How Time Travel Works 

Time is malleable. 

 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 are 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. 

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. 

In this article, we'll look at the real-life; everyday methods of time travel in our universe, as well as some of the more far-fetched methods of dancing through the fourth dimension.

Time Travel Into the Future 

If you want to advance through the years a little faster than the next person, you'll need to exploit space-time. 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 the surface, the planet's mass drags on time and slows it down in small measures. 

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.   

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]. 

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?

 Time Travel Into the Past 

We've 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 light-years 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? 

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. 

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. 

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.

What's on the other side of a black hole?


Black Holes and Kerr Rings 

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. 

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 Earth's sun [source: Kaku]. 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 wouldn't have a singularity, Kerr believed it would be safe to enter without fear of the infinite gravitational force at its center. 

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. And while a tremendously advanced civilization might develop a means of calibrating such a method of time travel, there's no telling where or when a "wild" Kerr black hole might leave you.

Imagine space as a curved two-dimensional plane. Wormholes like this could form when two masses apply enough force on space-time to create a tunnel connecting distant points.



Theoretical Kerr black holes aren't the only possible cosmic shortcut to the past or future. As made popular by everything from "Star Trek: Deep Space Nine" to "Donnie Darko," there's also the equally theoretical Einstein-Rosen bridge to consider. But of course you know this better as a wormhole. 

Einstein's 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 bed sheet up and stretching it tight. If one person were to place a baseball on the bed sheet, 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 bed sheet it would travel toward the baseball because of the curve.

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, it's 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.

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 you're 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.

The right cosmic anomaly could turn any spaceship into a time machine. 

Cosmic String

We've blown through black holes and wormholes, but there's 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 string like objects that some scientists believe were formed in the early universe.

These strings may weave throughout the entire universe, thinner than an atom and under immense pressure. Naturally, this means they'd 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 what's called a closed time like curve.

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, you'd have to split half the atoms in the galaxy to power your time machine. And, as with any time machine, you couldn't go back farther than the point at which the time machine was created. 

Oh yes, and then there are the time paradoxes. 

Time Travel Paradoxes

As we mentioned before, the concept of traveling into the past becomes a bit murky the second causality rears its head. Cause comes before effect, at least in this universe, which manages to muck up even the best-laid time traveling plans.

For starters, if you traveled back in time 200 years, you'd 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 we're dealing with here, consider the famous grandfather paradox. You're 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 father's father. You raise your laser blaster, but just what happens when you pull the trigger?

Think about it. You haven't been born yet. Neither has your father. If you kill your own grandfather in the past, he'll never have a son. That son will never have you, and you'll never happen to take that job as a time-traveling assassin. You wouldn't exist to pull the trigger, thus negating the entire string of events. We call this an inconsistent 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.

Then there's 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.

But then there's 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 doesn't 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. 

Confused yet? Welcome to the world of time travel.



•Cleland, Andrew. Personal interview. April 2010.

•Davies, Paul. "How to Build a Time Machine." Penguin. March 25, 2003.

•Davies, Paul. Personal interview. April 2010.

•Franknoi, Andrew. "Light as a Cosmic Time Machine." PBS: Seeing in the Dark. March 2008. (March 1, 2011)

•Hawking, Stephen. "How to build a time machine." Mail Online. May 3, 2010. (March 1, 2011)

•"Into the Universe with Stephen Hawking." Discovery Channel.

•Kaku, Michio. "Parallel Worlds: A Journey Through Creation, Higher Dimensions, and the Future of the Cosmos." Anchor. Feb. 14, 2006.

•"Kerr Black Holes and time travel." NASA. Dec. 8, 2008. (March 1, 2011)

•Sanders, Laura. "Physicists Tame Time Travel by Forbidding You to Kill Your Grandfather." WIRED. July 20, 2010. (Mach 1, 2011)

Time Travel

The urge to hug a departed loved one again or prevent atrocities are among the compelling reasons that keep the notion of time travel alive in the minds of many.

While the idea makes for great fiction, some scientists now say traveling to the past is impossible.

There are a handful of scenarios that theorists have suggested for how one might travel to the past, said Brian Greene, author of the bestseller, “The Elegant Universe” and a physicist at Columbia University.“And almost all of them, if you look at them closely, brush up right at the edge of physics as we understand it. Most of us think that almost all of them can be ruled out.

The fourth dimension

In physics, time is described as a dimension much like length, width, and height. When you travel from your house to the grocery store, you’re traveling through a direction in space, making headway in all the spatial dimensions—length, width and height. But you’re also traveling forward in time, the fourth dimension.

