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# Time Travel: Possibilities and theories

Time travel means moving between different points in time. In other words, traveling from the present to the future or past. This has been a popular topic for science fiction for many decades. Franchises ranging from “Doctor Who” to “Star Trek” to “Back to the Future”; we have seen humans get in a vehicle of some sort and arrive in the past or future, ready to take on new adventures. Each of these movies has its own time travel theories.

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

## Understanding Time

What is time? A lot of people think of time as a constant. Theoretical physicist, Albert Einstein has shown us that time is an illusion; it is relative — it can vary for different observers depending on a person’s speed through space. Einstein’s theory of special relativity says that time slows down or speeds up depending on how fast you move relative to something else. When we approach the speed of light, a person inside a spaceship would age much slower than his twin at home. Also, according to Einstein’s theory of general relativity, gravity can bend time.

Imagine there’s a four-dimensional fabric called space-time. When any matter sits on that piece of fabric, it causes the 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.

# Time travel theories

Wormhole theory

The 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 such possibility could be to go faster than light, which travels at 186,282 miles per second (299,792 kilometres 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 could be done.

A linked possibility, NASA stated, would be to create “wormholes”; between points in space-time. A wormhole is a sort of a tunnel that connects two distant points in space-time allowing to travel between them. While Einstein’s equations support them, they would collapse very quickly and would only be suitable for very small particles. Also, scientists haven’t actually observed these wormholes yet. The technology needed to create a wormhole is far beyond anything we have today.

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 and roll it into a 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.

Black holes

Another such 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 that 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 Machine

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.”

To accomplish this, time machines often are thought to need an extraordinary form of matter with so-called “negative energy density.” Such extraordinary 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.

## Time travel in fiction

Some famous methods used in fiction include:

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

Time travel by moving through higher dimensions: In the movie “Interstellar”, there is something called “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 through a tesseract. The book, however, uses supernatural beings to make the travel possible.

Traveling faster than light: “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.

So is time travel possible?

While time travel does not appear possible — at least, possible in the sense that humans would survive it — with the physics that we use today, the field is constantly changing. Advances in quantum theories could perhaps provide some understanding of how to overcome time travel paradoxes.

One possibility, although it would not necessarily lead to time travel, is solving the mystery of how certain particles can communicate instantaneously with each other faster than the speed of light.

In the meantime, however, interested time travelers can at least experience it vicariously through movies, television, and books.

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