Whether a quantum object behaves like a wave or like a particle depends (according to the Copenhagen interpretation) on the choice of measurement apparatus used for observing the system, and therefore on the type of measurement performed.
Anton Zeilinger's team of physicists at the University of Vienna and the Austrian Academy of Sciences has recently taken this phenomenon further than ever. Whether a certain photon behaves like a particle or like a wave depends on the measurement performed on a second photon. In the new experiment, this second photon is so far separated from the first photon that no transfer of information whatsoever (the velocity of which can never exceed the speed of light) would be fast enough. Yet, the first photon behaves like a wave or like a particle, still depending on the measurement performed on the second. While the results of such experiments are fully consistent with quantum physics, a clear explanation in terms of causality is impossible, as, according to Einstein's relativity theory, any transfer of information is limited to the speed of light. The science article "Quantum erasure with causally disconnected choice" has appeared in the current issue of the renowned science journal Proceedings of the National Academy of Sciences (PNAS).