Storm in a Teacup: The Physics of Everyday Life(5)
ONE LAST INTAKE of air, one calm flick of its fleshy tail, and the giant leaves the atmosphere behind. Everything this sperm whale needs to live for the next forty-five minutes is stored in its body, and the hunt begins. The prize is a giant squid, a rubbery monster armed with tentacles, vicious suckers, and a fearsome beak. To find its prey, the whale must venture deep into the real darkness of the ocean, to the places never touched by sunlight. Routine dives will reach 1,600–3,200 feet, and the measured record is around 1.2 miles. The whale probes the blackness with highly directional sonar, waiting for the faint echo that suggests dinner might be close. And the giant squid floats unaware and unsuspecting, because it is deaf.
The most precious treasure the whale carries down into the gloom is oxygen, needed to sustain the chemical reactions that power the swimming muscles, and the whale’s very life. But the gaseous oxygen supplied by the atmosphere becomes a liability in the deep—in fact, as soon as the whale leaves the surface, the air in its lungs becomes a problem. For every additional yard it swims downward, the weight of one extra yard of water presses inward. Nitrogen and oxygen molecules are bouncing off each other and the lung walls, and each collision provides a minuscule push. At the surface, the inward and outward pushes on the whale balance. But as the giant sinks, it is squashed by the additional weight of the water above it, and the push of the outside overwhelms the push from the inside. So the walls of the lungs move inward until equilibrium, the point where the pushes are balanced once again. A balance is reached because as the whale’s lung compresses, each of the molecules has less space and collisions between them become more common. That means that there are more molecules hammering outward on each bit of the lungs, so the pressure inside increases until the hammering molecules can compete equally with those outside. Thirty-two feet of water depth is enough to exert additional pressure equivalent to a whole extra atmosphere. So even at that depth, while it could still easily see the surface (if it were looking), the whale’s lungs reduce to half the volume that they were. That means there are twice as many molecular collisions on the walls, matching the doubled pressure from outside. But the squid might be half a mile below the surface, and at that depth the vast pressure of water could reduce the lungs to less than 1 percent of the volume they have at the surface.
Eventually, the whale hears the reflection of one of its loud clicks. With shrunken lungs, and only sonar to guide it, it must now prepare for battle in the vast darkness. The giant squid is armed, and even if it eventually succumbs, the whale may well swim away with horrific scars. Without oxygen from its lungs, how does it even have the energy to fight?
The problem of the shrunken lungs is that if their volume is only one-hundredth of what it was at the surface, the pressure of the gas in there will be one hundred times greater than atmospheric pressure. At the alveoli, the delicate part of the lungs where oxygen and carbon dioxide are exchanged into and out of the blood, this pressure would push both extra nitrogen and extra oxygen to dissolve in the whale’s bloodstream. The result would be an extreme case of what divers call “the bends,” and as the whale returned to the surface the extra nitrogen would bubble up in its blood, doing all sorts of damage. The evolutionary solution is to shut off the alveoli completely, from the moment the whale leaves the surface. There is no alternative. But the whale can access its energy reserves because its blood and muscles can store an extraordinary amount of oxygen. A sperm whale has twice as much hemoglobin as a human, and about ten times as much myoglobin (the protein used to store energy in the muscles). While it was at the surface, the whale was recharging these vast reservoirs. Sperm whales are never breathing from their lungs when they make these deep dives. It’s far too dangerous. And they’re not just using their one last breath while they’re underwater. They’re living—and fighting—on the surplus that’s stored in their muscles, the cache gathered during the time they spent at the surface.
No one has ever seen the battle between a sperm whale and a giant squid. But the stomachs of dead sperm whales contain collections of squid beaks, the only part of the squid that can’t be digested. So each whale carries its own internal tally of fights won. As a successful whale swims back toward the sunlight, its lungs gradually reinflate and reconnect with its blood supply. As the pressure decreases, the volume once again increases until it has reached its original starting point.
Oddly, the combination of complex molecular behavior with statistics (not usually associated with simplicity) produces a relatively straightforward outcome in practice. There are indeed lots of molecules and lots of collisions and lots of different speeds, but the only two important factors are the range of speeds that the molecules are moving at, and the average number of times they collide with the walls of their container. The number of collisions, and the strength of each collision (due to the speed and mass of that molecule) determine the pressure. The push made by all that compared with the push from the outside determines the volume. And then the temperature has a slightly different effect.
“WHO WOULD NORMALLY be worried at this point?” Our teacher, Adam, is wearing a white tunic stretched over a happily solid belly, exactly what central casting demands of a jolly baker. The strong cockney accent is just a bonus. He pokes at the sad splat of dough on the table in front of him, and it clings on as though it’s alive—which, of course, it is. “What we need for good bread,” he announces, “is air.” I’m at a bakery school being taught how to make focaccia, a traditional Italian bread. I’m pretty sure I haven’t worn an apron since I was ten. And although I’ve baked lots of bread, I’ve never seen dough that looked like the splat, so I’m learning already.