Chapter 13 12. Time travel
In the novel Janus Equation, author G. Spruill explores a harrowing question raised by time travel.In this story, a brilliant mathematician with the goal of discovering time travel meets an unusually beautiful woman and they become lovers, even though he knows nothing about her past.He became interested in finding out who she really was.Eventually, he discovered that she had undergone plastic surgery to change her appearance and had undergone sex reassignment surgery.In the end, he discovers that "she" is actually a time traveler from the future, and that "she" is actually himself, but from the future.That means he made love to himself.Curious, what if they had a child?If the child went back in time and grew up to be the mathematician at the beginning of the story, would it be possible to be his own mother and father and son and daughter?
Time is one of the greatest mysteries in the universe.We are all pushed forward by the river of time, against our will.Around 400 BC, Saint Augustine wrote extensively about the paradoxical nature of time: "How can the past and the future be true when the past is no longer the past and the future is not yet the future? As for the present, If it is always in the present and never moves forward to make it past, it ceases to be time and becomes eternity." If we push St. Augustine's logic a step further, we find that time cannot be real.For the past has passed, the future does not yet exist, and the present exists only for a moment. (St. Augustine thus raised meaningful theological questions about how time would affect God. "If God is omnipotent," he wrote, "is he then bound by the passage of time? In other words, would God be like us mortals , and have to hurry because of fear of being late for an appointment?” St. Augustine finally concluded that God, being omnipotent, could not be governed by time, and therefore must exist “outside time.” The notion that something outside this may seem absurd, but it is a recurring construct in modern physics, as we shall see.)
Like St. Augustine, we've all at some point been curious about the quirky nature of time and how it differs from space.If we can move back and forth in space, why not in time?We also all wonder what the future will look like in the years and months after our lifespan ends.Human life is limited, but we are curious about what will happen to us after we are gone.
Although our desire to time travel may be as old as human history, it seems that the first story about time travel was written in 1733 by Samuel Madden in "Memoirs of the 20th Century". of the Twentieth Century), about an angel from 1997 who travels 250 years into the past in order to deliver documents describing the future world to a British ambassador.
There are many such stories. The 1838 anonymous short story "Missing One's Coach: An Anachronism" is about a man waiting for a coach who suddenly finds himself in a place 1,000 years in the past.He meets a monk from an ancient monastery and tries to explain to him how history will develop in the next 1,000 years, and then he finds himself transported back to the present as magically as before, except that he has Missed the carriage.
Even Charles Dickens' 1843 novel A Christmas Carol is a sort of time travel story, as Ebenezer Scrooge is transported back in time and the future, witnessing the world before him and the world after his death.
In American literature, time travel first appeared in Mark Twain's 1889 novel A Connecticut Yankeein King Arthur's Court.A 19th-century Yankee is dragged back in time, ending up at King Arthur's court in 528 BC.He is thrown in jail and about to be burned at the stake, but he randomly announces that he has the power to wipe out the sun because he knows there will be a solar eclipse on that day.When the sun was eclipsed, the mob, terrified, agreed to release him, promising him favors in exchange for returning the sun.
But the first novel to seriously attempt time travel was HG Wells' classic (The Time Machine), in which the protagonist is sent hundreds of years into the future.In that distant future, humanity itself is genetically divided into two races: the sinister Moorlocks, who maintain their grimy underground machines, and the useless, childish Eloi. , they dance in the above world, never realizing their terrible fate (being eaten by Morox).
Since then, time travel has become a common feature of science fiction, from "Star Trek" to "Back to the Future."In Superman I, when Superman learns that Lois Lane is dead, he desperately decides to turn back time by sending himself flying around the Earth faster than the speed of light until time itself is reversed.The Earth slows down, stops, and eventually spins in the opposite direction until all clocks on Earth run backwards.The flood waters receded quickly, the ruptured levee miraculously repaired itself, and Louis Lane returned from the realm of the dead.
From a scientific point of view, time travel is impossible in Newton's universe.In Newton's universe, time is regarded as an arrow, once shot, it can never be turned back. A second on the earth is a second in the entire universe. This idea was overturned by Einstein, who proved Time is more like a river that meanders through the universe, speeding up or slowing down as it snakes through the stars and galaxies, so a second on Earth is not absolute, as we travel around the universe , the times vary.
As I discussed earlier, according to Einstein's special theory of relativity, the faster a rocket moves, the slower time passes inside it.Science fiction writers speculate that if you can break the light barrier, you can go back in time.But that's impossible, because it would have to have infinite mass to reach the speed of light, the ultimate obstacle for all rockets. In "Star Trek IV: Saving the Future", the crew of the "Enterprise" hijacked a Klingon spaceship and used it to fly around the sun, breaking the light barrier like a slingshot and reaching the 20th century San Francisco in the 1960s.But this violates the laws of physics.
