Acta Geod. Geoph. Hung ., Vol . 34(1- 2) , pp . 181 -186 (1999)

SOME ASPECTS IN EMIL WIECHERT'SSCIENTIFIC WORK

W SCHRODERl and H-J TREDER2

[Manuscript received December 12, 1997]

Some extracts from Emil Wiechert's publications and letters relating to geo­physics and physics are presented as to give some idea of his contribution to thissubject during the 19th and 20th centuries. These are set in context with othercontemporary geophysical and seismological research.

Keywords: history ofgeophysics; historyof physics; seismology; Wiechert, Emil

Life and work of Emil Johann Wiechert , born on December 26, 1861 in Tilsitshould be significant for two disciplines, namely for physics and for geophysics. Hegot strong impulses already during his school-days towards physical and philosoph­ical problems. Following the secondary school he started in 1881 to study at theAlbertus University in Konigsberg (today Kaliningrad). In the focus of his studieswere Physics, Mathematics, Astronomy, Geology and Philosophy. Moreover he par­ticipated strongly in the activity of the Society of Physics-Economy, as well as inthe Mathematical Society. Due to economical problems his studies were elongated,therefore he presented his dissertation in 1889 "On the elastic after-effect" and gotthe doctor's degree. As soon as in 1892 he also obtained the degree dr. habil. Inthe years to follow, he treated the atomic structure of electricity and the structureof matter as well experimented with cathode rays.

His tutor Woldemar Voigt went in the meanwhile to Gottingen and carriedWiechert with himself. In the framework of Felix Klein's reform efforts the foun­dation of a Geophysical Institute appeared, too. Wiechert hoped in that time thathe would obtain an invitation to the professorship in theoretical physics. As hedid not receive this on time, he accepted Klein's invitation to the directorship ofthe Geophysical Institute together with the professorship in geophysics. He becamesoon also a member of the Royal Society of Sciences in Gottingen and developedconsequently the field of geophysics in the framework of the university. Externalstations in the Southern Pacific, in China, in Argentina as well as in Germany aimedat helping the phenomenon earthquake. In addition to this activity, Wiechert con­tinued to be interested in the great problems of theoretical physics. He died in 1926in Gottingen.

In the eighties of the last century four students were matriculated at the fac­ulty of philosophy of the Konigsberg University, who were well acquainted witheach other and who remained in their following life in close connection. Theywere as listed here according to their age: Emil Johann Wiechert (1861-1928),

1Hechelstrasse 8, D-28777 Bremen-Roennebeck, Germany2Rosa-Luxemburg-Strasse 17a, D-14282 Potsdam, Germany

1217-8977/99/$ 5.00 @1999 Akademiai Kiad6, Budapest

182 W SCHRODER and H-J TREDER

David Hilbert (1862-1943), Hermann Minkowski (1864-1909) and Arnold Som­merfeld (1868-1951). They were all later in Gottingen, partly at the same time:Wiechert since 1897 (as professor of geophysics), Hilbert since 1898 (as professor ofmathematics), and since 1902, Minkowski, too. Sommerfeld was between 1896 and1900 private-docent in Gottingen. In Konigsberg Wiechert, the oldest of the four,was the most impressive personality, as Sommerfeld remembered to this time. Heserved as opponent of Hilbert's doctor thesis as early as in 1885.

All the four scientists had a significant influence on the development of Ein­stein's special (1905) and general (1915) relativity theory. Wiechert, Minkowskiand Sommerfeld belonged to the co-founders of electrodynamics in the form of elec­tron theory (Wiechert), relativistic electrodynamics moving bodies (Minkowski) andquantum electrodynamics (Sommerfeld) .

The tradition of mathematical physics at the University Konigsberg was foundedby G G Jacobi (1804-1851) and Franz Neumann (1798-1895). Wiechert was person­ally acquainted with the latter. Neumann was the first who lectured on "Theoreticaland mathematical physics" at a German university. Following his retirement he wasfollowed in 1875 by Woldemar Voigt (1850-1919), who left in 1883 to Gottingen.

