India, Jan. 18 -- LONDON: Imagine this. A video appears on social media showing Narendra Modi announcing that he has converted to Islam and that his aim is to change India into an Islamic state. Impossible and outrageous you think; it must be fake. Then you look carefully at the synchronisation of his mouth and words, and it appears perfect. You have the voice on the video analysed, and it is unquestionably his. It simply cannot be true and you are now utterly bewildered. Welcome to the world of the deepfake.

This example is absurd, of course, as the gap between what you see and reality is huge and would never be believed. But what if you see a video when the gap is small, such that your eyes overcome your judgement? As a benign example,...