Time in a side view

One of the best articles written in Hebrew analyzing the journey at the time was penned by the late Dr. Nachman Gavoli (died 1996) from "Fantasia, 2000" issue number 6 June 1979

By: Dr. Nachman Gaboli

Frederic Brown's story 'The First Time Machine' that appears in this issue is one of the many stories on this topic, which is one of the most popular in MDB literature and at the same time - one of the most disturbing. Every veteran MDB enthusiast has surely tried more than once to 'break his head' in an attempt to get out of the tangle of paradoxes arising on the topic of time travel.
The magazine 'Science' devoted a number of articles to this topic two years ago, and for the pleasure of the readers of 'Fantasia' 2000 we decided to bring them here in a concentrated form, with only minor changes.

Nachman Gaboli is a veteran computer expert, and the editor of the 'Mathematical Riddles and Fun' section in the 'Mada' newspaper. At the time, accept Mr. Gaboli from the pages of 'Ma'da' because there is no place in the country suitable for the publication of the stories of Madev. Well, there is already one, and one of Med Gavoli's pieces will surely find their place in our magazine. Science fiction' is the accepted Hebrew translation. The English term science fiction - a translation that is not successful at all. The science is not fictional in these stories, but on the contrary - the fiction is scientific. The exact - and simple - translation is science stories. The more scientifically based and plausible the story, the more reliable and therefore more interesting the story. In a good science story, the author does not come up with any idea that contradicts the known laws of nature. Stories about flights to distant stars, about new technological inventions, about the development of the human race in the future from a social, genetic, psychological point of view - all of these can be beautifully written, and yet they will be foolish if they are not based on reasonable scientific assumptions. A scientific story, like any other story, must first be believable in order to be interesting.

"Time machine"

One of the most controversial motifs in this respect in science fiction is the theme of time travel, to the future or to the past. According to the simplistic science fiction approach, you sit in a sort of cage, press a button or turn a lever that sets the year so and so AD or BC and you're on your way. Many see it as a vanity idea that can never be realized; And it is possible that if it wasn't for H.J. Wells, who wrote the classic story on this subject - 'The Time Machine' - was not the idea so accepted in this type of literature. It cannot be said that Wells exhausted the subject, but all the stories about time travel that followed him repeated, knowingly or unknowingly, some of his ideas and descriptions: the explanation that time is a fourth dimension, in which one can move just as one can move lengthwise, horizontally or vertically; The description of the journey itself: of the heavenly bodies moving in the sky with incredible speed; The days and nights change at an increasing pace; The season of the passing year flies by; The appearance of the changing landscape, such as buildings collapsing under them and appearing, trees growing in the blink of an eye, etc. - all this was already mentioned in Wells, and appeared again in different versions in many other stories.

"Flight to the Future"

The writer of these lines did not shy away from this topic either: in 1959, Kol Israel broadcast a play of mine, called 'Flight to the Future', with the participation of Shmuel Rodansky, Bezalel London, Yehuda Fox and others, in which I tried to show that it is possible to deal with this issue without getting into the usual tangle, which is The weak point of the stories about time travel - the paradoxes. The fact that in most of these stories, the author and his heroes find themselves in a paradoxical situation, is seemingly proof that time travel is impossible, and will never be possible. One of the mathematical ways to prove that something is impossible is to show that it entails a contradiction. In stories about time travel, a clear contradiction is usually revealed: you cannot travel to yesterday, because then you would be there (or were there) twice at the same time. If you meet yourself, it will not only be a strange situation, but a contradiction to the facts, because you remember very well that you did not meet yourself yesterday. Furthermore, if traveling to the past is possible, what would happen if you went back a few generations and one of your ancestors died? If you do this before Hela had time to conceive and give birth - you will not travel to the past and your victim will be saved from death. In this case, things will go well, and when the time comes you will be born... and God forbid.

Even traveling to the future is not free of paradoxes. If you travel to the next century, for example, and bring from there a technological invention that will only be discovered in 50 years, you are putting too much behind the times. And you invention will no longer be new when it is invented. Now it will be impossible to determine at all when he invented it.

Settlement of the paradoxes

Such contradictions can be overcome in several ways. The simple way is to say that it is only possible to travel to the future, but not to the past, and therefore it is not possible to return from the past to the present. This limitation is also acceptable in light of the fact that, in fact, we are all always traveling into the future - at a fixed 'speed' of 60 minutes per hour! It is therefore possible that we will be able to sense the speed of travel, but we will not be able to reverse its direction.

