National Geographic : 2014 Mar
102 national geographic • march 2014 old stars in the galactic center and pipe scald- ing gases generated in this process to the galaxy’s outer parts. The gas cools, coalesces, and eventu- ally forms new stars, refreshing the galaxy like a fountain of youth. It’s important to clarify a couple of things about black holes. First is the idea, popularized in science fiction, that black holes are trying to suck us all in. A black hole has no more vacuum- ing power than a regular star; it just possesses extraordinary grip for its size. If our sun suddenly were to become a black hole—not going to hap- pen, but let’s pretend—it would retain the same mass, yet its diameter would shrink from 865,000 miles to less than four miles. Earth would be dark and cold, but our orbit around the sun wouldn’t change. This black hole sun would exert the same gravitational tug on our planet as the full-size one. Likewise, if the Earth were to become a black hole, it would retain its current weight of more than six sextillion tons (that’s a six followed by 21 zeros) but be shrunk in size to smaller than an eyeball. The moon, though, wouldn’t move. So black holes don’t suck. Easy. The next topic, time, is way more of a mind bender. Time and black holes have a very strange relationship. Actu- ally time itself—forgetting about black holes for a moment—is an unusual concept. You proba- bly know the phrase “time is relative.” What this means is that time doesn’t move at the same speed for everybody. Time, as Einstein discovered, is af- fected by gravity. If you place extremely accurate clocks on every floor of a skyscraper, they will all tick at different rates. The clocks on the lower floors—closer to the center of the Earth, where gravity is stronger—will tick a little slower than the ones on the top floors. You never notice this because the variances are fantastically small, a spare billionth of a second here and there. Clocks on global positioning satellites have to be set to tick slightly slower than those on Earth’s surface. If they didn’t, GPS wouldn’t be accurate. Black holes, with their incredible gravitational pull, are basically time machines. Get on a rocket, travel to Sgr A*. Ease extremely close to the event horizon, but don’t cross it. For every minute you spend there, a thousand years will pass on Earth. It’s hard to believe, but that’s what happens. Grav- ity trumps time. And if you do cross the event horizon, then what? A person watching from the outside will not see you fall in. You will appear frozen at the hole’s edge. Frozen for an infinite amount of time. Though technically not infinite. Nothing lasts forever, not even black holes. Stephen Hawk- ing, the British physicist, proved that black holes leak—the seepage is called Hawking radiation— and given enough time, will evaporate entirely. But we’re talking trillions upon trillions upon many more trillions of years. Long enough so that in the far future, black holes may be the only objects remaining in our universe. While an outside observer would never see you slip into a black hole, what would happen to you? Sgr A* is so large that its event horizon is about eight million miles from its center. There’s some debate in the physics community about the moment you cross over. It’s possible there exists what’s called a fire wall, and that upon reaching the event horizon, you promptly burn up. General relativity theory predicts, however, that something else happens when you cross the event horizon: Nothing. You just pass through, unaware that you’re now lost to the rest of the universe. You’re fine. Your watch on your wrist ticks along as usual. It’s often said that black holes are infinitely deep, but this is not true. There is a bottom. You won’t live to see it. Gravity, as you fall, will grow stronger. The pull on your feet, if you’re falling feet first, will be so much greater than the tug on your head that you’ll be stretched until you’re ripped apart. Physicists call this be- ing “spaghettified.” But pieces of you will reach the bottom. At the center of a black hole is a conundrum called a singularity. To understand a singularity would be one of the greatest scientific breakthroughs in his- tory. You’d first need to invent a new theory—one Michael Finkel wrote on Australian Aboriginals in the June 2013 issue. Illustrator Mark A. Garlick’s latest book is Cosmic Menagerie.