“Space and time are tangled together in a sort of a four-dimensional fabric called space-time,” said Charles Liu, an astrophysicist with the City University of New York, College of Staten Island and co-author of the book “One Universe: At Home In The Cosmos.”

Space-time, Liu explains, can be thought of as a piece of spandex with four dimensions. “When something that has mass—you and I, an object, a planet, or any star—sits in that piece of four-dimensional spandex, it causes it to create a dimple,” he said. “That dimple is a manifestation of space-time bending to accommodate this mass.”

The bending of space-time causes objects to move on a curved path and that curvature of space is what we know as gravity.

Mathematically one can go backwards or forwards in the three spatial dimensions. But time doesn’t share this multi-directional freedom.

“In this four-dimensional space-time, you’re only able to move forward in time,” Liu told LiveScience.

Tunneling to the past

A handful of proposals exist for time travel. The most developed of these approaches involves a wormhole—a hypothetical tunnel connecting two regions of space-time. The regions bridged could be two completely different universes or two parts of one universe. Matter can travel through either mouth of the wormhole to reach a destination on the other side.

“Wormholes are the future, wormholes are the past,” said Michio Kaku, author of “Hyperspace” and “Parallel Worlds” and a physicist at the City University of New York. “But we have to be very careful. The gasoline necessary to energize a time machine is far beyond anything that we can assemble with today’s technology.”

To punch a hole into the fabric of space-time, Kaku explained, would require the energy of a star or negative energy, an exotic entity with an energy of less than nothing.

Greene, an expert on string theory—which views matter in a minimum of 10 dimensions and tries to bridge the gap between particle physics and nature's fundamental forces, questioned this scenario.

“Many people who study the subject doubt that that approach has any chance of working,” Greene said in an interview . “But the basic idea if you’re very, very optimistic is that if you fiddle with the wormhole openings, you can make it not only a shortcut from a point in space to another point in space, but a shortcut from one moment in time to another moment in time.

Quantum Time Machine Solves Grandfather Paradox

Of all the weird consequences of quantum mechanics, one of the strangest is the notion of post selection: the ability to trigger a computation that automatically disregards certain results.

Here’s an example: suppose you have a long, tortuous expression in which there are a frighteningly large number of variables. The question you want answering is which combination of variables makes the expression logically true. And the conventional way to solve it is by brute force: try every combination of variable until you find one that works. That’s hard.

Postselection, however, makes the solution easy to find. Simply allow the variables to take any value at random and then postselect on the condition that the answer must be true. This automatically disregards any wrong’uns that come up.

Postselection is controversial because it leads to all kinds of fantastical predictions about the power of quantum computers. Nobody is quite sure if these kinds of computations are possible or how to achieve them but quantum mechanics seems to allow them.

Now postselection gets even weirder thanks to some new ideas put forward by Seth Lloyd at the Massachusetts Institute of Technology and a few buddies. They say that if you combine postselection with another strange quantum behaviour called teleportation and you can build a time machine.

Before we look at how this idea works, a quick reminder about quantum teleportation. This uses the phenomenon of entanglement to reproduce in one point in space a quantum state that previously existed at another point in space.

Lloyd and cos idea is to use postselection to make this process happen in reverse. Postselection ensures that only a certain type of state can be teleported. This immediately places a limit on the state the original particle must have been in before it was teleported. In effect, the state of this particle has travelled back in time.

What’s amazing about this time machine is that it is not plagued by the usual paradoxes of time travel, such as the grandfather paradox, in which a particle travels back in time and some how prevents itself from existing in the first place.

Lloyd’s time machine gets around this because of the probabilistic nature of quantum mechanics: anything that this time machine allows can also happen with finite probability anyway, thanks to these probabilistic laws.

Another interesting feature of this machine is that it does not require any of the distortions of spacetime that traditional time machines rely on. In these, the fabric of spacetime has to be ruthlessly twisted in a way that allows the time travel to occur. These conditions may exist in the universe’s extreme environments such as inside black holes but probably not anywhere else.

The fact that similar time machines may also be possible when quantum mechanics is pushed to its limits suggests an avenue that may prove fruitful in uniting this disparate areas of science. “Our hope is that this theory may prove useful in formulating a quantum theory of gravity,” say Lloyd and buddies.

So where might their time machine be built. That’s a tricky question too. Postselection can only occur if quantum mechanics is nonlinear, something that seems possible in theory but has never been observed in practice. All the evidence so far is that quantum mechanics is linear. In fact some theorists propose that the seemingly impossible things that postselection allows is a kind of proof that quantum mechanics must be linear.

However, if nonlinear behaviour is allowed, time travel will be possible wherever it takes place. As Lloyd and co say: “It is possible for particles (and, in principle, people) to tunnel from the future to the past. “

Fire up the Delorean.

