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CONTENTS


Beginning How time travel works Time travel into future Time travel into past

Ready To Go Time travel methods Methods from the movies

Future Fact or Fiction Conservation Laws Future

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Chapter 01 ✴

Time Travel Beginning How it works Into the future Into the past

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We do not see space, time and the universe the way they are; we see space, time and the universe the way we are. — David Lewis Anderson


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How does Time Travel Work From millennium–skipping Victorians to phone booth–hopping teenagers, the term time travel often summons our most fantastic visions of what it means to move through the fourth dimension. But of course you don’t need a time machine or a fancy wormhole to jaunt through the years. As you’ve probably noticed, we’re all constantly engaged in the act of time travel. At its most basic level, time is the rate of change in the universe —and like it or not, we’re constantly undergoing change. We age, the planets move around the sun, and things fall apart.

We measure the passage of time in seconds, minutes, hours and years, but this doesn’t mean time flows at a constant rate. Just as the water in a river rushes or slows depending on the size of the channel, time flows at different rates in different places. In other words, time is relative.

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The term time travel often summons our most fantastic visions of what it means to move through the fourth dimension. But what causes this fluctuation along our one – way trek from the cradle to the grave? It all comes down to the relationship between time and space. Human beings frolic about in the three spatial dimensions of length, width and depth. Time joins the party as that most crucial fourth dimension. Time can’t exist without space, and space can’t exist without time. The two exist as one: the space –time continuum. Any event that occurs in the universe has to involve both space and time.

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Into the Future Any event that occurs in the universe has to involve both space and time. Gravity does not just pull on space; it also pulls on time. If you want to advance through the years a little faster than the next person, you’ll need to exploit spacetime. Global positioning satellites pull this off every day, accruing an extra third–of–a–billionth of a second daily. Time passes faster in orbit, because satellites are farther away from the mass of the Earth. Down here on surface, the planet’s mass drags on time and slows it down in small measures.

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We call this effect gravitational time dilation. According to Einstein’s theory of general relativity, gravity is a curve in space–time and astronomers regularly observe this phenomenon when they study light moving near a sufficiently massive object. Particularly large suns, for instance, can cause an otherwise straight beam of light to curve in what we call the gravitational lensing effect.

What does this have to do with time? Remember: Any event that occurs in the universe has to involve both space and time. Gravity doesn’t just pull on space; it also pulls on time.

Our heirs will explore space and time to degrees we cannot currently fathom.


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The science of realities: Stephen Hawking on “Time Travel to the Future”


Entrance to wormhole

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Exit of wormhole

Earth

Our Universe

Other Universe


You wouldn’t be able to notice minute changes in the flow of time, but a sufficiently massive object would make a huge difference— say, like the supermassive black hole Sagittarius A at the center of our galaxy. Here, the mass of 4 million suns exists as a single, infinitely dense point, known as a singularity [source: NASA]. Circle this black hole for a while (without falling in) and you’d experience time at half the Earth rate. In other words, you’d round out a five –year journey to discover an entire decade had passed on Earth [source: Davies]. Speed also plays a role in the rate at which we experience time. Time passes more slowly the closer you approach the unbreakable cosmic speed limit we call the speed of light. For instance, the hands of a clock in a speeding train move more slowly than those of a stationary clock. A human passenger wouldn’t feel the difference, but at the end of the trip the speeding clock would be slowed by billionths of a second. If such a train could attain 99.999 percent of light speed, only one year would pass onboard for every 223 years back at the train station [source: Davies].

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(1-(U/c)2)1/2 (U=speed, c=speed of light) The time dilation factor theory says that the faster you go the slower time gets for you, because of the fact that time is elastic and relative, not constant. "Time is not absolutely defined," said Einstein in his theory of special relativity. He tried to explain how time and space are connected and that this space –time could be stretched and shrunk. The method for stretching and shrinking time is very simple —all you have to do is move very fast.


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History would be constantly changing every time someone spun the dial of a time machine. — Michio Kaku

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Into the Past One event happens in our universe, and it leads to yet another in an endless one way string of events. We have established that time travel into the future happens all the time. Scientists have proven it in experiments, and the idea is a fundamental aspect of Einstein’s theory of relativity. You’ll make it to the future; it’s just a question of how fast the trip will be. But what about travel into the past? A glance into the night sky should supply an answer. The Milky Way galaxy is roughly 100,000 lightyears wide, so light from its more distant stars can take thousands upon thousands of years to reach Earth.

Glimpse that light, and you’re essentially looking back in time. When astronomers measure the cosmic microwave background radiation, they stare back more than 10 billion years into a primordial cosmic age. But, can we do better than this?


The times they are a changing.

