One of the greatest challenges facing our physicists today are the mysteries of black holes. After being coined by John Wheeler in 1967, the phrase ‘black hole’ has spread throughout the planet. But yet we know very little about black holes.
Black holes are too far out into space for us to approach. And even if we had the technology to travel that far into space, black holes are certainly too dangerous to approach. Most of what scientists believe about black holes is based on theory alone. Our only means of actually observing black holes are viewing the effect of the gravity surrounding the black hole on other celestial bodies, and measuring the radiation emitted by black holes.
Physicists theorised that black holes are formed on the deathbed of stars, when the star collapses inwards due to its own gravity. The result is a force of gravity so large that it bends the very fabric of space, curving into an infinite hole.
If we were to send a space probe into a black hole, current physicists believe that it would be torn into pieces before being completely crushed and sucked into the center of the black hole. Additionally, all signals that it tries to send back to us would be instead be sucked to the center of the black hole. Not even light can escape the insane gravity of the black hole.
Despite our lack of information, over the past two centuries physicists have come up with a theory for what is at the center of a black hole. At the center of the black hole, gravity is at its most extreme levels. It focuses into a singularity around which all matter is crushed into an infinitely small point of an infinitely large density. However, this point is so small and the force of gravity is so large that it exists outside dimensions.
Imagine a ball of foam. You can apply a force with your hands to squeeze that ball. As you do so, the ball loses air pockets and becomes denser. At the same time, that ball becomes smaller. The more you squeeze the ball the small and denser it becomes. Now imagine that you can squeeze it so much that its size is next to non-existent. If you replace the ball with a star, then you would have the center of a black hole. Your hands would act as the gravity surrounding the black hole. Such a center is an infinitely small and infinitely dense piece of matter surrounding a point of singularity of gravity.
Theoretically, there are two possibilities surrounding what would happen should one fall towards the singularity. For Schwarzschild (non-rotating) black holes, any mass falling towards the singularity would be crushed into an infinitely small size of an infinitely large density, which would become part of the mass at the center of the black hole. However, a mass falling into a Reissner-Nordström (rotating) back hole could potentially avoid the singularity. This theory extends to suggest that it is possible to exit such a black hole in another time. However, traveling back in time creates a grandfather paradox.
Grandfather paradox: Physicists theorize that it may be possible to travel to the future but not to the past. This is because if one travels to the past, it would be possible to conduct an action which prevents them from time-traveling. However, in that case they wouldn’t have time-traveled and hence wouldn’t have prevented themselves from time traveling. This leads to the creation of two separate situations in the same time, which is theoretically impossible. It is called the grandfather clause since the original example was that the time-traveler killed his own grandfather, which prevented his birth, hence preventing his action of traveling back in time and killing his grandfather.