Physics: The Fundamental science was laid down during precisely the time when the great majority of the small swimming animals in the marine waters of that region had disappeared. What made it even more exciting was the fact that this time also coincided with the dis The layer of clay -observed worldwide and known as the K-t boundary layer-contains an unusually high concentration of the element iridium. This element is present in very small amounts in the earth's crust but is much more abundant in meteorites. The father and son team thought that by measuring the amount of iridium present in the clay they could determine how long the layer had taken to form. They assumed that iridium could have rained down on the earth from meteoritic dust at a fairly steady rate during the thousand years that it took to form. If that were the case, they could measure the amount of iridium in the clay and in the rocks above the clay(formed ter)and below(formed earlier) and determine the time it had taken for the iridium to accumulate. To that effect, they enlisted the help of Frank Asaro and Helen Michel, nuclear chemists at the Lawrence Berkeley Laboratory. Asaro and Michel showed that the clay layer contains three hundred times as much iridium as the layers above and below The source of this unusual amount of iridium had to be extraterrestrial, Luis Alvarez reasoned. Meteorites, which are igure 1.5. K-T boundary layer with a high concentration of iridium. curtesy Alessandro Montanari
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SUPERSTRINGS AND OTHER THINGS extraterrestrial, have fallen on the earth since its formation If the iridium came from the meteorites, why this sudden increase in the meteorite rate during this particular time and why did it decrease again to normal levels? What was so special about this particular time in the history of the earth? More importantly why did it coincide with the extinction of about 50 percent of the species in existence then? The Alvarez team first proposed that the iridium could have come from the explosion of a supernova near the solar system Astrophysicists had proposed that the mass extinctions could have been caused by such an explosion. Since these tremendous plosions produce heavy elements Luis Alvarez proposed ana- lyzing the samples taken from the clay for their presence Detailed measurements revealed no heavy elements, however, and the supernova idea had to be abandoned While Walter Alvarez returned to Italy to collect more clay samples, his father worked on theory, inventing"a new scheme every week for six weeks and [shooting them down one by one,as he wrote later. Luis Alvarez then considered the possi- bility of an asteroid or a comet passing through the atmosphere, breaking up into dust which would eventually fall to the ground, like the comet that broke up over Tunguska, Siberia, in 1908 Calculations showed him that a larger asteroid, of 10 kilometers in diameter, for example, would not break up into pieces. The Tunguska comet was smaller Alvarez then concluded that some 65 million years ago, a 10- kilometer comet or asteroid struck the earth, disintegrated, and threw dust into the atmosphere. The dust remained in the atmos- phere for several years, blocking sunlight, turning day into night and preventing photosynthesis, the process by which, in the presence of light, plants convert water, carbon dioxide, and miner als into oxygen and other compounds. Without plants to eat, animals starved to death. We see the remnants of dust today as the global K-T boundary layer between the Cretaceous and Terciary layers. Alvarez calculated the diameter of the object from the known concentration of iridium in meteorites and his group's data on the iridium content of the Italian clay samples Other scientists proposed the idea that intense volcanic erup tions could account for the mass extinctions. These scientists found high levels of iridium in tiny airborne particles released
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Physics: The Fundamental science by the Kilauea volcano in Hawaii and concluded that iridium from the inner earth can reach the surface. For a few years after they were proposed, both ideas could be used to explain the K-T extinctions. However, different predictions could be drawn from the two competing ideas and scientists scurried to find new evidence in support of the different predictions. Recent find- ings, however, appear to confirm the predictions of the impact theory According to the scientific method, however, no theory can ever be proved correct. One of the theories will eventually be shown to be incorrect, leaving the remaining theories stronger, but not proven. You will never convince some [scientists] that an impact killed the dinosaurs unless you find a dinosaur skele- ton with a crushed skull and a ring of iridium around the hole, ked a scientist at a conference on the subject Sizes of things: measurement Most work in physics depends upon observation and mea- surement. To describe the phenomena encountered in nature and to be able to make observations, physicists must agree on a consistent set of units Throughout history, several different systems of units were developed. It began with the Babylonians and the Egyptians, thousands of years ago. The earliest recorded unit of mea- surement, the cubit, based on the length of the arm, appeared in Egyptian papyrus texts. According to Genesis, Noah,'s Ark was 300 cubits long(about 150 m). Because the length of th varies from person to person, so did the cubits used among vari ous civilizations. The Egyptians used a short cubit of 0. 45 m and a royal cubit of 0.524 m. The ancient Romans used the mille passus, 1000 double steps by a Roman legionary, which was equal to 5000 Roman feet. In the 15th century, Queen Bess of England dded 280 feet to the mile to make it eight"furrow-longs"or furlongs In 1790, Thomas Jefferson proposed a system based on units of 10 where 10 feet would be a decad. 10 decads a road, 10 roads a furlong, and 10 furlongs a mile. Congress did not approve
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SUPERSTRINGS AND OTHER THINGS Merr. Ppis Figure 1.6. The meter was originally defined as the 1/10 000000 of the length of the Earth's meridian from the North Pole to the Equator In france, however, the French Revolution brought an inter- est in science and another base 10 system, the metric system, was born. This system, based on the meter, from the Greek metron meaning measure", was more scientific. Instead of usin human anatomy, the meter, as approved by the French National Convention in 1795, was defined as 1/10000000 of the length of Earths meridian between the Equator and the North Pole(figure 16) Once the meter was defined, a unit of volume, the liter, could be defined by cubing a tenth of a meter. From the liter, the kilogram as a unit of mass was derived. Multiples of 10 provided larger units indicated by Greek prefixes, and for smaller units, Latin prefixes were used Due to the consistency and uniformity of the system and the easiness of defining new units merely by adding a greek or a Latin prefix, the metric system was adopted in Europe in the 19th century. Today, an expanded version of the system, SI units, for Le Systeme International d'unites, is used by 95 percent of the world,'s population and is the official system in science. In Table 1.1 we list the standard prefixes used in the SI system Notice in Table 1.1 that for large and small numbers, it is easier to use scientific notation. In the scientific notation numbers are written as a number between 1 and 10 multiplied by a pow of 10. The radius of the earth, for example, which is 6380 km,can be written in scientific notation as 6.380 x 10km. To see why
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Physics: The Fundamental science able 1.1 alue Prefix 03s=10000000000 1015=1000000000000 102=100000000 10=1000000 102=100 10-3=0.00 milli 10-6=0000001 unpf 10-18=0.0000000000000 atto note that we can write the number 1000 as follows 1000=10×10×10=103 The radius of the earth is, then 6380km=6.38×1000km=6.38×103km Fundamental units All mechanical properties can be expressed in terms of three fundamental physical quantities: length, mass, and time. The SI fundamental units are: Quantity Fundamental unit meter kilogram Time The General Conference on Weights and Measures held in Paris in 1983 defined the meter as the distance traveled by light through space in 1/299792458 seconds. Notice that the unit of length is defined with such high precision in terms of the unit of time. This is possible because the second is known to better than 1 part in 10 trillion. The 1967 General Conference on Weights and Measures
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