Thanks for this informative video. It is worth mentioning, in this context, that even a single photon (or electron) can interfere (manifest its wave nature) with 'itself' !
🍎 Want to learn quantum mechanics by solving real problems? I'm running a course Jan 6-31st for people who've always wanted to understand quantum mechanics properly! For 4 weeks, you'll have homework and small-group tutorials with me ✏️ No advanced math prerequisites needed - it's designed for curious minds from all backgrounds! If you've been wanting to understand what quantum mechanics really says (beyond the usual vague analogies), come learn with us! More info here: looking-glass-universe.teachable.com/p/quantum-mechanics-fundamentals1
@@zamplify I don’t subscribe to this channel, but the algorithm pushed it at me (probably because I’m subscribed to channels like Hossenfelder’s). I have a similar thought: why is this TH-camr speaking as though quantum theory is understood? It’s an incomplete theory that uses undefined terms.
Hi there I had some query on quantum mechanics so I tried to reach you through your email but I am unable to do so I used the mail Id that’s mentioned in your channel description I am unable to send that mail
While there is no universally accepted definition of what constitutes a ‘measurement’ in quantum mechanics, it is generally modeled as an interaction that extracts classical information from a quantum system. The specifics of how this happens and what it means remain open questions in quantum theory (“the measurement problem”).
Who cares of those specifics. Wait until the general model is known to be correct, then care about the specifics. Also, there is no measurement problem.
@@truejim And some physicists know that those other physicists are out to lunch, and we can prove it. We simply ask them to be clear about what the measurement problem is, and they cannot do it. In physics we observe phenomena and describe it mathematically, and try to make sense of it (as E did with gravity, for example). That is what physics is about. It's not about something else to that. Physics requires phenomena, which is observed, which then gets described and explained (if possible). That is what physics IS. Anyone who says there is a Measurement Problem needs to show what had happened in Nature (what phenomenon) such that this happening = a problem which is being called the measurement problem. They cannot do it. They just cannot. There is no MP. It is a conceptual accident to entertain it, and it typically is an accident that happens to philosophers (not physicists), but it can happen to some physicists too.
@@truejim There is no measurement problem, as I said. Now you asking a different thing. "What is measurement". Are you saying that the measurement problem is that it is not known what measurement means?
Ah, I remember back in undergrad days doing calculations on calcite crystals for building a laser frequency tripler. Calcite was my first exposure to a nonlinear birefringent optical material. You can model its refractive indices with a fancy index ellipsoid.
@@LookingGlassUniverse Unfortunately, I can't give you any good book recommendations on nonlinear optics since I've usually done most of my work empirically or referencing random research publications. However, I can tell you the Sellmeier equation and index ellipsoid (indicatrix) are the general concepts used for most nonlinear optics. Also, thanks a lot for the quality videos. You have some of the most novel and intuitive physics explanations I've ever seen anywhere online
If I've understood correclty, black body radiation can have both linear and circular polarization for emitted photons. If you use linear polarizer filter you remove all circular polarized light in addition to the linear polarization light in ortogonal orientation your filter.
Man TH-cam is hard. Algorithm either loves you or forgets you exist. Best of luck with the channel. Unsolicited opinion incoming I really love the way Arvin Ash explains stuff. No shame in taking inspiration from other creators
The statement at 9:03 that the single photon is a (split-in-two-halves) wave until it reaches the wall of detectors is inconsistent with the Locality axiom, because one of the two half-waves was outside the past lightcone of the detection event a moment before the detection event. So it's not only entanglement phenomena that undermines confidence in the Locality axiom... unless one considers the two half-waves to be "self-entangled."
...unless one considers the two half-waves to be "self-entangled." YES, and? Not a pro, but I view the two halfs as parts of single wave-function (yes, that sounds like "self-entangled" to me). So, no non-local problem aside from what we already know: en.wikipedia.org/wiki/Bell%27s_theorem#Experiments
@firdacz : Your point is unclear to me. A wavefunction is a mathematical description: a prediction of what would be observed if a detection interaction occurs, assuming the detection device has been pre-arranged in a specific way consistent with the arbitrarily-chosen eigenbasis. A wavefunction is therefore an abstract object, not physical. But the two half-waves are physical elements of this model, not descriptions nor predictions. So the half-waves cannot be "part of the wavefunction." The two half-waves may be considered part of the same wave, distributed in space (until it's entirely absorbed by an electron of the detector). The distribution in space guarantees one of the two half-waves was outside the past lightcone of the absorption event, a moment before the absorption. The special property that makes the wave "quantum" (not classical) is that when it interacts, it interacts as if it's located entirely at the point of the interaction, regardless of how widely distributed it was a moment earlier. Thus this model with two physical half-waves and quantized absorption violates Locality. Bell was in fact a fan of the DeBroglie-Bohm interpretation of QM, which is a nonlocal theory older than Bell's analysis of the possible nonlocality involved in entanglement experiments. So I don't understand why you say the nonlocality of the "two half-waves" model is no different from the nonlocality of two entangled "particles." It takes care & effort to entangle two particles, and entanglement is fragile so the particles must be unnaturally isolated from other interactions in order to do Bell Test experiments. Waves, however, are the ubiquitous normal state of matter/energy. Their distribution in space makes their quantized interaction property nonlocal. In other words, _all_ interactions are nonlocal, in this model.
