In the last few months pretty much all the available time I have – I’ve been thinking about the implications of the Delayed Choice Quantum Eraser. All my life I understood the double-slit experiment to need ran observer in order to collapse the wave-like nature of reality. I have, as perhaps others, never grasped the deeper, the more subtle implication – the idea that:-
If we can know the path the entangled photon takes, that in itself is enough to bring about a collapse.
In quantum mechanics, the quantum eraser experiment is a double-slit experiment that demonstrates several fundamental aspects of the quantum theory, including quantum entanglement and complementarity. Brian Greene describes the experiment in his recent book The Fabric of the Universe. Consider
“A simple version of the quantum eraser experiment makes use of the double-slit set up, modified in the following way. A tagging device is placed in front of each slit; it marks any passing photon so that when the photon is examined later, you can tell through which slit it passed…when this double-slit-tagging experiment is run, the photons do not build up an interference pattern.
What if just before the photon hits the detection screen, you eliminate the possibility of determining through which slit it passed by erasing the mark imprinted by the tagging device?
As we know, it turns out, that the interference pattern shows up again. Which, is unexpected, counter initiative and strange to say the least. But again it gets even stranger with the delayed-choice quantum eraser. Greene describes it thus,
It begins with [the set-up of the quantum eraser], modified by inserting two so-called down-converters, one on each pathway. Down-converters are devices that take one photon as input and produce two photons as output, each with half the energy (“down converted”) of the signal. One of the photons (called the signal photon) is directed along the path that the original would have followed toward the detector screen. The other photon produced by the down-converter (called the idler photon) is sent in a different direction altogether. On each run of the experiment we can determine which oath a signal photon takes to the screen by observing which down-converter spits out the idler photon partner. And once again, the ability to gleen which-path information about the signal photons– even though it is totally indirect, since we are not interacting with any signal photons at all– has the effect of preventing an interference pattern from forming.
So all we know about the signal photon we learn by observing the idler photon. But even so, we get the photons acting like particles. Greene again
What if we manipulate the experiment so as to make it impossible to determine from which down-converter a given idler photon emerged? What if, that is, we erase the which-path information embodied by the idler photon? Well, something amazing happens: even though we’ve done nothing directly to the signal photons, by erasing which-path information carried by their idler partners we can recover an interference pattern from the signal photons.
The implication here is the fact that we can know which slit the photon exits is exactly what causes the interference pattern to collapse. In the double slit experiment it isn’t the detector (the measurement) that collapsing the waveform. It is that fact that we can know that collapses the wave.
Now if a God can always knows which-path information a photon is to pass, he/she can therefore never see the interference pattern, the wave like nature of reality will be hidden from such a God.