Sun in a Bottle (6 page)

This baffled Thomson. The sun was emitting energy, and things that emit energy tend to cool down over time. So how can the sun stay so hot? Perhaps it was able to replenish its energy reserves by burning fuel—but what sort of fuel could it be using? It couldn’t be burning like a giant charcoal; burning is merely a chemical reaction, and no known chemical reaction could release the sorts of energy that the sun was radiating. No fire could generate that much heat. Could the sun be getting energy from another source—by gravity, perhaps? Meteors occasionally strike the sun, adding energy to the solar furnace, but there aren’t that many meteors around, and their energy is just a tiny drop in the tsunami of power coming from the sun. Thomson knew of no possible way to generate the quantity of energy that was escaping the sun every second, yet the first law of thermodynamics dictates that the energy has to come from somewhere.

It was clear to the physicist: no mechanism—chemical, gravitational, electrical, or whatever—existed that could generate the amount of energy the sun was emitting every moment. Since energy can’t be created out of nothing, the sun must be depleting its energy reserves at an enormous rate. And that meant that the sun must be slowly getting colder and colder. Unable to replenish its reserves through any means known to man, the sun, presumed to be a gigantic ball of hot liquid, must slowly be cooling down.

Once Thomson reached that conclusion, he wondered: If the sun is merely a huge molten sphere of liquid, where did it get its energy in the first place? The only answer he could think of was the energy due to gravitation. Imagine that the sun came from an enormous cloud of tiny rocks. Those rocks are attracted to one another by the force of gravity. Under their mutual attraction, they begin falling toward one another. As they fall inward toward the center of the cloud, they move ever faster. The cloud of rocks begins to collapse. The individual rocks speed inward quicker and quicker, because their gravitational energy is being converted into kinetic energy—the energy of their motion. As the fast-moving rocks stream toward the center of the cloud and collide, their kinetic energy gets converted into heat energy: the cloud heats up. Eventually, it gets so hot it glows.

Thomson calculated how hot such a protosun could have been. Then he calculated how long it would have taken to cool to its present temperature. Not long. The sun wasn’t more than a few tens of millions of years old, not long enough for the long, slow process of evolution Darwin proposed.

In fact, Darwin was deeply shaken by the calculations. He considered Thomson’s challenge to evolution “one of the gravest” that the theory had to face, and he could do little to counter it other than argue that scientists did not have a perfect understanding of the nature of the universe.

It was an impasse. The laws of physics seemed to say one thing, while the observations of biologists seemed to tell another.

Physicists would have to follow a tortuous path before they could resolve the contradiction—a path that led, first, to understanding the mystery of matter.

 

 

By the end of the nineteenth century, physicists and chemists had unraveled many of the mysteries of the universe. Isaac Newton had divined the physical laws that govern how objects move and how gravity works. James Clerk Maxwell had figured out the subtle interrelationships between electric and magnetic forces. Thermodynamicists had codified the laws of energy and heat. At the same time, though, scientists did not know much about matter; they had little idea what sort of stuff made up stars and planets and people. That was soon to change, as they came rapidly to the conclusion that matter was composed of tiny building blocks known as atoms.

Atomic theory, in its most primitive form, goes back to the ancient Greeks. In the fifth century BCE, the philosopher Democritus held that all matter was created out of little indivisible particles. These particles, far too tiny to see, were considered to be uncuttable. Democritus’s idea was just one of a huge number of competing theories about the universe. Some philosophers argued that everything was made of fire; others thought that objects were made from a mixture of earth, air, fire, and water. Some argued that matter was infinitely divisible; others, like Democritus, argued that there was a limit to how finely you could slice an object. Though we now know that Democritus’s idea was closest to the truth, for millennia it had no special status.

More than two thousand years later, a steady march of experimentation and observation led scientists to the conclusion that Democritus was essentially correct: matter is made up of tiny atoms. Chemists had led the way; the work of chemists such as the Briton John Dalton, the Italian Amedeo Avogadro, and the Russian Dmitri Mendeleev began to produce a picture in which all matter consisted of a collection of invisible “elemental” particles. Water, for example, was made up of two particles of hydrogen and one of oxygen; alcohol had two of carbon, six of hydrogen, and one of oxygen.

