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Time Travel: Theories, Paradoxes & Possibilities

 
Time travel may be theoretically possible, but it is beyond our current technological capabilities.

Time travel may be theoretically possible, but it is beyond our current technological capabilities.

Credit: argus  Shutterstock

Time travel — moving between different points in time — has been a popular topic for science fiction for decades. Franchises ranging from "Doctor Who" to "Star Trek" to "Back to the Future" have seen humans get in a vehicle of some sort and arrive in the past or future, ready to take on new adventures.

The reality, however, is more muddled. Not all scientists believe that time travel is possible. Some even say that an attempt would be fatal to any human who chooses to undertake it.

Understanding time

What is time? While most people think of time as a constant, physicist Albert Einstein showed that time is an illusion; it is relative — it can vary for different observers depending on your speed through space. To Einstein, time is the "fourth dimension." Space is described as a three-dimensional arena, which provides a traveler with coordinates — such as length, width and height —showing location. Time provides another coordinate — direction — although conventionally, it only moves forward. (Conversely, a new theory asserts that time is "real.")

Most physicists think time is a subjective illusion, but what if time is real?
Most physicists think time is a subjective illusion, but what if time is real?
Credit: Shutterstock/Kim D. French

Einstein's theory of special relativity says that time slows down or speeds up depending on how fast you move relative to something else. Approaching the speed of light, a person inside a spaceship would age much slower than his twin at home. Also, under Einstein's theory of general relativity, gravity can bend time.

Picture a four-dimensional fabric called space-time. When anything that has mass sits on that piece of fabric, it causes a dimple or a bending of space-time. The bending of space-time causes objects to move on a curved path and that curvature of space is what we know as gravity.

Both the general and special relativity theories have been proven with GPS satellite technology that has very accurate timepieces on board. The effects of gravity, as well as the satellites' increased speed above the Earth relative to observers on the ground, make the unadjusted clocks gain 38 microseconds a day. (Engineers make calibrations to account for the difference.)

In a sense, this effect, called time dilation, means astronauts are time travelers, as they return to Earth very, very slightly younger than their identical twins that remain on the planet.

Through the wormhole

General relativity also provides scenarios that could allow travelers to go back in time, according to NASA. The equations, however, might be difficult to physically achieve.

One possibility could be to go faster than light, which travels at 186,282 miles per second (299,792 kilometers per second) in a vacuum. Einstein's equations, though, show that an object at the speed of light would have both infinite mass and a length of 0. This appears to be physically impossible, although some scientists have extended his equations and said it might be done.

A linked possibility, NASA stated, would be to create "wormholes" between points in space-time. While Einstein's equations provide for them, they would collapse very quickly and would only be suitable for very small particles. Also, scientists haven't actually observed these wormholes yet. Also, the technology needed to create a wormhole is far beyond anything we have today. 

Alternate time travel theories

While Einstein's theories appear to make time travel difficult, some groups have proposed alternate solutions to jump back and forth in time.

Infinite cylinder

Astronomer Frank Tipler proposed a mechanism (sometimes known as a Tipler Cylinder) where one would take matter that is 10 times the sun's mass, then roll it into very long but very dense cylinder.

After spinning this up a few billion revolutions per minute, a spaceship nearby — following a very precise spiral around this cylinder — could get itself on a "closed, time-like curve", according to the Anderson Institute. There are limitations with this method, however, including the fact that the cylinder needs to be infinitely long for this to work.

An artist's impression of a black hole like the one weighed in this work, sitting in the core of a disk galaxy. The black-hole in NGC4526 weighs 450,000,000 times more than our own Sun.
An artist's impression of a black hole like the one weighed in this work, sitting in the core of a disk galaxy. The black-hole in NGC4526 weighs 450,000,000 times more than our own Sun.
Credit: NASA/JPL-Caltech 

Black holes

Another possibility would be to move a ship rapidly around a black hole, or to artificially create that condition with a huge, rotating structure.

"Around and around they'd go, experiencing just half the time of everyone far away from the black hole. The ship and its crew would be traveling through time," physicist Stephen Hawking wrote in the Daily Mail in 2010.

"Imagine they circled the black hole for five of their years. Ten years would pass elsewhere. When they got home, everyone on Earth would have aged five years more than they had."

