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Gravitational lens

A gravitational lens is matter, such as a cluster of galaxies or a point particle, that bends light from a distant source as it travels toward an observer. The amount of gravitational lensing is described by Albert Einstein's general theory of relativity. If light is treated as corpuscles travelling at the speed of light, Newtonian physics also predicts the bending of light, but only half of that predicted by general relativity.

Orest Khvolson (1924) and Frantisek Link (1936) are generally credited with being the first to discuss the effect in print, but it is more commonly associated with Einstein, who made unpublished calculations on it in 1912 and published an article on the subject in 1936.

In 1937, Fritz Zwicky posited that galaxy clusters could act as gravitational lenses, a claim confirmed in 1979 by observation of the Twin QSO SBS 0957+561.

Description

Unlike an optical lens, a point-like gravitational lens produces a maximum deflection of light that passes closest to its center, and a minimum deflection of light that travels furthest from its center. Consequently, a gravitational lens has no single focal point, but a focal line. The term "lens" in the context of gravitational light deflection was first used by O.J. Lodge, who remarked that it is "not permissible to say that the solar gravitational field acts like a lens, for it has no focal length". If the (light) source, the massive lensing object, and the observer lie in a straight line, the original light source will appear as a ring around the massive lensing object (provided the lens has circular symmetry). If there is any misalignment, the observer will see an arc segment instead.

This phenomenon was first mentioned in 1924 by the St. Petersburg physicist Orest Khvolson, and quantified by Albert Einstein in 1936. It is usually referred to in the literature as an Einstein ring, since Khvolson did not concern himself with the flux or radius of the ring image. More commonly, where the lensing mass is complex (such as a galaxy group or cluster) and does not cause a spherical distortion of spacetime, the source will resemble partial arcs scattered around the lens. The observer may then see multiple distorted images of the same source; the number and shape of these depending upon the relative positions of the source, lens, and observer, and the shape of the gravitational well of the lensing object.

There are three classes of gravitational lensing:

 * Strong lensing: Where there are easily visible distortions such as the formation of Einstein rings, arcs, and multiple images. Despite being considered "strong", the effect is in general relatively small, such that even a galaxy with a mass more than 100 billion times that of the Sun will produce multiple images separated by only a few arcseconds. Galaxy clusters can produce separations of several arcminutes. In both cases the galaxies and sources are quite distant, many hundreds of megaparsecs away from our Galaxy.
 * Weak lensing: Where the distortions of background sources are much smaller and can only be detected by analyzing large numbers of sources in a statistical way to find coherent distortions of only a few percent. The lensing shows up statistically as a preferred stretching of the background objects perpendicular to the direction to the centre of the lens. By measuring the shapes and orientations of large numbers of distant galaxies, their orientations can be averaged to measure the shear of the lensing field in any region. This, in turn, can be used to reconstruct the mass distribution in the area: in particular, the background distribution of dark matter can be reconstructed. Since galaxies are intrinsically elliptical and the weak gravitational lensing signal is small, a very large number of galaxies must be used in these surveys. These weak lensing surveys must carefully avoid a number of important sources of systematic error: the intrinsic shape of galaxies, the tendency of a camera's point spread function to distort the shape of a galaxy and the tendency of atmospheric seeing to distort images must be understood and carefully accounted for. The results of these surveys are important for cosmological parameter estimation, to better understand and improve upon the Lambda-CDM model, and to provide a consistency check on other cosmological observations. They may also provide an important future constraint on dark energy.
 * Microlensing: Where no distortion in shape can be seen but the amount of light received from a background object changes in time. The lensing object may be stars in the Milky Way in one typical case, with the background source being stars in a remote galaxy, or, in another case, an even more distant quasar. In extreme cases, a star in a distant galaxy can act as a microlens and magnify another star much farther away. The first example of this was the star MACS J1149 Lensed Star 1 (also known as Icarus), thanks to the boost in flux due to the microlensing effect.

Gravitational lenses act equally on all kinds of electromagnetic radiation, not just visible light, and also in non-electromagnetic radiation, like gravitational waves. Weak lensing effects are being studied for the cosmic microwave background as well as galaxy surveys. Strong lenses have been observed in radio and x-ray regimes as well. If a strong lens produces multiple images, there will be a relative time delay between two paths: that is, in one image the lensed object will be observed before the other image.

History

Henry Cavendish in 1784 (in an unpublished manuscript) and Johann Georg von Soldner in 1801 (published in 1804) had pointed out that Newtonian gravity predicts that starlight will bend around a massive object as had already been supposed by Isaac Newton in 1704 in his Queries No.1 in his book Opticks. The same value as Soldner's was calculated by Einstein in 1911 based on the equivalence principle alone. However, Einstein noted in 1915, in the process of completing general relativity, that his (and thus Soldner's) 1911-result is only half of the correct value. Einstein became the first to calculate the correct value for light bending.