In this way paradoxes can indeed be avoided, but still it is not an obligation on the authors of the stories, because if the traveler cannot return to the present, how will he tell us his story? Another way to resolve the paradoxes, which is very common in science fiction, is to say that when we travel to the past and cause a change in the present or the future as a result of some action, we create a kind of branching in the flow of time. Time splits in that moment into two branches: in one the future will be in the mouth that was 'before', and in the other the change caused by our action will apply. Time therefore does not flow as a single stream, but in a kind of door that has infinite arms, and you may be found in each of them in a different version: here you are married, here you are single, here you live, here you die, and here you were not born at all....

Despite the fact that this explanation is quite acceptable in the literature, it does not seem to me, because in my opinion it causes more problems than it solves: now the question arises, if it is not possible to travel from present to present - that is - laterally, instead of lengthwise. Time therefore becomes not one dimension added to the three dimensions of space, but a plane with two dimensions. For all the new paradoxes this creates.

The return to the present

I proposed a third way to avoid paradoxes in the aforementioned play. There, a trip to the future was described, with a return to the present. During their visit in the future, the passengers - Prof. Miller and his friend Dr. Weiss - were asked that on their return they would perform a certain action, which would cause a deliberate change in the future. The passengers agreed to do so, but this raised concerns and doubts in them. Below is an excerpt:

Weiss: Do you know what I find strangest of all? The indifference with which we were received. No journalists, no celebrations, no curiosity, even. After all, we were the first guests from the past!

Miller: Mmm? The first ones from the past? Where did you come from? It is possible that people who left after us arrived before us!
Weiss: Excuse me?
Miller: (impatiently) Don't you understand anything? We traveled from the twentieth century to the twenty-third century. Maybe there were others who traveled from the twenty-first century to the twenty-second century? In fact, I am guaranteed that after the success of this experiment there were many more trips to the future and also to the years before this one. We are not the first here.

Weiss: Weird. I didn't worry about it. A strange paradox!

Miller: There is no paradox in this! As I have already emphasized, time travel does not involve any paradoxes

Weiss: Really? And what do you think of the whole idea of ​​Son? He wanted us to keep that girl away from the country, what's her name? - Mira - so that you don't get married here, and her offspring will be born somewhere else and be different. In this way he wants to prevent the birth of his rival. Assuming we do, what will become of him, Son's opponent? Will he suddenly disappear from the world?
From a child: He will not disappear, because he will not be born at all. Weiss: What does "not be born" mean? He exists!
Miller: Now it exists, but when we return to the twentieth century we can say that it does not exist, and there is no certainty that it will exist.
Weiss: I don't understand that. Let's assume that we do this and keep Mira out of the country. If we come here after that, won't we find Sun's rival here?
Miller: Mmm? It turns out that it is, and there is no paradox in this.
Weiss: One moment, Professor Miller. Let's say you stay here, and I go back to the twentieth century and get rid of Mira. You sit here and talk to her offspring, and suddenly you see him disappear from the world?
Miller: Hmm! This is a somewhat strange situation, admittedly. (laughter in the background). Ah, there's Professor Minkowski, standing behind us and laughing. We didn't hear you come, Professor.
Minkowski: (can't stop laughing) I'm sorry. I heard your interesting argument, and I didn't want to stop it (continues to laugh).
Weiss: Since our ideas are so funny, maybe you can give us the correct explanation?

The explanation

Professor Minkogsky evades an answer, but the correct explanation of the way in which these paradoxes are avoided becomes clear towards the end of the play. During the return trip, all the processes in the universe take place in the opposite direction, seemingly: the sun rises in the west and sets in the east, the trees grow younger, the people walk backwards. This description is also present in Wells, but he did not draw the conclusions from it. The main thing is that the consciousness of the passengers then also flows in the opposite direction: their memory, instead of being enriched with new experiences, is being erased, and they completely forget not a minute the details of their trip, but even the very fact that they have already visited the future! When they reach the present, to the moment from which they left, they only know that they have in mind to sail now to the future... for a similar reason it is also impossible to bring souvenirs from the future to the present. At the time of return, everything returns to its current state in the present. Therefore, only things that came out of the present can return to it, while everything else that originates in the future, including objects, as well as ideas, plans and memories, will disappear from the time machine during the return trip

speed of light

Traveling to the past contradicts a fundamental law of nature - the one that says that it is not possible to transmit signals, or any information, at a speed higher than the speed of light. Let's say we have to send an urgent message to a spaceship, located somewhere in the cosmos, ten light years away from Earth. The laws of physics say that there is no possibility that the information will reach the spacecraft in less than ten years. But if we travel to the past, ten years ago, and from there we broadcast the news on the radio, the waves will spread ten years in space, and reach the spaceship today! Traveling to the past therefore allows information to be transferred quickly, which is practically unlimited.

