Any of you recognize this?
To those who don’t, it probably looks like a pretty unimpressive, blurry ring. In fact, this is the first ever image of a black hole, taken with an interferometer the size of the Earth.
If you’re a science geek, you’ve no doubt seen tons of artists’ conceptions of black holes on the internet. Most use a great deal of artistic license. Some of my favorite “images” of black holes used to be the ones that look like ripples in the fabric of space. Imagine my disappointment when I realized that’s not the case at all.
Black holes are singularities—infinitely dense places of zero radius with at least 3 M☉ (solar masses) of star stuff—surrounded by an event horizon, inside of which gravity is so strong that even light cannot escape. That’s why it’s called a black hole.
But they are not “holes” in the usual sense. They are not giant space potholes that you can easily stumble into, and you certainly don’t fall into them the same way you would a pothole.
So…what are black holes really like?
Let’s start by exploring what would happen if you were to jump into a black hole. And while I’m at it, I’ll disabuse you all of a few notions science fiction has perpetuated.
The first movie scene I’m going to examine is the one that bugs me the most: the part in J.J. Abrams’ Star Trek reboot when the bad guy is falling into a black hole and the good guys almost get caught, too. Though in that scene’s defense, the part where they eject a bunch of warp cores to give them some extra thrust is pretty accurate. You do need to leave something behind in order to escape a strong gravitational pull. (Interstellar comments on this.)
So. Anyone notice that as the Narada (enemy ship) is falling into the black hole…we visibly see it get crunched, and it actually looks like it’s half poking out of the black hole?
Uh…no. That’s not how it works.
In real life, the Narada would have slowly, slowly, slowwwwly approached the black hole’s event horizon…and then seemed to stop.
Yup. An outside observer wouldn’t see the Narada actually fall into the black hole. I dunno why it’s half sticking out of the hole, because—like I said above—this isn’t like some kind of space pothole. It’s a black hole. It’s a place where at least 3 M☉, squeezed to a size of zero radius, produces a gravitational field so strong it warps time itself.
That’s time dilation for you. It’s a result of time slowing down in curved space-time.
Now, here’s the part of that scene that actually makes sense (besides the warp core ejection). We actually get a camera shot of the main baddie bracing as his ship gets crushed by insane tidal forces.
One: the whole “crunching ship” bit actually makes sense. After all, gravity changes dramatically close to a black hole, and its pull would actually be dramatically different on the near and far sides of the Narada. A tidal force would also squeeze the ship from either side, causing its frame to lengthen and narrow. It would be under tremendous stress, so yes, it would crunch.
Two: Nero, the main baddie, would be very well aware of his fall past the event horizon, as opposed to the observers on the Enterprise (good guys) who should see his ship slowly, slowly, slowly approach the event horizon. Here’s why.
Human beings—and, if we’re including aliens, all other creatures that perceive our same four dimensions the way we do—are trapped in time. We can move however we please within the three physical dimensions, but we can only exist moment to moment in time, never moving forward or backward.
Time, therefore, becomes our main reference frame. It’s perceived as unchangeable because we cannot change it or move within it.
So…what if we’re in a place where time moves differently, such as in the gravity well of a black hole?
Well, your thought patterns, and the way you perceive reality itself, are locked into time. Your synapses will fire slower in a place with slower time, but you won’t notice it, because you can’t perceive it. Relative to yourself, your synapses are just firing “normally.”
What does that mean? Well, to the Enterprise, time near the black hole has slowed almost to a stop. The Narada never appears to breach the event horizon. But to Nero, time is moving normally. He’ll perceive that slower time as if it were normal. To him, his fall towards the event horizon takes a matter of months.
Wait a second…in the movie, it took mere moments…
Okay, okay, I’ll give them a pass for that. It would be a bit weird for an action movie to show months passing as the bad guy falls to his death.
The other scene that bugs me is in the same movie: the destruction of the planet Vulcan.
Okay, two things. First…people…why does Vulcan collapse like it’s quicksand or something?
The movie is making the same error as up above: this is not a giant space pothole.
It’s a singularity surrounded by an event horizon. More generally, it’s a compact object. When stuff is falling into or onto a compact object, there is always an accretion disk. Why? For the same reason water whirls into a mini-whirlpool when it swirls down your bathtub drain, instead of just falling straight down. That’s conservation of angular momentum for ya.
In binary systems, when mass is transferred to a compact object from a companion star, the system looks something like this:
Let’s assume that the event horizon of this black hole—the one that seems to gobble up Vulcan—is tiny, way smaller than the planet’s original radius. By that assumption, it makes sense that Vulcan doesn’t just instantly disappear from sight—most of the planet is still outside the range of distance from the singularity where even light can’t escape.
That means that the accretion disk should be somewhere inside Vulcan’s radius. We should see the planet flatten into a disk of material that whirls around the central black hole, but—just like with Nero’s situation—never seems to actually fall inside it.
Which brings me to the second thing that bothers me. In the movie, Vulcan disappears from sight completely. It shouldn’t. We should never see a single iota of its mass disappear. If we could see it while it was falling into the black hole—meaning, it hasn’t crossed the event horizon—we should never see it disappear.
There’s actually a third, relatively minor thing that bugs me about this bit, and its name is gravitational redshift.
As you may remember from my post on the Doppler effect, redshift in general means that the wavelength of any radiation appears to be shifted towards longer (redder) wavelengths whenever it moves away from you.
Radiation also gets shifted to longer wavelengths when it travels out of a gravitational field, but not because it’s necessarily moving away from the observer—it’s because it loses energy by escaping gravity, and longer wavelengths have less energy.
So…if all radiation close to a black hole should glow in the longer (redder) wavelengths of the electromagnetic spectrum…then why the heck is there bright blue light coming from inside this collapsing planet?
That just about covers most common misconceptions people have about black holes, thanks to the media and pop culture. If you have any questions that I haven’t covered, please do let me know. And stay tuned for my next post, in which I’ll actually dive into how to find black holes!