George's Cosmic Treasure Hunt by Lucy Hawking Read Free Book Online

be much quicker for the astronauts themselves. Indeed, if one could travel at the speed of light, no time at all would pass!
    No spaceship can travel as fast as light, but one could still approach this maximum speed. The time experienced would then be much shorter than that on Earth. For example, a journey across the Galaxy at nearly the speed of light would seem to take only thirty years, while much more time would have passed on Earth. One could therefore return to Earth in one’s own lifetime, although one’s friends would have died long ago. If one continued to accelerate beyond the Galaxy for a century, one could, in principle, travel to the edge of the currently observable Universe!
    Einstein’s general theory of relativity (1915) could allow even more exotic possibilities. For example, maybe astronauts could one day use wormholes or space warp effects—just like in Star Trek and other popular science-fiction series—to make these journeys even faster and get home again without losing any friends. But this is all very speculative.
    The Voyage Through the History of Human Thought
    To the ancient Greeks, the Earth ( geos ) was the center of the Universe ( cosmos ), with the planets, the Sun ( helios ), and the stars being relatively close. This geocentric view was demolished in the sixteenth century, when Copernicus showed that the Earth and other planets move around the Sun. However, this heliocentric picture did not last very long. Several decades later, Galileo used his newly invented telescope to show that the Milky Way—then known only as a band of light in the sky—consists of numerous stars like the Sun. This discovery not only diminished the status of the Sun, it also vastly increased the size of the known Universe.
    By the eighteenth century it was accepted that the Milky Way is a disk of stars (the Galaxy) held together by gravity. However, most astronomers still assumed that the Milky Way comprised the whole Universe, and this galactocentric ( galactos = milk) view persisted well into the twentieth century. Then, in 1924, Edwin Hubble measured the distance to our nearest neighboring galaxy (Andromeda) and showed that it had to be well outside the Milky Way. Another shift in the size of the Universe!
    Within a few more years, Hubble had obtained data on several dozen nearby galaxies, which showed that they are all moving away from us at a speed that is proportional to their distance from us. The easiest way to picture this is to think of space itself as expanding, just like the surface of an inflating balloon onto which the galaxies are painted. This expansion is known as Hubble’s Law, and it has now been shown to apply up to distances of tens of billions of light-years, a region containing hundreds of billions of galaxies. Yet another huge shift of scale!
    The cosmocentric view regards this as the final shift in the size of the Universe. This is because the cosmicos expansion means that as one goes back in time the galaxies get closer together and eventually merge. Before that, the density just continues to increase—back to the Big Bang fourteen billion years ago—and we can never see beyond the distance traveled by light since then. However, recently there has been an interesting observational development. Although one expects the expansion of the Universe to slow down because of gravity, current observations suggest that it is actually accelerating . Theories to explain this suggest that our observable Universe could be a part of a much larger “bubble.” And this bubble could itself be just one of many bubbles, as in the multiverse proposal!!
    What Next?
    So the end point of all three of our journeys—the first back through time, the second across space, and the third retracing the history of human thought—is the same: those unobservable universes that can only be glimpsed through theories and visited in our minds!
    I wonder what tomorrow’s