The complete guide for the time traveler

Einstein's theory of relativity laid the scientific foundation for examining the physical meaning and feasibility of time travel. Will we be able to visit in the future, or meet our parents when they were children?

Yoram Kirsh, Galileo

Imagine that you enter a closed cell on June 1, 2007 at 20.00:1 p.m. You press a few buttons, the cabin vibrates a little and its door opens. You get out of the cell and ask people: What is today's date? And they say: today is June 2009, 1. Your wristwatch, which also has a date, shows: June 2007, 20.10 time, XNUMX. That means there is a time gap (of about two years, in our example) between your watch and the watches around you. This is an important point - if your watch also shows that two years have passed, then you were probably just in a coma and woke up after two years. But the gap between the watches shows that you have really traveled to the future.

In the journey to the past, the opposite happens; The people outside will tell you that the date is earlier than when you entered the cell. A "time machine" of this kind, which can bring us to the past and the future, is described in many science fiction books, the pioneer of which is probably the "time machine" of the British writer and historian H.G. Wells (Wells), published in 1895. Wells refers to time in his book as the fourth dimension, ten years before Einstein published the special theory of relativity!

Previous books that described time travel, such as Mark Twain's book "A Yankee in King Arthur's Court", did not describe the journey as scientifically as Wells' book. There are also stories and legends about people who fell asleep for decades and thus made a sort of journey into the future. For example, the story about Rip Van Winkle by the American writer Washington Irving (Irving), published in 1819, or the Talmudic legend about the dwellers of the circle who fell asleep for seventy years. Such a journey into the future can be referred to as "fake time travel".

Journey into the future according to the theory of special relativity

Until Einstein formulated the theory of special relativity in 1905, the laws of physics did not allow time travel at all (except for the "fake travel" to the future, described above). The special theory of relativity was mainly formulated to explain the fact that it is impossible to change the speed of light in a vacuum; The measurement always gives the same result (close to 300,000 km per second) regardless of the relative movement between the observer, who measures the speed of light, and the light source.

The fact that the speed of light is constant became a basic premise of the theory of special relativity, but from this basic premise it follows that time and space are not absolute but depend on the movement of the measurer. Movement can affect the rate of the passage of time, in such a way that a moving clock will seem slower to us than a clock at rest. The phenomenon is called "time dilation". The speeds we are used to in our day-to-day life are very small compared to the speed of light, so we do not feel the phenomenon. For example, the watch of a pilot in a fighter plane whose average speed is 1,800 km/h (1.5 times the speed of sound), will be behind by only one millionth of a second after two hundred hours of flight.

We find more impressive examples in science fiction books, where there are no technical limitations on the speed of flight. For example, let's say that an astronaut whose spacecraft flies at a speed of 98 percent of the speed of light wants to reach a star that is five light years from Earth ("a light year" is the distance that light travels in one year. It is roughly equal to about ten million-million kilometers). According to Earth clock, the flight will last a little over five years. However, according to the astronaut's watch, the journey will last only one year. If its speed gets even closer to the speed of light, it will be able to complete the journey in a few hours, although according to the control room on Earth, the flight will still take a little more than five years (according to special relativity, the speed of the spacecraft cannot exceed the speed of light).

The phenomenon of time dilation has many experimental proofs. Most of them are related to the particles that disintegrate shortly after their formation. It was found that when these particles are moving at high speeds, they decay after a longer period of time than when they are at rest or when they are moving slowly. The reason is that the rate of the "internal clock" of the particle slows down compared to a clock that is at rest. In 1971, the phenomenon was demonstrated for the first time using a real watch. Two physicists from the United States flew around the world with very precise atomic clocks, and it was found that a clock that leaves a certain place, is in motion for a period of time and returns to the place from which it left behind - during this time compared to a second clock that remains at rest.

