Space and time are intertwined, called space-time, and gravity has the ability to stretch space-time. Objects with a large mass will be able to stretch space-time to the point where our perception of it changes, known as time dilation. The more mass an object has, the more it stretches and slows down time.
For example, Sagittarius A* – the gigantic black hole at the centre of the galaxy – would almost be able to stretch time to a point where it almost comes to a complete standstill.
Sagittarius A* has a radius of 22 million kilometres and a mass of more than four million times that of the Sun.
In other words, it is very dense.
And because it is so heavy, it has the ability to completely stretch out space-time, and travelling towards its centre means time would almost come to a standstill for you.
However, if you were somehow able to travel back out of a black hole, you could theoretically reverse the arrow of time.
Jeff Koch, a former physics professor, wrote on Q&A site Quora: “Time does lose its arrow in a black hole. You could move back and forth in time.”
But there is a major stumbling block if you were trying top achieve this.
The gravitational pull of a black hole is so immense that not even light can escape its grasp, so once you are closing into the singularity, a one-dimensional point where gravity becomes infinite and space and time become curved, there is no turning back.
There are a few ways in which a black hole can form.
Scientists believe the most common instance is when a star, thousands of times the size of the Sun, collapses in on itself when it dies – known as a supernova.
Another way is when a large amount of matter, which can be in the form of a gas cloud or a star collapses in on itself through its own gravitational pull.
Finally, the collision of two neutron stars can cause a black hole.
The gist of all three ways is that a massive amount of mass located in one spot can cause a black hole.