However, he added, the crew would need to travel around the speed of light for this to work. Physicist Amos Iron at the Technion-Israel Institute of Technology in Haifa, Israel pointed out another limitation if one used a machine: it might fall apart before being able to rotate that quickly.

Cosmic strings

Another theory for potential time travelers involves something called cosmic strings — narrow tubes of energy stretched across the entire length of the ever-expanding universe. These thin regions, left over from the early cosmos, are predicted to contain huge amounts of mass and therefore could warp the space-time around them.

Cosmic strings are either infinite or they’re in loops, with no ends, scientists say. The approach of two such strings parallel to each other would bend space-time so vigorously and in such a particular configuration that might make time travel possible, in theory.

Time machines

It is generally understood that traveling forward or back in time would require a device — a time machine — to take you there. Time machine research often involves bending space-time so far that time lines turn back on themselves to form a loop, technically known as a "closed time-like curve."

The Doctor's time machine is the TARDIS, which stands for Time and Relative Dimensions in Space.
The Doctor's time machine is the TARDIS, which stands for Time and Relative Dimensions in Space.
Credit: BBCAmerica 

To accomplish this, time machines often are thought to need an exotic form of matter with so-called "negative energy density." Such exotic matter has bizarre properties, including moving in the opposite direction of normal matter when pushed. Such matter could theoretically exist, but if it did, it might be present only in quantities too small for the construction of a time machine.

However, time-travel research suggests time machines are possible without exotic matter. The work begins with a doughnut-shaped hole enveloped within a sphere of normal matter. Inside this doughnut-shaped vacuum, space-time could get bent upon itself using focused gravitational fields to form a closed time-like curve. To go back in time, a traveler would race around inside the doughnut, going further back into the past with each lap. This theory has a number of obstacles, however. The gravitational fields required to make such a closed time-like curve would have to be very strong, and manipulating them would have to be very precise. [Related: Warp Speed, Scotty? Star Trek's FTL Drive May Actually Work]

Grandfather paradox

Besides the physics problems, time travel may also come with some unique situations. A classic example is the grandfather paradox, in which a time traveler goes back and kills his parents or his grandfather — the major plot line in the "Terminator" movies — or otherwise interferes in their relationship — think "Back to the Future" — so that he is never born or his life is forever altered.

If that were to happen, some physicists say, you would be not be born in one parallel universe but still born in another. Others say that the photons that make up light prefer self-consistency in timelines, which would interfere with your evil, suicidal plan.

Some scientists disagree with the options mentioned above and say time travel is impossible no matter what your method. The faster-than-light one in particular drew derision from American Museum of Natural History astrophysicist Charles Lu.

That "simply, mathematically, doesn't work," he said in a past interview with sister site LiveScience.

Also, humans may not be able to withstand time travel at all. Traveling nearly the speed of light would only take a centrifuge, but that would be lethal, said Jeff Tollaksen, a professor of physics at Chapman University, in 2012.

Using gravity would also be deadly. To experience time dilation, one could stand on a neutron star, but the forces a person would experience would rip you apart first.

Time travel in fiction

Two 2015 articles by Space.comdescribed different ways in which time travel works in fiction, and the best time-travel machines ever. Some methods used in fiction include:

One-way travel to the future: The traveler leaves home, but the people he or she left behind might age or be dead by the time the traveler returns. Examples: "Interstellar" (2014), "Ikarie XB-1" (1963)

Time travel by moving through higher dimensions: In "Interstellar" (2014), there are "tesseracts" available in which astronauts can travel because the vessel represents time as a dimension of space. A similar concept is expressed in Madeleine L'Engle's "A Wrinkle In Time" (2018, based on the book series that started in 1963), where time is folded by means of a tesseract. The book, however, uses supernatural beings to make the travel possible.

Travelling the space-time vortex: The famous "Doctor Who" (1963-present) TARDIS ("Time And Relative Dimension In Space") uses an extra-dimensional vortex to go through time, while the travelers inside feel time passing normally.

Instantaneous time jumping:Examples include "The Girl Who Leapt Through Time" (2006), the DeLorean from "Back To The Future" (1985), and the Mr. Peabody's WABAC machine from "The Rocky and Bullwinkle Show" (1959-64).

Time travelling while standing still:Both the "Time Machine" (1895 book) and Hermione Granger's Time-Turner from "Harry Potter" keep the traveler still while they move through time.