@@brothermine2292 First of all, your original comment was unlcear to me, especially "Locality axiom" - what are you referring to? That is also a reason why I referenced Bell, because I though you were referring to "Local realism". Overall I do not see what is your point, what are you trying to (dis)proove. "A wavefunction is therefore an abstract object, not physical. ... But the two half-waves are physical elements of this model" - I disagree, there is only one photon, but again, not an expert. (20 years out of Math-Phys University, became a programmer.) "DeBroglie-Bohm" - Pilot Wave I hope, liked that too for some time, then discarded it together with Many-Worlds because of Occam's Razor - why making it complicated when it does not bring anything new/useful. As for EPR etc.: The random nature of the "collapse" prevents it to break causality (faster-than-light communication), so I simply see no problem. "it interacts as if it's located entirely at the point of the interaction, regardless of how widely distributed it was a moment earlier." - again not sure if we see the same picture here, I see random collapse (exchange of something - energy, momentum, ...) happening with the calculated probability (and I do not care about any other world with possibly different result or over-complicated non-local guiding wave). P.S.: You: So I don't understand why you say the nonlocality of the "two half-waves" model is no different from the nonlocality of two entangled "particles." Me: never said that, on the contrary: I view the two halfs as parts of single wave-function (yes, that sounds like "self-entangled" to me). In other words: single wave-function or two entangled parts sound like the same thing to me. And again, see no problem in it.
>firdacz : It seems like your latest reply is mostly asking me to repeat myself. For example, I implicitly defined the Locality axiom in my initial comment by describing how it's violated by an effect from outside the past lightcone. Your description of the two half-waves as "a single photon" doesn't clarify anything. If the two half-waves are physically real, meaning the photon's quantum of energy is distributed (not localized) while the photon travels, it follows that the localized absorption of the entire quantum involves a nonlocal phenomenon. This model isn't "local realism." It's nonlocal realism: in this model, the two half-waves really exist physically prior to the absorption of the entire quantum. Nonlocal real half-waves is consistent with the language used in the video at 9:03, and it may be the simplest explanation of how the interference pattern is generated in the double-slit single-photons experiment. It's often claimed that the Compton scattering experiment in 1922 falsified the waves model. But it actually only falsified the _classical waves_ model, not a model in which waves have the quantized interaction property described in my previous reply.
@@brothermine2292 Nope, I explicitly searched for "Locality axiom", which you never specified, nor provided a link for, found nothing that would look like you may be talking about, so, your "lightcone" was viewed by me as "referring to relativity and causality". And I simply refuse to continue these obvious "parallel monologs". Have a nice day :)
The measurement takes place at the wall, not at the calcite, right? A single photon's wave function would go through the calcite and interfere with itself past the calcite. All of that evolution is described by Schrödinger's equation. When the wave function hits the wall, a measurement takes place by collapsing the wave function on one of the base states of the calcite. It's not the calcite that does the quantum measurement, it's the wall!
Strictly speaking, the calcite measures the polarization by deflecting the photon according to its polarization, and then the wall makes a position measurement.
@JonBrase So you're saying the photon is not in a superposition of diagonal and anti-diagonal after the interaction with the calcite as I thought, but its wave function is already collapsed to either diagonal or anti-diagonal? In that case you're right, the calcite does a polarisation measurement.
I think you're half correct: the calcite coherently modifies the polarization via double refraction; it is not making any measurement. However, the linearly-polarizing absorption films (i.e. polaroids) ARE detectors (and effectively perform polarization measurements). The concept of wavefunction collapse is not a requirement for measurement, though various degrees of phase-space reduction often accompany measurement.
Great discussion! I glossed over this in the video but yes, the calcite by itself isn’t a measurement, it’s the calcite plus measuring at the wall. But how do we know? Couldn’t it collapse when it goes through the calcite? Well, this is the issue in quantum mechanics- the rules never explain what counts as a measurement. But still, we can be pretty sure the calcite by itself can’t collapse the state, because it’s possible to rejoin the two beams up and show that the light must have been in a superposition of both paths when it went through the calcite. So in this case, if there is measurement collapse, it’s happening at the wall
@@sebastiandierks7919 After the interaction, the photon is in a superposition of polarization states, but it's now also in a superposition of momentum states (determining its direction of travel) that's entangled with the superposition of polarization states. As it travels, the superposition of momentum states leads to a superposition of position states, and the wall eventually measures the position. But the calcite has set things up so that measuring the photon's polarization at a particular point on the wall will only ever return one result (because if the photon has the other polarization, it won't be at that position to be measured).
When I first heard these vectors, I didn't recognize more than two vectors: two measurement basis vectors (two vectors) and a photon's state vector. The photon's state vector can be written as a linear combination of two basis vectors. That was a bit confusing. But it was soon clear. Thanks a lot!😀
Thanks for this. I still disagree with calling this a measurement though as it interferes with light, it doesn't benignly measure it like we do in classical measurements. When you look at the mechanisms in detail, the polarizer on the laser filters the light, the calcite filters it again, the paper at the end filters again, and then our eyes/camera are a detector (technically a series of filters with our photoreceptors being the only detector). This is my issue with the language of QM, it's not just unintuitive, but blatantly opposite of what we'd say in classical. I can't figure out why this persists in a field that's usually good about inventing and precisely using new jargon.
