When you die, the last eight minutes of your life remains electromagnetically imprinted in your brain. If you could inject someone into that persistent time memory and pull them back afterwards, you could send investigators into crises just before they happened and have them identify who committed the crime. It’s a fascinating premise for a sci-fi thriller and director Duncan Jones pulls it off splendidly in Source Code.
The handwave explanation for what doesn’t really quite hold together as a scientific explanation is that the electromagnetic imprinting is something to do with “quantum physics”. Dr. Rutledge (Jeffrey Wright), the amoral scientist in charge of the Source Code project, explains to Stevens that the experiences he’s having are within the framework of that eight minute window, but that any changes he makes cannot affect whether people die or not: it’s a sort of parallel universe. Except for when… well… whatever. Don’t see this film with a physicist and you’ll be able to just accept the internal inconsistencies of the premise without any further worries.