Time travel isn't a weapon, necessarily, but neither is a B-52 Stratofortress. The latter is a bomber, a delivery system for weapons. Likewise, if our future operates the way it did in the Terminator films, we might use time travel to assassinate future military leaders before they rise to power. Or perhaps we'll actually wage full-blown wars across four dimensions, such as the Time War waged between the Time Lords and the Daleks of TV's "Doctor Who."
Real time travel is much trickier than movies would lead us to believe, but it is within the laws of physics. Einstein taught us that time slows down -- or to think of it visually, stretches out -- when you travel close to the speed of light. You only need an airplane to notice the effect. In a famous experiment, physicists synchronized five atomic clocks, then kept one on the ground and put the rest on two very fast airplanes (one heading eastward, the other westward). After the airplanes landed, their clocks measured less time than the ground clock. The difference was tens to hundreds of nanoseconds [source: Nave]. If the clocks were people, the airplane clocks would be younger than the ground clock.
Of course, nanoseconds don't interest us. We want to meet ourselves as children or old folks. Serious time travel requires more than an airplane; it requires us to play around with black holes, wormholes or cosmic strings, all of which we're still studying as phenomena.
We can't really say whether we'll ever be able to instantly go forward or back in years (except by aging), but the physical framework is real enough for it to make our list.
Next, we'll play with more physics.