Quantum mechanics appears to say that the world at its smallest scales is fuzzy. Little particles, such as electrons, don't have precise locations in space and they don't travel at well-defined speeds, for example. It's only when we look, that is, when we make a measurement, that reality somehow "snaps" into place and particles are found sitting at well-defined places and moving with well-defined speeds. The exact values of properties like location and speed appear to be chosen at random.
Jeremy Butterfield, philosopher of physics at the University of Cambridge, explains contextuality.
The predictions of quantum mechanics have been tested endlessly in experiments and they hold true, but could it be that the theory simply isn't complete? Could there be hidden variables — some extra information — which, if we include them, give us a theory that isn't random or fuzzy? The answer is yes, but not without a price. Such a theory will always exhibit something called contextuality: the outcome of a measurement will be heavily distorted by your experimental set-up, so try as you might, you can never be an impartial observer. The following articles and video explore this concept of contextuality and related topics.
These articles and videos are part of our Who's watching: The physics of observers project, run in collaboration with the Foundational Questions Institute.