“To see a world in a grain of sand,
And a heaven in a wild flower,
Hold infinity in the palm of your hand,
And eternity in an hour.”
William Blake (1803)1
Everything in the material Universe is composed of countless unseen, moving particles. Some of them are atoms or molecules; others are sub-atomic particles like electrons, neutrons, or protons. Invisible atoms are moving about in the solid chair you are sitting in; unseen electrons travel along wires to light your room; and the molecules in the air you breathe are darting about with a speed that increases with their temperature. Large numbers of imperceptible, electrified particles are even moving out from the Sun, many of them enveloping the Earth.
Invisible, Immortal Atoms
What are things made out of? To find out, you might try breaking any object into smaller and smaller pieces, until you reach a stage when the smallest piece cannot be broken apart. That last step in this imaginary decomposition arrives at the atoma, a Greek word that means “unable to cut.”
The belief that all visible objects are composed of tiny unseen, immortal atoms dates back thousands of years, to ancient Greek and Roman philosophers, and it has never been discounted. The incredible atoms are supposed to be in continual motion within all that exists throughout all eternity, coming together to create visible things and dispersing when objects fall apart. They are the former ingredients of all that existed in the past and will be the seeds of everything that might exist in the future.
As the great English astronomer Isaac Newton wrote in 1704:
“It seems probable to me that God in the Beginning formed Matter in solid, massy, hard, impenetrable, moveable Particles … even so very hard as never to wear or break in pieces; no ordinary Power being able to divide what God himself made one in the first Creation.”2
In the early 19th century, the English chemist John Dalton proposed that all the various kinds of materials are composed of pure, unalterable substances, the atomic elements, which cannot be divided by chemical means.3 Any material object can be decomposed into these simple, elemental atoms that remain exactly identical wherever and whenever they are found. Nowadays, these elements are specified by atomic numbers, with hydrogen the lightest element numbered 1, through carbon 6 and oxygen 8, and up to gold 79 and beyond.
Elemental atoms are combined and bound together in thousands and millions of ways to form molecules, from the Latin for “little mass,” and a countless number of them are hidden in everything we see. Every time you breathe, you take in roughly one million, billion, billion, or 1024, molecules of oxygen from the air. A drop of water contains a comparable number of atoms, close to the number of stars in the Universe.
The tiny elemental atoms are much smaller than a speck of dust, and as invisible as germs, a phantom, or a spirit. As the Little Prince learned, “what is essential is invisible to the eye; the house, the stars, the desert — what gives them beauty is something that is invisible.”4
Radioactivity
The most abundant atoms found on the Earth are exceptionally durable, and have been around for billions of years. Only relatively rare, naturally occurring elements are temporary and unstable, like uranium and other radioactive elements. Their discovery began at the close of the 19th century, at the Sorbonne in Paris, when Henri Becquerel found that minerals containing uranium emitted strong, invisible radiation that fogged photographic plates.5 This mysterious, penetrating emission was being sent out at a regular pace in intense light or pitch darkness and at high or low temperatures. It eventually became known as radioactivity, with the term radio implying “radiation,” so the radioactive atoms were radiation-active. No one knew just what the imperceptible rays were, how they were energized, or why the radioactive materials kept pouring out energy, seemingly nonstop.
Hearing of Becquerel’s discovery, Pierre Curie, also a Professor of Physics at the Sorbonne, and the young graduate student he had recently married, Manya (Marie) Sklodowska Curie, began to investigate the new kind of rays.6 Marie Curie developed methods of measuring the amounts being released, and she found that impure uranium ores emitted more rays than could be explained in terms of the uranium they contained.
The couple began a laborious two-year search for the unknown substance that was emitting the powerful rays. From one ton of uranium ore known as pitchblende they extracted just a few grams of powerful new radioactive elements that had not been previously known. One of them, called radium, released an energy that surpassed anything that had been achieved by chemical reactions. Crystals containing radium would light up an otherwise dark room, and also burn the skin, as Pierre Curie discovered to his dismay.
These findings became an international sensation. In 1903, the Nobel Prize in Physics was, for example, awarded jointly to Bequerel, for his discovery of spontaneous radioactivity, and to Pierre and Marie Curie for their joint researches on this radiation phenomenon. [Marie Curie was the first woman to win a Nobel Prize, the first person to win twice (in 1911 for Chemistry), and the first woman to become a Professor at the University of Paris. She died in 1934 due to illness brought on by exposure to radium and x-rays.]
The emanations of radium were investigated in greater detail by the New-Zealand born physicist Ernest Rutherford and the English radio-chemist Frederick Soddy while they were working at the McGill University in Montreal, Canada. They found that the radioactive rays included at least two distinct types, termed α rays and β rays, which are not waves of radiation but instead beams of energetic, fast-moving particles.7 By using electric and magnetic fields, the two types of particles could eventually be separated and their physical properties determined. 8
The rocks and soil under our feet are still radioactive billions of years after the Earth formed, and they still heat the interior of the planet.9 So the radioactive atoms have been around for a very long time, and are just slowly falling apart. That’s because it is ever so hard for a α particle to overcome the forces holding it within an atom.
