IMHO the part in the Discussion section about no usable information traveling backward in time is both unnecessary and wrong First of all I never said anything about anything travelling faster than the speed of light And as for usable information travelling backward in time read the experimental setup as described on bottomlayer com The whole point is that the delayed choice DOES change the result of the observation on the primary detector It changes the results of an observation made in the past that is the whole point All that nonsense about causality not being violated is a bunch of arm waving Prove it I think that the most interesting result of this experiment also as described on bottomlayer com is this shows the underlying flaws in the copenhagen explanation of the double slit experiment As Wheeler observed an experimenter may choose to know a property after the event should already have taken place In other words as described in the bottomlayer com explanation the observer s choice would determine the outcome of the experiment regardless of whether the outcome should logically have been determined long ago Frankly this DOES violate causality I believe this shows that causality itself is an illusion QM doesn t care about causaltiy This is the direct result of the copenhagen explanation of the double slit expderiment and it is correct Furthermore Wheeler s thought experiment has been experimentally verified by the results from Kim et al So I think that comment about causality not being violated really takes away from the point of this experiment Wheeler was disturbed by the fact that causality seemed to be violated by the Copenhagen interpretation and Kim et al have shown experimentally that this is true I think that a lot of leading QM researchers have done science a dis service by avoiding this issue It is a fundamental paradox that has never been resolved The more you look at it the worse it gets Copenhagen himself said that he himself did not understand it and anyone who said they did was a liar This result clearly makes no sense yet it can be shown to be true But as long as people go around spouting nonsense like this is only qunatum information which is violating causality and you can t actually do anything with it at the qunatum level is wrong It is like sweeping the problem under the rug While it is true that QM is mathematically bullet proof it contains at its core this fundamental paradox which no one has ever been able to explain I wish more experimental physicists like Kim and others would take this problem seriously and try to understand what is really happening I personally think that the so called multi world intepretation is the only way to resolve this paradox and experiments like this show that any other interpretation leads to crazy results like violations of causality