The first observation of light deflection was performed by noting the change in position of stars as they passed near the Sun on the celestial sphere. The observations were performed in 1919 by Arthur Eddington, Frank Watson Dyson, and their collaborators during the total solar eclipse on May 29. The solar eclipse allowed the stars near the Sun to be observed. Observations were made simultaneously in the cities of Sobral, Ceará, Brazil and in São Tomé and Príncipe on the west coast of Africa. The observations demonstrated that the light from stars passing close to the Sun was slightly bent, so that stars appeared slightly out of position.

The result was considered spectacular news and made the front page of most major newspapers. It made Einstein and his theory of general relativity world-famous. When asked by his assistant what his reaction would have been if general relativity had not been confirmed by Eddington and Dyson in 1919, Einstein said "Then I would feel sorry for the dear Lord. The theory is correct anyway." In 1912, Einstein had speculated that an observer could see multiple images of a single light source, if the light were deflected around a mass. This effect would make the mass act as a kind of gravitational lens. However, as he only considered the effect of deflection around a single star, he seemed to conclude that the phenomenon was unlikely to be observed for the foreseeable future since the necessary alignments between stars and observer would be highly improbable. Several other physicists speculated about gravitational lensing as well, but all reached the same conclusion that it would be nearly impossible to observe.

Although Einstein made unpublished calculations on the subject, the first discussion of the gravitational lens in print was by Khvolson, in a short article discussing the "halo effect" of gravitation when the source, lens, and observer are in near-perfect alignment, now referred to as the Einstein ring.

In 1936, after some urging by Rudi W. Mandl, Einstein reluctantly published the short article "Lens-Like Action of a Star By the Deviation of Light In the Gravitational Field" in the journal Science.

In 1937, Fritz Zwicky first considered the case where the newly discovered galaxies (which were called 'nebulae' at the time) could act as both source and lens, and that, because of the mass and sizes involved, the effect was much more likely to be observed.

In 1963 Yu. G. Klimov, S. Liebes, and Sjur Refsdal recognized independently that quasars are an ideal light source for the gravitational lens effect.

It was not until 1979 that the first gravitational lens would be discovered. It became known as the "Twin QSO" since it initially looked like two identical quasistellar objects. (It is officially named SBS 0957+561.) This gravitational lens was discovered by Dennis Walsh, Bob Carswell, and Ray Weymann using the Kitt Peak National Observatory 2.1 meter telescope.

In the 1980s, astronomers realized that the combination of CCD imagers and computers would allow the brightness of millions of stars to be measured each night. In a dense field, such as the galactic center or the Magellanic clouds, many microlensing events per year could potentially be found. This led to efforts such as Optical Gravitational Lensing Experiment, or OGLE, that have characterized hundreds of such events, including those of OGLE-2016-BLG-1190Lb and OGLE-2016-BLG-1195Lb.

Approximate Newtonian description
Newton wondered if light, in the form of corpuscles, would be bent due to gravity. The Newtonian prediction for light deflection refers to the amount of deflection a corpuscle would feel under the effect of gravity, and therefore one should read "Newtonian" in this context as the referring to the following calculations and not a belief that Newton held in the validity of these calculations.

For a gravitational point-mass lens of mass $$M$$, a corpuscle of mass $$m$$ feels a force

 * $$\vec F = -\frac{GMm}{r^2} \hat r ~,$$

where $$r$$ is the lens-corpuscle separation. If we equate this force with Newton's second law, we can solve for the acceleration that the light undergoes

 * $$\vec a = -\frac{GM}{r^2}\hat r$$.

The light interacts with the lens from initial time $$t=0$$ to $$t$$, and the velocity boost the corpuscle receives is

 * $$\Delta \vec v = -\int_0^{t} dt' ~ \frac{GM}{r(t')^2} \hat r(t') ~.$$

If one assumes that initially the light is far enough from the lens to neglect gravity, the perpendicular distance between the light's initial trajectory and the lens is b (the impact parameter), and the parallel distance is $$r_{\parallel}$$ such that $$r^2 = b^2 + r_{\parallel}^2$$. We additionally assume a constant speed of light along the parallel direction, $$dr_{\parallel} \approx c dt$$, and that the light is only being deflected a small amount. After plugging these assumptions into the above equation and further simplifying, one can solve for the velocity boost in the perpendicular direction. The angle of deflection between the corpuscle’s initial and final trajectories is therefore (see, e.g., M. Meneghetti 2021)

 * $$\theta = \frac{2GM}{c^2 r} ~.$$

Although this result appears to be half the prediction from general relativity, classical physics predicts that the speed of light $$c$$ is observer-dependent (see, e.g., L. Susskind and A. Friedman 2018) which was superseded by a universal speed of light in special relativity.