The young Russian physicist George Gamow explained the mystery in 1928, while at the University of Göttingen, in what is now Germany. He proposed that the α particles have a range of possible behaviors with varying probability. Although they usually lack the energy to overcome the forces holding them within a radioactive atom, some of the α particles have a small nonzero possibility of penetrating that barrier and escaping to the outside world.10
In this surreal world of sub-atomic probability, one could relentlessly throw a ball against a wall, watching it bounce back countless times, until eventually the ball would tunnel through the wall, breaking out to the other side. As Ahab stated in Herman Melville’s Moby Dick: “How can the prisoner reach outside except by thrusting through the wall.”11
The known, steady decay rates of radioactive atoms have been used to clock the Earth’s age. By combining these rates with measurements of the relative amounts of radioactive parent elements and stable, non-radioactive daughters, an age of about 4.6 billion years has been determined for the Earth.12
All the atoms, from the durable ones to the radioactive ones, are in a state of continual motion, with speeds that depend on their temperature.
Ever Moving Atoms and Molecules
Atoms and molecules are always in motion, pushing and jostling each other, coming together and pulling apart, in constant invisible turmoil. This incessant movement can be noticed if you add some wine to a glass of water; the two liquids do not remain separated but dissolve and move into each other. A similar thing happens when two gases are mixed together, as when smoke from a chimney moves out into the air and disperses and disappears within it.
Molecules are always moving about because they are hot. That is, their energy of movement, and their state of agitation, increases with the temperature. The higher the temperature the faster a molecule moves, and at a specific temperature, lighter molecules tend to move faster than heavier ones. When the temperature drops to absolute zero, every molecule will cease to move, becoming totally immobile and completely at rest.
How fast and far do molecules move? Each molecule in the air of your room is moving with speeds of about 500 meters per second. That is about as fast as a rifle bullet moves when fired. But no molecule ever travels very far. There are so many molecules in the air that a given molecule hits another one after traveling only one ten-millionth of a meter. In just one second, this molecule collides with other ones and is deflected from its course five billion times.
Figure 3.1 Speed distribution The speeds of particles with the same mass and three different temperatures. The peak of this distribution shifts to higher speeds at higher temperatures. The peak also shifts to higher speeds at lower mass when the temperature is unchanged.
It was James Clerk Maxwell, from Scotland, who in 1860 realized that gas molecules gain or lose speed by collisions with one another. He showed that these encounters produce a statistical distribution of speeds, in which all the speeds might occur with a different and known probability.13 At any given instant, the speed of most of the molecules in a gas is very close to the average value that increases with the temperature, but there is always a small percentage that move faster or slower than the average and this range also increases with the temperature (Fig. 3.1).
Maxwell’s discovery was a true moment of unexpected insight, made at a time when no one else even imagined that a gas is composed of molecules whose motions determine its physical properties. In addition to atoms or molecules, his distribution can be used to describe the speeds of numerous sub-atomic particles inside the Sun. It applies to the motions of many different kinds of particles, as long as there are a lot of them in a state of thermal equilibrium, characterized by a single temperature.
As a result of the frequent collisions between each other, molecules are in haphazard movement and no molecule ever takes a direct path anywhere. They move about in a disoriented and random way, first moving in one direction and then in another one. This meandering journey can be observed by watching small particles or grains suspended in water.14
The observed motion does not originate either in the particles themselves or from currents in the water, but instead arises from water molecules that relentlessly strike nearby pollen particles that travel on a lively, disoriented path (Fig. 3.2). Watching the movement caused by the unseen molecular collisions is somewhat like inferring the motion of sea waves by observing the rocking motion of a ship.
Figure 3.2 Brownian motion The irregular movement of a microscopic grain. [Adapted from Jean Perrin’s Les Atomes, published in 1913].
The particles deep within a star are in a heightened state of movement due to the exceptionally high temperature down there. These atoms are so hot, and the collisions between them so frequent, that they are torn into their internal parts.
Sub-Atomic Particles
When bombarding gold leaf with beams of α particles in the second decade of the 20th century, Ernest Rutherford found that most of the α particles went right through the material as if it was not there.15 To his amazement, only about 1 in 20,000 of them bounced right back from where they had come from, while all the others passed through the gold. This indicated that the mass of an atom is concentrated in a nucleus that is 100,000 times smaller than the atom. It meant that the material part of an atom is concentrated almost entirely at its center, and that atoms are mostly empty space.
