You can make those internal spinning forces so strong that things that would never separate out under gravity alone can be teased apart. For example, if you ever have a blood test for anemia, laboratory technicians will put a sample of your blood in a centrifuge and spin it so fast that it experiences an outward force perhaps twenty thousand times as great as gravity. Red blood cells are far too small to separate out under gravity in any normal circumstances, but they can’t resist the forces generated by the centrifuge. Under those conditions, it takes only five minutes for almost all the red blood cells to be pulled outward from the center of the centrifuge, toward the bottom of the tube. They’re more dense than the liquid they’re in, so they win the race to the bottom. Once they’re all there, the tube can be lifted out and it’s possible to directly measure the percentage of your blood that is made up of red blood cells, just by measuring the thickness of the lowest layer. This is a simple test that can indicate a range of health problems, and it’s also used to check for blood doping in athletes. If it wasn’t for the forces generated by spinning, this measurement would be much harder and much more expensive to carry out. And these forces can be applied to much larger things than blood samples. One of the biggest centrifuges in the world is designed to spin an entire human being.
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.