Explanation in terms of spacetime curvature
In general relativity, light follows the curvature of spacetime, hence when light passes around a massive object, it is bent. This means that the light from an object on the other side will be bent towards an observer's eye, just like an ordinary lens. In general relativity the path of light depends on the shape of space (i.e. the metric). The gravitational attraction can be viewed as the motion of undisturbed objects in a background curved geometry or alternatively as the response of objects to a force in a flat geometry. The angle of deflection is:

 * $$\theta = \frac{4GM}{c^2 r}$$

toward the mass M at a distance r from the affected radiation, where G is the universal constant of gravitation and c is the speed of light in vacuum.

Since the Schwarzschild radius $$r_\text{s}$$ is defined as $$r_\text{s} = {2Gm}/{c^2}$$ and escape velocity $$v_\text{e}$$ is defined as $v_\text{e} = \sqrt{2Gm/r} = \beta_\text{e} c$, this can also be expressed in simple form as

 * $$\theta = 2 \frac{r_\text{s}}{r} = 2 \left(\frac{v_\text{e}}{c}\right)^2 = 2\beta_\text{e}^2$$

Search for gravitational lenses

Most of the gravitational lenses in the past have been discovered accidentally. A search for gravitational lenses in the northern hemisphere (Cosmic Lens All Sky Survey, CLASS), done in radio frequencies using the Very Large Array (VLA) in New Mexico, led to the discovery of 22 new lensing systems, a major milestone. This has opened a whole new avenue for research ranging from finding very distant objects to finding values for cosmological parameters so we can understand the universe better.

A similar search in the southern hemisphere would be a very good step towards complementing the northern hemisphere search as well as obtaining other objectives for study. If such a search is done using well-calibrated and well-parameterized instrument and data, a result similar to the northern survey can be expected. The use of the Australia Telescope 20 GHz (AT20G) Survey data collected using the Australia Telescope Compact Array (ATCA) stands to be such a collection of data. As the data were collected using the same instrument maintaining a very stringent quality of data we should expect to obtain good results from the search. The AT20G survey is a blind survey at 20 GHz frequency in the radio domain of the electromagnetic spectrum. Due to the high frequency used, the chances of finding gravitational lenses increases as the relative number of compact core objects (e.g. quasars) are higher (Sadler et al. 2006). This is important as the lensing is easier to detect and identify in simple objects compared to objects with complexity in them. This search involves the use of interferometric methods to identify candidates and follow them up at higher resolution to identify them. Full detail of the project is currently under works for publication.

Microlensing techniques have been used to search for planets outside our solar system. A statistical analysis of specific cases of observed microlensing over the time period of 2002 to 2007 found that most stars in the Milky Way galaxy hosted at least one orbiting planet within 0.5 to 10 AU.

In 2009, weak gravitational lensing was used to extend the mass-X-ray-luminosity relation to older and smaller structures than was previously possible to improve measurements of distant galaxies.

the most distant gravitational lens galaxy, J1000+0221, had been found using NASA's Hubble Space Telescope. While it remains the most distant quad-image lensing galaxy known, an even more distant two-image lensing galaxy was subsequently discovered by an international team of astronomers using a combination of Hubble Space Telescope and Keck telescope imaging and spectroscopy. The discovery and analysis of the IRC 0218 lens was published in the Astrophysical Journal Letters on June 23, 2014.

Research published Sep 30, 2013 in the online edition of Physical Review Letters, led by McGill University in Montreal, Québec, Canada, has discovered the B-modes, that are formed due to gravitational lensing effect, using National Science Foundation's South Pole Telescope and with help from the Herschel space observatory. This discovery would open the possibilities of testing the theories of how our universe originated.

Solar gravitational lens
Albert Einstein predicted in 1936 that rays of light from the same direction that skirt the edges of the Sun would converge to a focal point approximately 542 AU from the Sun. Thus, a probe positioned at this distance (or greater) from the Sun could use the Sun as a gravitational lens for magnifying distant objects on the opposite side of the Sun. A probe's location could shift around as needed to select different targets relative to the Sun.

This distance is far beyond the progress and equipment capabilities of space probes such as Voyager 1, and beyond the known planets and dwarf planets, though over thousands of years 90377 Sedna will move farther away on its highly elliptical orbit. The high gain for potentially detecting signals through this lens, such as microwaves at the 21-cm hydrogen line, led to the suggestion by Frank Drake in the early days of SETI that a probe could be sent to this distance. A multipurpose probe SETISAIL and later FOCAL was proposed to the ESA in 1993, but is expected to be a difficult task. If a probe does pass 542 AU, magnification capabilities of the lens will continue to act at farther distances, as the rays that come to a focus at larger distances pass further away from the distortions of the Sun's corona. A critique of the concept was given by Landis, who discussed issues including interference of the solar corona, the high magnification of the target, which will make the design of the mission focal plane difficult, and an analysis of the inherent spherical aberration of the lens.

In 2020, NASA physicist Slava Turyshev presented his idea of Direct Multipixel Imaging and Spectroscopy of an Exoplanet with a Solar Gravitational Lens Mission. The lens could reconstruct the exoplanet image with ~25 km-scale surface resolution, enough to see surface features and signs of habitability.