However, time travel to the future is possible and has been proven experimentally millions of times.The protagonist's journey to the distant future is actually physically possible.For example, if an astronaut is moving close to the speed of light, let's say it takes him 1 minute to get to the nearest star, 4 years have elapsed on Earth, but only 1 minute has passed for him because the rocket inside Time will slow down.So he has traveled 4 years into the future based on what has already been experienced on Earth (our astronauts actually go into the future briefly every time they go into space. When they are moving above Earth at 18,000 miles per hour , their clocks run slightly slower than clocks on Earth. So after a one-year mission on the International Space Station, they returned to Earth less than a second into the future. The world record for going into the future is currently held by Russian cosmonaut Sergei Avdeyev maintains that he spent 748 days in space and thus arrived 0.02 seconds into the future).
Therefore, a time machine that can take us into the future is consistent with Einstein's special theory of relativity.But what about going back in time?
If we could go back in time, history would be impossible to write.Once historians have recorded the history of the past, someone can go back and rewrite it.Not only would a time machine put historians out of work, but it would allow us to alter the course of time at will.For example, if we go back to the time of the dinosaurs and accidentally step on some mammal that happened to be our ancestor, maybe we will accidentally wipe out the entire human race.History becomes an endless, topsy-turvy episode as visitors from the future mess with historical events and try to find the best camera angles.
Perhaps the man best known for the dense mathematical equations of black holes and time machines is cosmologist Stephen Hawking.Unlike other students who studied relativity theory who gained attention early on in mathematical physics, Hawking was not an excellent student when he was young.He was obviously extremely bright, but his teachers often noticed that he was not concentrating on his studies and never reaching his full potential.But a turning point came in 1962, when he started noticing symptoms of ALS (amyotrophic lateral sclerosis, also known as Greco's disease) after graduating from Oxford.He was devastated by the news that he had an incurable disease of the motor neurons that would deprive him of all motor function and could kill him quickly.The news made him extremely sad at first. What's the use of getting a PhD if he's going to die soon anyway?
But when he got past the initial shock, he was able to focus for the first time in his life.Realizing that he didn't have much time to live, he began storming some of the most difficult problems in general relativity.In the early 1970s, he published a series of landmark papers demonstrating the "singularity" in Einstein's theory. instant) is an integral part of the theory of relativity and cannot be dismissed easily (as Einstein thought).In 1974, Hawking also proved that black holes are not completely black, but gradually emit radiation, which is now called "Hawking radiation" because the radiation penetrates even the black hole's gravitational field.This paper was the first important application of quantum theory to relativity and is his most famous work.
As expected, ALS slowly paralyzed his hands, legs, and vocal cords, but at a much slower pace than doctors initially expected.So he's already lived through many of the life's major events of the common man, becoming a father of three (he's now a grandfather), divorcing his first wife in 1991, and marrying his speech synthesizer four years later The maker's wife, and filed for divorce from his second wife in 2006.In 2007, he boarded a jet plane and took a zero-gravity flight, achieving a lifelong dream with great fanfare.His next goal is to go into space.
Today he is almost completely paralyzed in a wheelchair, communicating with the outside world through the movement of his eyes.Yet despite his excruciating disability, he was still able to tell jokes, write papers, give speeches, and engage in polemics.He was far more productive by moving his eyes than a team of scientists who had full control over his body (his colleague at Cambridge University, Sir Martin Reese, the Queen's Appointed Astronomer Royal, once confided to me that Hawking's disability did prevent him from performing monotonous calculations required to maintain his top position in his research field. So he instead concentrated on coming up with novel ideas rather than performing difficult calculations, which could be done by his students).
In 1990, Hawking read a paper by a colleague proposing a type of time machine, and he was immediately skeptical.His habits tell him that time travel is impossible because there are no travelers from the future.If time travel is as common as Sunday picnics in the park, then time travelers from the future should be pestering us with cameras, asking us to pose for their photo albums.
Hawking also issued a challenge to physicists around the world.He announced that there should be a law that makes time travel impossible, and he proposed the "chronological protection conjecture" to prohibit time travel from the laws of physics in order to "guarantee the safety of history for historians".
Sadly, however, no matter how hard physicists try, they can't figure out a law that doesn't allow time travel.It appears that time travel is consistent with the known laws of physics.Unable to figure out any laws of physics that prohibit time travel, Hawking recently had a change of heart.His statement that "time travel may be possible, but it's not practical" was widely reported in the London newspapers.
