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Comment by semi-extrinsic | Hacker News Reader

This is a Hard Problem, to define what process is a "measurement" and what is not. But if we think about it very simply: a photon hitting a detector (or analog film strip) can be a measurement.

Since this process converts quantum information to classical information (a digital or analog signal), we know that it must lose information. Only by making many measurements can we infer statistical properties of the entire quantum state. This is exactly what happens with the two-slit experiment.

I personally subscribe to the "shut up and calculate" interpretation of QM (or perhaps the projection-valued operator approach when I'm feeling mathematical). The above intuition has served me well so far.

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> Since this process converts quantum information to classical information (a digital or analog signal), we know that it must lose information. Only by making many measurements can we infer statistical properties of the entire quantum state. This is exactly what happens with the two-slit experiment.

Wow, I've never heard it explained that way. That's brilliant - did you come up with it? Interference patterns are the result of a huge number of individual photons interfering with each other at an enormous rate, and the result are laid out spatially...really lovely.

> Interference patterns are the result of a huge number of individual photons interfering with each other

It's stranger than that. A single photon can interfere with itself, see for example: https://en.wikipedia.org/wiki/Double-slit_experiment#Interfe...

Are you still comfortable with this explanation if the photons are released with a 1-second gap between them and the tally is visibly shown building up into the interference effect on a computer screen? "Interfering with each other at an enormous rate" might mislead you a bit; neither "each other" nor "enormous rate" is particularly important in the double-slit interference pattern.

Actually, you can't know this until you get a lot of measurements to see the pattern. E.g. if you just put one photon through you get one dot, and you'd be hard pressed to infer anything from that. So in the end you still need a lot of measurements to detect interference.

Also I bet you could mess with the interference pattern by pulsing the light source. This would be the lesser known form of the Uncertainty principle, where energy and time are unknowable.

I can't say I can claim any credit for the idea, no, this is what is you learn once you chase the QM rabbit hole deep enough. But it's maybe not a common explanation in Intro to QM class or in popular science writing?

Actually (technical nitpick) it's even more lovely than what you describe: people have done the two slit experiment with verifiably single-photon sources... and you still get interference patterns. Really, we know that only a single photon exists between source and detector for the entire lifetime of the photon, and yet you get interference. The only explanation is that it interferes with itself, which is what QM says happens.

The original "Wow! QM!" two slit experiments were done with electrons, which were thought of as competely particle-like, whereas light was thought of as completely wave-light. QM of course says both are both.

I've been struggling to understand double-slit.

I had thought the pattern represented interference between the probability distributions of where the photon might be found (which themselves follow "wavelength" and produce wave-like behavior). With two slits you get two distinct probability distributions that overlap spatially at the screen, which when added together cause denser/less-dense areas of photon detection and looks like an interference pattern.

That would have explained why streams of single photons "self-interfere" (they wouldn't--it's the underlying probability of their path that interferes with itself, so the pattern would eventually appear at any emission rate) but that seems so simplistic as to be unlikely to be the truth of the matter, especially with delayed choice and quantum erasure effects.

Do you happen to know a good layman's source for a solid explanation of what we know about double slit right now? I'd really like to understand it, if only at a high level.

> QM of course says both are both.

No, they're both something else entirely that if you look at them the right way could look like they share properties with either.

> this process converts quantum information to classical information

This view fails to predict the outcome of the quantum eraser experiment.

https://en.wikipedia.org/wiki/Quantum_eraser_experiment

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