While writers have produced some great ideas for time machines over the years, a real-life time machine has yet to be built. Most theories of time travel don't rely on machines at all. Instead, time travel will likely be done by way of natural phenomena that will transport us instantly from one point in time to another. These space phenomena, which I am not even sure exist, include:
Cosmic Strings 1) According to me the best bet for time travel would be - "Wormholes", if they do exist. Wormholes are considered possible based on Einstein's theory of relativity, which states that any mass curves space-time. To understand this lets think about two people holding a bed sheet up and stretching that sheet 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.
Here, space is shown as a two-dimensional plane rather than the four dimensions that actually make up space-time. Now imagine that this sheet is folded, 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 is 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 one of the ways how wormholes may form, really fascinating, isnt it?
In space, masses that place pressure on different parts of the universe could eventually come together to form a tunnel -- this is a wormhole. We could then travel from Earth to another galaxy and back relatively quickly (within a lifetime). I am saying a bit quickly because hypothetically speaking, we are traveling for a shorter distance at much high speed. Why we need to travel through the worm holes at the speed of light is something that I cant put here in this blog. If wormholes could be discovered, it might allow us to travel to the past as well as the future, though there are infinite complications and intricacies involved in this mind blowing journey.
2) Another way to time travel is by using the concept of cosmic strings -which are string-like objects that some scientists believe were formed in the early universe. These strings may line the entire length of the universe and are under immense pressure -- millions upon millions of tons. These cosmic strings, which are thinner than an atom, can generate an enormous amount of gravitational pull on any objects that pass near them, just like the black holes. Objects attached to a cosmic string could travel at incredible speeds, and because their gravitational force distorts space-time, they could be used for time travel. By pulling two cosmic strings close together, or one string close to a black hole, it might be possible to warp space-time enough to create closed time-like curves.
A spacecraft could be turned into a time machine by using the gravity produced by the two cosmic strings, or the string and black hole, to propel itself into the past. To do this, it would loop around the cosmic strings. However, I read somewhere 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. Also it has been speculated that one cant go back farther than the point at which the time machine itself was created.