Death by Black Hole: And Other Cosmic Quandaries - Neil deGrasse Tyson (2014)



Without a doubt, the most spectacular way to die in space is to fall into a black hole. Where else in the universe can you lose your life by being ripped apart atom by atom?

Black holes are regions of space where the gravity is so high that the fabric of space and time has curved back on itself, taking the exit doors with it. Another way to look at the dilemma: the speed required to escape a black hole is greater than the speed of light itself. As we saw back in Section 3, light travels at exactly 299,792,458 meters per second in a vacuum and is the fastest stuff in the universe. If light cannot escape, then neither can you, which is why, of course, we call these things black holes.

All objects have escape speeds. Earth’s escape speed is a mere 11 kilometers per second, so light escapes freely, as would anything else launched faster than 11 kilometers per second. Please tell all those people who like to proclaim, “What goes up must come down!” that they are misinformed.

Albert Einstein’s general theory of relativity, published in 1916, provides the insight to understand the bizarre structure of space and time in a high-gravity environment. Later research by the American physicist John A. Wheeler, and others, helped to formulate a vocabulary as well as the mathematical tools to describe and predict what a black hole will do to its surroundings. For example, the exact boundary between where light can and cannot escape, which also separates what’s in the universe and what’s forever lost to the black hole, is poetically known as the “event horizon.” And by convention, the size of a black hole is the size of its event horizon, which is a clean quantity to calculate and to measure. Meanwhile, the stuff within the event horizon has collapsed to an infinitesimal point at the black hole’s center. So black holes are not so much deadly objects as they are deadly regions of space.

Let’s explore in detail what black holes do to a human body that wanders a little too close.

If you stumbled upon a black hole and found yourself falling feet-first toward its center, then as you got closer, the black hole’s force of gravity would grow astronomically. Curiously, you would not feel this force at all because, like anything in free fall, you are weightless. What you do feel, however, is something far more sinister. While you fall, the black hole’s force of gravity at your two feet, they being closer to the black hole’s center, accelerates them faster than does the weaker force of gravity at your head. The difference between the two is known officially as the tidal force, which grows precipitously as you draw nearer to the black hole’s center. For Earth, and for most cosmic places, the tidal force across the length of your body is minuscule and goes unnoticed. But in your feet-first fall toward a black hole the tidal forces are all you notice.

If you were made of rubber then you would just stretch in response. But humans are composed of other materials such as bones and muscles and organs. Your body would stay whole until the instant the tidal force exceeded your body’s molecular bonds. (If the Inquisition had access to black holes, this, instead of the rack, would surely have become the stretching device of choice.)

That’s the gory moment when your body snaps into two segments, breaking apart at your midsection. Upon falling further, the difference in gravity continues to grow, and each of your two body segments snaps into two segments. Shortly thereafter, those segments each snap into two segments of their own, and so forth, and so forth, bifurcating your body into an ever-increasing number of parts: 1, 2, 4, 8, 16, 32, 64, 128, etc. After you’ve been ripped into shreds of organic molecules, the molecules themselves begin to feel the continually growing tidal forces. Eventually, they too snap apart, creating a stream of their constituent atoms. And then, of course, the atoms themselves snap apart, leaving an unrecognizable parade of particles that, minutes earlier, had been you.

But there is more bad news.

All parts of your body are moving toward the same spot—the black hole’s center. So while you’re getting ripped apart head to toe, you will also extrude through the fabric of space and time, like toothpaste squeezed through a tube.

To all the words in the English language that describe ways to die (e.g., homicide, suicide, electrocution, suffocation, starvation) we add the term “spaghettification.”

AS A BLACK HOLE eats, its diameter grows in direct proportion to its mass. If, for example, a black hole eats enough to triple its mass, then it will have grown three times as wide. For this reason, black holes in the universe can be almost any size, but not all of them will spaghettify you before you cross the event horizon. Only “small” black holes will do that. Why? For a graphic, spectacular death, all that matters is the tidal force. And as a general rule, the tidal force on you is greatest if your size is large compared with your distance to the center of the object.