Measuring weak lensing
Kaiser, Squires and Broadhurst (1995), Luppino & Kaiser (1997) and Hoekstra et al. (1998) prescribed a method to invert the effects of the point spread function (PSF) smearing and shearing, recovering a shear estimator uncontaminated by the systematic distortion of the PSF. This method (KSB+) is the most widely used method in weak lensing shear measurements. Galaxies have random rotations and inclinations. As a result, the shear effects in weak lensing need to be determined by statistically preferred orientations. The primary source of error in lensing measurement is due to the convolution of the PSF with the lensed image. The KSB method measures the ellipticity of a galaxy image. The shear is proportional to the ellipticity. The objects in lensed images are parameterized according to their weighted quadrupole moments. For a perfect ellipse, the weighted quadrupole moments are related to the weighted ellipticity. KSB calculate how a weighted ellipticity measure is related to the shear and use the same formalism to remove the effects of the PSF.

KSB's primary advantages are its mathematical ease and relatively simple implementation. However, KSB is based on a key assumption that the PSF is circular with an anisotropic distortion. This is a reasonable assumption for cosmic shear surveys, but the next generation of surveys (e.g. LSST) may need much better accuracy than KSB can provide.

title: Gravitational lens


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Wittenberg

Wittenberg, officially Lutherstadt Wittenberg, is the fourth-largest town in Saxony-Anhalt, Germany. It is situated on the River Elbe, 60 km north of Leipzig and 90 km south-west of Berlin, and has a population of 46,008 (2018).

Wittenberg is famous for its close connection with Martin Luther and the Protestant Reformation, for which it received the honorific Lutherstadt and has been called the "cradle of the Reformation" and "cradle of Protestantism". Several of Wittenberg's buildings are associated with the events, including a preserved part of the Augustinian monastery in which Luther lived, first as a monk and later as owner with his wife Katharina von Bora and family, considered to be the world's premier museum dedicated to Luther. Wittenberg was also the seat of the Elector of Saxony, a dignity held by the dukes of Saxe-Wittenberg, making it one of the most powerful cities in the Holy Roman Empire.

Today Wittenberg is an industrial centre and popular tourist destination, best known for its intact historic centre and various memorial sites dedicated to Martin Luther and Philip Melanchthon. The buildings associated with those two figures were added to the UNESCO World Heritage list in 1996, along with other sites in Eisleben, because of their religious significance and testimony to one of the most influential movements of medieval Europe.

History

Historical documents first mention the settlement in 1180 as a small village founded by Flemish colonists under the rule of the House of Ascania. In 1260 this village became the residence of the dukes of Saxe-Wittenberg, and in 1293 the settlement was granted its town charter as a free-standing town.

Wittenberg developed into an important trade centre during the following few centuries because of its central location. When the local branch of the Ascanians died out in 1422, control of Saxe-Wittenberg passed to the House of Wettin. The town became an important regional political and cultural centre at the end of the 15th century when Frederick III "the Wise", the Elector of Saxony from 1486 to 1525, made his residence in Wittenberg. Several parts of the boundaries of the town were extended soon afterwards. The second bridge over the Elbe River was built between 1487 and 1490 and the castle church (the Schlosskirche in German) was erected between 1496 and 1506. The Elector's palace was rebuilt at the same time.

In 1502 Elector Frederick founded the University of Wittenberg, which attracted some writers such as Martin Luther—a professor of theology beginning in 1508—and Philipp Melanchthon—a professor of Greek starting in 1518.

On 31 October 1517, according to legend, Luther nailed his 95 theses against the selling of indulgences to the door of the All Saints', the Castle Church – an event taken as marking the beginning of the Protestant Reformation. The Anabaptist movement had one of its earliest homes in Wittenberg, when the Zwickau prophets moved there in late 1521, only to be suppressed by Luther when he returned from the Wartburg in spring of 1522.

The Capitulation of Wittenberg (1547) is the name given to the treaty by which John Frederick the Magnanimous was compelled to resign the electoral dignity and most of his territory to the Albertine branch of the House of Wettin.

In 1760, during the Seven Years' War, the Austrians bombarded the Prussian-occupied town. The French took control in 1806, and Napoleon commanded the refortification of the town in 1813. In 1814 the Prussian Army under Tauentzien stormed Wittenberg; he received the title of "von Wittenberg" as a reward. In 1815 Wittenberg became part of Prussia, administered within the Province of Saxony. Wittenberg continued to be a fortress of the third class until the reorganisation of German defences after the foundation of the new German Empire led to its dismantling in 1873.

It contained a prisoner of war camp from 1914 to 1918. A camp 10½ acres in area was set up at Klein Wittenberg, 2 miles from the city. Eight compounds held 13,000 men. During the typhus epidemic of 1914–1915, conditions were harsh. The camp medical officer, Dr. Aschenbach, was awarded the Iron Cross for his part in the epidemic. The award was questioned by the Allies. The use of dogs to attack POW's was criticised by American Ambassador James W. Gerard in his book "Four Years in Germany".

