By Eudora Donne

科学幻想简称“科幻”（Sci-fi），是根据有限的科学假设，在不与人类最大的可知信息量冲突的前提下，虚构可能发生的事情。

Science fiction，which is often shortened to sci-fi or SF，is a kind of writing.Science fiction stories can be novels,movies,TV shows,and other literature.

SF is often about the future.It can be about imaginary new science and inventions such as spaceships,time machines,and robots.The world of SF is different from the real world.It can have science and tools that do not exist in reality.Science fiction stories often take place in other worlds.There are often alien（外星的） creatures.

Science fiction is different from fantasy.Fantasy（幻想）stories often have magic and other things that do not exist.Isaac Asimov was a famous science fiction writer.He once said that science fiction is possible,but fantasy is not.Writers often use SF to explain everyday questions or problems by putting them in the future.Usually they invent a very different world to help people be aware of important ideas.

Two broad genres （体裁）of science fiction are Hard SF and Soft SF.

Hard science fiction,or “Hard SF”,is special.It uses true facts and theories from sciences.These sciences are very important in Hard SF like physics,astrophysics（天体物理学）,and chemistry.Also,Hard SF can show worlds that more advanced technology may be possible.Many correct predictions of the future come from the hard science fiction subgenre （分体裁）.However,there have been many incorrect ideas about the future,too.

Soft science fiction stories take ideas from social sciences such as psychology,economics,political science and so on.It can be mostly about character and emotion.Ray Bradbury won a prize called the SFWA Grand Master and wrote in this style.The Soviet Union produced social science fiction too.

The first important time travel novel is Mark Twain's A Connecticut Yankee in King Arthur's Court.The most famous is H.G.Wells's 1895 novel The Time Machine.Wells's book uses a machine that allows an operator to travel to an exact time.The term“time machine”,was invented by Wells.Now it is the name for any vehicle that can take a rider to a one date.Time travel stories can be complicated（复杂的）.Time travel is a popular subject in modern science fiction.