In this post, fundamental concepts related to time travel are presented. We would first see some related basic concepts and then try to explore some ways in which time travel may be achieved.
Spacetime
Spacetime is the most important fundamental topic in order to understand the advanced topics related to time travel. It is defined as a mathematical model that unifies the three spatial dimensions and the one time dimension into a fabric of fourdimensional manifold.
Acceleration due to gravity
Whenever an object goes up, it falls down. Now, when that object falls down, then it does that with some acceleration, and this acceleration is caused by the gravity. Irrespective of the size of an object, when it falls down, it does that with the same constant rate of accelerations due to gravity. In scientific terms, we denote gravity as \(g = 9.81 m/s^2\).
Theory of Special Relativity
The faster you move through space, the slower you move through time. This implies that time also appears to flow slower for a person standing still from the perspective of one who is moving.
TimeDilation effect
According to Einstein’s general theory of relativity, the closer you are to the center of the earth, the more gravity you feel, the slower will be the time. In a way, it can also be said that the if you are living at some higher altitude then time will move faster than if you have been at some lower altitude. This is often termed as timedilation effect. Spacetime is warped or distorted more in regions which is impacted more by gravity. Speed of light is constant in a vacuum and therefore the distance of light increases with the presence of such warped/distorted spacetime. in turn, the time gets slower for the light to travel such a distance.
In the figure shown below, the red line is the path of the beam of light as if there were no gravitational influence, and shown in blue is the light traveling through the warped gravitational field at the same speed.
What are Tachyons?
Tachyons are considered a theoretical, and in some sense, imaginary particles that is considered to have a potential of travelling faster than the speed of light. In reality, there is nothing in this universe that can travel faster than the speed of light.
What is a wormhole?
In theory, wormhole (sometimes called Einstein–Rosen bridge) can be thought of as a bridge/tunnel that connects one point to another in spacetime. It can be seen in the picture shown below:
Wormhole 
What is a black hole?
A black hole is essentially a region of space within which light doesn’t come out. It is because of the graviational singularity, a onedimensional point which contains huge mass in an infinitely small space, where density and gravity become infinite and spacetime curves infinitely, and where the laws of physics as we know them cease to operate.
Extra Dimensions
The dimension is usually referred to as geometric setting for estimating the position of an object. We often talk in terms of the usual 3dimensional space as there are observable three spatial dimensions, but we can have more, possibly time and some non observable dimensions. In general, the hierarchy of the dimensions can be described as:

One dimension – Length (move left and/or move right)

Twodimension – Length (move left and/or move right) and Width (move forward and/or move backward)

Three Dimension – Length (move left and/or move right), Width (move forward and/or move backward) and Height (move up and/or move down)

Fourth Dimension – Length (move left and/or move right), Width (move forward and/or move backward) , Height (move up and/or move down). Time forms the fourth dimension and together with the spatial dimensions it forms the observable spacetime.

Fifth Dimension – All the four dimensions that can be seen in a variation of the world we are currently living. Here, variation implies the world starts in the similar fashion as our own world, however, the occurrence of events between the other world might be different or varying.

Sixth Dimension – Plane of all variations of possible world seen in fifth dimension.

Seventh Dimension – Possible worlds starting with different intial configurations.

Eigth Dimension – Plane of all such possible worlds seen in the seventh dimension.

Ninth Dimension – We can see all the possible worlds with different initial histories and subsequent different variation of occurrence of events.

Tenth Dimension – We can see every possible variations of every world. The complete unimaginable observation power comes in this dimension.
The dimensions (510) often consists of something that can’t be percieved in practice, however, this forms a nice basis for string theory.
String Theory
It is a theoretical framework that unifies quantumn mechanics and einstein’s theory of general relativity. Instead of having a zerodimensional point particles, string theory suggests to have onedimensional stringlike objects.
Useful Resources
Five ways to travel through time
NIST Clock Experiment Demonstrates That Your Head is Older Than Your Feet