This is the name of an experiment first proposed in 1982, and is the one that I have had in mind when talking about God and quantum mechanics. I realized that most of the comments I have received seem to be taking me to be talking only about the standard double-slit experiment; this is of course my own fault since I haven’t done a very good job of indicating what I had in mind. So, let me describe these results and then reformulate the argument.
We have to build up to this, so let’s start with the quantum eraser experiment. Here is how Brian Greene describes the experiment in his recent book The Fabric of the Universe.
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.
As he goes on to point out, this is what we would expect. Since we measure the photon’s path, we get the photons acting like particles. But then it gets weirder. As Green continues, the quantum eraser asks,
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?
The answer, as it turns out, is that the interference pattern shows up again. Which, is , uh, weird. But again it gets weirder 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.
OK, so far so good. This is just a fancier version of what we have already talked about, with the exception that we are now no longer causally interacting with the signal photon. Everything we know about the signal photon we learn by observing the idler photon. But even so, we get the photons acting like particles. But we aren’t done yet. Again Greene
Now for the weirder part. 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[!!!!!!]
OK, so what this seems to show is that it is not anything that we do to the photon that determines which way it will behave. Rather what determines this is whether or not we are able to know which path the photon takes to the detector. Nothing changes here except our ability to know which path the photon took.
We can hammer home this point with one further modification of the experiment. Suppose that we set it up so that we could only get which-path information from some of the photons (and further that which ones we get this information about is random). Again Greene.
Does this erasure of some of the which-path information– even though we have done nothing directly to the signal photons– mean that the interference effects are recovered? Indeed it does– but only for those signal photons whose idler photons [had their which-path information erased]…If we hook up equipment so that the screen displays a red dot for the position of each photon whose idler photons [had their which-path information erased] and a green dot for all others, someone who was color-blind would see no interference pattern, but everyone else would see that the red dots we arranded with bright and dark bands– an interference pattern.
So, it is the knowledge of which-path information that determines which way the photons behave. Since God always has which-path information, whether he obtains it in such a way as to effect the physical world or not, He will never see the interference pattern. Or in other words, the wave like nature of reality will be hidden from Him.
Sheez! That took longer than I thought!!