I guess the issue is that the classical definition of measurement is incoherent in quantum mechanics. Why should our conscious viewing of it change anything? In quantum mechanics, the information interacting with our brains is no different than the information interacting with the environment. The linearly polarized light can be thought of as a superposition of 2 different polarizations. I agree that this isn't really evidence of anything quantum. All that is happening is that each photon causes the atoms in the crystal to wiggle in 2 different ways at the same time, producing the 2 weaker photons. There is a correlation between the polarizations, but this can be understood classically.
@ Our consciousness has nothing to do with it. We get the same results when we use mechanical means to perform the same detections and translate those into machine output. And I haven't yet seen evidence of a superposition of states that couldn't also be described as just a componentization of the actual state in the way a point or vector can be written as a combination of coordinates and magnitude. Every experimental wave function collapse I've seen is due to forcing the system to adhere to the strictly defined filters and detectors, not some magical quantum property. It's like we invented all these new concepts to explain the weirdness we saw, then eventually explained the weird mechanically/classically and kept the same concepts despite not needing them.
@@jokeyxero What about the wave function collapse of the emissions of a photon from a high energy electron in an atom. That spontaneously happens and we see the light. It is true that superpositions look like vector components being added. But so do 2 photons in an electromagnetic field. Just because the EM field can be written all together as photon1+photon2 doesn't mean that there aren't 2 objects. I agree that consciousness is not a part of it and that we can have mechanisms that force wave function collapse, but it happens spontaneously all the time. We have detectors (rather than experiment) that measure the result of the collapse of a neutrino wave function interacting with ice. The data can only be explained if the neutrino was traveling in superposition of 3 different states. That in itself is not quantum, but it has quantum interference which (along with entanglement) is not describable by classical physics.
@@jokeyxeroThe difference between quantum and classical mechanics here is that when we use filters and detectors that we would expect from classical mechanics to perform measurements of the exact state vector of the particle or system under observation with minimal interference, what we find instead is that they only give us a componentized measurement, that for single measurements (as opposed to the many measurements that we see with a beam of light bright enough to see passing through calcite) each individual measurement falls exactly along one of the basis vectors of the componentization, and that the subsequent state after the measurement is forced to the measured state. To my understanding, the calcite would produce a smear classically, not two dots. Whatever the classical case for calcite, there are other experiments that definitely would not produce a componentized result classically, and we need QM to explain why the result gets componentized.
@jokeyxero I agree. The calcite coherently projects the incoming light polarization onto the crystal basis via double refraction; it is not making any measurement! However, the linearly-polarizing absorption films (i.e. polaroids) ARE detectors (and effectively perform polarization measurements).
It makes sense that imaginary numbers would be needed for spin. I really like the explanation for how they get into polarization, but what about in the position basis? It would make sense for momentum since a Fourier transform is most concise using exponantials and Euler's formula. Since Schrodinger's equation has i in it, is that, de facto, where it comes from?
I would be happy if someone would make a video series "Quantum at Home". Parts and material should be affordable or even possible to take from broken TVs, microwaves or video projectors. I am sure there are a lot of DIYers with special technical skills out there. Some of them might come with new ideas, not only replicating the experiments from the videos.
If I ever found a magic lamp I'd definitely use one of the wishes so that your channel would get the millions of followers it deserves. It would be well spent.
If you only tested one photon, and it was randomly measured to have one polarization, would that mean that some conserved quantity was not conserved? Same if the universe only tested one particle’s spin, etc. over multiple particles, the randomness balances out. But in the moment, it seems like you can create a net change.
7:38 Doesn't the fact that we get two dots, instead of a smear, itself indicate that this is a quantum measurement without having to go to the single photon case? I know that in the Stern-Gerlach experiment we get discretized results where we'd expect a continuous spectrum of results classically, and I think(?) that's what's going on here too?
Hey, great question!! The answer is hard… it depends what you count as classical. There was already a wave theory of light that explained polarisation and would predict 2 blobs. But if people didn’t realise light is wavelike then yes, they’d probably predict a smear. Nice insight!
You seem to be implying that the quantum nature of the absorption of photons by an atom is what gives light its quantum property, and that outside of the atom, the light is not quantum but a wave, regardless of how low the intensity is. But my understanding is that the light is quantum regardless of atoms and their quantized absorption. So which point of view is correct? And how is it proven?
Light is not a wave. We measure it as a localised point of energy all the time. What it is is its properties, as measured. What it is outside of measurement is outside of what it is. It's like asking how someone felt outside of how they felt when told they passed the exam. What kind of question is that? It's not asking how they felt when they were studying, else it would have been "How did you feel when you were studying?". It's asking some other gibberish thing, like "Putting aside how you felt when you past the exam, how did you feel when you passed it?". It's just nonsense. What light is is what its properties are, as measured. The question is not What is light, but "What is this effect here, made by light? How does this effect arise? Is light (which we do know what it is) something that travels from point A to point B? If not, then how does it show up there? Those types of questions are the deep quantum mechanical questions. The question "what is light" is not.
