Cause and effect my darling, cause and effect.
After the latest post on the delayed choice experiment I’ve done some research on latest papers that comment on the experiment.Fortunately enough I found one recent paper http://arxiv.org/pdf/1007.3977 The paper tries to explain the delayed choice experiment and the confusion it may causes. The author is not of the opinion that the choice “changes” the past and he has a very good point; Special Relativity.
As you might know according to special relativity there is no preferred system of reference in the world. For example in the past that humans believed that the Earth is flat, and is the center of the world, they were assuming that our system of reference (or in simple words; our seat in the the theater of the universe) is privileged and unique. We were assuming that someone that lives on the surface of the Earth has the “best” or more accurate view of the universe. Of course Galileo first talked about the principal of relativity but until Einstein, scientists never gave up the idea of the privileged system of reference. Einstein with the theory of special relativity started from the idea that light has the same velocity for all observers which lead to the result that there is no absolute time. Time depends on the system of reference.
Going back to the delayed choice experiment and introducing relativity, then we have trouble identifying the series of the events; for some observer the event of producing the photon and measuring it can be concurrent, this observer will not experience a “changing the past” effect. According to the author;
“The (well-known) point stated in the introduction was to distinguish correlation from causation. The lesson we draw here is that this very correlation between distant measurements does not feel their relative time ordering: it does not distinguish between future and past. This implies backwards correlation but still precludes backwards causation or any other tension with relativity, effectively demystifying the delayed choice experiments. the only place where something physical happens is the place of the measurement, and the implications on conditional probabilities hold for other measurements throughout the entire space time, present and past. “
So, according to our understanding, the measurement does not affect the past but there is a statistical correlation between the measurement and the choice¹. Correlation does not prove cause and effect relationship. Let’s understand this what one example;
There was a research that stated that families that have big libraries at home, tend to have children that succeed better in life . Someone can think that libraries are the cause for children performance, but in a second thought this is a naive; Families with big libraries are most likely to belong to middle and upper classes of society. It’s obvious that upper classes children succeed better in life. The cause for the success is not the libraries but social status!
Of course what correlation can mean in the context of quantum mechanics is not easy to understand, and the example above cannot help because it implies that there is a causation relationship (social status) but is hidden from us. This does not happen in quantum mechanics, but this is another story!
¹ Please check the delayed choice experiment.