[mh7]

It's probably a case of me not knowing what I don't know, but I've never understood what the big deal with 'observation' is in QM. I dabble a lot in electronics and I'm painfully aware of how measurements affect a circuit - try to measure current and you introduce a voltage drop, stick a probe somewhere and you add an antenna, add capacitance, etc.

So to me when trying to measure anything it seems so blatantly obvious that it has to change the outcome - you will interact in some way with the system to get any information out, and this will change the tracejctory, energy, momentum or whatever of the particles.

I mean is that it? That's the mystery?

[ko27]

If you read up on the Delayed-choice quantum eraser experiment [1] you'll see how your "simplistic" explanation leads to paradoxes such as the present altering events that occurred in the past. But if you describe unmeasured states as being in a superposition then there is no paradox.

[1] https://en.wikipedia.org/wiki/Delayed-choice_quantum_eraser

[prettyStandard]

Does anyone know what the Many World's interpretation of the delayed choice quantum eraser experiment is?

[lmm]

As with most things, under many worlds you barely need any interpretation. You either read the measurement and thereby entangle your own state with what was measured (putting yourself in a superposition of having seen one waveform or the other), or you don't and you see the interference pattern. There's not really anything to interpret or explain.

[russdill]

That there is no delayed choice or quantum eraser. The naming of the experiment is based on waveform collapse interpretations.

[zeroonetwothree]

If you allow hidden variables it’s also easily resolved ;)

[ko27]

Good thing we ruled out local hidden variables. https://en.wikipedia.org/wiki/Bell_test

[NoThisIsMe]

The local part is important. We haven't ruled out hidden variables. Bell himself preferred hidden variable interpretations.

[ko27]

Yes, I am aware of that, that's why I made sure to mention it. There is a world of difference between local and non-local theories. And in no way do non-local theories fit in any of the simple explanations the original poster had in mind.

> We haven't ruled out hidden variables

No, but we ruled out every non-local or non-real one. Which is a far more interesting achievement.

[mal-2]

Superdeterminism hasn't been ruled out yet.

[EliRivers]

In your examples interacting with a circuit, you affect what you're observing because you have physically changed it. You have poked it with a metal stick and you have physically changed the system you are observing.

With QM, it's NOT that you are poking it with a stick. It's NOT that you are physically interacting with the system and physically changing it because you've poked it with a stick (be it literally or metaphorically).

With QM, having knowledge of the system is what changes it. Observing it is what changes it; not some physical change you make to the system because you're poking it with a tool. The fact that it is observed is what changes it.

Having knowledge about the system changes it, no matter how you got that knowledge, even if you got that knowledge in a way that cannot possibly have physically affected the system.

[zeroonetwothree]

It’s not “knowledge” that changes it—this seems to give some mystical power to human minds. It’s that it interacts with the “outside world”, ie decoherence. Which isn’t very magical in itself.

[EliRivers]

By "knowledge", I include any kind of record. Anything. Something different in the universe. Even if nobody actually looked at it, even if it wasn't actually recorded. Not magical human minds indeed; just that the measurement happened. Information created. However we like to word it. English isn't a great language for this. I suspect no language really suits.

Which isn’t very magical in itself.

Well, it's also supremely magical. That's the whole weirdness of it all.

[zeroonetwothree]

I don’t think decoherence is magical, indeed it’s what happens every day all the time such that we never perceive quantum effects in our everyday lives!

[s0ulphire]

You do realise the definition is "beautiful or delightful in a way that seems removed from everyday life"...seems like you're set on defining "magical" as "related to druids" lol, when nobody is using the term in that way.

[rsp1984]

even if you got that knowledge in a way that cannot possibly have physically affected the system.

How can that be possible? Any examples?

[TheOtherHobbes]

https://en.wikipedia.org/wiki/Elitzur%E2%80%93Vaidman_bomb_t...

[dinkumthinkum]

No, it’s not like sticking a voltmeter on a circuit. The double slit experiment definitely does not show something obvious. If you look into it a bit more about the wave function collapse, you will see very surprising results, if you are not familiar with it. I never heard anyone say it was obvious when they learned it. :)

[changoplatanero]

Imagine you set up Schrodinger's cat experiment where you have a photon pass through a double slit and if it goes through the left slit then it electrocutes the cat in the box and if it goes through the right slit then the cat is spared. You set up the experiment and leave the box for two days and then come open it to "measure" if the cat is alive or dead. The mystery is that its hard to understand how performing the measurement of opening the box can change the outcome of seeing a healthy cat or one that's been dead for two days.

[djmips]

The cat thing isn't really used anymore. The cat isn't going to be in superposition. All the things that could have been in superposition will have already collapsed including the cascade of things that lead to a dead cat or an alive cat. A tree falls it makes sound. It doesn't need an conscious observer. They universe has plenty of 'observers' that are just plain matter.

[zeroonetwothree]

This example isn’t very compelling because we might as well say the cat died two days ago you just found out later.

[Nomentatus]

Except that that's precisely what some Copenhagen Interpretation guys actually wanted to say; that the collapse of superposition didn't happen two days ago. Hence S's large-living-object example. You want to be sensible, they (according S) didn't. Their "lack of sense" tends to force them to many-worlds views.

But Einstein and S's being sensible pushed them towards thinking entanglement couldn't be a real thing. Although their best default position is to toss up their hands and say that there's gotta be a non-local hidden variable we just haven't found yet. But there are no candidates, as yet. (Unless you like Bohm, I suppose.)

[SAI_Peregrinus]

Schrodinger's cat thought experiment was meant to argue against the collapse-based interpretations of QM.

[mongol]

I don't get it. At what point in this hypothetical experiment did the photon pass one of the slits? Two days before you open the box?

