SUPERSTRINGS AND OTHER THINGS 上自 石 3 g 型
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Physics: The Fundamental science Figure 1.8. A cesium atomic clock at the National Institute of Standards and Technology in Washington, DC.( Courtesy National Institute Standards and Technology. defined the second as the duration of 9192 770 periods(dura tions of one oscillation) of a particular radiation emitted by the cesium atom. The device that permits this measurement is the cesium clock, an instrument of such high precision that it would lose or gain only 3 seconds in one million years(figure 1.8) The last fundamental mechanical quantity is mass. Mass is a measure of the resistance that an object offers to a change in its condition of motion. For an object at rest with respect to us, mass is a measure of the amount of matter present in the object The standard unit of mass is the standard kilogram, a solid plati- num-iridium cylinder carefully preserved at the Bureau of eights and Measures in Sevres, near Paris. The kilogram is now derived from the meter, which is derived from the second. copy of the standard kilogram, the Prototype Kilogram No 20, is kept at the National Bureau of Standards in Washington, DC. A high precision balance, especially designed for the National Bureau of Standards, allows the e com Ison masses of other bodies within a few parts in a billion. 17
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SUPERSTRINGS AND OTHER THINGS MyeB5525031me印 R LUNcH (Cartoon by Sydney Harris. The mass of an atom cannot be measured by comparison ith the standard kilogram with such a high degree of precisio The masses of atoms, however, can be compared with each other with high accuracy. For this reason, the masses of atoms are given in atomic mass units(amu). The mass of carbon in these units has been assigned a value of 12 atomic mass units. In kilograms, an atomic mass unit is 1amu=16605402×10-2kg Physics and mathematics Physics and mathematics are closely intertwined. Mathematics is an invention of the human mind inspired by our capacity to deal 18
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Physics: The Fundamental science Frontiers of physics: Very small numbers What does a mass like 1.6605402 x 10-kg mean? Suppose that you start with one grain of salt, which has a mass of about one ten-thousandth of a gram and with a very precise cutting instrument you divide it into ten equal parts, take ach one of the tenths, divide them into ten new equal parts, and so on. You will not arrive at single electrons this way because, as we shall see in chapters 7 and 8, the electron is one of the several constituents of atoms. Although atoms can be split, you cannot do it with a cutting instrument Suppose, however, that we divide the grain of salt into the smallest amounts of salt possible, single molecules of salt. One single molecule of table salt has a mass of about 9 x 10g. Let's round this number up to 10g. If your instrument takes one second, say, to take each piece of salt and divide it into ten equal parts, how long would it take to end up with individual molecules of salt? The answer is Astrophysicists estimate that the age of the universe is of the order of 10 years. It would take our hypothetical instru- nt roughly the age of the universe to arrive at a single Grains of salt, magnified 100 times. Courtesy V Cummings, NASA with abstract ideas; physics deals with the real material world Yet, mathematical concepts invented by mathematicians who did not foresee their applications outside the abstract world mathematics have been applied by physicists to describe natural 19
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SUPERSTRINGS AND OTHER THINGS Pioneers of physics: Measuring the circumference of the earth The meter, as we saw, was defined in 1795 as 1/10000000 of the length of the earth's meridian from the Equator to the North pole. For that definition to make sense, an accurate knowledge of the Earths dimensions was needed. That is, the actual length of the meridian from the Equator to the North Pole had to be known with good precision. How did we come to know the earth's dimensions before the advent of twentieth century technology? *krano The dimensions of the earth have been known since the time of the ancient Greeks. The greek astronomer eratos thenes, who lived in the third century Bc in Alexandria (Egypt), came up with a very clever method for obtaining the circumference of the earth eratosthenes had heard that in the city of Syene, an ancient city on the Nile,near todays Aswan, on the first day of summer, the sun shone on the bottom of a vertical well at noon however in his native Alexandria, the sun's rays did not fall vertically down but at an angle of o to the vertical. This angle of 7o was about one-fiftieth of a circle and that meant that the
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