When, if ever, can we travel back and forth in time?
Amnon Jaconte
The class seems like the beginning of a joke: a psychologist, a doctor, a mayor and a neurotic redhead gather in a distinguished guest room for a conversation with a time traveler. The continuation is far from funny: "It is clear that every tangible body has four dimensions," explains the time traveler: "Length, width, thickness and life span. For some reason, there is a tendency to separate the first three, which belong to space, and the fourth, which is related to the dimension of time, but in reality there is no difference between the three dimensions of space and the dimension of time, except for the difference found in our consciousness."
Thus, with ingenious simplicity, the science fiction writer H.J. Wells, in his book "The Time Machine", explains the great change that has taken place in recent decades in the perception of time. Until the beginning of the 20th century, people believed in absolute time, whose values are equal for every body. Thus, apparently,
A tree, a book, a building, a person or a loaf of bread can be carried on the stream of time only in one direction - forward - and only at a uniform rate. Our consciousness cannot accept another state, partly because the observation of the world around us proves that most of the processes that take place in it flow from a state of certain order to a state of irreversible disorder: a vase placed on the table falls, shatters and its water is absorbed by the carpet without finding any way to turn it back into a whole vase It should contain a whole lot of water. Even if the vase does not fall, the flowers in it will wither over time, they will fall on them, the stems will dry and the water in which they are soaked will go rancid. Time rushing forward thwarts any possibility of restoring the old order, and tomorrow will be different in thousands of small and large details from today and yesterday.
The arrow of time, as we perceive it, is rooted in our world through physical elements (for example, the length of time the flowers in the vase withered), cosmological (the time during which the universe continues to expand), and psychological (Bodil Jenson, in her book "Stop the Clock", which was recently translated into Hebrew, illustrates this well through the difference between the feeling of a young woman who says "time is running out" and that of a middle-aged woman who repeats the same sentence knowing that the finish line of her life is closer). These elements shape our perception and determine our attitude to different bodies and situations. But if we ignore them, will we be able to perceive time in the way that Wells suggests, as another dimension of space, and travel forward and backward, along and across, in and out?
Presumably so, but this is about as possible as responding to Archimedes' well-known suggestion "Give me a fulcrum and I will lift the earth". Theoretically, there is no doubt that a fulcrum in space plus a suitable lever may move the Earth. Practically, this is impossible, and not only for technological reasons but also for fundamental reasons: will the earth that is found in another point in the universe, under a different regime of sunlight and Venus and the moon, cosmic winds and magnetic fields, remain the earth? Will a person returning from a trip during which he reached the 19th century remain a 21st century person who traveled in the 19th century, or will his mere presence in that century cause sweeping changes that will also affect the 21st century from which he left?
The famous "grandfather paradox" illustrates the question in all its seriousness: suppose that man met his grandfather on his journey, and for some reason killed him. Did he not thereby cancel his own existence? In terms of the logic of time as we understand it, isn't the Gollel just wasting his chances of returning to his place in the 21st century? Let's say he met his grandmother and had a close relationship with her, didn't he become his grandfather or give birth to his mother?
no time
Paradoxes do not eliminate phenomena, they only sharpen them. One of the solutions to solving the phenomenon was proposed by scientists who sought to limit the idea of time travel to the areas of logic that we understand, that is: to determine that it can only be carried out if it does not contradict basic laws and does not retroactively impose disorder in areas in which a known order to which the traveler is related has already been established. These scientists see the past as a collection of time-space slices, and state that even when it is possible to move from slice to slice, this will be limited by the fact that the transition will not break logical principles, such as the principle of historical continuity in the "Grandfather's Paradox".
But here the question arises: Is the collection of principles we call "logic" the only logic in the universe?
It seems not. You don't have to take off into space to understand this. Even here, on Earth, a change in perspective on this or that event may reveal the existence of a parallel, completely different logic: imagine a line of cars passing a pack of dogs. From the dogs' point of view, the well-known saying "the dogs bark and the caravan passes" is completely accurate and logical. From the point of view of those sitting in the cars, it was the dogs that passed, while the convoy remained as it was in terms of its components, order, speed, direction and all other characteristics. If in one of the cars there is also a dog that will continue to bark enthusiastically, it will be possible to claim justice
that "the dogs pass and the caravan barks".