Franz Neumann was together with C F Gauss (1777-1855), Wilhelm Weber(1804-1891), Bernhard Riemann (1826-1866) and his son Carl Neumann (1832­1925), all from Gottingen, the founder of the so-called "German school of electro­dynamics" that introduced remotely active, velocity dependent interaction poten­tials. After 1887 - following the proof of the existence of electromagnetic waves byHeinrich Hertz (1857-1894) the Faraday-Maxwellian field theory became generallyaccepted. According to Hertz, electrodynamics was from that time "Theory of theMaxwellian equations" . The big problem was the deduction of the interaction be­tween electromagnetic fields and electric charges, and not the interaction betweencharges and currents.

Weber , Neumann and Riemann supposed in the framework of their interactiontheory that all electric currents are moving (positive and negative) charges andthat charge carriers in the electric conductors are point-like particles of a very (orinfinitely) small mass which carry the same charge (e) without distinction as to sign.In his famous Faraday-lecture (1881) H von Helmholtz deduced such an "atomisticof electric charges" - the existence of an elementary charge with an absolute valuecorresponding to Faraday's equivalence law of electrochemistry. The correspondinghypothetical charge carriers (according to the remote effect electrodynamics) werecalled by J Storey (1826-1911) in 1891 "electrons". Sir William Crokes (1832-1919)supposed that the cathode rays in gas discharge tubes currents of such electronsare. Hertz and his pupil, P Lenard (1862-1947), however, who used as first freeelectrons supposed at first that he could prove that cathode are rays are no chargedparticles, but they are a particular form of "ether waves" .

Just at this point J J Thompson (1856-1940) in Cambridge and Emil Wiechertintervened. They discovered the error in Hertz's and Lenard's work and provedthat cathode rays consist of particles with negative charges. These charges have thevalue e and the masses are small as compared to the mass of the chemical atoms. WKaufmann's (1871-1947) experiments who continued Wiechert's work with cathode

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WIECHERT'S SCIENTIFIC WORK 183

rays of higher velocity (after 1896) confirmed this supposition (Kaufmann was from1908 till 1935 professor in Konigsberg) . Wiechert tried to couple his experimentalresults about electric currents as moving electrons with the Maxwellian equationsand thus to substantiate electrodynamics in form of electron dynamics. He madehere, however, an elementary error in the computation which prevented him fromreaching a consistent result. Therefore he throw the manuscript away.

In the same time , H A Lorentz (1853-1928) studied in Leiden the influenceof the electromagnetic field according to the Maxwellian equations on spatiallyextended charges and then he considered electrons as a limiting case. The result,the deduction of the Lorentz' force was published in 1895. Having seen Lorentz'publication, Wiechert saw at once the computational error in his manuscript andhe presented in 1896 a summarising report on the "Theory of electrodynamics" atthe Konigsberg Physical-Economical Society.

Lorentz' and Wiechert 's postulates differ so far that Lorentz took extendedcharges into account , i.e, he integrated at first according to time , while Wiechertstarted with point-like electrons, i.e, integrated at first according to space. That ishow differ retarded potentials defined by Lorentz (1895) from the Lienard-Wiechertpotentials (Lienard 1898, Wiechert 1900). In the form as given by Wiechert thesepotentials can be brought into a relativistic form, according to Einstein and Minkowski:

-exi~i=kkx x

(i,k = 1,2,3,4).

The basic problem was after Lorentz' and Wiechert's publications that the in­ternal symmetries of the Maxwellian field equations and of the Newtonian dynamics- their movement groups and relativity principles - differ from each other. (TheMaxwellian equations are Lorentz-invariant, the Newtonian mechanics is in con­trast Galilei-invariant). This fact was discovered in full details by Poincare and byEinstein in 1905.

