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# Time travel

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Time travel is one of my favorite topics! I wrote some time travel stories in junior high school that used a machine of my own invention to travel backwards in time, and I have continued to study this fascinating concept as the years have gone by. We all travel in time. During the last year, I've moved forward one year and so have you. Another way to say that is that we travel in time at the rate of 1 hour per hour.

But the question is, can we travel in time faster or slower than "1 hour per hour"? Or can we actually travel backward in time, going back, say 2 hours per hour, or 10 or 100 years per hour?

It is mind-boggling to think about time travel. What if you went back in time and prevented your father and mother from meeting? You would prevent yourself from ever having been born! But then if you hadn't been born, you could not have gone back in time to prevent them from meeting.

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### Time travel

1. 1. TIME TRAVELtravelling through time…Aravind_n_c@yahoo.co.inFacebook.com/aravindnc
2. 2. Introduction At its most basic level, time is the rate of  Human beings frolic about in the change in the universe -- and like it or not, we are constantly undergoing change. three spatial dimensions of length, We age, the planets move around the sun width and depth. etc. We measure the passage of time in sec,  Time joins the party as that most min, hrs and years, but this doesnt mean crucial fourth dimension. time flows at a constant rate. Just as the water in a river rushes or slows depending  Time cant exist without space, and on the size of the channel, time flows at space cant exist without time. different rates in different places. In other words, time is relative.  The two exist as one: the space-time continuum.  Any event that occurs in the universe has to involve both space and time.
3. 3. Time Travel Into the FutureTravelling through time…
4. 4. Time Travel Into the Future If you want to advance through the years a little faster than the next person, youll need to exploit space-time. GPS 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 the surface, the planets mass drags on time and slows it down in small measures. We call this effect gravitational time dilation.
5. 5. Gravitational Lensing Effect According to Einsteins theory of general relativity, gravity is a curve in space-time. When light is 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.
6. 6. What does this have to do with time? Remember: Any event that occurs in the universe has to involve both space and time. Gravity doesnt just pull on space;  it also pulls on time.
7. 7.  You wouldnt 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. Circle this black hole for a while (without falling in) and youd experience time at half the Earth rate. In other words, youd round out a five-year journey to discover an entire decade had passed on Earth
8. 8.  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 wouldnt 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
9. 9.  In effect, this hypothetical commuter would have traveled into the future. But what about the past? Could the fastest starship imaginable turn back the clock?
10. 10. Time Travel Into the PastTravelling through time…
11. 11.  Weve 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 Einsteins theory of relativity. Youll make it to the future; its 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.
12. 12.  The Milky Way galaxy is roughly 100,000 light-years wide,  so light from its more distant stars can take thousands upon thousands of years to reach Earth. Glimpse that light, and youre 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.
13. 13. Law of Causality Theres nothing in Einsteins 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, a person’s birth(cause) and the person (effect)  Travels to his past, and now we can see the effect is before the cause which is not possible.  Exception (BBT)
14. 14. Could we avoid causality ? 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-future energy costs.
15. 15.  But what if time travel into the past and future depends less on speculative space propulsion technology and more on existing cosmic Yes, Set a course phenomena? for the black hole.
16. 16. Black Holes and Kerr RingsTravelling through time…
17. 17.  Circle a black hole long enough, and gravitational time dilation will take you into the future. But what would happen if you flew right into the maw of this cosmic titan? Most scientists agree the black hole would probably crush you, but one unique variety of black hole might not: the Kerr black hole or Kerr ring.
18. 18.  In 1963, New Zealand mathematician Roy Kerr proposed the first realistic theory for a rotating black hole. The concept hinges on neutron stars, which are massive collapsed stars the size of Manhattan but with the mass of Earths sun. Kerr postulated that if dying stars collapsed into a rotating ring of neutron stars, their centrifugal force would prevent them from turning into a singularity. Since the black hole wouldnt have a singularity, Kerr believed it would be safe to enter without fear of the infinite gravitational force at its center.