Ref: The Quantum Mechanics Of Time Travel Through Post-Selected Teleportation

Source: Technology Review


Temporal paradox 


Temporal paradox (also known as time paradox and time travel paradox) is a theoretical paradoxical situation that happens because of time travel. A time traveler goes to the past, and does something that would prevent him from time travel in the first place. If he does not go back in time, he does not do anything that would prevent his traveling to the past, so time travel would be possible for him. However, if he goes back in time and does something that would prevent the time travel, he will not go back in time. Thus each possibility seems to imply its own negation - a type of logical paradox.
A typical example of this kind is the grandfather paradox, where a person goes back in time to kill their grandfather before he had any biological descendant. If they succeed, one of their parents would never exist and they themselves would never exist either. This would make it impossible for them to go back in time in the first place, making them unable to kill their grandfather, who would continue to produce offspring and restart the situation. But if they fail, their grandfather would be alive and produce offspring, one of whom would eventually conceive the time traveler and the whole scenario would start over.

Temporal paradox has been used to argue that time travel must be impossible, because it is capable of resulting in a paradox. Kip S Thorne, however, said that none of the supposed paradoxes formulated in time travel stories can actually be formulated at a precise physical level: that is, any situation in a time travel story turns out to permit many consistent solutions.
Time line protection hypothesis
This theory states that a time traveler, no matter what he had done, would not be able to create a time paradox. A person traveling back in time to terminate his grandfather, could have appeared in a wrong place, or had his gun jammed, thereby allowing his grandfather to have descendants. The natural law would prevent the traveler to alter the time travel he had done in the first place.
The animated television series Futurama shows a more lighthearted side of the paradox. In the episode "Roswell That Ends Well", the main character, Philip J. Fry, travels back in time with his friends to 1947 in Roswell, New Mexico. Remembering that his grandfather works at the base, and told that killing the man would nullify his own existence, Fry becomes obsessed with protecting him. Fry's efforts prove counterproductive: he locks the man in a shack to protect him, failing to realize that an atomic bomb is being tested on the grounds. When he does not disappear, he assumes that the man could not have been his grandfather and thus proceeds (unknowingly) to sleep with and (accidentally) impregnate a beautiful woman, who apparently is the younger version of his grandmother, thereby becoming his own grandfather.
In the 1972 Doctor Who adventure Day of the Daleks, Sir Reginald Styles is targeted by 22nd Century guerrillas, who believe he's behind the deaths of VIP delegates. Because of those deaths, the Daleks were able to take over Earth in their time. However, a fellow guerrilla who was left behind was to blame, which was the true cause of their timeline ensuing.
In the game "Ratchet and Clank Future: A Crack in Time", Alister Azimuth tries to fix his mistake of causing the fall of the Lombaxes, but if he was successful in traveling to the past, time would rip itself apart and the universe would collapse on itself.
In the machinima series Red vs. Blue, Church is sent back in time via the combined energy of a bomb and a weather machine. He attempts to fix past events, as well as preventing the explosion, but his attempts are ultimately responsible for most of the events that took place beforehand in the series, including his own death.
In the Japanese manga Doraemon, the future grandson of the lead character—Nobita—comes back in time to meet his grandfather in his primary school days intentionally to change the life he is in. Nobita questions the existence of his grandson that if he did marry the girl he likes (instead of the one he dislikes but conceived the father of his future grandson), what would happen to his grandson. His grandson, along with Doraemon, replies that there are multiple paths leading to the same future, and they will still exist even if Nobita married another girl. The plot never explicitly told of when the history was altered, but later events in the plot did show a future where Nobita married the girl he likes and lived a better, wealthier life and yet, future characters showed no signs of remembering the original history. However, when being asked of Doraemon's reason of being in the past, he replies that his role is to make sure Nobita marries the girl he likes, which mentally still reserves logic.
In the television series Lost, Jack and Locke enter an underground research facility on the island in 2004, and watch an orientation film made by the mysterious DHARMA Initiative organisation in 1980. The film makes reference to an "incident" having occurred on the island at some point in the 1970s, necessitating the construction of the underground bunker, as a method of containing a limitless amount of electromagnetic energy, and preventing a global catastrophe. In the show's fifth season, some of the main characters begin moving erratically through time, before eventually becoming stuck in 1974. Three years later, these same characters unwittingly become the cause of the incident.
Novikov self-consistency principle
This Novikov self-consistency principle says that anything a time traveler does in the past must have been part of history all along, so although the time traveler can have a causal influence on events in the past, it is impossible for anything the time traveler does to "change" history. So, for example, any attempt by the time traveler to kill one of his ancestors before they became a parent would be guaranteed to fail for some reason or another (perhaps the gun would jam, or perhaps the traveler would just have a change of heart), so the grandfather paradox would be avoided. This theory, however, is capable of causing bootstrap paradox.
Multiple universes hypothesis
This theory states that there are infinite number of universes, all-together known as multiverse. If a person is about to travel back in time, he will end up in a parallel universe. So if he kills "his" grandfather, a paradox would not occur because the grandfather that he had killed is the grandfather who lives in the universe he currently is in.
