Asked by: Carl Redgrove, Milton Keynes
The only hope of tailoring gravity to our needs lies with quantum vacuum effects, in which energy and particles and anti-particles appear out of nowhere. These are predicted to have anti-gravitational properties, and may already be propelling the expansion of the Universe in the form of 'dark energy'. But as yet no-one has any idea how to generate and manipulate such effects to order.
Get more fascinating Q&As from BBC Focus magazine by following @sciencefocusQA