They’re some of the most formidable entities in the universe: their strength is unmatched, they make their own rules, and if you get into a serious tussle with one, you’re almost guaranteed to lose. No, I’m not talking about moms. I’m talking about black holes, of course! Duh…
If you’ve ever learned about astronomy, chances are you’ve bumped into the idea of a black hole, “a place in space where gravity pulls so much that even light [cannot] get out,” (source). As for why it’s called a “black hole”: we can’t see into it! Because light cannot get out, the entity appears black.
In this blog post, I’m going to tell you about black holes and white holes and wormholes, oh my! It’s enough to make Dorothy wish she could go back to Kansas even more…
Let’s start with some groundwork: every object with mass exerts a gravitational force on every other object with mass. The larger an object’s mass, the larger the force of gravity it exerts on other objects. If we pack tons and tons of mass together, we get an object that exerts huge gravitational forces on other objects, and if we pack all that mass into a small enough space, we get a singularity. “In the center of a black hole is a gravitational singularity, a one-dimensional point which contains a huge mass in an infinitely small space, where density and gravity become infinite and space-time[*] curves infinitely, and where the laws of physics as we know them cease to operate,” (source).
In other words, singularities create “wells” in spacetime so deep that nothing – not even light (which is the fastest entity in the universe) – can escape. We call those gravitational wells “black holes,” and we call the points of no return for those black holes their “event horizons” (after entering a black hole and passing its event horizon, nothing – including light – can escape the singularity’s gravitational pull). And as you’ll recall, because light cannot escape from inside black holes’ event horizons, we can’t see inside black holes.
*Model of a singularity and black hole:
Note how spacetime curves (droops downward) as a result of the singularity’s gigantic mass – the singularity causes a huge gravitational well, and objects in space can fall into that well (and if they fall far enough inside – past the event horizon – they won’t be able to escape). Also, note that this image depicts the warping of two-dimensional spacetime into the third dimension; in reality, black holes warp three-dimensional spacetime into the fourth dimension. (If this talk of four dimensions doesn’t make sense, don’t worry too much about it yet.) Image courtesy of The Physics of the Universe.
From where do we get the idea of black holes? Einstein’s equations – the math for his Theory of Relativity – yield a solution suggesting the existence of black holes, such as those which form from the deaths and collapses of supermassive stars (source). But is that all there is to his equations – i.e., are black holes the only implication of his theory? As it turns out, no. There’s another solution: white holes (source).
What are white holes? According to Robert Matthews, a science writer for BBC’s Science Focus, “[a] white hole is a bizarre cosmic object which is intensely bright, and from which matter gushes rather than disappears. In other words, it’s the exact opposite of a black hole,” (source).
Do white holes and black holes share anything in common? According to PBS’ NOVA television series, “[a] white whole [sic] is, roughly speaking, the opposite of a black hole. ‘A black hole is a place where you can go in but you can never escape; a white hole is a place where you can leave but you can never go back,’ says Caltech physicist Sean Carroll. ‘Otherwise, [both share] exactly the same mathematics, exactly the same geometry.’ That boils down to a few essential features: a singularity, where mass is squeezed into a point of infinite density, and an event horizon, the invisible ‘point of no return’ first described mathematically by the German physicist Karl Schwarzschild in 1916. For a black hole, the event horizon represents a one-way entrance; for a white hole, it’s exit-only,” (source).
This talk is all well and good, but do white holes actually exist? According to astronomy author Fraiser Cain, “if white holes did exist, which they probably don’t, they would behave like reverse black holes – just like the math predicts. Instead of pulling material inward, a white hole would blast material out into space like some kind of white chocolate fountain. So generous, these white holes and their chocolate,” (source).
The takeaway: no, white holes probably don’t exist. But do we no for sure? Another no. After all, it has been only approximately one hundred years since Einstein predicted black holes’ existence and only approximately fifty years since the first black hole was discovered (source). So, is it possible white holes exist and we haven’t found them yet? Yes. But is that likely? Probably not.
If they did exist, how would white holes appear? According to science journalist Charlie Wood, “[t]o a spaceship crew watching from afar, a white hole looks exactly like a black hole. It has mass. It might spin. A ring of dust and gas could gather around the event horizon – the bubble boundary separating the object from the rest of the universe. But if they kept watching, the crew might witness an event impossible for a black hole – a belch. ‘It’s only in the moment when things come out that you can say, ‘ah, this is a white hole,’’ said Carlo Rovelli, a theoretical physicist at the Centre de Physique Théorique in France,” (source).
What do we know so far? To recap, black holes suck in matter, and white holes eject matter. Black holes exist, but we don’t think white holes do. But if they do exist, white holes will look just like black holes.
You’ll notice a similarity between black and white holes – all the magic happens at the event horizons. For black holes, the event horizon is the point of no return, and for white holes, the event horizon is the point at which matter appears. The similarity is striking, and it leads us to a simple question: could the matter coming out of white holes be the matter falling into black holes?
To answer this question, we first need to examine how black and white holes form: “[t]here is excellent evidence that black holes really exist, and astrophysicists have a robust understanding of what it takes to make one. To imagine how a white hole might form, though, we have to go out on a bit of an astronomical limb. One possibility involves a spinning black hole [moving in a way unique relative to how we expect most other black holes to spin]. According to Einstein’s general theory of relativity, the rotation smears the singularity into a ring, making it possible in theory to travel through the swirling black hole without being crushed,” (source). From there, we can examine what would happen if someone were to enter a swirling black hole: “[g]eneral relativity’s equations suggest that someone falling into such a black hole could fall through a tunnel in space-time called a wormhole and emerge from a white hole that spits its contents into a different region of space or period of time.”.