@ That makes a lot of sense. Yet it feels unsatisfactory. When we ask what something IS, we never expect a complete answer down to every possible aspect of it, down to the position of each atom. But we do generally know enough about many things to answer many questions about it and to feel like we "know" it. Say a pot on my stove. If I describe it briefly to someone, they might say "I know you've got a pot but I really don't know that pot. But if I describe it in extreme detail and allow them to examine it with all their senses and read about the properties of the materials out of which it is made, they might say "I know that pot well." Other things we feel we know but clearly only understand a small part of. A flower, for example. So, is light, or a photon, something we can EVER say "I know it well?" We really can ever know only measurable observable properties of ANYTHING. That's what you are saying. If something has unmeasurable properties, do they matter? I guess so, they could AFFECT measurable properties. In which case you might infer or try to guess what those unmeasurable properties are. I suppose that's what is done in QM.
Hi! Is it a way you can answer this problem? : Imagine a question like that because is believed even in scientific world you cannot go in the past: two brothers go towards each other with spaceships able to reach 50% of the speed of light and want to meet at the halfway from Earth to Mars. Within a last minute before the meeting,the brother who came from Mars called (speed of light) a cousin who left in the same time like him from Mars but with a spaceship able to reach 90% of speed of light and ask him to say hi if he meets his brother on the way to Earth taking in consideration he is more advanced like him, probably close to Earth? Does that information go in the past of the brother who came from Earth?!
I see it like this: a _measurement_ is an _interaction_ that is observed by a being capable of getting information from it. Every interaction, even one we can't see that occurs billions of light years away, collapses a wave function.* In order for an interaction to qualify as a measurement, though, we have to observe it, gaining information by doing so. This description means that an observer is not necessary for quantum interactions to be real. Schrödinger's cat is either dead or alive, not both, because the particles from the radioactive decay in the experiment interact with the rest of the system whether or not we observe those interactions, and the things that result from the interactions happen because of that. (Or not, if no decay has yet taken place.) We simply lack information about the cat's state until we open the box. Alternatively,you could say that because at root an observation is nothing more than the interaction of particles, the "observer" of an interaction is the collection of particles surrounding the interaction. The collection does what it does as a result of the interaction. If that collection, or part of it at least, is someone conducting an experiment and that someone is paying attention (or an apparatus records the results of the interaction, which is merely an indirect way of "paying attention"), then a measurement occurs. If not, then the consequences of the interaction still happen, "observed" but not measured. Thus Jupiter's red spot is real even when it has rotated away from us. It must be, because if it weren't then it wouldn't be there when that side of Jupiter rotates around to face us. ----- *If you use the Copenhagen interpretation of QM,. Other interpretations are beyond the scope of this comment.
No, it wouldn’t work for measuring polarisation, because a regular beam splitter doesn’t split the light according to polarisation, it will just split it in half, regardless of polarisation. But this is a polarising beam splitter. Those work
I have a small issue with this kind of talk about quantum mechanics in general. Whate er you can see with the naked eye is already in the classical limit, and so there should always be classical theory capable of explaining it. :)
But what's the point if you can't actually do the calculation. Honestly magical things like the uncertainty principle just pop out from the Fourier transform
It’s very cool, but not unexpected. There’s a tonne of misunderstanding about what it actually does- it sounds like it’s a trillion times better than a regular computer, but it’s actually only a trillion times better at a super specific task (which is useless). It’s an impressive engineering feat to make it, but it’s not yet clear what quantum computers will be useful for
Another way to do quantum measurements at home is to shine a laser pointer directly into your eye (don't do this). You will observe trillions of photons in a very short period of time.
I don’t think so, and the last bit about the “single photon” thing explains my reasoning. It wasn’t a prediction of classical mechanics that light could become clumpy when measured. I think it’s very fair to call this quantum: black body radiation and the photoelectric effect are both due to this phenomenon and they are considered the earliest results in quantum mechanics
I think if you are teaching a subject like quantum measurement you should up your language from "the wave would be less wavey". Nice attempt though, to explain a complex concept.
Thanks for this informative video. It is worth mentioning, in this context, that even a single photon (or electron) can interfere (manifest its wave nature) with 'itself' !
That sounds like how subatomic particles insult one another. "Ahhh, go interfere with yourself!"
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For 4 weeks, you'll have homework and small-group tutorials with me ✏️ No advanced math prerequisites needed - it's designed for curious minds from all backgrounds!
If you've been wanting to understand what quantum mechanics really says (beyond the usual vague analogies), come learn with us! More info here:
looking-glass-universe.teachable.com/p/quantum-mechanics-fundamentals1
Without the math??
The web page says the cost is $380. I believe you’re in Australia; is that in Australian dollars?
@@zamplify I don’t subscribe to this channel, but the algorithm pushed it at me (probably because I’m subscribed to channels like Hossenfelder’s). I have a similar thought: why is this TH-camr speaking as though quantum theory is understood? It’s an incomplete theory that uses undefined terms.
Hi there
I had some query on quantum mechanics so I tried to reach you through your email but I am unable to do so
I used the mail Id that’s mentioned in your channel description
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While there is no universally accepted definition of what constitutes a ‘measurement’ in quantum mechanics, it is generally modeled as an interaction that extracts classical information from a quantum system. The specifics of how this happens and what it means remain open questions in quantum theory (“the measurement problem”).
