Kill the grandfather, and stay alive: on the paradoxes of time travel

Of course, no physicist studies time travel, at least not officially. If you declare that you are trying to design a time machine, you will discover that there is only one substance in the universe that is capable of moving faster than the speed of light, and that is the letter from the university administration announcing the termination of funding for your lab. That's why physicists study 'closed time loops', which is completely different

The idea of ​​time travel is one of the most intriguing topics in science today. A little over a hundred years ago, no one would have thought to seriously investigate the possibility of traveling back and forth in time. It was only after Albert Einstein published his theory of relativity that scientists began to turn their attention to this field. According to the theory of relativity, time is 'fluid', flexible: time can slow down or speed up, shorten or lengthen like a spring. And if time is so flexible, maybe it is also possible to fold it, make it flow in the opposite direction?

Of course, no physicist studies time travel, at least not formally. If you declare that you are trying to design a time machine, you will discover that there is only one substance in the universe that is capable of moving faster than the speed of light, and that is the letter from the university administration announcing the termination of funding for your lab. That's why physicists study 'closed time loops', which is completely different.

In this chapter, then, I will talk about paradoxes of time travel. In other words: if a time machine were possible here and now, what strange problems and paradoxes would arise in its wake.

What is a paradox? A paradox is a contradiction. In a paradox, you start from a basic premise and following a series of logical steps you reach a conclusion that contradicts the basic premise from which we started.

A classic example of a paradox not related to time travel appears in the excellent book 'Catch 22' by Joseph Heller. The protagonist of the book, Captain Yossarian, is a crew member in a bomber plane during World War II. Nature has gifted Yossarian with a highly developed sense of survival, and he tries to free himself from military service in any way possible. The most logical way to be released from the army is to pretend to be crazy, but here comes the paradox: to get released from the army you must prove that you are crazy, but if you are afraid of dying and try to be released - that means you are sane. In other words: to prove you're crazy and get a release from the army, you have to volunteer for dangerous bombing missions. This paradox was named 'Catch 22'. The desperate Yossarian turns to the military psychiatrist and tries to convince him that he is suffering from paranoia. He tells the psychiatrist: "I think everyone is trying to kill me." The psychiatrist agrees with him: "You're right, they're really trying to kill you, they're trying to kill us all."

Time travel is a particularly magical idea for science fiction writers, but good ideas don't come for free. Time travel leaves behind a trail of paradoxes and logical problems that the author or screenwriter has to deal with and solve, or hope that the special effects department will do a particularly good job and then no one will notice.

A famous paradox of time travel is the 'grandfather paradox'. The name 'The Grandfather Paradox' is given to this problem because the general idea is this: the time traveler goes back in time and then shoots his grandfather when he was young. Since the grandfather died, the traveler's father (or mother) was not born and then the traveler himself never came into the world. But the traveler exists, otherwise who killed the grandfather? And that is the paradox. It is interesting to note that almost all examples of time paradoxes involve the murder of someone in the family. Sometimes they go back to the past to stab their grandmother, sometimes they run over their mother and some investors kill themselves when they were babies. When you read about the paradoxes of time, it seems as if no one goes back to the past to enjoy a good meal in a restaurant that has already closed a long time ago - everyone is trying to settle a score with grandfather who didn't give them enough pocket money on Hanukkah.

So here is another, more positive example from the movie 'Back to the Future'. Marty McFly, played by Michael J. Fox, returns to the past using his friend scientist Doc Brown's D-Lorain time machine. When he reaches the year 1950, Marty saves his father from a car accident. Pay attention, theoretical physicists, saves father: does not prey, does not stab and does not drown - saves him. Marty's troubles begin when it turns out that precisely following the accident, Marty's mother was supposed to take pity on his father, take care of him and then they would fall in love, etc., etc. Now the mother-to-be falls in love with Marty, her son. If things stay as they are, Marty's mom and dad will never get married and he won't be born. A classic paradox - but how to solve it?

The solution of the screenwriters in the movie 'Back to the Future' was to make the deterioration of the family unit a gradual process: as time passes and Marty is unable to solve the problem, he and his brother and sister begin to disappear little by little from the shared family picture, as if they never existed. In the time that passes until he himself is in danger of evaporating, Marty tries to correct the mistake and make his father and mother fall in love with each other.

This solution sounds like a successful solution, until chaos theory is introduced into this cauldron. According to chaos theory, seemingly small and insignificant actions can have dramatic effects on the world as time passes. If the screenwriters had considered chaos theory and incorporated it into the script, they would have been in deep trouble because any action Marty took in the past could have caused dramatic changes in the future. If, for example, Marty had startled a small butterfly sitting on a flower, the flapping of that butterfly's wings might have eventually caused a hurricane that would have completely destroyed the city. We got the 'grandfather paradox' multiplied many times over.

The screenwriters of 'Back to the Future' decided to elegantly ignore the chaos theory, but the scientists actually tried to deal with this paradox. One of the possible solutions is the 'parallel universes' theory. According to this theory, when the time traveler returns to the past and changes something in it - whether a dramatic change or a tiny change - the universe splits at that moment into two parallel universes.

One universe is the original universe, in which nothing has changed and everything remains the same. If we continue the previous example, this is the universe where Marty failed to save his father from the accident. The second universe is the universe where Marty did manage to save his father from the accident. The critical point for understanding this solution is that the time traveler always remains in the other universe, the universe of change. In this universe, he was really never born but there is no paradox because he comes from the parallel universe, the one where the accident happened, the parents fall in love and everything continues as usual.