There had been already efforts to solve this problem by referring electrody­namics to a particular system of reference, to Lorentz' "rigid world ether" or byinterpreting the bodies of mechanics , especially the electrons by hypothetical mod­els as constituents of the electromagnetic field. (This was the topics of studies bye.g. G Larmor and M Abraham.) Wiechert looked for a common ether of mat­ter and electromagnetism. (He wrote on this topics in 1899 in his "Principles ofElectrodynamics" , which constituted together with Hilbert's famous "Principles ofGeometry" , the content of the Gottingen Gauss-Weber-Festschrift.)

Einstein special theory of relativity brought in 1905 a general solution by thebasic idea that the Lorentz-group is not the relativity principle of a special physicalentity, namely that of the electromagnetic field, but it is a fundamental symmetrygroup of space and time which is fulfilled by all physical laws. The Maxwelliantheory is in itself specially relativistic. The mechanical equations are to be formu­lated so that they became Lorentz-invariant, too . This was made by Einstein in theyear 1905 and Max Planck in 1906. The special relativistic form of electrodynamicswas founded by Einstein (1905) and by Minkowski (1907) and these established theconnection to Lorentz' study from 1895.

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184 W SCHRODER and H-J TREDER

Wiechert considered Einstein's work on general relativity which yielded a geo­metrical theory of gravity, a confirmation of his ether conception. In the generalrelativity theory the planar homogenous Minkowskian space-time-world of the spe­cial relativity is substituted by a curved Riemannian manyfoldness which was theninterpreted by Wiechert as a structured ether. Thus Wiechert confronted an ethertheory of gravity and electrodynamics with Einstein gravity theory. In this ethertheory the Newtonian gravity together with the first "after-Newtonian effects" givenby Einstein (rotation of the perihelion, red shift, light deflection) is represented bya Riemann-Neumannian interaction potential which contains in addition to the ve­locity of light c and the gravity constant 'Y three further constants as structuralconstants of the ether. These constants can be chosen so that exactly the Ein­steinian effects occur (Wiechert 1920).

Einstein's general relativity theory is in contrast a theory of principle whichdoes not contain any optional constants in addition to c and 'Y. This theory doesnot fit ad hoc to the measurement results, but he prognosticates them. Wiechertadmitted this in 1925, but he referred to the fact that this theory did not yield aunified theory of space, time and matter and that Einstein's and H Weyl's efforts toreach to such a theory returned to the idea of ether. Wiechert referred to Einstein'slectures (1920) on ether and relativity theory and Weyl (1921) emphasised "theoverwhelming power of ether" over matter.

In spite of the fact that Wiechert's efforts toward a uniform ether physics provedabortive, they added to the clarification of the principles of the theory of relativityand to the field theory. Thus they belong to the history of relativistic physics.

A further field of Wiechert's research was geophysics, especially seismology al­ready in his Konigsberg years. F Neumann had already lectured on "Theory ofthe Earth" , therefore geophysical topics were not unknown both to him and to hisstudents. The later Professor Paul Volkmann supported discussion in geophysicalproblems, too and he dealt with the history of this new academic direction, too .Wiechert treated the mass distribution within the Earth during his Konigsbergyears . In a lecture at the Physical-Economical Society on January 9, 1896 heexpressed his opinion that the Earth has an iron core (1896, 1897). In 1897 hepresented an Earth model with the following parameters: density of the mantle3.2 gjern", that of the core 8.21 gjern". The core would have in this model a ra­dius 0.779 times the Earth radius, corresponding to a depth of 1408 km. Wiechertconcluded from the difference between the density of surface rocks and that of themean density that the Earth has a heavy iron core. His ideas in these 1896 and 1897publication were of a hypothetical character, nevertheless the earthquake observa­tions carried out in Gottingen in the years to follow gave a possibility to confirm ordisprove this idea by experimental methods, namely by the observation of distantearthquakes. It was found that the changes of the velocity vs. depth are not contin­uous. The first computations traced the waves down to depths of about 2500 km. Alinear increase by about 50 percent of the velocity was found from the surface downto a depth of about 1200 km; from 1200 to 2500 km, a remarkable constant velocitywas found. Thus the Earth seemed to consist of two parts. In this situation arrivedthe observations from Samoa where a Gottingen station existed. With the help of

