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Comment by ggm | Hacker News Reader

I am sure others will say it better, but the cat-in-the-box experiment is a shockingly bad metaphor for the idea behind quantum states and observer effect.

I will commit the first sin, by declaring without fear of contradiction the cat actually IS either alive or dead. it is not in a superposition of states. What is unknown is our knowledge of the state, and what collapses is that uncertainty.

If you shift this to the particle, not the cat, what changes? because if very much changes, my first comment about the unsuitability of the metaphor is upheld, and if very little changes, my comment has been disproven.

It would be clear I am neither a physicist nor a logician.

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Well you are in luck because that was the point of Schroedingers cat; it was constructed to show the impossibly odd implications of quantum mechanics.

From the wikipedia page: “This thought experiment was devised by physicist Erwin Schrödinger in 1935 in a discussion with Albert Einstein to illustrate what Schrödinger saw as the problems of Niels Bohr and Werner Heisenberg's philosophical views on quantum mechanics.”

There are various theories about what's actually happening in quantum mechanics. Some theories have hidden variables, in which case the issue is simply one of measurement (i.e. there really is an "objectively correct" value, but it only looks to us like there isn't).[0] However, this is not known to be the case, and many theories really do claim that position and momentum fundamentally cannot both be well-defined at once. (The "default" Copenhagen interpretation is in the latter camp; AFAIK it's convenient in practice, and as a result it's implicitly assumed in introductory QM classes.)

[0] Well, and the hidden variables are non-local, which is a whole 'nother can of highly non-intuitive worms.

I'm not qualified to say. But, because of inductive reasoning, I have some concern that underneath the next level of "oooh we found the hidden variable" will be a Feynman moment of saying "yea, thats defined by the as-yet unproven hidden-hidden variables, about which much conjecture is being made but no objective evidence exists, but if you fund this very large machine...."

Along similar lines, the double-slit experiment, seems simple. Two slits let light though and you get bands where they constructively or destructively interfere, just like waves.

However I still find it crazy that when you slow down the laser and one photon at a time goes through either slit you still get the bands. Which begs the question, what exactly is it constructively or destructively interfering with?

Still seems like there's much to be learned about the quantum world, gravity, and things like dark energy vs MOND.

I had a conversation about this in HN some months back. It's a surprisingly modern experiment. It demanded an ability to reliably emit single photons. Young's theory may be 1800 but single photon emission is 1970-80.

(This is what I was told, exploring my belief it's always been fringes in streams of photons not emerging over repeated applications of single photons and I was wrong)

To get single photons, you just need to stack up enough stained glass infront of a light source. That's been acheivable for aeons (the photon will go through at random time though).

The difficult part is single photon _detectors_, they're the key technology to explore the single-photon version of Young's experiment (which originally showed that light has wave-like properties).

The most simple answer here is the "fields are real, particles are excitation patterns of fields." And that's generally the practical way most physicists think of it today as I understand it.

If I make the equivalent of a double slit experiment in a swimming pool, then generate a vortex that propagates towards my plywood slits or whatever, it's not really surprising that the extended volume of the vortex interacts with both slots even though it looks like a singular "particle."

And yet if you place a detector at the slits to know which slit the single photon goes through, you get no interference pattern at the end.

> However I still find it crazy that when you slow down the laser and one photon at a time goes through either slit you still get the bands.

why does nobody mention the fact the photon doesnt keep going through the same hole? like why is it randomly moving through the air in this brownian way? the laser gun doesnt move, the slit doesnt move, so why do different photons end up going through different holes?

> What is unknown is our knowledge of the state, and what collapses is that uncertainty.

Unless you believe in a hidden-variables theory, this is provably false, though, see Bertlmann's Socks etc.

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