Storm in a Teacup: The Physics of Everyday Life(84)
Our bodies are immense machines; even a single cell might contain a billion molecules, and there are around ten million million (1013) cells in our body. We need impressive signaling and transport systems to coordinate all these constituent parts, and that coordination takes time. No human has “lightning reactions,” because the cost of our wonderful complexity is the huge amount of time it takes us to get anything done. The shortest time that we can appreciate is approximately the blink of an eye (about a third of a second), but in that time millions of proteins have been built inside us and billions of ions have diffused across our nerve synapses, while the simpler world outside our bodies has just been getting on with things.
Our internal information engine carries on churning as we walk from one room to another. But this gigantic system needs information about what’s around it. Just at this moment, we need to find water. We have sensors built in, body parts that change in response to the environment and share that information with our brain. The sense that we’re most aware of is probably our sight.
We live submerged in light, but our body keeps most of it out. That sea of light carries information about the world, because light’s nature offers clues to its origins, but most of this information goes straight past us. A tiny fraction of this luminous cornucopia falls on to the pupils of our eyes, two circles at most 0.1 inch in diameter. A small subset of what arrives at the pupils, the visible light rays, is allowed in. From this tiny sample comes all of the visual richness that we take for granted. As they cross the boundary, these light waves must be marshaled so that the information can be harvested. Our windows on the world are guarded by soft, transparent lenses which slow the light to 60 percent of its speed in air. As these light rays are slowed down, they swerve, and the lens shape is tweaked by tiny muscles to make sure that all the rays from a single object out there, outside the body, will meet again at the back of the eye. This selection process is astonishing. We assume we see all there is, but really we sample only the tiniest fraction of what is out there to build our picture.
The light rays hitting our retina may have traveled from the Moon or from our fingers, but they have the same effect. A single photon is absorbed by a single opsin molecule, twisting the molecule around to start a chain of dominoes that sends an electronic signal into our control systems. As our thirsty body walked into the kitchen, photons that had bounced off a sink, a tap, and a kettle were streaming into our eyes, and our brain processes that information in the blink of an eye to tell us what to pick up first. If it’s slightly dark in the kitchen, we turn on a light bulb, releasing a fountain of light waves. They radiate outward, and as soon as their journey starts they’re being modified by the world, reflected, refracted, and absorbed until perhaps our eyes pick up what’s left. And it isn’t just light that’s flowing around us.
Humans are sociable animals. We hold our social networks together via communication, sending and receiving signals from others. Our voice is one of our most special features, a flexible musical instrument capable of producing and shaping sound waves which are then broadcasted through our surroundings. No Brit could imagine making a hot drink without asking others in the room whether they wanted to share the experience, and we ask with sound. The others in the room pick up the signal with their ears, and hearing the question will trigger a new flow of information inside their bodies, splitting and rejoining and gathering meaning until their nerve fibers instruct their vocal muscles to provide the appropriate response. Once the message has come back to us, we alter the world appropriately, rearranging the ceramics and metals in front of us.
Many different atoms make up our bodies, and wonderful though the variety is, there are limits to what we can do directly because of the way those atoms are arranged. But humans are experts at manipulating the world to produce tools that can do what we can’t. We can’t hold water in our hands as it boils, but a steel kettle can. We can’t turn a part of ourselves into an air-proof container for dried leaves, but a glass jar will do that job. We don’t have claws or a shell or tusks, but we can make knives and clothes and can openers. A ceramic container can hold a hot drink for us without transmitting the heat energy to our vulnerable and sensitive fingers. Metals, plastics, glass, and ceramics are all our proxies in the world, helped by the materials with biological origins: wood, paper, and leather.
The kettle has held the water molecules while giving them energy in the form of vibrations at the tiniest level. Now they’re jostling far more quickly than before, and we transfer them to their new ceramic home. Frustratingly, we can only see a hint of the splash of milk that bounces back up just after we add it to the drink. It’s right there, right in front of you in plain sight, but it’s virtually invisible because we just can’t process the signals fast enough. You can no longer see the bottom of the cup: The liquid that had been partly transparent is now opaque because light is bouncing off millions of tiny fat droplets.
As we’re manipulating the world around us, we take for granted that we are stuck to the floor by a force that’s manageable because our bodies have evolved to cope with it. If the Earth’s gravity were stronger, we’d probably need sturdier legs and we might find bipedal life difficult. If gravity were weaker, we might have evolved to be taller, but life would be slower because everything would take longer to fall. As we lift one leg to step, we’re relying on the pull of gravity to make us fall forward. We pivot around our stationary foot and by the time we stop the fall with the stepping foot, our whole body has moved forward. Walking doesn’t work without gravity, and our bodies have evolved to suit the Earth’s gravity. We’re just the right size and shape for bipedal walking to work for us. As we pick up the drink and move to the door, we’re using our own bodies as an upside-down pendulum, swinging each leg forward as we pivot about our other foot and hip. The rhythm in our walking caused by that regular swinging affects the liquid in the cup, forcing it to slosh with the same rhythm.