Beyond the Black Horizon

By Dr. John C. (“Jack”) Adler, as told to Bill DeSmedt

When we left off last time, we’d just got done scrunching several suns’ worth of mass down into a black hole. Now it’s time to take a look inside, at what’s called a “singularity.”

Calling something a singularity is just a polite way of saying it’s impossible. Whenever an equation breaks down and starts churning out meaningless results — infinities and the like — well, us astrophysicists find that real “singular.”

It turns out that Einstein’s theory of relativity suffers just that kind of breakdown when it tries to describe what goes on inside a black hole. In particular, the field equations predict that what we’ll find at the center of the hole is a dimensionless point of infinite mass, infinite density, infinity curvature, infinite what-have-you. In short, a singularity.

All this is really bad news for modern physics. And I don’t just mean the part about the tensor calculus spitting out impossibilities. No, all kinds of bad things go on in Mr. Singularity’s Neighborhood. General Relativity says that gravitation is equivalent to acceleration, you see, and that enough acceleration will do really weird stuff to time and space. So an infinite gravitational field is something physicists would just as soon not deal with.

Luckily enough, they mostly don’t have to. Because there’s a silver lining to this particular black cloud — namely, that singularities just naturally wall themselves off from the rest of the universe.

It’s called an “event horizon.” You could think of it as job security for physicists. It’s there to make sure that all the paradoxes riddling the inside of a black hole can never get out to pollute the universe at large.

Now, for something so simple, this event horizon idea seems to’ve stirred up an awful lot of confusion over the years. Sometimes you’ll hear people talk about it like it was a physical barrier or some such. There’s even a Star Trek episode (the one called “Parallax”) where the starship Voyager escapes from a black hole by scooting through a “crack” in the event horizon!

That episode won a place of honor on Lawrence’s Krauss’s top-ten list of all-time Star Trek bloopers. Because, with apologies to Captain Janeway, the whole notion is just plain silly. It’s not like an event horizon was the sort of thing that could develop a hairline fracture. In fact, it’s not a physical object of any kind, no more so than that old, familiar horizon we watch the sun set behind every day.

With one big difference: you could walk forever and never reach the horizon here on earth, whereas it’d be all too easy to reach, and cross over, the event horizon surrounding a black hole.

Because what an event horizon really is, is just the mathematically-defined dividing line (dividing sphere?) between a singularity and the rest of the universe. It’s the point — the collection of points, actually — of no return. Cross it, and there’s just no turning back. Not even for a beam of light.

Any light that falls into a black hole gets trapped there too, remember?

Why is that, exactly? After all, it’s not as if gravity, even a black hole’s unimaginable gravity, could actually slow light down. Physicists have recently learned how to do that in a laboratory, using a special state of matter called a Bose-Einstein condensate, but it never happens in nature, far as we know.

So, how does a black hole trap light?

Here’s one way to think about it: Imagine you’re trying to climb a ladder out of a really, really deep hole. Doesn’t matter if you go fast or slow; as long as you keep climbing, you can’t help but make it eventually — or so you think.

Now, it’s going to be a long climb, so you’re going to want to bring some food along and stop every now and again to have a snack. Let’s say you go up a mile, then take a Snickers break. That gives you the energy to keep going.

But what if the pull of the gravity you’re fighting against is really strong, so strong it takes more energy to lift that candy bar a mile than you’ll get back by eating it? That makes your Snickers break a losing proposition — you’d have done better to leave the food at the bottom.

Of course, even weighing in at just a few ounces, your average candy bar’s pretty heavy compared to its nutritional value. You’d be a lot better off with something you could turn completely into energy — a candy bar, say, where the ingredients label reads “chocolate, sugar, almonds, anti-matter.”

But even that anti- Snickers has still got a finite energy content. If the gravity’s strong enough and the ladder’s tall enough, you’re still going to wind up burning more energy lifting that candybar than it can ever give you back. Meaning that, at some point short of the top, you’re just plain going to run out of steam.

Even so, it’s not like gravity’s crushing you to the floor. You can still climb. You just can’t climb all the way to the top. All the gravity in the universe won’t slow light or hold it back — but it can rob it of energy. Partway out of the hole’s gravity well, it just runs out of energy and ceases to exist! And light’s pure energy, no excess baggage. If light can’t make it to the top, neither can anything else.