Slow time travel: In "Primer" (2004), a traveler stays in a box while time traveling. For each minute they want to go back in time, they need to stay in the box for a minute. If they want to go back a day in time, they have to stay there for 24 hours.

Traveling faster than light: In "Superman: The Movie" (1979), Superman flies faster than light to go back in time and rescue Lois Lane before she is killed. The concept was also used in the 1980 novel "Timescape" by Gregory Benford, in which the protagonist sends (hypothetical) faster-than-light tachyon particles back to Earth in 1962 to warn of disaster. In several "Star Trek" episodes and movies, the Enterprise travels through time by going faster than light. In the comic book and TV series "The Flash," the super-speedster uses a cosmic treadmill to travel through time.

Difficult methods to categorize:There's a rocket sled in "Timecop" (1994) that pops in and out of view when it's being used, which has led to much speculation about what's going on. There's also the Time Displacement Equipment in "The Terminator" movie series, which shows off how to fight a war in four dimensions (including time)

find full article on space.com

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Quote

 

E.g., that thing Arceus does in Super Smash Bros. 4. :lol:

 

Quote

Black holes

 

If Super Paper Mario and the Super Mario Galaxy duology taught me anything, it's that you want to stay TF away from those. :lol:

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It's interesting that time travel via consciousness is never mentioned on that list. For example, X-Men: Days of Future Past (2014) had Logan traveling through time by transferring his consciousness back into his body in 1973. The mechanics are never explained (beyond the mere fact that Kitty Pryde has this power), but it clearly has tangible consequences.

Slaughterhouse-Five has a similar dynamic in which the Tralfamadorians view the past, present, and future simultaneously. However, their view of time is fatalistic: they know the future but make no effort to change it. The same holds true for Billy Pilgrim. Granted, the implication is that he has suffered a breakdown; thus the fatalism is a reflection of his coping with traumatic experiences.

The major hurdle I see in all of this is our tendency to view time as linear. The term "timeline" itself suggests a straight line with a beginning and end. I think it's a strong working approximation based on our experience -- but if it is indeed possible to physically "move" through time, then I suspect we will have to revamp our concept of time altogether. It is like trying to study the ocean with a two-dimensional map as the only guide.

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You forgot a way:

 

The Slingshot Effect. It's shown several times throughout the Star Trek franchise. It would require a vessel to go around warp 8 (it supposedly had to be done around Warp 8, which is approximately 512 times the speed of light) and rotate around a massive object (usually a sun, since it would probably not be good to do it around a black hole) several times until the ship accelerated so fast that it traveled back in time. 

 

In general, I'm pretty unsure about the concept, but I don't see how it could be an impossibility altogether.

Edited by Duzzki

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7 hours ago, Duzzki said:

You forgot a way:

 

The Slingshot Effect. It's shown several times throughout the Star Trek franchise. It would require a vessel to go around warp 8 (it supposedly had to be done around Warp 8, which is approximately 512 times the speed of light) and rotate around a massive object (usually a sun, since it would probably not be good to do it around a black hole) several times until the ship accelerated so fast that it traveled back in time.

I believe that method is alluded to in the "faster than light" section. I like your description better, though, because in Star Trek ships regularly travel faster than the speed of light. The critical difference was that a vessel had to exceed Warp 10 for the slingshot method to work.

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I would say: yes. Some prospects are: extreme speed, gravity, and light. Travel onwards the future might be possible -- traveling into the past is much, much more difficult. 

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When I saw the title of this forum I said to myself (yup if we are traveling into the future and definitely nope if we are traveling into the past). But after reading your post I am starting to reconsider:twilighthat:.

Wormholes, closed-timelike curves and cosmic strings does sound like cool and plausible ways to travel through time! Although it would cost a pretty penny to build such a machine :P . 

