Speaking of going back in time, we have to mention four-dimensional space. Space consists of three dimensions: length, width and height. When we move from one place to another, our coordinates in three-dimensional space are also changing. However, it is difficult for us to realize that we are also moving quietly in the dimension of time.
Charles Liu, an astrophysicist at City University of new york, said that time and space combine to form a four-dimensional space-time which can be regarded as Zhang Siwei's organic fiber cloth. When a mass object, such as a person, an object or a star, is placed on this cloth, wrinkles will be formed on it. This fold is a space-time bend to accommodate mass.
Two theories of time reversal
Brian Green, an author of The Perfect Universe and a physicist at Columbia University, said that there are many theoretical models about time reversal in the scientific community, but most of them do not conform to the principles of physics.
The most mature theory at present is called "wormhole theory". A wormhole is an imaginary channel connecting two space-time regions. The two ends of the connection can be two different universes or two parts of the same universe. Objects can pass through wormholes.
Micio Kaku, a physicist at City University of new york, said that wormholes can lead to the future or the past. However, the energy required by this theory far exceeds the energy that can be provided by today's technology. This energy is equivalent to the total energy contained in a star, and even requires theoretical "negative energy".
Another popular theory is called Cosmic String. The theory holds that the early universe left a narrow energy pipeline, which extended the length of the whole universe. This area has supermassive matter, which can distort the surrounding space-time. Richard Gert, a professor at Princeton University, said that cosmic strings are infinitely long, or closed. If two cosmic strings are put together, space-time will be greatly distorted, which may make time to go back. However, this theory also needs huge energy to support.
Scientists say that although we can find a way to go back to the past mathematically, no scheme is feasible at present, and the required technology far exceeds what human beings have today. However, scientists believe that it is very possible for human beings to invent a time machine on the basis of new theories one day.