19. 19.  If Kerr black holes exist, scientists speculate that we might pass through them and exit through a white hole.  Think of this as the exhaust end of a black hole.  Instead of pulling everything into its gravitational force, the white hole would push everything out and away from it -- perhaps into another time or even another universe. Kerr black holes are purely theoretical, but if they do exist they offer the adventurous time traveler a one- way trip into the past or future.
20. 20. Einstein-Rosen bridgeTravelling through time…
21. 21.  Theoretical Kerr black holes arent the only possible cosmic shortcut to the past or future. As made popular by everything from "Star Trek: Deep Space Nine" to "Donnie Darko," theres also the equally theoretical Einstein-Rosen bridge to consider. But of course you know this better as a wormhole .
22. 22.  Einsteins general theory of relativity allows for the existence of wormholes since it states that any mass curves space-time. To understand this curvature, think about two people holding a bedsheet up and stretching it tight. If one person were to place a baseball on the bedsheet, the weight of the baseball would roll to the middle of the sheet and cause the sheet to curve at that point. Now, if a marble were placed on the edge of the same bedsheet it would travel toward the baseball because of the curve.
23. 23.  In this simplified example, space is depicted as a two-dimensional plane rather than a four-dimensional one. Imagine that this sheet is folded over, leaving a space between the top and bottom. Placing the baseball on the top side will cause a curvature to form. If an equal mass were placed on the bottom part of the sheet at a point that corresponds with the location of the baseball on the top, the second mass would eventually meet with the baseball. This is similar to how wormholes might develop. In space, masses that place pressure on different parts of the universe could combine eventually to create a kind of tunnel. This tunnel would, in theory, join two separate times and allow passage between them. Of course, its also possible that some unforeseen physical or quantum property prevents such a wormhole from occurring. And even if they do exist, they may be incredibly unstable.
24. 24.  According to astrophysicist Stephen Hawking, wormholes may exist in quantum foam, the smallest environment in the universe. Here, tiny tunnels constantly blink in and out of existence, momentarily linking separate places and time like an ever-changing game of "Chutes and Ladders." Wormholes such as these might prove too small and too brief for human time travel, but might we one day learn to capture, stabilize and enlarge them? Certainly, says Hawking, provided youre prepared for some feedback. If we were to artificially prolong the life of a tunnel through folded space-time, a radiation feedback loop might occur, destroying the time tunnel in the same way audio feedback can wreck a speaker.
25. 25. Cosmic StringTravelling through time…
26. 26.  Weve blown through black holes and wormholes, but theres yet another possible means of time traveling via theoretic cosmic phenomena. For this scheme, we turn to physicist J. Richard Gott, who introduced the idea of cosmic string back in 1991. As the name suggests, these are stringlike objects that some scientists believe were formed in the early universe.
27. 27.  These strings may weave throughout the entire universe, thinner than an atom and under immense pressure. Naturally, this means theyd pack quite a gravitational pull on anything that passes near them, enabling objects attached to a cosmic string to travel at incredible speeds and benefit from time dilation. By pulling two cosmic strings close together or stretching one string close to a black hole, it might be possible to warp space-time enough to create whats called a closed timelike curve.
28. 28.  Using the gravity produced by the two cosmic strings (or the string and black hole), a spaceship theoretically could propel itself into the past. To do this, it would loop around the cosmic strings. Quantum strings are highly speculative, however. Gott himself said that in order to travel back in time even one year, it would take a loop of string that contained half the mass- energy of an entire galaxy. In other words, youd have to split half the atoms in the galaxy to power your time machine. And, as with any time machine, you couldnt go back farther than the point at which the time machine was created.
29. 29. …. Oh yes, and thenthere are the timeparadoxes.
30. 30. You’ll Never Go Back In Time !!!
