The Delayed Choice Quantum Eraser experiment is already explained with really extraordinary clarity on line by Ross Rhodes at bottomlayer.com here. People who are already quite familiar with QM (quantum mechanics) will enjoy his explanation, in part because it reproduces text and figures from the paper by Kim et al that reports the experiment formally. But mostly because of the grace and clarity of mind at work.

Absolute beginners with a little time on their hands can figure everything out by using Mr. Rhodes’ explanations starting here. Both beginner and advanced, however, should at some point end up reflecting on the 2001 article in the American Journal of Physics, “Do we really understand quantum mechanics? Strange correlations, paradoxes, and theorems” by Franck Laloë. My emphasis.

For purposes of my own argument from physics, I will start by recalling Amit Goswami’s statement that, “since our observation magically resolves the dichotomy of the cat, it must be us — our consciousness — that collapses the cat’s wave function.” As a rough first approximation, collapsing the wave function amounts to taking a measure­ment of something, finding out where it is or which of several properties it has.

And it is in the relationship between the measure-taking person, the observer, and the outcome of the measurement that the mystery lies.

I’ve heard an expert describe the mystery in terms of the old paradox of the tree falling in the forest - if no one hears it, is there really a loud sound? Most of us would definitely say that of course there’s a loud sound, it’s just that nobody happened to be there to hear it. But physics today has shown repeatedly that you get a sound when there’s somebody there, but otherwise you don’t. And by “loud sound” they mean acoustic vibrations of the air near the tree when it falls. It’s as if the observer caused the sound to appear by listening for it. Or just by being there.

Another random note, from Chris Carter, Science and the Afterlife Experience, quoting Nick Herbert in Elemental Mind, p 172: “In the von Neumann interpretation of quantum theory, consciousness is a process lying outside the laws that govern the material world, and it is just this immunity from the quantum rules that allows mind to turn possibility into actuality.”

The tree-in-forest bit is only a clari­fying metaphor. So far experiments have shown this observer effect in nothing larger than a 60-atom Buckey­ball of carbon. (Pretty big as QM ex­per­iments go.)