Why black holes are the scariest things in the universe
Halloween is a time for ghosts, goblins, and ghouls to haunt you, but nothing in the universe is scarier than a black hole.
Black holes, which are regions of space where gravity is so strong that nothing can escape, are a popular topic in the media these days. Roger Penrose received half of the 2020 Nobel Prize in Physics for his mathematical work demonstrating that black holes are an unavoidable consequence of Einstein’s theory of gravity. Andrea Ghez and Reinhard Genzel contributed the other half, which demonstrated the presence of a massive black hole at the center of our galaxy.
Black holes are scary for three reasons. You would be shredded if you fell into a black hole created when a star died. Moreover, the massive black holes seen at the heart of all galaxies have insatiable appetites. And black holes are places where physics’ laws are obliterated.
For over 30 years, I’ve been researching black holes. I’ve focused my efforts on the supermassive black holes that lurk at the heart of galaxies. They are inactive most of the time, but when they are active and eat stars and gas, the region near the black hole can outshine the entire galaxy that hosts them. Galaxies where the black holes are active are called quasars. With all we’ve learned about black holes over the past few decades, there are still many mysteries to solve.
Death by black hole
When a massive star dies, black holes are expected to form. When the star’s nuclear fuel runs out, its core collapses to the densest potential state of matter, a hundred times denser than an atomic nucleus. Protons, neutrons, and electrons are no longer discrete particles in this density. Since black holes are dark, they are found when they orbit a normal star. Astronomers can infer the properties of a normal star from the properties of its dark companion, a black hole.
Cygnus X-1, the brightest X-ray source in the Cygnus constellation, was the first confirmed black hole. Around 50 black holes have been discovered since then in systems where a normal star orbits a black hole. They are the nearest examples of about 10 million that are expected to be scattered through the Milky Way.
Black holes are matter’s tombs; nothing, not even light, can escape them. The fate of anyone falling into a black hole would be a painful “spaghettification,” an idea popularized by Stephen Hawking in his book “A Brief History of Time.” In spaghettification, the black hole’s intense gravity would pull you apart, separating your bones, muscles, sinews, and even molecules. In his poem Divine Comedy, Dante described the words over the gates of hell as “abandon hope, all ye who enter here.”
A hungry beast in every galaxy
Observations with the Hubble Space Telescope over the last 30 years have revealed that all galaxies have black holes at their centers. Bigger galaxies have bigger black holes.
Nature knows how to make black holes over a staggering range of masses, from star corpses a few times the mass of the Sun to monsters tens of billions of times more massive. That’s like comparing an apple to the Great Pyramid of Giza.
Astronomers released the first image of a black hole and its event horizon, a 7-billion-solar-mass beast at the center of the M87 elliptical galaxy.
It’s over a thousand times greater than our galaxy’s black hole, the discovery of which earned this year’s Nobel Prize. These black holes are dark most of the time, but when their gravity pulls in nearby stars and gas, they flare into intense activity and pump out a huge amount of radiation. Massive black holes represent a threat in two ways. If you get too close, you’ll be sucked in by the enormous gravity. And if they are in their active quasar phase, you’ll be blasted by high-energy radiation.
How bright is a quasar? Consider flying over a large city at night, such as Los Angeles. The city’s approximately 100 million lights from cars, houses, and streets correspond to the stars in a galaxy. In this analogy, the black hole in its active state is like a light source 1 inch in diameter 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
So far, the largest black hole discovered weighs 40 billion times the mass of the Sun, or 20 times the size of the solar system. Whereas our solar system’s outer planets orbit once every 250 years, this much larger object spins once every three months. Its outer edge moves at a half-light speed. A huge black hole, like all black holes, is hidden from view by an event horizon. At their cores is a singularity, a point in space with infinite density. Because the laws of physics fail, we can not comprehend the interior of a black hole. At the singularity, time stops moving and gravity becomes infinite.
The good news about massive black holes is that you might be able to survive if you fall into one. Although their gravity is greater, the stretching force is less than that of a small black hole and would not kill you. The bad news is that the event horizon marks the abyss’s edge. Nothing can escape the event horizon, so you couldn’t try to escape or report on your experience.
According to Stephen Hawking, black holes are slowly evaporating. In the far future of the universe, long after all stars have died and galaxies have been wrenched from view by the accelerating cosmic expansion, black holes will be the last surviving objects.
The most massive black holes are estimated to take 10 to the 100th power, or 10 with 100 zeroes after it, years to evaporate. The most terrifying objects in the universe are nearly eternal.