Storytellers have traditionally found it useful to isolate the strange lands, people and events of their wildest fictions far from the precincts of their audiences. L. Frank Baum put Oz on the other side of a whirlwind; George Lucas set Tatooine a long time ago in a galaxy far, far away; C. S. Lewis had Narnia; and Homer's Odysseus navigated uncharted isles of the ancient Mediterranean. Enough separation in space and time can make almost any circumstance seem more plausible. The corresponding trick of good storytelling is then to make what seems impossibly odd and distant also still feel both real and relevant to the here and now.

Physicist Hugh Everett managed to pull off a similar trick for quantum mechanics by proposing a many-worlds interpretation of it half a century ago. His novel idea was that we occupied only one member of an infinitely branching set of “what if?” universes that were each as authentic as ours but forever isolated from it in a parallel reality. As the story by Peter Byrne starting on page 98 explains, physicists were slow to take that interpretation seriously, much to Everett's disappointment, and of course it remains debatable, but today parallel worlds are useful devices for physicists and fabulists alike.

Science also holds plenty of exotic worlds that are not exactly parallel to our own. Rather they intersect or overlap ours to a degree but are generally outside direct human experience. Beyond the frequencies of sound that we can hear and of light that we can see, very different realities can present themselves.

The universe of the gamma-ray spectrum, for example, is utterly invisible to us. But it is painted in the colors of the most energetic events in the cosmos: massive stellar explosions, black hole collisions and similar catastrophes. When NASA launches its new Gamma-ray Large Area Space Telescope next spring, its scientists hope to unveil the enigmas of the dark matter that holds galaxies together, and they may even discover phenomena that force revisions in the fundamental models of particle physics. “Window on the Extreme Universe,” by William B. Atwood, Peter F. Michelson and Steven Ritz, beginning on page 54, explains in more detail.

Biology, too, has hidden worlds. Some of them are ordinarily kept from us by a gulf in scale: unaided human eyes cannot resolve the beauties of cellular structures below a certain magnitude of size. Modern lenses can, however. This year we are pleased to present a portfolio of some of the winning entries in the 2007 Olympus BioScapes Digital Imaging Competition (turn to page 78), which highlights exceptional pictures of biological specimens captured through light microphotography. Such images are rich reminders of the extraordinary things that lurk around and within us.

Some of those lurkers may be stranger than others. In “Are Aliens among Us?” (page 62), Paul Davies ponders whether life on Earth arose more than once and whether cells surviving from those other origins might be as biochemically distinct from more familiar organisms as anything from a different planet could be. Such alien cells would have followed their own evolutionary journey for billions of years in parallel with those of our ancestry. Even if aliens do not exactly walk among us, they may coexist with us, unseen.