"We are not alone", declared the poster and this tale of Richard Dreyfus' escalating obsession with alien visitors against a backdrop of a secretive, omniscient government agency has provided the core of science fiction ever since.
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Our expert panel votes for the top 10 sci-fi films

Whether you prefer the original theatrical version (with a bored-sounding narration and without the famed unicorn scenes) or the director's cut of a few years later (sans narration and unicorn duly re-inserted), Blade Runner was the runaway favourite in our poll.

The story revolves around Harrison Ford's policeman, Rick Deckard, and his hunt for four cloned humanoids, known as replicants, in a dystopian version of Los Angeles. Replicants have been deemed illegal and Deckard is a blade runner, a specialist in exterminating them.

The film is loosely based on Philip K Dick's short story, Do Androids Dream of Electric Sheep? "Blade Runner is the best movie ever made," says Stephen Minger, stem cell biologist at King's College London. "It was so far ahead of its time and the whole premise of the story - what is it to be human and who are we, where we come from? It's the age-old questions."

It also discusses consciousness with an attempt to formulate a way to tell a human from a machine. The Voight-Kampff empathy test is used by the police in the film to identify the replicants - who have memories implanted and are programmed with artificial emotions. "The Voight-Kampff empathy test is not far away from the sort of thing that cognitive neuroscientists are actually doing today," says Chris Frith of the Institute of Cognitive Neuroscience at University College, London.

Debates rage on whether Deckard himself is a replicant. Ridley Scott says that he is artificial, but Harrison Ford argues that during filming Scott told him Deckard was human. Whatever the answer, it is a worthy winner also because of the quality of the film-making: Vangelis' brooding score, Rutger Hauer's replicant's seminal "I've seen things..." speech and that shot of the future LA cityscape, which kicks off the story. 2001: A Space Odyssey2. 2001: A Space Odyssey (1968) Dir: Stanley Kubrick

A very close second, this mystifying story came out of a collaboration between Kubrick and science fiction writer Arthur C Clarke. It achieved enormous fame for its then revolutionary special effects.

Spacecraft consultants Frederick Ordway and Harry Lange, who had worked for Nasa, persuaded companies such as Boeing and IBM to supply prototypes and technical documents for use in the film. Astronauts visiting the set at Borehamwood referred to it as "Nasa East".

Aubrey Manning, emeritus professor of natural history at Edinburgh, praises 2001 for "the brilliance of the simulations - still never done better despite all the modern computer graphics. The brilliance of using Brazilian tapirs as 'prehistoric animals'. The brilliance of the cut from the stick as club, to the space shuttle. Kubrick declaring that once tool use begins - the rest is inevitable. Hal: the first of the super computers with its honeyed East-Coast-Establishment voice." Darth Vader3. Star Wars (1977)/Empire Strikes Back (1980)

The first two films of the original Star Wars trilogy make it onto the list probably for reasons of nostalgia rather than science.

Essentially westerns set in space, they both cover the universal themes of good versus evil while making lead actors Harrison Ford, Mark Hamill and Carrie Fisher spit out mind-boggling technospeak on a regular basis. There is also an element of mysticism (which some say sets them apart from the rest of science fiction), with the idea of an all-pervading "force" that can be harnessed by certain people for good or evil.

The epic saga revolves around the battle between the all-enslaving Empire (led by the Emperor, a force-wielding maniac and his part-human part-machine henchman Darth Vader) and a small band of rebels.

Its use of science is sketchy at best - light-speed travel is dealt with by the use of a "hyperspace" where the normal laws of physics don't seem to apply and force-wielding Jedi fight with theoretically impossible lightsabers - but the emphasis here is certainly not on answering the problems of the human condition. Two of the first blockbusters, they also started the franchises for toys, games and replicas that no science fiction film can do without nowadays. Alien4. Alien (1979) Dir: Ridley Scott

Remembered for the iconic scene of an infant creature bursting bloodily through John Hurt's chest, but Alien was about much more. An interstellar mining vessel takes onboard a lifeform with concentrated acid for blood and two sets of jaws, which then messily dispatches the crew.