Unlike many other historic German cities during World War II, Wittenberg's town centre was spared destruction during the conflict. The Allies agreed not to bomb Wittenberg, though fighting took place in the town, with bullet pock-marks visible on the statues of Luther and Melanchthon in the market square – or so the popular version of the town's history goes. In actual fact the Luther statue was not even present in the town square during much of the war but in storage at Luther Brunnen, a roadhouse a few kilometres north of the town.

Wittenberg's reputation as a town protected from Allied bombing is largely accurate. However, just outside Wittenberg the government had built the Arado Flugzeugwerke (the Arado Aircraft Factory), which produced components of airplanes for the Luftwaffe. This war factory was worked by Jews, Russians, Poles, political prisoners and even a few Americans—all prisoners engaging in forced labour, including POW's who were supposed to be exempt from this sort of labor. American and British planes bombed the factory near the end of the war, and in destroying it killed over one thousand of the prisoners and POW's placed by the Germans in this war plant. The 1995 publication of "...und morgen war Krieg!" by Renate Gruber-Lieblich attempts to document this tragic bombing outside Wittenberg.

In 1945, Wittenberg issued 19 of its own postage stamps, each depicting Hitler but with a large black round overprint covering his face. At the end of the war, Soviet forces occupied Wittenberg; it became part of East Germany in 1949. During the East German period, it formed part of Halle District. By means of the peaceful revolution in 1989, the communist régime dissolved and the town has been governed democratically since 1990.

Wittenberg is currently characterized by renovation and new construction work, an economic recovery and tourism development as a "place of pilgrimage for the Reformation". With the Luther Decade starting in 2008, the city began preparing for the 500th anniversary of the Reformation, which took place in 2017. Numerous buildings have been restored, the infrastructure has been partially renewed and numerous new attractions have been created in the city (e.g. the new Lutherstadt Wittenberg Hauptbahnhof, Arsenal shopping centre, Luthergarden or the Panometer).

In 2014 Lutherstadt Wittenberg was awarded the honorary title European City of the Reformation by the Community of Protestant Churches in Europe.

Historical population
The figures are given for the metropolitan district at the point in time. Up to 1791 the figures are generally estimated, later figures are from census or local authorities.

Source: 2012 census.

Divisions
The town Wittenberg consists of Wittenberg proper and the following Ortschaften or municipal divisions:

 * Abtsdorf
 * Apollensdorf
 * Boßdorf
 * Griebo
 * Kropstädt
 * Mochau
 * Nudersdorf
 * Pratau
 * Reinsdorf
 * Schmilkendorf
 * Seegrehna
 * Straach

Sights and culture
Lutherstadt Wittenberg has a wide range of cultural sites. Most are located within the historic old town along the Wittenberg Culture Mile.

Wittenberg is home to numerous historical sites, as well as portraits and other paintings by Lucas Cranach the Elder and Younger. On the doors of All Saints' Church, the Schlosskirche ("castle church", built in 1496–1506) Martin Luther is said to have nailed his 95 theses in 1517. It was seriously damaged by fire in 1760 during a bombardment by the French during the Seven Years' War, was practically rebuilt, and was later (1885–1892) restored. The wooden doors, burnt in 1760, were replaced in 1858 by bronze doors, bearing the Latin text of the theses. Inside the church are the tombs of Martin Luther, Philipp Melanchthon, Johannes Bugenhagen, Paul Eber and of the electors Frederick the Wise (by Peter Vischer the Younger, 1527) and John the Constant (by Hans Vischer), and portraits of the reformers by Lucas Cranach the Younger, who is also buried in the church.

St. Mary's Church, the parish church in which Luther often preached, was built in the 14th century, but has been much altered since Luther's time. It contains a painting by Lucas Cranach the Elder, representing the Last Supper (with the faces of Luther and other reformers), Baptism and Confession, also a font by Hermann Vischer the Elder (1457). In addition, there are numerous historic paintings in the church.

The ancient electoral palace is another of the buildings that suffered severely in 1760; it now contains archives.

Martin Luther's home, the Lutherhaus, where he studied and lived both before and after the Reformation, is now a museum containing many artifacts from his life. Melanchthon's house and the house of Lucas Cranach the Elder, mayor of Wittenberg, can also be found here. Statues of Luther (by Schadow), Melanchthon and Bugenhagen embellish the town. The spot outside the Elster Gate where Luther publicly burned the papal bull in 1520 is marked by an oak tree.

The original Wittenberg University quadrangle also lies in the city centre.

