[HenryBraxton (OP)]

Double slit experiment shows a wave pattern when "which path" info is unknown.

When which path the photon took is known, a particle pattern exists.

If the which path information is collected and stored, say, on a harddrive, but nobody looks at the data, then that info is still unknown and a wave pattern exists? Or does the data need to be destroyed from the drive to show a wave pattern?

[evan_au]

You  destroy the wave pattern in the process of measuring which slit the photon passed through.
- You need to measure it to write it to the hard disk.
- It doesn't matter whether you read the hard disk or not.

[RobC]

I find it unsettling that even after 50 years since the single electron double-slit diffraction was first demonstrated by Giulio Pozzi that no plausible explanation has been given.

[yor_on]

Don't know about your 'quantum erasing' but there is a explanation. It's called HUP (Heisenberg's Uncertainty Principle).

It's not that complicated to get, although it in some ways tells you just as much as 'c' do, that is nothing. Unless you're prepared to step out of what we think.

[yor_on]

If you do, I better warn you, you will probably seem slightly deranged to most people.

[yor_on]

As for if a 'passive  recording' becomes a 'observation'?
Yes, as far as I'm concerned.


Otherwise there would be no 'universe' to observe.

[redlionfour]

Richard Feynmann noted more than once that complementarity is the central mystery that lies at the heart of quantum theory. Complementarity rules the world of the very small… the quantum world, and surmises that particles and waves are indistinguishable from one other. That they are one and the same. That it is nonsensical to think of something, or even try to visualize that something as an individual “particle” or a “wave.” That the particle/wave/whatever-you-want-to-call-it is in this sort of superposition, where it is neither particle nor wave. It is only the act of trying to measure what it is that disengages the cloaking device and the particle or wave nature is revealed. Look for a particle, and you’ll find a particle. Look for a wave instead, and instead you’ll find a wave.

[Colin2B]

It is only the act of trying to measure what it is that disengages the cloaking device and the particle or wave nature is revealed. Look for a particle, and you’ll find a particle. Look for a wave instead, and instead you’ll find a wave.

Except there is no cloaking device, it just has to do with the interaction between the wave/particle and our measuring apparatus.
As has been said here many times, what we are detecting is neither a particle nor a wave, but can be modelled as either/both.

[elwishseven]

I'm going to briefly go over your questions about your understanding, then analyze the experiment from the video in detail, then explain why I think this experiment is not very interesting (since the video also tries to hype it).

[hamdani yusuf]

There was an experiment trying to determine which way photon goes through double slit experiment by "tagging" the slits using different orientation of polarizers. The result shows no apparent interference pattern. Quantum eraser basically works by rotating the orientation of light beam through one of the path so their polarizations are the same again, which gives interference pattern again.
I answered the same question in Quora.
https://www.quora.com/What-happens-in-a-double-slit-experiment-if-you-put-relatively-perpendicular-polarization-filters-on-the-slits/answer/Hamdani-Yusuf

Let’s say you put the vertical polarizer on the right slit while horizontal polarizer is put on the left slit. The screen still receive the result of superposition of light coming from both slit, with different phases according to their distances. Since two combining light sources have perpendicular polarization to each other, the combined light is generally elliptically polarized.

Different points on the screen will get circularly polarized light with various eccentricity. The eccentricity is 1 when the combining lights have the same phase (or multiple of 180⁰), and 0 when there is 90⁰ phase difference (or its odd multiple). So, when light coming from right and left slit have the same phase, the result is linearly polarized light with 45⁰ polarization angle. When there is 90⁰ phase difference, the result is circularly polarized light.

So if you observe the screen using an additional polarization filter in front of your eyes, you can see dark fringes when the light from some area on the screen is blocked by the polarizer. You can use either left or right circular polarizer, or use linear polarizer rotated at positive or negative 45⁰.

You can also get interference pattern by producing interleaving dark spots on the screen using additional polarization filter in front of the screen which will block the light on some area while allowing light from other area.

[hamdani yusuf]

All of physics is either impossible or trivial. It is impossible until you understand it, and then it becomes trivial.
Ernest Rutherford.

Extraordinary claims, such as that there is no objective reality or causality, requires extraordinary evidences.

If our experiments give unexpected results, we must have made one or more false assumptions. Most likely, they are implicit and taken for granted. So when we find ourselves in this situation, use Rand's razor and explicitly state all of assumptions we used, especially the hidden ones. Then scrutinize each one of them and see if its falsity can give us a better conclusion.