Latest / Art of the Question / Everything You Want To Know About Quantum Mechanics - Maria Violaris - #22

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Everything You Want To Know About Quantum Mechanics - Maria Violaris - #22 · Art of the Question

• speaker-1: Thanks for inviting me to the podcast. ⁓ I first became interested in quantum mechanics in general near the end of high school. I started coming across some popular science, like books and magazines that were mentioning quantum mechanics. Then I was reading about it and reading about the strange phenomena of things being in two places at once. the different interpretations of quantum mechanics, which kind of sounded a bit insane to me. I was motivated by reading about these things to study physics as a degree so that I could find out more about, ⁓ try and actually kind of understand how they work. then, yeah. I remember reading about the many worlds interpretation of quantum mechanics and Schrodinger's cat. ⁓ so yeah, the different explanations for how to solve the, problem of Schrodinger's cat. ⁓ and, ⁓ yeah, I remember seeing this, one of the explanations being this many worlds interpretation that the, the cat is both dead and alive at the same time. It's in a superposition of being dead and alive. And it really is in both of those states. then when you look at it, you enter both of those states. So one of you, one version of you sees it dead, one version of you sees it alive. And yeah, I kind of thought like, ⁓ that sounds a bit, I don't know why scientists are seriously considering that. And that there was this kind of Copenhagen interpretation that if you observe the cat, it collapses into a single state. And I thought, yeah, that sounds more reasonable. yeah, I was wondering why you can't just count the cat as an observer and then the cat observes the atom in superposition and collapses it. And I thought I'd solved the problem by thinking that at the time. But yeah, upon finding out more about the problem, I understood why people actually take seriously the multiverse. explanation and why the Copenhagen one isn't enough to actually give an actual theory of what's going on. it kind of shouldn't even really count. Yeah. ⁓ Yeah, I guess most broadly, it's the best theory we have for how matter behaves. So the equations of quantum mechanics are our best description of the dynamics of how particles move or how matter behaves on the most fundamental level. And for describing that behavior, it's been extremely well tested, very high. degree of precision. It's kind of seen as one of either one of or perhaps the most successful theory of physics in that sense of kind of really having predicted how matter behaves to a really high degree of precision. And yeah, it's typically thought of as the science of the very small. So kind of zooming into the quantum level refers to kind of zooming into small scales. And then you start to get this deviation from standard classical physics when you zoom into the small scales and then you have to shift to these equations of quantum mechanics in order to explain what's happening at those scales. Like if you want to describe how chemistry is working and how molecules are interacting, then normal equations of physics aren't enough to explain that anymore. ⁓ I guess a couple of branches from that. What is that? It's not actually just the size of the very small, even though that's kind of how we tend to think of it. You need quantum mechanics to explain kind of large scale phenomena as well, like why the sky is blue, for example, comes from understanding the way that light scatters from atoms in the sky. it's a quantum mechanical explanation for kind of a large effect, to understand the hell. solids work and so on. And another aspect is that what actually interests me and a lot of people that think about the fundamentals of quantum, think, isn't so much these kind of equations that describe molecules really well, for example, it's more the aspect of quantum information science. thinking about what happens when you store information in the systems described by the equations of quantum mechanics. When you think about that level, you get bit less physics-y in a way. You're more on the information level and that's where you can describe things like superposition and entanglement very naturally on this. You're thinking about how you manipulate information stored in systems described by these equations. ⁓ Yeah, it's kind of, I guess to summarise this kind of this fundamental theory of matter that then gives rise to quantum information theory, which I think is where some of the most interesting phenomena are. ⁓ It gives a good way of approaching some of the most kind of interesting and counterintuitive aspects of the theory. Yeah, that sounds about right. think it was Planck that was suggesting that there's quanta of light and then there was some phenomena that were being observed in terms of looking at black body radiation. So certain type of radiation that could only be explained when you kind of switch to this model where you have discrete chunks of light instead of ⁓ a kind of continuous ⁓ version and ⁓ switching to this kind of quantized, so discrete chunks version led to being able to solve ⁓ that problem. So I think that's kind of seen as the first step. then there were subsequent steps by other kind of founding fathers of quantum theory like Erwin Schroedinger. developed the Schroedinger equation, is an equation that describes what we call the wave function, which is the abstract quantum wave that describes a system. Then there was a different approach from Heisenberg based on a mathematical object called a matrix instead of a wave. That was known as matrix mechanics. ⁓ He developed his own kind of equations of quantum theory in this matrix based version. And then at some point, I