Schlosskirche
One of the town's main attractions is the Schlosskirche (Castle Church), most notably associated with the publication of Martin Luther's 95 Theses, a bold attack on the Roman Catholic Church's practice of indulgences. The castle church was remodelled between 1883 and 1892. According to the intentions of the builders, the world-historical significance of the castle church was to be expressed. In the spirit of the historicism period, the statement was related to the precursors and bearers of the Reformation, thus creating a Reformation memorial. On the occasion of the 500th anniversary of the posting of the theses in 2017, the church was extensively renovated.

Stadtkirche Wittenberg
St. Mary's Town Church and Parish Church is the mother church of the Reformation. The first Protestant service was held in it in 1521 by Justus Jonas the Elder and Andreas Bodenstein of Karlstadt. As Martin Luther's preaching church, it was the Reformation official church of the general superintendents of the Saxon Kurkreis. After the Congress of Vienna it became the official residence of the Wittenberg superintendents. The artistic decoration is well preserved and includes works by Lucas Cranach the Elder and Lucas Cranach the Younger. Epitaphs on the interior and exterior walls refer to the work of many important personalities. The cemetery chapel of the Holy Corpse stands to the south of the town church and once belonged to the walled cemetery area of the church.

Unbefleckte Empfängnis Catholic Church
The church on Mauerstraße was consecrated in 1872 by Bishop Konrad Martin. It was renovated in 1999/2000.

Christ Church
Christ Church was built as another church in 1907/1908 in the suburb of Kleinwittenberg in historicist forms.

Augusteum and Luther House
The representative showcase building Augusteum was once an extension of the University of Wittenberg "Leucorea". In the inner courtyard of the Augusteum is the former home of Martin Luther. Today, the building houses the Reformation History Museum with its collections of pictures, writings and contemporary exhibits from the Reformation era. During the renovation of the Luther House, its medieval charm was partially lost.

Melanchthon House
The architectural style of the Melanchthon House in Collegienstraße, which expresses the self-confident modernity of the Renaissance, is an architectural sight. In this house lived and died the reformer Philipp Melanchthon lived and died in this house. It houses an exhibition. In 2013, the house received an extension according to plans by the architects Dietsch & Weber from Halle made of grey brick.

University Leucorea Wittenberg
Founded in 1502, the University of Leucorea in Collegienstraße was and is not only a teaching institution. For example, the famous personalities of the Reformation Martin Luther and Philipp Melanchthon spent time here. Over the history of the university, which is rich in tradition, developments took place that had an impact not only on Germany but also on large parts of the world.

Hamlethaus
The Hamlethaus in Collegienstraße blends into the ensemble of the town.

Bugenhagenhaus
The Bugenhagen House next to the town church is the oldest Protestant vicarage in the world and is one of the most significant memorials to the Reformation. Until 1997, it was the residence and place of work of the Wittenberg superintendents without interruption since the Reformation. Johannes Bugenhagen was the first pastor to live here until his death in 1558. Between 2004 and 2007, the building was thoroughly renovated and now serves as a spiritual community and meeting centre.

Stadthaus
The Stadthaus is a modern building complex on Arsenalplatz that includes several historic buildings. Their origins date back to the 13th century. The town house contains significant remains of the medieval building fabric of the former monastery church of the Franciscans, which in turn was used as the burial place of the Ascanians. As early as 1536, the building was converted into a granary by Conrad Theiß and heavily remodelled by adding several levels. The medieval windows were closed and pouring openings were installed. During the Seven Years' War, the building was severely damaged and rebuilt with a lower building height. Several building activities in the following period have greatly changed the appearance of the building. At the end of the 19th century, for example, one storey was added and large window openings were broken in on the south side. Between 1945 and 1992, Arsenal Square with the town house was occupied by the Red Army and therefore inaccessible. On the site of the former Franciscan monastery, the city's Central Visitors' Reception was opened in 2014/2015. It includes the event centre Stadthaus, the Historical City Information, the Council Archive and the Tourist and City Information Centre.

Altes Rathaus and Marktplatz
In the centre of the old town is the generously proportioned market square, where a harmonious ensemble of town houses has grown up over the centuries. On it are the Renaissance town hall, the monuments to Martin Luther (designed by Schadow) and Philipp Melanchthon (by Drake), as well as the Marktbrunnen. Since the town administration has been located in the former Tauentzien barracks in Lutherstraße beginning in 2000 (New Town Hall), the town hall on the market square has been called the Old Town Hall and has since served more representative purposes.

Cranach-Höfe
The Cranach Courts attest to the work of Lucas Cranach the Elder, Lucas Cranach the Younger, Hans Cranach, Augustin Cranach, and Lucas Cranach III in Wittenberg. The building at Markt 4 and the pharmacy with the courtyard at Schlossstraße 1 show points of contact with Lucas Cranach and his descendants.

Lucas Cranach the Elder set up his own print shop in the once-famous printing town of Wittenberg. Among other things, the 95 Theses, the first part of the Luther Bible, Luther's Table Talks, and numerous woodcuts were printed here. After the reconstruction of the Cranach courtyards, a historic print shop was re-established in Schlossstrasse, where texts (e.g. Luther's table speeches) and illustrations (in linocut) are now produced as privately printed matter using the letterpress process. The historical Gutenberg press in the Cranachhof at Markt 4 demonstrates how printing was done in the Middle Ages.

