A fiendish quantum thought experiment starts with the toss of a coin darkmatterphotography/Getty

The theory of quantum mechanics can give contradictory answers when applied to large objects like people and quantum computers, suggesting it may not be a valid description of how nature works at all levels.

Standard quantum theory explains the behaviour of microscopic things like electrons and atoms. We know that in practice, it is very difficult to observe quantum behaviour in large objects without their fragile quantum state collapsing, but in principle the theory should still apply – physicists have even used quantum theory to understand black holes …