An example of this occurs in the Japanese anime series Dragonball Z in which Trunks, the son of Vegeta and Bulma, comes from the future. In the present, Trunks was not even born. He warns of the arrival of androids which are more powerful than even Goku and Vegeta, and forewarns the death of Goku in his past. However, by giving the medicine which can cure Goku and undertaking extensive training, Trunks and the Z Fighters manage to defeat the androids as well as Cell. When Trunks returns to his present, though, his universe is still the same with Goku and other Z-fighters still dead, the only difference being that at that point of time, Trunks, because of his training with the Z-fighters, had become strong enough to defeat the androids and Cell of his own time.
Branching universe hypothesis
This theory says that to travel back in time would cause time to branch.
An example of this occurs in The Legend of Zelda. Toward the middle of The Legend of Zelda: Ocarina of Time, the protagonist, Link, is sealed in slumber for seven years. When he awakens, he finds the world in a ruined state. At the end of the game, the antagonist, Ganon, is defeated and sealed within the Sacred Realm. Princess Zelda, then, sends Link into the past to right the wrongs Ganon created. By doing so, he creates two worlds, one in which Ganon is sealed away in the Sacred Realm, and one in which Ganon was merely imprisoned by the King. Additional evidence is stated by Eiji Aonuma when conversing on timeline placement of The Legend of Zelda: The Wind Waker and The Legend of Zelda: Twilight Princess, where he says that the games are parallel.
This was also in a DC anthology, which included among other series, one comic with Superman. In the comic, Superman attempts to change many of the world's great events, including Lincoln's assassination. When he comes back to his own time, nothing had changed. However, he discovers a parallel version of the world with the altered history.
Timeline corruption hypothesis
Another idea is that any change in the timeline, even without personal interaction, while allowable, would cause a "butterfly effect" in the timeline. All history after the time traveler visited would be affected by minute changes the traveler had made in the past, and the history, depending on how severe the time traveller's actions were, would sooner or later be completely changed. This has been coined the "timeline corruption hypothesis." The 2004 film The Butterfly Effect and the Multiverser RPG system prefer this view. There's also the Ray Bradbury science fiction short story "A Sound of Thunder", in which the butterfly effect is caused by a real butterfly.
Note that the timeline corruption hypothesis is not intended to solve the temporal paradox. It seems to be part of the multiple universes hypothesis, in which a change in the timeline creates a new universe.
The most well-known example of this theory is the 1985 film Back to the Future, in which the protagonist Marty McFly goes back in time and interferes with his parents' first romantic encounter, thus erasing his own existence (as well as that of his siblings). However, the effect only happens gradually, exemplified by a family photo in his possession: each of his siblings begins to disappear limb by limb, starting with the oldest and working down to him (the youngest of the three). This allows Marty to correct the error and restore the timeline, albeit with a few minor changes that are due to his interference. This effect has one contradiction - If a person, somehow, causes himself to not be born in the past, then he would not have been able to do the thing that caused him to not be born for he would have not existed, thus causing time to corrupt.
This theory also figures prominently in the 1989 sequel Back to the Future Part 2 in which McFly's enemy, the now-elderly Biff Tannen, travels from 2015 back to 1955 to give his younger self a copy of a sports almanac with the final scores of professional sports games from 1950 to 2000. The younger Biff uses this information to change history, so when Doc and McFly return to 1985 from their own mission in 2015, they find Hill Valley drastically changed. Marty proposes going back to 2015 to stop Biff from going back to 1955, but Doc explains that it would do no good since they were on a different timeline and 2015 would also be different. The only way to restore the timeline is to return to 1955 and take the almanac away from Biff so he will not use it to change history. Note that this does not create a bootstrap paradox because Biff, from timeline A, traveled back in time and created timeline B, thus there is a clear(ish) logical reason as to where the almanac came from.
This idea also appears in a Family Guy episode, in which Peter goes back in time with the help of "Death" so that he can relive his teen life. When he arrives in the past, rather than spending time with his present-day wife, Lois, Peter ignores her. His actions cause a corruption in the timeline, and when Peter returns to the present day, all of reality is radically different.
The timeline corruption hypothesis is also used in the Red Dwarf episode, "Tikka to Ride", when the Red Dwarf crew travels back in time (with Lister's intention of ordering 500 curries) and accidentally kills Lee Harvey Oswald, saving Kennedy's life. Three years later, in an alternate reality, it is revealed that through a series of chain events, the USSR won the Space Race and put the first man on the moon, meaning that the Dwarfers never travelled into deep space in the first place, trapped in an alternate 1966. The Dwarfers manage to correct this by trying to make Oswald shoot from a different floor, before making an impeached Kennedy assassinate himself, as the man on the grassy knoll.
Philip K. Dick also explored timeline corruption paradoxes. In the story "Orpheus with Clay Feet", Slade, a character from the future, goes on a time travel vacation to the past where he can visit famous science fiction writer Jack Dowland and become his muse. Slade, however, fails to inspire Dowland as he had hoped, and Dowland never becomes the master he should have been.
Timeline corruption is an important motif in Star Trek: Enterprise. Captain Archer and the Enterprise crew become embroiled in an ongoing Temporal Cold War with the Suliban Cabal, a race of hostile aliens from the future who deliberately manipulate the timeline for their own ends. One of the results was that Earth's early 20th-Century history was changed so that the Nazis controlled much of Europe and proceeded to invade North America. Of course, the humans had to find a way to stop the Suliban and restore the timeline. Similar events occur in other Star Trek series.
