Knocking on Heaven's Door - Lisa Randall [40]
I sometimes find it remarkable how constrained our comfort zone can be. The NBA basketball player Joakim Noah is a friend of my cousin. My family and I never tire of commenting on his height. We can look at photos or marks on a door frame charting his height at various ages and marvel at him blocking a smaller guy’s shot. Joakim is mesmerizingly tall. But the fact is, he is only about 15 percent taller than the average human being, and his body works pretty much like everyone else’s. The exact proportions might be different, sometimes giving a mechanical advantage and sometimes not. But the rules his bones and muscles follow are pretty much the same that yours do.
Newton’s laws of motion, written down in 1687, still tell us what happens when we apply force to a given mass. They apply to the bones in our body and they apply to the ball Joakim throws. With these laws we can calculate the trajectory of a ball he tosses here on Earth and predict the path the planet Mercury takes when orbiting the Sun. In all cases, Newton’s laws tell us that motion will continue at the same speed unless a force acts on the object. That force will accelerate an object in accordance with its mass. An action will induce an equal and opposite reaction.
Newton’s laws work admirably for a well-understood range of lengths, speeds, and densities. Disparities appear only at the very small distances where quantum mechanics changes the rules, at extremely high speeds where relativity applies, or at enormous densities such as those in a black hole where general relativity takes over.
The effects of any of the new theories that supersede Newton’s laws are too small to ever be observed at ordinary distances, speeds, or densities. But with determination and technology we can reach the regimes where we encounter these limitations.
JOURNEY INSIDE
We have to travel a ways down before we encounter new physics components and new physical laws. But a lot goes on in the range of scales between a meter and the size of an atom. Many of the objects we encounter in our daily existence as well as in life itself have important features we can notice only when we explore smaller systems where different behaviors or substructures become prominent. (See Figure 13 for some scales that we refer to in this chapter.)
Of course, a lot of objects we’re familiar with are made by simply putting together a single fundamental unit many times, with few details or any internal structure of interest. These extensive systems grow like walls of bricks. We can make walls bigger or smaller by adding more or fewer bricks, but the basic functional unit is always the same. A large wall is in many respects just like a small wall. This type of scaling is exemplified in many large systems that grow with the number of repeated elementary components. This applies, for example, to many large organizations as well as computer memory chips that are composed of large numbers of identical transistors.
A different type of scaling that applies to other types of large systems is exponential growth, which occurs when the connections, rather than the fundamental elements, determine a system’s behavior. Although such systems too grow by adding many similar units, the behavior depends on the number of connections—not just the number of basic units. These connections don’t extend just to an adjacent part, as with bricks, but can extend to other units across the system. Neural systems composed of many synaptic connections, cells with many interacting proteins, and the Internet with a large number of connected computers are all examples. This is a worthy subject of study in itself, and some forms of physics also deal with related emergent macroscopic behavior.
[ FIGURE 13 ] A tour of small scales, and the length units that are used to describe them.
But elementary particle physics is not about complex multi-unit systems. It focuses on identifying elementary components and the