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WIECHERT'S SCIENTIFIC WORK 185

these Samoa values the waves could be traced farther near to the mid-point of theEarth. The constant velocity remained from a depth of 1200 km farther, but onlyto a depth of 2900 km. Below this depth, the velocity decreased suddenly by 30percent, followed by a slow increase in even greater depths. Thus the threepartitestructure of the Earth was discovered: two shells around a core, which was calledby Wiechert as the "Samoa-core". This discovery was made by Wiechert's studentBeno Gutenberg in the year 1914.

Evidently the studies in geophysics which Wiechert started in Konigsberg weremostly continued in Gottingen. Here in Gottingen he was engaged to establish theGeophysical Institute, to reform the curriculum of geophysics, as well as to organisea world-wide net of research stations (Pacific Ocean, China, Argentine). The aimof all these efforts was to record earthquakes world-wide without gaps in order toconstruct a clear idea about the Earth's structure. Other areas, however, as e.g.atmospheric electricity, aurora as well as practical application of seismology wereselected to be subjects of his studies. He paid attention to the organisation ofgeophysics, too: thus he participated in the organisation of the international collab­oration of this discipline. In Germany, he was strongly engaged in the foundationof the Society of Seismology, being a precursor of the present "German GeophysicalSociety" .

Thus Wiechert's life-work belong to both disciplines, to physics as well as togeophysics. His early Konigsberg studies and his publications in physics and ingeophysics from Gottingen made him soon internationally known. His partners inscientific discussions included several great names of science as e.g. Ludwig Boltz­mann, Max Born, Peter Debye, Albert Einstein, James Franck, David Hilbert, FelixKlein, Max von Laue etc. Concerning the merit of his activity it is evident thathe contributed mainly to the interdisciplinary discussion, especially of physics, geo­physics and theory of science. In this respect Wiechert's life was always concentratedaround his general interest in nature.

Acknowledgement

WS' studies about Wiechert were supported by Professor Hund and by the GottingenAcademy of Sciences. He is most grateful for this support.

References

Einstein A 1905: Ann. Physik, 17, 891.Einstein A 1920: Ather und Relativitatstheorie, BerlinGutenberg B 1914: Nachr. Kg. Ges. Wiss . Gottingen. 1907-191Hund F 1987: Die Geschichte der Gottinger Physik. Vandenhoek and Ruprecht, GottingenLorentz H A 1895: Versuche einer Theorie der elektrischen und optischen Erscheinungen

in bewegten Korpern. Leiden, GottingenLorentz H A 1900: Ann. Phys . und Chemie, 4, Folge 4, 1901, 667.Lorentz H A 1914: Physik, Leipzig, 343-357 .Minkovski H 1911: Gesammelte Abhandlungen. hrsg . von D Hilbert, Leipzig, Berlin

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Neuma nn F E 1848: Uber ein allgemeines Prinzip der ma th ematischen Theorie der in­duzi erten Strome, Berlin

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Schroder W 1985: Nachr . Akad . Wiss. in Gottingen , II. Math.-Phys. Klasse, Nr . 2Schroder W 1988: Arch. Rist . Ex. Sci., 157-171.Schroder W 1990: Ann. Physik, 47, 475.Weyl R 1921: Phy s. Z., 22, 473.Wiechert E 1896a: Schriften phys.-okon. Ges., Konigsberg , 37, 1.Wiechert E 1896b: Schrift en phys.-okon. Ges., Konigsberg, 37, 4.Wiecher t E 1897: Schrift en phys.-okon . Ges., Konigsberg, 38, 221.Wiechert E 1899: Grundlagen der Elektrodyn amik. Gottin genWiechert E 1900: Arch. neerl ., livre jubi laire dedie a.Wiechert E 1914: Physik. Leipzig , 1Wiechert E 1920: Nachr . Ges. Wiss. Got ting en , Math.-phys. Kl. , 1.

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