In the videogame World of Warcraft, the Bronze Dragonflight (tasked with the safety of the timelines) frequently asks the heroes (players) to help them fight the Infinite Dragonflight, who would want to change important events of the past. Although this may vary depending on the point of the view of the player, most of the events are negative ones - Thrall's escape from his prison, leading to the formation of the new Horde, enemies of the Alliance; the opening of the Dark Portal, in which a corrupted Medivh opens up a link with the world of Draenor, starting the orcish invasions of Azeroth and ensuing wars; and the Culling of Stratholme, a defining moment in which Prince Arthas's fall to madness leads to the rise of the undead Scourge and his eventual merge with the Lich King - but they insist that the outcome of preventing such events would be "much worse".
The videogame series Command & Conquer contains a miniseries called Red Alert, whose three games depict what "happened" when Albert Einstein traveled back in time to prevent Hitler from rising to power. This resulted in the Nazi regime never coming to power, allowing the Soviet Union to become a formidable threat to the Allied Nations; instead of just tension during the Cold War, the Soviets launched a full-scale invasion against the United States. Later in the series, the Soviets steal the time-travelling technology for themselves to kill Einstein, preventing his technology from helping the Allies; however, this leads to Japan's Empire of the Rising Sun emerging as a new superpower.
In the videogame The Journeyman Project, a Temporal Distortion Wave threatens to alter the present. The player is tasked with retrieving a CD-ROM disk that has the correct timeline stored on it. Upon returning to the present from the far distant past, it is revealed that the timeline has indeed been corrupted by four key events in the recent past. This idea of avoiding corrupted timelines is seen throughout the Journeyman Project Trilogy. Although, it is only a story element in the later games as opposed to a major gameplay moment.
Realistically, there is absolutely no way to travel into the past without causing irreparable damage to the timeline as there are far too many variables. Example: One atom travels back in time one million years. Its slight gravitational effect changes the position of every subatomic particle in the universe to eventually move less than a micron. That change may be enough to cause a neuron in one person's brain to fire or not fire causing or preventing a cascade effect. This changes what this person thinks and does thus altering far too many variables to list therefore changing the entire course of history. The effects of any larger object would be far more devastating. If the theory of temporal corruption is correct, time travel would be extremely impractical to say the least.
Temporal merging hypothesis
This is the opposite of the multiple universes hypothesis, in that each action committed in time travel actually overlaps one reality with another. For instance, if a time traveler were to meet his double from another time, the double would merge with the time traveler, making the traveler a part of the time he is visiting. The same would hold true for events. Two events would merge into the nearest event which does not produce a paradox (a dead grandfather in one universe but not in another would either create a dead grandfather in both universes, but alter the person's heritage so as to allow this, merge both timelines so that the person would fade from all timelines upon return, or produce a mean between life and death such as a coma).
An example of this is seen in the film The One, in which a character travels across time or dimensions, destroying copies of himself to cause them to merge — thus increasing power for the original character. A similar idea is seen in Margaret Peterson Haddix's book series, The Missing, in which any records of original time are kept in tracers, and those traveling back in time can merge with their own tracers.
Choice timeline hypothesis
In the choice timeline hypothesis, history changes the instant the time traveler decides to travel back in time, thereby rendering his actions in that regard pre-destined. This theory is the most consistent with our understanding of the dimension of time.
Bill & Ted's Excellent Adventure, as well as its sequel and television spin-offs (Bill & Ted's Excellent Adventures - both animated and live-action), feature numerous uses of this hypothesis. Bill and Ted, constantly realizing that their plans are foiled by the lack of a certain item, decide to later travel back in time and deliver themselves the necessary item, often indicating a specific place in which the item will appear. Upon searching the location, the item is invariably there.
Can-Not Because Has-Not
This theory states that the present is not the forefront of time, and so we are our future selves' past. Thus, if sometime in the future a time travel device were created, someone from the future would have already brought it back to us, thus establishing itself as "already" existing in our time as a result - and likely copied and recopied. Since our present selves are still wondering about time travel, this theory states that we will never be able to build a time machine, because if we are still wondering, then no one from the future has built a time machine and brought it back with them to us, and if no one in the future has built a time machine, then we in the future will not build a time machine, and no one can ever build a time machine because no one in the future has built one. This theory is demonstrated in an episode of the TV show Big Bang Theory where Leonard and Sheldon sign a roommate agreement that states if either one of them ever invents time travel their first trip back would be to their present location 5 seconds after the signing of the contract, they are subsequently disappointed when the 5 seconds pass and nothing happens. This theory, however, creates a Causality Loop. Also if it is that any time machine made will allow only one way travel, i.e. into the future, or if it is that we can go into the future, but while returning, we can only return to the present (i.e. not set the dial to go back more than we used to go into the future, or before the time machine is invented), this theory fails. Wormholes, which are suggested to allow backwards time travel, demonstrate this, as you cannot go further back in time than when the wormhole was created.
