A place for majestic STEMLORD peacocking, as well as memes about the realities of working in a lab.
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This sort of comic always bugs me. Observation in QM is not the same as observation in layman terms.
Best think of it as hit it and watch the pieces fly. When you get small enough, you can't approximate out the impacts. It's akin to studying road traffic by sending an overloaded freight truck the wrong way and counting tires that hit the verge. It might also affect the current traffic's motion.
This is something that I noticed is hard to understand to most people. My SO got served a video of the double slit experiment and thought it was like magic, until I tried to explain to him that at this scale, "observing" doesn't just mean looking at it. Observation makes you part of the system and causes the system to change.
That is part of what bugs me.
Quantum mechanics isn't magical or unknowable. It's just an area of physics where some of our base assumptions/approximations break down. It's not even that hard to wrap your head around, it just seems most people don't want to try.
Quantum mechanics is an area of physics that leans on measurement as a central concept. What things are doing between measurements is not its' concern, and don't even stop and think about it, you'll go nuts, just shut up and calculate!
There is a magic to perception nothing else can replicate. I'm pretty sure awareness is existence, so it's attention has to change reality.
Perception and observation are different things. Air molecules can be "observers" when looking at electrons etc.
"Observation" in a QM sense has nothing to do with sapience of anything like that. It literally just means that something needed the information and it collapses the waveform. It's any time it influences another particle. It doesn't matter what that particle is. If it needs to know the state or the particle we care about then it "observed" it. Humans or animals need not be involved.
Tbh, I think no one who hasn’t solved the Schrödinger equation at least once (at least time independent), should be allowed to talk about quantum.
Like, the uncertainty principle is really really fucking cool when you understand why it works mathematically. But without differential equations and linear algebra, I don’t think it’s possible to really conceptualize what’s going on in quantum.
Idk, I always try to explain to students the deficiencies of the Bohr model and explain the significance of the electron cloud, but probability is hard.
One of my favorite things in quantum was deriving the “quantum numbers” they have you memorize in chemistry (if you don’t remember, you probably got a SPeeDy F) It’s beautiful to watch the way they emerge from the second order diff eq.
I don't think solving the Schrodinger equation really gives you a good idea of why quantum mechanics is even interesting. You also shouldstudy very specific applications of it where it yields counterintuitive outcomes to see why it is interesting, such as in the GHZ experiment.
While I'm rusty as hell, my physics degree was actually focused quite a lot into QM.
It's perfectly possible to get a reasonable understanding of what's going on without going head first into the maths. There are definitely areas however that we don't have a good conceptual model of yet. For those, the maths definitely leads the way. 90% of QM is comprehendible with relatively little maths. You only need the maths when you start to get predictive.
I don’t think you can get the intuitive feel/the “why” without the maths.
I guess I get frustrated when I have to teach algebra based introductory physics for similar reasons - everything makes so much more sense when you understand how the pieces fit together. (Why make them memorize d=d0+v0t+1/2at^2 when all that is integrating a constant twice? That you can set v=0 to find the time of maximum height, because you’re using a derivative to find a max! And then that helps you get why it works, and then even how to possibly explore non constant acceleration!)
I got really fucked over because I didn’t take linear (at all - advising in my physics department was non existent which lead to things like taking classical before Diff Eq lol) and so things like eigenvalues - which tbh I think is kinda the money shot - that things end up quantized and discrete - that took a while for me to get what that meant.
I find QM quite confusing, in that one can observe only the eigenvalues and not the state itself. Why is it specifically, or is this wrong conceptualization? Also, how does particle-ness relate to the eigenvalues?
Eigenvalues come from linear algebra. I think a difficult think in general with understanding them is often the failure of most middle/high school math teachers to teach matrix operations at all. (I’m guessing because matrix multiplication never shows up on SAT/ACT). Here’s a good explanation for the math on finding eigenvalues and eigenvectors.
But basically eigenvalues are going to be associated with certain matrixes/vectors. You take a “Hamiltonian” of a system, which is a way of describing possible energy values in the system, and it’ll give you a set of possible answers - pairs of eigenvalues and eigenvectors that describe the system.
