According to physicist Albert Einstein, time is not a constant phenomenon as it appears, but is an illusion, and can vary from different vantage points.
Space is three-dimensional, and Time, according to the famous physicist, is the fourth dimension. It also speeds up or slows down, so is actually subjective, as stated in his Theory Of Special Relativity.

Understanding Time

Various fictional and semi-fictional methods are discussed in order to build a time machine. It is supposed to have ‘antimatter’, ‘negative-energy density’ and other exotic technology in order to work. 
New research in Time-Travel suggests it is possible using focused gravitational fields in a doughnut-shaped hole.

Time Machine

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<li>Infinite Cylinder: Also known as Tipler Cylinder is a hypothetical theory of a cylinder capable of handling matter that is 10 times the mass of the sun and rolling it inside.</li>
<li>Cosmic Strings: Is a theory for potential time travel is to make use of the narrow tubes of energy found in the universe, which is believed to be leftover ‘strings’ from the early cosmos, stretching infinitely.</li>
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Theories Of Time Travel

Einstein's theory of general relativity says that Time can be bent, stretched and squeezed, and the four-dimensional fabric with a huge mass creates a dimple, or bending of Space-Time, causing gravity.
This effect of time dilation is proven using GPS satellite technology in space, making astronauts not only travel space but in a slightly different time than the earthlings.

Space-Time

The Theory of general relativity suggests that if an object is faster than the speed of light, which is 299,792 km per second, and in a vacuum(no air). This also would mean that such objects would have an infinite mass and a zero-length, which is not possible.

Light Speed

The Science Fiction’s holy grail, Time Travel has been a popular topic in various books, movies and TV series for decades.
Time Travel is the ability to ‘travel’ between two different points of time, mostly involving a vehicle of some sort, which is able to arrive in the past or future.

Time Travel

Wormholes are points in Space-Time in which the two different points are connected, allowing instant transportation of matter.
Blackholes too can be a possibility for time travel, according to another famous physicist, Stephan Hawking, who envisioned circling around them in the opposite direction.

Wormholes And Blackholes

Time Travel has been popularized in movies like Interstellar, episodes of Doctor Who, Back To The Future, and even the first Superman movie made in the 70s.
Time Travel is an engaging concept, which has plenty of questions, which the scientists are working on. New developments in quantum theories and particle study may provide answers to the various paradoxes.

Culture And the Future

Time Travel, in pop culture and theory, presents many unique situations and paradoxes, the most famous of them being the Grandfather Paradox, which is dealt with in the Terminator movies, along with Back To The Future.
The paradox states that if you travel back to the past and kill your grandfather, then you cannot be born in the first place, at least in that timeline. Some scientists have alternate theories involving a parallel universe to answer this.

The Grandfather Paradox

Traveling through time — possible in theory — is beyond our current technological capabilities. Here are some of the leading time travel theories, delving into the fourth dimension and space-time.