31. 31. grandfather paradox For starters, if you traveled back in time 200 years, youd emerge in a time before you were born. Think about that for a second. In the flow of time, the effect (you) would exist before the cause (your birth). To better understand what were dealing with here, consider the famous grandfather paradox. Youre a time-traveling assassin, and your target just happens to be your own grandfather. So you pop through the nearest wormhole and walk up to a spry 18-year-old version of your fathers father. You raise your laser blaster, but just what happens when you pull the trigger?
32. 32. inconsistent causal loop Think about it. You havent been born yet. Neither has your father. If you kill your own grandfather in the past, hell never have a son. That son will never have you, and youll never happen to take that job as a time-traveling assassin. You wouldnt exist to pull the trigger, thus negating the entire string of events. We call this an inconsistent causal loop.
33. 33. consistent causal loop On the other hand, we have to consider the idea of a consistent causal loop. While equally thought- provoking, this theoretical model of time travel is paradox free. According to physicist Paul Davies, such a loop might play out like this: A math professor travels into the future and steals a groundbreaking math theorem. The professor then gives the theorem to a promising student. Then, that promising student grows up to be the very person from whom the professor stole the theorem to begin with.
34. 34. post-selected model Then theres the post-selected model of time travel, which involves distorted probability close to any paradoxical situation [source: Sanders]. What does this mean? Well, put yourself in the shoes of the time- traveling assassin again. This time travel model would make your grandfather virtually death proof. You can pull the trigger, but the laser will malfunction. Perhaps a bird will poop at just the right moment, but some quantum fluctuation will occur to prevent a paradoxical situation from taking place.
35. 35. Parallel universe But then theres another possibility: The future or past you travel into might just be a parallel universe. Think of it as a separate sandbox: You can build or destroy all the castles you want in it, but it doesnt affect your home sandbox in the slightest. So if the past you travel into exists in a separate timeline, killing your grandfather in cold blood is no big whoop. Of course, this might mean that every time jaunt would land you in a new parallel universe and you might never return to your original sandbox.
36. 36. The Bootstrap Paradox The bootstrap paradox is a paradox of time travel in which information or objects can exist without having been created. After information or an object is sent back in time, it is recovered in the present and becomes the very object/information that was initially brought back in time in the first place. Alternate history is a popular concept of time travel and centers on the premise of changing history, whether accidentally or deliberately, while traveling back through it. One counter to this is the claim that any change a time traveler makes to history is precisely what was always supposed to happen
37. 37. Weak Cosmic CensorshipHypothesis Stephen Hawking has spent his career working with black holes, and most of what we know about them is based on his work. The surface of a black hole is the event horizon, and once any object crosses this and enters the hole, it no longer exists in our spacetime continuum. It is drawn by extreme gravity into an infinitely thin strand of energy called a singularity.   Hawking’s work theorizes that only the terrific energy of a black hole can create a singularity. The weak cosmic censorship hypothesis asserts that there can be no singularity unhidden by a black hole, and thus, no singularity can ever be observed. The singularity is a major talking point of cosmology, because one theory of black holes paints them as gravitational pulls so strong that they impart faster-than-light speed to any object entering them. The singularity is the engine of a black hole’s gravity.   So if a spacecraft wanted to break the light barrier, it would need only to travel through a black hole, and upon emergence from the other side would still be traveling at this speed – namely, jump-starting a spacecraft past light speed so it can return to Earth at some point in the past.   But no object can survive a black hole’s singularity. Here, matter may actually be destroyed, apparently violating the law of conservation of mass. Hence, until singularities are proven to exist outside black holes, this method of traveling into the past is impossible.
