What would happen to you if you fell into a black...

Halloween is a time haunted by ghosts, goblins and ghouls, but nothing in the universe is more frightening than a black hole.

Black holes – regions in space where gravity is so strong that nothing can escape – is a hot topic on the news these days. Half of the 2020 Nobel Prize in Physics was awarded to Roger Penrose for his mathematical work showing that black holes are an inevitable consequence of Einstein’s theory of gravity. Andrea Ghez and Reinhard Genzel split the other half to show that there is a massive black hole in the center of our galaxy.

Black holes are scary for three reasons. If you fell into a black hole left over when a star died, you would be shredded. The massive black holes at the center of all galaxies also have insatiable appetites. And black holes are places where the laws of physics are wiped out.

I’ve been studying black holes for over 30 years. In particular, I focused on the supermassive black holes that lurk in the center of galaxies. Most of the time they are inactive, but when they are active and eating stars and gas, the region near the black hole can outshine the entire galaxy in which they are located. Galaxies in which the black holes are active are called quasars. With everything we’ve learned about black holes over the past few decades, there are still many puzzles to be solved.

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Death by black hole

Black holes are expected to form when a massive star dies. After the star’s nuclear fuel is exhausted, its core collapses into the densest conceivable state of matter, a hundred times denser than an atomic nucleus. It’s so dense that protons, neutrons and electrons are no longer discrete particles. Because black holes are dark, they are found when they orbit a normal star. The properties of the normal star allow astronomers to infer the properties of its dark companion, a black hole.

The first confirmed black hole was Cygnus X-1, the brightest X-ray source in the Cygnus constellation. Since then, around 50 black holes have been discovered in systems in which a normal star orbits a black hole. They are the closest examples of roughly 10 million expected to be scattered across the Milky Way.

Black holes are graves of matter; nothing can escape them, not even light. The fate of a person who falls into a black hole would be a painful “spaghettification”, an idea that Stephen Hawking popularized in his book “A Brief History of Time”. In spaghettification, the black hole’s intense gravity would pull you apart, separating your bones, muscles, tendons, and even molecules. As the poet Dante described the words above the gates of hell in his poem Divine Comedy: give up hope, all who enter here.

A hungry animal in every galaxy

Over the past 30 years, observations with the Hubble Space Telescope have shown that all galaxies have black holes at their centers. Bigger galaxies have bigger black holes.

Nature knows how to make black holes over an amazing range of masses, from star bodies a few times the mass of the sun to monsters ten billion times as massive. It’s like the difference between an apple and the Great Pyramid of Giza.

Just last year, astronomers released the first image of a black hole and its event horizon, a 7 billion solar-mass animal at the center of the M87 elliptical galaxy.

It is more than a thousand times larger than the black hole in our galaxy, the discoverer of which grabbed this year’s Nobel Prize. These black holes are dark most of the time, but when their gravity attracts nearby stars and gases, they flicker in intense activity and pump out a large amount of radiation. Massive black holes are dangerous in two ways. If you get too close, enormous gravity will suck you in. And when they are in their active quasar phase, they are emitted by high-energy radiation.

How bright is a quasar? Imagine floating over a big city like Los Angeles at night. The 100 million or so lights on cars, houses and streets in the city correspond to the stars in a galaxy. In this analogy, the black hole in its active state is like a 1-inch diameter light source in downtown LA that outshines the city by a factor of hundreds or thousands. Quasars are the brightest objects in the universe.

Supermassive black holes are strange

The largest black hole discovered to date weighs 40 billion times the mass of the Sun or 20 times the size of the solar system. While the outer planets of our solar system orbit once every 250 years, this much more massive object rotates every three months. Its outer edge moves at half the speed of light. Like all black holes, the giant ones are protected from view by an event horizon. At their centers there is a singularity, a point in space where the density is infinite. We cannot understand the inside of a black hole because the laws of physics are breaking down. Time freezes at the event horizon and gravity becomes infinite at the singularity.

The good news about massive black holes is that you could survive falling into one. Although its gravity is stronger, the stretching force is weaker than a small black hole and would not kill you. The bad news is that the event horizon marks the edge of the abyss. Nothing can escape within the event horizon, so you cannot escape or share your experiences.

According to Stephen Hawking, black holes are slowly evaporating. In the universe’s distant future, black holes will be the last surviving objects long after all stars have died and galaxies have been torn from view by accelerated cosmic expansion.

It will take an unimaginable number of years for the most massive black holes to evaporate, estimated at 10 to the power of 100, or 10 with 100 zeros after that. The scariest objects in the universe are almost forever.

This article was republished from The Conversation by Chris Impey, professor of astronomy at the University of Arizona, under a Creative Commons license. Read the original article.