Anders Jonas Ångström

Anders Jonas Ångström: The Man Who Read Sunlight

In 1862 a Swedish physicist pointed a spectroscope at the Sun, photographed what came back, and announced that the Sun's atmosphere contains hydrogen. It was a claim of astonishing scope — that the chemical composition of an object 150 million kilometres away could be determined from a pattern of dark lines — and it was correct. Astronomy has been chemistry ever since.

From Medelpad to Uppsala

Ångström was born on 13 August 1814 at Lögdö in Timrå, in the northern Swedish province of Medelpad, the son of Johan Ångström. He was schooled at Härnösand and moved to Uppsala in 1833 to enter the university, where he became a docent in physics in 1839 — at twenty-five, and at an institution that would hold him for the rest of his career.

His early path ran through astronomy rather than laboratory optics. In 1842 he went to the Stockholm Observatory to acquire practical observing experience, and in 1843 he was appointed keeper of the Uppsala Astronomical Observatory. The combination shaped everything that followed: he was a physicist who knew how to point an instrument at the sky and a working astronomer who understood what light was made of.

The 1853 Paper

His foundational work, Optiska undersökningar — Optical Investigations — was submitted to the Stockholm Academy in February 1853, having been published under an 1852 imprint. In it he demonstrated that the electric spark yields two superposed spectra, one belonging to the metal of the electrode and the other to the gas through which the spark passes. Disentangling the two is the essential first move of spectrum analysis: it establishes that a spectrum can be decomposed into the separate contributions of the substances producing it.

The deeper result came from reasoning rather than apparatus. Drawing on Euler's theory of resonance, Ångström deduced that an incandescent gas emits luminous rays of the same refrangibility as those it can absorb — that a substance gives out precisely the wavelengths it takes in. This is the principle that makes the dark lines in the solar spectrum legible as chemical signatures. He had formulated, in 1853, a law of absorption that would be somewhat modified and afterwards known by another man's name: Kirchhoff's law of thermal radiation.

Overlooked, Then Not

His contribution was, in the plain wording of the record, overlooked for a number of years. Recognition came late and from abroad. In 1870 he was elected a Foreign Member of the Royal Society, and in 1872 that body awarded him the Rumford Medal, with Edward Sabine remarking that the principle he had established "entitles him to rank as one of the founders of spectroscopy." In 1873 he was elected to the Institut de France. He had joined the Royal Swedish Academy of Sciences long before, in 1850.

In 1858 he succeeded Adolph Ferdinand Svanberg in the chair of physics at Uppsala, and from 1861 he turned his attention almost wholly to the solar spectrum.

The Map of the Sun

The hydrogen result of 1862 depended on marrying the spectroscope to photography, which allowed lines to be recorded rather than merely glimpsed and sketched. Six years later, in 1868, came his masterwork: Recherches sur le spectre solaire, a map of the normal solar spectrum containing detailed measurements of more than a thousand spectral lines. It became the standard reference for a generation of astronomers.

It also contained an error that says a good deal about the scrupulousness of the work. His measurements were inexact by roughly one part in seven or eight thousand — not because his technique was poor, but because the metre standard he had used was very slightly too short. The mistake was in the ruler, not the eye, and its size is the measure of his precision: an error of one part in eight thousand is what counts as a flaw in Ångström's work.

In 1867 he became the first person to examine the spectrum of the aurora borealis, detecting and measuring the characteristic bright line in its yellow-green region. He was wrong about one implication, believing the same line appeared in the zodiacal light, but the observation itself opened the spectroscopic study of the upper atmosphere.

Magnetism and Heat

Spectroscopy was not his only occupation. He studied heat transfer and terrestrial magnetism, recording magnetic fluctuations across Sweden in 1852 and 1853. The Stockholm Academy of Sciences also handed him the magnetic data collected during the round-the-world voyage of HSwMS Eugenie between 1851 and 1853, a reduction he completed only shortly before his death — unglamorous, laborious national service of exactly the kind that swallows careers.

Why Anders Is Called a Genius

The quality is precision married to inference — the ability to make measurements fine enough to carry an argument, and then to see what argument they carry. Two moves define him. The first, in 1853, was theoretical: reasoning from Euler's resonance theory to the conclusion that a glowing gas emits exactly the wavelengths it absorbs, which converts a spectrum from a curiosity into a chemical fingerprint. The second, from 1861 onward, was executional: measuring a thousand solar lines accurately enough that the resulting atlas governed the field, and using the result to establish that a star contains hydrogen. The gap between those two things — an idea about resonance, and knowing what the Sun is made of — is the whole of astrophysics, and he crossed it.

The honest counter-case is that he shares the credit and, on the largest question, lost the naming. Kirchhoff's law bears Kirchhoff's name; Ångström's version was formulated first but was somewhat modified afterwards, and his work sat unnoticed for years while others built on the same ground. He founded no school of thought and left no general theory of matter and radiation. His great atlas carried a systematic error inherited from a defective standard metre, and his aurora work included a mistaken claim about the zodiacal light. What is not arguable is Sabine's verdict when the Royal Society finally handed him the Rumford Medal in 1872: the principle he established entitles him to rank as one of the founders of spectroscopy. Whole sciences do not have many founders.

Legacy

He married Augusta Bedoire in 1845, daughter of a Stockholm merchant family of Huguenot origin; their son Knut, born in 1857, became a physicist in his turn. Ångström died of meningitis at Uppsala on 21 June 1874, aged fifty-nine, with the Eugenie magnetic reductions barely finished.

His name survives in three places. A crater on the Moon carries it. The Ångström Laboratory at Uppsala University, housing physics, mathematics, engineering, chemistry and space physics, carries it. And most durably, the ångström — one ten-billionth of a metre, the unit in which wavelengths of light and the spacing between atoms in solids are measured — carries it into every crystallography and spectroscopy paper still being written. A man whose great work was undone by a slightly short metre ended up as a unit of length.