So, in short, yes – the matter coming out of a white hole could, in fact, be the matter falling into a black hole, and that matter may go through what’s called a wormhole to travel from the black hole to the white hole. Note that the wormhole is path through the fourth dimension** that matter would take to travel between the holes; the wormhole itself wouldn’t be a physical thing in space.
**We know space is three-dimensional (think about how you can move left and right (dimension number one), forward and backward (dimension number two), and jump up and down (dimension number three)). Wormholes would let you travel through space via a special shortcut. Simply, wormholes would take you through a higher dimension – the fourth dimension – thereby letting you bypass the “regular” distance you’d have to travel otherwise. See the paragraph about Interstellar below (including the video clips) for more information.
Let’s look at another analogy, this one to describe the business of black holes, wormholes, and white holes all together: think about a subway station (let’s call it Station A). Station A itself is a black hole, its train platform is the singularity (the platform warrants the station, just as singularities create black holes), and the part of the street hanging over the station’s stairs is the event horizon (that part of the street prevents others from seeing into Station A, just as event horizons prevent us from seeing into black holes – it’s all about blocking the view inside the station/ black hole). After entering Station A and passing under overhanging street (read: after falling past the event horizon), you (read: matter falling into the black hole) are in the station (read: in the black hole). You then wait for your train to arrive, and once it does, you get on and travel to Station Z (travel to another hole). Think of the train as being a wormhole – it connects Station A (black hole) with Station Z (another hole). Finally, you take stairs up and out of Station Z, returning to the world. Now, we can clearly see that Station Z is not another black hole – it’s a white hole! The train (the wormhole) connects Stations A and Z (connects the black and white holes).
To take the analogy one step further, we know Stations A and Z are part of a larger subway system – there are Stations C, D, E, X, Y, and everything in between (though not Station B; that was already taken by Vanderbilt’s post office). We know Station A (black hole) connects with Station Z (white hole), but it’s also possible that Station A also connects with Stations X and Y (each a white hole). For that matter, Stations C, D, and E (black holes) could also connect with Stations X, Y, and Z (white holes). Or, maybe only Station C connects with Stations X, Y, and Z, leaving Stations D and E to connect with other white-hole stations (Stations P, Q, and R). Or, maybe it’s a different combination of these black-hole stations and white-hole stations. The takeaway: it’s possible that each black-hole station connects with only one white-hole station (i.e., the wormholes run in parallel… but don’t get bogged down by the directional meaning of “parallel” given that we’re talking about four dimensions), such as if Station A connects only with Station Z, Station B only with Station Y, etc. It’s also possible that each black-hole station connects with more than one white-hole station (i.e., the wormholes intersect and can deliver matter to different stations), such as if you could enter the subway system at Station A and exit at Station X, Y, or Z. We just don’t know. (Also, recall we don’t think white holes actually exist, so this is essentially just a big rabbit hole… and note that’s a type of hole distinct from black and white holes).
If you’ve ever seen the movie Interstellar, you may already have an understanding of all this, though it will be imperfect because the movie is not fully accurate in its depiction of this theory. (Please note the remainder of this paragraph contains spoilers for the movie.) As you’ll recall, our team of astronauts enters a wormhole by Saturn to travel to a distant part of space (click here to see that clip). The visual effects are compelling, but remember that according to this theory, the astronauts would have traveled through a wormhole only after falling into a black hole – they could not have traveled through a shiny orb as depicted in the film given that wormholes are the subway trains, not the stations themselves (i.e., you can’t enter a wormhole directly). Later in the movie, we have a marginally-better depiction of this travel: one of the characters falls into a black hole (click here to see that clip), and he later pops out right next to Saturn, where the original wormhole was (click here to see that clip). To make sense of this part of the film, just think about the beginning and ending of his journey: falling into the black hole and reappearing by Saturn (where there would actually have existed a white hole connected with that black hole). Because the clips are from a Hollywood movie and not a science documentary, it’s an imperfect depiction of the theory, but it nonetheless is helpful in trying to understand the idea.
Though I just said it, it’s worth repeating: this black-hole-wormhole-white-hole idea is just a theory. And like all good theories, it has competition: “some theorists think that a combination of Einstein’s theory and quantum theory points to a new way of thinking about white holes. Instead of being the ‘exit’ from a wormhole, they may be a slow-motion replay of the formation of the original black hole. The process starts when an old massive star collapses under its own weight and forms a black hole[.] … But then, quantum effects occurring around the surface of the black hole halt further collapse to a singularity, and instead begin to gradually turn the black hole into a white hole that’s spewing out the original star matter again. The process is mind-bendingly slow, though, so we may be in for a very long wait to find out if white holes really exist,” (source). So, could the matter coming out of white holes be the matter falling into black holes? Maybe. Could it actually be the original star matter that formed the black hole? Also maybe. Does any of this really matter given that we doubt white holes exist in the first place? I’ll let you decide.
There are lots of takeaways from all this, with my favorite being the following: we don’t know everything about the theories we claim to understand (or understand mostly, at least). We can grasp the Theory of Relativity and what it tells us about black holes, but we lose some of that grasp when it comes to understanding the alternate suggestion of white holes, a mathematical solution we don’t think exists in reality (recall that Einstein’s equations can be solved in such a way as to suggest the existence of black holes – which exist – and white holes – which probably don’t exist). Ultimately, though, I don’t believe that alternate, likely-false solution diminishes our understanding of relativity and black holes; it just adds a bit of nuanced flavor. In other words, taking together the black and white holes illuminates just how much gray area there is.
What’s your takeaway from all this?
In addition to Interstellar, here’s a bonus example of black holes in pop culture:
Superman holding a black hole in his hand. Image courtesy of Comic Vine.
