Storm in a Teacup: The Physics of Everyday Life(62)

Many people envy astronauts for their adventures: their amazing views of our home planet, all the technical toys they get to play with, the accumulation of fabulous stories to tell, and the accolades of having one of the rarest and most hard-earned job descriptions in the world. But ask most people what they envy the most and you almost always get the same answer: weightlessness. All that floating about without “up” or “down” being a problem sounds both highly exciting and very relaxing. So it might seem slightly strange that astronauts in training need to be just as well prepared for the opposite problem: forces that far exceed gravity. The only current way to get to space is to sit on top of a rocket that’s accelerating pretty quickly. And it’s even worse on the way back down: reentry into the Earth’s atmosphere can generate forces four to eight times as strong as gravity, the sort of forces that a fighter pilot doing tight turns at high speed might have to deal with. If you feel slightly queasy as an elevator accelerates, this might not be for you. Depending on the direction of the additional g-forces, extra blood will be pushed toward or away from your brain, possibly even bursting the tiny capillary blood vessels in your skin. The details aren’t necessarily pleasant. But humans can not only survive these forces, they can also work while subjected to them (as you have to if you’re piloting a spacecraft back to Earth), and they do both better if they’re used to it. So a way has been found to train them.

All current astronauts and cosmonauts will spend considerable periods of time at the Yuri Gagarin Cosmonaut Training Center in Star City, just to the northeast of Moscow. Among the lecture halls, medical facilities, and spacecraft mock-ups sits the TsF-18 centrifuge. From the center of a huge circular room, the arm of the centrifuge stretches 60 feet outward. The capsule at the end can be changed depending on what’s needed on any given day. The tests that any budding astronaut has to pass involve sitting in the capsule while the arm rotates once every two or four seconds, which doesn’t sound like much until you calculate that the capsule itself must therefore be traveling around at either 120 or 60 mph. Once they’ve shown that they’ve got the right stuff, astronauts can practice working in these conditions, and are constantly monitored to check on how their bodies are responding. And it’s not just astronauts—test pilots and fighter pilots can also train here. The center even offers the experience to members of the public who can afford to pay for it. Be warned, though: The only thing about it everyone seems to agree on is that it’s very uncomfortable. But if you want to experience a consistent very high force, taking a spin is the way to do it.

The centrifuge is one way of exploiting the forces generated when something spins: by taking advantage of the ability to generate a very strong force in a single direction, and treating it like artificial gravity. But there is also a second way of employing forces from spinning. The tea and the cyclist and the astronaut were all confined—they were all forced to move in a circle because there was a solid barrier pushing back on them, preventing them from moving outward. But what if you’re spinning and there’s nothing external to trap you on a fixed circular path? This is a pretty common scenario. Rugby balls, spinning tops, and frisbees all spin without anything external pushing them inward. But the best way of seeing what’s going on is far more fun, and also edible: pizza.

To my mind, the perfect pizza should have a thin crispy base, the vital but understated foundation that lets the toppings shine. Raw pizza dough starts out as a rotund blob, a living lump that needs to be kneaded and nurtured to bring out the best in it. Transforming the blob into a delicate sheet without breaking it is an essential skill for a pizza maker, and some go a step further, taking the basic skill and turning it into theater. The chefs who toss pizzas have mastered the trick of letting the spinning do the work for them. Why push and prod each part of the dough with your fingers when you can just let physics sort out all those messy details? Especially when the flying disk gives you the mysterious aura of a dough wizard.

Tossing pizza dough has evolved into a proper spectator sport of its own; there’s now a world championship every year. There are even those who call themselves “pizza acrobats,” whose party piece is keeping a constantly spinning pizza base (or two) flying and somersaulting around their body for several minutes at a time. No one seems to eat pizza made from such well-traveled dough, but it definitely looks impressive. However, there are plenty of pizza chefs out there who spin their pizza dough briefly without making a cabaret act of it, and who have every intention of turning it into someone’s dinner. What is the spinning actually doing?

Some pizza-mad friends of mine recently took me to a very friendly restaurant with an open kitchen, and I asked whether I could watch someone spinning pizza dough. The young Italian chefs giggled a bit, but then gathered around the one who was brave enough to volunteer. Half embarrassed and half proud to show off, he patted a ball of dough to flatten it slightly, then picked it up and with a slight flick of the wrist, sent it twirling into the air.

What happened next happened very quickly. As the circle of dough left his hand, it was suddenly free of anything external pulling and pushing on it. It’s helpful to think about a single point on the edge. It’s traveling around in a circle, but only because the rest of the dough is stuck to it, pulling it inward. That inward pull is always necessary for something to rotate. In the case of the cyclist, the track is constantly pushing the bike from the outside, so that the cyclist must curve inward toward the center instead of continuing on a straight line. For the pizza dough, it’s a pull from the middle that makes the edge of the dough curve around toward the center. Either way, there has to be a force directed toward the middle of the spin. But dough is soft and elastic. If you pull on it, it stretches. The middle of the dough is pulling the edge inward, but that means there’s a pulling force across the dough. And so the dough has to stretch. When any solid object spins, the spinning generates forces inside it that you can’t see. The internal pull that’s keeping the pizza together is also stretching the dough, and the edge is getting farther and farther from the center. The brilliance of this for a pizza chef is that the internal pull is smooth and symmetrical. All of the pizza is spinning, so all of it stretches out away from the center.