The connection between traveling at a higher speed than light and traveling in time is also revealed in the opposite way: suppose that a spaceship leaves the earth today and moves away from it at twice the speed of light. Ten years later, in 1989, the spacecraft reaches a distance of 20 light years from the earth. Here the spacecraft stops, and its pilots observe the Earth with a powerful telescope. The light now arriving from the earth to the spacecraft has traveled a distance of 20 light-years, so it took 20 years to reach the point where the spacecraft is.

The light therefore left the earth in 1969. The space pilots therefore see the earth as it will be in 1969.

Lost idea?

And yet, in spite of all these contradictions, the idea of ​​time travel is not completely lost. The science stories also discuss other forms of such a journey, which are not so scientifically far-fetched. In fact, certain forms of gasman travel are completely acceptable from a scientific point of view. One of them is, again, related to flying in space, but this time it is not about speeds higher than the speed of light, but "only" those approaching the speed of light. According to the theory of relativity, the flow of time in a spaceship moving relative to the earth is not the same as the flow of time on the earth, but there is a phenomenon called 'time dilation', according to Einstein's following formula

delta (T1) is equal to delta T divided by the square root of 1 less (V squared divided by C squared)
Delta (T) is some amount of time in the spacecraft, while Delta(T1) is the amount of time on Earth, as it appears to the pilots in the spacecraft. V is the speed of the spaceship, and C is the speed of the orbit, which is a 'normal' spaceship, whose speed is very small compared to the speed of light, the ratio between the two speeds is close to zero (for example - this ratio for today's spaceships is on the order of one to one hundred thousand), and therefore The root value is close to and . Under these conditions, the time intervals in the spaceship and on Earth are almost identical.
On the other hand, when the speed of the spaceship reaches 99% of the speed of light, the time on Earth becomes 7 times faster than the time in the spaceship. The passengers who will leave the Earth in such a spaceship and stay in space for 10 years, will discover upon their return that in the meantime they have spent 70 years on Earth! All their acquaintances and friends will already be extremely old, and their children, who remained on earth and were babies at the time of separation, will now be older than the passengers themselves. Isn't this a trip to the future for everything?
This formula of Einstein's, as killing the principles of the theory of relativity, seems no less strange than the most imaginary scientific story; But today no one doubts its correctness anymore. It has been verified in recent years in practical experiments, both with regard to elementary particles, whose velocities are close to the speed of light, and in flight in a regular airplane, using very precise time meters.

Prof. Gamov's space flight

The principle of time dilation is well used in science fiction, because it opens up the possibility of reaching distant stars, which a normal spaceship would not be able to reach in closed time. The well-known astrophysicist, Judge Gamow, who contributed a lot to the development of the universe, writes in his popular book: "One is two; Three... infinity' "From the phenomenon of time slowing down in moving systems, an interesting conclusion regarding interstellar travel arises. Suppose you want to visit one of the planets of Abarak (Sirius) which is nine light years away from us, and you use for this purpose a spaceship, whose speed is almost the speed of light. You must think that the trip to Avrak and back will take at least eighteen years... In fact, if you fly, for example, at a speed that is 99.99999999 percent of the speed of light, then your clock, your heart, your lungs, your digestive system and your thought processes will slow down 70,000 times and those 18 years (from the point of view of inhabitants of the Earth), needed for this journey, will seem to you to be only a number of hours.
Indeed, if you leave Earth right after breakfast, you'll be more or less ready to eat lunch while your spaceship lands on one of Avrak's planets. If you hurry back home right after the meal, you can eat dinner on Earth. However, here you will have a surprise if you forgot the laws of relativity: it will become clear to you that your friends and relatives despaired of you a long time ago, and believed that you were lost in space, because 18 years had passed between them - during that period that seemed like only one day to you."
Arthur S. Clark, also a renowned scientist, emphasizes in his book 'Profiles of the Future' that it is important to understand that the space pilots will not be able to notice, in any way, that something strange is happening to them. Everything in the spaceship will appear completely normal... Only when they return to Earth will they discover that much more time has passed in it than in the spaceship... A person will be able to return to Israel hundreds or thousands of years after leaving, while he himself has aged only a few years. For a person familiar with the theory of relativity there is no wonder in this: it is nothing but a natural result of the structure of space and time:
And Prof. Gamov continues: "And what about faster-than-light flight? A partial answer to this is given in Hamshir, which refers to the theory of relativity:

