**Some extracts from Emil Wiechert´s publications
and letters to geophysics and physics are presented as to give some idea of his
contribution to this subject during the 19 ^{th} and 20^{th}
centuries. These are set in context with other contemporary geophysical and
seismological research.**

**Keywords**: history of
geophysics; history of physics; seismology; Wiechert, Emil

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

His
tutor Woldemar Voigt went in the meanwhile to Goettingen and carried Wiechert
with himself. In the framework of Felix Klein's reform efforts the foundation of
a Geophysical Institute appeared, too. Wiechert hoped in that time that he would
obtain an invitation to the professorship in theoretical physics. As he did not
receive this on time, he accepted Klein's invitation to the directorship of the
Geophysical Institute together with the professorship in geophysics. He became
soon also a member of the Royal Society of Siences in Goettingen and developed
consequently the field of geophysics in the framework of the university.
External stations in the Southern Pacific, in China, in Argentina as well as in
Germany aimed at helping the phenomenon earthquake. In addition to this activity,
Wiechert continued to be interested in the great Problems of theoretical physies.
He died in 1926 in Goettingen.

In
the eighties of the last century four students were matriculated at the faculty
of philosophy of the Koenigsberg University, who were well acquainted with each
other and who remained in their following life in close connection. They were as
listed here according to their age- Emil Johann Wiechert (1861-1928), David Hilbert (1862-1943), Hermann Minkowski (1864-1909) and Arnold
Sommerfeld (1868-1951). They were all later in Goettingen, partly at the same
time: Wiechert since 1897 (as professor of geophysics), Hilbert since 1898 (as
professor of mathematics), and since 1902, Minkowski, too. Sommerfeld
was between 1896 and 1900 private-docent in Goettingen. In Koenigsberg
Wiechert, the oldest of the four, was the most impressive personality, as
Sommerfeld remembered to this time. He served as opponent of Hilbert's doctor
thesis as early as in 1885.

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

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

Franz Neumann was together with C F Gauss
(1777-1855), Wilhelm Weber (1804-1891), Bernhard Riemann (1826-1866) and his son
Carl Neumann (1832- 1925), all from Goettingen, 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 by Heinrich Hertz (1857-1894) the
Faraday-Maxwellian field theory became generally accepted. According to Hertz,
electrodynamics was from that time "Theory of the Maxwellian equations".
The big problem was the deduction of the interaction between electromagnetic
fields and electric charges, and
not the interaction between charges and currents.

Weber, Neumann and Riemann supposed in the
framework of their interaction theory that all electric currents are moving
(positive and negative) charges and that charge carriers in the electric
conductors are point-like particles of a very (or infinitely) 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 "atomistic of
electric charges" - the existence of an elementary charge with an absolute
value corresponding to Faraday's equivalence law of electrochemistry. The
corresponding hypothetical charge carriers (according to the remote effect
eleetrodynamies) were called 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 electrons are. Hertz and his pupil, P Lenard (1862-1947),
however, who used as first free electrons supposed at first that he could prove
that cathode rays are no charged particles, buet they are a particular form of
"ether waves".

Just at this point J J Thompson (1856-1940) in
Cambridge and Emil Wiechert intervened. They discovered the error in Hertz's and Lenard's
work and proved that cathode rays consist of particles with negative charges. These charges have the value e and the masses are small as cornpared to
the mass of the chemical atoms. W Kaufmann's (1871-1947) experiments who
continued Wiechert's work with cathode rays of higher velocity (after 1896) confirmed this
supposition (Kaufmann was from 1908 till 1935 Professor in Koenigsberg).
Wiechert tried to couple his experimental results about electric currents as
moving electrons with the Maxwellian equations and thus to substantiate
electrodynamics in form of electron dynamics. He made here, however, an
elementary error in the computation which prevented him from reaching a
consistent result. Therefore he threw the manuscript away.

In
the same time, H A Lorentz (1853-1928) studied in Leiden the influence of the
electromagnetic field according to the Maxwellian equations on spatially
extended 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 and
he presented in 1896 a summarising report on the "Theory of electrodynamics'
at the Koenigsberg Physical-Economical Society.

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

(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. (The
Maxwellian equations are Lorentz-invariant, the Newtonian mechanics is in
contrast Galilei-Invariant). This fact was discovered in full details by Poincaré
and by Einstein 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 by
interpreting the bodies of mechanics, especially the electrons by hypothetical
models as constituents of the electromagnetic field. (This was the topics of
studies by e.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
of Electrodynarnies", which constituted together with Hilbert's famous
"Principles of Geometry", the content of the Goettingen
Gauss-Weber-Festschrift.)