Hugh Price's book "The Arrow of Time and the Archimedes Point" (translation: Emmanuel Lotham) transfers these principles to the cosmic environment. Price suggests avoiding a subjective view of time and try to look at it from an external fulcrum, just like the one Archimedes asked himself to swing the world. This point - which he calls "no time" - is similar to the Archimedes point in another respect: the most efficient place to place it is outside the earth. Thus, for example, observing the particles that were created on a distant star in space and make their way to Earth will show that in terms of Earth's time concept, the particle's journey takes several light years, but in terms of their age, they will only be one or two seconds old the moment they reach Earth. The logical gap described in the event of the caravan and the dogs becomes, in the extreme conditions
that prevail in the universe, to a physical gap.
The fulcrum "no time" therefore makes it possible to understand that time is relative and is measured differently in each system. It also illustrates that the element that causes this relativity is the speed of any system compared to the speed of light. Just as the psychological component makes the 50-year-old woman in Bodil Janson's book feel that time "runs faster" as she approaches the end of her life, so in a propulsion system at a speed close to the speed of light, private time becomes slower. Theoretically, clocks sent traveling at the speed of light should show a different time than they would show if they remained on Earth, physiological and chemical processes should be slower and the aging of organisms should be delayed.
The twin paradox
The principle according to which time is not an absolute concept but an essence arising from the speed at which every body moves, was formulated convincingly in Einstein's special theory of relativity and was even tested and confirmed in physical experiments performed on particles. In real life, this is almost impossible to test, since the proof is only possible at a speed close to the speed of light, while the fastest a person has reached - the speed of a rocket launched into space - is still 30 thousand times less than that.
Without a choice, we have to make do with the illustration provided by the science fiction films. In many of them ("The Planet of the Monkeys", for example) the "twin paradox" is applied: one of two identical twins goes on a journey at the speed of light, while the other remains on Earth. When the twin returned from his journey, he found that his brother had become many years older than him. From a subjective point of view, the traveling twin meets his future, while the twin who remains on earth meets his past. From an objective point of view, that is: observing from the point of "no time", it is possible to identify here two systems of time, the difference between which is due to the speed of their movement, just as Einstein predicted.
The practical application of all these phenomena is still far away. His day will only come when they find spaceships that will touch the stars at a speed close to the speed of light. A journey to a star that is a thousand light-years away from us will then take about a thousand years in terms of Earth's time, but as far as the passengers are concerned - the journey will be much shorter and they will be able to get there in their lifetime.
The day the service becomes commercial, there will surely also be those who will try to attract passengers by offering shortcuts. The ads he will publish will probably carry slogans like: "To other worlds - the black guys" or "If you want to know the future - go there worm guy".
ridiculous? unbelievable? Scary? It is enough if we recall the learned explanations issued by scientists at the beginning of the operation of the car and the train, according to which no living organism would survive at a speed exceeding 40 kilometers per hour, or the diagnoses of experts regarding inventions such as the radio or the computer ("the radio craze", predicted Thomas Alva Edison in ,'22" will soon die out"; while Thomas Watson, chairman of the board of IBM stated in 43: "I think there is a market in the world for five computers at the most"). If we imagine the space of the future to be a network of highways where spaceships will move at a speed close to the speed of light - the black holes will serve as a kind of tunnels through which the passengers will penetrate into the depths of the mountains of time and space and will exit at other places and times in the universe.
dark objects
In essence, black holes are not a new discovery. Already 200 years ago, several mathematicians and physicists, the most prominent of whom was the Frenchman Pierre Simon Laplace, predicted that under certain conditions, objects with such a strong gravitational force would form in the universe, that even light rays could not escape from them. These ideas hardly resonated and were even considered wrong, since according to Newton's descriptions, gravity should not act on light rays.
At the beginning of the 20th century, when Albert Einstein published his theory of relativity, in which it was determined, among other things, that light rays are indeed affected by the force of gravity, Laplace's theory was removed from scientific femininity, and the idea that "dark objects" (as Laplace called them) would exist no longer seemed far-fetched at all. so.
A few months after the publication of Einstein's theory, another physicist, Carl Schwarzschild, began to back it up with mathematical equations. One of them proved beyond any doubt that, at least in theory, there could be an environment similar to those "dark objects" described by Michel and Place, and that the gravitational force of such an environment is so enormous that nothing, including light rays, could escape from it.