[changoplatanero]

Yes

[DontchaKnowit]

It very obviously does not change yhe outcome here right? This is an odd example

[changoplatanero]

The fact that checking on the cat is obviously unrelated to the path of the photon is what makes it an interesting example of how quantum mechanics is different from ordinary intuition.

[DontchaKnowit]

I see. Makes sense.

I still refuse to believe the unintuitive interpretations of QM. I am a show me stater, after all. (Only kind if joking)

[awb]

Einstein would have agreed with you

[oneoff786]

If an electron can get to the same outcome with equal probability it does so equally. Or rather we get the sample distribution weighted outcome of each.

If you put something in the middle that would need to physically change to experience either end state (screw measurements, imagine little closed doors), you can’t have gotten to the end by taking either path. It must be clear which path you’ve taken. So the creation of the physical paper trail means we get only the outcomes corresponding the possible pasts.

[bhattid]

What you're describing is called the "observer effect", which is different from the "measurement effect" that's used to describe the quantum mechanics problem. The misunderstanding is understandable though, because it's difficult to properly explain why 'observation' in quantum mechanics is so weird. What constitutes observation is a bit controversial, but you can more or less interpret it as taking a measurement - measuring voltage with a voltmeter, looking at something with your eyes, touching something, etc.

I feel like Schrodinger's cat is used as an example a lot for this, but imo it's a bad example because it doesn't properly distinguish between our classical intuition (the cat is either alive or dead) and the quantum interpretation (the cat is in a superposition between being alive and dead until observed). If I recall correctly, when Schrodinger originally proposed the thought-experiment, it was more of a jab against quantum theory, since the concept of a cat being in a superposition of being alive and dead sounds nonsensical (and probably is, since most would agree that a cat, or any conscious entity, measures things constantly).

Also, in case it's not clear, saying an object is in a superposition between X or Y does not mean that the object is either in a state of X or Y. I don't think there's an intuitive way to describe it without referencing some math. If you've taken some linear algebra, imagine that X and Y are linearly independent vectors in a vector space. Then classical mechanics says that an object can either be in state X or state Y. Quantum mechanics says that the object can be in X, Y, or a linear combination of the two vectors.

To work with something concrete, let's say that our object is an electron and X is spin-up and Y is spin-down (disclaimer: spin is bad name since they don't correspond physically to something spinning). I'm hoping this might be familiar to you since you like electronics, but let's just say that we've created a context where these are the only two states the electron is ever observed in.

In the classical interpretation, the electron is only ever in a spin-up or spin-down position, regardless of whether we're observing it or not. In the quantum interpretation, it's possible for the electron to be in a superposition of spin-up and spin-down when we're not observing it, and when we observe it, it "collapses" into either spin-up or spin-down. Put this way, it sounds like cheating; quantum mechanics is saying we can only observe spin-up or spin-down anyway, so what's the difference! Well, fortunately, there ARE experiments that can distinguish between the classical and quantum based on what they're doing 'behind the scenes' when we're not observing them.

Imagine now that we have photons of light. Instead of spin-down and spin-up, these photons are either horizontally polarized or vertically polarized. The experiment I'm about to explain would also work for the electron example above, but I'm only switching to photons since I know experiments for this have been performed (https://en.wikipedia.org/w/index.php?title=GHZ_experiment&ol...).

Suppose that we've entangled three photons of light together. If you're unfamiliar with entanglement, it just means that we've produced the photons in such a way that they're either all horizontally polarized or all vertically polarized. We can confirm this by using a horizontal polarizer (or vertical polarizer if you prefer). Whenever we shoot the horizontal polarizer with the three photons, they either all go through or none of them go through. Maybe we switch the horizontal polarizer with a vertical polarizer just to be sure, and indeed, we observe the exact same thing happen. Right now, the classical and quantum interpretations agree that this is what we should observe.

Now let's do something that sounds a bit silly. Horizontal and vertical polarization aren't absolute things, they're relative. What this means is that we're testing for polarization at angles, say 0 degrees and 90 degrees. This also means we can rotate our polarizer to a 45 degree angle.

Just for fun, let's say we shoot our three polarized photons through the polarizer which is now at a 45 degree angle. If you're thinking classically, you might think that maybe all will go through or all won't go through. Maybe some will go through sometimes and others will go through other times (probabilistic).

The standard classical interpretation says that you'll observe either: 1. All three photons go through. 2. None of the photons go through.

This is where the classical and quantum disagree. The quantum interpretation also says you'll observe one of two scenarios as well, but those scenarios are: 1. Two photons will pass through, one photon will not 2. One photon will pass through, two photons will not

And lo and behold, experiments show (within experimental error) that the quantum interpretation is correct!

There's still plenty of room for disagreement. Maybe you or someone might argue that the photons are interacting with each other or something funny is going on with the polarizer in question. However, we still observe results aligned with the quantum interpretation regardless if we use different polarizers for each photon, have them sent on a delay, or so on (although, I don't know how many variations have been tested by others for this specific experiment).

Hopefully, I haven't been much of a bore, or wasn't overly confusing. :)

There are ways to "save" classical mechanics using non-local hidden variables and other fancy things, but (if you can take my word for it) at that point, classical mechanics starts losing its intuition anyway. I'm not very knowledgeable about these alternate theories of classical mechanics, but my impression is that they don't make strong predictions, which I'm guessing is why quantum mechanics is more heavily favored.

If you're interested in reading more on the topic, an experiment related to Bell's Inequality was a major piece of evidence in favor of the quantum model. It's similar to the GHZ experiment I described, but simpler. The tradeoff is that its predicted result is inherently probabilistic.