Can the double-slit experiment distinguish between quantum interpretations?
arxiv.org
arxiv.org
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?
I still refuse to believe the unintuitive interpretations of QM. I am a show me stater, after all. (Only kind if joking)
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.)
[1] https://en.wikipedia.org/wiki/Delayed-choice_quantum_eraser
> 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.
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.
How can that be possible? Any examples?
Which isn’t very magical in itself.
Well, it's also supremely magical. That's the whole weirdness of it all.
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.
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.
(To be fair, I have never seen any discussion or comparison of QM interpretations that uses QFT as its framework; they all use non-relativistic QM, even though we know that's just an approximation. But it's still an issue even if it's an extremely common one.)
Can these other equations be derived from the Schrödinger equation using a suitable Hamiltonian?
And do you mean that you need time to be in the Hamiltonian as an observable to satisfy Lorentz invariance?
No, it's the other way around: given a quantum field theory, under certain conditions (the main one I'm aware of is choosing a particular frame and taking the non-relativistic limit), you can derive a Schrodinger equation.
> And do you mean that you need time to be in the Hamiltonian as an observable to satisfy Lorentz invariance?
I mean that a fully relativistically correct treatment needs to treat time and space on the same footing. In quantum field theory that is usually done by making the quantum fields local operators, i.e., a quantum field is a mapping of points in spacetime to operators. In the simplest case, a free scalar field, these are the creation and annihilation operators of an infinite set of harmonic oscillators.
[0] https://www.thepublicdiscourse.com/2023/01/86512/
[1] https://edwardfeser.blogspot.com/2023/01/koons-on-aristotle-...
> Given the documented failures of common sense in mathematics and physics [...], why should anyone think common sense is a reliable metric to how things are?
I think the most serious objection is that categorical dismissal of common sense is a form of skepticism, and as a consequence, you undermine the very claims you are appealing to. All science takes place within a context and that context is going to be common sense, ultimately; the alternative is some truncation or corruption of it. So you might as well own it and own it to the fullest. All skepticisms suffer from the same problem, namely, the strange belief that you can know something while undermining the very conditions possibility of knowing it.
Note that by "common sense", we mostly mean that we take the human apprehension of the world as basically accurate, even if it is fuzzy around the edges or needs correction or refinement [0]. So at the very least, I think that the presumption is in favor of common sense. Your question does not provide a reason for doubting common sense categorically or even rejecting common sense interpretations of QM. It is a better idea to engage with the proposed interpretation, to understand it, and make specific criticisms instead.
[0] https://www.firstthings.com/web-exclusives/2012/10/aristotle...
This is incorrect. That's now how Pyrrhonism works. They have the same critiques of other skeptics.
I think these conditions are overstated. The human brain is a paraconsistent reasoning machine: it is made to be robust to inconsistency and contradiction. I think it is obvious that there exist logical contradictions in the belief systems of every human being, and it is equally obvious that we can reason productively in spite of them, so is it really that big of a deal?
It is not clear to me that we ought to believe something merely to avoid an inconsistency. If our common sense is indeed error-ridden, I would argue that it is ultimately better to accept the skeptic position and let our brains deal with the internal inconsistency than to accept a falsehood merely to preserve consistency.
In science, common sense has been wrong so consistently its wrongness is practically empirical.
QM, relativity, thermodynamics, gravity, astrophysics, electromagnetism, and math itself are profoundly and consistently unintuitive - to the extent that if someone starts a claim about reality with "Well, obviously..." you can pretty much bet they're wrong.
I'm having trouble locating what I'm thinking of but I thought it had been established that at least one of three very non-common-sense interpretations of QM had to hold at this point.
It seems to pass a first scan here.
“On the one hand, according to the (generalized) standard canonical interpretation, the arrival distribution is considered as a generalized observable, which is described by a positive-operator-valued measure (POVM), satisfying some required symmetries [10, 11, 30, 31]. On the other hand, in the realistic- trajectory-based formulations of quantum theory, such as the Bohmian mechanics [32], Nelson stochastic mechanics [33], and many interacting worlds interpretation [34], the arrival time distribution could be obtained from particles trajectories [7, 18, 35, 36].”
I’d be interested to hear a definition of each of those interpretations.
Or maybe I should have stuck around to move the science along. I didn’t have the grit for that
And I’m not saying we should be having philosophical discussions.
But pretending that there isn’t a problem with the copenhagen interpretation is also an insult to the students intellect.
What I like is that the first use was ironic, as he continues "But I won't."
In a nutshell, you are a quantum system as well, when two quantum systems interact, then you get the wavefunction collapse, but it doesn't collapse, it just appears that way.
Then I got to density matrices and understood what that "parallel universes" thing actually is... It removed a major source of confusion.
[edited for spelling errors]
(Observation can also literally freeze the state of a system, via the Quantum Zeno Effect https://en.wikipedia.org/wiki/Quantum_Zeno_effect but that's based on observing the same system over and over again, not different systems interacting)
https://www.theguardian.com/world/2018/jan/09/iran-reopens-i...
"Today, you can see that Germany and Japan have the strongest economies in the world. These same two countries were prohibited from having military forces after the Second World War. When a country is at war, it spends so much money on its military. With no military spending, these countries could use that extra money on science and production and were able to create a science-based economy for themselves. As a result, they are no longer fragile. The door has been opened to a similar process in Iran."
https://www.al-monitor.com/originals/2016/09/ayatollah-hashe...