Wittenberg Castle
Wittenberg Castle is the former residence of the Saxon Electors. After being destroyed in 1760 and 1814, it was used as a barracks as part of the defensive fortifications. Since the First World War, the castle has served civilian purposes.

Museums

 * Cranach-Höfe, Markt 4 (changing exhibitions)
 * Altes Rathaus (changing exhibitions)
 * Christian Art Foundation Wittenberg with a permanent exhibition and changing special exhibitions in Wittenberg Castle (access via visitor centre)
 * Haus der Geschichte (20th century housing culture and in particular that of the former GDR), Schlossstraße 6
 * Museum of Municipal Collections in the Zeughaus (city history; Julius Riemer collection: natural history, ethnology), Arsenalplatz
 * Science Center futurea, Markt 25
 * Historical City Information and burial place of the Ascanians in the monastery church, Arsenalplatz

Other sights

 * Luthereiche at the southern end of Lutherstraße, where Martin Luther burned the Bull of Banishment of the Pope in 1520, thus completing his separation from the Roman Catholic Church.
 * Röhrwasser, historical water supply system. In the 16th century, the steady growth of the population of the town of Wittenberg made a supply of fresh and clear water increasingly necessary. The existing wells and streams were no longer sufficient, as they were increasingly polluted with rubbish. The tubewater changed all that. With the tube water connection, fresh spring water was available to the citizens day and night, summer and winter. It was not until 1883 that the tubewater lost its importance due to the central water supply. Today, the Wittenberg tube water system with its 20 or so existing wells is the only functioning tube water supply system from the Middle Ages north of the Alps and thus a technical monument. Since 2002, parts of the streams flowing through the town have been opened up.
 * Ratsarchiv Wittenberg, Juristentrasse 16.
 * Kirchliches Forschungsheim, Friedrichstrasse 1a.
 * Luthergarten, park with 270 trees from all over the world near the castle on Kurfürstenring (formerly Hallesche Straße).
 * Panoramic circular image Luther 1517 by Yadegar Asisi, Wilhelm-Weber-Straße near the Lutherhaus
 * Tierpark Wittenberg (Zoological Garden), Juristenstraße.
 * Alaris Butterfly Park, Rothemarkstraße.
 * K-Building (Kommandantengebäude), to house the planned study collection of the municipal collections, with attached lecture and conference centre, Juristenstrasse 14

Coat of arms
Wittenberg's civic coat of arms conveys with its various heraldic elements something of the town's history. On 27 June 1293, Wittenberg was granted town rights by Duke Albert II. There then arose a mediaeval town whose highest governing body was its council. This council, known to have existed as early as 1317, was given the job of administering the town in its care through law and legislation, and of handling the town's revenue. For documentation, the administration used its own seal. One version of what is believed to be the town's oldest town seal, which the council used, and which dated from the first half of the 14th century, set the pattern with its elements for various civic coats of arms down to the present day.

The coat of arms symbolizes, with its crenelated wall and the towers within and each side, a town that was already strongly fortified by 1409. The two shields in the centre form the coat of arms of the Electorate of Saxony with the Saxon arms on the right, whose gold and black stripes recall the Ascanian rulers' house colours with the Rautenkranz or crancelin (literally "rue wreath"; see the Saxony article for more) across them symbolizing the town's founder Duke Albrecht II since 1262, when it appeared in his arms. The shield on the left is the Wittenberg district's arms. In 1356, Emperor Charles IV bestowed upon the Duke of Saxony-Wittenberg the honour of Elector. Wittenberg became an Electoral residence. The shield with its crossed swords stands for the office of "Arch-Marshal of the Holy Roman Empire" inextricably joined by the Electorate, brought to Wittenberg by Rudolf I. Both coats of arms continued to be used by the Wettins after the Ascanians died out. The flowing water at the foot of the shield symbolizes Wittenberg's location on the River Elbe. The fish is a salmon, which as once abundant in the Elbe. The fishermen, like all professions in town, got their own order in 1422, and the fish found its way onto their coat of arms.

Economy and infrastructure
The town is an important centre of chemical industry with the. The whole area of the industrial park covers more than 220 hectares with more than 1,500 workers. Wittenberg is also the headquarters of the eco-friendly web search engine Ecosia. Tourism plays a major role. Wittenberg is one of the top destinations in Saxony-Anhalt.

Lutherstadt Wittenberg station is the main railway station. It connects Wittenberg hourly with Berlin to the north and Leipzig and Halle (Saale) to the south. The station was rebuilt to be more environmentally friendly and re-opened in December 2016.

