  A unifying principle explaining the numerical bounds of quantum correlations
remains elusive despite the efforts devoted to identifying it. Here we show
that these bounds are indeed not exclusive to quantum theory: for any abstract
correlation scenario with compatible measurements, models based on classical
waves produce probability distributions indistinguishable from those of quantum
theory and, therefore, share the same bounds. We demonstrate this finding by
implementing classical microwaves that propagate along meter-size
transmission-line circuits and reproduce the probabilities of three emblematic
quantum experiments. Our results show that the "quantum" bounds would also
occur in a classical universe without quanta. The implications of this
observation are discussed.