The twin paradox

What will happen if two twins (who are of course the same age) are separated from each other; One will fly in a spaceship that will move at a speed close to the speed of light and eventually return to Earth, and the other will remain on Earth? As we said, the moving twin's clock will be slower than the stationary twin's, so the moving twin will age more slowly than its stationary sibling. If the speed of the spaceship is 99.5 percent of the speed of light, for every year that passes by the astronaut twin, ten years will pass by his brother who lives in the house. When the astronaut returns after five years according to his watch, he will find that fifty years have passed on Earth. Therefore, a vehicle that moves at a speed close to the speed of light can be used as a "time machine" and create a shortcut to the future.

The phenomenon we described is known as the "twin paradox". The paradox arises from the fact that from the point of view of the twin in the spaceship, he is at rest while his brother and the entire Earth are moving away from him. Therefore he (the twin in the spaceship) had to age faster. The solution to the paradox is related to the fact that in order to be able to compare the clocks of the two twins, the spacecraft has to reverse direction and return to Earth. The change of direction is an accelerated movement, and it breaks the symmetry between the spacecraft and the Earth. When you consider this, you accept that the flying twin will indeed age more slowly.

General Relativity

The theory of special relativity deals with systems that move relative to each other at a constant speed. In 1915, Einstein published the theory of general relativity, which deals with systems that move at a non-constant speed (that is, with acceleration) and systems that are under the influence of strong gravitational forces. According to the theory of general relativity, the force of gravity results from the fact that a mass causes the curvature of space-time, and this affects the movement of other masses in its vicinity.

General relativity pointed to another mechanism that could allow time travel. According to this theory, even a gravitational field can cause time to slow down, and it is possible in principle to take advantage of this for a journey into the future. For example, you could launch a spacecraft that would attack a black hole (without getting too close to it) and then come back and move away. The astronauts in the spaceship will find that while they were in the strong gravitational field of the black hole, the course of their time slowed down compared to the outside world.

They may find that according to their time the journey took several days while in the outside world tens or hundreds of years have passed. The science fiction writer Frederick Pohl made use of this phenomenon in his book Beyond the Blue Horizon ("The Threshold of the Event" in Hebrew). An astronaut named Clara is in a spacecraft that is trapped near a black hole. Her younger lover, Robin, manages to escape, and then he suffers for many years from feelings of guilt. Decades after the event, another spaceship arrives and rescues Clara, and it turns out that, according to Season, she circled the black hole for only a few minutes. When Clara and Robin meet again, she is still young and he is already seven days old.

Is it possible to travel in time?

We have seen that according to the theory of relativity, on its two parts, a journey into the future is possible, in principle. And what about a journey into the past? Is there any chance we will ever see the status of Mount Sinai? Are you close to Troon in the war of liberation? And maybe we can warn the captain of the Titanic and prevent her sinking? Does physics allow us, in some way, to build a time machine that will take us back to the past?

Logic says that this is not possible, because a journey into the past can change the present. For example, a person who will make a journey into the past could murder his grandmother before she gave birth to his mother, and then the question arises, how did he come into the world? In fact, one should not reach an extreme act like murder. Everything done in the past can affect the present and create a paradox. For example, imagine a time traveler who sets off on his journey on Sunday at 8 am, ends it that day at 7 am, and just shakes his hand before setting off. But when that traveler went on his way, he did not shake hands with someone who looked like his twin brother! Was there a handshake or was there not?

Indeed, a journey into the past contradicts a fundamental law of nature called the principle of causality. This principle states that according to any observer, the cause always precedes the effect. According to the principle of causality, time travel can only be to the future. A time traveler who reaches the future will not be able to return to the present from which he left, because it will already be a journey to the past. Because of the principle of causality, we will also not be able to build a device that will allow us to see the future, even as passive observers. Suppose there is such a facility, similar to a television receiver, with the help of which you can see what will happen on Allenby Street in Tel Aviv in ten years. To do this, you need to absorb photons, or other particles, from the future, and transfer them to the present, and we have already said that a journey from the future to the present is impossible.
Furthermore, not only is it impossible to see the future, it is also impossible, in principle, to predict it accurately. Suppose someone is gifted with perfect foresight, and he predicts what the correct results of the lottery will be in a week, and how many winners will share the first prize. He could, of course, send a winning form, but this would increase the number of winners, and hence his vision of the future was not perfect.