Some people think that this solution, splitting into two parallel universes, somewhat defeats the whole purpose of time travel because the time traveler did not really affect "his" universe, the universe from which he originally came. If, for example, the time traveler came back to kill Hitler and prevent World War II, he cannot truly prevent the war. There will be two parallel universes - one where the war, the holocaust and human suffering remained exactly as they were - and a parallel universe where none of this happened.

Another paradox that may arise as a result of time travel is the 'causality' paradox. Common sense tells us that everything has a reason. If every night I open the fridge, eliminate an entire box of cream ice cream and at work they tell me that I have gained weight, then there is obviously a reason for this, and that is that I have heavy bones - genetics.

Here is an example of the causality paradox. In the movie 'The Terminator', a robot from the future (Arnold Schwarzenegger) is sent to eliminate Sarah Connor. The one who sent the robot is a supercomputer called Skynet that in the future tries to destroy humanity. Sarah Connor is the mother, or rather - will be the mother - of the leader of humanity who fought Skynet. By eliminating her Skynet hopes to get rid of the rebel leader.

In the second movie of the 'Terminator' series, we find out that Skynet was built by the engineers by analyzing and reverse-engineering the remnants of the robot from the first movie.

If Skynet sent the robot from the future, and through the knowledge taken from the robot, the engineers were able to design Skynet - what came before what? If Skynet sent the robot and the robot itself caused Skynet's formation, where did this knowledge come from? According to all the rules of logic, someone had to invent the engineering knowledge that led to the creation of the computer and the robot, but the paradox forces us to believe that this knowledge was there all along, no one invented it.

The causality paradox also has a possible solution. It was the physicist Roger Penrose who thought of it in 1969, and the solution was named the 'Cosmic Censorship Hypothesis'.

According to this hypothesis, if time travel is possible at all, then it can only be done in very certain places in the universe, places that are 'singular points' - like in the hearts of black holes. A singular point is a point where the laws of physics break down and break, and there, perhaps, there is no longer any meaning to causality and what preceded what. The only problem is that matter that is 'sucked' into the heart of a black hole, for example, can never escape. Hence, if a scientist managed to return to the past inside a black hole, he would not be able to get out of it to tell Khabra. This is the 'cosmic censorship' and as of today it is only a hypothesis.

Let's go back to 'Back to the Future' and Marty McFly. In one of the scenes of the first movie, Marty goes on stage and plays the song "Johnny B. Good" by Chuck Berry. In the second film in the series we see Marty, who has returned to the past again at the end of the first film, climbs the scaffolding above the stage and looks at Marty from the first film as he plays the song.

Hence, at the same time, in the same universe, there are two exactly identical copies of Michael G. Fox. It's strange, it might even make the production of the film very expensive, but it's still not the paradox.

The real problem begins when you realize that, in principle, you can produce an infinite number of folding marts with this method. For example, if every time Marty from the future returns to the past a few seconds earlier than the previous Marty. Theoretically, the universe could be filled with identical copies of Marty McFly until there is no room for anything else.

In the film 'Twelve Monkeys' by director Terry Gilliam, a completely different approach to solving the problem of paradoxes in time is presented.

According to the film, in the future a plague breaks out that wipes out most of humanity and forces the rest to live underground. James Cole, played by Bruce Willis, is sent to the past to try and prevent the outbreak of the epidemic. His every step and every action brings him closer, unbeknownst to him, to the inevitable disaster. This is a very common approach in the world of science fiction to solve the paradoxes, according to which there is some mysterious force in the universe that prevents the time traveler from changing the past. For example, Cole meets Jeffrey - Brad Pitt in an amazing role - in an insane asylum, in which he suspects that he is the distributor of the original virus. At a later point in the film, Jeffrey claims that he had never thought of the idea of ​​destroying humanity until Cole brought it up in a conversation between them.

At first glance, this solution sounds like running away from the problem instead of dealing with it. It is very easy to solve the problem of paradoxes by declaring that they do not exist, and that there is some mysterious law of nature that will fix anything the time traveler might screw up, or conversely, will screw up anything the time traveler tries to fix.

But it is possible that this solution is a real possible solution to the problems posed by time travel, and it is based on a theory put forward by Dr. Igor Novikov in the XNUMXs. Novikov's principle of self-consistency is based on one of the important principles of quantum theory, according to which every possible occurrence in the universe has a certain statistical chance of it occurring. For example, if you lock an unstable radioactive atom in a closed box, there is a chance that it will decay and there is a chance that it won't.

According to the self-consistency principle, the basic quantum law states that the probability that the time traveler will do something that changes the future is always zero. I will refine the point: the time traveler can affect the past, but he cannot change it, and hence the probability that his action will change the future is zero.

The discussion of paradoxes of time travel is long and winding and in this chapter I have covered only a small part of it. For example, there are scientists who claim that if time travel is possible, it will be possible to go back in time only to the point where the time machine was built and therefore we do not see tourists from the future around us. There are science fiction writers who speculated that if some kind of time paradox occurred, then the universe would decide it had had enough of everything and kill itself. It is likely that we will not get an answer to our questions...at least not in the near future. And if time travel were possible, what would you change in your past?

On the same topic on the science website

A fascinating article by the late Nachman Givoli - Time in a side view

The complete guide for time travelers

Paradoxes of time travel - Marius Cohen

Technion researchers have developed an innovative theoretical model for time travel

This article is taken from the show's script.Making history!', a bi-weekly podcast about the history of science and technology.