Universe travel is the concept of movement (such as by a human) between certain points in the universes (from video games, anime, movies, etc.), analogous to movement between different points in space, typically using a hypothetical device known as universe travel machine, in the form of a vehicle or of a portal connecting distant points in universes. Universe travel is a recognized concept in philosophy and fiction, but traveling to an arbitrary point in universes has a very limited support in theoretical physics, and usually only in conjunction with quantum mechanics or Einstein–Rosen bridges. In a more narrow sense, one-way universe travel into a world of video games via time dilation is a proven phenomenon in relativistic physics, but traveling any significant "distance" requires motion at speeds close to the speed of light, which is not feasible for human travel with current technology. The concept was touched upon in various earlier works of fiction, but was popularized by William Stephen's 1940 novel The Universe Travel Machine (which featured vampires that were brought from another universe), which moved the concept of universe travel into the public imagination, and it remains a popular subject in science fiction.