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— Bob Dylan

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There’s nothing in Einstein’s theory that precludes time travel into the past, but the very premise of pushing a button and going back to yesterday violates the law of causality, or cause and effect. One event happens in our universe, and it leads to yet another in an endless one–way string of events. In every instance, the cause occurs before the effect. Just try to imagine a different reality, say, in which a murder victim dies of his or her gunshot wound before being shot. It violates reality as we know it; thus, many scientists dismiss time travel into the past as an impossibility.


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The stars above Flagstaff, Ariz., provide a backward view through time.

The very premise of pushing a button and going back to yesterday violates the law of causality, or cause and effect. Some scientists have proposed the idea of using faster–than–light travel to journey back in time. After all, if time slows as an object approaches the speed of light, then might exceeding that speed cause time to flow backward? Of course, as an object nears the speed of light, its relativistic mass increases until, at the speed of light, it becomes infinite. Accelerating an infinite mass any faster than that is impossible. Warp speed technology could theoretically cheat the universal speed limit by propelling a bubble of space–time across the universe, but even this would come with colossal, far-–uture energy costs. But what if time travel into the past and future depends less on speculative space propulsion technology and more on existing cosmic phenomena? Set a course for the black hole.

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Chapter 02 ✴

time travel d Ready To Go Time travel methods Methods from the movies

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Once confined to fantasy and science fiction, time travel is now simply an engineering problem. — Michio Kaku, Wired Magazine

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Time Travel Methods A look at the types of time travel that exist in stories How we often long to travel back in time and change our pasts, to stop some horrible event, to rewrite history. Movies often indulge and inspire us with their time travel adventures, but how many of these have any basis in real science? Let’s, for this purpose, ignore how much the movies tug at the heartstrings, entertain us, or tickle our funny bone. We can never forget great ones like “Back to the Future,” “Bill and Ted’s Excellent Adventure,” “Time Cop” and many others. But this is a focus on how the heck we are going to get these movies to come true.

Ron Mallett, professor of physics at the University of Connecticut and author of Time Traveler: A Scientist’s Personal Mission to Make Time Travel a Reality, has spent his whole career studying the possibilities of time travel and he weighed in on what aspects of the movies are on the edge of possibility and which ones are not. Derived from the expertise of Mallett, here are the top five time travel movies ranked on their basis in science and their true feasibility.

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Our heirs will explore space and time to degrees we cannot currently fathom and create new melodies in the music of time. — Clifford Pickover


Frequency (2000) Frank Sullivan communicates with his son John 30 years into the future through a radio. They work together to save Frank’s life and to find John’s mother’s would–be–killer. Frequency’s time travel is largely based around an unusual environmental phenomenon in the aurora borealis. This solar disturbance causes Frank’s radio to send its signal to the same radio in the future, where John now has it in the same home. The idea that the energy from a solar disturbance could alter spacetime in some way that sends radio waves through time is very much outside the realms of possibility.

The aurora borealis could never produce that much energy and if it were to do so, there would likely be some disastrous side effects. But being outside the realms of possibility isn’t a problem when it comes to sci–fi time travel. However, Brian Greene, physicist and author of several books including The Elegant Universe, was a consultant for the film, so his input certainly gave the movie some extra weight as far as feasibility goes.

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Déjà Vu (2006) After a ferry bombing, Agent Doug Carlin (Denzel Washington) joins forces with a special team that has technology to see four days into the past. This technology turns out to be a “time window” which Carlin convinces them to use as a time machine and send him into the past. He ends up saving the ferry from the bomber. This is a stretch and would involve technology and an understanding of wormholes that we just don’t have right now. The “Snow White” project, as it’s called in Déjà Vu, somehow controls wormholes or what they call a “time window” to travel through time and space.

Wormholes, also called Einstein–Rosen bridges, are a valid theoretical method for time travel, but solely theoretical. Their existence has never been proven and requires something called “exotic matter” to keep them stable. Yet, Mallett says that any movie involving theories of Einstein for time travel is more feasible than those that do not. The movie takes the theory to the extreme in a way that is very entertaining and smart.

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The Time Machine (1960) H.G. Wells wrote the original book and the movie’s lead character is named H. George Wells after him. He uses his machine to travel from the year 1900 through the future seeing two world wars and accidentally ending up in the year 802,701, where he finds a future people who he tries to help. Mallett has this high on his list because it’s the only movie that accurately states time as a fourth dimension. Although in modern relativistic physics, space and time are combined into one metric called “spacetime.”

The machine itself; not so feasible. A chair with spinning parts and a lever that controls time travel is something today’s scientists have yet to find viable. It’s quite miraculous that the fictional H. George Wells was able to invent it in the early 1900s. Another positive note for this version it does not have him traveling to the past beyond the invention of his machine. The movie doesn’t explain this rule, but it does follow the true physics of time travel.


