The original paper by the authors who first performed the Delayed Choice Quantum Eraser implied retro causality. But retro causality is true only if you assume a classical way of thinking. But that’s not the way quantum mechanics works, and I was wrong for interpreting it that way in my original 2019 video. When viewed with the standard interpretation of quantum mechanics where a particle is always a wave until the moment it is measured, there is no retro causality.
How the double slit experiment works: If you send photons one at a time through the slits, at first you will see what looks like a random distribution of dots. But after a while, you will see that those dots create an interference pattern.
If you then put detectors on the slits to measure which slit the photon passes through, you see a pattern like you would if you were sending individual particles through the slits. The act of measuring seems to affect the results. But the change is due to the nature of quantum mechanics. All quantum objects like photons and electrons are really waves. But if they interact with anything, that is, if an irreversible energy exchange takes place, their waves become localized like a particle. This is called “wave collapse.” Wave collapse also occurs when the photon interacts with the screen in the back. And we this as a dot on the screen.
The Delayed Choice Quantum Eraser is like the double slit experiment on steroids. First, I want to point out that if you have a detector that measures the path, you don’t really get two clean lines of photons like it’s usually illustrated. You get a single spread out distribution of photons.
How does the delayed choice experiment work?
It starts with the double slit, but first the photons go through a special optical device called a Barium Borate crystal. It splits a single photon into a pair of entangled photons with half the energy each of the original. Note that the process of creating entangled photons effectively results in a measurement. In other words, the wave function of the photon collapses so that it is now a particle. And since the path from the top slit to detector 1 is slightly different than the path from detector 2, the which way information of the photon is known. Thus the pattern that will show up at detector 1 will always be a spread out pattern, not an interference pattern. It doesn’t matter what happens at any of the other detectors.
So why is it illustrated as changing depending on what happens at the other detectors? This is the center of the confusion, and where the idea of retro causality comes in.
Well the confusion is from the way this experiment is presented – as D1 changing its pattern to match the interference pattern at D4 or D5 when the photons end up there, but showing a different pattern, a spread out pattern, if the photons end up at D2 or D3.
So this implies that what happens at D2, D3, D4 or D5 influences what happens at D1. But since the path to D1 is shorter than the path to any of the other detectors, the photons reach D1 BEFORE they reach D2, D3, D4 or D5. So the implication is that the pattern at D1 which would be in the past, is being affected by what happens in the future at D2, D3, D4 or D5. So people have naturally been led to think that this means retro causality. This is wrong.
The quantum eraser has no effect on the original screen. What’s really happening is that the changing patterns are due to the scientists, conducting this experiment, selecting subsets of the photons in D1 to show the same patterns as at each of the other detectors. This can be done because the particles hitting the screen at D1 and the particles going to the other detectors are entangled.
#delayedchoicequantumeraser
#quantumphysics
So in the presentations that you see, including the one I originally made, the interference pattern you see get at D1 is nothing but a hand-selected subset of the actual original spread out pattern at D1, corresponding to photons that ended up at D4 or D5. This is done post-experiment by hand! The patterns do not change on their own. The future does not affect the past.
