Emma is one of my regular guest bloggers. I feel really thrilled about the possibility to post one educating post of this great blogger once a month over the next couple of months. Thank you so much, Emma, for sharing these great posts with us! If you would like to check out the previous guest posts, this amazing blogger wrote for me, head over here.
Black holes. They’re some of the most incredible, and most elusive, phenomena in the universe.
Most of the things we know about the universe, we know through data gathered using the electromagnetic spectrum. That’s basically a spectrum of energy wavelengths. Visible light is a tiny, tiny part of it. And everything in the universe emits electromagnetic radiation.
Well, almost everything.
Black holes defy analysis. We can’t reach out and touch the universe, so we need the electromagnetic spectrum—in the form of visible light, infrared radiation, X rays, and others—to know what’s out there. But black holes don’t emit anything.
They just gobble up everything. Their immense, powerful gravitational fields suck in all matter, energy, light, time, etc. that is unfortunate enough to slip past what we call the “event horizon.”
So if we can’t reach out and touch them, and we can’t see them, how do we know they’re there?
First of all, we may not be able to see black holes, but we can see the stuff that gets pulled into them. The problem is, it’s hard to tell if it’s a black hole we’re seeing, or just a faraway object that looks too dim to see from our corner of the universe.
The most direct way to observe a black hole is by using something astronomers call “redshift.” To get a better idea of redshift, let’s take a look at some sound waves.
Have you heard the ice cream truck recently?
I don’t know if the ice cream truck is a worldwide thing, but where I grew up, we heard it all the time. It would drive down the road, its merry tune growing louder and louder as it got closer.
And then as it moved away, it would get quieter and quieter, until we didn’t hear it at all.
I’ll tell you what’s happening here: a simple, sound wave version of redshift and blueshift.
The volume of a sound is determined by the wavelength. Wavelength is a little different for sound than for light, but let’s not argue technicalities here.
Basically, the longer the wavelength, the quieter the sound. The shorter the wavelength, the louder the sound.
As an ice cream truck, ambulance, or really any kind of vehicle making a noise moves toward you, the wavelength of the sound doesn’t change. It seems to get squished, as you see above. The wavelengths thus seem to be shorter, so it seems to be getting louder.
The same goes for anything moving away from you. As the distance between you and the vehicle widens, the wavelength still doesn’t change. But it seems to get stretched, so the sound seems to be getting quieter.
So how does this work for black holes?
Well, this whole redshift/blueshift thing works in a similar way for light. Redshift is what we call it when the light wavelengths get stretched…because red light is just visible light wavelengths being stretched as far as they can go without becoming invisible.
And blueshift is what we call it when light wavelengths get squished, because blue light is just visible light wavelengths being squished as much as they can without becoming invisible.
All this is important because the one defining characteristic of a black hole—at least as far as we can easily observe—is that stuff whizzes around it fast.
The orbital velocities around a black hole are absolutely extraordinary. If black holes are the racetracks of the universe, every single star in orbit receives a grand prize. And this comes in really handy for us—because extraordinary velocities mean dramatic redshift.
Here’s the redshift we observe from stars orbiting around a black hole. Usually, when we observe redshift, it’s miniscule. Like, a fraction of what you see above. That’s because stars don’t usually move all that fast.
When we’re looking at a black hole, though, we’re seeing tons of stars whizzing around at dizzying velocities—dizzying, that is, if you’re in space. The funny thing is, time in space moves a lot slower than you’re used to on Earth. Maybe it’s because of our short lifespans—we live each day moment by moment. Stars, on the other hand, take their sweet time. Because time may wait for no man, but it certainly waits for the universe.
Think about it. If stars have orbits that are angled so that half the time, they move towards you, and half the time, they’re swinging off away from you, you’re going to see redshift and blueshift. And if they’re whizzing around a black hole, it will be dramatic.
I just spent the last couple paragraphs talking about knowing it’s a black hole by stars orbiting around it at fantastic speeds. What happened to everything in the vicinity being gobbled up?
Well…every heard of the Schwarzschild radius?
A black hole is often referred to as a singularity—a single, infinitely dense point in time and space where matter is sucked in…and, as far as we can tell, sucked out of the known universe.
Beyond this singularity, unimaginable gravitational force wreaks havoc on anything in the vicinity. If you’re between the singularity and the event horizon, you’re basically dead.
Well…spaghettified is a more accurate description. You’ll die of being stretched beyond your body’s tolerance.
The Schwarzschild radius is the distance between the singularity and any point on the event horizon. And if you’re outside the event horizon, you’re safe. That’s why stars can continue to orbit around a black hole…right until the day they swing too close, and then begin the inexorable descent into uncharted territory.
Because it’s literally impossible to see beyond the event horizon, no one’s entirely sure what happens inside it. Last I heard, it was suspected that time itself gets a bit warped—gravity is caused by a dent in the fabric of space-time, after all, and time is part of that equation.
What happens when you reach the singularity? I’m not sure anyone knows. There have been all sorts of theories…
What if a black hole opens up into a white hole?
Mind you, this one’s purely theoretical, and it’s only called a white hole because it’s the opposite of a black hole.
Anyway…let’s get out of the theoretical, and back into the realm of “beyond a reasonable doubt,” as scientists like to say.
So, how does a black hole form, anyway?
A black hole is basically the result of gravitational collapse. An object’s gravity becomes so powerful that it can’t sustain its own density, and the object falls through the fabric of space-time to become a singularity. And it can happen to anything.
Even the Earth.
This is highly unlikely to ever happen, but if the Earth got smushed down to the size of a marble without losing any of its mass, then it would become a singularity.
Planets don’t usually get smushed down into black holes, though. It’s more common—and more in the range of reasonable doubt—for a black hole to form at the end of a star’s life cycle.
A star begins its life within a nebula, where dust and gas collects together until the star has enough gravity to draw in even more dust and gas. A star is born once its gravity drives pressures beneath its surface to generate incredible amounts of energy.
The star can then support a solar system, and starts its family by drawing gas and dust into orbit around it. That gas and dust collects to create planets in individual orbits, and the solar wind from the star blows the remaining gases away.
An average star, like our sun, burns bright for billions of years before eventually expanding, blowing off its outer layers, and fading away into a white dwarf.
A larger, more massive star, on the other hand, burns bright for only around 20 or so million years before expanding. Its demise is more spectacular, however, and it implodes into a supernova. That’s right—a supernova is not an explosion, it’s an implosion. It’s the beginning of that gravitational collapse we talked about.
After that, the supernova might just settle into existence as a neutron star. But if it has enough mass, the gravitational collapse and matter implosion will continue and form a singularity. And thus, a black hole is born.
These black holes are believed—beyond a reasonable doubt—to be at the center of every large galaxy. There’s one at the center of ours. But don’t worry about getting gobbled up in the near future—the central bulge of the galaxy, where all the stars near the black hole are whizzing around it, will be the first to go.
And it’s not going anywhere anytime soon. It’s been around for millennia. Technically, billennia, though that’s not really a word. In fact, I wouldn’t worry about our black hole at all—our sun will expand and swallow us whole before any black hole comes to get us.
Not comforting, I know. But that’s all still generations away.