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 of space where gravity is so strong that nothing can escape – is a hot topic in 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 result of Einstein’s theory of gravity. Andrea Ghez and Reinhard Genzel shared the other half to prove the existence of a supermassive black hole at the center of our galaxy.
Black holes are scary for three reasons. If you fall into a black hole with a star behind, you will be sliced. And the huge black holes seen in the center of all galaxies have an insatiable appetite. And black holes are places where the laws of physics are being blurred.
I have been studying black holes for over 30 years. In particular, it has focused on supermassive black holes that lie at the center of galaxies. It is often inactive, but when it is active and eating stars and gas, the region near the black hole can overtake the entire galaxy that hosts it. The galaxies in which black holes are active are called quasars. With everything we’ve learned about black holes over the past few decades, there are still many mysteries to be solved.
Black hole death
Black holes are expected to form when a massive star dies. After a star’s nuclear fuel is exhausted, its core collapses into the densest state of matter imaginable, which is a hundred times denser than the atomic nucleus. This is so dense that the protons, neutrons, and electrons are no longer separate particles. Because black holes are dark, they are found when they orbit an ordinary star. The properties of a natural star allow astronomers to infer those of its dark companion, the black hole.
The first confirmed black hole was Cygnus X-1, the brightest X-ray source in the constellation Cygnus. Since then, about 50 black holes have been discovered in systems in which an ordinary star orbits a black hole. They are the earliest examples of the approximately 10 million expected to spread into the Milky Way.
Black holes are graveyards of matter. Nothing can escape from it, not even the light. The fate of anyone who falls into a black hole will be a painful “spaghetti”, an idea promoted by Stephen Hawking in his book “A Brief History of Time.” In the case of sweating, the intense gravitational pull of the black hole will separate you, separating your bones, muscles, tendons, and even particles. As the poet Dante described the words above the gates of Hell in his poem The Divine Comedy: Let go of hope, everyone who enters here.
Hungry monster 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. Larger galaxies contain larger black holes.
Nature knows how to create black holes over an astonishing range of masses, from corpses of stars a few times the mass of the sun to monsters tens of billions of times mass. This is like the difference between an apple and the Great Pyramid of Giza.
Just last year, astronomers published the first-ever image of a black hole and the event horizon, a monster of 7 billion solar masses at the center of the elliptical galaxy M87.
It is more than a thousand times larger than the black hole in our galaxy, for which its discoverers were awarded this year’s Nobel Prize. These black holes are dark most of the time, but when their gravity pulls in nearby stars and gases, they ignite in intense activity and pump out an enormous amount of radiation. Supermassive black holes are dangerous in two ways. If you get too close, the sheer gravity will absorb you. If it is in the active quasar phase, it will be exposed to high energy radiation.
How bright is the quasar? Imagine hovering over a big city like Los Angeles at night. Nearly 100 million lights from the cars, homes and streets of the city correspond to the stars in the galaxy. In this analogy, a black hole in its active state resembles a light source one inch in diameter in downtown Los Angeles that outperforms the city by a factor of hundreds or thousands. Quasars are the brightest things in the universe.
Supermassive black holes are strange
The largest black hole discovered so far weighs 40 billion times the mass of the sun, or 20 times the size of the solar system. Whereas the exoplanets in the orbit of our solar system revolve once every 250 years, this massive body rotates once every three months. Its outermost tip is moving at half the speed of light. Like all black holes, massive holes are shielded from view by the event horizon. In their centers there is a singularity, a point in space where the density is infinite. We cannot understand the interior of a black hole because the laws of physics are disrupted. Time freezes at the event horizon and gravity becomes infinite on singularity.
The good news about supermassive black holes is that you can survive falling into one. Although its attraction is stronger, its expansion force is weaker than that of a small black hole and it will not kill you. The bad news is that the event horizon marks the edge of an abyss. Nothing can escape from within the event horizon, so you cannot escape or report your experience.
According to Stephen Hawking, black holes are slowly evaporating. In the distant future of the universe, long after all stars have died and galaxies have been kept out of sight by accelerating cosmic expansion, black holes will be the last remaining objects.
The most massive black holes would take an unimaginable number of years to evaporate, estimated at 10 to 100 forces, or 10 with 100 zeros after them. The scariest things in the universe are almost eternal.
Chris Embee, Distinguished University Professor in Astronomy, University of Arizona
This article is republished from The Conversation under a Creative Commons license. Read the original article.