Part 9 (2/2)
-STEPHEN HAWKING
”[Tiainst reason,” said Filby
”What reason?” said the Time Traveler
-H G WELLS
In the novel Janus Equation, writer G Spruill explored one of the harrowing problems with tioal is to discover the secret of tie, beautiful wo about her past He beco out her true identity Eventually he discovers that she once had plastic surgery to change her features And that she had a sex change operation Finally, he discovers that ”she” is actually a time traveler from the future, and that ”she” is actually himself, but from the future This , ould have happened if they had had a child? And if this child went back into the past, to grow up to beco of the story, then is it possible to be your own hter?
CHANGING THE PAST
Tireat mysteries of the universe We are all swept up in the river of tiustine wrote extensively about the paradoxical nature of tier is, and the future is not yet? As for the present, if it were always present and never moved on to become the past, it would not be tiic further, we see that tione, the future does not exist, and the present exists only for an instant (Saint Augustine then asked profound theological questions about how time must influence God, questions that are relevant even today If God is omnipotent and all-powerful, he wrote, then is He bound by the passing of time? In other words, does God, like the rest of us mortals, have to rush because He is late for an appointustine eventually concluded that God is omnipotent and hence cannot be constrained by time and would therefore have to exist ”outside of ti outside of ti in ustine, all of us have at soe nature of time and how it differs from space If we can move forward and backward in space, why not in time? All of us have also wondered what the future may hold for us, in the time beyond our years Humans have a finite lifetime, but we are intensely curious about events that will happen long after we are gone
Although our longing to travel in time is probably as ancient as humanity, apparently the very first written time travel story is Memoirs of the Twentieth Century, written in 1733 by Sael froive documents to a British ambassador that describe the world of the future
There would beOne's Coach: An Anachronis for a coach who suddenly finds himself a thousand years in the past He meets a monk from an ancient ress for the next thousand years Afterward he suddenly finds himself just as mysteriously transported back to the present, except that he has missed his coach
Even the 1843 Charles dickens novel, A Christmas Carol, is a kind of tie is taken into the past and into the future to witness the world before the present and after his death
In American literature the first appearance of time travel dates back to Mark Twain's 1889 novel, A Connecticut Yankee in King Arthur's Court A nineteenth-century Yankee is wrenched backward through ti Arthur's court in AD 528 He is taken prisoner and is about to be burned at the stake, but then he declares he has the power to blot out the sun, knowing that an eclipse of the sun would happen on that very day When the sun is eclipsed, the rant hie for the return of the sun
But the first serious attempt to explore time travel in fiction was H G Wells's classic The Time Machine, in which the hero is sent hundreds of thousands of years into the future In that distant future, huenetically split into two races, the round machines, and the useless, childlike Eloi who dance in the sunlight in the world above, never realizing their awful fate (to be eaten by the Moorlocks)
Since then, tiular feature of science fiction, from Star Trek to Back to the Future In Superman I, when Superman learns that Lois Lane has died, he decides in desperation to turn back the hands of ti hiht, until tioes backward The Earth slon, stops, and eventually spins in the opposite direction, until all clocks on the Earth beat backward Floodwaters rage backward, broken dams miraculously heal themselves, and Lois Lane comes back from the dead
From the perspective of science, time travel was impossible in Newton's universe, where time was seen as an arrow Once fired, it could never deviate frohout the universe This conception was overthrown by Einstein, who showed that time wasup and slowing down as it snaked across stars and galaxies So one second on the Earth is not absolute; time varies e move around the universe
As I discussed earlier, according to Einstein's special theory of relativity, time slon inside a rocket the faster it moves Science fiction writers have speculated that if you could break the light barrier, you could go back in time But this is not possible, since you would have to have infinite ht is the ultimate barrier for any rocket The crew of the Enterprise in Star Trek IV: The Voyage Hoon spaceshi+p and used it to whip around the sun like a slingshot and break the light barrier to wind up in San Francisco in the 1960s But this defies the laws of physics
Nonetheless, time travel to the future is possible, and has been experimentally verified millions of times The journey of the hero of The Time Machine into the far future is actually physically possible If an astronaut were to travel near the speed of light, it ht take him, say, one minute to reach the nearest stars Four years would have elapsed on the Earth, but for him only one minute would have passed, because time would have slowed down inside the rocket shi+p Hence he would have traveled four years into the future, as experienced here on Earth (Our astronauts actually take a short trip into the future every tio into outer space As they travel at 18,000 miles per hour above the Earth, their clocks beat a tiny bit slower than clocks on the Earth Hence, after a yearlong mission on the space station, they have actually journeyed a fraction of a second into the future by the ti into the future is currently held by Russian cosei Avdeyev, who orbited for 748 days and was hence hurled 02 seconds into the future) So a time machine that can take us into the future is consistent with Einstein's special theory of relativity But what about going backward in time?