In a simple but extreme example, if a six-foot man (who is not otherwise prone to ripping apart) falls feet-first toward a six-foot black hole, then at the event horizon, his head is twice as far away from the black hole’s center as his feet. Here, the difference in the force of gravity from his feet to his head would be very large. But if the black hole were 6,000 feet across, then the same man’s feet would be only one-tenth of 1 percent closer to the center than his head, and the difference in gravity—the tidal force—would be correspondingly small.

Equivalently, one can ask the simple question: How quickly does the force of gravity change as you draw nearer to an object? The equations of gravity show that gravity changes more and more swiftly as you near the center of an object. Smaller black holes allow you to get much closer to their centers before you enter their event horizons, so the change of gravity over small distances can be devastating to fallers-in.

A common variety of black hole contains several times the mass of the Sun, but packs it all within an event horizon only about a dozen miles across. These are what most astronomers discuss in casual conversations on the subject. In a fall toward this beast, your body would begin to break apart within 100 miles of the center. Another common variety of black hole reaches a billion times the mass of the Sun and is contained within an event horizon that is nearly the size of the entire solar system. Black holes such as these are what lurk in the centers of galaxies. While their total gravity is monstrous, the difference in gravity from your head to your toes near their event horizons is relatively small. Indeed, the tidal force can be so weak that you will likely fall through the event horizon in one piece—you just wouldn’t ever be able to come back out and tell anybody about your trip. And when you do finally get ripped apart, deep within the event horizon, nobody outside the hole will be able to watch.

As far as I know, nobody has ever been eaten by a black hole, but there is compelling evidence to suggest that black holes in the universe routinely dine upon wayward stars and unsuspecting gas clouds. As a cloud approaches a black hole, it hardly ever falls straight in. Unlike your choreographed feet-first fall, a gas cloud is typically drawn into orbit before it spirals to its destruction. The parts of the cloud that are closer to the black hole will orbit faster than the parts that are farther away. Known as differential rotation, this simple shearing can have extraordinary astrophysical consequences. As the cloud layers spiral closer to the event horizon they heat up, from internal friction, to upwards of a million degrees—much hotter than any known star. The gas glows blue-hot as it becomes a copious source of ultraviolet and x-ray energy. What started as an isolated, invisible black hole (minding its own business) has now become an invisible black hole encircled by a gaseous speedway, ablaze with high-energy radiation.

Since stars are 100 percent certified balls of gas, they are not immune from the fate that greeted our hapless clouds. If one star in a binary system becomes a black hole, then the black hole does not get to eat until late in the companion star’s life, when it swells to become a red giant. If the red giant grows large enough, then it will ultimately get flayed, as the black hole peels and eats the star, layer by layer. But for a star that just happens to wander into the neighborhood, tidal forces will initially stretch it, but eventually, differential rotation will shear the star into a friction-heated disk of highly luminous gas.

Whenever a theoretical astrophysicist needs an energy source in a tiny space to explain a phenomenon, well-fed black holes become prime ammunition. For example, as we saw earlier, the distant and mysterious quasars wield hundreds or thousands of times the luminosity of the entire Milky Way galaxy. But their energy emanates primarily from a volume that is not much larger than our solar system. Without invoking a supermassive black hole as the quasar’s central engine, we are at a loss to find an alternative explanation.

We now know that supermassive black holes are common in the cores of galaxies. For some galaxies, a suspiciously high luminosity in a suspiciously small volume provides the needed smoking gun, but the actual luminosity depends heavily on whether stars and gas are available for the black hole to shear them apart. Other galaxies may have one too, in spite of an unremarkable central luminosity. These black holes may have already eaten all the surrounding stars and gas, leaving no evidence behind. But stars near the center, in close orbit to the black hole (not too close to be consumed), will have sharply increased speeds.

These speeds, when combined with the stars’ distance from the center of the galaxy, are a direct measure of the total mass contained within their orbits. Armed with these data, we can use the back of an envelope to calculate whether the attracting central mass is, indeed, concentrated enough to be a black hole. The largest known black holes are typically a billion solar masses, such as what lurks within the titanic elliptical galaxy M87, the largest in the Virgo Cluster of galaxies. Far down the list, but still large, is the 30-million solar mass black hole in the center of the Andromeda galaxy, our near neighbor in space.

Beginning to feel “black hole envy”? You are entirely justified: the one in the Milky Way’s center checks in at a mere 4-million solar masses. But no matter the mass, death and destruction are their business.