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Black Holes for Dummies

When you read about black holes and the complicated physics behind them, it’s hard not to feel that your own brain is falling into a mental black hole and your IQ is becoming infinitely smaller. Although our puny brains may never understand the complex mathematics behind this phenomenon, the theories that they support can be explained relatively easily. Let’s start with the formation of black holes: how have these all-consuming monsters come to be?

To be clear, astronomers are still unsure as to how supermassive black holes form (which are black holes found at the center of most galaxies), so we’ll only be discussing the formation of stellar black holes. These black holes arise from the death of stars when they run out of nuclear fuel. However, not every dying star will become a black hole. The greater the mass of the star, the greater the effect of gravity, and the greater the collapse of the remaining object. Stars that are about the size of our Sun or smaller will die peacefully by collapsing into a white dwarf. Stars that are six times the size of our Sun or greater will undergo a violent explosion, called a supernova. This explosion sometimes leads to the formation of a neutron star, which is when the star’s core collapses and its protons and electrons essentially melt to form neutrons. Other times, the supernova forms a black hole.


The formation of a black hole (Image credit: geek.com)

During the normal life of a star, there is a constant interplay between gravity pulling in and pressure pulling out. For a majority of the star’s life, gravity and pressure balance each other, keeping the star stable. However, when the star runs out of nuclear fuel and explodes, gravity will have the upper hand and will forcibly compress the core of the star. The large dying star’s outer parts will fly wildly out into space and its remaining core will collapse completely into an infinite space-time curvature at a single point.

In order to truly wrap your head around that last sentence, it’s appropriate to refer back to Einstein’s theory of general relativity. When Einstein wrote the theory, he discovered something revolutionary about gravity. It was not a force, as described by Sir Isaac Newton, but a consequence of a distortion in space and time. Einstein proposed that there are three spatial dimensions (up-down, forward-back, and left-right) and one time dimension (measured in seconds). Based on their mass, objects warp the fabric of space-time. The greater the mass of the object, the greater the distortion.

Imagine a bowling ball sitting at the center of a trampoline, causing the fabric to dimple around it. This is also how planets and stars distort space-time. Now if we placed a marble onto the trampoline, it would follow the curve down toward the bowling ball. The planets in our solar system are, like the marble, following the curve of space-time produced by the Sun, much like our bowling ball. The planets do not fall into the Sun because the speed at which they are traveling is enough to escape the gravitational pull.


A representation of space-time (Image credit: NASA)

John Wheeler, who coined the phrase “Black Hole,” described this concept beautifully:

“Matter tells space-time how to curve, and space-time tells matter how to move.”

Along with this concept of space-time, another discovery was made by yet another notable German physicist, Karl Schwarzschild. He found that the escape velocity from the surface of a given object is determined by its mass and its radius. For instance, a rocket must reach a speed of 11.2 kilometers per second in order to depart Earth. Departing the Moon, in contrast, only requires an escape velocity of 2.4 kilometers per second because the Moon is one fourth the size of our planet and possesses less than 1% of the Earth’s mass.

Therefore, theoretically, if an object has a radius small enough, the escape velocity will increase until it reaches the speed of light, the fastest speed our universe knows. At this point, neither matter nor light can escape the object’s surface. Also, the atomic and subatomic forces are unable to keep the object from collapsing in on itself, which means that the object caves-in to an infinitesimal point, which we call the singularity. These ideas gave birth to the theory of black holes.

Although this theory has been touted for decades, no one has actually ever physically observed a black hole. Scientists cannot identify black holes as they normally would using telescopes that detect light, x-rays, or other electromagnetic radiation. However, we can detect their effect on matter nearby and infer their existence. If a black hole passes through a cloud of interstellar matter, it will draw that matter inward by way of a process called accretion. The attracted matter will accelerate and heat up, emitting x-ray radiation that can we can measure. Black holes also have a noticeable affect on light; the incredible mass of the black hole creates gravitational fields that bend light as it passes by. This phenomenon is called gravitational lensing.


Gravitational lensing by a black hole (Image credit: Oliver James et al (2015), Classical and Quantum Gravity)

Now that we have an understanding of the forces that govern black holes, we’ll allow our minds to wander into the “what-if” scenarios. If we could directly observe a black hole, what would it look like?

It would be quite difficult to know when you came upon a black hole because they swallow and do not reflect light, meaning that they would be colorless, spherical entities. However, if you got close enough to the event horizon, which is the edge of the black hole after which there is no escape, you would soon realize that something was afoot. Assuming that you fell toward the stellar black hole head-first, the incredible force of gravity would pull your head away from your feet, making you a human laffy taffy. Eventually, you would simply become a stream of atoms, traveling down to the singularity.


An object falling into a black hole (Image credit: DNews)

An outside observer could actually watch you as you are stretched to death, but it would take a very long time. As you fall toward the event horizon, photons take longer and longer to climb out of the gravitational well of the black hole, which to the observer, makes it look as if you have slowed down. When you finally reach the horizon, any photon emitted will take an infinite amount of time to reach the observer. You would appear to freeze! However, this is an illusion, as you will eventually pass the event horizon.

Apart from just being incredibly mind-blowing, the physics of black holes can give us clues to the formation of the universe. They are also capable of clarifying and verifying the laws that govern physics. Of course, as with any theory, this black hole dogma is still a hot debate in the physics community. So, as we cannot prove anything for certain about black holes, you are welcome to venture down the mental rabbit hole of the physical possibilities.

Sources & Citations

“Black Holes.” Physics Central. American Physical Society, n.d. Web. 31 Jan. 2016.

“Black Holes – NASA Science.” Black Holes – NASA Science. NASA, n.d. Web. 31 Jan. 2016.

GaBany, R. J. “Stretching the Fabric of Space.” Cosmotography. N.p., n.d. Web. 31 Jan. 2016.

“HubbleSite – Reference Desk – FAQs.” HubbleSite – Reference Desk – FAQs. STScl, n.d. Web. 31 Jan. 2016.

Jones, Andrew Zimmerman. “What Exactly Is a Black Hole?” About.com Education. About.com, 15 Dec. 2014. Web. 31 Jan. 2016.


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