What’s Up? When You Get Down to It, It’s Hard to Say

Chicken Little was right. The sky is falling. Chunks of ice and rock from outer space continually bombard our little blue planet, lighting up the sky with green streaks from wandering meteors, wiping out dinosaurs (and scores of other species), and giving Hollywood scriptwriters a chance to write horror stories about killer asteroids.

In a few days, a comet being hailed as the brightest in a century will start hanging around the twilight sky. Known as Hale-Bopp, it is already visible to the naked eye just before dawn.

The only thing wrong with the chicken’s prediction is the idea that the sky is falling “down.” In fact, the objects we see in the sky (as well as those we don’t) are falling everywhere at once. “If anything,” says Griffith Observatory director Ed Krupp, “we’re falling into them.”

Once you look farther into the cosmos than your own square foot of Earth, “down” becomes a complicated matter. In fact, “down” turns up in the most unlikely places, and even scientists can be muddled about which end is up.

Consider your own personal sense of “up.” Wherever it is, you can bet that your “up” is “down” for someone standing on the other side of the Earth.

We in the Northern Hemisphere tend to think that up is somehow north. But if you are at the South Pole, north is down. Indeed, the only reason that north is up on globes and maps, and south is relegated to Down Under, is because Northern navigators made the first maps and naturally put themselves at the top of the world.

Physically speaking, down is toward the center of mass of the Earth--in other words, toward the center. But if the Earth were shaped like a doughnut, and you happened to find yourself in the hole in the middle, down would be toward the outside and up would be toward the center.

For the same reason, plants grown on a rotating platform get confused about the direction of down and grow toward the center and up in parabolas. The outward pull of centrifugal force becomes a kind of pseudo-gravity, and the flowers think “up” is opposite to its force, or inward.

If we move off the Earth and into the galaxy, “down” gets even stranger. Because the galaxy also holds most of its mass in the core, the general down direction is toward the center. And the sun gets pulled in an orbit around the center just as the Earth gets pulled around the sun.

But with all those massive stars floating round, gravity in the galaxy is far more diffuse and fragmented than gravity in a spherical Earth or sun. So as the sun orbits, it also gets pulled to and fro by nearby stars. The result is that it rides around in its orbit like a horse in a merry-go-round, round and round and up and down--never traveling the exact same orbit twice.

Beyond our local neighborhood of galaxies, some scientists believe that a huge invisible glob of mass (think Jabba the Hutt) called the Great Attractor pulls everything toward it. The star-spangled rivers of galaxies flowing toward the unknown object could also be said to be falling, or flowing, down.

And what about the universe? Do we know what’s up?

“When you talk about the whole universe, all bets are off,” Krupp said. For one thing, the universe is so large that you have to consider all four dimensions of space/time--not only the three spatial dimensions, but also the fourth dimension of time. So any discussion of where down is also has to consider when it is.

In addition, space/time is curved, so that “down” curls around. If you stand on the curved surface of the Earth and beam a hypothetical laser, say, east, and it travels all the way around the world, it will reappear behind you on the west. Similarly, in four-dimensional space, some speculate that you can point something in the down direction and have it reappear behind you as up.

In fact, in the universe at large, down is everywhere at once because the center of the universe is everywhere at once. Time and space both began with the big bang--the ultimate center of everything, and therefore also the ultimate “down.” There’s no point in asking “where” the big bang happened, because it created space, time and everything. At the beginning of the universe, where is everywhere, and so is down.

After the big bang, everything started expanding outward, or away from the center, which is to say, up. You could say that things have been looking up ever since.