Knocking on Heaven's Door - Lisa Randall [42]
DNA can itself be categorized on different scales. With its twisty convoluted molecular structure, the total length of human DNA can be measured in meters. But DNA strands are only about two thousandths of a micron—two nanometers wide. That’s a little smaller than the current smallest transistor gate of a microprocessor, which is about 30 nanometers in size. A single nucleotide is only 0.33 nm long, comparable in size to a water molecule. A gene is about 1,000-100,000 nucleotides long. The most useful description of a gene will involve different types of questions than those we would confer on individual nucleotides. DNA therefore operates in different ways on different length scales. With DNA, scientists ask different questions and use different descriptions on different scales.
Biology resembles physics in the way that smaller units give rise to the structure that we see at large scales. But biology involves far more than understanding the individual elements of living systems. Biology’s goals are far more ambitious. Although ultimately we believe the laws of physics underlie the processes at work in the human body, functional biological systems are complex and intricate and often have difficult-to- anticipate consequences. Disentangling the basic units and the complicated feedback mechanisms is enormously difficult—complicated further by the combinatorics of the genetic code. Even with knowledge of the basic units, we still have the formidable task of resolving more complicated emergent science, notably that responsible for life.
Physicists too can’t always understand processes at larger scales through understanding the structure of individual subunits, but most physics systems are simpler in this respect than biological ones. Although composite structure is complex and can have very different properties than the smaller units, feedback mechanisms and evolving structure usually play less of a role. For physicists, finding the simplest, most elementary component is an important goal.
ATOMIC SCALES
As we move away from the mechanics of living systems and descend further in scale to understand basic physical elements themselves, the next length at which we will momentarily pause is the atomic scale, 100 picometers, which is about 10,000 million (1010) times smaller than a meter. The precise scale of an atom is difficult to pin down since it involves electrons that circulate around a nucleus but are never static. However, it is customary to categorize the average distance of the electron from the nucleus and label that as an atom’s size.
People conjure up pictures to explain physical processes on these small scales, but they are necessarily based on analogies. We have no choice but to apply descriptions we’re familiar with from our experiences at ordinary length scales in order to describe a completely different structure that exhibits strange and unintuitive behavior.
Faithfully drawing the interior of an atom is impossible with the physiology most readily at our disposal—namely, our senses and our human-sized manual dexterity. Our vision, for example, relies on phenomena made visible by light composed of electromagnetic waves. These light waves—the ones in the optical spectrum—have a wavelength that varies between about 380 and 750 nanometers. That is far larger than the size of an atom, which is only about a tenth of a nanometer. (See Figure 14.)
[ FIGURE 14 ] An individual atom is a mere speck relative to even the smallest wavelength of visible light.
This means that probing within the atom with visual light to try to see directly with our eyes is as impossible as threading a needle with mittens on. The wavelengths involved force us to implicitly smear over the smaller sizes that these overly extended waves could never resolve. So when we want to literally “see” quarks