There are also other possibilities: someone in the future will build a time machine and many people will use it to travel back in the past, but we do not know this because: "no one can travel so far in the past to reach our time" or "no one has revealed that time-traveling is possible" or "no one that revealed that time-traveling is possible, was trusted" or "anyone with the knowledge to sufficiently understand temporal mechanics enough to build such a device, will also be enlightened enough not to interfere with the known written past" or "the time machine will be destroyed if you go back before it is created."
Self-Healing hypothesis
This theory states that time would heal itself from the paradox. A person is about to alter an event in the present by traveling back to the past and altering the events that cause the events in the present day to happen. Then there will be another set of events that will cause the present-day events to happen, and so there will be reason for him to travel back in time, and thus the paradox would never happen.
An example of this can be seen in the film The Time Machine, where the main character creates a time machine to save his dead fiancé, but upon doing so, she is killed another way. When traveling to the future, he finally finds out that if she did not die he would never create the time machine. Thus it is impossible to save her. Even though this does not create a physical paradox, it creates a mental one in which the time traveler, who still builds the time machine for the same reason, remembers a different cause for that reason.
Another example is the TV series Doraemon, where in one episode the main character travels back in time to save someone from being hit by a car. Doraemon then states that 'if history is altered, it will always repeat itself to make up for the event that did not occur'. However he also states that 'the event might happen on another person, or be a completely different event altogether', raising the question how did the time traveler know about what was supposed to happen in the first place?
This theory also happens in the TV series Lost when Desmond Hume, who can see into the future, sees Charlie Pace die when struck by lightning. Desmond saves him by building a big metal pole to absorb the shock instead. When Charlie doesn't die this way, Desmond sees him dying by trying to save Claire from drowning, trying to get Claire a bird, and while getting shot with an arrow from a trap in the jungle- all from which he is saved. Eventually Charlie accepts his fate and drowns in the DHARMA Initiative facility "the Looking Glass".
This theory also makes an appearance on the first season of the TV series Star Trek Where Captain Kurk tries to save a person that he shares a love for from being hit by car, but must let her to let America win World War II. Going back in time to save his love from being hit by the car would allow her to succeed in a plan to pull out of the war, and allow the Nazi Empire to grow until they overwhelmed America, thus making it impossible for the inventor of the time machine invent it, due to them never existing.
Destruction resolution
This theory states that any paradox would cause the destruction of a universe, or at least the part of the time & space affected by the paradox. In this hypothesis, if a person travels back to kill his grandfather before one of his parents is conceived, then it will cause himself to disappear. History would erase all traces of the person's existence, and the death of the grandfather would be caused by another reason. Thus, the paradox would never occur from the historical viewpoint.
A variation of this example occurs in the Season 3 finale of FRINGE when Peter Bishop uses the Doomsday machine. When he tells both Walter and Walternate he punched holes in both universes in the past and created a bridge between universes in order to repair the damage, Peter disappears because his actions caused the younger versions of himself in both of the new timelines to die in 1985.
Temporal Modification Negation Theory
This theory is partially similar to other theories on time travel. While stating that if time travel is possible it would be impossible to violate the grandfather paradox, it goes further to state that any action taken that itself negates the time travel event cannot occur. The consequences of such an event would in some way negate that event, by either voiding the memory of what one is doing before doing it, by preventing the action in some way, or even by destroying the universe among other possible consequences. It states, therefore, that to successfully change the past one must do so incidentally.
For example, if one tried to stop the murder of one's parents, he would fail. On the other hand, if one traveled back and did something to some random person that as a result prevented the death of someone else's parents, then such an event would be successful, because the reason for the journey, and therefore the journey itself, remain unchanged preventing a paradox.
In addition, if this event had some colossal change in the history of mankind, and such an event would not void the ability or purpose of the journey back, it would occur, and would hold. In such a case, the memory of the event would immediately be modified in the mind of the time traveler.
An example of this would be for someone to travel back to observe life in Austria in 1887 and while there shoot five people, one of whom was Hitler's parent. Hitler would therefore never have existed, but since this would not prevent the invention of the means for time travel, or the purpose of the trip, such a change would hold. Also for it to hold, every element that influenced the trip must remain unchanged. This would void someone convincing another party to travel back to kill the people without knowing who they are and making the time line stick, because by being successful, they would void the first party's influence and therefore the second party's actions.
hanges allowed, without resolution to paradoxes
Another theory, though much less commonly utilized,[citation needed] is one in which the altering of an event in the past would always be allowed, even if such alteration would logically cause a paradox.
Ad exemplum, a time traveler who embarks on a trip to the past would be able to kill his grandfather, even though such an act would prevent one of his parents from being born and thus prevent the time traveler himself from being born, without causing the resulting paradox to change any other part of the past. Though, with this theory, the time traveler, who would still remember having a grandfather and even parents, would appear to have not been born. This seems to generate a problem resembling the bootstrap paradox.