Once upon a time there was a man named Dror
who flew back and forth to him
He just came out today
but he came back yesterday,
In the faster-than-light plane

Einstein's formula on time dilation, which we gave above, proves that it is not possible to fly at a speed higher than light, since, if the ratio between the speed of the spacecraft and the speed of light is greater than 1, the value under the root sign becomes negative, and as we know, there is no actual root for a negative number. And what happens at the moment of transition from sub-light speed to super-light speed (at subsonic and supersonic weight)? Arthur Eddington, also a well-known astrophysicist, writes about this in his book "Space, Time and Gravity" It is interesting to ask what happens when the speed of the pilot reaches the speed of light. The lengths in the direction of flight shrink to zero (according to another principle of relativity), the pilot and the objects with him are reduced to only two dimensions. We don't have to strain our brains to understand how life can be conducted in two dimensions, since nothing is right. Time has completely stopped. Yes, it seems to an observer from the country.
The pilot himself does not notice anything out of the ordinary; He does not notice that he has stopped moving. He waits for the next moment to move; And the fact that time has stopped means that he does not notice that the next moment is so delayed."

A space odyssey

Indeed, the flight to Abarak mentioned by Gamov is nothing compared to the long journeys we read about in science fiction. These tell of flights to much more distant stars, and even to other galaxies. Here, even slowing down time 70,000 times will not be enough to reach the destination within the years of a normal person's life. The authors therefore resort to another seasoning, an example of which we saw in the movie 'Odyssey in Space', according to Arthur S. Clark's book: deep slumber. The pilots are fast asleep for most of the flight, under the supervision of an automatic mechanism that wakes them up when the time comes.
At this time of year, all the processes in their bodies stop completely: the heartbeat, breathing, metabolism, cell culture and their degeneration, the speed of the flight will cause time to slow down, but even at this slow rate, thousands of years will pass in the spaceship (which are millions of years on Earth); But as for the space pilots, the sleepers, it will be seen in the blink of an eye.

Two stories

This brings us to the topic of subjective, or psychological time, as opposed to objective, or physical time, which we dealt with earlier. It is important to understand that the phenomenon of time dilation is not a psychological phenomenon, but is actually physical. The astronaut in the almost Orith spaceship does not imagine to himself that fat is slower than fat on earth, but that is how it really is. But now we will leave the spaceship and return to the solid ground of the earth. Let's suppose that a person sleeps, as the circle ends, for seventy consecutive years, and his sleep is so deep that at the end of the day he does not feel at all the time that has passed. Wouldn't it be like traveling to the future?
To this one can reply that 'as if' does not mean 'really'. And it won't be a real trip to the future. is that so ? Let's check it out. Let's compare two apparently different science stories. One tells about a man sitting in a cage, which is the time machine. The man presses a button -, and the cage starts moving into the future. In its journey the cage passes rapidly through all the intermediate periods. Today goes by in a flash, followed by tomorrow, the two days after, the whole month, the year and the years after that, and so on.
There are people standing around the cage, watching the time-traveler. They see him sitting motionless for the whole day, and when they visit there the next day he is still sitting there, and also a month later and a year later as the time machine goes through all these periods. The man, of course, does not move, since all this time passes for him in the blink of an eye. Even if he moves, his every movement will take years for the observers from the outside, and they will not be able to observe it. We will also have to assume that the man is not able to see anything of what is happening outside the cage, since the reflection of the images on the retina of his eyes does not last, for him, more than a zero fraction of a second. Finally, after 70 years, let's say, the machine stops, and the man gets out of it - into the future. Also in the other story, a man sits on a chair inside a cage, except that this is an anesthesia machine, not a time machine. As soon as the man presses the button, he falls into a deep unconsciousness-like slumber. All processes in his body stop completely. The people around him see him sitting motionless, day after day and year after year. Finally, 70 years later, the anesthesia machine stops working, and the man wakes up. He comes out of the cage and looks around in amazement. Everything is different from what he knows, and when he asks what the date is, it turns out that he has arrived in the future.