Or here’s another way to look at it: Like we said last time, General Relativity tells us that gravity warps spacetime. What that means is an object with enough gravity can take all the possible escape routes away from that object and bend them back on themselves. Light’s still travelling as fast as ever, but it’s doing that travelling inside a curved space, where all roads lead back to the singularity at the center.

So whatever crosses the event horizon can’t ever “talk” to anybody on the outside ever again. A careless experimenter who slips and falls into the hole can’t tell us what his instruments read. And, considering how weird things can get down at the singularity, that’s a good thing.

Other than that, though, there’s really nothing to the event horizon itself. With a large enough hole — here I’m talking millions or billions of solar masses, like the ones at the centers of galaxies — with a hole like that you could cross right over the event horizon and never notice the difference. Until you realized you couldn’t get back out again, that is. Once you’re trapped inside an event horizon, all roads lead downward, to the singularity.

Here’s maybe the place to point out, though, that not all physicists are convinced this story’s right. In particular it’s been suggested that the gravity of a supermassive object should slow time itself to a dead stop at the event horizon. That’d mean it would take an eternity or more for anything to actually fall in — all the infalling stuff would just get “stuck” at the horizon. That’s from the perspective of the outside universe, anyway. You’d tell a different tale if you were the one doing the falling — as we’ll see in a minute.

Meanwhile, precisely because nothing can get ever out, black holes as seen from the outside are really simple objects. It doesn’t matter what the matter that went into them was in its previous life: Two or three solar masses worth of used TV sets’ll do just as well as the same weight of lottery tickets or butterflies or (in the case of a supernova) stellar core material. Or anti-matter, or pure energy, even. The end result is always the same. All that the final collapse leaves behind is mass and, optionally, spin and/or electromagnetic charge.

And that’s it. As Princeton physicist John Wheeler put it: black holes have no “hair” — no other distinguishing traits. Like a prisoner-of-war refusing to give his interrogators any more thanjust  his name, rank, and serial number, a black hole will tell you its mass, spin, and charge, but nothing else.

(Well, and maybe not. Toward the end of his life, Stephen Hawking came up with some new work that threatens to upset the applecart once again. Seems quantum fluctuations at the event horizon may cause a hole to eventually regurgitate, in a “mangled” form, all the matter it’s ever swallowed. The jury’s still out on this one, but folks like Leonard Susskind are hoping it’s right, because if matter and energy and information can really be destroyed the way classical black hole theory claims, then quantum mechanics is wrong.)

But, be that as it may, even given only mass, spin, and charge to work with, black holes can still conjure up some mighty strange effects.

Like frame drag, for instance. That’s where a spinning black hole pulls space itself along behind it in the direction of its rotation. That long-predicted effect was actually observed back in 2005, by a team out of the Harvard-Smithsonian Center for Astrophysics.

Or, my personal favorite, the tides.

Because, when you get right down to it, tides are nothing but a gravitational effect. Think about it this way: the moon’s gravity pulls on every atom of the earth. But that pull varies with distance. The atoms directly beneath the moon feel it strongest because they’re the closest, so they get dragged up toward the moon, away from the bulk of the planet. Those on the opposite side of the world, the ones furthest away, are getting pulled on the least, so they get left behind, relatively speaking. The upshot is: the whole planet gets stretched a little.

Now, the solid body of the earth itself is reasonably rigid, so it stays more or less round no matter how hard it gets pulled on. But the oceans are a different story; they get stretched out into an ellipsoid, with a bulge at either end. What that gives you is two standing waves of seawater moving through the oceans at twelve-hour intervals as the earth rotates beneath them. In other words, the tides.

With me so far? Then add this: That tidal effect isn’t peculiar to earth. It happens everywhere there’s a gravitational field. And the more intensely that field’s strength changes with distance, the higher the tides become. Get in really close to a singularity and its gravity gradient can produce tidal distortions across distances measured in micrometers or less. An object would need phenomenal tensile strength to survive a fly-by. As for a human being, you can forget about it! Come in too close, and you’d be stretched out like a piece of saltwater taffy — torn limb from limb, then atom from atom. It’s a process called “spaghettification.”

Bad as all that is, we can take comfort from the fact that the event horizon is always there to shield us from even worse.

… Or is it?