Who cares of those specifics. Wait until the general model is known to be correct, then care about the specifics. Also, there is no measurement problem.
@@TimoBlacks Some physicists believe a general theory can’t be developed until the measurement problem is solved.
@@truejim And some physicists know that those other physicists are out to lunch, and we can prove it. We simply ask them to be clear about what the measurement problem is, and they cannot do it. In physics we observe phenomena and describe it mathematically, and try to make sense of it (as E did with gravity, for example). That is what physics is about. It's not about something else to that. Physics requires phenomena, which is observed, which then gets described and explained (if possible). That is what physics IS. Anyone who says there is a Measurement Problem needs to show what had happened in Nature (what phenomenon) such that this happening = a problem which is being called the measurement problem. They cannot do it. They just cannot. There is no MP. It is a conceptual accident to entertain it, and it typically is an accident that happens to philosophers (not physicists), but it can happen to some physicists too.
@ A quantum state collapses into a classical state when measured. What constitutes a measurement?
@@truejim There is no measurement problem, as I said.
Now you asking a different thing. "What is measurement". Are you saying that the measurement problem is that it is not known what measurement means?
Ah, I remember back in undergrad days doing calculations on calcite crystals for building a laser frequency tripler. Calcite was my first exposure to a nonlinear birefringent optical material. You can model its refractive indices with a fancy index ellipsoid.
Wow! I know very little about nonlinear crystals but I would love to learn! Whats a good book?
@@LookingGlassUniverse Unfortunately, I can't give you any good book recommendations on nonlinear optics since I've usually done most of my work empirically or referencing random research publications. However, I can tell you the Sellmeier equation and index ellipsoid (indicatrix) are the general concepts used for most nonlinear optics. Also, thanks a lot for the quality videos. You have some of the most novel and intuitive physics explanations I've ever seen anywhere online
In a quantum way I’m both just here for the physics and not just here for the physics at the same time.
Thank you for this careful and clear explanation. It was very helpful to me.
If I've understood correclty, black body radiation can have both linear and circular polarization for emitted photons. If you use linear polarizer filter you remove all circular polarized light in addition to the linear polarization light in ortogonal orientation your filter.
Man TH-cam is hard. Algorithm either loves you or forgets you exist. Best of luck with the channel.
Unsolicited opinion incoming
I really love the way Arvin Ash explains stuff. No shame in taking inspiration from other creators
The statement at 9:03 that the single photon is a (split-in-two-halves) wave until it reaches the wall of detectors is inconsistent with the Locality axiom, because one of the two half-waves was outside the past lightcone of the detection event a moment before the detection event.
So it's not only entanglement phenomena that undermines confidence in the Locality axiom... unless one considers the two half-waves to be "self-entangled."
...unless one considers the two half-waves to be "self-entangled."
YES, and? Not a pro, but I view the two halfs as parts of single wave-function (yes, that sounds like "self-entangled" to me). So, no non-local problem aside from what we already know:
en.wikipedia.org/wiki/Bell%27s_theorem#Experiments
@firdacz : Your point is unclear to me. A wavefunction is a mathematical description: a prediction of what would be observed if a detection interaction occurs, assuming the detection device has been pre-arranged in a specific way consistent with the arbitrarily-chosen eigenbasis. A wavefunction is therefore an abstract object, not physical. But the two half-waves are physical elements of this model, not descriptions nor predictions. So the half-waves cannot be "part of the wavefunction."
The two half-waves may be considered part of the same wave, distributed in space (until it's entirely absorbed by an electron of the detector). The distribution in space guarantees one of the two half-waves was outside the past lightcone of the absorption event, a moment before the absorption. The special property that makes the wave "quantum" (not classical) is that when it interacts, it interacts as if it's located entirely at the point of the interaction, regardless of how widely distributed it was a moment earlier. Thus this model with two physical half-waves and quantized absorption violates Locality.
Bell was in fact a fan of the DeBroglie-Bohm interpretation of QM, which is a nonlocal theory older than Bell's analysis of the possible nonlocality involved in entanglement experiments. So I don't understand why you say the nonlocality of the "two half-waves" model is no different from the nonlocality of two entangled "particles." It takes care & effort to entangle two particles, and entanglement is fragile so the particles must be unnaturally isolated from other interactions in order to do Bell Test experiments. Waves, however, are the ubiquitous normal state of matter/energy. Their distribution in space makes their quantized interaction property nonlocal. In other words, _all_ interactions are nonlocal, in this model.
@@brothermine2292 First of all, your original comment was unlcear to me, especially "Locality axiom" - what are you referring to? That is also a reason why I referenced Bell, because I though you were referring to "Local realism". Overall I do not see what is your point, what are you trying to (dis)proove.
"A wavefunction is therefore an abstract object, not physical. ... But the two half-waves are physical elements of this model" - I disagree, there is only one photon, but again, not an expert. (20 years out of Math-Phys University, became a programmer.)
"DeBroglie-Bohm" - Pilot Wave I hope, liked that too for some time, then discarded it together with Many-Worlds because of Occam's Razor - why making it complicated when it does not bring anything new/useful.
As for EPR etc.: The random nature of the "collapse" prevents it to break causality (faster-than-light communication), so I simply see no problem.
