Yes, effectively the photon goes through both slits, and the interference pattern or ballistic pattern relates to when decoherence occurs, either at the point a which slit measurement is made or at the point it hits the detector.
So the double slit experiment was a starting point, and there’s been a bunch of variations testing different aspects.
One of which is that we can ‘tag’ the photons that go through path A with an indication it went through path A and tag photons passing through path B accordingly.
As would be expected, when tagged this way both result in ballistic patterns as if particles and not waves.
But the neat part is that if you add a polarizer after they are tagged which removes any way of recovering the tagging information about which path they went through, the interference pattern comes back and they behave like waves again.
If the explanation for why it goes from a wave to a particle in the first place is something like “it was disturbed by the act of measurement”, adding additional disturbance would seem like the last thing to get it back to behaving like a wave, right?
The variation suggests that the collapse of the wave function relates to the continued existence of recoverable information about the photon, not necessarily the physical mechanics of its measurement at that instant in time.
As for the other comments I made, the TL;DR is that there’s easily a dozen different interpretations of why quantum weirdness occurs among physicists, and so very often when you see someone saying “this is how it works” what you are really seeing is “this is how it works in the theory I subscribe to” but a different physicist might have a very different explanation.
The only explanation/representation that everyone can agree on is the mathematical representation, but translating the math into a physical reality is still very much disputed from physicist to physicist.
So for example, the Bohemian mechanics view would have disagreed with the idea that the probabilistic nature of the photon before measurement is a physical reality, instead claiming that it is just a reflection of what we can know about the photon, and that it already physically is a point in spacetime that’s being guided by a wave, which is why it has wave-like behavior. But don’t worry too much about it - just keep in mind it’s worth taking any online explanations of why quanta behave in a certain way with a giant grain of salt as it’s not a settled topic.
So how do you explain the quantum eraser experiment (note for those who might be excited to respond with recent YouTube physics videos, this is not the delayed choice quantum eraser, and has nothing to do with retrocausality claims)?
Also, their interpretation of what’s happening largely falls apart with the quantum eraser variations.
If it’s collapse from mechanical measurement side effects, why does it go back to an interference pattern when which path information is erased by a polarizer?
One of the key points that the “it’s just mechanical interactions, bro” crowd should be more aware of though are the quantum eraser variations (not the delayed choice quantum eraser).
There is still something rather bizarre about mechanical interactions that measure which path information being sufficient to collapse on their own but suddenly insufficient when something like polarization which erases which path information is added back in later in the chain.
Also, it’s worth declaring when giving an answer like this that you are operating under the assumptions of QFT, and that this isn’t necessarily for sure what’s going on. For example, I’d imagine there’s Bohemian mechanics adherents still around somewhere that would take issue with your “it doesn’t exist as a single point in spacetime until it’s measured.”
It’s straight up some of the ways in which various accredited physicists were explaining how and why it does the weird things it does.
Von Neuman arguably started it by correctly pointing out that the collapse could be taking place anywhere between the measurement device to the subjective perception of that measurement.
The latter boundary was favored at the time by people like Fritz London, a five times Nobel nominee.
Thinking outside the box and from all different angles to try and understand counterintuitive experimental results.
Some of those theories have since been extrapolated from by popsci and new age circles to claim ridiculous things, but the existence of “quantum stickers” to cure your ills doesn’t mean Dirac and Schrodinger were crackpots, and so neither does someone claiming “The Secret” like powers based on quantum theory mean that folks like Wigner or Penrose are conspiracy theorists.
It’s a legitimate interpretation with a number of very experienced physicists in favor of it over the years, even if not a popular one.
While it doesn’t address the topic of consciousness, you might find some of how this sort of “backwards in time change” is being applied today interesting:
Which led to what’s currently my favorite titled paper, Stable facts, relative facts: arxiv.org/abs/2006.15543
So one of the challenges that would arise from layers of delayed/hidden observations would be whether you’d even have universal agreement at the final review. i.e. The computer might have observed the cat as alive and baked a cake celebrating it, but then you open the box to a dead cat, each having correctly observed a result, just separated enough that they didn’t need to agree.
Again, the theory would be that collapse (including the state of what is on the paper) occurs upon review of the paper.
Consciousness collapse theories are particularly interesting in the context of the quantum eraser variations of the double slit experiment.
Personally my favorite interpretations ever since reading the Asking photons where they’ve been paper have been ones incorporating forward and backwards wave functions like the two-state vector formalism or the transactional interpretation.
It’s thought provoking to look at experimental results under different interpretation contexts, and is one of the things that frustrating in people thinking there’s merit to trying to “pick a team.”
Not everything needs to be a team sport, and a variety of interpretations tends to be a good thing as each prompts different types of experiments by their various supporters.
Which is why QM interpretations are considered to be part of Physics philosophy as you can see the link to the weighty writeup on the Copenhagen Interpretation is part of Stanford’s Encyclopedia of Philosophy.
But all interpretations are part of philosophy and are currently not falsifiable. Not just the ones someone may not like.
When which slit a photon goes through is unobserved, it behaves like a wave and self interferes so many photons create an interference pattern with stripes where self-interactions prevented any photons from appearing.
When the photon is interacted with in a way which leaves permanent information about which slit it went through, it behaves like a particle and the pattern from many photons looks ‘ballistic’ like you were shooting tiny balls through each slit.
So in the meme when he’s not looking at the slits, there’s stripes, and when he’s looking it’s a ballistic pattern.
There’s actually no way to prove or disprove consciousness collapse theories, as even if an unmonitored detector causes collapse, you only know about it when a consciousness is reviewing the data. So at best it can be said that direct consciousness collapse theories aren’t true, but AFAIK the ones still around are all indirect (i.e. collapse occurs at the point you are reviewing the data).
We could similarly talk about the “woo woo” of multiverse theories and how there’s no proof for Everett’s interpretation (despite being one of the few popular theories not to need an invalidation of an assumption in the Frauchiger-Renner paradox).
But no proof doesn’t equal “not true.”
All QM interpretations are up in the air, and an appeal to Copenhagen interpretation is probably one of the most nonsensical given a specific interpretation doesn’t even exist for that one and it’s effectively just become euphemistic for “shut up and calculate.”
Do you include the 1804 massacres of the French with the mass rape of women and killing of children by Dessalines which followed the Haitian revolt in that intrinsic good?
The real issue at hand is whether or not we’re talking about moral relativism or absolutism.
If we are endorsing relativism, then all actions have a relative frame of reference by which they are justified (i.e. Bob’s killing Jane).
My stance is that in terms of absolutism, there is no such thing as justified mass violence, and that while it is certainly possible for mass violence to be a lesser evil absolutely, and thus easily argued as a moral good relative to the alternative, that ultimately it remains an evil under all circumstances objectively, and at best can be a lesser evil regarded absolutely.