Hydrogen: The Essential Element

Why a biography of hydrogen? The book justifies itself.

Hydrogen is not only the most common element in the universe, the source of energy production in stars, and the simplest of all elements. It’s also the historical lab specimen/crash test dummy for physicists.

Theory, lab experimentation, and observation place hydrogen at a focus for the understanding of the universe from the very, very small in particle physics to the very, very large in cosmology.

Why hydrogen? Its abundance is one thing, at least if we just think about “normal” matter in the universe. But it’s really its simplicity — one proton and one electron. It’s as if the universe swept away the complexity so that we could see atomic structure at its cleanest and show more measure parameters without noise.

And deuterium, hydrogen’s heavier isotope, provides the simplest example for studying the atomic nucleus. Deuterium’s nucleus (“deuteron”) contains just one proton and one neutron, so that the relationship between the two and how they are bound together through the strong force can also be studied without complication.

The book is organized into chapters focused on landmarks in the study of hydrogen that generate more general theoretical breakthroughs in physics.

Rigden begins with William Prout’s 1815 speculations about hydrogen as the basic building block for all of the elements, a speculation that is both “wrong” but conceptually provocative. It provides a conceptual framework for the atom, taking up ancient clues from the Greek atomists. The idea of a fundamental building block, out of which all the elements are made is not obvious, and Prout’s speculation carries forward a way of thinking that, even though wrong in its particulars, provides a conceptual framework into which atomic structure later fits.

Other early chapters focus on the Balmer spectral lines for hydrogen and the Bohr model. Those two set out a basic question — how can we understand the spectral emission lines (and absorption lines) associated with hydrogen in relationship to the structure of the hydrogen atom? Hydrogen emits energy at those particular wavelengths for a reason — can we derive them from hydrogen’s atomic structure, in particular the behavior of its one electron?

Bohr’s original model of the atom did tie atomic structure to those Balmer lines. But it was specific to hydrogen. It contained ad hoc values to place transitions between electron energy levels and orbits in sync with the Balmer spectral lines.

Not until more generalized theories of the atom and the energy states of electrons (through work by Sommerfeld, Dirac, Schrodinger, and others) did this initial model of the atom, with its quantized conceptual character, evolve into what we now know as quantum theory. Rigden traces this theoretical and experimental progress through succeeding chapters of the book.

Later chapters make the transition from particle physics to cosmology and some more speculative topics. Some of these chapters made me appreciate the continuity between the study of atomic structure, understanding the energy states for hydrogen’s electron at the particle physics level, and application of that same understanding to cosmological questions.

Rigden does a good job of showing, for example, how spectral analysis of hydrogen emission lines from interstellar hydrogen gas enables astronomers to map the structure of our galaxy.

Ordinarily, in trying to understand our galaxy’s structure, our view is blocked, like someone unable to see the forest for the trees. As we look out into the sky, we see stars, gas and dust clouds, etc. — what’s right in front of us blocks our view of the whole.

Mapping the galaxy’s structure was only possible because one of the energy state transitions of hydrogen’s electron, a transition that was missed in the Bohr model but discovered as part of the “hyperfine structure” understood through quantum theory, results in an emission at 21 centimeters wavelength. That emission, in the radio spectrum, passes transparently through the objects (stars, planets, dust, gas, . . ) that would otherwise block our view of the galaxy’s structure as a whole. In the radio spectrum we can see hydrogen gas clouds emitting signals of their presence anywhere they exist in the galaxy, giving us a map of the galaxy’s structure.

Hydrogen has always provided a link between the very big and the very small. As the simplest of elements it has played this key role in the development of particle physics, and especially quantum theory. But hydrogen was also the first element to form after the Big Bang, by far the most common element in the universe, and a key to understanding the universe’s structure and geometry.

Also, hydrogen’s isotope, deuterium, provides both a validation of Big Bang cosmology and a mystery about the current makeup of the universe.

The Big Bang is the only (known) appreciable source of deuterium in the universe. From Big Bang theory, we can derive an expected abundance of deuterium relative to hydrogen per se (really the lighter isotope without deuterium’s neutron). Then by comparing that predicted abundance with the actual observed abundance, we can provide one of the validating pillars of Big Bang theory.

(I’m skipping a complication — deuterium, although it can be created only in the Big Bang, can be depleted in the fusion processes at the cores of stars, so we need “primordial” sources from which to sample deuterium’s actual abundance — fortunately quasars supply such sources).

The comparison confirms the Big Bang prediction. But the accuracy of the prediction poses a problem. If we now look at the actual, observable amounts of deuterium in galaxies, we can, given its expected abundance, derive some estimate of the amount of hydrogen in the galaxy, and, because we know the abundance of hydrogen itself relative to the galaxy’s mass, we can derive an estimate of the galaxy’s total mass.

When we do that, the estimate is way too low. In order for galaxies to interact gravitationally the way we see them do in galaxy clusters, and in order for galaxies to rotate and retain their observed shapes, they must have much greater masses than the estimate provides. The missing mass problem is a principal rationale for “dark matter” theories.

The final chapters concern some more esoteric work relating to antimatter (including antihydrogen), the novel state of matter known as the Bose-Einstein condensate, and “hydrogen-like atoms” (artificially assembled atomic-scale entities with similar structure to hydrogen but different components), along with technological applications.

I learned a lot from Rigden’s book. To be a bit critical, it is a little uneven in level of explanation. That, to me, is generally a problem with physics-related books for the so-called “general reader.” To explain EVERYTHING would make the book unreadable. At least with Rigden’s book, my questions were comprehensible enough that I could go off on my own to try to resolve them separately. show less

Seduced by simplicity, physicists find themselves endlessly fascinated by hydrogen, the simplest of atoms. Hydrogen has shocked, it has surprised, it has embarrassed, it has humbled--and again and again it has guided physicists to the edge of new vistas where the promise of basic understanding and momentous insights beckoned. The allure of hydrogen, crucial to life and critical to scientific discovery, is at the center of this book, which tells a story that begins with the big bang and continues to unfold today.

In this biography of hydrogen, John Rigden shows how this singular atom, the most abundant in the universe, has helped unify our understanding of the material world from the smallest scale, the elementary particles, to the show more largest, the universe itself. It is a tale of startling discoveries and dazzling practical benefits spanning more than one hundred years--from the first attempt to identify the basic building block of atoms in the mid-nineteenth century to the discovery of the Bose-Einstein condensate only a few years ago. With Rigden as an expert and engaging guide, we see how hydrogen captured the imagination of many great scientists--such as Heisenberg, Pauli, Schrödinger, Dirac, and Rabi--and how their theories and experiments with this simple atom led to such complex technical innovations as magnetic resonance imaging, the maser clock, and global positioning systems. Along the way, we witness the transformation of science from an endeavor of inspired individuals to a monumental enterprise often requiring the cooperation of hundreds of scientists around the world.

Still, any biography of hydrogen has to end with a question: What new surprises await us? show less