Arguably the weirdest objects in space, black holes are both understood and a mystery, even though almost every galaxy has one at its center.  The nature of black holes and what they are go back to two fundamental aspects in physics: general relativity and quantum mechanics.  General relativity is a theory of gravitation first developed by Albert Einstein.  This theory deals with massive objects.  It also points out the existence of spacetime. Spacetime is like a stretchy sheet that encompasses four dimensions: length, width, height, and time. Spacetime can be curved by massive objects like planets, stars, and black holes, and we perceive that as gravity.  The more massive the object, the more curved spacetime gets, the stronger the gravity gets, the slower the passage of time gets.  While general relativity deals with the massive, quantum mechanics deals with the super tiny, such as atoms and how they act.  Physicists are on a large quest to find a ‘theory of everything’, which unites both general relativity and quantum mechanics, the super large and the super tiny.  The data to reconcile these two theories lies within black holes.

To understand why this is true, it first must be established how black holes are formed.  Black holes occur when a star’s core over 2.8 times the mass of our own sun dies and collapses.  This is due to the star’s hydrogen atoms collapsing under their own gravity.  Nuclear fusion creates tremendous amounts of energy that push against gravity.  One of the fused elements created is iron, which doesn’t produce energy to push against gravity.  Eventually, the iron builds up so much that the balance of gravity pushing against the nuclear fusion is suddenly broken, making the core collapse as the star dies in a supernova.  Since the star’s core is over 2.8 times the mass of our sun, the entire mass of the core collapses into a single point, and a black hole is formed.  Once the core’s size is about 18 km, the velocity needed to fling an object off and escape the black hole is equal to the speed of light, which is the fastest anything can travel.  This means nothing, not even light, can escape from a black hole’s gravity.  This is why black holes are called, well, “black” holes. 

The “hole” part comes from the fact that at the very center of a black hole is a single extremely tiny point called the singularity, where all matter is crushed to an infinite density.  At this point, spacetime is curved to be infinitely steep.  Because the singularity is such a minuscule point but the complete size of the black hole is extremely massive, general relativity and quantum mechanics have to exist with each other, breaking the laws of physics known at this point, but providing the information for a theory of everything.

As mentioned before, the velocity needed to escape a black hole is equal to the speed of light when a star’s core size is about 18 km.  One part of a black hole, called the event horizon, is a radius around the singularity.  Once something passes the event horizon, it must be faster than the speed of light to escape, due to the vast gravity of the black hole.  Once it passes the event horizon, it can’t be known to us since nothing travels faster than the speed of light.

What would happen if you were to fall into a black hole? Two very extraordinary things.  The first is that you would experience a process called spaghettification.  This occurs because, assuming you fell feet first, the gravity at your feet would be much stronger than the gravity at your head, resulting in your body getting stretched out long and your body getting thinner, resembling a noodle.  If the black hole is small, you would get ripped apart before you cross the event horizon.  Otherwise, you would cross the event horizon unharmed but eventually get crushed by the immense gravity and merge with the singularity, where all matter goes that falls into a black hole. During the process of spaghettification, there would also be a major time dilation.  From your perspective, time would pass normally for you yourself, but looking at the outside world you would see time speed up drastically, millions or billions of years passing by in seconds.  It may be possible to see the end of the world. 

 An outside observer sees something different.  They would see you fall, and soon it would appear that you are frozen in time.  They would never see you cross the event horizon.  Rather, you would become redshifted, which is when wavelengths of light get stretched toward the red side of the spectrum due to gravity.  As you approach the event horizon, the gravitational force is so strong that any light you emit or reflect would become infinitely stretched, making it seem to the outside observer that you become redder and more faded.  You eventually disappear, never to be seen again.

Some fun facts about black holes:

• The black hole at the center of the Milky Way is called Sagittarius A* (pronounced ay-star) and is 4 million times the sun’s mass.
• The circle around a black hole is called an accretion disk and is made of gas and dust that heats up and orbits the black hole.
• The light you see on the top and bottom isn’t really there, but rather light reflects the accretion disk both up and down and also curves it.
• Stephen Hawking discovered that black holes can eventually evaporate in a process called Hawking Radiation, where particle-antiparticle pairs separate near the event horizon.  One particle goes into the black hole, decreasing the black hole’s mass, while the other particle escapes.
• Black holes are not vacuums.  You can only get sucked into one if you get close enough.
• The largest black hole, TON 618, is more than 60 billion times the mass of our sun, and light from this black hole takes more than 10 billion years to reach us.