Zoom: From Atoms and Galaxies to Blizzards and Bees: How Everything Moves (21 page)

Most people completely misunderstand what’s afoot. The moon does not yank directly on ocean water. If it did, there might be something to the New Age belief that the moon’s pull affects human lives—after all, our bodies are 65 percent water. Instead, the real story involves the moon’s gravity in a very specific way. Because our cratered neighbor is so nearby, and because tidal forces vary with the cube (not the square) of distance between the earth and the moon, the moon exerts a greater “pull” on the side of the earth that’s facing it than it does on the far side. This difference is not what causes the tidal effect. It is the tidal effect.

A tidal effect is not gravity, but the difference in gravity between two locations.

This is the critical point. For when the moon passes overhead, there is no effective difference in its distance to your head versus its distance to your feet, just five or six feet farther from it. The difference is basically zero. No difference means no tidal effect. Your bodily fluids remain in their customary locations.

But Earth’s eight-thousand-mile diameter is another story. That’s nearly 4 percent of the distance between the moon and the earth. So the difference between the lunar force on Earth’s moon-facing hemisphere versus its force on the opposite hemisphere sets up a bit of torque that results in a three-foot bulge of ocean water.5 The moon mostly calls the shots when it comes to creating tides only because it’s so nearby. In truth, the sun exerts a far greater gravitational pull on us—177 times more than the moon does. After all, it’s twenty-seven million times more massive! But because the sun lies so far away, there’s just not much difference between its strength on the opposite sides of our planet. And—I can’t emphasize this enough—it’s the difference that matters, not the overall gravity.

But tides are quirky. In Tahiti there are no moon-caused tides at all. French Polynesia only experiences a single daily solar tide of a paltry one-foot height. As the tidal bulge of water travels around the various seas, there’s a rocking, an oscillation, and Tahiti happens to lie at a swivel point. It’s like carrying a shallow pan of water. A back-and-forth sloshing quickly arises. But in the middle of the pan the water scarcely moves. Tahiti sits at that fulcrum spot in the Pacific. In other places, tides arrive at illogical times, thanks to the shape of the harbor or bay.6

Currents are the second mover of the seas. These are powerful rivers of seawater that have enormous influence. Ocean waters move continuously. Whenever we’ve swum or sailed in the ocean we’ve probably felt horizontal currents. Some come and go and shift with the wind or affect only a small beach area. But other currents can run through much of an entire hemisphere as a response to tropical heat and the prevailing wind.

Currents can flow anywhere from 0.5 to 5.6 miles per hour—generally the same as a river’s speed. The Gulf Stream, which carries warm water from the Caribbean up the eastern coast of the United States and then to Europe, is one of the very fastest. Much more laid-back is the California Current, which brings chilly Alaska water down past Oregon to San Francisco, making its beaches fit for seals and nobody else. Also slow is the famous cold Humboldt Current, which moves up western South America from Antarctica, letting penguins lounge closer to the equator than anyone would think possible.

About 40 percent of global heat transfer is carried by ocean-surface currents, which are generally less than a thousand feet deep. They are created and steered mostly by the prevailing wind.

Our final mover of the seas is waves—the most visually obvious of them all. Here, too, nearly all the energy comes from wind. Waves in the open sea are usually between five and fifteen feet high and run at forty-five miles per hour. It’s important to remember that although a wave appears to be in motion, each individual drop of water does not move, except in a tiny circular path a few inches wide. After a wave passes, each drop of water is pretty much back in its original position. We see this clearly when watching floating debris.

Out at sea, waves are typically four hundred feet apart (the wavelength) and pass a given location every few seconds. In an individual series of waves, the interval between one wave and the next—sometimes as long as nine seconds but almost never more than that—never changes; the waves chug across the vast ocean in lockstep day after day.

All those days of lockstep monotony end when a wave reaches shallow water. As soon as its trough is half a wavelength’s distance from the bottom, friction starts acting on the wave’s base and increasingly slows it down. Meanwhile, momentum still carries its top forward at the previous rate. The result is that the wave’s top rises while also leaning farther and farther ahead. When the steepness ratio reaches 1:7 (i.e., the wave’s height is one-seventh of its length), it cannot support itself, and it “breaks.”

