IV. Translate at sight

Planet Earth

One day the Sun will die. One day the Sun will "eat" planet Earth and life on Earth will stop. Is that scary? Oh yes it is, but do not worry. The sun will shine the next five billion years before it becomes the "Death Star".

Nothing lasts forever, not even the sun. During the next five billion years the sun will continue shining and giving us energy and chance to live on our planet. After five billion years the sun has used all its "fuel". At this time several chemical reactions happen, causing that the outer layers of the sun start expanding. During few hundred million years the sun becomes a red giant that will "eat" Mercury and Venus. The new red giant can then be seen as far as 160 light years away.

The core of the Sun will activate again, which will blow out the gas layer by layer. A planetary nebula is created. In the middle of that planetary nebula is our old sun, which has now become a white dwarf. The new white dwarf will become cooler step by step until all chemical reactions stop. The sun has died.

The ftiture does not sound good. It is a bit dramatic and scary. Anyhow, we do not need to be afraid at all. When we think in which scale these phenomena happen, we realize that there is not need to worry at all. The universe was born 13.7 billion years ago and the sun will continue shining the next five billion years. Human being has measured the time only few thousand years, which is really nothing compared to

figures such as 5 billion years and 13.7 billion years. So, there is no need to be afraid of the future of the sun because the things happen during very very long time. Our life on Earth is not in danger when it comes to the sun, but the fact is that one day the sun will die, for sure. When that has happened, there is no life on Earth.

V. Retell the text.

SPACE AND TIME

Einstein made the revolutionary suggestion that gravity is not a force like other forces, but is a consequence of the fact that space-time is not flat, as had been previously assumed: it is curved by the distribution of mass and energy in it. Bodies like the earth are not made to move on curved orbits by a force called gravity; instead, they follow the nearest thing to a straight path in a curved space, which is called a geodesic. A geodesic is the shortest (or longest) path between two nearby points. For example, the surface of the earth is a two-dimensional curved space. A geodesic on the earth is called a great circle, and is the shortest route between two points.

In general relativity, bodies always follow straight lines in four-dimensional space-time, but they nevertheless appear to us to move along curved paths in our three-dimensional space. (This is rather like watching an airplane flying over hilly ground. Although it follows a straight line in three-dimensional space, its shadow follows a curved path on the two-dimensional ground.)

The mass of the sun curves space-time in such a way that although the earth follows a straight path in four-dimensional space-time, it appears to us to move along a circular orbit in three-dimensional space..

Light rays too must follow geodesies in space-time. Again, the fact that space is curved means that light no longer appears to travel in straight lines in space. So general relativity predicts that light should be bent by gravitational fields. This means that light from a distant star that happened to pass near the sun would be deflected through a small angle, causing the star to appear in a different position to an observer on the earth. Of course, if the light from the star always passed close to the sun, we would not be able to tell whether the light was being deflected or if instead the star was really where we see it. However, as the earth orbits around the sun, different stars appear to pass behind the sun and have their light deflected. They therefore change their apparent position relative to other stars. It is normally very difficult to see this effect, because the light from the sun makes it impossible to observe stars that appear near to the sun the sky. However, it is possible to do so during an eclipse of the sun, when the sun's light is blocked out by the moon j

Another prediction of general relativity is that time should appear to slower near a massive body like the earth. This is because there is a relation between the energy of light and its frequency (that is, the number of waves of light per second): the greater the energy, the higher frequency. As light travels upward in the earth's gravitational field, it loses energy, and so its frequency goes down. (This means that the length of time between one wave crest and the next goes up.)

To someone high up, it would appear that everything down below was making longer to happen. This prediction was tested in 1962, using a pair of very accurate clocks mounted at the top and bottom of a water tower. The clock at the bottom, which was nearer the earth, was found to run slower, in exact agreement with general relativity.

Newton's laws of motion put an end to the idea of absolute position in space. The theory of relativity gets rid of absolute time. Consider a pair of twins. Suppose that one twin goes to live on the top of a mountain while the other stays at sea level. The first twin would age faster than the second. Thus, if they met again, one would be older than the other. In this case, the difference in ages would be very small, but it would be much larger if one of the twins went for a long trip in a spaceship at nearly the speed of light. When he returned, he would be much younger than the one who stayed on earth. This is known as the twins paradox, but it is a paradox only if one has the idea of absolute time at the back of one's mind. In the theory of relativity there is no unique absolute time, but instead each individual has his own personal measure of time that depends on where he is and how he is moving.

Before 1915, space and time were thought of as a fixed arena in which events took place, but which was not affected by what happened in it. This was true even of the special theory of relativity. Bodies moved, forces attracted and repelled, but time and space simply continued, unaffected. It was natural to think that space and time went on forever.

The situation, however, is quite different in the general theory of relativity. Space and time are now dynamic quantities: when a body moves, or a force acts, it affects the curvature of space and time - and in turn the structure of space-time affects the way in which bodies move and forces act. Space and time not only affect but also are affected by everything that happens in the universe. Just as one cannot talk about events in the universe without the notions of space and time, so in general relativity it became meaningless to talk about space and time outside the limits of the universe.

In the following decades this new understanding of space and time was to revolutionize our view of the universe. The old idea of an essentially unchanging universe that could have existed, and could continue to exist, forever was replaced by the notion of a dynamic, expanding universe that seemed to have begun a finite time ago, and that might end at a finite time in the future. That revolution forms the subject of the next chapter. And years later, it was also to be the starting point in theoretical physics. Einstein's general theory of relativity implied that the universe must have a beginning and, possibly, an end.

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