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The intimate link between space and time was well known in astronomy, in a different context, long before relativity theory. Astronomers and astrophysicists deal with extremely large distances, and here again the fact that light needs some time to travel from the observed object to the observer is important. Because of the finite velocity of light, the astronomer never looks at the universe in its present state, but always looks back into the past. It takes light eight minutes to travel from the Sun to the Earth, and hence we see the Sun, at any moment, as it existed eight minutes ago. Similarly, we see the nearest star as it existed four years ago, and with our powerful telescopes we can see galaxies as they existed millions of years ago. <\/p>\n\n\n\n
The finite velocity of light is by no means a handicap for astronomers but is a great advantage. It allows them to observe the evolution of stars, star clusters or galaxies at all stages just by looking out into space and back into time. All types of phenomena that happened during the past millions of years can actually be observed somewhere in the skies. Astronomers are thus used to the importance of the link between space and time. What relativity theory tells us is that this link is important not only when we deal with large distances, but also when we deal with high velocities. Even here on Earth, the measurement of any distance is not independent of time, because it involves the specification of the observer\u2019s state of motion and thus a reference to time. <\/p>\n\n\n\n