Einstein
special theory of relativity brought in 1905 a general solution by the basic
idea that the Lorentz-group is not the relativity principle of a special
physical entity, namely that of the electromagnetic field, but it is a
fundamental symmetry group of space and time which is fulfilled by all physical
laws. The Maxwellian theory 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 the year 1905 and Max Planck in 1906. The special
relativistic form of electrodynamics was founded by Einstein (1905) and by
Minkowski (1907) and these established the connection to Lorentz' study from
1895.

Wiechert considered Einstein's work on
general relativity which yielded a geometrical theory of gravity, a confirmation
of his ether conception. In the general relativity theory the planar homogenous
Minkowskian space-time-world of the special relativity is substituted by a
curved Riemannian manyfoldness which was then interpreted by Wiechert as a
structured ether. Thus Wiechert confronted an ether theory of gravity and
electrodynamics with Einstein gravity theory. In this ether theory the Newtonian
gravity together with the first "after-Newtonian effects" given by
Einstein (rotation of the perihelion, red shift, light deflection) is
represented by a Riemann-Neumannian interaction Potential which contains in
addition to the velocity of light c and the gravity constant g
three further constants as structural constants 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 which does not contain any optional constants in addition to c and g. This theory does not fit ad hoc to the measurement results, but he prognosticates them. Wiechert admitted this in 1925, but he referred to the fact that this theory did not yield a unified theory of space, time and matter and that Einstein's and H Weyl's efforts to reach to such a theory returned to the idea of ether. Wiechert referred to Einstein's lectures (1920) on ether and relativity theory and Weyl (1921) emphasised "the overwhelming power of ether" over matter.

In spite of the fact that Wiechert's
efforts toward a uniform ether physics proved abortive, they added to the
clarification of the principles of the theory of relativity and 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 Koenigsberg years. F
Neumann had already lectured on "Theory of the Earth", therefore
geophysical topics were not unknown both to him and to his students. The later
Professor Paul Volkmann supported discussion in geophysical problems too and he
dealt with the history of this new academic direction, too. Wiechert treated the
mass distribution within the Earth during his Koenigsberg years. In a lecture at
the Physical-Economical Society on January 9, 1896 he expressed his opinion that
the Earth has an iron core (1896, 1897). In 1897 he presented an Earth model
with the following parameters- density of the mantle 3.2 g/cm', that of the core
8.21 g/cm'. The core would have in this model a raius 0.779 times the Earth
radius, corresponding to a depth of 1408 km. Wiechert concluded from the
difference between the density of surface rocks and that of the mean density
that the Earth has a heavy iron core. His ideas in these 1896 and 1897
publication were of a hypothetical character, nevertheless the earthquake
observations carried out in Goettingen in the years to follow gave a possibility
to confirm or disprove this idea by experimental methods, namely by the
observation of distant earthquakes. 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. A linear increase by about 50 percent of the
velocity was found from the surface down to a depth of about 1200 km; from 1200
to 2500 km, a remarkable constant velocity was found. Thus the Earth seemed to
consist of two parts. In this situation arrived the observations from Samoa
where. a Goettingen station existed. With the help of these Samoa values the
waves could be traced farther near to the mid-point of the Earth. The constant
velocity remained from a depth of 1200 km farther, but only to a depth of 2900
km. Below this depth, the velocity decreased suddenly by 30 percent, followed by
a slow increase in even greater depths. Thus the threepartite structure of the
Earth was discovered: two shells around a core, which was called by Wiechert as
the "Samoa-core". This discovery was made by Wiechert's student Beno
Gutenberg in the year 1914.

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

Thus
Wiechert's life-work belong to both disciplines, to physics as well as to
geophysics. His early Koenigsberg studies and his publications in physics and in
geophysics from Goettingen made him soon internationally known. His Partners in
scientific discussions included several great names of science as e.g. Ludwig
Boltzmann, Max Born, Peter Debye, Albert Einstein, James Franck, David Hilbert,
Felix Klein, Max von Laue etc. Concerning the merit of his activity it is
evident that he contributed mainly to the interdisciplinary discussion,
especially of physics, geophysics and theory of science. In this respect
Wiechert's life was always concentrated around his general interest in nature.

My studies about Wiechert were supported by Professor Hund and by the
Goettingen
Academy of Sciences. I am most grateful for this support.

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