There was only a handful of known elements, and they each had different properties. For example, the atoms of some elements, such as hydrogen, oxygen, and carbon, were very light. Other elements’ atoms, like those of mercury, lead, and uranium, were very heavy. And these particles—these atoms—were fixed in their properties; it was impossible to transmute an atom of hydrogen, say, into an atom of lead.

This picture explained the nature of matter extremely well. Within a century, atomic theory changed the subject of chemistry from a quasi-mystical hodgepodge of contradictory ideas into a real science. Physicists soon joined the chemists in their support of atomic theory; they began to provide evidence for the existence of tiny atomic particles. Theorists like Ludwig Boltzmann realized that you could explain the properties of gases simply by imagining matter as a collection of atoms madly bouncing around. Observers even saw the random motion of atoms indirectly: the jostling of water molecules makes a tiny pollen grain swim erratically about. (Albert Einstein helped explain this phenomenon—Brownian motion—in 1905.) Though a few stubborn holdouts absolutely refused to believe in atomic theory,
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by the beginning of the twentieth century the scientific community was convinced. Matter was made of invisible atoms of various kinds: hydrogen atoms, oxygen atoms, carbon atoms, iron atoms, gold atoms, uranium atoms, and a few dozen others. But, as scientists were soon to find out, atoms are not quite as uncuttable as the ancient Greeks thought. Indeed, to figure out why different elements have different properties, physicists had to slice the atom into pieces.

The first piece came off in 1898. The Cambridge physicist J. J. Thomson was studying a mysterious phenomenon known as cathode rays. He used electric and magnetic fields to deflect the rays and came to the correct conclusion that the rays were made up of negatively charged particles that had been stripped away from atoms. These very, very light particles came to be known as electrons.

Since an atom is, on balance, neither positively nor negatively charged, the positive and negative charges in the atom must be equal and opposite; the charges in the atom have to cancel each other out. This means that for every electron in an atom, there has to be something else in the atom that carries the equivalent positive charge. About a decade after the discovery of the electron, the physicist Ernest Rutherford found out where that equal and opposite charge sits. It resides in tiny, but extremely solid, nucleus at the very center of the atom. This nucleus is quite heavy, thousands of times heavier than an electron, so the nucleus of an atom had to be made of stuff very different from electrons. Rutherford soon figured out what that positively charged stuff was: he realized that the positive charge is cloistered inside a heavy particle known as a proton.

For every electron zipping around in the outer regions of the atom, a proton had to be sitting in the nucleus. Since positively charged objects attract negatively charged ones, the nucleus attracts the electrons through electrical forces, in roughly the same way that the sun attracts its planets with gravitational forces. Rutherford took this analogy fairly literally; he imagined the atom to be like a miniature solar system. At the center is a heavy, dense, positively charged nucleus. Quite a distance away, lighter, quick-moving, negatively charged electrons are in “orbit” around it.
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In between, there is empty space—lots of it.

When physicists discovered the proton and electron, they sparked a revolution in the scientific understanding of matter. Two subatomic particles suddenly explained the properties of the elements. No longer were atoms of different elements considered to be fundamentally different objects; an atom of gold need not be thought of as a different sort of creature compared with an atom of lead. Gold and lead were essentially the same kind of object: bundles of protons surrounded by bundles of electrons. Gold has properties different from those of lead—and they both have properties different from the other elements—because they have different numbers of protons in their nuclei (and, hence, different numbers of electrons). A hydrogen atom has one proton per nucleus, helium has two. Oxygen has eight; gold, forty-seven; lead, eighty-two; uranium, ninety-two. In each case, the number of protons in a nucleus—known as an atom’s atomic number—determines how the atom behaves chemically. It tells you which atoms it will react with and which it won’t; it tells you whether a collection of atoms is likely to be a gas or a metal, whether it will burn in oxygen or explode in water or refuse to react with anything at all. This theory was a tremendous success for science. The uncuttable atom had been dissected into its component parts. But one piece was still missing.

