If God did create the world by a word, the word would have been hydrogen. —Harlow Shapley
The story of hydrogen begins before there was anyone to notice. Long before the Earth and its planetary siblings existed, before the Sun and the Milky Way existed, and even before chemical elements like oxygen, sodium, iron, and gold existed, the hydrogen atom was old, old news.
According to current wisdom, our universe began about 15 billion years ago at a point with infinite density and infinite temperature. That was the beginning of time; that was the origin of space. Since then, the original point has expanded in all directions to the dimensions of the current universe. As the universe expanded, the cosmic clock ticked and the temperature cooled: at 0.01 second after the big bang, the temperature was 100,000 million degrees K; 0.12 second, 30,000 million degrees K; 1.10 seconds, 10,000 million degrees K; 13.83 seconds, 3,000 million degrees K. By the time the universe was four minutes old, the basic ingredients required for all that was to follow were present and their basic modes of interaction were established. The stage was set for everything that followed.1
Hydrogen is the simplest of all atoms. In its dominant form, hydrogen consists of one electron and one proton; in its rare form, called deuterium, there are three particles: an electron, proton, and a neutron. By contrast, ordinary water, a simple mole cule, consists of twenty-eight particles: ten electrons, ten protons, and eight neutrons. The water molecule is very complicated when compared to the hydrogen or deuterium atoms. Because of its simplicity, hydrogen dominates the 15 billion-year tale of our universe. Approximately 300,000 years after the origin of our universe, the temperature had cooled to approximately 3,000 degrees and the hydrogen and helium atoms took their characteristic forms. Even this early, a particular kind of universe was inevitable: a universe that would eventually become a hospitable haven for life.
When atoms first began to take form, the ingredients available were limited. There were photons (particles of light) and neutrinos, and elementary particles of matter—electrons and protons (the nucleus of the hydrogen atom is a proton). There were composites of elementary particles—deuterons, a proton plus a neutron (the deuteron is a special part of the story told in this book because it is the nucleus of the heavy hydrogen atom, deuterium), and alpha particles, two protons plus two neutrons (the nucleus of the helium atom is an alpha particle). By the time the universe was 300,000 years old, neutrinos were aloof from their surroundings and did not participate in the birth of atoms, and photons were not essential to the atom-forming process. So, to form the first atoms of our universe there were electrons, protons, deuterons, and alpha particles. In this mix, protons outnumbered alpha particles by about eleven to one. The deuteron was a mere sprinkling in the mix. Thus, when atoms formed, the ingredients present coupled with the particle recipes for hydrogen and helium resulted in an atomic mix of about 92 percent hydrogen, 8 percent helium, and a fraction of a percent deuterium. Today, 15 billion years after hydrogen and helium were first formed, these elements remain the most abundant throughout the cosmos: hydrogen makes up approximately 90 percent of the total, whereas helium comes in at about 9 percent.
Since the ingredients for hydrogen and helium atoms—elec-
trons, protons, and neutrons—were present in the earliest seconds of the universe, why did it take 300,000 years before atoms appeared? Dropping temperatures over this span of years slowed the rapidly moving protons and electrons to speeds that allowed the electrical attraction between them to challenge their independent motions, bring them together, and form stable atoms. In fact, even the strongest force of nature, the nuclear force, was not strong enough to pull the frantic protons and neutrons together into nuclei during the earliest seconds of the universe. It was not until the universe was about fourteen seconds old and had expanded and cooled considerably that the first nuclei, alpha particles, formed. The early formation of alpha particles testifies to their stability. Deuterons, while simpler than alpha particles, are not as stable. Consequently, they did not form until the universe was almost four minutes old.
The primordial period of nuclear synthesis was all over by the time the universe was four minutes old. Nuclei heavier than that of helium—nuclei of beryllium, boron, and carbon, for example— did not form because these heavier nuclei could not compete with the inherent stability of the helium nucleus. Thus, all the free neutrons that were still available at the four-minute point took refuge in either the helium nucleus or the heavy hydrogen nucleus.
Essentially all the heavy hydrogen in the universe today originated during the first minutes of cosmic time. One thousand tons of heavy water, used to detect solar neutrinos, fill the tank at the Sudbury Neutrino Observatory in Sudbury, Ontario. This heavy water, each molecule of which consists of one oxygen atom, one hydrogen atom, and one deuterium atom, brings together deuterium that was formed when the universe was about four minutes old. When you hold a tube of heavy water in your hand, you hold primordial atoms, remnants from the first moments after the big bang.
Today, 473 million billion seconds after the big bang, the temperature of the universe has dropped to three degrees above absolute zero. Embedded in this frigid environment are galactic systems distributed across the far reaches of the observable universe. Each galaxy consists of stars and dust clouds. Each star, each dust cloud in each and every galaxy consists of about 90 percent hydrogen atoms and 9 percent helium atoms. Because of this composition, established approximately 15 billion years (or 473 million billion seconds) ago, the stars twinkle and the Sun shines.
The Sun is a typical star. The composition of the Sun (as well as other stars) reflects the cosmic abundance: about 90 percent of the atoms making up the Sun are hydrogen. And it is the fusion of hydrogen that fuels the Sun. Every second, 600 million tons of hydrogen are fused into helium in the core of the Sun, releasing prodigious energy that slowly makes its way from the core to the Sun's surface, heating it to a temperature of 5,800 K. The Earth, 92 million miles away, basks in this life-giving warmth.
Approximately 3.5 billion years ago, life emerged on at least one planet orbiting one star. There may be planets other than Earth that nurture life: we simply do not know. On planet Earth, hydrogen remained obscure for many centuries. Paracelsus (born Theophrastus Bombast von Hohenheim) noted during the early years of the sixteenth century that when acids attacked metals, flammable gas was a by-product. He had unknowingly observed hydrogen. Other chemists and physicists produced hydrogen and in 1671 Robert Boyle described its properties. As is frequently the case in science, the credit for discovering hydrogen rests on how "discovery" is defined. The credit for isolating and characterizing hydrogen goes to Henry Cavendish, who isolated hydrogen and determined its density in 1776. The French chemist Antoine-Laurent Lavoisier, whose head was severed by the guillotine on May 8, 1794, gave hydrogen its name.
The world as we know it is a consequence of the balance between the number of hydrogen nuclei and the number of helium nuclei, established in the early moments after the big bang. Perhaps it is preferable to say that the world is a consequence of the basic laws that produced this particular blend of hydrogen and helium. Did the laws of nature exist prior to the origin of the universe? Did the laws of nature take their present form at the instant of the big bang? One millionth of a second after the big bang? No one can say. Looking back, however, we can say the following: if the weak force had been just a little weaker, the free neutron would decay a little more slowly and, as a result, the universe would have started out as predominantly helium rather than hydrogen. A world without hydrogen is a world without water, a world without carbohydrates, a world without proteins—a world without life.
So take your pick. We can say that the world is the way it is because the laws of nature are the way they are. Or we can say that the world is the way it is because hydrogen is the way it is. Whichever you select, one or the other, is a matter of preference. Either way, the little hydrogen atom commands the stage on which the long and enchanting drama of our universe, the story of galaxies, stars, planets, and life, unfolds.
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