Praised for the gothic set design and Sigourney Weaver's portrayal of reluctant hero Ellen Ripley, it is notable for its underlying themes of motherhood, penetration and birth. But for UCL space physiologist Kevin Fong it's the mundanity of the crew's lifestyle that makes it stand out.

"For the first time we got the idea that, in the far-flung future, people who live and work in space might be a bunch of Average Joe slobs sitting around with leftover pizza, smoking and playing cards to pass the time," he says. "It captures much of what long duration space flight is about now: dirty, sweaty and claustrophobic with long periods of boredom followed by moments of sheer terror." Solaris5. Solaris (1972) Dir: Andrei Tarkovsky

Remade by Steven Soderbergh in 2002, but the original still holds a fascination for fans of the novel by Stanislaw Lem. A psychologist travels to a base on a remote planet to replace a mysteriously deceased scientist. There he encounters the secretive survivors - and his dead wife. Reality is supplanted by the increasingly attractive alternative of the planet's alien intelligence.

"The 1972 Solaris is perhaps the only film to address the limits of science set by our constrained human perceptions, categories and tendency to anthropomorphise," says Gregory Benford, professor of physics at University of California, Irvine and author of Timescape. "That it is also a compelling, tragic drama, not a mere illustrated lecture, makes it even more important." Terminator 26. Terminator (1984)/T2: Judgment day (1991) Dir: James Cameron

Robots from 2029 send a relentless cyborg (Arnold Schwarzenegger) back to 1980s Los Angeles to assassinate the mother of a future human rebel. One of a few films to deal with problems of time travel, such as the grandfather paradox: if you travel back in time and kill your grandfather, you wouldn't exist so wouldn't be able to travel back in time to...

The sequel featured another cyborg made of shapeshifting metal. "Despite the incoherent fictional science, it is a perfect piece of film-making in its genre, which I would call 'action movie' rather than 'sci-fi movie' if it were not for the fact that there are very few, if any, movies that genuinely deserve to be called sci-fi," says David Deutsch, quantum physicist at Oxford. US flag7. The Day the Earth Stood Still (1951) Dir: Robert Wise

Set amid the cold war paranoia of postwar America, a flying saucer lands in Washington DC and a humanoid alien, Klaatu emerges, accompanied by his robot, Gort.

Klaatu (who pronounces: "I'm impatient with stupidity. My people have learned to live without it") tries to convince the world's leaders - and when they won't listen, scientists - to stop the rush toward mutual destruction.

It is cited by Beagle 2 project leader Colin Pillinger as one of his favourite sci-fi films. "During the showing, the cinema manager pulled a classic Orson Welles stunt and stopped the film to announce that a spaceship had landed." War of the Worlds8. War of the Worlds (1953) Dir: Byron Haskin

Famously adapted for radio by Orson Welles, HG Wells' tale of a Martian invasion of Earth became another cold war movie.

"The idea that there could be life that's developed in completely other circumstances in a completely different world which you would never recognise. That's a very appealing idea," says Seth Shostak, senior astronomer at the Search for Extraterrestrial Intelligence, California. Matrix9. The Matrix (1999) Dir: Andy & Larry Wachowski

Cod philosophy, fetish clothing and incredibly cool special effects combined in 1999 for a fresh take on man-made artificial intelligence enslaving the planet.

The science behind the fiction is conspicuously absent, being replaced with the permanently befuddled Keanu Reeves stumbling around being confused by nonsense about spoons, and jumping off buildings. Tak Mak, a cell biologist at University of Toronto, doesn't think this matters: "It's good old-fashioned entertainment value ... Future bad guys fighting future good guys." Close Encounters (100 moments)10. Close Encounters of the Third Kind (1977) Dir: Steven Spielberg

"We are not alone", declared the poster and this tale of Richard Dreyfus' escalating obsession with alien visitors against a backdrop of a secretive, omniscient government agency has provided the core of science fiction ever since.

"While it is highly unlikely that 'they' will rock up in a vehicle that looks like a giant, inverted Christmas tree or make their presence known by doing Jean Michel Jarre impressions on a cosmic synthesiser, Close Encounters is for me still the classiest alien visitation story in celluloid history," says UCL's Kevin Fong.



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