Quantum time machine 'allows paradox-free time travel'

Quantum physicists at the Massachusetts Institute of Technology believe it is possible to create a time machine which could affect the past without creating a "grandfather paradox".

have for some years been able to 'teleport' quantum states from one place to another. Now Seth Lloyd and his MIT team say that, using the same principles and a further strange quantum effect known as 'postselection', it should be possible to do the same backwards in time. Lloyd told the Technology Review: "It is possible for particles (and, in principle, people) to tunnel from the future to the past."

Postselection is a vital part of the nascent science of quantum computing. In traditional computing, if a user needs to determine which set of variables in an equation leads to the answer being true, the computer must try every combination until it hits upon one that works. In quantum computing, due to the weird parallel behaviour of subatomic particles, it seems to be possible to simplify the procedure by running all possible variations simultaneously, and selecting only the combinations that make the answer true.

Professor Lloyd and his team say that, by combining teleportation and postselection, it would be possible to carry out the quantum teleportation effect in reverse; that is, to decide after the teleportation what the quantum state must have been before it. This works as postselection allows you to dictate which quantum states can be teleported, limiting what state it can have been in before the teleportation. The state of the particle post-teleportation has therefore, in effect, travelled back in time.

Dr Richard Low, a quantum computing scientist from the University of Bristol, says: "You could think of it as postselection affecting the history of the particle, sending the state back in time."

Unlike previous theories of teleportation, this apparently avoids the "grandfather paradox" - or, to Back to the Future fans, the Marty McFly problem. If you go back and change time, and accidentally end up killing your own grandparent, you create a paradox - you will not be born, so you cannot go back and affect time. Even with subatomic particles, this is still a problem: upon travelling back in time, the particle could somehow destroy its earlier self or move it, thus preventing it from travelling.

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However, because of the probabilistic nature of quantum mechanics, Prof Lloyd's method seems to avoid this. Anything caused by the time travel must have had a finite probability of happening anyway, so paradoxical impossibilities are out.

Further, this time travel method does not involve bending spacetime, unlike other proposed systems. At the moment, the only conditions known that would bend spacetime sufficiently exist in black holes, which would be impractical at best.

It is a controversial theory, to say the least. Some physicists claim that the apparently impossible things implied by postselection prove that it cannot work, which would destroy Prof Lloyd's theory before it got off the ground.

The theory is unlikely to lead to DeLorean time machines or anything remotely similar. Instead, Prof Lloyd and his team hope that it can advance our understanding of physics: "Our hope is that this theory may prove useful in formulating a quantum theory of gravity," he says.

