Knocking on Heaven's Door (14 page)

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FIGURE 13
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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 physical laws they obey. Particle physics zones in on basic physical quantities and their interactions. These smaller components are of course relevant to complex physical behaviors that involve many components interacting in interesting ways. But identifying the smallest basic components and the way they behave is our focus here.

With technology and biological systems, the individual components of the larger systems have internal structure too. After all, computers are built from microprocessors built from transistors. And when doctors look inside human beings, they find organs and blood vessels and everything else that one encounters upon dissection that are in turn built from cells and DNA that one can see only with more advanced technology. The operation of those internal elements is nothing like what we see when we observe only the surface. The elements change at smaller scales. The best description for the rules those elements follow changes as well.

Since the history of the study of physiology is in some ways analogous to the study of physical laws, and covers some of the interesting length scales for humans, let’s take a moment to think a bit about ourselves and how some aspects of the more familiar inner workings of the body were understood before turning to physics and the external world.

The collarbone is an interesting example for which the function could only be understood upon internal dissection. It has its name because on the surface it seems like a collar. But when scientists probed inside the human body they found a key-like piece to the bone that gave it another name we often use: the clavicle.

Nor did anyone understand blood circulation or the capillary system connecting arteries and veins until the early seventeenth century when William Harvey did meticulous experiments to explore the details of hearts and blood networks in animals and humans. Harvey, though English, studied medicine at the University of Padua, where he learned quite a lot from his mentor Hieronymus Fabricius, who was interested in blood flow as well but misunderstood the role of veins and their valves.

Not only did Harvey change our picture of the actual objects involved—here we have networks of arteries and veins carrying blood in a branching network to capillaries working on smaller and smaller scales—but Harvey also discovered a process. Blood is transferred back and forth to cells in ways that no one anticipated until they actually looked. Harvey discovered more than a catalog—he discovered a whole new system.

However, Harvey did not yet have the tools to physically discover the capillary system, which Marcello Malpighi succeeded in doing only in 1661. Harvey’s suggestions had included hypotheses based on theoretical arguments that were only later validated by experiments. Although Harvey made detailed illustrations, he couldn’t achieve the same level of resolution that users of the microscope such as Leeuwenhoek would subsequently attain.

Our circulatory system contains red blood cells. Those internal elements are only seven micrometers long—roughly one hundred thousandth the size of a meter stick. That’s 100 times smaller than the thickness of a credit card—about the size of a fog droplet and about 10 times smaller than what we see with the naked eye (which is in turn a bit smaller than a human hair).

Blood flow and circulation is certainly not the only human process doctors have deciphered over time. Nor has the exploration of inner structure in human beings stopped at the micrometer scale. The discovery of entirely new elements and systems has since been repeated at successively smaller scales, in humans as much as in inanimate physical systems.

Coming down in size to about a tenth of a micron—10 million times smaller than a meter—we find DNA, the fundamental building block of living beings that encodes genetic information. That size is still about 1,000 times bigger than an atom, but is nonetheless a scale where molecular physics (that is, chemistry) plays an important role. Although still not fully understood, the molecular processes occurring within DNA underlie the abundantly broad spectrum of life that covers the globe. DNA molecules contain millions of nucleotides, so the significant role of quantum mechanical atomic bonds should not be surprising.

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.)

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FIGURE 14
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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 or even a proton, we’re asking for something intrinsically impossible. We simply don’t have the capacity to accurately visualize what is there.

But confusing our ability to picture phenomena with our confidence in their reality is a mistake that scientists cannot afford to make. Not seeing or even having a mental image doesn’t mean that we can’t deduce the physical elements or processes that are happening at these scales.

From our hypothetical vantage point on the scale of an atom, the world would appear incredible because the rules of physics are extremely different from those that apply to the scales we tick off on our measuring sticks at familiar lengths. The world of an atom looks nothing like what we think of when we visualize matter. (See Figure 15.)

Parts of the Atom

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FIGURE 15
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An atom consists of electrons orbiting a central nucleus, which consists of positively charged protons, each of charge one, and neutral neutrons, which have zero charge.

Perhaps the first and most striking observation one might make would be that the atom consists primarily of empty space.
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The nucleus, the center of an atom, is about 10,000 times smaller in radius than the electron orbits. An average nucleus is roughly 10-14 meters, 10 femto-meters, in size. A hydrogen nucleus is about 10 times smaller than that. The nucleus is as small compared to the radius of an atom as the radius of the Sun is when compared to the size of the solar system. An atom is mostly empty. The volume of a nucleus is a mere trillionth of the volume of an atom.

That’s not what we observe or touch when we pound our fist on a door or drink cool liquid through a straw. Our senses lead us to think of matter as continuous. Yet on atomic scales we find that matter is mostly devoid of anything substantial. It is only because our senses average over smaller sizes that matter appears to be solid and continuous. On atomic scales, it is not.

Near emptiness is not all that is surprising about matter on the scale of an atom. What took the physics world by storm and still mystifies physicists and nonphysicists alike is that even the most basic premises of Newtonian physics break down at this tiny distance. The wave nature of matter and the uncertainty principle—key elements of quantum mechanics—are critical to understanding atomic electrons. They don’t follow simple curves describing the definite paths that we often see drawn. According to quantum mechanics, no one can measure both the location and the momentum of a particle with infinite precision, a necessary prerequisite for following an object’s path through time. Heisenberg’s uncertainty principle, developed by Werner Heisenberg in 1926, tell us that the accuracy with which position is known limits the maximum precision with which one can measure momentum.
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If electrons were to follow classical trajectories, we would know at any given time exactly where the electron is and how fast and in what direction it is moving so that we could know where it will be at any later time, contradicting Heisenberg’s principle.