If we could journey back into the past, history would be impossible to write As soon as a historian recorded the history of the past, soo back into the past and rewrite it Not only would time machines put historians out of business, but they would enable us to alter the course of tio back to the era of the dinosaurs and accidentally step on a mammal that happens to be our ancestor, perhaps ould accidentally wipe out the entire hu, madcap Monty Python episode, as tourists froet the best cale
TIME TRAVEL: PHYSICISTS' PLAYGROUND Perhaps the person who has distinguished himself the most on the dense matheist Stephen Hawking Unlike other students of relativity who often distinguish the was actually not an outstanding student as a youth He was obviously extreht, but his teachers would often notice that he was not focused on his studies and never lived up to his full potential But a turning point caan to notice the sy's disease) He was rocked by the news that he was suffering from this incurable motor neuron disease that would rob him of all motor functions and likely soon kill hi What would be the use of getting a PhD if he was going to die soon anyway?
But once he got over the initial shock he beca that he did not have long to live, he began to ferociously tackle soeneral relativity In the early 1970s he published a landularities” in Einstein's theory (where the gravitational field becomes infinite, like at the center of black holes and at the instant of the big bang) were an essential feature of relativity and could not be easily dis also proved that black holes are not entirely black, but gradually e radiation, because radiation can tunnel through the gravity field of even a black hole This paper was the first major application of the quantum theory to relativity theory, and it represents his best knoork
As predicted, ALS slowly led to paralysis of his hands, legs, and even his vocal cords, but at a inally predicted As a result, he has passedthree children (he is now a grandfather), divorcing his first wife in 1991, four years laterthe wife of thefor divorce from his second wife in 2006 In 2007 he made headlines when he went aboard a jet airplane that sent hi wish of his His next goal is to blast off into outer space
Today he is al to the outside world viadisability, he still cracks jokes, writes papers, gives lectures, and engages in controversy He ishis two eyes than are teams of scientists who have full control over their bodies (His colleague at Cae University, Sir Martin Rees, as appointed Astrono's disability does prevent hi the tedious calculations necessary to keep at the top of his ga new and fresh ideas rather than cranking out difficult calculations, which can be done by his students) In 1990 Hawking read papers of his colleagues proposing their version of a time machine, and he was immediately skeptical His intuition told him that time travel was not possible because there are no tourists fro a Sunday picnic in the park, then ti us with their ca us to pose for their picture albue to the world of physics There ought to be a law, he proclaiy Protection Conjecture” to ban time travel from the laws of physics in order to ” thing, however, was that no matter how hard physicists tried, they could not find a law to prevent time travel Apparently time travel seems to be consistent with the knos of physics Unable to find any physical law that ed his mind He made headlines in the London papers when he said, ”Time travel may be possible, but it is not practical”
Once considered to be fringe science, tiround for theoretical physicists Physicist Kip Thorne of Cal Tech writes, ”Time travel was once solely the province of science fiction writers Serious scientists avoided it like the plague-even riting fiction under pseudonyed! One now finds scholarly analyses of time travel in serious scientific journals, written by ee? Because we physicists have realized that the nature of time is too important an issue to be left solely in the hands of science fiction writers”
The reason for all this confusion and excitement is that Einstein's equations allow for many kinds of ties from the quantum theory, however, is still in doubt) In Einstein's theory, in fact, we often encounter so called ”closed time-like curves,” which is the technical term for paths that allow for time travel into the past If we followed the path of a closed time-like curve, ould set out on a journey and return before we left
The first time machine involves a wormhole There are many solutions of Einstein's equations that connect two distant points in space But since space and time are intimately intertwined in Einstein's theory, this sa down the wormhole, you could journey (at least mathematically) into the past Conceivably, you could then journey to the original starting point and meet yourself before you left But as we h the wormhole at the center of a black hole is a one-way trip As physicist Richard Gott has said, ”I don't think there's any question that a person could travel back in time while in a black hole The question is whether he could ever e about it”
Another ti universe In 1949 mathematician Kurt Godel found the first solution of Einstein's equations involving time travel If the universe spins, then, if you traveled around the universe fast enough, you ht find yourself in the past and arrive before you left A trip around the universe is therefore also a trip into the past When astronomers would visit the Institute for Advanced Study, Godel would often ask the He was disappointed when they told him that there was clearly evidence that the universe expanded, but the net spin of the universe was probably zero (Otherwise, tiht be commonplace, and history as we knoould collapse) Third, if you walk around an infinitely long, rotating cylinder, you also ht arrive before you left (This solution was found by W J van Stocku solution, but van Stockum was apparently unaware that his solution allowed for ti May Pole on May Day, you ht find yourself in the n, however, is that the cylinder th and spin so fast that most materials would fly apart) The most recent example of time travel was found by Richard Gott of Princeton in 1991 His solution was based on finding gigantic cos) He assus were about to collide If you quickly traveled around these colliding cose of this type of ti cylinders, spinning universes, or black holes (The problee cos in space and then make theoing back in ti loop of string large enough to allow you to circle it once and go back in tiy of an entire galaxy”
But the n for a time machine is the ”transversable wormhole,” mentioned in the last chapter, a hole in space-time in which a person could freely walk back and forth in time On paper, transversable worht travel, but also travel in tiy
A transversable wormhole time machine would consist of two chambers Each chamber would consist of two concentric spheres, which would be separated by a tiny distance By i the outer sphere, the two spheres would create a Casiy assu a wor one from the space-time foam) Next, take the first chaht-speed velocities Tier in synchronization Time beats at different rates inside the two chambers, which are connected by a wormhole
If you are in the second chah the wormhole to the first chaone backward in tin The wormhole may be quite tiny, much smaller than an atoth distances to create enough negative energy Lastly, you would be able to go back in time only to the point when the time machines were built Before then, ti at the same rate
PARADOXES AND TIME CONUNDRUMS
Time travel poses all sorts of probleal, and ethical issues are raised by Larry Dwyer, who notes, ”Should a tier self (or vice versa) be charged with assault? Should the time traveler who murders someone and then flees into the past for sanctuary be tried in the past for crimes he committed in the future? If he h his other ill not be born for almost 5,000 years?”
