Time machine
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A time machine is a device which allows its user to travel through time, either to the past or the future. In general, such a device is believed to be against the laws of physics, though a one-way trip to the future could theoretically be organized in a number of ways that do not violate these laws. If real time machines exist they would involve tampering with black holes billions of miles out in space, and would be massively expensive and impractical.
Time machines have played an important role in science fiction, ever since H.G. Wells's famous book of the same name over a hundred years ago.