Time travel is extremely common in SF, but there are stories involving time travel which just barely qualify as SF at all (or only barely). It's method that makes the difference. Time travel by just wishing, or magic, or unexplained methods, can't really be called science fiction (although Octavia Butler's excellent book Kindred is usually called SF, and the method her time traveller uses is certainly not explained..oh, well). For a time travel story to be science fiction, there generally has to be an explanation, or at least some handwaving.

Time travel to the future often involves a machine?frequently invented by our familiar mad scientist . But there are a couple of other methods. One of these is suspended animation. In this method, a person is "frozen" (according to Larry Niven, the person is made into a "corpsicle") or otherwise put into a state of deep sleep, in which the body's metabolism is slowed down to the point where aging does not occur. You go to sleep and wake up in 2121 (or whatever year). Some people () even promote this as a real-life method of "life extension" (minimum price, $28,000, and I do not recommend it--remember that word, "corpsicle", as in corpse) This method was used to great success in stories by H.G. Wells ("The Sleeper Wakes") and by Washington Irving ("Rip Van Winkle"--which few would really call SF) to name just two.

There is another method of time travel to the future, which is a little more complicated to explain. If you travel in a spacehsip at a speed approaching that of light, Einstein's theory of relativity predicts that for you, time will pass at greater rate, compared to someone left behind on earth. If you go faster than light (impossible in our current technological/ theoretical state--ask your physics teacher--but common in SF), this phenomenon is exaggerated. So if you travel to the stars, and come back, your five year mission will be over, five years will have passed on your ship and for you, but for the people back on Earth it will have been maybe a hundred years, everyone you know will be dead and gone, your whole world will have changed?you come back to the future. This is hard to visualize, but you can take my word for it, or look at this site which explains it more clearly. Or this one , which lets you figure it out the same way Einstein did.

You can also check this page from Marshall Brain's (it's his real name, honest) excellent "How Stuff Works" site.

There are also SF stories in which you can just view or preview the past or the future, sometimes by esp (extra-sensory perception--another common SF theme), or sometimes by machines, without actually going there (a kind of virtual time travel--a method of avoiding the "grandfather paradox," which we'll see more about later).

Whatever the method, there must be a method for the story to be SF. Even then, what really matters, in most stories, is not how you travel through time, but why. And what you find when you get there.

Go to the discussion board. When would you travel to? Past or Future? Why specifically would you want to visit that time? What would you want to see?