INTRODUCTION BY ROBERT SILVERBERG

      TIME AND TIME AGAIN, YES: throughout a career of nearly sixty years I would return almost obsessively to the theme of traveling freely in time. It has always seemed to me the essence of what science fiction is about, offering liberation from the bonds of the quotidian, the freedom to move in unhindered leaps through the unknown realms of the future and the nearly-as-mysterious realms of the past. Even though one could argue with some justification that the space-travel story also provides such freedom, I still think that a fictional space voyage, even if it takes us to the ends of the universe, still ties us to our three-dimensional world, whereas an imagined trip in time, sending us backward even to the moment of creation or forward to the final day of all, or, indeed, down some parallel track in a reality that is not our reality, adds a fourth dimension, a sense of boundless adventure, of infinite possibility.

      And so for me, time travel has always been the basic theme. I have assembled here an assortment of my time travel stories, spanning the full length of a long writing career. One story from my earliest days as a writer is in this book, and one of the last that I wrote before I decided, around the year 2009, that I had written enough fiction for one lifetime—and in the decades between “Absolutely Inflexible” of 1955 and “Against the Current” of 2009 I rang every imaginable change I could on the idea that it might somehow be possible to move forward or backward along the stream of time. Which is an idea, by the way, that I happen to think is scientifically impossible, a violation of the laws of thermodynamics, and one which I think is philosophically implausible as well, running us aground on the shoals of paradox whichever way we try to go. But science fiction is not necessarily limited by the boundaries of the possible: quite the contrary. As Robert A. Heinlein once suggested, it really ought to be called speculative fiction. There often isn’t much science in it, but there’s a powerful element of speculation, a what-if element, that allows a sufficiently skillful writer to transcend mere scientific or philosophical implausibility for the sake of telling a good story and to arouse the reader’s sense of wonder. The trick is to make one’s speculations seem plausible: to achieve what Samuel Taylor Coleridge called, long ago, the willing suspension of disbelief. It is by achieving that suspension of disbelief that we can make the gateway swing open and undertake the otherwise impossible journey to some otherwise inaccessible distant reach of time.

      Four works of science fiction in particular imprinted this lifelong fascination with time travel on me before I was thirteen years old.

      The first, which I encountered in the Brooklyn Public Library when I was ten or eleven, was H.G. Wells’ short novel The Time Machine. I had already read Jules Verne’s Twenty Thousand Leagues Under the Sea, which only marginally qualifies as science fiction but which does convey the kind of extraordinary venture into the unknown that science fiction would later provide for me, and when a librarian