think someone showed that they were equivalent. You could kind of map between one to the other. so nowadays we kind of call those the Schrodinger picture and the Heisebo picture. And they kind of have, yeah, the different implications and people in different fields like to use a different one depending on what they're working on, but they're kind of the same. equations. ⁓ Yeah. And I think that those equations, from what I understand, were kind of developed with bit of trial and errors, trying to figure out what are the right equations to describe this kind of behavior that we're seeing. ⁓ But I will caveat that I'm not an expert on the history of quantum mechanics. So this is kind of what I believe is true from what I've heard in the community. yeah. Yeah, think there's the idea of God, the kind of famous quote of God did not play dice with the universe, which was a kind of resistance to the idea that there's a fundamental randomness in quantum mechanics that there isn't in classical physics. ⁓ So I guess, yeah, in quantum mechanics, have a fundamental part of it is ⁓ probabilistic nature of measurements that if you have a system that's in a quantum superposition of two different states and then you measure it, you'll get some outcome with some probability. And in the past, probabilities were kind of constructed for certain situations like gambling. I think that might be where probability was developed, they weren't seen as kind of part of physics. so then with quantum theory, was kind of the first time where we were getting probabilities appearing in actual physics. a lot of people, including Einstein, I think found that strange or that seemed ⁓ wrong to have something kind of just randomly happening ⁓ as like part of the fundamental law. So I think that's where he was coming from with the dice comment. ⁓ And then there's the other aspect of the explanation for entanglement. So quantum entanglement is this ⁓ phenomenon where if you let two quantum systems interact and then take them far apart, you can measure these correlations between the systems that go beyond what you could measure if they were described by classical physics. We say that they're quantum entangled because they can have these kind of extra strong correlations between them. And there's a famous quote from Einstein describing this as spooky action at a distance. Because under some kind of interpretations of what's going on, it can seem like there's instantaneous influences between them. And that seems very, to the contrary to kind of the core principles of Einstein's theories of ⁓ special and general relativity where the idea is that you is very cool to those theories that you've got ⁓ locality, which means that if you do something to one region of space, it doesn't kind of automatically influence something in a different region of space. ⁓ So that seems to be kind of violated by entanglements. the story goes like he's kind of wasn't comfortable with that, but wasn't able to resolve these issues in his lifetime. So I think that's why the narrative comes to, well, he had to accept these problems, these aspects of quantum theory weren't the way that he would expect physics is. Yeah, maybe we can come onto it later, but I think that in our modern understanding of quantum theory, in my opinion, least I think we've got some satisfying explanations for those points that were troubling him and maybe he wasn't as wrong as the ⁓ conventional media might suggest he was sometimes when the headlines say Einstein was wrong. Yeah, it's very interesting debate that is still being discussed. yeah, as you said, there were kind of experiments in the run-up where you would do one experiment and it would show some kind of particle-like behavior of light and you do another one and it shows some wave-like behavior. And I think the... There are different variations of the double slit experiment and the one that is useful to consider for the is a quantum particle of light, a wave or a particle is the single photon double slit experiment. So photon is a single particle of light. And so the idea is that you imagine you have your double slit. kind of a barrier with two gaps in it, your two slits. And then you've got a source of your particles of light, your photons, a photon source, and that kind of fires photons one by one towards this barrier. And then on the other side of the barrier, you have a screen. And when a photon reaches the screen, ⁓ it'll then kind of register somewhere on the screen as a ⁓ hit. And so if you find lots of photons at the screen, then you'll get a buildup of all the points where the photons landed, and then you can look at the pattern on the screen. So that's the setup, and then you can consider... ⁓ So what happens when you start to send these particles in? So ⁓ these photons in. ⁓ So if you just send a single photon through, the double slits and then look at the pattern that builds up on the screen. ⁓ What you'll find is that you get what we call bright and dark fringes, which means that you've got a series of regions with a load of detections and then a region that's dark and then a bright region again where you've had a load of detections and then dark. You have these strips of bright, dark, bright, dark, bright, dark. ⁓ And the way to explain that is, well, you imagine, what would I expect to happen if I sent a wave through these two slits? And if you just imagine a water wave going through this kind of two-slit setup, you would get the waves kind of spreading out through both slits as though was two sources of the wave, and then they would interfere with each other and you'd end up with... this kind of series of intense and less intense regions because you get some parts of the waves adding up, some parts cancelling out. So that's what's called constructive and destructive interference of the waves. So