"it interacts as if it's located entirely at the point of the interaction, regardless of how widely distributed it was a moment earlier." - again not sure if we see the same picture here, I see random collapse (exchange of something - energy, momentum, ...) happening with the calculated probability (and I do not care about any other world with possibly different result or over-complicated non-local guiding wave).
P.S.: You: So I don't understand why you say the nonlocality of the "two half-waves" model is no different from the nonlocality of two entangled "particles."
Me: never said that, on the contrary: I view the two halfs as parts of single wave-function (yes, that sounds like "self-entangled" to me). In other words: single wave-function or two entangled parts sound like the same thing to me. And again, see no problem in it.
>firdacz : It seems like your latest reply is mostly asking me to repeat myself. For example, I implicitly defined the Locality axiom in my initial comment by describing how it's violated by an effect from outside the past lightcone.
Your description of the two half-waves as "a single photon" doesn't clarify anything. If the two half-waves are physically real, meaning the photon's quantum of energy is distributed (not localized) while the photon travels, it follows that the localized absorption of the entire quantum involves a nonlocal phenomenon. This model isn't "local realism." It's nonlocal realism: in this model, the two half-waves really exist physically prior to the absorption of the entire quantum. Nonlocal real half-waves is consistent with the language used in the video at 9:03, and it may be the simplest explanation of how the interference pattern is generated in the double-slit single-photons experiment.
It's often claimed that the Compton scattering experiment in 1922 falsified the waves model. But it actually only falsified the _classical waves_ model, not a model in which waves have the quantized interaction property described in my previous reply.
@@brothermine2292 Nope, I explicitly searched for "Locality axiom", which you never specified, nor provided a link for, found nothing that would look like you may be talking about, so, your "lightcone" was viewed by me as "referring to relativity and causality". And I simply refuse to continue these obvious "parallel monologs". Have a nice day :)
The measurement takes place at the wall, not at the calcite, right? A single photon's wave function would go through the calcite and interfere with itself past the calcite. All of that evolution is described by Schrödinger's equation. When the wave function hits the wall, a measurement takes place by collapsing the wave function on one of the base states of the calcite. It's not the calcite that does the quantum measurement, it's the wall!
Strictly speaking, the calcite measures the polarization by deflecting the photon according to its polarization, and then the wall makes a position measurement.
@JonBrase So you're saying the photon is not in a superposition of diagonal and anti-diagonal after the interaction with the calcite as I thought, but its wave function is already collapsed to either diagonal or anti-diagonal? In that case you're right, the calcite does a polarisation measurement.
I think you're half correct: the calcite coherently modifies the polarization via double refraction; it is not making any measurement. However, the linearly-polarizing absorption films (i.e. polaroids) ARE detectors (and effectively perform polarization measurements). The concept of wavefunction collapse is not a requirement for measurement, though various degrees of phase-space reduction often accompany measurement.
Great discussion! I glossed over this in the video but yes, the calcite by itself isn’t a measurement, it’s the calcite plus measuring at the wall. But how do we know? Couldn’t it collapse when it goes through the calcite? Well, this is the issue in quantum mechanics- the rules never explain what counts as a measurement. But still, we can be pretty sure the calcite by itself can’t collapse the state, because it’s possible to rejoin the two beams up and show that the light must have been in a superposition of both paths when it went through the calcite. So in this case, if there is measurement collapse, it’s happening at the wall
@@sebastiandierks7919 After the interaction, the photon is in a superposition of polarization states, but it's now also in a superposition of momentum states (determining its direction of travel) that's entangled with the superposition of polarization states. As it travels, the superposition of momentum states leads to a superposition of position states, and the wall eventually measures the position. But the calcite has set things up so that measuring the photon's polarization at a particular point on the wall will only ever return one result (because if the photon has the other polarization, it won't be at that position to be measured).
When I first heard these vectors, I didn't recognize more than two vectors: two measurement basis vectors (two vectors) and a photon's state vector. The photon's state vector can be written as a linear combination of two basis vectors. That was a bit confusing. But it was soon clear. Thanks a lot!😀
Thanks Hitoshi! It’s a really confusing concept for sure
This may seem trivial but I would like to see the polarized filter between the paper and the crystal, and then rotated 360°.
Thanks for this ❤
Being as physicists,i used to watch your all videos thank you for this clearance.
English much?
Absolutely beautiful
Thanks for this. I still disagree with calling this a measurement though as it interferes with light, it doesn't benignly measure it like we do in classical measurements. When you look at the mechanisms in detail, the polarizer on the laser filters the light, the calcite filters it again, the paper at the end filters again, and then our eyes/camera are a detector (technically a series of filters with our photoreceptors being the only detector). This is my issue with the language of QM, it's not just unintuitive, but blatantly opposite of what we'd say in classical. I can't figure out why this persists in a field that's usually good about inventing and precisely using new jargon.
I guess the issue is that the classical definition of measurement is incoherent in quantum mechanics.
Why should our conscious viewing of it change anything? In quantum mechanics, the information interacting with our brains is no different than the information interacting with the environment.
The linearly polarized light can be thought of as a superposition of 2 different polarizations. I agree that this isn't really evidence of anything quantum. All that is happening is that each photon causes the atoms in the crystal to wiggle in 2 different ways at the same time, producing the 2 weaker photons. There is a correlation between the polarizations, but this can be understood classically.
