(1803-1853) Austrian Theoretical Physicist (Acoustics, Optics)
Christian Doppler discovered the so-called Doppler effect, an apparent change in the frequency of a wave motion caused by relative motion between the source and the observer. It was quickly and easily verified with respect to sound waves, for which the frequency increases as source and observer are in motion toward one another and decreases as they are in motion away from one another. The Doppler effect would have its greatest impact, however, when applied to light waves by astronomers, who found in it an indispensable tool for measuring the velocities and distances of celestial bodies.
Christian Doppler was born in Salzburg, Austria, on November 29, 1803, into a family that had operated a successful stone-masonry business since 1674. He was heir to a family tradition that decreed the son would take over the family business, but for the frail Christian, a less physically arduous path seemed advisable. He progressed from primary school in Salzburg to secondary school in Linz, where his talent for mathematics was increasingly evident. Upon graduating, he attended the Polytechnic Institute in Vienna from 1822 to 1825; there he continued to excel in mathematics and other studies. He then returned to his native city, where he attended lectures at the Salzburg Lyceum and continued his studies privately while tutoring in physics and math.
By 1819, Doppler was back in Vienna, where he studied higher mathematics, mechanics, and astronomy at the university and graduated in 1833. Remaining in Vienna to work as assistant to the professor of higher mathematics and mechanics, A. Burg, he wrote his first papers on math and electricity. Doppler, who, at 30, was somewhat older than most assistants, began seeking a permanent academic post. In the Austro-Hungarian Empire of that time aspiring academics had to compete for vacant professorships (a term applied to secondary as well as university teaching positions) by taking state exams and giving demonstration lectures. Doppler entered several of these competitions, on both the secondary school and university levels, while supporting himself as a bookkeeper at a cotton-spinning factory.
When success continued to elude him, he decided to emigrate to the United States in 1835. But he did an abrupt about-face, just before setting out, when a job teaching mathematics at the Technical Secondary School in Prague was offered to him, two years after he had competed for it. Doppler accepted, glad to have any teaching position, even if it was not the one he dreamed of. He competed for a job teaching advanced mathematics in Prague, but the closest he came to realizing his ambitions was a four-hour-a-week teaching assignment at a technical college. He had married in 1836 and was grateful for the extra income. Not until 1841 would he obtain a professorship in mathematics at the State Technical Academy in Prague. At the same time, his research was producing exciting results.
On May 25, 1842, at a meeting of the Royal Bohemian Society of Sciences, Doppler read his paper “On the Colored Light of the Double Stars and Certain Other Stars of the Heavens.” Here he first presented what came to be called the Doppler effect, the relation of the frequency of a source to its velocity relative to an observer. Doppler derived his principle by treating both light and sound as longitudinal waves in the ether and matter, respectively. He pointed out that sound waves from a source moving toward an observer reach the observer at a greater frequency than if the source is stationary, thus increasing the observed frequency and raising the pitch of the sound. Similarly, sound waves from a source moving away from the observer reach the observer more slowly, resulting in a decreased frequency and a lowering of pitch. In 1845, the first experimental test of Doppler’s principle was made at Utrecht in the Netherlands. A locomotive was used to carry a group of trumpeters in an open carriage back and forth past a second group of musicians, who jotted down the pitch of the notes being played. The variation of pitch produced by the motion of the trumpeters verified Doppler’s equations for the case of sound waves. He would later publish an improved version of his principle, which took into account both the motion of the source and the motion of the observer.
In the 1842 paper, Doppler hypothesized that his principle would apply to any wave motion, including motion of light waves, and predicted that this would be of particular value to astronomers.
Time would prove Doppler eminently correct. In 1848 armand-hippolyte-louis fizeau suggested that applying the Doppler principle to observed shifts in the spectral lines of stars would enable astronomers to determine their motion. Twenty years later, William Huggins applied this idea when he determined that Sirius was moving away from our solar system by detecting a small red shift, that is, a displacement of lines toward the red end of the visible spectrum, where wavelengths are longer. This is caused by the Doppler effect and indicates that the observed body is moving away from the Earth. Later observations found that the spectra of some astronomical objects show a blue shift, indicating movement toward the observer. Both red and blue shifts became known as Doppler shifts.
While making his important discovery, Doppler was finding his long-desired professorship, involving the examination of hundreds of students in different scientific and mathematical areas, more than he could handle. In 1844, with his health failing, he requested a leave. He would not return to teaching until 1846. The following year he eagerly accepted a professorship of mathematics, physics, and mechanics at the Academy of Mines and Forests in Banska Stiavnica. Only a year later political unrest throughout the Austro-Hungarian Empire forced him to uproot himself once more. That year he was elected to membership in the Academy of Sciences in Vienna and was awarded an honorary doctorate from the University of Prague. With a solid reputation as a researcher established, he then returned to Vienna and in 1850 became director of the new Physical Institute and professor of experimental physics at the Royal Imperial University of Vienna. This was the apogee of his career. His lung problems steadily worsened until, in November 1852, he traveled to Venice, seeking the healing influence of the warmer climate. His devoted wife, who had remained in Vienna with their three sons and two daughters, rushed to his side when it became clear that he was failing. She was with him when he died in Venice on March 17, 1853.
Doppler’s legacy is most readily reflected in its impact on astronomy, in which, applied to the case of light waves, measuring the Doppler shift of celestial bodies remains a fundamental method of estimating their relative velocities and distances. When, in 1929, Edwin Hubble linked the velocity of a galaxy to its distance from Earth, it became possible to use the red shift to determine, not only the distances of galaxies, but, ultimately, the size and structure of the universe.
