In the movie Back to the Future, hero Marty McFly travels bacl to November 5, 1955. At that point in time, he meets his parents-to-be as high school students. For a while it seems as if Marty's presence might change the past so that his mm and dad will not fall in love. And if they never go on to marry and have children, how could Marty ever be born to travel in the past? Fortunately, this remaking of his family history is avoided in the nick of time. Still, when Marty arrives back in he 1980s, he find shis family to be somewhat different than he had remembered it!

The idea of traveling through time has been explored in many books and films over the years. For instance, in The Time Machine, the English author H. G. Wells tells of a remarkable journey into the year 802,701. In this short story, human beings have been divided into two very different races – the gentle, childlkie "eloi" and the terrible "morlocks." The morlocks live underground. There they work with machinery and provide for the daily needs of the eloi, who dwell on the surface. But the morlocks also treat the eloi like fatted cattle, taking a few of them each night for food.

Other writers have whisked theor eroes and heroines into a more hopeful future. Still others have described trips into the past, to the year of the French Revolution or the long-ago age of dinosaurs. But could time travel ever become more than just an author's fantasy? Could it ever happen in the real world?

In one sense, we travel through time every moment of our lives. A few seconds into the future, you will be reading the words of the next sentence in this book. Now it is happening, and you are actually reading them. But now, they are no longer in the future, or the present, but th epast. You have just done a little time-traveling of your own!

Yet this everyday kind of movement through time is not very exciting. What we really mean by time travel is being able to jump back into the past or forward into the future, as far or as quickly as we like. Will that ever be possible? Could you, in fact, ever build a time machine?