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.

Exercises:

I. Memorise the following phrases and word combinations:

curved by the distribution of mass and energy - искривлено распределенными в нем массой и энергией; it appears to us to move along a circular orbit - мы видим, что она движется по круговой орбите; light from a distant star would be deflected – луч света от далекой звезды должен отклониться; eclipse of the sun - солнечное затмение; the sun's light is blocked out by the moon - Луна перекрывает солнечный свет; a relation between the energy of light and its frequency - соотношение между энергией света и его частотой; length of time between one wave crest and the next - интервал времени между гребнями двух соседних волн; accurate clocks mounted at the top and bottom of a water tower - точные часы, расположенные: одни на самом верху водонапорной башни, а второй - у ее подножья; general theory of relativity implies - согласно общей теории относительности; starting point in theoretical physics - отправная точка исследований в теоретической физике; was replaced by the notion of a dynamic, expanding universe - сменилось картиной динамической, расширяющейся Вселенной.

II. Translate into English using the active vocabulary of the lesson:

1. Такие тела, как Земля, вовсе не принуждаются двигаться по искривленным орбитам гравитационной силой; они движутся по линиям, которые в искривленном пространстве более всего соответствуют прямым в обычном пространстве и называются геодезическими.

2. Согласно общей теории относительности, тела всегда перемещаются по прямым в четырехмерном пространстве-времени, но мы видим, что в нашем трехмерном пространстве они движутся по искривленным траекториям.

3. Луч света от далекой звезды, проходящий рядом с Солнцем, должен отклониться на небольшой угол, и наблюдатель, находящийся на Земле, увидит эту звезду в другой точке.

4. Еще одно предсказание общей теории относительности состоит в том, что вблизи массивного тела типа Земли время должно течь медленнее. Это следует из того, что должно выполняться определенное соотношение между энергией света и его частотой (т. с. числом световых волн в секунду): чем больше энергия, тем выше частота.

5. В обшей теории относительности нет единого абсолютного времени; каждый индивидуум имеет свой собственный масштаб времени, зависящий от того, где этот индивидуум находится и как он движется.

6. Пространство и время не только влияют на все, что происходит во Вселенной, но и сами изменяются под влиянием всего в ней происходящего.

7. В последующие десятилетия новому пониманию пространства и времени предстояло произвести переворот в наших взглядах на Вселенную.

III. Read and translate the Russian sentences into English and then give their Russian version again:

	Черная дыра - область про- странства-времени, из которой ничто, даже свет, не может выбраться наружу, потому что в ней чрезвычайно сильно действие гравитации.	Black hole: A region of space-time from which nothing, not even light, can escape, because gravity is so strong.
Предел Чандраеекара - макси­	Chandrasekhar limit: The
мально возможная масса стабильной	maximum possible mass of a stable cold
холодной звезды, выше которой звезда	star, above which it must collapse into a
должна сколлапсировать в черную	black hole.
дыру.
Закон сохранения энергии -	Conservation of energy: The law
закон науки, согласно которому энер­	of science that states that energy (or its
гия (или ее массовый эквивалент) не	equivalent in mass) can neither be
может ни создаваться, ни уничто­	created nor destroyed.
жаться.
Координаты - числа, определяю­	Coordinates: Numbers that specify
щие положение точки в пространстве и	the position of a point in space and time.
во времени.
Космологическая постоянная -	Cosmological constant: A
математическая вспомогательная вели­	mathematical device used by Einstein to
чина, введенная Эйнштейном для того,	give space-time an inbuilt tendency to
чтобы пространство-время приобрело	expand.
тенденцию к расширению.
Космология - наука, занимаю­	Cosmology: The study of the
щаяся изучением Вселенной как	universe as a whole.
целого.
Теорема о сингулярности -	Singularity theorem: A theorem
теорема, в которой доказывается, что	that shows that a singularity must exist
при определенных условиях сингуляр­	under certain circumstances - in
ность должна существовать и что, в	particular, that the universe must have
частности, началом Вселенной должна	started with a singularity.
быть сингулярность.
Пространство-время - четырех­	Space-time: The four-dimensional
мерное пространство, точки которого	space whose points are events.
отвечают событиям.
Пространственное измерение -	Spatial dimension: Any of the three
любое из трех пространственно-подоб-	dimensions that are space like - that is,
ных измерений пространства-времени.	any except the time dimension.
т.е. любое измерение, кроме времен­
ного. Специальная теория относи­	Special relativity: Einstein's theory
тельности - теория Эйнштейна,	based on the idea that the laws of science
отправная точка которой состоит в	should be the same for all observers, no
том, что законы науки должны быть	matter how they are moving, in the
одинаковы для всех свободно	absence of gravitational phenomena.
движущихся наблюдателей независимо
от их скоростей.

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