Apr 10 · 10 min read

Black holes have fascinated scientists (and the general public) for a century. They are among the most bizarre things in the Universe, bringing together the greatest forces in the Universe. It is at this juncture where greatest extremes of gravity meet the smallest scales in physics, as they operate within the strange world of quantum mechanics. The ideas, terms, and concepts can be challenging to understand, even for those who dedicate their lives to studying these bizarre features of nature.

Here are the top 10 things any layperson interested in astronomy (or science in general) should know about black holes.

## #1 | It’s All About the Escape Velocity

In order to escape the gravitational pull of Earth, a rocket must travel at a minimum speed of around 11 kilometers (seven miles) per second. This escape velocity is determined by the mass of the planet, as well as it’s radius. If the Earth were more massive (and the same size), or if our planet had it’s current mass stuffed into a smaller ball, its escape velocity would be greater than it is under current conditions. Once an object shrinks to its Schwarzschild radius (a size dependent on the mass of the shrinking body), the escape velocity from its surface would reach the speed of light, closing the body off from the rest of the Universe.

If one were to shrink the Earth down to the size of a small marble, the escape velocity of our planet would increase to that of the speed of light. Since no object can travel this quickly (thanks for the insight, Einstein!), not even light could escape the surface of our diminutive world, leaving The Earth as a black hole. Or, if you were to assemble a sphere of water having a radius of 40 astronomical units (roughly the size of the orbit of Pluto), its mass would be so great, the escape velocity of the body would be greater than the speed of light, creating a (much larger) black hole.

## #2 | Black Holes are Marked by Event Horizons

The closer you are to a black hole, the greater your escape velocity will be from the object. There is a point where the escape velocity is equal to the speed of light, and everything closer to that region is invisible to the outside, which is where we get the term “black hole.” Anything — or anyone — journeying past the event horizon would never return to the other side.

This event horizon, set at the Schwarzschild radius of the black hole, would be accompanied by a photon sphere, where a ring of light orbits the body, setting the region aglow.

## #3 | The Accretion Disk Surrounds it all

Outside the event horizon, we can see matter slowly spiral toward the black hole, emitting vast quantities of radiation. Looking like a spiral galaxy, or water going down a bathtub drain, this feature of a black hole is what most people picture when they imagine what it would be like to watch one of these bodies up-close.

As friction builds up between colliding particles, the materials heat up within the surrounding disk. Additional energy is released due to the rotational effects of the accretion disk. The accretion disk glows in energy (including visible light), as it rotates around the black “shadow” of its dark parent. Much like a spiral in a bathtub drain, accretion disks feed gas and dust to their ravenous companion.

## #4 | Black Holes Come in Four Varieties

There are four ways black holes can form, each possessing unique properties.

Primordial black holes — The oldest of all black holes, these formed in the earliest age of matter in the Universe. They are thought to be have very small event horizons (roughly one millimeter, or 1/25”) in diameter. The famed physicist Stephen Hawking stated these miniature black holes could be responsible for the gravitational effects we attribute to dark matter. His theory now appears to be wrong, as there are not nearly enough primordial black holes to create the effects we see.

Stellar black holes — These are the most familiar to most people. Stars spend their lives in a balance between nuclear forces at their core driving mass outward, and gravity squeezing the ball together. As a massive star runs out of fuel, it will collapse under its own weight, fusing heavier and heavier elements. Once the largest stars produce iron, nuclear fusion stops and the star explodes as a supernova. If enough material is left behind, nothing can stop the collapse, the atoms of the star are crushed, and the stellar corpse shrinks until all that is left behind is a black hole. Roughly one out of a thousand stars are large enough to form a black hole.

Supermassive Black Holes — These are the largest of all types of black holes, and are found at the center of nearly every galaxy. The one at the center of the Milky Way Galaxy, Sagittarius A* (Sgr A*), has a mass 400 million times greater than our Sun, caught in a region just over three times larger than the orbit of Neptune.

Intermediate-Sized Black Holes — These black holes are found in masses larger than stars, but they are not behemoths. They are the most-recently discovered form of black holes.

“In the mass range between stellar-mass and supermassive black holes — that is hundreds to hundreds-of-thousands of solar masses — are intermediate-mass black holes. Astronomers have spotted evidence for a handful of candidates, but none have been conclusively detected. Theorists believe there are three scenarios for their formation: They could be primordial black holes, they might have formed in environments dense with stars, or they formed from mergers of stellar-mass black holes,” the National Science Foundation explains.

## #5 | There is More than One Way a Black Hole Can Kill You

Tidal Forces — If you were to travel toward a black hole, the gravitational forces on the edge of your body or spacecraft closest to your target would be far greater than the amount of gravity pulling on you from behind. This would stretch any approaching spacecraft toward the black hole, tearing it into pieces, scattering the debris into a line, swirling inward to the abyss. Astronomers term this effect “spaghettification,” for obvious reasons.

Fried like a Charred Sandwich— Friction between particles in the accretion disk turns gaseous debris there into a highly-charged plasma, frying anything in the area with vast amounts of high-energy radiation.