But Schwarzschild, like Einstein, let his prejudices overcome his judgment and the degree of trust he had in the theory, and saw his discovery as a stain on the theory of relativity. At the same time, one of Einstein's students, Prof. Natan Rosen from the Technion in Haifa, was also rescued, and until the end of his life he tried to build versions of the theory of relativity in which "dark objects" do not exist. Only about 50 years after the publication of the theory of relativity, another physicist, John Wheeler, confirmed the fact of the existence of dark objects, and also gave them a new name: "black holes".
Today, black holes are one of the most studied fields in physics. Small, dark objects have been observed in the universe which, although they have not been identified with certainty as black holes, but there is no other explanation for them having such a large mass and such small volume dimensions. Astronomers have discovered particles of matter moving around the black hole and are in various stages of falling into it. In the period of time before the fall, the particles of matter are compressed and the bodies that will be swallowed emit light radiation with a greater intensity than usual, but once they have passed the horizon surrounding the black hole, they are no longer visible.
Astronomy even provided a tangible explanation for the mathematical phenomenon that Einstein and Schwarzschild hesitated to predict, stating that a "black hole" is one of the three forms in which stars die. Some of them continue to accumulate inactive gases and become "white dwarfs" and some emit the rest of the energy that bubbled up in them in a spectacular firework explosion called a "supernova", and then begin to shrink. There are among those who shrink into a massive and relatively large mass called a "neutron star", and there are those who shrink even more and receive the same fate as those who predicted the universe like him when they still assumed that it would shrink (see previous article): shrinking to the size of a small and very dense point, whose gravity creates a horizon Spherical, which any substance that crosses it is drawn in without being able to get out.
wormholes
The contribution of black hole research to the idea of time travel is expressed in a very specific type of black holes, called "worm holes". Ironically, the existence of such a hole was already predicted by those who, as mentioned, did their best to deny the existence of black holes - Einstein and Rosen - and was even called the Einstein & Rosen Bridge in the past. The two scientists discovered that according to Einstein's theory, it is possible for two black holes to be connected. To this in a kind of worm-like internal canal, where one hole is an entry point to the canal (or "the bridge", as they say) while the other is an exit point.
The practical application can be described simply: on the day when suitable spacecraft will operate, the power of their engines allows for an "escape" from the gravitational forces of the point at the bottom of the black holes, it will be possible to use a wormhole to travel within a few seconds to distant regions of the universe, a journey to which under normal conditions may take hundreds or thousands of years. In such a case, a spaceship will enter one black hole, reach the point at its bottom and then, by the power of its engines, it will be freed from the influence of the gravitational force of that point and pass through the tunnel to the other hole. When you get out of there, it will turn out that you traveled at a speed that exceeds the speed of light (after all, even light is swallowed by the black hole and does not leave), from one point in the universe to another point, located in a different time and space and usually very far from the point of entry. It will become clear to the passengers that many years have passed in the universe, but they aged only in a few seconds.
Human failure
In the last ten years, the insights related to time travel have become more and more refined, and from a theoretical point of view it is clear that this is possible. So what stands between the idea and its implementation?
First of all, a technological capability is necessary that will enable the development of spacecraft capable of moving at speeds close to the speed of light (and for those who doubt the ability of science and technology to achieve such capability in the future, it is worth mentioning that until a few decades ago the speed of sound was considered an insurmountable barrier). In addition, the spacecraft will be required to withstand extremely intense radiation, of the type found on the horizon of black holes, to navigate in the vast void of the wormholes and to have a supply of fuel that will also allow it to return from the distant point in space and time to which it will arrive.
But all of these are minor problems compared to the social and psychological anarchy that real time travel can create. Assuming that those who go on a journey will also return from there, loaded with gifts or products from interstellar duty free shops, what kind of world will they find? A vacation that for you lasted a few days, usually means a few ages on earth. Those dear to them will no longer exist, the jobs in which they were employed have been canceled or given to others, and the worst of all - their psychological identity, which consisted of the accumulation of images they had with those around them, was completely erased in the absence of someone to remember them.
This is perhaps another proof that technology does not necessarily bring about the improvement of life on earth. Sometimes it actually highlights the most basic human failings. Long before the implementation of time travel, it will be necessary to check its necessity. Just as Alfred Nobel was dismayed when he discovered that the substance he invented to make mining easier - dynamite - had become a deadly device, so too may the person of the 22nd or 23rd century discover that movement in time is nothing but a journey in one direction - loneliness.
Next week: Aliens
