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Real World Physics Problems Newsletter - Black Holes, Issue #40
April 02, 2017

# Black Holes

The first time you heard about black holes you may have thought that they were pure fiction. They certainly have been a part of many science fiction movies. Their amazing properties are certainly the stuff of science fiction, and yet science has proven that they definitely exist. For example, there is a black hole in the center of the Milky Way galaxy, which is the galaxy that we inhabit. It has a mass that is millions of times greater than the mass of the Sun.

Black holes typically form from a dying star which has used up its nuclear fuel. The action of fusing atoms together in the star's core to produce energy ceases, and the resulting internal pressure is no longer able to maintain the star's volume. So the star collapses in on itself due to the very large mass of the star, which in turn creates a tremendous gravitational force that crushes the star matter together. The volume of the star gets smaller and smaller, and increasingly denser as a result, until it reaches a point of essentially zero volume. The gravitational force is so strong that not even the forces between atoms can prevent the collapse. This point of zero volume and infinite density is called a "singularity".

From a gravitational perspective, a black hole is the same as any other celestial object in the universe, provided that you are far enough away from it. It has the same gravitational effect on you as, say, another star of equal mass if you were located the same distance away from it. But if you get close enough to a black hole, what ends up happening turns out to be very different. There is a so-called point of no return that surrounds a black hole, called the event horizon. If you move closer to the black hole than this point you will not be able to ever come back. Beyond this point, the gravity is so strong that not even light can escape. This explains why black holes are black, because they absorb all light that enters them. In fact, the diameter of a black hole is not actually the diameter of it. It is just the border of the event horizon. It can only be seen against a background of star light.

What happens to objects that get trapped inside black holes is that they get torn apart due to the immense gravitational tidal forces, which causes the part of the object located closer to the black hole center (the singularity) to be pulled with much more force than the part of the object located further away from the center. This is simply a consequence of the very large gravitational force gradient that exists in the vicinity of black holes.

But we don't really know exactly what will happen inside the event horizon of a black hole. Eventually, black holes evaporate and disappear over time, which can take place over billions or trillions of years, due to a phenomenon known as "hawking radiation" (crediting the renowned physicist Stephen Hawking for that particular discovery). The movie Interstellar was pretty good at portraying how you will age much slower near a black hole than far from it. This is due to relativity, as proven by Einstein.

Stephen Hawking has suggested that black holes might even be doorways to another universe. This is what can happen when the laws of physics as we know them break down and become incomprehensible, as is the case inside the singularity of a black hole.

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