On a side note, I think the consequences of time traveling is quite interesting as well. One interesting thing about timetraveling I've been thinking about is to imagine traveling back in time to the early 1900s and say the only thing you did was accidently spilling coffee on another guy's shoe. This could set off a butterfly effect that changes the course of action of maybe 100 people in a day and then spread to a million over a month. For example, two weeks after you spilled your coffee, it may lead to a chain of events leading to a high school teacher living 1000km away to reach school three seconds later than he was supposed to. These changes might seem insignificant at first, but note a slight change in a person's life is very likely result in a different sperm that fertlizes the egg during the conception of their child. Now the couple's baby is likely to look very different than he/she is in the original timeline . So one could argue, every child that was born after your timetraveling event are now drastically different from their original timeline. Now the effect of timetravel is no longer insignificant and these babies (even if say their personalities are exactly the same but look slightly different) are likely to go on and live a completely different life than in the original timeline. As a result, even a very small change in our (recent) timeline may lead to a completely different and unrecognizable now. 

But.... I am by no means an expert in time-travel (or biology) :D and aside from ethical concerns, I now do believe time-travel is possible! (At least in paper and maybe not physically possible in the near future. :) ).

(Following contains Spoilers for Season 6&7):

Spoiler

Personally I think the closed-timelike curves mentioned above offers the best chance of timetravel because: There are sooooooooooooo few exact solutions to Einstein's equations and one of them is the Gödel metric. What is unique about this metric is that it gives closed-timelike curves. So it is mathematically consistent with the physics we know really really well, just that we havn't discovered it yet. But unfortunately, it requires the universe itself to be rotating and so far all our experiments says that it doesn't, so the scientific consensus is that it does not exist. Although I have yet to hear a good mathematical argument against it. (More information can be found on String Theory for Dummies by A.Zimmerman)

And perhaps, they do exist in Equestria and maybe.... just maybe.... that is how Starlight Glimmer and Twilight travels back in time. Note how after a certain period of time they returned to the future (just like going in a closed loop ;) .)  

 

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 Of course! :squee:82300142-17CD-4B1E-B281-8D810E0D2FAD.jpeg602F7F29-13D0-4C60-A01F-DA2059C52AF4.jpeg

66B5A6BF-74EE-463B-9906-35A4551F54E0.jpeg

7D07648B-2FDC-4C4D-A183-2E677DC20060.png

69580EC0-379A-4FD4-BF99-EABE9BC98A90.jpeg

*Comes out of fiction mode* (Sad, I know...)

To be honest, I don’t think that time travel is possible, and I almost hope it isn’t, since people would probably fail to learn from all the sci-fis and mess time up if it  were to be discovered. 

But then again, most people would probably have said that touch screen phones weren’t possible 20 years ago. So you never know!

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I have the blue prints on how to make a time machine, And am not kidding either as it was very hard to come by online. 

Getting the parts are hard so all have to make some of them. 

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I think we could more forward but not backward. The best explanation I heard is it's like being in a car with the gas pedal permanently stuck to the floor and the steering wheel doesn't go left or right you can only move forward with the car itself. (Like an Astronaut being propelled through the vaccum of space sorta.)

Edited by Ph0enix

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On 12/27/2017 at 8:11 PM, meck-can-ik pony said:

I have the blue prints on how to make a time machine, And am not kidding either as it was very hard to come by online. 

Getting the parts are hard so all have to make some of them. 

There's both classified and declassified documents about time travel and time machines. Unless you're really rich...I don't see it happening. 

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I firmly believe that anything is possible and time travel is no exception if you take into account every concept available. Lots of movies have come up with some interesting theories. Notable are Looper, Your Name, and Back to the Future. Some of these deal with transferal of consciousness rather than the physical body. A lot depends on what a time traveler wanted to accomplish. Does he/she want to observe past events without altering them? Take a peek at a possible future? If anyone traveled into the past there would be almost no way they could avoid altering future events and risking a paradox. If one only traveled to the future it might be workable since it's unwritten and any alterations made are simply the effect of the present on things to come, as is all of life. The concept I find most interesting is the transfer of mind, in an ethereal state or sending the consciousness into the mind/body of someone else in a different time, and seeing the world through their eyes. In that case, it might be confusing for both parties. 

Under any circumstances it would be tricky at best and require a lot of discipline and responsibility on the part of the time traveler. That's probably where the weakest point would be. 

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It is possible, but as you said, beyond us currently. 

There is one way to see back in time, though. It is cheap, easy, and can be done with no major technology. All you have to do is look at the stars in the sky. You see into the past every time you look up. When you admire Aldebaran, for instance, you're seeing it as it was 65 years ago. You see Betelgeuse as it was around 640 years ago.