It’s stating the obvious that crashing waves exert enormous power. Their remorseless cycle of punishment lies almost beyond human appreciation. A single sea wave weighs thousands of tons. During storms, high waves can make the ground tremble with each impact, delivering a ton of force to each square foot of whatever substance—preferably inexpensive—receives its brunt.

Waves moving at forty-five miles per hour arrive from the open sea every five to eight seconds and “break” as soon as their height-to-length ratio reaches 1:7.

Needless to say, the wave phenomenon reaches its terrifying extreme in a tsunami. Even in our own times, had it not been for the widely videotaped and heartbreaking events in the Indian Ocean in 2004 and in northeastern Japan in 2011, a tsunami would still be popularly misconstrued as a single looming tidal wave moving toward shore. Now few would make that mistake. Whereas the average normal ocean wave travels at forty-five miles per hour, a tsunami moves at around five hundred miles per hour, rivaling a jet aircraft. And yet, the Noah story aside, the ancient world, too, seemed largely unaware of the possibility of the ocean utterly changing its behavior and taking countless lives.

It is true that, in prehistory, a fierce tsunami devastated the Norwegian Sea around 6000 BCE, but no records existed to warn Middle Eastern, Persian, and Mediterranean civilizations of tsunamis’ devastating power. And nearly the entire island of Thera, now called Santorini, was destroyed by an explosive eruption around 1650 BCE that created a tsunami so strong it wiped out the advanced Minoan civilization on nearby Crete. Yet a millennium later, when parts of the Bible were penned and the first thinkers of classical Greece were observing nature, this, too, had been essentially forgotten. A thousand years is, after all, a long time.

This lack of awareness changed in the summer of 426 BCE, when a modest tsunami startled the sailors in the ships armed for the Peloponnesian War. In his written history of that conflict, the Greek historian Thucydides openly mused about what could possibly cause the ocean to behave so strangely and correctly concluded that it must have been an undersea earthquake. He thus became the first to link the movement of solid earth with that of the liquid seas.

A half century later, in 373 BCE, a tsunami permanently submerged the town of Helike in Greece, obliterating its population and perhaps inspiring Plato, who was in his midfifties at the time, to speculate about a lost civilization that he called Atlantis. And yet this, too, was a largely localized event.

The same was not true 738 years later.

On July 21, 365 CE, an enormous undersea quake, the likes of which occurs only every few thousand years, struck the eastern Mediterranean between Crete and Egypt. Although everyone in the region felt the ground shake violently, it passed without widespread destruction—except on Crete, which received no warning that an astonishing hundred-foot-high wall of water was radiating outward. When we recall the eighty-foot tidal wave of the 2004 tsunami that killed a quarter million people in places such as Banda Aceh, Indonesia, or the seventy-seven-foot-high tsunami that wiped out Japan’s Fukushima Daiichi nuclear power plant in 2011 (thanks to the back-up diesel generators having been sited, bewilderingly, on the ground floor), we can appreciate how horrible a hundred-foot—ten-story—wall of water must be.

We have an actual eyewitness account of the 365 CE tsunami from a survivor. And not just any survivor but the Roman historian Ammianus Marcellinus, known for his accurate, unembellished accounts of the everyday life of his time. It was he who watched in astonishment as an event that was anything but everyday unfolded and recounted it almost matter-of-factly in Book 26 of his epic, Res Gestae:

Slightly after daybreak… the solidity of the whole earth was made to shake and shudder, and the sea was driven away… and it disappeared, so that the abyss of the depths was uncovered and many-shaped varieties of sea creatures were seen stuck in the slime.… Many ships, then, were stranded as if on dry land, and people wandered at will… to collect fish and the like in their hands; then the roaring sea… rises back in turn, and through the teeming shoals dashed itself violently on islands and extensive tracts of the mainland, and flattened innumerable buildings in towns.… For the mass of waters returning when least expected killed many thousands by drowning.… [H]uge ships, thrust out by the mad blasts, perched on the roofs of houses… others were hurled nearly two miles from the shore.

