DailyGalaxy has an interesting article on quantum entanglement, and how certain properties of quantum mechanics can seem to break the chain of cause-and-effect:
The authors experimentally realized a “Gedankenexperiment” called “delayed-choice entanglement swapping”, formulated by Asher Peres in the year 2000. Two pairs of entangled photons are produced, and one photon from each pair is sent to a party called Victor. Of the two remaining photons, one photon is sent to the party Alice and one is sent to the party Bob. Victor can now choose between two kinds of measurements. If he decides to measure his two photons in a way such that they are forced to be in an entangled state, then also Alice’s and Bob’s photon pair becomes entangled. If Victor chooses to measure his particles individually, Alice’s and Bob’s photon pair ends up in a separable state.
Modern quantum optics technology allowed the team to delay Victor’s choice and measurement with respect to the measurements which Alice and Bob perform on their photons.
“We found that whether Alice’s and Bob’s photons are entangled and show quantum correlations or are separable and show classical correlations can be decided after they have been measured”, explains Xiao-song Ma, lead author of the study.
“Within a naïve classical word view, quantum mechanics can even mimic an influence of future actions on past events”, says Anton Zeilinger at the Institute for Quantum Optics and Quantum Information, the University of Vienna.
I’ve heard of this being done with the Double Slit Experiment, too. In short, the double slit experiment shows that light behaves both as a wave and as a particle, depending on whether or not you observe the photons as they pass through the slits. If you’re watching them they act like particles, and if you’re not watching, they behave like waves.
The creep/cool thing is that you can make that choice after the fact. If you record which slit photons move through and then look at the pattern they create, you’ll see particles. But if you record the data and then destroy it without examining it, you’ll see a wave.
The universe is a strange, strange place.