The discovery of the electron had come from Thomson’s investigations into cathode rays. Cathode rays come from a fairly simple piece of laboratory equipment: put a couple of pieces of metal in a vacuum tube, hook them to a battery, and
radiation
streams from one end to the other.

The concept of radiation was a new phenomenon at the turn of the twentieth century. Scientists knew little about it, but they were beginning to detect it everywhere. Marie Curie’s radium emitted a substance—particles or rays or something as yet unknown—that carried energy; something was fogging a photographic plate. That was one kind of radiation. The German scientist William Roentgen discovered another kind in 1895. When he sent electrical current through an evacuated tube, he noticed it would generate invisible rays that could make fluorescent screens glow. Like the rays coming from radioactive elements like radium and uranium, Roentgen’s
x-rays
could expose a photographic plate. X-ray radiation, too, carries energy. (It turned out that x-rays are beams of light so energetic that they pass right through flesh.) Then there were the mysterious rays coming from Thomson’s cathode. By the turn of the century, scientists across the world were finding all sorts of rays in strange places. The scientific world was going radiation crazy.

We now know that these “radiations” are not all the same thing. Some, like x-rays, are varieties of light. (Gamma rays, too, are light beams even more energetic, and more penetrating, than x-rays.) These high-energy light rays penetrate matter relatively easily. Not all the radiations had this property. Thomson’s cathode rays couldn’t penetrate very far into an object before being absorbed. Neither could
beta
radiation, another type of emanation that streams from certain kinds of unstable atoms. Alpha radiation, which comes from yet other varieties of unstable atoms, penetrates even less than beta rays. It turns out that cathode rays, beta rays, and alpha rays are all subatomic fragments. Cathode rays and beta rays are both made up of electrons; alpha rays are made up of heavier, positively charged pieces of large atoms.
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Not surprisingly, researchers were so excited about finding new kinds of radiation that some of their discoveries were entirely fictional. In 1903, the French physicist René Blondlot thought he had discovered a new type, which he dubbed the “N-ray.” But Blondlot had deceived himself; his desire to believe in N-rays made him ignore the evidence against them. When a skeptical researcher removed a crucial component of the experimental apparatus and the unsuspecting Blondlot continued to observe the N-rays, N-rays were exposed as a fiction. Blondlot was made a laughingstock.

There was one type of radiation, though, that did not fit neatly into the pattern scientists had been seeing. High-energy light, such as x-rays or gamma rays, penetrates matter easily; its beams are hard to block. Fragments of atoms—charged particles like protons and electrons and alpha particles—tend not to penetrate matter much at all. Because of their charge, they get tangled in the electrons and protons in a given hunk of matter and quickly slow to a halt. But a new type of radiation, discovered in the 1930s, seemed like a weird cross between light and atom fragment. Scientists generated this bizarre radiation by shining a beam of alpha particles upon certain kinds of atoms (such as beryllium atoms). This new kind of radiation did not have an electric charge: it was unaffected by electric or magnetic fields. It penetrated matter as readily as gamma rays did, but it did not behave as a light beam should. It behaved like a heavy particle: it would hit a block of paraffin and knock protons out; mere light couldn’t do that so easily. In 1932, the British physicist James Chadwick concluded, correctly, that this new type of radiation consisted of particles almost identical to protons but for one major difference: they had no electric charge. Chadwick won the Nobel Prize for his discovery: the neutron.

The neutron is just a tiny fraction of a percent heavier than a proton, so it has quite a bit of oomph. But because it is electrically neutral, it doesn’t “feel” the electrical charges of the electrons and protons in a material. It is only affected by an atom when it slams directly into the nucleus. However, since atoms are mostly empty space and atomic nuclei are very small, a neutron can zoom straight through a chunk of matter without ever encountering something that deflects it. Neutrons penetrate matter extremely well, going through lead bricks almost as if they didn’t exist. But when a neutron does, by chance, hit an atomic nucleus, it packs a punch. A light atom (such as hydrogen) might be kicked out of the substance altogether. A heavy atom (such as uranium) might shiver and break apart when struck with the right amount of force. (As described in chapter 1, neutrons doing just this is what causes the chain reaction at the heart of the atom bomb.)