Another historian, Thucydides, said that “without an earthquake it does not appear to me that such a thing could happen.”

He was not entirely correct. Any large mass, such as a meteorite, hitting the sea can displace enough water to do the job. The largest wave ever recorded, an astonishing 1,720 feet tall, or about 50 percent higher than the Empire State Building, raged through Alaska’s Lituya Bay on July 9, 1958. This was the tallest tsunami in history. It did begin with an earthquake, but not a particularly big one; yet the tremor knocked loose a mass of rock that plunged three thousand feet into the Gilbert Inlet, displacing enough water to create the monster wave. The sheer scale of such motion defies visualization. After all, each cubic mile of the ocean weighs five billion tons.

The 365 CE tsunami went on to annihilate much of Alexandria, Egypt; Crete; and coastal Libya, and marched up the Nile River delta, hurling ships two miles inland. The quake permanently raised the coast of Crete by thirty feet—to this day the record elevation gain resulting from a single sudden event. This sunrise tsunami was so devastating that its anniversary was commemorated each year in Alexandria as “the day of horror”—for the next two centuries! It was quietly forgotten only at the end of the sixth century.7

Observing waves is everyone’s idle pastime. Waves have many cool attributes, as people such as Otis Redding have perenially noticed while they’re “sitting on the dock of the bay, watching the tide roll away.” One favorite attribute involves diffraction. We experience this principle when we turn on the car radio and the FM stations fade in and out as we pass close to hills or buildings. But the AM signals are much steadier and don’t vanish so readily. This is due to the bending of electromagnetic radiation around obstacles—diffraction. Longer wavelengths diffract more readily. AM stations broadcast waves hundreds of times longer than those on the FM band, so they bend around obstacles much more easily and are therefore not as readily blocked by obstructions. In other words, we don’t get into radio shadows as easily when we listen to AM’s widely spaced waves.

Now back to the ocean, whose waves are also pretty long—hundreds of feet—so they’re not easily stopped by a small obstacle. Notice how waves encountering a little rocky lighthouse island soon fill in again behind the island and continue on their way. If the sea waves were closer together, there’d be more of a “shadow” zone behind small islands in which the ocean is permanently calm. By noticing this phenomenon behind jetties, docks, and other obstructions of various sizes you can see the diffraction effect in action.

Major mysteries of maritime motion remain. For example, south of New York City, much of the coastline as far down as Florida is peppered with barrier islands—low offshore sand ridges that run parallel to the coast. Waves crash onto them, and thus they shield the tranquil lagoons behind them, where boaters enjoy miles of protected sailing without having to take to the rough open sea.

The question is: Why should barrier islands endure? All shorelines suffer major modifications from the relentless pounding of the sea and storms. Logic tells us that these barrier islands should not last long. Their continued existence is a mystery, although this doesn’t stop oceanographers from guessing.

Do breakers heap up sand scoured from the bottom of the sea and continually deposit it on the shore, replenishing the islands? Their sands are much higher than the high-tide mark, so any sand must be deposited during extreme storms. But such storms might just as well wash away these long, narrow, fragile islands, so we’re back to square one. Or are the islands perhaps remnants of giant sand dunes, maybe deposited during the last glaciation period? If so, then perhaps it’s the calm sea between them and the mainland that requires explanation—does it conceal a lowland running parallel to the dune ridges that got submerged when the sea level rose?

This lone example—and one could easily find hundreds—illustrates that even some simple aspects of the powerful moving sea and its relation to the long-suffering coastline remain uncertain. Our current science is not always up to the task of fully appreciating the ceaseless aquatic pageants.

Perhaps, as so many before us did, it’s sometimes better to simply sit “on the dock of the bay,” wasting time by watching the waves.

The Odd Entities Zooming Through Our Bodies

Yesterday, upon the stair,

I met a man who wasn’t there…

—HUGHES MEARNS, “ANTIGONISH” (1899)

Before the twentieth century, most people believed in ghosts or spirits. Yet no one in all of history suspected that tiny invisible entities zoom right through our bodies 24-7. Expressing such a belief would have gotten you thrown into a medieval insane asylum, where they probably didn’t even accept MediSerf.