Hello everyone, I actually have three questions: 1. Am I missing an important detail in my understanding of how the delayed choice quantum eraser experiment is done? 2. How does one account for what takes place in the experiment without using the concept of "retrocausality" (effect before cause)? 3. If the photon passes through both slits, wouldn't the BBO crystal produce 4 photons? If it does, what happens in that case, and if it doesn't, then why? Okay so here is my understanding of the experiment. A laser fires a photon at a double slit. It can either go through slit A (red), slit B (blue), or both. After the double slit, there's a nonlinear optical crystal (BBO) that converts the photon into two entangled photons. A Glan-Thompson prism diverges these two entangled photons. One of them (called the signal photon) goes towards the detector D0 while the other (idler photon) goes towards a prism PS and is deflected depending on whether it follows path A or path B. An idler photon following path A passes through a beam splitter BSb where it can either reflect and go to D4 or transmit, reflect off of mirror Mb and then either reflect off of BSc and enter D2 or transmit and enter D1 (sorry about the run-on sentences, trying to keep this short). An idler photon following path B will either reflect off of BSa or transmit and reflect off Ma and arrive at BSc where it will either go to D2 or D1. Detectors D1 and D2 always give interference patterns, while D3 and D4 only show diffraction without interference. If the idler photon enters D4, then we know that it passed through slit A, if D3, then slit B. What ends up happening though is that whether or not the signal photon displays interference at D0 depends on whether the idler photon enters D1/D2 or D3/D4. If the idler photon enters D1/D2, there will be an interference pattern at D0. If the idler photon enters D3/D4, there will not be an interference pattern at D0. I'm not studying this for a class or anything, I've just been having a discussion with someone about the role of consciousness within these double-slit experiments. They used this as an example of how consciousness can effect matter. I, however, have a very hard time accepting this. There just has to be another explanation that does not involve retrocausality. If there isn't, then my friend would have to be right; somehow the signal photon knows whether or not we will have the path information (it is "erased" at D1/D2). I know some people believe consciousness plays a role in the original double-slit experiment, but I know that it doesn't. In that experiment, the reason why the photon acts like a particle is not because it knows a physicist is attempting to measure it, but because of the way it physically interacts with the detector. The delayed time version can't be explained this way. I'm not really too familiar with entangled particles, I only understand the main concept. All 5 detectors are the same kind of detector correct? The only difference between the last 4 is that we the observers know that D3/D4 will let us know the path information, while D1/D2 will not. How in the world would a photon "know" this??? There just has to be something I'm missing here. I would really like to know what the explanations that don't involve retrocausality (Ex. of explanation using retrocausality: if the idler photon arrives at say D3/D4, then it will "go back in time" and make sure that the original photon only passes through one slit, even if it originally passed through both) are. Wikipedia says that this paper provides such an explanation, however, I'm having some trouble understanding it http://arxiv.org/abs/1103.0117. Thank you for taking the time to read all of this.