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Comment by z92 | Hacker News Reader

Actually the mysteries are deeper. This one can't explain "Delayed Choice Quantum Erasure" experiments, which is also a double slit experiment. But the arrangement allows you to detect the path AFTER the photon has traveled through one of the slits and hit the surface. Now after the photon has hit the surface you can decide whether to detect its path or not. A delayed choice. But based on your later choice the interference pattern of the photon from a time from the past appears, or doesn't.

This is the main reason why some scientists come up with many world interpretation. This article doesn't even touch the surface of it.

https://en.wikipedia.org/wiki/Delayed-choice_quantum_eraser

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Indeed, a quick video from space time for those being first introduced. It’ll likely make you even more confused to the double slit experiment.

https://youtu.be/8ORLN_KwAgs

> allows you to detect the path AFTER the photon has traveled through

It is NOT AFTER: "Why delayed choice experiments do Not imply retrocausality"

https://link.springer.com/content/pdf/10.1007%2Fs40509-014-0...

tl;dr: the detector collapses the state, detection does not mean both paths were not "taken". This is the same as detecting a photon on a screen after a double slit.

Disclaimer: I only skim QM stuff, I am not a physicist

Question: what's the name for this experiment where light is interfering with a "slower" beam of itself (legend: ⫽ are beam splitters, \ and / are mirrors, arrows are photon):

  → ⫽ → ⫽ → detector

    ↓   ↑

    \ → /

How can a "thought experiment" demonstrate anything?

The same way a logical argument or a mathematical proof can demonstrate anything.

It just can't prove it empirically -- but it can very well demonstrate it.

Your question is also a "though experiment" (it's pure thought) and yet you aim it to demonstrate something.

Common example is dropping two balls of different mass connected by a chain off a tower.

The point is not that is demonstrates something empirically, which of course it doesn't. But it does say something about which hypotheses make sense within whatever assumptions you're making.

All mathematical proofs are thought experiments, for starters. Similarly, all predictive output of scientific models is technically thought experiments, although we nowadays use computers to achieve superhuman precision and accuracy.

Well, from the article on the dual-slit experiment ...

> What many don’t realize is that the double slit experiment (with particles), proposed by Feynman in 1963, was for decades only a thought experiment. Finally, in 2013, it was successfully performed with electrons.

The author implied it was a thought experiment for "particles" like electron, though actually demonstrated from the very beginning for non-particles such as photons.

That claim still might not be true.

The author is mistaken: https://news.ycombinator.com/item?id=21363496

The quantum Erasure doesn't have anything to do with wave particle duality imo. They are purely testing the effects of entanglement and choice.

I see entanglement differently than most people.

They tend to believe it's a state of identically positioned particles acting in unison. To quote the wiki "that converts the photon (from either slit) into two identical, orthogonally polarized entangled photons with 1/2 the frequency of the original photon"

But entanglement and super position are all about balance. Too much interference and your state is lost.

Our galaxies and solar systems are macro representations of entanglement. I mean is got to be pretty obvious. We are essentially using the same technology to look at both.

So if entanglement is all about balance, no shit whatever you measure on one will be the opposite for the other.

There are two sides to a coin.

We are messing up where the axiom of choice is taking place. It always happens at the point of entanglement. Based on that choice every step from there is deterministic "in theory".

Not even wrong.

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