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Star Trek (2009) Time travel is featured in the “Star Trek” series many times, but the way the most recent movie dealt with time travel makes it very easy to understand without having to be a physics expert. “They actually brought in a number of current ideas,” says Mallett. The movie’s use of parallel universes is done very well and explained very well. There is a valid scientific theory in quantum mechanics that says there are many parallel universes. Plus, when you go back into the past you actually

arrive in the past of a parallel universe, where you can change the future of that universe, but in your time those things have already happened. It’s also important to know that once you are in this alternate reality you cannot, as another time travel movie suggests, go back to the future.

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Black holes are a very popular method for sci–fi time travel and one that could actually be possible. Einstein’s general theory of relativity basically says that gravity effects time and since black holes have gravity so strong that light can’t even escape, they create the possibility for time travel. If you were able to get close to a black hole, time would slow down.

When you escaped, time outside would have passed a lot more quickly. Making things more complicated, when the black hole is rotating, it can cause time to be twisted into a loop that allows you to go into the past. That still leaves some scientific flaws in the movie, but nobody’s perfect.

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Planet of the Apes (1968) Put briefly, astronaut Taylor (Charlton Heston) unknowingly goes on a journey to a future Earth where apes have guns, don’t believe in the possibility of flight, and rule the planet. Humans are still around in this future, but are mute and quite unintelligent. The best thing about this movie is that they talk about real scientific theories in explaining how they traveled through time. “According to Dr. Hasslein’s theory of a vehicle traveling near the speed of light, the earth has aged nearly 700 years since we left it, while

we’ve aged hardly at all,” says Taylor in the opening scene of the movie. Actually, it’s based more on theories of a man named Albert Einstein. His special theory of relativity says that time for a moving clock slows down.

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We will able to go back in time and fix the things that are messed up. — Craig Ferguson


In 1971, the Hafele–Keating experiment proved this with very accurate atomic clocks. One was on Earth and another was flown around the world on a passenger jet. When the jet landed, the clock on the jet was about 50 nanoseconds behind the one on Earth, just like Einstein predicted in 1905. “This means, for a space traveler traveling close to the speed of light, that this effect will happen dramatically. A few years will pass for those on board, but when the rocket lands, decades will have passed on Earth.” explains Mallett.

So as Heston and his crew travel near the speed of light, they would in fact be traveling through time relative to those of us on Earth. Getting a ship close to the speed of light is the practical challenge in this case, but very plausible in theory.

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Chapter 03 ✴

Time Travelbl Conclusion Fact or Fiction Conservation Laws Future

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Fact or Fiction We define time travel to mean that departure from a certain place and time followed (from the traveller’s point of view) by arrival at the same place at an earlier (from the sedentary observer’s point of view) time. Time travel paradoxes arise from the fact that departure occurs after arrival according to one observer and before arrival according to another. In the terminology of special relativity time travel implies that the timelike ordering of events is not invariant. This violates our intuitive notions of causality. However, intuition is not an

infallible guide, so we must be careful. Is time travel really impossible, or is it merely another phenomenon where “impossible” means “nature is weirder than we think?” The answer is more interesting than you might think.

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Conservation Laws It is sometimes argued time travel violates conservation laws. For example, sending mass back in time increases the amount of energy that exists at that time. Doesn’t this violate conservation of energy? This argument uses the concept of a global conservation law, whereas relativistically invariant formulations of the equations of physics only imply local conservation. A local conservation law tells us that the amount of stuff inside a small volume changes only when stuff flows in or out through the surface. A global conservation

law is derived from this by integrating over all space and assuming that there is no flow in or out at infinity. If this integral cannot be performed, then global conservation does not follow. Therefore, sending mass back in time might be all right, but it implies that something strange is happening.


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I can not travel into my past, without consent of the future me. — Toba Beta


Future I look at time as something that we can not cheat. It is something that when is passed, is passed. When an action took place, it did take place, and when something bad happened, it did happen. Maybe everything, including us, is predetermined in one way or another, but I am not aware of it. I definitely cannot see my future and don’t know what holds in it for me.

The only thing that I can do is to guess my future, which is not gonna help that much. While time passes, and events take place, either they are predetermined or not, I tend to live. As they say, things happen for reasons, and we don’t always know the reasons.

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Angie Cheng San Francisco, CA angie33520@gmail.com Š 2012 ANGIE CHENG Designer: Angie Cheng Typeface: Avenir, Scala Instructor: David Hake GR_601_02 Type Systems The Academy of Art University San Francisco, CA Printed in USA


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