Once considered a fringe science, time travel has suddenly become a playground for theoretical physicists.Kip Thorne, a physicist at Caltech, writes: "Time travel was once the province of science fiction writers. Serious scientists avoided it like the plague—even when writing novels under a pseudonym or Time to read in private. Times have really changed! It is now possible to find scholarly analyzes of time travel in serious scientific journals, by renowned theoretical physicists...Why has this changed? Because our physics Scientists have realized that the nature of time is an extremely important issue that cannot be left in the hands of science fiction writers alone."
The reason for all this confusion and excitement is that Einstein's equations allow for many kinds of time machines (however, it is uncertain whether they will stand up to the challenges of quantum theory).In Einstein's theory, we actually often come across something called "closed time-like curves," which is the technical term for time-travel paths that allow going back in time.If we follow a closed time-like curve path, we will take a journey back to the time before we started.
The first type of time machine contained a wormhole.Einstein's equations have many solutions connecting two distant points in space.But because space and time are inextricably entangled in Einstein's theory, this same wormhole could also connect two points in time.Fall into a wormhole and you can (at least mathematically) travel back in time.It is conceivable that you can then go back to the moment you started and meet yourself before you left.But, as we mentioned in previous chapters, going through a wormhole at the center of a black hole is a one-way trip.As the physicist Richard Gott said: "I don't think there is any doubt that a person can travel back in time in a black hole. The question is whether he can reappear and boast about his experience. .”
Another kind of time machine involves a spinning universe. In 1949, mathematician Kurt Gödel discovered the first solution to Einstein's equations involving time travel.If the universe spins, then if you move fast enough through the universe, you might find yourself in the past and in a time before you started.So a trip around the universe is also a trip back in time.When astronomers visited the Institute for Advanced Study, Gödel would often ask if they had found evidence that the universe was spinning, and he was disappointed when they told him that there was clear evidence that the universe was expanding, but that the net rotation of the universe might be zero. (Otherwise, time travel might be commonplace, and history would collapse as we know it).
Third, if you walk around an infinitely long, rotating cylinder, you may also find yourself back where you left off (this solution was discovered by WJ van Stockum in 1936 , earlier than Gödel's time travel solution, but van Stockum seems unaware that his solution allows time travel).That being the case, if you danced around a rotating maypole on Mayday, you might find yourself in April (the problem with this scheme, however, is that the column would have to be infinitely long and spin very fast , so that most of the material will be thrown out).
The most recent example of time travel was discovered in 1991 by Richard Gott of Princeton.His solution is based on the discovery of gigantic cosmic strings, possibly remnants of the original Big Bang.He hypothesized that two large cosmic strings were about to collide.If you move quickly around these colliding cosmic strings, you will travel back in time.The advantage of this type of time machine is that it doesn't require infinite cylinders, spinning universes, or black holes (the problem with this solution, however, is that you must first find giant cosmic strings floating in space and then make them collide in a precise fashion . the possibility of going back in time is only available for a short time), Gott said: "A constriction loop so large that it would take you a year to go around it would require more than half the mass-energy of a string to go around it. "
But the most promising proposal for a time machine is the "traversable wormhole" mentioned in the previous chapter, a hole in space-time through which one can freely travel forward and backward in time.In theory, traversable wormholes can not only provide faster-than-light travel, but also time travel.The key to traversable wormholes is negative energy.
A traversable wormhole time machine would consist of two chambers, each consisting of two concentric spheres separated by a small distance.Make the outer spheres implode inwards, and the two spheres will create the Casimir effect, and thus negative energy.Suppose a Type III civilization is able to connect these two rooms with a wormhole (possibly harvesting one from the space-time bubble).Subsequently, the first chamber was sent into space at nearly the speed of light.Time in that room will slow down so that the two clocks are no longer in sync.Time passes at different speeds in the two rooms, which are connected by a wormhole.
If you are in the second room, you can immediately go through the wormhole to the first room, where the time is earlier.This way you are back in time.
This program faces insurmountable problems.Wormholes could be tiny, much smaller than an atom.The metal plates would probably have to be tightened inwards by a distance of the Planck length to create enough negative energy.In the end, you can only go back to the point in time when the time machine was created.Until then, time passes at the same speed in both rooms.
Time travel raises all kinds of problems, both technical and social.Larry Dwyer addresses ethical, legal and moral issues, recording: "Should a time traveler who punches his younger self (or vice versa) be charged with assault? Murder Should a time traveler who escaped back in time seeking asylum be tried in the past for crimes he committed in the future? Should he be tried for bigamy if he was married in the past? It won't be born for 5,000 years."
But perhaps the most poignant problem is the logical paradox raised by time travel.For example, what if we killed our parents before we were born?This is a logical impossibility, and it's sometimes called the "grandfather paradox."
