Okay, good question. How the heck do you find an object that emits no radiation? Astronomers find—and study—just about everything in the universe using the radiation it emits or reflects. So…what happens when the object we’re looking for has such a strong gravitational pull that even light can’t escape?
Well, that’s when we need to turn to the theoretical science behind black holes. What measurable effects do they have on objects in their vicinity? Can we detect them indirectly?
Of course, some of you might be screaming at me that we’ve already photographed a black hole—in visual wavelengths! Yes, astronomers did make that achievement—we now have visual proof that what we’ve been theorizing all along is indeed real.
But that black hole was so faint, it took an interferometer the size of the Earth to image. We had to know exactly where to look in order to get that picture.
So how the heck do we find one in the first place?
The secret lies not with trying to search for the black hole itself, but with searching for an accretion disk—the disk of matter that swirls in toward the black hole.
Accretion disks result from mass transfer, when stars in binary systems exchange mass. So we know right off the bat that it will be next to impossible to find a lone black hole, but substantially easier to find one in a binary system.
So…what, specifically, about a binary system can clue us in that there’s a black hole there?
Well, the first thing we have to realize is that accretion disks are violent places. Matter falling on the accretion disk gets hot enough to light up at X-ray wavelengths. So our best bet to find a black hole is to look for X-ray binaries (binary systems that emit X-rays).
But…there’s one huge caveat. And that is, neutron stars also have accretion disks that emit X-rays.
So how do we tell the difference?
Well, remember that many neutron stars appear to us as pulsars, objects that pulse rapidly and rhythmically. They do so whenever one of the beams of radiation at a neutron star’s magnetic poles sweeps directly over Earth—like a rotating lighthouse beam.
Pulsars cannot be black holes. Why? The “hot spots” that cause the radiation beams must be anchored on a solid surface. A black hole has no solid surface. That’s because a black hole is a singularity–an infinitely dense object with absolutely zero radius.
Yeah, that bit trips my brain up a little. As I’ve commented before, an actual object that takes up no space whatsoever but has a ton of mass is kind of hard to compute. But the important thing is, if something doesn’t take up any space at all, it can’t have a solid surface, can it?
Alright, so we know that an X-ray source that’s a pulsar can’t be a black hole.
But what about all the neutron stars whose radiation beams don’t pass directly over Earth, and don’t show up on our instruments as pulsars? How can we tell the difference then?
Well, then it’s a simple matter of determining the mass of the object in question. If it’s greater than about 3 M☉, it can’t be supported by the material strength of degenerate neutrons, and it definitely can’t be supported by the material strength of degenerate electrons. When those material strengths both fail, matter must collapse into a black hole.
How do we find out the mass, anyway?
You might remember from one of my posts ages ago that, while mass can only be estimated with lone stars, it’s relatively easy to calculate it for binary systems.
All orbits are defined by mass.
You can think of a binary star system as like a teeter-totter, as shown above. If you’ve ever played on a teeter-totter as a kid, you probably know that if one of you is significantly older/heavier than the other, that kid will plonk straight to the ground.
But if that kid just scoots closer to the fulcrum–the balance point–of the teeter-totter, you can balance out your masses and have fun. Just as long as the older kid doesn’t mind a wedgie from sitting on the actual pole of the teeter-totter.
It’s the same with a binary star system. Neither star orbits the other star–they both orbit a center of mass, and the more massive star will always orbit closer to that center of mass.
So…if you have an X-ray source in a binary system, you can easily find the mass of the object. And if it’s greater than 3 M☉, you know it’s a black hole.
There’s one more way to confirm that an X-ray source is a black hole, and its name is the signature event horizon.
The event horizon is a feature that is unique to singularities, so if one is detected, the X-ray source can’t be anything other than a black hole.
But what exactly is an event horizon?
Simply put, the event horizon is the distance from the singularity where mass can safely orbit and not fall into the gravity well.
Ever wonder why even light can’t escape a black hole’s gravity?
Light is a mysterious phenomenon–and simultaneously an astronomer’s most valuable tool. The key to understanding the universe is understanding light. And one key characteristic of light is that it does not travel instantaneously from a source–it has a set speed that never changes.
The speed of light is often called the “universal speed limit.” And that’s true. For some reason, one of the laws of physics of our universe is that nothing can travel faster than light.
Bad news for sci-fi dreams of FTL travel, perhaps. Maybe one day we’ll find a workaround. But more importantly, it’s bad news for anything within a black hole’s event horizon.
That’s because within the event horizon, the escape velocity of the singularity–that is, the velocity needed to escape its gravitational pull–is greater than the speed of light.
That means that within the event horizon, the laws of physics have determined that it is impossible to reach escape velocity.
But how can we see evidence of an event horizon?
Well…let’s go back to the idea of accretion disks.
Accretion disks are full of infalling matter, swirling ever closer to the compact object. What happens to that matter depends on whether the compact object has a solid surface.
In the case of a white dwarf or neutron star, the matter will fall onto the surface of the compact object and cause a violent explosion–a type Ia supernova or an X-ray burster. Astronomers can see that explosion as a final burst of energy.
But in the case of a black hole…the matter will appear to ever-so-slowly approach the event horizon, and then simply disappear.
Astronomers will never see that matter actually cross the event horizon–just like I described in my post on what movies get wrong about black holes. But it apparently becomes undetectable as it approaches the event horizon.
In any case, the fact alone that no violent explosion occurs indicates that this X-ray source has no solid surface for infalling material to land on, and therefore must be a black hole.
I grew up thinking of black holes as elusive ideas existing mostly in the realm of science fiction. They’re extreme objects–more than three suns worth of mass squeezed into zero space, gravity so strong even light can’t escape, with accretion disks emitting insane amounts of energy. Especially given the fact that they are, practically by definition, invisible, you’d think they’d be harder to find and confirm.
As it turns out, astronomers have definitively confirmed the existence of at least 20 black holes, and still more are strong candidates.
Between looking for X-ray sources, confirming the lack of a solid surface, measuring sufficient mass, and observing evidence of an event horizon, black holes aren’t so elusive after all. We know that these incredible objects actually exist in our universe and in our own galaxy. Is that cool or what?