Pieter Zeeman

Pieter Zeeman: Splitting Light With a Magnet

On 12 March 1862, near the end of his working life, Michael Faraday set up a light source between the poles of a magnet and looked through a spectroscope for any change. His notebook records the result: "not the slightest effect demonstrable either with polarized or unpolarized light." Eight years later James Clerk Maxwell wrote that no force in nature could alter even very slightly the mass or period of oscillation of the particles emitting light. Between them, the two greatest authorities in the field had closed the question. In August 1896 a thirty-one-year-old Dutch lecturer opened it again.

Zonnemaire to Leiden

Pieter Zeeman was born on 25 May 1865 at Zonnemaire, in the Dutch province of Zeeland. He studied at the University of Leiden under Hendrik Antoon Lorentz, then the outstanding theoretical physicist in the Netherlands, and began lecturing there in 1890. Six years later he did the experiment that made his name.

The Kerr Effect, and an Idea

The thought did not arrive out of nowhere. "The idea came to me," Zeeman told the Nobel audience in 1902, "during an investigation of the effect discovered by Kerr on light reflected by magnetic mirrors." If magnetism could alter light that had bounced off a magnetised surface, why should it not alter light at the moment of its emission? Faraday had already established that a magnetic field rotates the plane of polarisation of light passing through a medium. Faraday had also gone looking for the more fundamental effect and found nothing, and Faraday's negative result had, as Zeeman noted, probably discouraged anyone else from trying for decades.

August 1896

At Leiden, Zeeman placed a sodium flame between the poles of a powerful electromagnet and examined its light with a Rowland diffraction grating — at that date the finest spectroscopic instrument in existence. Sodium produces two sharp yellow lines, among the most familiar features of any spectrum. With the magnet energised, each line broadened.

Broadening is a suspicious result; it is what you get from heat, pressure, contamination, and a dozen other mundane causes. What Zeeman established was that the flame was emitting not merely its original frequency but additional oscillations of both greater and smaller period — the line was not smeared but split. The magnitude was tiny, roughly one-thirtieth of the gap between the two sodium lines. Detecting it at all was a feat of instrumentation.

Lorentz's Three Lines

Lorentz supplied the explanation, and it was the explanation that made the result historic. On his theory, atoms contain small electrically charged particles — electrons — whose vibration produces light. Put such a vibrating charge in a magnetic field and its motion resolves into three components: one straight-line oscillation along the field, unaffected, and two circular oscillations in opposite senses whose periods are shifted, one up and one down. That predicts something very specific. Viewed at right angles to the field you should see three lines, the middle one polarised parallel to the field and the outer two perpendicular to it. Viewed along the field you should see only two, and they should be circularly polarised in opposite directions.

Zeeman looked, and every one of those predictions held. "You see how beautifully the consequences following from Prof. Lorentz's theory were confirmed by observation," he said in Stockholm.

A Colossal Number

The deepest result came from the size of the splitting, because the shift depends on the ratio of the oscillating particle's charge to its mass. Zeeman worked the number out and got roughly 10⁷ c.g.s. units per gram — "a colossal number," about a thousand times larger than the corresponding figure derived from electrolysis. Whatever was vibrating inside the atom to make light was around a thousandth of the mass of the atom itself. He was blunt about what this meant for his own fortunes: had the ratio not turned out unexpectedly large, "I should not have had the honour of addressing you in Stockholm today."

He went further. From the direction of the doublet and the sense of the polarisation he determined the sign: the oscillating electrons are negatively charged. And the properties matched those being measured for cathode rays, which suggested — in his own careful formulation — that what vibrates in the light source is the same thing that travels in a cathode-ray tube. This was 1896 and 1897, precisely the moment the electron was becoming a real object rather than a theoretical convenience.

Amsterdam and After

Zeeman became professor of physics at the University of Amsterdam in 1900 and director of its Physical Institute from 1908. In 1902 he and Lorentz shared the Nobel Prize in Physics, Zeeman's citation naming his discovery of the effect that carries his name. His later research turned to the propagation of light through moving media — water, quartz, flint glass — delicate optical work of the kind he was uniquely equipped to do. He died in Amsterdam on 9 October 1943, aged seventy-eight.

Why Pieter Is Called a Genius

The quality is a specific and underrated one: the willingness to reopen a question that the field's greatest names had declared closed, combined with the experimental skill to see something a thousand times smaller than anyone had looked for. Faraday had tried and failed. Maxwell had pronounced it impossible in principle. Zeeman went ahead, and the decisive factor was not boldness alone but craft — knowing that a Rowland grating could resolve what a mid-century spectroscope could not, recognising a genuine splitting inside what looked like ordinary broadening, and then methodically testing Lorentz's polarisation predictions instead of settling for the headline. Extracting a charge-to-mass ratio from the size of a spectral shift, and a *sign* from the handedness of circular polarisation, is inference of a high order.

The counter-case is real and Zeeman would have been the first to make it. The theory was Lorentz's, not his; the Nobel was shared, and much of what makes the discovery important — the electron model, the predicted triplet, the polarisation rules — came from his teacher. He was an experimentalist who happened to be positioned beside the right theorist at the right moment. He also could not explain the complicated splitting patterns later called the anomalous Zeeman effect, which turned out to require electron spin and quantum mechanics — machinery two decades away. And J. J. Thomson, working independently in Cambridge, generally receives the credit for the electron. Zeeman's is a genius of measurement rather than of ideas, which is a lesser kind of fame and not a lesser kind of science.

Legacy

The Zeeman effect turned into one of the most productive diagnostic tools in physics. Because the separation of the lines is proportional to the field strength, a spectrum becomes a magnetometer: astronomers use it to measure magnetic fields in stars and sunspots, at distances where nothing can be measured directly. In the laboratory it verifies atomic energy levels and reveals spin and nuclear properties, and it underlies electron paramagnetic resonance and nuclear magnetic resonance spectroscopy — the ancestor, by a long line of descent, of the MRI scanner. Faraday, who had grandly titled his own attempts "the magnetization of light" and "the illumination of lines of force," was right about everything except whether it could be done.

Achievements

• Rumford Medal — 1922
• Franklin Medal — 1925
• Nobel Prize in Physics — 1902
• Foreign Member of the Royal Society — 1921
• Matteucci Medal — 1969
• Notable work: Zeeman effect
• Held posts at Delft University of Technology, Leiden University and University of Amsterdam
• Educated at Leiden University
• Fields of research: Zeeman effect, magnetic field, magnetism and physics