Hugh Everett III in 1957-multi-world Interpretation (many world interpretation) [Note 4].
We know that an important feature of quantum mechanics is that the results of measuring quantum systems are often not unique. So, how exactly does a specific measurement result occur? Physicists have made many different points of view.
Some physicists believe that when we measure the quantum system, the state of the system collapses, and the measurement result we observe is a collapsed state. In this view, the collapse of state is an unpredictable process.
In contrast, Everett et al.'S multi-world interpretation believes that there is no such unpredictable state collapse, and the result of quantum measurement is that the world splits into a set of parallel universes. All possible measurement results in quantum mechanics are real, except that they exist in their respective parallel universes rather than in a single world. The measurement result obtained by the observer is only a specific result in the parallel universe where he (she) is located [Note 5].
If we apply this view to time travel and think that time travellers not only cross time, but also across different parallel universes, then all the paradoxes are solved [Note 6].
For example, time travellers preventing their parents from getting acquainted are no longer a paradox, because all this happens in a different parallel universe. In that universe, his parents did not know each other, and he himself was never born. This is not inconsistent with preventing the time traveler whose parents know each other from appearing in that universe, because the time traveler is from another parallel universe in which his parents still know each other.
In this view, the history of each parallel universe is still unique, but all the history permitted by the laws of physics will be realized in a parallel universe. Although time travellers cannot change the history of any parallel universe, they are free. To choose which parallel universe to enter, he cannot change history, but he can choose history [Note 7].
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