Prof Lloyd's paper, The quantum mechanics of time travel through post-selected teleportation, is published in - Quantum Physics.

Solving The UFO Enigma: How Modern Physics is Revealing the Technology of UFOs, Published by Author Robert L. Schroeder

BOSTON/EWORLDWIRE/Dec. 15, 2011 --- According to Robert L. Schroeder's new book, "Solving the UFO Enigma: How Modern Physics is Revealing the Technology of UFOs," the technology behind the UFO phenomenon can now be explained by modern physics and by a close examination of the more credible UFO reports (Quantum Universe Books, November 28, 2011. ISBN-10: 1452836817, BISAC: Science/Physics, trade paperback, 236 pages, $21.95; available on Amazon, Kindle [$9.95] and at select retail outlets).

"The idea that a subset of all UFO reports represent advanced craft of extraterrestrial origin has been around for a long time - but only recently has the science of physics developed theories which, when correlated with selected UFO data, point directly toward a likely explanation of this alien technology," said Schroeder.

"Modern physics in its attempt to unify the fundamental forces of nature in a 'Theory of Everything' has proposed theories that require the existence of up to 11 space-time dimensions and the possibility of entire universes existing side by side with our own," added Schroeder. The author not only shows how physics has evolved to this point, but he also looks at the unanswered questions of modern physics such as the origin of inertia, the mystery of quantum non-locality, the Higgs or "God" particle, and the puzzle of the relative weakness of gravity.

"It is these unanswered questions, along with certain conjectures and pertinent UFO characteristics, that, when incorporated with the latest cutting edge physics theories, look to be the final keys to explaining UFO technology. The book does contain some of the math of the most important historical physics theories. Although the math isn't essential to understanding the basic theme of the book, it is there for those who would like an even more in-depth knowledge of modern physics," stated Schroeder.

Schroeder's proposed solution to UFO technology is based on some remarkable and rather exotic physics theories that are currently being tested at the Large Hadron Collider particle accelerator in Geneva, Switzerland. "Should these theories prove correct then there is a real possibility that UFO technology is within the reach of our earth civilization. Its development on our planet would open up whole new worlds, both literally and figuratively, to be discovered and explored."

About the Author: Robert Schroeder recently retired after 26 years with a major computer company where he was involved in operations and product management. His educational background includes a B.A. in mathematics from Rutgers University, an M.B.A. and an A.A. in aerospace engineering. He has closely followed developments in modern physics over the years and has researched the UFO phenomenon since the 1960s.

The Dec. 18, 2011, author event at Tatnuck Bookseller in Westborough, Mass., includes a slideshow describing the evolution of modern physics, its relationship to the UFO phenomenon, and excerpts from DVDs of commercial and military pilot sightings of UFOs. The physic's slideshow is at a layman's level and is suitable for general audiences.

Bob Schroeder
Author Robert L. Schroeder
PHONE. 9784439018

Time Cloak Hides Very Brief Events [Animation]

For years physicists have been refining invisibility cloaks—physical setups that cleverly reroute light around a region in space, effectively concealing any object that might be inside. But now researchers at Cornell University have built the first temporal cloak, a device that obscures an object or event not at a particular point in space but at a specific moment in time.

In a preliminary demonstration, Cornell postdoctoral researcher Moti Fridman and his colleagues shone a laser beam through an experimental apparatus and into a detector. A physical object or even another beam of light in the laser beam’s path could create a change in the laser light that the detector would register. But with some clever optics, Fridman and his colleagues were able to open up a brief time gap in the beam and then close it back up as if the beam had gone undisturbed, and such that the detector did not register the interruption. The gap allows anything that would have otherwise affected the beam to instead slip right through [see animation below], leaving no trace for the detector to pick up.

Click link for animation -

The researchers used the cloak to obscure an optical pulse that ordinarily interacts with the laser beam to produce a telltale spike at a certain wavelength. When the event was cloaked, however, the telltale spike was basically undetectable.

The cloak, described in the January 5 issue of Nature, relies on the fact that light of different colors moves at different speeds through certain media. (Scientific American is part of Nature Publishing Group.) Using a device that they call a “time lens,” the researchers split a single-color laser beam into a spread of wavelengths, then slowed half of those wavelengths while speeding up the others. That created a very brief time gap that could be closed again before the beam reached the detector by reversing the lensing process, restoring the beam to a single, seemingly undisturbed wavelength.

The gap achieved by Fridman and his colleagues was extremely small—just 50 picoseconds, or 50 trillionths of a second, in duration. The researchers note that it is possible to extend the gap somewhat, but that scattering and dispersion effects limit the scope of the temporal cloak to a few nanoseconds.

Animation by Rose Eveleth

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