Yes, you are correct to read that and understand the Ayatollah Rafsanjani was explaining the benefits for Islamic Republic to throw in the towel and join the ranks of former enemies now prosperous allies of US: Germany and Japan.
https://www.newyorker.com/news/news-desk/rafsanjani-irans-wi...
Cartoon by "Dr Quantum"
For starters it depicts photons as balls, when it would be much less confusing and more intuitive to instead think of them as waves with discreet energy amounts. If you think about it as a wave, what's happening is a wave with a photon-amount of energy passes through both slits which causes an interference pattern. When it hits the wall it coalesces into a single particle/dot.
Also, it talks about 'observer' and strongly indicates the observer needs to be a conscious entity. Instead of observer, just use the word "detector", and bear in mind that detector interacts with the photon (whereas at the classical level you can watch or observe something without affecting the system).
The difficulty is most video are either way to technical or way too high-level.
I’ve also always wondered if it matters what side of the slits the detector is on? The experiment as depicted in that video has the detector on the gun side of the slits. Would you get the same thing if you moved it to the wall side of the slits? Also, does the wall itself count as a detector/observer?
Everything has a de Broglie wavelength, but the more massive it is the shorter the wavelength, so you'd need closer slits and higher-resolution detectors to notice anything. I guess if you used a really big particle then you'd reach a point where it would be bigger than its own wavelength and you couldn't observe any wave behaviour.
> I’ve also always wondered if it matters what side of the slits the detector is on?
No, the important thing is observing which path it takes
> Also, does the wall itself count as a detector/observer?
Not unless there's a way for you to tell when and where it hits the wall.
A finitely bounded wave is better described as a wavelet.
https://en.wikipedia.org/wiki/Wavelet
Particles are the wavelets that when summed, give us the field as a whole, ie, particles are the wavelet transform of a field.
I'm a layperson who reads a bit so I could be misinterpreting. I was always partial to the idea of probabilistic squeezing rather than collapse. In the absence of quantum gravity, probabilities can propagate only as far as their gravitational effects allow. As particles interact the compatible possible positions reduces to eventually squeeze the location to an infinitesimal size. It may be utterly wrong as an idea but it would negate the need for observers, detectors, or many worlds which makes it a bit compelling.
- It shows the single-slit case as a vertical band. Actually, the single slit case is also a smear[1][2]. A smear just a wide as the double slit pattern (but less rippled). Photons don't switch between moving like bullets and moving like waves, they consistently move like waves and consistently hit like bullets.
- It shows the photon detector as an eye off to the side of the experiment. You gotta stick it in the beam path for it to interact with the photon. You can't see photon flying by in front of you, you can only see photons bounced into your eye. Note that I'm not saying the detector has to absorb the photon, but it does have to interact with the photon and it can't interact with the photon if it's way off to the side like that.
Ultimately, it's playing into the common misunderstanding that a human glancing towards a quantum experiment has some profound effect upon it.
1: https://www.youtube.com/watch?v=h53PCmEMAGo
2: http://backreaction.blogspot.com/2021/10/the-delayed-choice-...
In an experimental setup, you might have many different levels of interaction: the photon either hits one sensor or another, that results in one or another message being sent to your recording device, which records one or the other result on your harddrive, etc. As the scale goes up, more and more stuff gets dependent on the result of your measurement, until eventually everything is, yourself included. And if everything is dependent on the result of your experiment, they must all be consistent with a given result of your experiment. Thus, you end up with exactly one answer.
Since the slit edge isn't changed by the diffraction of light, there is no inconsistency between whether the photon passed through its slit or the other.
(And if you observe it, but nobody else observes you, then you are in a superposition as far as the rest of the world is concerned.)
You can think of the barrier with two slits as a device that measures (a) whether the photon hit the barrier or missed barrier and additionally (b) when the photon hits the barrier, where did it hit the barrier. Notably, if the photon misses the barrier, it doesn't tell you where it missed. This is why, when the photon doesn't hit the barrier, it can end up in a superposition of passing through both slits.
There's some classic thought experiment, I think it's [1], thinking about an atom decaying into a superposition expanding in a spherical shape, and how you can make barriers to shape the outgoing superposition. For example, to make an expanding half sphere, block off the other half sphere and focus on the times the decay didn't hit the barrier.
1: https://en.wikipedia.org/wiki/Renninger_negative-result_expe...
I still believe the video I linked to is the best for a general audience.
I'm not sure someone without prior knowledge would follow that video.
Someone who watched the Dr. Quantum video has learned a bunch of keywords and phrases associated with quantum mechanics. They've now heard of waves and particles and detectors and whatnot. But they have been actively misled with respect to how those things behave. I bet it's possible to put together a reasonable looking test where watching the Dr Quantum video caused scores to go down.
She never discusses the detector portion of the experiment and how it's been observed to make light switch from "wave" to "particle".
Is that not accurate in the video I linked?
Or event to get more eccentric, your ocular lobes can interpret quantum because they receive photon interactions with the rod cells.
Just pointing out the obvious problems with tongue-in-cheek.