Even seeing the past has limitations. Those who entertain the idea of ​​a miracle television, which we can direct at will into the past to see, for example, what exactly happened at Mount Sinai, or during the destruction of the Second Temple in 70 AD, should know that such a device is not possible, because it requires the launch of a detector (or at least instructions) from the present to the past, and this is equivalent to a journey into the past (however, the light that reaches us from distant galaxies tells us a story from the past, because it set out millions of years ago).

A journey into the past in general relativity

We mentioned that according to general relativity it is possible to use a strong gravitational field for a journey into the future. However, surprisingly, the theory of general relativity apparently also allows a journey into the past. To understand this, one must be familiar with the concept of "world line": it is a continuous line in the four-dimensional space-time, which describes the location of a certain particle at every moment. As you remember, according to general relativity, mass causes the curvature of space-time around it, and therefore also the curvature of the world lines that pass through the area. A strong enough curvature can cause a certain world line to become a closed loop, and this is equivalent to returning to the past.

The first to point out the phenomenon was the famous mathematician Kurt Gedel. Gadel was born in 1906 to a Jewish family living in Austria, and studied mathematics at the University of Vienna. When he was 25 years old he proved a revolutionary theorem known as "Gadel's Incompleteness Theorem". Until then, it was common to assume that in any mathematical theory it is possible to build a system of axioms - basic theorems - and with the help of it prove all the theorems of the theory. Godel refuted this assumption. He proved that even arithmetic, which is considered a relatively simple mathematical theory, is incomplete, in the sense that no matter how we formulate its axioms, there will be arithmetic theorems that we cannot prove to be true, nor can we prove that they are not true.

After the Germans took over Austria in 1933, Gadel moved to the Institute for Advanced Studies in Princeton in the USA, where Einstein also started working that same year. The two became close friends and Godel became interested in general relativity. In 1949 he formulated a solution to Einstein's equations (the basic equations of general relativity) which describe a universe that does not expand and does not contract but rotates. Godel showed that in this universe there may be world lines in the form of closed loops. Such a loop is called a closed time-like loop or LDS (CTL in English).

In the universe described by Godel, a spaceship can embark on a journey that will eventually return it to the same point in space-time from which it came, while traveling to the past part of the way. In order to create a large-scale LDS, the universe has to rotate around itself once every 70 billion years. The spacecraft's journey would have to last at least 100 billion years (according to the spacecraft's clock), even if its speed is close to the speed of light. Such a "time machine" is impractical, due to the extreme time required to complete the journey according to the spacecraft's clock. However, Godel's article showed that, in principle, general relativity allows for closed loops in space-time, for all the paradoxes that arise from this.

From the sixties of the 20th century, more solutions to Einstein's equations began to appear, which contained DCs and enabled time travel into the past. One of the physicists who dealt with this was Frank Tipler (Tipler) from the USA. In 1974 he showed that if we build a cylinder of infinite length, along its axis a singularity prevails (infinite curvature of space-time, as in the center of a black hole), and make it rotate around its axis at a sufficient speed, it will be possible to return to the past by circling This roll.

Building an infinite cylinder is of course impossible, but Tiffler believed that a non-infinite cylinder could also be used for the same purpose, if it were compressed enough and long enough. His assessment was that the cylinder should have a length of about one hundred kilometers, a diameter of ten to twenty kilometers and a material density similar to that of a neutron star. It has to rotate on its axis at a rate of two thousand revolutions per second, a rate that only slightly exceeds the maximum rate of rotation of known pulsars, which are neutron stars that rotate around their axis.