In effect - you get things like the quantum numbers. That the 1st energy level has 1 subshell can hold 2 electrons, both with opposing spins. That the 2nd energy level has a 2s subshell that holds two, that 2p holds six. You get your n (1st energy level, 2nd so on as you go down periods of the periodic table), l (subshell - don’t get a SPeeDy F), m (which breaks down where in the subshell they are) and the need for opposing spins.
Thank you for in-depth explanation! Though I already know the eigenvalues and eigenvectors, as a math major. What I am curious of is: why can't we only observe e.g. energy values? I heard that one can only observe commutative operators or something, but honestly why is quite unclear.
I’ll try to dig out Griffith for a better explanation but has to do with the fact that when you do a partial derivative you kinda lose information I guess?
(Idk, this is heady trying to make math into reality shit and I got a “c” in the class (for reasons partially related to other things) - also, there might be a way to do latex in markdown but I’m a bit too stoned to figure out, look up Schrödinger equation on wiki for maybe a helpful visual aid)
So go back how often we do implicit differential because it’s just an opportunity to look at how sexy the chain rule [edit: product rule, you get me though. math is easier sober] is. d(xy)/dx = xy’+x’y god fucking dammit that gorgeous
But okay. Think about position and velocity. Velocity is the derivative of position right (and also connected to energy - KE = 1/2mv^2 and E = mc^2 lol)
But since velocity is a derivative of position, it loses information. d(mx+b)/dx turns into m, no way to ever get b back with an initial value condition.
Then - omigod, when you take a partial - you have to ignore dependence. curlyd(xy+by)/curlydx turns into y and then things is really fucked if there was any dependence on y (ie, doing curlyd(xy+by)/curlydy would give you a different answer if you did that first order matters I guess)
There are some operators that are just exclusionary. Once you chose to look for one, you’ve discounted the chance of finding the other. Taking position versus taking energy/velocity. And then the fucky thing there is lots of shits mass is measured in eV/c^2
(I’m neglecting a proper discussion of momentum which is 100% where someone can come in and humiliate me. Please do so.)
You're right. But the thing that's interesting about the double slit experiment though is that it works on only a single photon. It's as if all the traffic was created by a single car. So classically you might not think that the single car should care if the freight truck is heading down a different lane than the car but in QM it does, because the car is in a superposition of occupying several lanes.
I'm probably driving the analogy straight into the ground of course
Which is what's so "magical" about it - Newtonian rules seem to break down at the quantum level.
It was an incredible discovery, and for practically anyone not a physicist, it's incredibly hard to comprehend. I say this as a not-a-physicist who struggled to comprehend it decades ago, and read several books on the subject to finally get my head around it (as much as a non-physicist can).
Also, it's just a meme mate.
I disagree with it being hard to comprehend. The maths is an absolute bitch, but the basic premise is fairly simple. Everything is (quantised) waves. The rest clicks, once you get your brain to accept this. Everything else is a consequence. Those consequences can lead you down deep dark tunnels, filled with evil maths and mind bending results, but the basic idea is simple.
I have a bit of an issue with memes that are actively misleading.
Well, famously, they're waves and particles. The double slit which way experiment will only set off the detector in one slit, as if it was a particle. Yet, without a detector it will interfere with itself as if it were a wave that passed through both slits.
QM entities are quantised waves. You can make a wave look very close to a particle quite easily, a particle can never behave like a wave.
Dumping the mental short hand of particle interactions is one of the main reasons most people can't get their heads around it.
That’s wrong though, and further belies my point about math, and perhaps needing to take a quantum class before talking about it.
They are particles and waves.
What is a particle, what is a wave? QM entities are neither. They are a 3rd thing. A quantised wave is the term my university professor used as a short hand. The nature of that wave is described by the Schroeder equation + its constraints. Certain interactions will bound it heavily, and so make it look particle like, others emphasise the wavelike properties.
