IMG28

Ernst Mach

sccond npparatus was ablc to dcmonstratc the Doppler cffcct, at least with regard to sound.

This npparatus (diagram i) “consists of a tubę AA' 6 fcct (183 cm) in length, capablc of turning about an a\is of RB' at its center. At one end is placcd a smali whistlc or reed G, which is blown by wind foreed along the axis of the tubc. An obscrvcr situated in the piane of rotation hcars a notę of fluctuating pilch, but if he placcs himsclf along the axis of rotation, the sound becomcs steady." ¹¹ Also, the faster the tubc is rotated the greater the diffcrcncc between the two tones. In this way Mach was able to show that Doppler was right. that the changc in pitch varied only with the dircction and speed of the sound sourcc, given a starionary listener.

Mach’s npparatus bccamc a frcqucntly uscd class demonstration dc.-vicc throughout Central Europc for many years, but this should not bc construcd to mcan that all opposition to Dopplcr’s theory thereby ceascd or that tlicre wcrc still not problcms with respcct to interpret-ing star colors and color changcs.

In any case, Mach was convinced, and hc remained an adamant sup-porter of Dopplcr's theory for the rest of his lifc. Mcanwhilc, ho\vcver, in the same ycar, 1860, Pctzval rencwcd his attack. Mach, in his rcply, attempted to show that both Doppler and Pctzval wcrc right in their opinions, but diat Pct/.val had misunderstood the Doppler theory and the rangę of application of Petzval’s own mathcmatical “law.” In fact, therc was no conflict bctwccn the two notions at all. Pctzval remained obdurate but at least refrained from using all the mcans at his disposal for settling the controvcrsy. We do not know Von Ettinghausen’s rcaction to Mach’s work, but prcsumably it was favorablc. On the other hand, as we shall see, it was unfortunatc that Mach bccamc involved in a conflict between two of his professors in such a way as could casily harm his own academic carecr.

Mach cvcntU3lly bccamc a party to other Doppler controvcrsies as wcll. In the 1870S a Professor Madlcr, an astronomer from Dorpat (Tartu) in Estonia, reopencd the controversy with blasts at both Doppler^ and Mach’s idcas. Mach countercd by rcissuing (1873) his carlicr Doppler articlcs, writing a ncw one (1878), and by carrying out threc public demonstrations. In the dead of winter in 1878 Mach dragged a group of Praguc professors and students outsidc into the cold, sat them on a hill, and madc them listen to whistles from approaching and dc-parting trains. On their return they signed prepared statements sup-porting the Doppler theory.¹” Evcn as lale as 1913, when a scientist had written him a letter asking his opinion on a possible incompatibility between Einsteins spccial theory of relativity and the Doppler theory. Mach went through the experimental cvidencc oncc again in an effort to show that Doppler’s ideas were still valid.¹³

Mach also madc the first elear proposal to determine the movement of the fixcd stars by examining spcctral lines, but it must be admitted that hc was unawarc of Armand Fizeau’s prior work in that area.¹⁴ Mach’s 1861 suggestion was followcd by imprcssivc results in 1889 when Edward Pickering describcd the first spectroscopic double star Ursa Maioris and H. C. Vogel in 1892 determined the radial com-ponents of the pcculiar motion of the fixcd stars by spectroscopic means.

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