There are three ways to resolve these paradoxes.First, perhaps you are simply repeating history when you go back in time, thus bringing the past to life.Then you don't have free will.You are forced to complete the past as already recorded.That way, if you go back in time and tell your younger self the secret of time travel, it's meant to happen that way.The secret of time travel comes from the future.It's fate (but that doesn't tell us where the original idea came from).
Second, you have free will, so you can change the past, but within limits.Your free will cannot create a time paradox.Whenever you try to kill your own parents before you are born, a mysterious force prevents you from doing so.This approach was strongly advocated by Russian physicist Igor Novikov (who argued that there is a law of physics that prevents us from walking on ceilings, even though we might want to. So, perhaps There's a law of physics that prevents us from killing our parents before we're born. Some weird law gets in the way).
Third, the universe splits into two universes.In one of the timelines, you kill people who just look like your parents, but they're not your parents because you're in a parallel universe.This last approach seems to be the only one consistent with quantum theory, as I will mention in a later chapter when I discuss the multiverse.
The second possibility was explored in the movie Terminator 3.Arnold Schwarzenegger plays a robot from the future.There, murderous robots have taken over.The few surviving humans are hunted like animals by machines, led by a great leader whom the machines cannot kill.The Robots, displeased with this, send a series of killer robots back before the Great Leader was born to kill his mother.But, after an epic battle, human civilization is finally destroyed at the end of the movie, as it was meant to be.
Back to the Future explores a third way.Dr. Brown invents a plutonium-fueled DeLorean car that's actually a time machine that can go back in time.Michael J. Fox (as Marty McFly) enters the machine, travels back in time, meets his teenage mother, and they fall in love.This presents a conundrum: If Marty McFly's mother had rejected his father, then they would not have married, and Michael J. Fox's character would never have been born.
Dr. Brown clarifies the issue a little.He went to the blackboard and drew a horizontal line representing the timeline of our universe.Then he draws a second line, branching off from the first, to represent a parallel universe that opens up when you change the past.In this way, whenever we go back to the river of time, the river forks in two, and one time axis becomes two time axes, or it is called the "many worlds" method, which we will discuss next discussed in the chapter.
This means that all time travel paradoxes can be resolved.If you killed your parents before you were born, it just means that you killed two people who were genetically identical to your parents, and had the same memories and personalities as your parents, but they were not you real parents.
The "many worlds" concept solves at least one major problem with time travel.For physicists, the biggest criticism of time travel (besides finding negative energy) is that the effects of radiation build up until you're either killed the moment you enter the machine, or the wormhole collapses on you.The radiation effect will be enhanced because any radiation that enters the time portal will be sent back to the past, and it will end up wandering in the universe until it reaches the present, and then falls into the wormhole again. Since the radiation can enter and exit the wormhole entrance countless times, the worm The radiation energy inside the hole is unbelievably strong -- strong enough to kill you, but the "many worlds" explanation solves that problem.If the radiation goes into the time machine and is sent back in time, then it goes into a new universe, it cannot go into the time machine again and again and again.This explicitly means that there are an infinite number of universes, one cycle per universe, and only one radiation per cycle, rather than an infinite amount of radiation.
This discussion was somewhat clarified in 1997 when a trio of physicists demonstrated that Hawking's scheme to ban time travel was inherently flawed.Bernard Kay, Marek Radzikowski and Robert Wald proved that time travel obeys all known laws of physics except one.When traveling in time, all possible problems converge at the event horizon (the location near the entrance of the wormhole).But the event horizon is exactly where we expect Einstein's theory to fail and quantum theory to kick in.The problem is that whenever we try to calculate the effects of radiation when entering a time machine, we have to use a theory that combines Einstein's general theory of relativity with the quantum theory of radiation.But whenever we naively try to combine the two theories, the resulting theory is meaningless: it yields an infinite series of answers that don't make sense.
This is a position where the truth of all things comes into play.All the problems that plague physicists about traveling through wormholes (for example, the stability of wormholes, the radiation that can kill people, and the closure of wormholes when entering them) focus on the event horizon, which is what Einstein's theory loses. place of meaning.
The key to understanding time travel, then, is understanding the physics of event horizons, which can only be explained by an axiom of everything.This is why most physicists today agree that the only way to definitively solve the problem of time travel is to come up with a complete theory of gravity and spacetime.
The Arithmetic unites the four forces of the universe and allows us to estimate what will happen when we step into a time machine.Only one theory of everything can successfully calculate the full effect of radiation produced by a wormhole, and definitely solve the problem of the stability of the wormhole when we enter the time machine.Even so, we may have to wait centuries or more for a machine to test these theories.
Since the principle of time travel is closely related to the physics of wormholes, time travel seems to meet the requirements of "second-class incredible".
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