Jets of Energy — The charged particles within the accretion disk is accompanied by its own, exceptionally powerful, magnetic field. Electrically-charged particles ride these magnetic waves at right angles to the accretion disk, forming powerful jets which head away from the system.

Did I mention radiation? — As the a space-traveler (human or robotic) began their doomed journey past the event horizon, they might be confronted with a powerful wall of radiation, the result of Hawking radiation (more on that later).

## #6 | Even if You Got Back, No One Would Have Witnessed Your Journey

If a unwary traveler were to undertake a foolhardy journey into a black hole (and somehow survived the journey), they wouldn’t experience anything unusual while crossing the event horizon (apart from the tidal forces and tremendous fields of radiation). Their instruments and bodies would all appear normal. However, a person observing the intrepid explorer would never see them pass into the abyss — due to the odd nature of relativity, the viewer would just watch as their partner-in-science seemed to approach the event horizon more and more slowly, never reaching their destination.

## #7 | There are Tens (or Hundreds) of Millions of Black Holes in the Milky Way — But Don’t Worry

The Milky Way Galaxy likely contains at least 100 billion stars, of various sizes and ages. Most of these stars are far too small to every develop into a black hole, but even if one in a 1,000 became a black hole, such numbers could still leave tens of millions of black holes in our galaxy. Fortunately, space is large. The low rate of black hole formation among stars and the vastness of space keep us safe from the threat of these objects, even with the vast number of black holes in the galaxy.

“Most of these are invisible to us, and only about a dozen have been identified. The nearest one is some 1,600 light years from Earth. In the region of the Universe visible from Earth, there are perhaps 100 billion galaxies. Each one has about 100 million stellar-mass black holes. And somewhere out there, a new stellar-mass black hole is born in a supernova every second,” The Hubble Site reports.

“Space is big. You just won’t believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it’s a long way down the road to the chemist’s, but that’s just peanuts to space.”
― Douglas Adams, The Hitchhiker’s Guide to the Galaxy

## #8 | There May be a Small Way Out (a Very Small One)

One of the greatest contributions made by Stephen Hawking to the universe of science was the theory of Hawking radiation, which may describe the only way anything ever leaves a black hole. Sub-atomic particles, created in pairs right at the Schwarzschild radius, could form on either side of the event horizon, allowing one of the partners to escape, while the other half of the pair remained, forever, inside the black hole. Perhaps, for some tiny particles, there remains this small chance to be created free from an eternal abyss.

Science-fiction stories often talk of “white-holes,” from which matter (including, we suppose, spacecraft and people) can travel through a black hole, arriving in another universe. There is some scientific reasoning behind such ideas, but such a journey would require certain types of black holes, in addition to being extremely hazardous. This might provide a very small chance for the survival of a (semi-) intelligent traveler into the great, dark, unknown. But the odds are stacked against such a journey.

## #9 | Black Holes Could Help Answer the Greatest Questions in Science

With the release of the first detailed images of the region surrounding a black hole, we may begin to better-understand the laws of gravitation in the Universe. If rings surrounding black holes are one shape, than relativity, as understood, holds true. If they have another shape, then modified theories of gravity may be needed to understand the data.

When tested by experiment, the predictions of relativity have been found correct to a remarkable degree. The special and general theories of relativity are some of the best-tested scientific predictions in history. It is both beautiful, and practical, perfectly predicting the forces of magnetism, and keeping our GPS accurate on Earth.

Like relativity, the equations of quantum theory, as currently understood, are nearly flawless. Due to our knowledge of these laws, we are able to build transistors, as well as nuclear weapons. Yet, the rules of quantum mechanics is unlike anything in our macroscopic world.

“I think I can safely say that nobody understands quantum mechanics” — Richard Feynman, The Character of Physical Law

The trouble is that both quantum theory and relativity are in opposition to each other. Attempts to reconcile this estranged couple of physics result in equations providing nonsensical answers, showing that one or the other theory, however right, may be fundamental wrong.

If relativity or quantum mechanics is proven conclusively correct, the other may need a good deal of rethinking.

## #10 | Black Holes have Already Revealed the Nature of Spacetime

In 2015, one century after Albert Einstein published his General Theory of Relativity outlining the nature of spacetime, astronomers first detected ripples in the very nature of the Universe.

In September 2015, researchers at the Laser Interferometer Gravitational-wave Observatory (LIGO) recorded ripples in spacetime passing through their detectors. Caused by the collision of a pair of black holes, these ripples traveled across 1.3 billion light years like ripples on a pond, formed as a stone is tossed in the water.

“Based on the observed signals, LIGO scientists estimate that the black holes in this event were about 29 and 36 times the mass of the sun, and the event took place 1.3 billion years ago. About three times the mass of the sun was converted into gravitational waves in a fraction of a second — with a peak power output about 50 times that of the whole visible universe,” the National Science Foundation (NSF) explains.

The following year, the LIGO detector found a second occurrence of gravitational waves. The formation of ripples in spacetime was one of the last predictions of Einstein proven to be correct.

There remains much left to learn about these bodies, and the knowledge we have of how black holes form, live out the existence, and slowly evaporate, will continue to grow. Best of all, with every answer we find, another question is born.

Written by

## Alexandria Science

#### The e-magazine of science, from astronomy to zoology

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