38. 38. The Chronology ProtectionConjecture This one was dreamed up by Hawking himself, and there is a LOT of mathematics without numbers involved in it. In a nutshell, the conjecture requires that there be no such thing as a closed timelike curve. A CTC is the closed path of any object as it travels through 4-dimensional spacetime; if the path brings the object back to its starting point, the path is said to be closed.   No mathematical theory can yet predict if CTCs exist. If their existence is demonstrated, Hawking’s conjecture is demonstrably false, and travel into the past may be possible, probably via the next entry. If CTCs do not exist, then the conjecture is true, and “historians throughout the Universe are protected,” as Hawking says.   Our most immediate chance of discovering whether CTCs exist lies in quantum gravity, the branch of mathematics devoted to combining all four forces of the Universe into a single blueprint that can describe all physical laws on both the macroscopic and subatomic scales. The four forces include: the weak force, which holds electrons in orbit around nuclei, initiates hydrogen fusion in stars, and causes the radioactive decay of all subatomic particles; the strong force, which holds protons and neutrons together as nuclei; electromagnetism; and gravity. The General Theory of Relativity reconciles all but electromagnetism; quantum gravity, using a different approach, reconciles all but gravity. Until quantum gravity is fully explored, CTCs can only be hypothesized, and in their absence, traveling into the past cannot be done.
39. 39. Wormholes Disobey the Laws ofPhysics All our understanding of time travel is based on what we know of the physical properties and interactions of the Universe. We have devised a branch of mathematics currently separate from physics to describe the laws of physics on a microscopic scale, and we call it quantum physics. This branch strongly theorizes the existence of Einstein-Rosen Bridges, named after the two scientists most responsible for our understanding of them.    They are more popularly called wormholes, and they are holes that have ripped through the fabric of spacetime. If we could make use of them, the shortest distance between two points would no longer be a straight line but zero, caused by puncturing spacetime at the point of origin and at the point of destination, just like poking holes through a sheet of paper; then spacetime is effectively folded until the two points overlap, and the traveler passes through from A to B, and spacetime is unfolded to its original state. No physical movement occurs, but the destination may be at the other end of the known Universe, and the spacecraft would have neither approached, nor surpassed the speed of light, but simply teleported.   This seems to allow the possibility of travel into the past by avoiding the speed of light altogether, but what it does not account for is what goes on inside a wormhole. Physics has no idea, except to say that the laws of physics do not exist as we know them, or do not exist at all, inside wormholes. If we attempt to comprehend travel through wormholes in our terms of physics, then we are not addressing the issue to begin with, and have not yet left square one.
40. 40. The Twin Paradox  This paradox deals more properly with travel into the future. It involves two newborn, identical twins, one who stays on Earth, and one who travels to Proxima Centauri, the nearest star, 4 light years away. If the spacecraft travels at 80% the speed of light, which amusingly seems more realistic, the round trip will take 10 years. That means the twin on Earth will be 10 years old when his brother returns.   But on the spacecraft, the crew observes Promixa Centauri and Earth also moving with relation to the craft, and this causes Points A and B to shorten to a distance of 2.4 light years, not 4. Each leg of the journey will take 2.4 light years divided by the speed, 80% of the speed of light, for a duration of 3 years one way, 6 round trip. Thus, the twin onboard will have aged 6 years in the same relative span of time. This much is not logically impossible.   What is impossible is the effect of one twin traveling 101% or more of the speed of light. This would, at least according to this scenario as we understand it, cause him to travel into the past and cease to exist, i.e. disappear from onboard, and not return to his brother on Earth.
41. 41. E = MC Squared The most famous equation in the history of mathematics describes the relationship of energy and mass. In 1942, it was notoriously seized upon as a great idea for a powerful new weapon. Einstein had no idea it could be used to build a bigger, better bomb, and wept when Enrico Fermi and Robert Oppenheimer explained what was going on at Oak Ridge, Tennessee.   Aside from explaining how much energy is contained in matter of any size, it also provides an exploration into what happens to mass when it travels faster. The faster something travels, the more energy is required to sustain its travel. As an object approaches the speed of light, it approaches infinite mass, and thus requires infinite energy to continue propelling it forward.   This does not prohibit traveling into the future, since all an object has to do is approach the light barrier. You approach it when you walk into the kitchen to get a beer. The distance into the future you have traveled is too insignificant to matter. But technically you gain an equally insignificant amount of mass. The energy required to propel a large object, like a spacecraft, any meaningful distance into the future, as that meaning relates to our frame of reference, would be greater than or equal to the energy currently in VY Canis Majoris, the largest star we know of.   But to break the light barrier would cause the traveler to go into the past, and this would require infinite, and then greater than infinite, energy. This is impossible to achieve.