According to Tipler, you can collect ten neutron stars that are celebrating at high speed, and attach them to each other to create a time machine. It should be emphasized that this time machine does not allow a journey to earlier times than the moment when it itself was built. If we were to build such a machine tomorrow, we would not be able to use it to test how the Exodus took place, or to participate in World War II. If the construction of the machine is completed by the first of January 2100, travelers in 2110 will be able to return with it to the year 2100 but not to the year 2099.

Note that uniqueness can have two opposing roles. Earlier we said that near the singularity of a black hole the clock slows down, which is equivalent to traveling into the future. The uniqueness of Tipler's Galilee creates the opposite phenomenon, of traveling to the past. The difference is due to the rotation of the cylinder - a rotating singularity drags the space-time with it and allows world lines to close and create DCs.

"Wormhole" as a time machine

After the publication of Tipler's article, other scientific articles appeared that dealt with the journey into the past, relying on various physical phenomena and the equations of general relativity. The most interesting proposal is based on theories that the singularity at the center of a black hole could be a corridor connecting to another universe or a distant region of our universe. This corridor is called a "wormhole".

In 1985, the astronomer and author Carl Sagan (Sagan) turned to his friend the physicist Kip Thorne (Thorne) from Caltech (Los Angeles) with the question: Do the laws of physics allow using a wormhole to travel great distances in space in a short time? Sagan needed an answer for a science fiction book he was writing at the time (the book was called Contact. It was published in 1986 and later turned into a movie starring Jodie Foster).

At that time it was common to assume that there was no possibility of using wormholes for a shortcut in space travel. However, Thorn believed that this assumption should be re-examined, and assigned it to two of his doctoral students, Michael Morris (Morris) and Ulvi Yurtsever (Yurtsever). The result was an article4 signed by Morris, Thorne and Yurtsbar published in Physical Review in 1988. The article, which was full of calculations and formulas of general relativity, claimed that it is possible to use a wormhole to move almost instantaneously between regions that are thousands of light years apart, and that the same wormhole can also be used as a time machine that will allow a journey into the past.

Time travel: possible solutions to the paradox

In the previous article on the subject, we saw that a journey into the past creates a paradox. The paradox arises from the fact that a closed loop of a world line changes the original world line, and according to the theory of relativity, both limited and general, it is impossible to change a world line (changing a world line actually means retroactively changing the history of the universe). On the other hand, general relativity itself presents a variety of possibilities for a journey into the past. How is this possible? This question has several possible answers.

1. The universe makes sure that the journey into the past does not create paradoxes. For example, if I wanted to murder my grandfather I would have a gun stop. Or they will catch me before I carry out my plan, and so on. This answer is unsatisfactory for two reasons: First, we do not know the laws of physics that consistently prevent us from doing things that we want and are supposedly able to do. Secondly, we have already said that it is not necessary to carry out something drastic, such as murder, to change the present, it is enough for a person or an object to be moved from the present to the past to create a paradox.

2. You can change the past and then the present also changes, and with it our memories. For example, the traveler while reaching the past and shaking hands with an earlier version of himself, will remember that occasion that before the start of the journey someone very similar to him appeared and shook his hand.
This solution is very popular in science fiction films dealing with time travel to the past. For example, in the movie "Back to the Future", whenever the hero's future mother is not satisfied with his future father (when they were not yet married) the hero looks at his picture from the present and sees that his image in the picture fades. In the movie "The Butterfly Effect" the hero can return to the past by reading an old diary, and change that past. He wakes up to a different present, but also remembers the previous ones. In the book "Making History" the heroes manage to prevent the birth of Hitler but someone else appears, much more sophisticated, who, unlike Hitler, manages to take over the world.

It should be noted that although this possibility seems interesting, it is not consistent with the laws of nature, and therefore cannot be a solution to the paradox.

