If a tree falls in a forest and no one is there to hear it, does it make a sound? Perhaps not, some say.

And if someone is there to hear it? If you think that means it obviously did make a sound, you might need to revise that opinion.

We have found a new paradox in quantum mechanics – one of our two most fundamental scientific theories, together with Einstein’s theory of relativity – that throws doubt on some common-sense ideas about physical reality.