Is there any objective difference between the two stories and how can the man himself determine if it was a dream or a "real" journey into the future? And how can the people who observe it determine this? Indeed, there is no such way, and the obvious conclusion is, therefore, that there is no real difference between these two possibilities. They are nothing but the same story itself, which only apparently has two different faces.

Stopping time

Deep anesthesia is therefore a way to jump into the future. Is there something parallel to it that allows a return to the past? What is the opposite of anesthesia? If we see in sleep a situation of stopping, or at least an extreme slowing down of the processes in the body, because then the opposite situation will be the maximum acceleration of these processes. If it were possible to cause, with the help of some drug or in some other way, an extreme acceleration of the body's processes, including the speed of sensation, reaction and thought, the person under the influence of the drug would feel as if everything around him was moving and moving in ridiculously slow motion. Time was slowing down, for him, the rate of his flood. The man felt that he was moving towards the future more slowly than usual. However, it is understood that this is not a return to the past. Even if the rate of his mind's activity increases to an infinite speed, this will not cause a return to the past, but a complete stop of time, as in Prof. Eddington's explanation of flying at the speed of light. The man will see everything still and frozen in place, without any movement or development. And again, H.J. It was Welles who wrote the classic story on this subject, called The New Accelerator. The reference here is not to an accelerator of elementary particles, but to a drug that accelerates the processes of the body and mind. Prof. Givran and his friend, the narrator, take the drug, and here is their conversation under its influence:
- Well ? I said
- Don't you feel anything?
-Nothing. A bit of elation, maybe. Nothing more.
- What about voices?
- Everything is quiet, I said. - Lordy! really quiet except for some kind of secret tapping,
Like rain falling on different objects. What is?
- Broken sounds, he said, as I imagine, but I'm not sure about that. He looked at the window. - Have you ever seen a curtain fixed like that in front of a window?
I followed his gaze, and saw the corner of the curtain pulled down and frozen,
Apparently, in a state of vigorous waving in the wind. - No, I said, - it's strange.
- And look, he said, and opened the palm of his hand that was holding the glass.
I subconsciously grimaced, expecting the glass to crash. Not only did it not crash, but apparently it did not move at all; She remained suspended in the air, motionless. - Objects fall at a speed of 5 meters per second Gibran said - about the first. This glass is now falling at a speed of 5 meters per second. But in fact she still didn't have time to fall for even one hundredth of a second...
(The years leave Prof. Gibran's apartment and reach the street, then -)...we looked at the frozen vehicles like statues. The upper edges of the wheels and the ends of the horses' legs, the tip of the whip and the lower jaw of the vehicle, which had just begun to yawn, moved a tiny amount, but the whole chariot seemed still, and completely still, except for a faint purr that emerged from the throat of one of the men...

The passengers sat in frozen positions, in gestures that were stopped by their means. A man and a girl smiled at each other, with a gentle smile that lasted as if forever... A man curled his mustache like a wax doll, and the second reached out a stiff and tired hand, with simple fingers, to his hat. We looked at them, we laughed in their faces, we made faces in front of them, until we got tired of them. - By God! cried Gibran suddenly. Look there! He pointed, and with the tip of his finger she glided through the air as her wings flapped.
Slow down at the pace of a lazy snail - a bee.

