Saturday, February 6, 2016

Telegraph Hill

No, I am not referring to the famous hill in San Francisco, neither to the one in London also known as Plowed Garlic Hill but to Potsdam's Telegrafenberg, a hill of 96 meter. Red Baron has visited Potsdam several times but I always missed one attraction and for a physicist a must: The expressionist Einstein Tower (Einsteinturm) in Potsdam's Albert Einstein Science Park. This astrophysical observatory was built by Erich Mendelssohn on Telegrafenberg in the years 1919 to 1922.

During my last trip to Berlin I took the S-Bahn to Potsdam Hauptbahnhof intending to take the uphill bus to Telegrafenberg. However the line only operates in the morning and the late afternoon transporting the employees of the science institutes up and down the hill. Eventually I took a taxi taking me to the guarded entrance of the Science Park. I told the man behind the counter that I wanted to pay a visit to the monument dedicated to my famous physics colleague. He smiled, stood up, and handed me a pound of paper information. Well documented I started my up-hill walk passing various information panels and institute buildings.

The Telegrafenberg got it's name from the mechanical telegraph that the Prussian government implemented in the first half of the 19th century. Following Napoleon's defeat and the Congress of Vienna in 1814/15 most of the old borders in Europe were restored with some noteworthy exceptions. Austria cleverly gave up its possessions on the Upper Rhine - note that on Napoleon's order the Breisgau with its capital Freiburg had already become "badisch" in 1806 - thus avoiding any future confrontation with France while Prussia acquired territories on the Lower Rhine including the coal-rich Ruhr district as well as the cities of Cologne and Coblenz. Thus the Prussians were assuming the Wacht am Rhein (The watch over the Rhine river) with respect to France.

Königlich-Preußische Optische Telegrafenlinie from Berlin to Koblenz

Reconstructed Prussian optical mechanical telegraph station in
 Cologne-Flittard. The many signal arms enabled the transmission
of information-rich messages (©Superbass/Wikipedia)

With such a vast territory spanning from Königsberg in East Prussia to Aachen at the Belgian border fast transmission of information in the kingdom was a challenge. It took messengers on horseback three to four days to cover the distance between Berlin and Coblenz. Thus Prussia began the construction of an optical mechanical telegraph line in 1833 with 62 relay stations covering a distance of 550 kilometers between Berlin and Coblenz. The line started at Dorotheenstraße in Berlin, joined the Saint Anna Church in Dahlem, passed the miserable hill of the Schäferberg (shepherd's mountain!), went to the Telegrafenberg at Potsdam, and continued to Magdeburg's Saint Jean Church. After having gone through many more relay stations the line crossed some regions belonging to the Duchy of Brunswick, regained Prussian territory in Westphalia at Paderborn, and eventually reached Coblenz via the Cologne-Flittard relay. With the new installation the transmission of a telegram took 1.5 hours.

On my way to the Einstein Tower I passed the street sign Schwarzschildweg honoring Karl Schwarzschild, the German astronomer. He was director of Potsdam's Astro-Physical Observatory from 1906 to his early death in 1916. In 1897, while in Vienna, he derived a formula to calculate the optical density of a photographic emulsion. Now the intensity of faint stellar light sources could be calculated with high precision from photographic measurements.

Whereas Einstein only gave an approximate solution for his field equations of general relativity around a single spherical non-rotating mass Schwarzschild while serving with the Prussian Army in Russia in 1915 calculated the exact solution. He wrote to Einstein: As you see, the war treated me kindly enough, in spite of the heavy gunfire, to allow me to get away from it all and take this walk in the land of your ideas. Needless to say, Einstein was impressed. Schwarzschild's calculations resulted in the famous Schwarzschild Radius being the size of the event horizon of a non-rotating black hole. Karl's son Martin became a distinguished professor of astronomy at Princeton University.

The first impressing landmark on my way to the Einstein Tower was the former astrophysical observatory built in the late 1870ies. The building has three observation towers with rotating domes initially equipped with telescopes. Later the building became known as Michelsonhaus for in 1881 the Nobel prize winner of 1907 Albert A. Michelson had performed his key experiments in its basement proving that the velocity of light is always constant. His discovery was the basis for Einstein's theory of relativity. Following important renovation work in 2000 the Institute for Climate Impact Research (PIK) moved into the building.

Großer Refraktor
Another impressive sight is the Big Refractor Building. The Großer Refraktor features two telescopes in parallel one for the observer and one for the camera. The installation was built in 1899, used until 1968, and completely refitted in 2006.

The Einsteinturm was built on the summit to house a solar telescope designed by the astronomer Erwin Finlay-Freundlich. The telescope supports experiments and observations to validate (or disprove) Einstein's relativity theory.

The Einstein Tower seen from the front

Here it is seen from the back with the Big Refractor in the background
On my way back to the gate I passed a nostalgic monument. The GDR was proud of having sent with Russian help the first German cosmonaut - the name distinct from NASA's astronauts - into space. The man was Sigmund Werner Paul Jähn who returned from space-station Salyut 6 to earth together with Russian Commander Valery Fyodorovich Bykovsky on the Soyus 29 mission. They both became Heros of the German Democratic Republic. In 1983 Jähn received a doctorate at the Zentralinstitut für Physik der Erde (Central Institute for the Physics of the Earth) specialized in remote sensing of the earth and located on Potsdam's Telegrafenberg.

Together on earth and in space

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