Science fiction film (or sci-fi film) is a genre that uses speculative, fictional science-based depictions of phenomena that are not fully accepted by mainstream science, such as extraterrestrial lifeforms, alien worlds, extrasensory perception and time travel, along with futuristic elements such as spacecraft, robots, cyborgs, interstellar travel or other technologies. Science fiction films have often been used to focus on political or social issues, and to explore philosophical issues like the human condition. In many cases, tropes derived from written science fiction may be used by filmmakers ignorant of or at best indifferent to the standards of scientific plausibility and plot logic to which written science fiction is traditionally held. The genre has existed since the early years of silent cinema, when Georges Melies' A Trip to the Moon (1902) employed trick photography effects. The next major example in the genre was the film Metropolis (1927). From the 1930s to the 1950s, the genre consisted mainly of low-budget B movies. After Stanley Kubrick's landmark 2001: A Space Odyssey (1968), the science fiction film genre was taken more seriously. In the late 1970s, big-budget science fiction films filled with special effects became popular with audiences after the success of Star Wars and paved the way for the blockbuster hits of subsequent decades.