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i dont think so,  though in spiritual sense if you can trancend the time itself i dont think it matter anymore

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Since Earth is hurling through space very fast,  I think that if you were to time travel, you’d not be on Earth, because it wasn’t there yet for that point in time.

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Time travel is partially possible; you can technically travel into the future... However, it hasn't been proven that you go back in time though.

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OP basically answered their own question. Theoretically, it would be more possible to travel forward than backward in time, if you're using a device. Not much left to add :/

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Traveling backwards would be possible only through anchoring a theoretical wormhole to a present point. When five years relativistic time passes you could jump back to said origin point. Unless this phenomenon exists in cosmology, absent its creation, time travel to historical moments would not be possible. 
 

I do love that @Yellow Diamond mentioned linear time as the main way we seem to comprehend it. That isn’t how time really works, as evidenced by all the tech you use that connects to satellites, yet we can only experience time at our relative perception. Your mass and the mass of objects around you ultimately determine your time traversal relative to others, and our brains may only be able to perceive time a set way. The common house fly and other animals perceive time slowed down relative to us which accounts for their continued existence in nature. We move slow so they can dodge easy. 

 

Time is weird 

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Time is a concept that is quite difficult to understand.... Unless it is linear. Then it is just a matter of math to do time travel! Except the butterfly effect.... Hmmm. I haven't figured that one out yet. :huh:

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(edited)

Even as a big sci-fi fan I've never really liked time travel. It's hard to explain but time travel just makes everything reality...mushy and meaningless. If everything can just be changed all the time it doesn't really exist at all.

So yeah, I don't believe it's really possible but what do I know?

Edited by Fluttershutter

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(edited)

My high school physics teacher (not the most reliable source) said it should be possible to jump forward in time if you move faster than the speed of light. It's just the issue of moving that fast that we can't do with today's technology.

In the BattleTech universe, it is possible to travel tens of light-years in the matter of seconds by going through hyperspace, called "jumping" (I know, that's a measurement for distance, not time. I'm getting there). Usually all goes well, but if the 'Ship's calculations are rushed or incomplete, if the 'Ship is trying to jump in or out of a large gravitational field other than its own for some stupid or crazy reason (such as jumping to or from inside a solar system), or if the crew is just particularly unlucky, a misjump can occur. Usually that only results in ending up a few kilometers or astronomical units from the 'Ship's planned destination, but 'Ships can end up light-years away from where they wanted to be, and sometimes even arrive with the 'Ship mangled and the crew injured or dead. This can also cause a 'Ship to jump well past the warp drive's (called a K-F Drive) maximum range, which is generally thought to be about thirty light-years. After that, the 'Ship needs to take a few days to recharge its K-F Drive before it can jump again. On such misjump put a 'Ship about 350 light years from its planned destination.

Outside of those more usual cases in a misjump, there has been at least one confirmed case where a 'Ship ended up at its planned destination, with the unexpected error of them arriving about fifty years in the future. This type of misjump has never been able to be recreated by scientists, and nobody is sure why or how it happened. So time travel is technically possible (in that universe, anyway), but nobody actually knows how it happens.

Sorry for the long explanation. I'm a bit of a BattleTech lore nut and I got carried away.

Edited by Booker

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As I understand it. time travel *forward* is the classic twins paradox - objects travelling close to the speed of light experience time dilation, and thus in their own personal timeline, only a few hours or days may pass while years or centuries pass outside of the ship.

We are unlikely to see that in practice though because

a) we don't know how to speed up something as big as a ship that much; we can barely speed up particles

b) slowing it back down again is MUCH harder than speeding it up (although if you give it enough time, just friction with interstellar gas and dust will do that for you)

c) as you speed up, ordinary particles going the other way are relatively coming at you much faster; if your ship doesn't burn up from hitting dust, the storm of hard radiation passing constantly though you will first kill then *cook* you. time dilation just makes that worse (as relative to your personal timeline, those particles are hitting you harder and faster)

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(edited)

Does sleeping all day count? :bedeyes:

Edited by Muffinnz

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1 hour ago, Muffinnz said:

Does sleeping all day count? :bedeyes:

Yes!

 

Also these sci-fi's that have the time loop episodes (thinking TNG and SG-1), where one or more people remember stuff from the last loop. Those are fun, but makes no sense!

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