Christian Doppler

Austrian physicist

• Born: November 29, 1803; Salzburg, Austria
• Died: March 17, 1853; Venice, Italy

Nineteenth-century physicist and mathematician Christian Doppler began his career as a professor of mathematics. At the age of thirty-nine, he published a paper on the color of binary stars, in which he first described the Doppler effect.

Primary field: Physics

Specialties: Astrophysics; optics; kinetics; acoustics

Early Life

Born in Salzburg, Austria, on November 29, 1803, Christian Andreas Doppler was expected to take over his family’s successful stonemasonry business. However, from a young age Doppler experienced poor health, preventing him from inheriting the family enterprise. Instead, he went on to study mathematics at Vienna’s Polytechnic Institute, where he quickly distinguished himself. After graduating in 1825, Doppler studied at the University of Vienna, completing his degree in 1829. He then worked as a mathematics assistant and tutor at the university for four years, during which time he published several papers on mathematical concepts. His first paper, published in 1829, was entitled, “A Contribution to the Theory of Parallels.”

For the rest of his life, Doppler’s career would be marked by a series of successes and apparent failures. He struggled at times to find employment as a professor, often criticized by his students for his excessively strict grading, and the strain of teaching large courses wore on his already weakened health. In the 1830s, he spent time as a bookkeeper at a cotton factory and briefly considered relocating to the United States in order to pursue his academic career in a fresh environment.

Doppler moved to Prague in 1835, which was then part of the Austrian Empire (now Czech Republic). The following year, he married Mathilde Sturm, also of Salzburg, with whom he would have five children. Doppler began teaching at Prague’s Technical Institute in 1837, but it was not until 1841 that the institute formally appointed him as a professor of mathematics.

An insightful and creative thinker, Doppler published papers that earned him some support among European scientists, who in turn championed his membership at institutions that provided him with an income and time to study. However, his unusual methods and sometimes sloppy mathematics drew criticism and occasionally prevented him from advancing in his field. As a result, Doppler bounced between positions in Prague, Slovakia (part of the Austrian Empire at the time), and Vienna, bringing his wife and young family with him while he studied astrophysics and optics.

Life’s Work

In 1842, Doppler presented the paper he is best-remembered for, “Über das farbige Licht der Doppelsterne und einiger anderer Gestirne des Himmels” (On the colored light of the double stars and certain other stars of the heavens), to the Royal Bohemian Society of the Sciences in Prague. At the time, Doppler was a relatively new member to the society, one of the leading scientific organizations of his day. His friend and supporter, the Italian-German mathematician Bernard Bolzano, had advocated for Doppler’s membership for years, but was continuously voted down by the Society. In 1840, however, Doppler’s minor papers earned him membership, which significantly furthered his career.

Doppler’s 1842 paper established the phenomenon that later became known as the Doppler effect, or Doppler shift. In the Doppler effect, the observed frequency of a light or sound wave is dependent on the speed of the wave’s source relative to the observer. A modern way of understanding this is through the example of an ambulance speeding past a person, with the sound waves from the ambulance changing pitch based on the motion of the ambulance itself. As the ambulance moves toward the observer, the waves are condensed closer together by its movement, and thus the pitch is higher. Once the ambulance passes, the waves are no longer condensed by the ambulance, and thus the pitch is lower. Doppler discovered this effect due to his interest in the light waves he observed from stars. He believed that the color of the waves could be used to better understand a star’s speed and location.

Although Doppler identified a fundamental principle of physics, the mathematical examples with which he tried to prove that principle were generally incorrect. For instance, he described the light waves of stars as longitudinal, meaning that vibrations moved in the same direction the waves traveled. Scientists later determined that stars have transverse waves of light. Doppler also believed that the slight changes in the color of binary stars could be measured in order to determine their movement in the universe. While modern scientists use exactly that principle in order to understand astrophysics, the telescopes and technology of Doppler’s day were not sophisticated enough to make any revealing analysis, and thus his conclusions about specific stars were all incorrect.

