Doppler, Johann Christian
Born Salzburg, (Austria), 29 November 1803
Died Venice, (Italy), 17 March 1853
Johann Doppler first proposed the famous effect named after him, which predicted a change in the frequency of sound or light waves emitted by a source when either the observer or the source is in motion along the line of sight. Doppler was the son of a stonemason and educated in Salzburg and Vienna. He held several academic appointments during his short life: first as a professor of mathematics at the Realschule (state secondary school) in Prague (from 1835), as a professor of mathematics at the State Technical Academy (from 1841) also in Prague, in 1847 at the Mining Academy in Chemnitz, and finally from 1850 as professor of experimental physics at the University of Vienna.
It was during his time at the Technical Academy in Prague that Doppler delivered a lecture, on 25 May 1842, to the Royal Bohemian Scientific Society entitled “On the colored light of double stars and of some other stars of the heavens.” The lecture was published in the society’s proceedings in 1843 and contained a mathematical derivation of the result that the frequency change would be proportional to the radial motion of either source or observer.
Although the Doppler effect was soon confirmed for sound waves (by C. H. D. Buys-Ballot in the Netherlands, who in 1845 played wind instruments on passing trains), its validity for light was a source of considerable controversy for many years. The early objections came notably from Buys-Ballot and also from a fellow Austrian of geometrical optics fame, J. Petzval.
Armand Fizeau in France delivered a lecture in 1848 on the wavelength (or frequency) shift expected in the absorption lines that had been observed by Joseph Fraunhofer in the spectra of the Sun and a number of stars, if such bodies were in motion. Unfortunately, his lecture was not published until 1870, so remained largely unknown. In France, the Doppler effect is today often referred to as the Doppler-Fizeau effect, evidently for good reason.
In Germany, Ernst Mach came to the same conclusion as Fizeau in 1860, as did James Maxwell in Scotland a few years later. None of these contributions invoked color changes for moving stars, but instead predicted small line shifts that might be detectable in the spectroscope. It should be recalled that Doppler’s paper had made no reference to spectroscopy, but only to the brightness and color changes of stars in motion relative to those at rest. Indeed, in 1842 the only significant observations of stellar spectra had been those of Fraunhofer in 1814/1815 and again in 1823. This made Fizeau’s and Mach’s insight into the application of the Doppler principle to stellar spectroscopy, which only experienced a rebirth from about 1862, all the more remarkable.
Doppler himself did not live to hear of this substantial modification to his effect when applied to starlight. His work was still enshrouded in controversy when he died while visiting Venice in hopes of improving his health. Both William Huggins in London and Angelo Secchi in Rome had around 1868 attempted to measure line shifts visually for bright stars through a spectroscope, but the shifts were too small to be reliably determined or substantiated.
Not until the 1870s did the careful observations of Secchi (1870) and Hermann Vogel (1872) demonstrate the reality of the line shifts from the spectrum of the equatorial region of the Sun arising from solar rotation. This demonstration opened up the way for a major new line of astronomical research – the measurement of Doppler shifts and hence of line-of-sight velocities for stars. This type of investigation was successfully undertaken from 1888 by Vogel and Julius Scheiner using spectrum photography at the newly established Potsdam Astrophysical Observatory. The discovery of spectroscopic binary stars by Vogel and Edward Pickering , using the Doppler effect, was also a major application of Doppler’s work from this time.
It would be wrong to suppose that Doppler completely misinterpreted the application of his effect to astronomy. For if stars were in fact to have significant velocities compared with the velocity of light, then Doppler’s predictions of color and magnitude changes would be upheld. Indeed, this is just the case with quasi-stellar objects. If the red shifts of these objects are cosmological, then they are receding at relativistic velocities and the photometric properties are affected accordingly, much as Doppler would have predicted.
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