Time Travel : A quick guide

Time Travel has been a well recognized concept in scientific philosophies and writer's fiction from an early time as long as the 1890's.

Well it all kick-started going crazy into the public after the debut of H.G Well's 1895 novel "The Time Machine", if you haven't you should give it a read. It is an awsome book for sci-fi lovers.

Let's get back on the scientific track. What made people think about time-travel?
It's the simple idea of how amazing it would be to see or change the past/future drives the idea of time-travel. 
Considering time as a dimension helped physicists think about moving between two points of time.
We all know there are three dimensions in the physical world which are labeled using the terms length, width, height, depth or breadth. Over the last century scientists have made a mathematical extension to these three dimensions making use of a fourth dimension ie. Time.

A visualisation of three dimensions

As per now there are no perfect visualisations of the 4 dimensions of reality but this should how it look:

The hypercube is also known as a "Tesseract"

Let's now get into some theories about time:

While most people think of time as a constant, physicist Albert Einstien 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.

Some other theories by physicists also suggest conversely time is real,

As quoted by Lee Smolin : "In the physicist's conception of nature, as developed from Newton to Einstein, time becomes a secondary concept. It becomes replaced by a notion of computation, so that a process carrying out in time and causing things to happen becomes modeled by a logical computation. Logic and mathematics are outside of time, and therefore if that modeling is completely accurate, time is unreal."
(Read more about Time and Black holes)

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 clock gains 38 microseconds per 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.

General relativity also provides us with theoretical passages through space-time fabric called wormholes (as seen in the movie "Intersetellar"). These passages are belived to be shortcuts through space which might possibly reduce the travelling time from one point to another, kind of making sense for FTL (faster than light) travel, effectively considering it time travel. But, This appears to be physically impossible, although some scientists have extended his equations and said it could be done.

A linked possibility, NASA states, would be to create wormholes. 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 know today. 

Though not technically viable, This would how a wormhole looks like

There also happens to be some historic mentions of time travel too:

Some ancient myths depict a character moving through time. In Hindu mythology, the Mahabharata mentions the story of King Raivata Kakudmi, who travels to heaven to meet the creator Bramha and is surprised to learn when he returns to Earth that many ages have passed. The Buddhist Pali Canon mentions the relativity of time. The Payasi Sutta tells of one of the Buddha's chief disciples, Kumara Kassapa, who explains to the skeptic Payasi that time in the Heavens passes differently than on Earth. The Japanese tale of "Urashima Taro", first described in the Nihongi (720) tells of a young fisherman named Urashima Taro who visits an undersea palace. After three days, he returns home to his village and finds himself 300 years in the future, where he has been forgotten, his house is in ruins, and his family has died. In Jewish tradition, the 1st-century BC scholar Honi-ha-M'agel is said to have fallen asleep and slept for seventy years. When waking up he returned home but found none of the people he knew, and no one believed he is who he claims to be.

Some other theories that could possibly suggest time travelling are :

Gravitational Time Dilation

Cosmic Strings

Human made Time Machines as depicted in movies, novels, comics etc.

                                                                                                                             

                                                                                                                                       - Vamsi k

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                                                                                                               Published by - Vamsi K