Most of us think that time traveling i.e. going to the future or to the past is impossible ,they are nothing but science fiction. But time machines existed in our universe from very early time ( before the first man came to earth). These time machines are not artificial but nature's most extreme resource -THE BLACK HOLES.
BLACK HOLES-Black holes are the evolutionary endpoints of stars 10-15 times as massive as the Sun.If a star that massive or larger undergoes a supernova explosion ,it may leave behind a fairly massive burned out stellar remnant.With no outward forces to oppose gravitational forces, the remnant will collapse in on itself. The star eventually collapses to the point zero volume and infinite density, creating what is known as a "singularity". As the density increases the path of light rays emitted from the star are bent and eventually wrapped irrevocably around the star .Any emitted photons are trapped into an orbit by intense gravitational field ; they will never leave it.Because no light escapes after the star reaches this infinite density ,it is called a black hole
SPECIAL THEORY OF RELATIVITY CONCERNING TIME TRAVELING- It says that when a particle travels at the speed of light it will go to the future.For an example consider a man in a rocket & the rocket is going at the speed of light then time slows down for the man like his heartbeat will slow down ,his brain will function slowly ,etc. Time runs so slowly for him that what is an hour for him could be a hundred years for the rest of us. Now going to the future is easy but in order to go to the past a body needs to break the speed of light .For example consider the rocket travels at a speed greater than the speed of light then not only time slows down for him but his clock runs backwards enabling him to go to the past.
FRAME DRAGGING - So time traveling is all about achieving and breaking the speed of light. Near a black hole the gravitational force is so strong that space itself is being dragged to the center of the black hole while circulating around it. This is called frame dragging and it occurs from the event horizon( the imaginary circular line around the black hole crossing which no particle can return back) to the center of the black holes . The gravitational force after the event horizon is so strong that even light cannot escape from it or in other words the speed of gravity is exciding the speed of light. So if a particle is thrown near the black hole it will automatically break the speed of light , thus enabling the particle to time travel.
Thus we can say that the BLACK HOLES are the TIME MACHINES of the universe.
N.B. In the point "THE SPECIAL THEORY OF RELATIVITY CONCERNING TIME TRVELING" I have mentioned man as an example to understand the concept .But only subatomic particles can time travel.How ever If a man is thrown into a black hole it may go back in time(since nobody has done it).