EPS Historic Site in Budapest to Honour the Eötvös Experiment
Budapest, October 2018 – The former physics building of the Loránd Eötvös University ‒ the physics department was moved from the centre of the city to a new campus about 20 years ago, the former building being occupied now by institutes of the Faculty of Humanities ‒ has been recognised as an EPS Historic Site. The inauguration of the memorial plaque took place on 12th October 2018 in the presence of Rüdiger Voss, president of EPS, David Lee, secretary general of EPS, and several other representatives of EPS, since the advisory editorial board of Europhysics News held its meeting in Budapest the next day, and many of the board members arrived early enough to participate on the Historic site ceremony.
After the welcome address by László Borhy, rector of the Eötvös University, Rüdiger Voss described the Historic Site programme of EPS. Jenő Sólyom, president of the Loránd Eötvös Physical Society talked about the significance of the building, the role it has played in the history of physics and about the personality of Loránd Eötvös himself.
Loránd Eötvös was professor of physics of the Budapest University from 1871 until his death in 1919. The physics building itself was built between 1883 and 1886 according to his ideas. He lived in this building during the winter season. Otherwise, when the weather permitted, he came on horseback from his estate in the outskirts of the city. He conducted most of his research here, especially his ground-breaking experiments in which he demonstrated to an extraordinary degree of accuracy, to 1 in 200 million, the proportionality of inertial and gravitational mass. Inertial mass is the mass experienced when a body is forced to accelerate and gravitational mass is the mass determining roughly speaking the weight of the body. In reality the weight of a body is not simply due to the gravitational force between the body and the Earth, since there is a tiny additional component to the weight, a centrifugal force due to the rotation of the Earth, and this component is proportional to the inertial mass. The proportionality or equivalence of the two masses – this distinction is just a matter of units ‒ has been suspected ever since Isaac Newton’s experiments, but the first precise measurements were carried out by Eötvös in 1885 and later, between 1906 and 1909, by himself and his collaborators, Dezső Pekár and Jenő Fekete. It was for these measurements that they won in 1909 the prestigious Beneke Prize from the philosophical faculty of the University of Göttingen. A few decades later a former student of Eötvös, János Renner improved the accuracy by one order of magnitude, and it took many more decades until Robert H. Dicke with his co-workers could achieve a further one order of magnitude increase in the accuracy. As we heard later on in the afternoon, even nowadays people are still trying hard to improve upon these results.
The equivalence of the two masses was in important ingredient when Albert Einstein developed his general theory of relativity. This theory is based on Einstein’s equivalence principle, a bold step beyond the equivalence of the two masses, namely on the complete physical equivalence of a gravitational field and a corresponding acceleration of the reference system.
Eötvös’s contribution to physics and geophysics, and to science, culture and education in general was much richer than that experiment. He developed the Eötvös torsion balance or Eötvös pendulum, an extremely sensitive instrument for measuring local variations in the gravitational field. Very modestly he wrote about this device as follows: „It is as simple as Hamlet’s flute, you only have to know how to play it. Just as the musician can delight you with splendid variations from his instrument, so the physicist can measure on this balance, with no less delight, the finest variations of gravity. In this way we can peer into the Earth’s crust at such depths that neither our eyes could penetrate, nor the longest drills could reach.” I wonder what he would have said if he had known that the use of his torsion balance would become instrumental in prospecting for oil fields in Texas and Venezuela.
Besides his scientific activity, he was minister for a short while responsible for public education and religious affairs, rector of the university for one term, president of the Hungarian Academy of Sciences for 16 years, and founder of the Hungarian Mathematical and Physical Society, the predecessor of the Roland Eötvös Physical Society. The international scientific community knows him, however, mostly for his achievements mentioned above. One hundred years after his death, his work is still deeply appreciated and his name still comes up in almost all textbooks presenting the experiments underlying Einstein’s theory. That is why UNESCO accepted the Hungarian proposal to be associated with the worldwide celebration of Loránd Eötvös in 2019 on the occasion of the 100th anniversary of his death.
After the unveiling ceremony a lecture series was organised in the main lecture hall where once Eötvös used to deliver his lectures. Clifford Martin Will of the University of Florida, Gainesville gave a talk on the Eötvös experiment showing how new techniques help us to impove upon the already fantastic accuracy achieved by Eötvös and his co-workers in demonstrating the equivalence of inertial and gravitational mass. The next speaker, Viktor Wesztergom, head of the Geodetic and Geophysical Institute of the Hungarian Academy of Sciences put Eötvös’s contribution to the foundations of geophysics into historical perspective, pointing out that our knowledge about the Earth’s interior is still unsatisfactory but expressing his belief that the intellectual heritage of Eötvös and the power of the modern geophysics may result in a breakthrough in understanding our planet.
This festive event was a good occasion for the Hungarian Physical Society to commemorate at the same time the 50th anniversary of the EPS. The president of EPS, Rüdiger Voss, talked about the fifty years of service of EPS to the European physical community, outlining its mission and actions, the challenges and opportunities, while a former president of EPS, Norbert Kroó talked ‒ illustrated with personal remembrances ‒ about the strong connections between EPS and Hungary from the very beginning, and how physics changed our lives in the last 50 years.