Again two fidush

As Wells explained, you can't pick up speech or music when your thought rate is many times faster than normal. All sounds (except those made by your friend, who is in the same situation) are, for you, hundreds of meters lower and completely beyond your hearing range. On the other hand, the people around you cannot see you, if you do not stand for a long time in one place, since every hour of your time is a fraction of a second for them. By the way, Wells also talked about other problems that this situation raises: you can't run for a long time, because your clothes will catch fire due to the friction in the air. When you put a glass on the table, it can crash if you don't do it very slowly. Actually there is another problem, which Wells didn't mention, you can't run at all, since your feet can crash from the impact of the speed on the ground - unless your body has become strong in it to the extent that its movements have become fast. The slowing down of time can also apparently be objective or subjective. Suppose it is not a drug, but, again, a cage in which a person sits. The influence of the cage is given, once again, to two interpretations: except that these interpretations are the opposite of the two above-mentioned stories about traveling to the future.
According to the one, objective interpretation, the cage causes the flow of time to slow down (and instead of sensing the flow of time inside the cage). According to the other, subjective interpretation, the cage causes the man's thought processes to accelerate (instead of putting him to sleep). What we tried to prove in all these stories is this: if you don't believe that 'real' time travel can ever be realized, maybe it will be easier for you to believe, that maybe one day you will find a way to speed up or slow down, to a very significant extent, the activity of the body and brain. (It seems that there are already drugs today that have a certain action in this direction). As we mentioned, this is also time travel, which is essentially no different from the previous one.
A trip to the past, after all

It turns out, therefore, that the only travel while it is impossible is the travel to the past. But in this matter it is impossible not to mention the surprising fact that even such a trip has some hold in modern science: the nuclear physicist Richard Feynman received a Nobel Prize in 1965 for his research on antiparticles. He showed that from the point of view of quantum mechanics, the anti-particles can be seen as if they were ordinary particles moving as if backwards in time, and there is no mathematical or experimental way to decide between these two interpretations! However, since anti-particles exist in our world, as we know, only for very short periods of time, it may be possible to conclude that traveling to the past is only possible for a zero fraction of a second and only for very tiny masses, as stated above, there is a connection between traveling to the past and flying at a speed higher than light. And here it turns out that even this topic is not considered a complete 'taboo' in modern science. The theoretical possibility of the existence of particles faster than light was raised, and we even gave them a name - tachyons (tachos - fast in Greek). However, the theory proves that even if it turns out that these particles do exist, it is not possible to use them to transmit any information or signal.
...
The publication of this article on time provoked at the time some of the readers of 'Science' to draw their own conclusions. Thus, for example, one of the readers claimed: Suppose we accept the assumption (which was expressed in the radio drama I mentioned - NG), that although it is possible to travel to the future, but when returning to the present, all the processes will work in the opposite direction. We must therefore apply this rule not only to the physical and mental processes, but also to the biological processes. Hence the conclusion that the returning traveler must become younger from moment to moment, at exactly the same rate as the rate at which he goes back. It therefore follows that travel on the past is possible only up to the date of the passenger's birth, but not to an earlier date. Accordingly, we must conclude that the trip to the future is only possible until the day of the traveler's death, but to a more distant date, since during the trip he must age at the rate at which time passes indeed. In fact, this method of drafting in time is so legitimate that we cannot prove that it does not actually happen! Who guarantees us that we don't skip, from time to time, backwards and forwards? If I suddenly go back to 1941, I'll be 20 years old again, I'll enlist in the British army, I'll live the wartime again, I'll see movies with Fred Astaire and Ginger Rogers, so on.
I will, of course, have no memories of a later period. On the other hand, if I suddenly skip to the year 2001, I will be 80 years old, I will lean on a cane, I will walk heavily, I will speak Yiddish, and I will have memories of the intermediate periods but no memory of the actual time travel. I will therefore not be able to know that there was such a trip. In other words, time travel according to this view is not your travel relative to the universe, indeed, I believe that the question of whether such travel does occur is meaningless, since there is no possibility to answer it positively or negatively.

Time is like a river

However, this kind of time travel is essentially different from the trips we talked about, which are accepted in science fiction. If we compare the flow of time to a river, we can illustrate the difference between these two views in the following way: we are all sailing in this river in boats, drifting with the current at a constant speed a. Traveling in time according to our interpretation is equivalent to sailing at a speed different from the speed of the current, forward or backward. On the other hand, according to the reader's interpretation, the travel in time is parallel to the change in the speed of the current and the boats together.
In this context it is worth noting that comparing the flow of time to a river involves some difficult logical problems. It is worth pondering the question - how are the historical events represented in this 'model', and how would the time traveler be able to see such an event?

law of energy convertion

Other readers also raised another problem: what happens, in such a trip, to the law of conservation of energy and mass? Does the traveler disappear from here and appear at another time, after all, he leaves a serious 'hole' here, while mass and energy suddenly appear there, in contradiction to the laws of nature. Moreover: what about the energy needed to operate the time machine?

