Space is the three-dimensional extension in which all things exist and move. We intuitively feel that we live in an unchanging space. In this space, the height of a tree or the length of a table is exactly the same for everybody. Einstein's special theory of relativity tells us that this intuitive feeling is really an illusion. Neither space nor time is the same for two people moving relative to each other. Only a combination of space and time, called space-time, is unchanged for everyone. Einstein's general theory of relativity tells us that the force of gravity is a result of a warping of this space-time by heavy objects, such as planets. According to the big bang theory of the origin of the universe, the expansion of the universe began from infinitely curved space-time. We still do not know whether this expansion will continue indefinitely or whether the universe will collapse again in a big crunch. Meanwhile, astronomers are learning more and more about outer space from terrestrial and orbiting telescopes, space probes sent to other planets in the solar system, and other scientific observations. This is just the beginning of the exploration of the unimaginably vast void, beyond the earth's outer atmosphere, in which a journey to the nearest star would take 3,000 years at a million miles an hour.
The difference in the perception of space and time, predicted by the special theory of relativity, can be observed only at very high velocities close to that of light. A man driving past at 50 MPH (80 km/h) will appear only a hundred million millionth of an inch thinner as you stand watching on the sidewalk. By themselves, three-dimensional space and one-dimensional time are different for different people. Taken together, however, they form a four-dimensional space-time in which distances are same for all observers. We can understand this idea by using a two-dimensional analogy. Let us suppose your definition of south and east is not the same as mine. I travel from city A to city B by going ten miles along my south and then five miles along my east. You travel from A to B by going two miles along your south and 11 miles along your east. Both of us, however, move exactly the same distance of 11.2 miles southeast from city A to B. In the same way, if we think of space as south and time as east, space-time is something like south-east.
The general theory of relativity tells us that gravity is the result of the curving of this four-dimensional space-time by objects with large mass. A flat stretched rubber membrane will sag if a heavy iron ball is placed on it. If you now place another ball on the membrane, the second ball will roll towards the first. This can be interpreted in two ways; as a consequence of the curvature of the membrane, or as the result of an attractive force exerted by the first ball on the second one. Similarly, the curvature of space-time is another way of interpreting the attraction of gravity. An extremely massive object can curve space-time around so much that not even light can escape from its attractive force. Such objects, called black holes, could very well exist in the universe. Astronomers believe that the disk found in 1994 by the Hubble telescope, at the center of the elliptical galaxy M87 near the center of the Virgo cluster, is material falling into a supermassive black hole estimated to have a mass three billion times the mass of the Sun.
The relativity of space and time and the curvature of space-time do not affect our daily lives. The high velocities and huge concentrations of matter, needed to manifest the effects of relativity, are found only in outer space on the scale of planets, stars, and galaxies. Our own Milky Way galaxy is a mere speck, 100,000 light years across, in a universe that spans ten billion light years. Though astronomers have studied this outer space with telescopes for hundreds of years, the modern space age began only in 1957 when the Soviet Union put the first artificial satellite, Sputnik 1, into orbit around the earth. At present, there are hundreds of satellites in orbit gathering information from distant stars, free of the distorting effect of the earth's atmosphere. Even though no manned spacecraft has landed on other worlds since the Apollo moon landings, several space probes, such as the Voyager 2 and the Magellan, have sent back photographs and information from the moon and from other planets in the solar system. There are many questions to be answered and much to be achieved in the exploration of space. The Hubble telescope, repaired in space in 1993, has sent back data that has raised new questions about the age, origin, and nature of the universe.
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