You require the maths to actually do anything useful with it, but not to get the basic concepts. It's no different to the rest of physics, in that. E.g. you can understand the concepts of orbital mechanics, without being able to calculate them.
psssst, it's just a meme
Lots of folks get their superficial education from memes and will be mislead by this...
Can't blame them I also try to act normal when people is watching.
I'm not weird. I act perfectly normal when no one is watching. honest.
I have been tryin to find an actual demostration of this experiement and it seem impossible to find. can someone sow me this experiment done on a single setup. where you have a light source, two slits a screen and an ''observer''. That swithes from interference pattern to two lines, by switching the observer on and off. I am convinced that the science is solid, in its theoric and applied aspect. but that this interpretation of it is complete bullcrap. And i am annoyed by the ''believe this cheap explanation''that is repeated and nauseam. What is the actual equipment required, wave lenghts, slit sizes. I know This is science meme sub, where is the sub where i can find an actual two slit operator?
https://en.m.wikipedia.org/wiki/Double-slit_experiment
Put a filter on one slit to measure the beams. Done.
Edit: this thinks about a related experiment, where you build an interferometer instead of a double slit. Now rephrase the experiment s.t. you have to beams serviced from a singular beam. interfere the two beams with each other. If you measure on one beam path (e.g. Pol filter) you destroy the interference pattern.
Physicists calling it an "observer" is the worst thing since Ben Franklin decided to name positive and negative electrical charges. "Observer" implies that it's someone watching the thing, when really it's just the light interacting with something.
Yes, that's what bothers me about the whole thing. All those ''popular science'' exposé telling us that if you just look at it, it will know and change from wave to particle. They put this big eye icon next to the slit and imply you are stupid if you dont just believe it as told. the experiment with polarized filter someone else showed me was interesting, but that's not merely observing.
What are you trying to see exactly? There's this video done with polarizers: https://youtu.be/unCXuRXpEhs Of course, it's not an instant on/off but having an instant on/off doesn't really change anything.
Thank you. Yes something like that. with lab grade equipment would be nice. I agree instant on/off is not required for my understanding.
You can put together a version good enough for high schoolers with a laser pointer and some pencils.
To conceptualize though, it might be better to set up some sort of wave table - some sort of tub of water you set up the slits in, then observe the patterns of constructive and destructive interference. (That’s what I did with students.)
Yes, thats a very easy demonstation of the wave nature of light, what bugs me is the demonstration that it's also a particle that I feel is misleading. Maybe particle is not the right word to describe it's nature.
You see it behave as a particle when you cover a slide and it simply passes through the single slit.
“Particle” is the right word to describe the nature of a photon, and so is “wave.” When you collapse a wave function by “observing it” - basically the wave/particle interacting with something - you find it as a particle somewhere.
Eg, imagine an atom surrounded by electrons. Until the electrons are “observed” - interact with something - they exist in a probabilistic “electron cloud.” Depending on energy level and sub shell (ie, the “quantum numbers” you might have encountered in chemistry - remember 1s^2 2s^2 2p^6?) there are places that the electron is more likely to be. An electron in 1s is likely to be closer to the atoms nucleus when observed. But it could also be on the opposite side of the universe - small chance, but possible. It exists as a probabilistic wave though until that wavefunction is “collapsed.” (One of the silly things I like to point out when teaching is that conceivably all of your electrons could be out for a trip, and you could just phase through your chair)
This is why the photons going through the a single slit will behave like particles, and those through the double slit will behave like waves. They aren’t “collapsed” at the double slit. The single slit will only allow them to pass through as a particle.
But the vocab is unambiguous and correct - they are both “particles” and “waves.”
You don't really need an on/off switch.
By default you are not observing what slit it goes trough so you should always see the wave pattern.
Wait, but we're looking at it...
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This makes me pretty annoyed, mostly because I get flashbacks to assholes pushing The Secret
Yeah they beamed with pride when asked about their work. They were so observant and laser focused and we hit the mark every time while they were here. It was pretty obvious they'd make waves when they quit. Still, no one expected it.
Thanks lemmy user!
Attractor