But perhaps the thorniest probleical paradoxes raised by time travel For example, what happens if we kill our parents before we are born? This is a logical irandfather paradox”
There are three ways to resolve these paradoxes First, perhaps you sio back in ti the past In this case, you have no free will You are forced to coo back into the past to give the secret of tier self, then it was meant to happen that way The secret of time travel came from the future It was destiny (But this does not tell us where the original idea cae the past, but within limits Your free will is not allowed to create a time paradox Whenever you try to kill your parents before you are born, a er This position has been advocated by the Russian physicist Igor Novikov (He argues that there is a law preventing us froht want to Hence thereour parents before we are born Soer) Third, the universe splits into two universes On one time line the people whom you killed look just like your parents, but they are different, because you are now in a parallel universe This latter possibility seems to be the one consistent with the quantum theory, as I will discuss later when I talk about the multiverse
The second possibility is explored in the er plays a robot from the future wherehureat leader whom the machines have been unable to kill Frustrated, the machines send a series of killer robots back to the past, before the great leader was born, to kill off his mother But after epic battles, human civilization is eventually destroyed at the end of the movie, as it was meant to be
Back to the Future explored the third possibility Dr Brown invents a plutoniu to the past Michael J Fox (Marty McFly) enters the e mother, who then falls in love with hie mother spurns his future father, then they never would have married, and Michael J Fox's character would never have been born
The probleoes to the blackboard and draws a horizontal line, representing the time line of our universe Then he draws a second line, which branches off the first line, representing a parallel universe that opens up when you change the past Thus, whenever we go back into the river of time, the river forks into two rivers, and one time line becomes two time lines, or what is called the ”many worlds” approach, which ill discuss in the next chapter
This means that all time travel paradoxes can be solved If you have killed your parents before you were born, it sienetically identical to your parents, with the same memories and personalities, but they are not your true parents
The ”many worlds” idea solves at least one main problem with time travel To a physicist, the nuative energy) is that radiation effects will build up until either you are killed the instant you enter the machine or the wormhole collapses on you Radiation effects build up because any radiation entering the time portal will be sent back into the past, where it will eventually wander around the universe until it reaches the present day, and then it will fall into the worain Since radiation can enter the mouth of the wormhole an infinite number of times, the radiation inside the worh to kill you But the ”many worlds” interpretation solves this probleoes into the time machine and is sent into the past, it then enters a new universe; it cannot reenter the tiain This simply means that there are an infinite number of universes, one for each cycle, and each cycle contains just one photon of radiation, not an infinite amount of radiation
In 1997, the debate was clarified a bit when three physicists finally proved that Hawking's program to ban time travel was inherently flawed Bernard Kay, Marek Radzikowski, and Robert Wald showed that time travel was consistent with all the knos of physics, except in one place When traveling in time, all the potential problems were concentrated at the event horizon (located near the entrance to the wormhole) But the horizon is precisely where we expect Einstein's theory to break down and quantum effects to take over The problem is that whenever we try to calculate radiation effects as we enter a time machine, we have to use a theory that coeneral relativity with the quantum theory of radiation But whenever we naively try totheory makes no sense: it yields a series of infinite answers that aretakes over All probleh a wor, the stability of the wor of the wormhole as you entered it) are concentrated at the event horizon, precisely where Einstein's theorytime travel is to understand the physics of the event horizon, and only a theory of everything can explain this This is the reason that ree that one way to definitively settle the tiravity and space-ti would unite the four forces of the universe and enable us to calculate ould happen e entered a ti could successfully calculate all the radiation effects created by a wormhole and definitively settle the question of how stable wormholes would be e entered the tiht have to wait for centuries or even longer to actually build a machine to test these theories
Because the laws of time travel are so closely linked to the physics of wormholes, time travel seems to qualify as a Class II impossibility
13: PARALLEL UNIVERSES