Theatre, culture and education
Wittenberg has a long tradition of cultural events. The Central German State Theatre (Mitteldeutsches Landestheater) reached great importance in GDR times. Since 1996, the town has staged open-air theatre shows based on the Lutheran history still alive in many historical places of the ancient town. As highlights, in 2001 and 2005, Fernando Scarpa became the artistic director of the "Bühne Wittenberg" (Stage Wittenberg), a project for theatre, art and culture in the whole of Germany which attracts many visitors to the town and whose success is known European-wide. On 2002 and 2003 Stefano Vagnini, Italian composer and organist created the music for Thesys and Luther Stories. Prince Hamlet is said to have studied in Wittenberg and it was the supposed home of Dr. Faustus.

Wittenberg is the seat of the Leucorea which is part of the Martin Luther University of Halle-Wittenberg, the largest university in Saxony-Anhalt.

Personalities from the 19th to the 21st century
Not only the men of the Reformation era Martin Luther, Philipp Melanchthon and Lucas Cranach left their mark on Wittenberg. Much more than in its role as the capital of Kursachsen and as the residential town of Saxony-Wittenberg, the city was shaped by what was probably the most important university in Central Europe at the end of the Middle Ages. It was the place of work for many personalities, who at the same time exerted influence on the city. Today, the names and dates of many personalities are recorded on memorial plaques on the houses in the old town.

1801–1850

 * Johann Friedrich von Brandt (* 1802 in Jüterbog; † 1879 in Merreküll), medical doctor and naturalist
 * Friedrich Drake (* 1805 in Pyrmont; † 1882 in Berlin), sculptor
 * Friedrich Wilhelm Ritschl (* 1806 in Großvargula; † 1876 in Leipzig), philologist.
 * Johann Hinrich Wichern (* 1808 in Hamburg; † 1881 in Hamburg), theologian
 * Johann Gottfried Galle (* 1812 in Pabsthaus; † 1910 in Potsdam), astronomer
 * Werner von Siemens (* 1816 in Lenthe; † 1892 Berlin), industrialist, founder of electrical engineering
 * Karl Wilhelm Nitzsch (* 1818 in Zerbst; † 1880 in Berlin), historian
 * Adalbert Falk (* 1827 in Metschkau; † 1900 in Hamm), Prussian minister of culture, honorary citizen of Wittenberg

1851–1900

 * Karl Lamprecht (* 1856 in Jessen; † 1915 in Leipzig), historian
 * Nathan Söderblom (* 1866 in Trönö; † 1931 in Uppsala), theologian, Nobel Peace Prize laureate, honorary citizen of Wittenberg
 * Otto Kleinschmidt (* 1870 in Geinsheim 1870; † 1954 in Wittenberg), natural scientist.
 * Otto Dibelius (* 1880 in Berlin; † 1967 in West Berlin), theologian
 * Julius Riemer (* 1880 in Berlin; 1958 in Wittenberg), museum founder
 * Else Hertzer (*1884 in Wittenberg; † 1978 in West Berlin), expressionist painter
 * Otto Rasch (* 1891 in Friedrichsruh; † 1948 in Nuremberg), Lord Mayor 1934–36, as commander of Einsatzgruppe C responsible, among other things, for the massacre of Babyn Yar
 * Hermann Oberth (* 1894 in Hermannstadt; † 1989 in Nuremberg), nuclear physicist

1901–1945

 * Erwin Wickert (* 1915 in Bralitz; † 2008 in Remagen), diplomat and writer
 * Konrad Wolf (* 1925 in Hechingen; † 1982 in Berlin), director
 * Ezard Haußmann (* 1935 in Potsdam; † 2010 in Potsdam), actor
 * Wolfgang Böhmer (* 1936 in Dürrhennersdorf), medical doctor, former Minister President of Saxony-Anhalt
 * Friedrich Schorlemmer (* 1944 in Wittenberge), theologian.

After 1945

 * Reiner Haseloff (* 1954 in Bülzig), politician (CDU), Minister-President of Saxony-Anhalt since 2011.
 * Frank Wartenberg (* 1955 in Prenzlau), track and field athlete
 * Christiane Wartenberg (* 1956 in Prenzlau), track and field athlete
 * Peter Fitzek (* 1965 in Halle an der Saale), activist of the Reich citizenship movement, founder of the fantasy state Kingdom of Germany
 * Fernando Scarpa (* 1968 in Milan), Italian film director
 * Nils Seethaler (* 1981 in Berlin), provenance researcher and museum initiator

Twin towns – sister cities
Wittenberg is twinned with:

 * 🇩🇪 Göttingen, Germany (1988)
 * 🇩🇪 Bretten, Germany (1990)
 * 🇺🇸 Springfield, United States (1995)
 * 🇭🇺 Békéscsaba, Hungary (1999)
 * Haderslev, Denmark (2004)
 * 🇧🇪 Beveren, Belgium (2019)
 * 🇷🇴 Mediaș, Romania (2019)
 * 🇧🇾 Mogilev, Belarus (2019)

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

title: Wittenberg