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.
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.
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.
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.
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.
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.
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.
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!
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.
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. :)
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.
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.
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.)
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.
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.
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.
The paper goes astray in my view right at the start, when it points out that time is a parameter in the Schrodinger equation, not an operator, so that equation gives no way to uniquely derive a probability distribution for measurement results as a function of time--but then fails to note the solution to that problem, which is to not use the Schrodinger equation in the first place. The correct framework in which to treat time on the same footing as other observables is relativistic quantum field theory. That is never even mentioned in this paper.
(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?
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.
A side note: Robert Koons at UT Austin has just published a book in which he examines the lesser known Aristotelian interpretation of QM. Here’s a short review of the book[0] (the book is quite cheap for the genre) and a lecture Koons gave last year.
Edward Feser published a review of Koon's book ([0], [1]). I was struck by the number of times Feser argued that a "common sense" view of QM is the one that is "basically correct." Given the documented failures of common sense in mathematics and physics (see especially Feynman's comments in the first 15 minutes of [2]), why should anyone think common sense is a reliable metric to how things are?
The article you provide a link to is the review I linked to (and that article mentions that Heisenberg was partial to the hylomorphist view of QM).
> 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.
No, common sense means we're using metaphors from a limited range of Earth-based everyday experience on a fairly cold planet where everything moves slowly to make predictions about physical phenomena.
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.
> 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.
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.
> 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.
This is incorrect. That's now how Pyrrhonism works. They have the same critiques of other skeptics.
I have no idea what a common sense interpretation of QM would be.
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.
Can someone in the physics community give us the consensus summary of whether this is serious/real/non-BS? Seems like this would be an absolutely earth-shattering theory result (especially since they claim it's verifiable with a feasible experiment), which seems hard to judge from outside based on the paper itself. Are people buying this, or are there questions?
It's not earth-shattering for theory, I can tell you that for sure. If you read the citations of the paper, you'll find that Das, Nöth and Dürr already proposed something similar in 2019 which was published in Phys. Rev. A, one of the most respected journals. What these authors are doing is basically to propose a refined (and possibly more general) experimental setup. Whether this actually works out in practice, or even represents a particularly good way to go about it, is beyond my fluency in this field; I would guess the practical effect is incremental at best.
It's earth-shattering, because the current prevailing opinion in physics is that some of those QM interpretations are indistinguishable in reality and that their difference is only a matter of philosophy.
“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.
If this paper is correct and we can experimentally verify different QM interpretations, then this is really groundbreaking stuff. I wonder why this news isn't everywhere.
I don't remember this being discussed at my alma mater. Not in lectures and not in colloquia. But then again, maybe it happened and I missed it. I don't want to throw shade.
Sean Carroll's many worlds is what I subscribe to.
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.
Yes agreed. I got lazy and didn't elaborate like I should have. I wanted to say Sean Caroll's talk on many worlds is the most coherent explanation I've seen of quantum mechanics so far.
I studied some QM on my own a while ago and while I could just do what the problems asked me to, I was really confused about how a quantum system is supposed to evolve assuming the state collapses.
Then I got to density matrices and understood what that "parallel universes" thing actually is... It removed a major source of confusion.
It would seem to me that the sheer amount of observation on the macro scales has the effect of freezing the quantum observables in space and time, giving rise to the common, agreed-upon, reality. Seems like a solution to a problem.
(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)
[...] freezing the quantum observables in space and time [...] also sounds like the quantum Zeno effect [1] which is kind of the opposite of decoherence [2].
I think the parent is using the terms here in a very loose way and it's not trying to refer to the quantum zeno effect but rather the emergence of the classical world.
Yes, "interesting". A cry for help from Sharif University, perhaps. Ayatollah Ali Akbar Rafsanjani gave up the ghost rather 'unexpectedly'. Prior to his sudden removal from the scene he had made remarks regarding Japan, Germany, and their ultimate relationship with USA. He was an eminently pragmatic ideologue. This internal contradiction clearly proved to have had adverse health consequences.
"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."
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.
High energy EM radiation is able to ionize atoms through the photoelectric effect. Cellphone radiation is unable to do this because it doesn't have enough energy. This is strictly the case because energy in these scales are quantized.
If you see something absurd in a HN comment, it's worthwhile to check their comment history. You aren't missing anything or miscomprehending - it's just a troll.
If you don’t like the theoretical explanation given in the sibling comment regarding energy carried by photons, try an experimental approach and get back to us with your results.
Oh god no. That video is very misleading and pretty much borders on pseudo-science.
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).
I'm not sure if detector is any better than observer. Isn't it the issue of when does it collapse/decide (also problematic terms) rather than the multiple options propagating into the detector? Observer get used because at least then someone is experiencing the decision. Many worlds punts on that option by making a new observer world each way.
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.
The video talks about shooting electrons rather than photons through the slits. Does it matter what kind of particle you shoot through? If I fired a stream of hydrogen atoms or neutrons through the slit, would I get an interference pattern?
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?
> Does it matter what kind of particle you shoot through? If I fired a stream of hydrogen atoms or neutrons through the slit, would I get an interference pattern?
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.
The effect applies to all particles, but it does matter what kind of particle you shoot because the sizes involved become very different. Apparently, the effect has been demonstrated even with molecules of 2000-ish atoms - https://www.nature.com/articles/s41567-019-0663-9
I recommend videos by Arvin Ash. He's one of the few who manages to explain things in a way that makes sense to me. A lot of Youtube videos either parrot the pop-sci explanations I get tired of reading in magazines or they're way above my head.
- 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.
"Plays into common misconceptions" is one of the worst properties that a video targeted at general audiences can have. This even happens if your video is technically correct but sounds sorta like the common misconception [1].
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.
IMO, detection/observation is the wrong framing of the problem. A better approach is about consistency. All interactions in a system must be consistent: if you have two Schrodinger's cat-boxes being controlled by the same measurement, then either both cats will be dead, or both will be alive, you will never find one dead and the other alive.
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.
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.
It is a detector, as is the wall. But you still don't know the state of that detector until you observe it, so now the photon+detector system is itself in superposition as far as you're concerned.
(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.)
Photons that interact with the edge go through a different process of absorption and re-emission + scattering (or not). They aren't the original photon.
This cartoon misled me until I realized that there is no observation without touching. Even "seeing" a particle implies that you bounced a photon off of it... and since we're talking about observing individual particles it makes sense that bouncing a photon off of it would have an effect on it.
I can't answer properly without seeing your video (again probably) but suspect that you and the video got flack because "how it's been observed to make light switch from 'wave' to 'particle'" puts its finger on the precise controversial point being debated here, and so restarts the debate in miniature, like a recursive dive.
That is about like saying photons can display quantum interpretations given that photons are the entities that interact with double slits in the first place.
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.
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?