3. All loops in time fit into the history of the world. That is, when a journey is made into the past, and a seemingly closed time-like loop is created, it actually already existed from the beginning of the history of the universe. This solution fits with a deterministic view of the world, according to which the past, present and future exist all the time; The experiences we have are like watching a movie (where the past, present and future exist all the time on the film reel or video tape). We only become aware of them in a certain order that creates the illusion of time passing. This view contradicts the accepted physical theory of time, according to which the present is the only thing that has reality - the past exists only in our memory, and the future has no reality at all until it occurs.

4. The theory of parallel universes. The theory proposed in 1957 by Hugh Everett (Everett) claims that every time a measurement is made in a quantum system, the entire universe splits into several parallel universes. The theory makes it possible to bypass the limitation on a journey into the past; The idea is that every trip to the past creates a split of the universe into two parallel universes. In one of the universes, history continues as usual, without the hero emerging from the past, and this is the universe we know. The hero who managed to reach the past lives in the second universe, and his actions there do not affect the original universe he came from.
The problem with this solution is that the theory of parallel universes is considered an unproven speculation (according to most physicists) and not a theory that can be used to solve physical problems.

5. Cosmic censorship. The fifth answer, which is probably the most serious, deals with a concept called "cosmic censorship" or "protecting the order of time", which we will now discuss.

The cosmic censorship

The concept of "cosmic censorship" was proposed by the physicist Roger Penrose (Penrose) from Oxford, in connection with paradoxical phenomena that can occur in the environment of the singularity at the center of a black hole. Penrose argued that these phenomena should not bother us, because matter and information cannot exit the black hole, therefore an observer outside the event horizon (the surface surrounding the black hole) cannot see these paradoxes, and an observer inside the event horizon will never be able to exit and tell about them.

For example, it can be shown that a rotating black hole can, in principle, allow DS and travel to the past. However, the LDS passes within the horizon of events, and its part which is a journey into the past is contained entirely within the horizon. The traveler whose journey is described by the LDS cannot leave the horizon of events. Therefore, the violation of causality in this case is completely hidden from the eyes of an outside observer. Penrose hypothesized that there is a universal principle, which he called "cosmic censorship", which does not allow the appearance of "naked uniqueness", that is, uniqueness that is not surrounded by an event horizon.

The concept of cosmic censorship can be extended to any process that allows DSs or journeys into the past. This means that when someone proposes a plan for a time machine that can reach the past, one of two things: or there are fundamental difficulties that will not allow the actual construction of the machine; Or the principle of cosmic censorship will ensure that the journey to the past remains hidden from view, and the violation of causality involved will not be revealed to the eyes of an outside observer.

In the nineties it was indeed clarified that the time machines proposed by various physicists are impractical, or cannot operate in a way that contradicts the principle of causality. For example, it was proven that in order for Tipler's cylinder to be a time machine, it must be infinite and therefore unbuildable. It also became clear that it is impossible to create a region of negative energy of the size and strength required for a wormhole that a macroscopic body, or even a single atom, could pass through.

Among the physicists who were involved in testing ideas for a journey into the past, the famous physicist Stephen Hawking ((Hawking from Cambridge) stood out A general principle, from which it follows that physics preserves the order of time between cause and effect at all costs. The principle is called "chronology protection" and can be seen as a version of Penrose's "cosmic censorship".

Solving the paradox of traveling to the past with the help of the principle of cosmic censorship seems particularly attractive, because similar principles already exist in physics. For example, Heisenberg's uncertainty principle is a kind of "quantum censorship", which allows the violation of the law of conservation of energy for a short period of time, which makes it impossible to measure the violation.

The principle of "cosmic censorship", or "protection of the order of time" in Hawking's terms, is still an assumed law of nature that has no theoretical proof. This is reminiscent of the situation that prevailed in the middle of the 19th century, when the law of conservation of energy had not yet been formulated, and many people tried to design a machine that performs work without the supply of energy (perpetuum mobilae). The fact that all these efforts failed eventually led to the formulation of the law of conservation of energy. It is possible that after many different plans for journeys into the past are tested and rejected, the principle of cosmic censorship will also become a basic law of physics.