@ Our consciousness has nothing to do with it. We get the same results when we use mechanical means to perform the same detections and translate those into machine output. And I haven't yet seen evidence of a superposition of states that couldn't also be described as just a componentization of the actual state in the way a point or vector can be written as a combination of coordinates and magnitude. Every experimental wave function collapse I've seen is due to forcing the system to adhere to the strictly defined filters and detectors, not some magical quantum property. It's like we invented all these new concepts to explain the weirdness we saw, then eventually explained the weird mechanically/classically and kept the same concepts despite not needing them.
@@jokeyxero What about the wave function collapse of the emissions of a photon from a high energy electron in an atom. That spontaneously happens and we see the light.
It is true that superpositions look like vector components being added. But so do 2 photons in an electromagnetic field. Just because the EM field can be written all together as photon1+photon2 doesn't mean that there aren't 2 objects.
I agree that consciousness is not a part of it and that we can have mechanisms that force wave function collapse, but it happens spontaneously all the time. We have detectors (rather than experiment) that measure the result of the collapse of a neutrino wave function interacting with ice. The data can only be explained if the neutrino was traveling in superposition of 3 different states.
That in itself is not quantum, but it has quantum interference which (along with entanglement) is not describable by classical physics.
@@jokeyxeroThe difference between quantum and classical mechanics here is that when we use filters and detectors that we would expect from classical mechanics to perform measurements of the exact state vector of the particle or system under observation with minimal interference, what we find instead is that they only give us a componentized measurement, that for single measurements (as opposed to the many measurements that we see with a beam of light bright enough to see passing through calcite) each individual measurement falls exactly along one of the basis vectors of the componentization, and that the subsequent state after the measurement is forced to the measured state.
To my understanding, the calcite would produce a smear classically, not two dots. Whatever the classical case for calcite, there are other experiments that definitely would not produce a componentized result classically, and we need QM to explain why the result gets componentized.
@jokeyxero I agree. The calcite coherently projects the incoming light polarization onto the crystal basis via double refraction; it is not making any measurement! However, the linearly-polarizing absorption films (i.e. polaroids) ARE detectors (and effectively perform polarization measurements).
It makes sense that imaginary numbers would be needed for spin. I really like the explanation for how they get into polarization, but what about in the position basis? It would make sense for momentum since a Fourier transform is most concise using exponantials and Euler's formula. Since Schrodinger's equation has i in it, is that, de facto, where it comes from?
Great question! I’m not 100% sure in the position basis where it comes from… something I’d like to look into!
I would be happy if someone would make a video series "Quantum at Home". Parts and material should be affordable or even possible to take from broken TVs, microwaves or video projectors. I am sure there are a lot of DIYers with special technical skills out there. Some of them might come with new ideas, not only replicating the experiments from the videos.
This is the plan :)
Thanks for the video, but can we just get the experiment with the weak light on the crystal?
Yeah, working on it
@@LookingGlassUniverse Nice! I will patiently wait 🙂
If I ever found a magic lamp I'd definitely use one of the wishes so that your channel would get the millions of followers it deserves. It would be well spent.
That’s very sweet, thank you!
lode stone? for navigating fog...is that the name?
If you only tested one photon, and it was randomly measured to have one polarization, would that mean that some conserved quantity was not conserved? Same if the universe only tested one particle’s spin, etc. over multiple particles, the randomness balances out. But in the moment, it seems like you can create a net change.
Great point! I don’t know the answer…
Why would it mean that? Same with spin. Why would it mean that?
I'll be sure to get one of those calcium something-or-other from my local gas station so I can do this at home.
7:38 Doesn't the fact that we get two dots, instead of a smear, itself indicate that this is a quantum measurement without having to go to the single photon case? I know that in the Stern-Gerlach experiment we get discretized results where we'd expect a continuous spectrum of results classically, and I think(?) that's what's going on here too?
Hey, great question!! The answer is hard… it depends what you count as classical. There was already a wave theory of light that explained polarisation and would predict 2 blobs. But if people didn’t realise light is wavelike then yes, they’d probably predict a smear. Nice insight!
You seem to be implying that the quantum nature of the absorption of photons by an atom is what gives light its quantum property, and that outside of the atom, the light is not quantum but a wave, regardless of how low the intensity is. But my understanding is that the light is quantum regardless of atoms and their quantized absorption. So which point of view is correct? And how is it proven?
Great question! I’d like to get back to this point soon
Light is not a wave. We measure it as a localised point of energy all the time. What it is is its properties, as measured. What it is outside of measurement is outside of what it is. It's like asking how someone felt outside of how they felt when told they passed the exam. What kind of question is that? It's not asking how they felt when they were studying, else it would have been "How did you feel when you were studying?". It's asking some other gibberish thing, like "Putting aside how you felt when you past the exam, how did you feel when you passed it?". It's just nonsense. What light is is what its properties are, as measured. The question is not What is light, but "What is this effect here, made by light? How does this effect arise? Is light (which we do know what it is) something that travels from point A to point B? If not, then how does it show up there? Those types of questions are the deep quantum mechanical questions. The question "what is light" is not.
