100 years Einstein Tower - A solar observatory connects art and science

Planning and construction of the Einstein Tower

The Einstein Tower is the first important building by architect Erich Mendelsohn. It was built between 1919 and 1924 under the direction of the astronomer Erwin Finlay Freundlich in collaboration with the physicist Albert Einstein and . The tower is a functional building that represents one of the rare links between science and art. Mendelsohn succeeded in fulfilling both the requirements of science and his own ideas on design.

On 6 December 1924, the Einstein Tower was officially opened during a meeting of the Einstein Foundation's Board of Trustees. The solar observatory was the most important scientific solar telescope in Europe until the Second World War.

Destruction during the World War II

In 1942, the tower was coated with a dark green and brownish mottled camouflage paint to prevent it from being a target during air raids. On 14 April 1945, an air mine exploded in the immediate vicinity. A fire broke out in the Einstein Tower, which was extinguished by an employee of the Astrophysical Observatory Potsdam (a predecessor institution of the Leibniz Institute for Astrophysics Potsdam), who spent the night in the workroom in the tower during the night of the bombing. The blast wave from the bomb also damaged the Einstein Tower. Two thirds of the dome was destroyed. The window sashes were torn out. There were splinter holes in the terrace and on the north side of the tower. The electrical cables were damaged. However, the tower's statics withstood the blast wave, which was confirmed by an examination of the tower in 1995 before the first major repair.

After the end of the war, the dome was provisionally renovated and work resumed at the beginning of October 1945. On the occasion of the 250th anniversary of the German Academy of Sciences in 1950, the building was thoroughly repaired.

Renovation of the Einstein Tower

Due to its mixed construction of brickwork and concrete, the Einstein Tower is susceptible to cracks through which moisture can penetrate. As a result, renovation work was necessary about every 10 to 15 years. In the 1990s, the Wüstenrot Foundation not only enabled the Einstein Tower to be fundamentally renovated, but also financed the first comprehensive structural and historical examination of the building since its construction. Between 2021 and 2023, the Einstein Tower was due for another refurbishment, which was again conducted and financed by the Wüstenrot Foundation.

Highlights from its history

In its hundred-year history, the Einstein Tower has experienced a number of highlights, including the visit of cosmonauts Sigmund Jähn and Calerie Bykowski in 1979, the royal visit of Prince Charles in 2009 and, more recently, a visit from the Nepalese president. The building has served as the backdrop for various ceremonies and cultural events, e.g. the production of ‘DAS NETZ’, and is still a world-famous visitor attraction to this day.

Science at the Einstein Tower as a solar observatory

The commissioning of the Einstein Tower in 1924 marked the beginning of a new era of modern solar research in Potsdam and Germany. The tower remained the largest solar research facility in Europe until the Second World War and was one of the most powerful facilities of its kind in the world. The Einstein Tower was originally built to provide evidence of the redshift of spectral lines in the gravitational field of the Sun predicted by Einstein's theory of relativity. However, turbulence on the surface of the Sun produces a blue shift of the same magnitude and superimposes the predicted effect, so that the observations remained inconclusive. Experimental confirmation of the effect was not achieved until the end of the 1950s.

In the centre of the building, a vertically fixed telescope with a lens of 60 cm diameter and 14 m focal length stands on its own foundation. A coelostat is located in the dome, whose two mirrors direct the sunlight into the telescope to the lens objective in the wooden tower. In the basement, the light is directed horizontally into the laboratory through another swivelling mirror. Its centrepiece is the spectrograph room, which is also around 14 m long and extends to the end of the earth wall on the south side of the tower. In this temperature-stabilised room, sunlight is broken down into its spectral components and analysed in order to obtain information about temperature, pressure, magnetic fields, eruptions and other physical processes and conditions on the Sun, in particular its outer layers.

Erwin Finley Freundlich used the Einstein Tower as a starting point for several expeditions to solar eclipses. On his trip to Sumatra in 1929 to see the solar eclipse, Walter Grotrian recorded the spectrum of the solar corona with his own device. In the following years, his analyses of the measurements revealed that the solar corona has a temperature of more than one million degrees Celsius. Grotrian also created a model of the physical nature of sunspots at the Einstein Tower. Harald von Klüber, one of Freundlich's early colleagues, developed a way of analysing the strength and direction of the magnetic field in sunspots. Another example of scientific success at the Einstein Tower is Horst Künzel's work on delta sunspots, published in 1960, which is still important for predicting radiation eruptions on the Sun.

Between 1943 and the mid-1980s, photographs of the Sun were taken at the Einstein Tower on glass photographic plates, on which in particular the dark sunspots can be seen. The approximately 3000 glass plates are stored in the Einstein Tower and have been digitised and published in the historical plate archive.

Today, the Einstein Tower is the AIP's in-house instrument for solar observations. Its still fully functional equipment enables participation in international measurement campaigns and long-term measurements. In particular, polarisation measurements of sunspots are carried out to investigate the magnetic and velocity field on the surface of the Sun. Researchers at the AIP also use the Einstein Tower to develop and test new experiments and devices before installing them on large-scale equipment such as the GREGOR telescope on Tenerife and to train students.

Among other things, parts of the instruments for the European Space Agency's (ESA) Solar Orbiter space probe, which was launched into space from Cape Canaveral in Florida on 9 February 2020 to study the Sun at close range, were developed in the Einstein Tower. On board the space probe is the STIX X-ray telescope, which was co-developed, tested and optimised at the Einstein Tower. A part of this telescope is still set up as a simulation in the laboratory of the Einstein Tower in order to be able to check the measurement results of Solar Orbiter.

The Einstein Tower today

After renovation and reopening in September 2023, the Einstein Tower is once again fully operational as a solar observatory. In order not to interfere with scientific activities, interior tours are only possible as part of guided tours and on special occasions.