In the movies, weird science, bad science and un-scientific science are nothing new. The fact that the cinema-going public can be assumed to know nothing about it has been used as a plot-enabler for futuristic hokum such as the Bride of Frankenstein or The Incredible Shrinking Man. If you need your hero to fly through the air and then turn into a toaster, well, bring on a white-coated, crazy-eyed boffin with a flying-plus-toaster formula bubbling away in his test-tube - and get the hero to drink. Science fiction is a tautology: in the movies, science is fiction .

But it doesn't stop us getting a little restive when bad science in the movies becomes self-important. The nature of consciousness is a hot topic at the moment. The idea of machines in The Matrix generating false consciousness in a conquered race of humans in a vast people-plantation is fine if it's just a bit of wacky fun: but some were uneasy at that movie's evident claims to be taken seriously.

The same goes for Steven Spielberg's AI: Artificial Intelligence, and its robot with supposed "conscious" self-awareness. It's a movie that solemnly behaves as if it is raising compelling and disturbing questions based on fact. In Ang Lee's recent version of The Hulk, the big green monster's transformation is avowedly derived from biologist Greg Szulgit's work in Hiram College, Ohio on sea cucumbers with mutable tissue allowing them to expand and contract. Professor Szulgit reportedly groaned that the incidental science in Hulk was "awful".

But what's more insidious is dodgy science in movies that are not obviously SF. A classic example is Lord of the Flies, which hinges on someone making fire by concentrating the sun's rays with short-sight spectacle lenses. In real life, it can't be done. Barry Levinson's tech thriller Disclosure (1994) had characters sending each other emails - a very space-age thing to do in those days - casually showing souped-up software graphics for incoming and outgoing messages far snazzier than anything available then or now. And many hi-tech movies semi-seriously imply the highly developed existence of cyber-consciousness and, of course, cyber-sex technology. Big-screen science is to be taken with a 0.0015mg dosage of NaCl.

· Peter Bradshaw is the Guardian's film critic