42. 42. Temporal Causality Loop This is a paradox as well, and deals with one specific scenario: the invention of the first time machine. The inventor travels back in time in an effort to make his grandfather and grandmother fall in love, only to accidentally kill his grandfather (see #2). Now, desperate to exist in the future, he sleeps with his future grandmother and fathers his own father, thus enabling himself, in the future, to travel back in time and father his father again.   This paradox is illogical because it describes an effect in the future occurring before its cause in the past. Suppose you were to travel back in time to before the Big Bang, somehow cause the Big Bang and thus create the Universe. In terms of fate, this would happen in order to enable you, 13.5 billion years later, to invent the time machine and travel back to create the Universe so the time machine could be invented. It is fundamentally insensible.
43. 43. Temporal Paradox This is essentially the negative version of #3, and is also called the Grandfather Paradox. Traveling into the past must be logically impossible because it would enable you to go back in time and kill yourself. But if you die, how will you travel into the past from the future to kill yourself? Critics, especially science fiction fans, are quick to point out that our understanding of mathematics expands every day thanks to people like Newton, Einstein, Hawking, and Michio Kaku, and with it comes an expanded understanding of the logic involved in time travel scenarios.   The best current counter to the temporal paradox is the Multiverse, which describes an infinite number of yous doing an infinite number of things at an infinite number of points throughout your life. You may be stabbed in a bar fight at 100 years old in another Universe, but die of cancer as a child in this one. Imagine a Universe without Listverse. Our current understandings of quantum mechanics and quantum physics lends strong credence to the possibility that the Multiverse is a reality. It would negate the temporal paradox, and several others, allowing you a future after you have killed yourself. But there is still no fully formed theory of the Multiverse’s existence, and until there is, this paradox stands.
44. 44. No Unified Field Theory  Frankly, all the previous entries are based more in terms of logic than in pure mathematics, precisely we can only surmise everything related to time travel according to our very superficial comprehension of it. Albert Einstein’s life work centered on what we now call Relativity. He postulated two theories of it, but the next step, an infinitely more important one, is to unify the General Theory of Relativity with electromagnetism. Einstein died working on this, and today’s eggheads have taken only baby steps forward. The “highest” form of mathematics to date is called “M Theory,” which is not even fully described yet. It’s practically a religion to mathematicians, because so little is understood about it that some don’t believe in it.   It identifies 11 dimensions in the Universe, not just 4, and its champions expect that it can unite the 5 differing string theories that preceded it, and take what may be the only step left beyond: a unification of the physical properties and laws of all 4 forces of the Universe. M Theory seeks a common ground between General Relativity and Quantum Gravity with the goal of combining all 4. To do so is to take a mathematical look at how the Universe appeared, and how it acted, when it was still an infinitely small point packing all the matter and energy that exist in it today. To comprehend such physics would enable a mathematical comprehension of how to manipulate spacetime itself and pre-vert to a time in the future or revert to a time in the past. Until someone unifies all 4 forces into a single physical quantity with a value for each point in spacetime, we aren’t going any-when.
45. 45. Confused yet?Welcome to the world of time travel.
46. 46. According to Albert Einstein, To travel into the future  we must approach the speed of light. To travel into the past  we must surpass the speed of light. The current record holder for time-traveling is Sergei Krikalev. He has traveled about 337 million miles in orbit at some 17,450 mph – reaching a grand total of 0.02 seconds into the future. This means that from now on, he takes a step two hundredths of a second before you see him take it.