Many of Doppler’s contemporaries, already incredulous of his theory, dismissed his argument because of its mistakes. Following the presentation of his paper, a flurry of debate occurred in the mathematics and physics communities. Doppler’s biggest opponent was the Hungarian-Slovakian mathematician Joseph Petzval. Both men were pursuing the same basic ideas, and ultimately many of their theories would overlap, but the specifics of their arguments and differences in technique left little room for agreement at the time.

Doppler continued to experiment in optics and astrophysics, refining his old theories and establishing new ideas. In 1845, with the help of Dutch meteorologist Christoph Buys Ballot, he conducted an experiment to prove the Doppler effect for sound. Musicians playing trumpets rode a train car that passed back and forth past a group of stationary musicians who recorded the pitches they heard. The closer the train approached, the higher the pitch was heard; the farther away the train traveled, the lower the pitch became. Doppler published a revised version of his 1842 paper in 1846, this time recognizing the role that the observer’s motion plays in the Doppler effect. The Doppler effect in relation to light would not be proven until after Doppler’s death.

In an effort to conduct a more precise study of the stars, Doppler worked on inventing new instruments and technological apparatus. While his papers continued to touch on theories that would occupy scientists into the twenty-first century, his lack of accurate technology and tendency toward unusual logic prevented him from any further breakthroughs. In 1850, he was rewarded with the position of director of the Institute of Physics at the University of Vienna. His health steadily on the decline, however, he left the position after a short time. Doppler moved to Venice, Italy, where he died on March 17, 1853.

Impact

The Doppler effect continues to have a significant impact on our understanding the universe. It has been expanded upon to apply to not only light and sound waves, but to other diverse phenomena as well, such as the force of gravity in general relativity. While its applications in diagnostic medicine, plasma physics, and satellite communication are all important, it is in astronomy, particularly astrophysics, that the Doppler effect has had the greatest impact.

Modern telescopes and other devices are able to detect alterations in the color of stars that were previously invisible to the human eye. Because the light emitted from stars follows the rules of the Doppler effect, astrophysicists are able to deduce the movement and speed of individual stars in relation to the planet Earth, greatly enhancing our map of the universe. When a planet is approaching Earth, its light has a higher frequency than normal and experiences what is called a “blue shift.” When a planet is moving away from Earth, the opposite, a “red shift,” occurs. Beyond this, scientists are also able to detect planets that would otherwise stay invisible from telescopes. Since planets exert gravity over the star they orbit, subtle variations in the star’s movement, observed as variations in the star’s Doppler effect, can reveal the presence of planets.

The information scientists have been able to extrapolate based on Doppler’s theories is not limited to the current location of stars and planets. The big bang theory, which models the universe as being in a constant state of expansion, is also based in large part on the Doppler effect. The movement of the stars consistently reflects the expansion of the universe. This changing speed creates a Doppler effect, which was first detected in a spiral galaxy in 1912. Although more elaborate than the initial experiments Doppler performed, modern applications of the Doppler effect have taken the same basic theory and used it to understand the origins and fundamental shape of the universe.

Bibliography

Eden, Alec. The Search for Christian Doppler. New York: Springer, 1992. Print. A comprehensive biography of Doppler’s scientific and personal life. Includes original documents, such as his paper introducing the Doppler effect. Illustrations, bibliography, index.

Hawley, John F., and Katherine A. Holcomb. Foundations of Modern Cosmology. New York: Oxford UP, 2005. Print. Emphasizes that theories of physics are the foundation for modern cosmological thought. Focuses on new information gained from advanced telescopes and satellites. Illustrations, bibliography, index.

Singh, Simon. Big Bang: The Origin of the Universe. New York: Harper, 2005. Print. An introduction to the theory of the big bang, with an overview of its scientific development and cosmology. Includes conclusions drawn directly from the Doppler effect. Illustrations, bibliography, index.