Applications of gravity

Most of the applications of gravity have not yet been experimentally proven for example the idea of a time machine simply sounds ridiculous but theoretically it seems like a good idea. The main problem with the time machine aspect is the lack of the materials like the environment needed for the shell method is not possible here on earth.
Even the twin paradox setup is practically not possible because nothing could move as fast as the speed of light although the sound barrier has been broken the speed require for the twin paradox setup is by far faster than the speed of sound at least 3 quarter that of the speed of light. Not all of the applications are theoretical the application of the equations derived from the gravitational field is one of them they have been used to solve so many problems involving gravitational fields. In the preparation of astronauts for space travel gravity is well used because the forces of gravity vary depending on the planet and for a person to perform or work in those conditions they have to be trained and taken through certain preparatory training exercises.
As the weakest of all the fundamental forces but yet the most important and most applied gravity and its applications have helped us to understand our universe and our immediate environment more and without the knowledge of it many inventions today would not be possible.