If traveling in one direction the machine consumes energy, and in the opposite direction it emits it? Or another claim: you may be able to go back to the distant past to the time before your birthday, but not to any event in your personal past. Why? Because if you do this, then when you return to the present, you will have a double memory of the same event, with a certain difference between the two events: one occurred, as you remember, in the normal course of affairs, while the other occurred at the end of time travel, therefore it cannot be said that it is exactly the same event itself And the conclusion is that you have not returned to the 'original' past at all.

Record the recovery

Another method of time travel, which does not contradict the known laws of nature. It is indeed a subjective journey, like the sleep of the circle dwellers; However, unlike sleep, which is like a leap into the future, this one has both a return to the past and a leap into the future, the idea borrowed from the concept of a 'recurrence record' familiar to computer people. As many know, there is no computer that does not 'fall' from time to time. Whether due to a power outage, or due to a fault in the computer itself, or due to human error
of the operator, it happens that the operation of the computer is stopped in the middle, and all the information stored in its immediate memory - as opposed to the auxiliary memory, such as records and magnetic tapes - is destroyed. In this case there is no choice, apparently, but to start the work again. If it is a long-term work, it is sometimes a serious waste of time and money. The usual way to overcome the lag is to copy the entire computer's memory to a disk (record system) at regular intervals, for example every 5 minutes. The computer continues its work, but in the event of a malfunction, you can use the block of information written on the disk, called a 'recovery record', and copy it back to the computer. In this way, the computer will be able to continue its work from the same point it reached when the last recovery record was recorded. Integration: the computer does not resume its work at the point of interruption, at the time of the malfunction, but it returns to the moment of registration. All the information created in his memory between the moment of the last registration and the moment of the malfunction - is lost and gone. The computer must therefore go back a few minutes, to the past...

A story in a drawer

A few years ago I wrote a story based on this topic, except it dealt with humans, not computers. Unfortunately, at that time there was still no stage in Israel suitable for original science stories, and the story is still in my desk drawer. Since it also has implications for the subject of time travel, I will tell it here briefly: the story takes place in the twenty-second century. Modern medicine is already capable of transplanting any organ in a person's body. These organs are not taken from other people's bodies, but are grown artificially from biological tissues, through appropriate nutritional and environmental conditions, just as they develop in the body of the fetus. A person whose organs, even the most important ones, have been damaged by disease or an accident, does not have to die, because new organs can always be transplanted under them. This also applies to the brain...
But the new brain will be a kind of smooth board - tabula rasa - without any knowledge or memories, like the brain of a baby. The man will have to relearn how to walk, talk, use his hands, etc. One day the scientists announce that they managed to overcome this problem as well. Every person can copy all the information stored in his mind to an auxiliary memory unit!
Now, if his brain is damaged and it is necessary to transplant a new brain at the top, the scientists will be able to return all the information and memories he had, by copying the restoration record, back into his brain. The first person who wants to take advantage of this new invention is an astronomer, who is about to embark on a very dangerous operation: landing on the surface of the planet Jupiter. The escape from the enormous gravity of this planet requires such a high acceleration that there is almost no doubt that the pilot's brain will be damaged during the return takeoff. The spacecraft will indeed be brought back to Earth, thanks to the automatic mechanisms installed in it, but the pilot will surely need his brain restored.

The man entered the room where the recording of the recovery record would be made. It is a kind of combination of an operating room with a computer room. In the center of the room there is a fixed seat, similar to a dentist's chair, and above it hangs a large helmet, connected by cables to the ceiling. The man sits, and the nurse shaves his head. The doctor explains the process to him. "The operation does not involve any pain, but it is necessary to stop the brain activity for a few seconds. You will of course lose your consciousness", says the doctor, "but when you wake up you will not feel any bad effect". The helmet comes down on the man's head and covers the face as well. The man stops seeing and hearing, and his consciousness is lost.
The helmet rises, and the man opens his eyes. Suddenly he stopped, and fixed his eyes on the doctor. This is not the same doctor, 'who are you' he asks. He looks at the nurse, and here she is not the same nurse either. "what happened here?" he asks.

Readers of 'Fantasia' 2000 will surely have no trouble explaining what happened here. (In any case, the explanation appears in the 'Science' issue 6)XNUMX

On the tombstone of a time traveler
"Kill my grandfather?" Jack wondered
It's impossible, that's obvious!
But when he kissed the grandmother
The grandfather put a bullet in him...

by Sherwood Springer