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On 2 December 1942, under the stands of a disused American football stadium at the University of Chicago, a team led by Enrico Fermi started the first controlled nuclear chain reaction. Their reactor, Chicago Pile-1, was a carefully built stack of graphite blocks and uranium.

Chicago, 2 December 1942

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In a chain reaction, each uranium atom that splits releases particles called neutrons, which go on to split more atoms. Fermi, an Italian physicist who had moved to the United States, worked with a team of about 30, including Leo Szilard and Leona Woods. They stacked some 45,000 graphite blocks, with uranium set among them, in a squash court beneath the stands of Stagg Field. The pile had no radiation shielding or cooling, because it ran at only about half a watt. After about four and a half minutes the team stopped it by putting control rods back in. The work was secret, part of the Manhattan Project, the wartime American programme to build an atomic bomb. Arthur Compton, the physicist in charge of the work in Chicago, reported the success by telephone in code: 'The Italian navigator has landed in the New World.'

Türkçe oku
Enrico Fermi'nin ekibi, 2 Aralık 1942'de Chicago Üniversitesi'nde, artık kullanılmayan bir stadyumun tribünleri altında ilk kontrollü nükleer zincirleme tepkimeyi başlattı. Chicago Pile-1 adlı reaktör, özenle üst üste dizilmiş grafit bloklarla uranyumdan oluşuyordu.
Chicago, 2 Aralık 1942

Zincirleme tepkimede bölünen her uranyum atomu, nötron denen parçacıklar saçar; bu nötronlar da başka atomları böler. ABD'ye yerleşmiş İtalyan fizikçi Fermi, aralarında Leo Szilard ile Leona Woods'un da bulunduğu 30 kadar kişiden oluşan bir ekiple çalıştı. Ekip, Stagg Field stadyumunun tribünleri altındaki squash kortunda 45.000 kadar grafit bloğu üst üste dizdi ve aralarına uranyum yerleştirdi. Reaktör yalnızca yarım watt kadar güçle çalıştığı için ne radyasyon kalkanına ne de soğutmaya ihtiyaç duyuldu. Yaklaşık dört buçuk dakika sonra ekip, kontrol çubuklarını yeniden içeri sokarak tepkimeyi durdurdu. Çalışma, ABD'nin savaş sırasında atom bombası yapmak için yürüttüğü gizli Manhattan Projesi'nin bir parçasıydı. Chicago'daki çalışmaların başındaki fizikçi Arthur Compton, başarıyı telefonda şifreli bir cümleyle bildirdi: “İtalyan denizci Yeni Dünya'ya ayak bastı.”

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WonderHistory

When Neil Armstrong radioed “Houston, Tranquility Base here. The Eagle has landed” on 20 July 1969, his words reached Earth about 1.3 seconds later. Radio waves travel at the speed of light, and the Moon is about 384,000 km (239,000 miles) away.

Eagle was the lunar module that carried Armstrong and Buzz Aldrin down to the surface. Light, and so radio, travels at almost 300,000 km per second. At the Moon's average distance of about 384,400 km, a signal takes roughly 1.28 seconds each way, so every exchange with Mission Control in Houston had a built-in pause of about 2.6 seconds before an answer could arrive. Large dish antennas at Goldstone in California and at Honeysuckle Creek and Parkes in Australia picked up the signals; the clearest pictures of the first steps came through Australia and were watched live by an estimated 600 million people. Before leaving, the crew set up a panel of reflectors on the surface. Scientists still fire lasers at it and time the light's return, which is how we know the Moon is drifting away from Earth by about 3.8 cm a year.

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WonderTechnology

The Apollo Guidance Computer that steered astronauts to the Moon had about 4 kilobytes of rewritable memory and 72 kilobytes of permanent memory. Its programs were woven by hand into “rope memory”: a wire threaded through a tiny magnetic ring stored a 1, and a wire passing it by stored a 0.

Designed at the MIT Instrumentation Laboratory and built by Raytheon, the computer was one of the first to use silicon chips (integrated circuits) and fitted into about 28 litres. Its memory was counted in “words” of 16 bits, or binary digits: 2,048 rewritable and 36,864 permanent. During Apollo 11's descent on 20 July 1969 it flashed 1202 and 1201 alarms: a radar left on in case the landing had to be abandoned was stealing processing time and overloading it. Its operating system, designed by J. Halcombe Laning, dropped less urgent jobs and restarted in a moment while the landing programs kept running, so Mission Control told the crew to carry on. The flight software was written by a team led by Margaret Hamilton, who received the Presidential Medal of Freedom in 2016. Its power was similar to that of the first home computers of the late 1970s.

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WonderTechnology

The Antikythera mechanism, an ancient Greek geared device probably made in the 2nd century BC, had a spiral dial covering 223 lunar months for predicting eclipses. Symbols on the dial marked the months when an eclipse of the Sun or Moon was expected, and inscriptions gave details such as the hour.

Sponge divers found the mechanism in 1901, a corroded lump among treasures from a Roman-era ship that sank off the Greek island of Antikythera around 70 to 60 BC. Its hand-turned bronze gears modelled the movements of the Sun and Moon, and it is called the oldest known analogue computer, a machine that calculates with moving parts rather than digits. The eclipse dial follows the Saros cycle: after 223 lunar months (new moon to new moon), about 18 years and 11 days, the Sun, Earth and Moon return to almost the same positions, so eclipses repeat in a similar pattern. Symbols in 51 of the dial's cells mark 38 lunar and 27 solar eclipses. Much of this was read only in the 2000s, when researchers including Mike Edmunds and Tony Freeth used X-ray scans to see inside the fragments. The front, which probably showed the planets, is largely lost.

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WonderTechnology

Paper money was first used in China. In the early 11th century, merchants in Sichuan began issuing paper notes called jiaozi in place of heavy iron coins, and in 1023 the Song government took over issuing them. Europe's first banknotes appeared in Sweden in 1661.

The idea had roots in the Tang dynasty (618–907): by 804, merchants were using “flying cash”, paper certificates that let them move money across China without carrying coins. In Sichuan, where iron coins were too heavy for large payments, private firms in the regional capital, Chengdu, began issuing jiaozi notes. When some issuers went bankrupt, the state set up its own office in 1023 and printed the first government notes the following year. Paper money rested on trust: a note was worth only what its issuer promised. That trust could be abused, and when the Song printed far more notes than they could back with coins, prices soared. Europeans learnt of paper money from travellers such as Marco Polo, who described the Mongol emperor Kublai Khan's notes in the 13th century. Europe's first banknotes came from Stockholms Banco in 1661; the bank failed six years later.

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WonderTechnology

The magnetic compass began in China as a tool for fortune-telling and geomancy, the art of siting houses and tombs in harmony with the land. The first records of Chinese sailors steering by a magnetised needle date from the early 12th century.

The first Chinese direction-finders were probably “south-pointing spoons” made of lodestone, a naturally magnetic iron ore. Placed on a smooth bronze board marked with directions, the spoon turned until its handle pointed south. Texts from the Han dynasty (206 BC to AD 220) appear to describe such devices, although archaeologists have not yet found one. The big step came under the Song dynasty. In 1088 the scholar Shen Kuo described needles magnetised by rubbing them on lodestone, and noticed that they do not point exactly north-south, the effect now called magnetic declination. A few decades later the writer Zhu Yu recorded that sailors steered by the stars at night and the sun by day, and turned to the south-pointing needle when the sky was overcast. In Europe, the first written mention of a compass dates from about 1190.

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