For any vehicle—airplane or car—to fly, there needs to be some force pushing it up so that it can overcome gravity. Airplane wings are specifically designed to create just such a force. As a plane moves forward, the wings push air down, and because for every action there's an equal and opposite reaction, this action creates an upward force on the wing, called lift.

Haven't you noticed the irony of asking the question "when?" about time travel in your March feature? Can't we assume that if it was ever going to happen we would have already heard?


Peter Rosenl
Ithaca, N.Y.

Start with a Black Hole ...

The physical possibility of time traveL is something of a catch-22. Any object that's surrounded by the twisted space-time that time travel requires must by its very nature be fantastically perilous, a maelstrom that would inevitably tear apart the foolhardy traveler. So physicists have labored to create a theoretically acceptable time machine that's free from nasty side effects like certain death. Their starting point: black holes.

If we wish to travel through time, we must be able to control it, to mold it to our own desires and specifications. This is possible only through Einstein's theory of relativity. Instead of imagining space as an immutable structure, part of Einstein's genius was realizing that space and time are interlinked in a single, pliable framework called space-time. Both space and time can be distorted, sometimes dramatically. Time travel requires nothing more than exploiting those distortions to create a path through time that ends earlier-or much later-than it begins.

When H.G. Wells sent the hero of The Time Machine into what Wells called "futurity," it was on a grim 30-million-year round-trip to pretty much the end of Earth time, when the last, poorest excuses for life were flopping around like squid under a darkening sun. Wells wasn't the first writer to imagine time travel, but he advanced the idea that a machine, rather than an angel or a bonk on the head, could accomplish it, and he pushed his machine to the limit.