Space and Time Are Interwoven

Space and Time Are Interwoven

Light travels very fast, but space is very empty, and the stars are far apart. Distances of seventy five light-ears or less are very small compared to other distances in astronomy.

From the Sun to the center of the Milky Way Galaxy is 30,000 light-years. From our galaxy to the nearest spiral galaxy M31, also in the constellation Andromeda, is 2,000,000 light-years. When the light we see today from M31 left for Earth, there were no humans on our planet, although our ancestors were evolving rapidly to our present form. The distance from the Earth to the most remote quasars is eight or ten billion light-years. We see them today as they were before the Earth accumulated, before the Milky Way was formed.

Space and time are interwoven. We cannot look out into space without looking back into time (quotations from Megan Jorgensen). Image : © Megan Jorgensen (Elena)

This not a situation restricted to astronomical objects, but only astronomical objects are so far away that the finite speed of light becomes important. If you are looking at a friend three meter away, at the other end of the room, you are not seeing her as she is now, but rather as she was a hundred millionth of a second ago, or a hundredth of microsecond. In this calculation we have merely divided the distance by the speed to get the travel time, but the difference between your friend now and now minus a hundred-millionth of a second is too small to notice.

On the other hand, when we look at a quasar eight billion light-years away, the fact that we are seeing it as it was eight billion light-years ago may be very important. For example, there are those who think that quasars are explosive events likely to happen only in the early history of galaxies. In that case, the more distant the galaxy, the earlier in its history we are observing it, and the more likely it is that we should see it as a quasar. Indeed, the number of quasars increases as we look to distances of more thant about five billion light-years.