Rutherford was able to show that the nuclei of different atoms contain various amounts of the nucleus of the simplest atom, hydrogen. He named this nuclear building block a proton, the name by which it has been known ever since. The term is derived from the Greek word proto for “first,” because it was the first particle to be found in the nucleus of an atom.16
The proton is positively charged, with a charge equal in amount to that of the electron but opposite in sign, and particles with an opposite sign to their electric charge attract each other. Negatively charged electrons surround the positively charged protons in an atom, whose total positive charge is equal to the total negative charge. An atom therefore has no net electrical charge and it is electrically isolated from external space. It is the relatively low-mass electrons that give the atom its space-filling extendedness and volume. As an example, a single electron gives the hydrogen atom its volume and balances the charge of its single proton at the center.
Particles with the same electric charge are driven apart by an electrical repulsion, due to their charged similarity. Rutherford therefore postulated the existence of an uncharged nuclear particle, later called the neutron, to act as a sort of glue to help hold protons together in the atomic nucleus, and keep the protons from dispersing as they repelled each other (Fig. 3.3). As the name implies, the neutron has no electric charge. [Rutherford became director of the Cavendish Laboratory at the University of Cambridge in 1919, and under his leadership the English physicist James Chadwick discovered the neutron in 1932 — after an eleven-year search.17]
The proton and neutron have about the same mass, which is nearly 2,000 times that of the electron. But the mass of an atomic nucleus is always just a bit less than the sum of the masses of its protons and neutrons because they have expended energy in order to bind themselves together. That is, energy has been set free in the formation of a nucleus, and the mass defect between the nucleus and the combined mass of its components is a measure of how much energy was lost in the creation of the atomic nucleus.
Once astronomers knew about the internal ingredients of atoms, they could estimate how hot it is inside the Sun, and how fast the sub-atomic particles are moving within it.
Figure 3.3 Helium The helium atom contains two electrons that swarm about the atom’s nuclear center in largely empty space. The atom is about 100,000 times bigger than its nucleus, which consists of two protons and two neutrons.
What’s Inside the Sun?
The American astronomer Jonathan Homer Lane first realized, in 1870, that the Sun would have to be very hot inside to support its enormous mass and retain its exceptionally large shape.18 Hot particles, which we now know are protons, move about rapidly and frequently collide with each other to produce the gas pressure that keeps the Sun from collapsing under its great mass. [Since the mass of the electron is negligible compared to the mass of the proton, the mass of the Sun is provided by the protons, which determine the interior particle number density, the central temperature and the central gas pressure of the Sun.]
When the temperature rises to a central value of 15.6 million degrees kelvin, equilibrium is reached between the outward pressure of moving protons and the inward gravitational pull at the Sun’s center. At this temperature, a proton within the Sun will be moving, on average, at a speed of about 500 kilometers per second.
In every layer within the Sun or any other star, the weight of the overlying gas must be equal to the outward-pushing pressure; otherwise the star would expand or contract, which is usually not observed. At greater distances from the center, there is less overlying material to support and the compression, pressure, and temperature are less, so the material gets progressively thinner and cooler. At the visible disk of the Sun, for example, the solar gas becomes far more rarefied than the air we breathe, the temperature has fallen to 5,780 degrees Kelvin, and the protons move at an average speed of about 10 kilometers per second.
What’s Outside the Sun?
Although usually invisible, the outer solar atmosphere, known as the corona, can be seen near the Sun when its light is blocked, or eclipsed, by the Moon. During such a total solar eclipse, the corona is seen at the limb, or apparent edge of the Sun, against the blackened sky as a faint halo of white light (Fig. 3.4).
The amazing thing about the corona is that it is incredibly hot, with a temperature of a few million Kelvins. The visible disk of the Sun is several hundred times cooler than the overlying corona, which was entirely unexpected. It violates common sense, as well as the second law of thermodynamics, which implies that heat cannot be continuously transferred from a cooler to a warmer body without doing work. When we sit far away from a fire, for example, it warms us less. Contemporary solar astronomers attribute the heat of the corona to the interaction of magnetic fields that are always coming out from, and going into, the interior of the Sun.
Figure 3.4 Eclipse corona streamers The million-degree solar atmosphere, known as the corona, is seen around the black disk of the Moon during a total solar eclipse on July 11, 1991. (Courtesy of the High Altitude Observatory, National Center for Atmospheric Research.)
The corona is so intensely hot that it cannot be entirely constrained by either the Sun’s inward gravitational pull or by its magnetic forces. Some of the corona’s solar electrons and protons escape into surrounding space, which means that the Sun’s radiation is not all that moves past the planets. An eternal solar wind of electrons and protons is forever blowing the outer solar atmosphere away in all directions. Some of it sweeps past the Earth and engulfs it, which means that we live inside the Sun.
Although the Sun is continuously blowing itself away, the outflow can continue for many billions of years without significantly reducing the Sun’s mass. Every second, the solar wind blows away a million tons, or a billion kilograms. That sounds like a lot of mass lost, but it is four times less than the amount of mass consumed every second during the nuclear reactions that make the Sun shine. This nuclear fuel is expected to last another 7 billion years, and that’s how long the Earth will continue to be illuminated and warmed by the Sun.
This brings us to the discovery of how sunlight and other kinds of radiation move through space.