The delayed-choice quantum eraser experiment does not rewrite the past

Weird imagery to show quantum stuff is happening, ooooooooh. Via Pixabay https://pixabay.com/photos/wormhole-time-travel-portal-vortex-2514312/

Simplifying the experiment

PBS Spacetime — How the Quantum Eraser Rewrites the Past

A delayed-choice CLASSICAL eraser experiment

Bob rewrites time

  • Didn’t shake the box and coin was even
  • Didn’t shake the box and coin was odd
  • Shook the box and coin was even
  • Shook the box and coin was odd

What the heck is going on?

  • Didn’t shake the box and coin was even
  • Didn’t shake the box and coin was odd
  • Shook the box and coin was even
  • Shook the box and coin was odd
  • The 2 objects definitely match and the coin was even
  • The 2 objects definitely match and the coin was odd
  • The 2 objects are no longer correlated and the coin was even
  • The 2 objects are no longer correlated and the coin was odd
  • Show me the results you got when I made choice x instead of choice y
  • Show me the results you got that match my results as opposed to the results that don’t

A classic(al) mistake

  1. Telling people that quantum mechanics is weird. The delayed choice quantum eraser experiment was proposed in the 1970s and physicists could predict what would happen if the experiment could be performed. Decades later physicists had the technology to create the real experiment and the results were as predicted. When scientists use theories and models to understand the world, then make successful predictions, it usually means something is being understood. The results of the experiment aren’t weird. Many features of quantum mechanics seem weird to some but are mundane to physicists and understood well enough to be used in engineering and technology. The big mystery is explaining exactly how more fundamental physics maps onto the everyday physics you experience in daily life. Physicists debate exactly what quantum mechanics is doing in the background but the experiment and results make sense. Telling people it’s all weird and mystifying sets people up to believe anything weird. Telling people to take the Copenhagen interpretation seriously doesn’t help either.
  2. Suggesting the supposed retrocausality is related to quantum mechanics at all. There are quantum effects in the experiment but they aren’t related to the misunderstanding that causes people to believe retrocausality is occuring. The rewriting of the past is presented as being a quality that only emerges because this is a quantum experiment but a classical analog of the experiment would cause the same supposed retrocausality. This isn’t about quantum mechanics.
  3. The complexity of the experiment. The main phenomena that’s causing us to confuse correlation for causation here doesn’t require the experiment to be as complex as it is. We can simplify the experiment considerably and maintain the supposed retrocausality, even to the point of removing all quantum features as mentioned above. It’s important to realise that the original quantum experiment is very difficult to perform in the real world, which is why it was first performed decades after it was thought up. The real experiment has more steps and equipment that is required conceptually but is just a consequence of the technology we have and what we’re able to do. The complexity of the experiment, involving waves and particles and slits and entanglement and crystals and mirrors and erasers and detectors and coincidence counters, obscures the nature of what’s going on when the results are provided.
  4. Suggesting the patterns emerge on the detectors themselves. It’s not the main misconception behind the misunderstanding but people are definitely misled by imagery showing patterns appearing on the detectors in the experiment. That suggests that the top photon (or Alice) sees patterns and then later the bottom photon (or Bob) makes choices that affect Alice’s patterns. In both the quantum and classical experiments, only a random pattern is found on the detection screens. We have to filter those results later to see patterns. People use this imagery as shorthand for the coincidence counter and the post-experiment filtering done to the data.
  5. The nature of the choice is about maintaining or destroying useful information. Explanations tend to describe the “choice” very loosely like it could be a choice to go left or right, or to be dry or wet but it’s all about correlation. The photons or classical objects are correlated to begin with so the choice is about maintaining that or not. Do you want useful information or useless information? Keep the match or lose it? Do you want information you could actually use in filtering later or none so you won’t be able to do any meaningful filtering later? That’s the choice. The patterns that emerge later when you filter the full results are a reflection of what you can or can’t know about the results given your PAST choice to keep or lose the correlation. Yes the choice matters to the pattern but it’s a case of normal causality: the choice in the past influences what you will filter in the future.

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Jennifer Harrison

Jennifer Harrison

Science, games, LGBTQIA, feminism, neurodiversity etc.

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