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Brief History of Time by Stephen Hawking

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The author who wrote this book Mr. Stephen Hawking is regarded as the most brilliant theoretical physicistt science Einstein. Stephen wrote this book explaining different theories that earlier scientists, philosphers and astronomers had about the univerise.

In his book he wrote about the different theories and opinions people have about the univerise. Why does it iexist? How the universe was created and where is it taking us? Some of the theories he wrote about discussed how some people thought the universe evolved and described the opinioins and ideas that events and natural phenomena were controlled by spirits with human emotions who acted in a very humanlike and unpredictble manner. he described how these spirits inhabited natural objects, like the rivers and mountains, includingcelestial bodies, like the sun and moon. Asking questions like why does the sun rise on the west side of the world and not on the east?

He talked about the Greek philosopher Aristotle and how he thought the earth was round rather than a hot plate. Aristotle realized this is when he noticed that the eclipses of the moon were caused by the earth coming between the sun and moon. The earth's shadow on the moon was always round, which would only be true if the earth was spherical.

He also explained why Aristotle thought the earth was stationary and that the sun, the moon, the planets, and the stars moved in circular orbits about the earth. he believed this because he felt, for mystical reasons, that thte earth was the center of the universe, and that tcircular motion was the most perfect. Thise idea was elaborated by Ptolemy in the second century AD into a complete cosmological model. The earth sttod at the center, surrounded by eight spheres that carried the moon, the sun, the stars, and the five planets known at the time, Mercury, Venus, Mars, Jupiter and Saturn.

How Isaac Newton not only put forward a theory of how bodies move in space and time, but e also developed the complIcated mathematics needed to analyze those motions. In addition, Newton created the law of universal gravitation according to which each body in the universe was attracted toward every other body by a force that was stronger the more massive the bodies and the closer they were to each other. It was this same force that caused objects to fall to the gorund.

Stephen explained how Galileo would experiment with laws of motion. he would show that the real effect of a force is always to change the speed of a body, rather than just to set it moving, as was previously thought. It also meant that whenever a body is not acted on by any forc, it will keep moving in a straight line at the same speed. This idea was first stated explicitly in Newton's Principia Mathematica, published in 1687, and is known as Newton's first law. What happens to a body when a force does act on it is given by Newton's second law? This states that the body will accelerate, or change its speed,at a rate that is proportional to the force.

Newton's law of gravity also tells us that the farther apart the bodies, the smaller the force. Newton's law of gravity says that the gravitational attraction of a star is exactly one quarter that of a similar star at half the distance. This law predicts the orbits of the earth, the moon, and the planets with great accuracy. If the law were that the gravitational attraction of a star went down faster of increased more rapidly with distance, the orbits of the planetswould not be elliptical, they would either spiral into the sun or escape from the sun.

The big difference between the ideas of Aristotle and those of Galileo and Newton is that Aristotle believed in a preferred state of rest, which any body would take up if it were not driven by some force or impulse. In particular, he thought that the earth was at rest. But it followsfrom newton's laws that there is no unique standard of rest.

We live in a galaxy that is about one hundred thousand light-years across and is slowly rotating; the stars in its spiral arms orbit around its center about once every several hundred million years. Our sun is just an ordinary, average-sized, yellow sar, near the inner edge of one of the spiral arms. We have certainly come a long way since Airstotle and Ptolemy, when thought that the earth was the center of the universe!

How I love to watch the stars at night stars are so far away that they appear to us to be just pinpoints of light. We cannot see their size of shape. So how can we tell different types of stars apart? For the vast majority of stars, there is only one characteristic feacture that we can observe - the color of their light. Newton discovered that if light from the sun passes through a triangular-shaped piece of glass, called a prism, it breaks up into its component colors (its spectrum) as in a rainbow.

Einstein's general theory of relativity sems to govern the large-scale structure of the universe. It is what is called a classical theory; that is, it does not take account of the uncertainty principle of quantum mechanics, as it should for consistency with other theories. The reason that this does not lead to any discrepancy with observation is that all the gravitational fields that we normally experience are very weak. However, the singularity theorems discussed earlier indicae that the gravitational field should get very strong in at least two situations, black holes and the big bang. In such strong fields the effects of quantum mechanics should be important. Thus, in a sense, classical general relativity, by predicting points of infinite density, predics its own downfall, just as classical (that is, nonquantum) mechanics predicted its downfall by suggesting that atoms should collapse to infinite density. We do not yet have a complete consistent theory that unifies general relativity and quantum mechanis, but we do know a number of the features it should have. The consequences that these would have for black holes and the big bang will be described in later chapters. For the moment, however, we shall turn to the recent attempts to bring together our understanding of the other forces of nature into a singel, unified uantum theory.

Aristotle believed that all the matter in the universe was made up of four basic elements - earth, air, fire and water. These elements were acted on by two forces: gravity, the tendency for earth and water to sink, and levity, the tendency for air and fire to rise. This division of the contents of the universe into matter and forces is still used today. Aristotle believed that matter was continuous, that is, one could diide a piece of matter into smaller and smaller bits without any limit: one never came up against a grain of matter



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