@ That makes a lot of sense. Yet it feels unsatisfactory. When we ask what something IS, we never expect a complete answer down to every possible aspect of it, down to the position of each atom. But we do generally know enough about many things to answer many questions about it and to feel like we "know" it. Say a pot on my stove. If I describe it briefly to someone, they might say "I know you've got a pot but I really don't know that pot. But if I describe it in extreme detail and allow them to examine it with all their senses and read about the properties of the materials out of which it is made, they might say "I know that pot well." Other things we feel we know but clearly only understand a small part of. A flower, for example. So, is light, or a photon, something we can EVER say "I know it well?" We really can ever know only measurable observable properties of ANYTHING. That's what you are saying. If something has unmeasurable properties, do they matter? I guess so, they could AFFECT measurable properties. In which case you might infer or try to guess what those unmeasurable properties are. I suppose that's what is done in QM.
But, how we can see a single photon at home?
Take it out to dinner first.
Thanks!
Hi! Is it a way you can answer this problem? : Imagine a question like that because is believed even in scientific world you cannot go in the past: two brothers go towards each other with spaceships able to reach 50% of the speed of light and want to meet at the halfway from Earth to Mars. Within a last minute before the meeting,the brother who came from Mars called (speed of light) a cousin who left in the same time like him from Mars but with a spaceship able to reach 90% of speed of light and ask him to say hi if he meets his brother on the way to Earth taking in consideration he is more advanced like him, probably close to Earth? Does that information go in the past of the brother who came from Earth?!
I think measurement and interaction are different things(?)
I see it like this: a _measurement_ is an _interaction_ that is observed by a being capable of getting information from it. Every interaction, even one we can't see that occurs billions of light years away, collapses a wave function.* In order for an interaction to qualify as a measurement, though, we have to observe it, gaining information by doing so.
This description means that an observer is not necessary for quantum interactions to be real. Schrödinger's cat is either dead or alive, not both, because the particles from the radioactive decay in the experiment interact with the rest of the system whether or not we observe those interactions, and the things that result from the interactions happen because of that. (Or not, if no decay has yet taken place.) We simply lack information about the cat's state until we open the box.
Alternatively,you could say that because at root an observation is nothing more than the interaction of particles, the "observer" of an interaction is the collection of particles surrounding the interaction. The collection does what it does as a result of the interaction. If that collection, or part of it at least, is someone conducting an experiment and that someone is paying attention (or an apparatus records the results of the interaction, which is merely an indirect way of "paying attention"), then a measurement occurs. If not, then the consequences of the interaction still happen, "observed" but not measured.
Thus Jupiter's red spot is real even when it has rotated away from us. It must be, because if it weren't then it wouldn't be there when that side of Jupiter rotates around to face us.
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*If you use the Copenhagen interpretation of QM,. Other interpretations are beyond the scope of this comment.
For all intents are purposes, measurements and interactions are treated the exact same in QM, there is only a human level difference in them.
Why do these things depend on geometry at all?
Wouldn’t a beam splitter work as well?
I think…. this IS a beam splitter… right?
No, it wouldn’t work for measuring polarisation, because a regular beam splitter doesn’t split the light according to polarisation, it will just split it in half, regardless of polarisation. But this is a polarising beam splitter. Those work
I have a small issue with this kind of talk about quantum mechanics in general. Whate er you can see with the naked eye is already in the classical limit, and so there should always be classical theory capable of explaining it. :)
How deep can you really go into quantum mechanics without hard-core mathematics until you hit the ceiling?
Surprisingly far!
But what's the point if you can't actually do the calculation. Honestly magical things like the uncertainty principle just pop out from the Fourier transform
You can get a decent understanding, but you can also get a decent understanding of the math, just try :)
Math is a language that describes reality. So do words
Math and concepts both have infinite boundaries
Did you see googles new quantum chip Willow? What are your thoughts?
It’s very cool, but not unexpected. There’s a tonne of misunderstanding about what it actually does- it sounds like it’s a trillion times better than a regular computer, but it’s actually only a trillion times better at a super specific task (which is useless). It’s an impressive engineering feat to make it, but it’s not yet clear what quantum computers will be useful for
@@LookingGlassUniverseI regularly see comments about quantum computers being able to make all computations much faster and it annoys me to no end
How does one make quantum measurements of quasi-particles? like phonon's?
Like phonon's what?
Photons are not really quantum, so you can really take quantum measurements of them
Not the duck taped hood laser lol
what equipment do you use to make tour videos
Fun
❤
Voice doesn't have enough croak
Another way to do quantum measurements at home is to shine a laser pointer directly into your eye (don't do this). You will observe trillions of photons in a very short period of time.
Warning: Do not look directly into laser with remaining eye.
Not merely _in_ a very short period of time but also _for_ a very short period of time.
Great video!
(Edited because i deleted stuff I was wrong about, lol)
I don’t think so, and the last bit about the “single photon” thing explains my reasoning. It wasn’t a prediction of classical mechanics that light could become clumpy when measured. I think it’s very fair to call this quantum: black body radiation and the photoelectric effect are both due to this phenomenon and they are considered the earliest results in quantum mechanics
@@LookingGlassUniverseYou're entirely right. I got confused with the down-conversion in BBO crystals. My bad, and good video!
She's pretty
I think if you are teaching a subject like quantum measurement you should up your language from "the wave would be less wavey". Nice attempt though, to explain a complex concept.