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General relativity is a theory of gravitation that was developed by Albert Einstein. According to general relativity, the observed gravitational attraction between masses results from the warping of space and time by those masses. Before the advent of general relativity, Newton's law of universal gravitation had been accepted. Experiments show that Einstein's description accounts for several effects that are unexplained by Newton's law. Some general relativity predictions have been confirmed by experiment, while others are the subject of ongoing research. General relativity has developed into an essential tool in modern astrophysics. It provides the foundation for the current understanding of black holes and of the standard Big Bang model of cosmology. Although general relativity is not the only relativistic theory of gravity, it is the simplest such theory that is consistent with the experimental data. Nevertheless, a number of open questions remain: the most fundamental is how general relativity can be reconciled with the laws of quantum physics to produce a complete and self-consistent theory of quantum gravity. (more...)

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• A pair of bombings on a center for displaced people near Kohat, Pakistan, kill more than 40 people.
• Volcanic ash from the eruption of Eyjafjallajökull (pictured) in Iceland disrupts air traffic across Europe.
• Kurmanbek Bakiyev officially resigns as President of Kyrgyzstan after he leaves the country for Kazakhstan.
• The United Kingdom holds a live, pre-election debate between the leaders of the largest political parties for the first time in the country's history.
• The GSAT-4 satellite is lost in the failure of the maiden flight of the GSLV Mk.II, India's first rocket with an indigenous cryogenic upper stage.
• A severe storm strikes Bangladesh and eastern India, killing more than 100 people and destroying over 50,000 homes.
• More than 1,000 people are killed in an earthquake in Yushu, Qinghai, People's Republic of China.
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April 18: Independence Day in Zimbabwe (1980)

• 1025 – Bolesław I Chrobry became the first King of Poland.
• 1506 – Construction of the current St. Peter's Basilica (interior pictured) in Vatican City, to replace the old St. Peter's Basilica built in the 4th century, began.
• 1906 – A major earthquake and resulting fires devastated San Francisco, killing at least 3,000 people and leaving more than half of the city's population homeless.
• 1942 – World War II: Sixteen B-25 Mitchell bombers from the aircraft carrier USS Hornet carried out the Doolittle Raid, the first Allied attack on the Japanese home islands.
• 1983 – A suicide bomber destroyed the United States Embassy in Beirut with a car bomb, killing over 60 people.
• 1996 – Israeli forces shelled Qana, Lebanon, during Operation Grapes of Wrath, killing over 100 civilians and injuring over 110 others at a UN compound.

More anniversaries: April 17 – April 18 – April 19