Anything with an escape velocity greater than the speed of light is a black hole. That’s the black hole’s defining characteristic: Gravity so intense nothing can escape. The radius around the black hole, where the escape velocity equals light speed, is called the event horizon, the point of no return. These extreme properties provide some opportunities for nerdy bumper stickers or t-shirts, such as Black Holes Suck, Black Holes are Out of Sight, or Black Holes are Where God Divided by Zero.
That last one is from the comedian Stephen Wright, a pretty weird guy in his own right, and derives from the notion that anything within the black hole collapses down into a point of infinite density, infinitely destructive. That point is called a singularity.
That’s all you get about black holes from the movies: They’re dark monsters flying through space sucking up planets and stars and anything else that gets in their way. But that barely touches all the wonderful weirdness that black holes bring to the universe.
Near a Semi-Local Black Hole
Let’s take a stellar-mass black hole in our own Milky Way. V-4641 Sagittarii is a binary star system that contains a regular star and a black hole joined together in their orbital dance. Astronomers believe that the most massive stars exploding as supernovas can leave behind collapsed cores. Any core more massive than about three times the mass of our own sun form into black holes.
Stellar mass black holes like this have a number of extreme properties that make them among the weirdest places in the universe. The first is the fact that gravity depends on distance, and the differences in the pull of gravity on the near side of an object compared to the far side is known as a tidal force. Stellar black holes possess wicked tidal forces that rip apart anything approaching too close like taffy. Illustrations in books and animations in documentaries show astronauts being stretched out like Mr. Fantastic. How do they survive like that? The short answer is that they don’t, not without science fictional technology like “tidal-gradient compensators” or the like. The astronauts would be pulled apart in a stream of blood and guts as in Larry Niven’s classic story “Neutron Star” (another dense object with wicked tidal forces), putting the worst scene in the movie Event Horizon to shame.
But while tidal forces can be extreme around black holes, they’re not totally alien. Milder tidal forces are visible here on Earth to people who live on the coasts and understand the moon’s effect on the oceans. Black hole tidal forces could make for gory pictures, but other things that go on near a black hole are much stranger.
The Weird Gets Weirder
The basis of Einstein’s general relativity is that that a gravitational field is indistinguishable from acceleration. If someone in a room with no windows were being accelerated through space at 9.8 meters per second every second, and they dropped a glass, it would crash to the ground at the same speed that a dropped glass on Earth would. If they jumped, they’d rise up and fall back down the same way they would on Earth. In other words, as long as that person didn’t open up a wall and look out a window—they’d think they were still on Earth, being held to the floor by gravity.
The gravitational field at the event horizon has a lot of similarities to pushing light speed, with a similar warping of space-time. If you were to callously toss your least favorite astronomer into a nearby black hole to watch him fall in, for educational purposes only, of course, two bizarre effects would become apparent.
First, he or she would experience gravitational time dilation. Time appears to pass more slowly for someone deeper in a gravitational field. Normally this effect isn’t large, but black holes are good at pushing effects to extremes. His watch would slow down, and he would appear to move more slowly. Basically, it would be like watching a slow motion Zack Snyder action sequence as in 300, only the cry would be “This is a black hole!” as our guinea pig falls in. And, approaching the event horizon, the slow motion would become infinitely slow. Now, our intrepid educator would experience the passage of time normally, but, from a safe distance, he would appear to stop, frozen in time at the event horizon, never quite falling in.
A quick aside. Before John Wheeler coined the term “black hole,” the term “frozen star” was in parlance. The collapsing core of a supernova would be seen to slow and freeze in time as it reached its event horizon. It’s not clear that every black hole was formed from a collapsing star, so “frozen star” might not be a good term, generally speaking, and certainly stars are not cold and frozen like ice, so it’s no wonder the mysterious phrase “black hole” won out in the end, although apparently it doesn’t translate well into French. (Yeah, it means something dirty.)
Why Black Holes are Black
So, if everything falling into a black hole slows down and never quite appears to fall all the way in, wouldn’t that mean that black holes aren’t black? Wouldn’t they look like some mishmash of everything they ever swallowed?
The short answer is no.
This is where the second seriously weird effect kicks in: Gravitational redshift. Imagine our poor astronomer trying to explain what’s happening as he falls toward his doom, signaling with his blue laser pointer (he’s on the cutting edge of technology). As he approached the event horizon, we would notice the color of his laser pointer changing. Different colors of light have different frequencies—blue being the most energetic. The frequency of any beam of light is the number of times the peak—or the trough, or the middle—of the waves pass a certain point in a certain amount of time. As time dilation occurs, the number of “peaks” that pass by in a certain amount of time decreases, and the frequency of the photons flying toward us would decrease, their wavelengths also being stretched out, losing energy. Blue photons, the most energetic visible-light photons, would shift along the spectrum, toward green, then yellow, orange, red, and into the infrared and beyond. That redshifting, toward lower energy and redder colors on the spectrum, would continue on through radio waves until individual photons would be so stretched out with such a low frequency that they effectively vanish.
At the event horizon, the redshift would become infinite and the laser would fade to black.
Black Holes Weirdness for Real and for Fun
Gravitational time dilatation and redshift are odd effects. They are so entirely foreign to everyday experience that they may seem the product of imagination rather than realistic effect. Incredible as they are, they have been verified in experiments here on Earth. Earth has a pretty measly gravitational field compared to that near a black hole, but we have the technology to measure the effect. Atomic clocks based near the Earth, and therefore in relatively high gravity, show different times than atomic clocks in low gravity far away from the Earth. Lasers do change color. Einstein’s predictions are matched perfectly.
Black holes may seem entirely destructive, but they’re not necessarily the malignant entities we see from the outside. It has been proposed that regions of stability that might exist inside rotating black holes, that they might be havens for alien civilizations to persist unmolested by the rest of the universe. Somewhere inside the event horizon might be sanctuary.
Black holes are fascinating and excellent fodder not only for science but also for science fiction. In some ways, they’re better explored in science fiction. While astronomers have only been discovering and studying black holes for a few decades in the most basic of ways, their wonderful weirdness can be enjoyed in fiction in all the infinite ways writers can imagine. Mathematicians may not like dividing by zero, but when nature does it, it’s fun, and writers can do it, too.
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