that's what happens with normal waves. If you just send an actual particle through, then you would expect to just get two bright regions that are just directly where each slit was because your particle just goes straight through. You're not going to have interference. You're not going to have adding up or canceling out. You're just going to get the particles going straight through. So when you send these single photons in one by one and you get this interference pattern, that seems really strange because you've got a wave-like pattern on the screen, but you've individually sent a photon in one by one. So the conclusion... according to quantum mechanics is that well, the photon is a quantum system which has this wave function which is this kind of abstract quantum wave that describes it and that wave goes through both slits and then interferes with itself in kind of a similar way to how water waves do and it ends up adding up and cancelling out with itself so that it forms these regions of intensity and darkness, but when it reaches the screen, you can think of it as like a wave of probability, and then when it hits the screen, it just lands on one possible point with a probability that's determined by how intense this wave is. And so you have to do this experiment again and again to see the pattern build up. because you just get kind of dots appearing with a certain probability. And so you have to do it lots of times to see how that, to see that probability emerging as a pattern. And this, think, ⁓ if it's the first time you hear about it, is like really ⁓ mind blowing, at least it was really mind blowing for me. This was another one of the popular science things I was reading about, or I watched a documentary about this kind single photon double-slit experiment. And I was like, how can a, it seems crazy that a single photon passing through one at a time ends up adding up to this wave-like pattern. Like it's really strange that can, that that happens even when you send them in one by one. ⁓ But then there's an extra twist that happens when you introduce the notion of measurement or observation. Because this is so strange, you're like, hey, well, I want to see what's happening. Which slit did the photon go through? You can imagine putting a detector to check which slit the photon goes through so you can figure out what's happening. If you put a detector that tells you whether it went through the left slit or the right slit, then what happens is that you sometimes get the left slit detector clicks, sometimes the right slit detector, but the pattern you get on the other side is actually the particle-like pattern. So you just end up with two bright regions, one at each slit instead of the fringes, the interference pattern. So by trying to observe where the particle went, where the photon went, ⁓ you end up with this particle pattern. So then it seems like when you don't look at it, the photon behaves as a wave. When you do look at it, it behaves as a particle. ⁓ And so you get this kind of aspect of wave-particle duality happening depending on if you... look at it or if you don't look at it. ⁓ Yeah, so I found that really strange for a long time, this, it's kind of a strange aspect of the narrative is exactly that point of how does the light know it's being observed. And what I found at some point was a very useful thought experiment for making sense of this and understanding how it knows it's observed is to imagine shrinking your detector down to itself being just like a single particle. So I'm imagining the kind of a... We have the kind of particle-like setup where you're going to use a detector to check if it's going to go through the left slit or through the right slit. But the detector, yeah, it's just like a single particle that you can imagine if the photon goes through the left slit, it kind of... the particle flips one way. if the photon goes through the right slit, the particle flips the other way. So the particle is like a literally single particle measurement device for which way the photon is going. And then you can ask the question, if you just introduced this particle, so no one else looks up the particle, I'm not going to look at it. No large device is going to look at it. No, nothing that... we typically call an observer is going to look at it. It's literally just this single particle is going to record whether the photon goes left or right. What are we going to see on the screen? Are we going to see the particle-like behavior, which is what we get when someone looks at it, or are we going to see the wave-like behavior, which is what we get when no one looks at it? ⁓ And when I've occasionally asked this, kind of giving talks and stuff, ⁓ kind of had divided responses from the audience of say like students. ⁓ So it's, yeah, it's maybe a bit counterintuitive, but when you work through it, you see that it actually behaves in the particle-like way, even if your detector is just a single quantum particle, but just stays there, doesn't interact with anything else. And that's because that single particle, in order to record which way the photon is going, it has to get quantum entangled with it. So it becomes correlated with which way the photon goes. And then once the photon has got entangled with something that's kind of a outside of just itself in isolation, it's no longer isolated and it can't, when it goes past the slits, it can no longer interfere with itself because it's become entangled with this other particle. If you wanted the photon to interfere with itself, you would have to bring that particle in and include it in this interference. So the fact that it's got entangled has basically kind of separated out those two directions that the photon can go in in different separate branches of the quantum wave function so they can't interfere anymore to produce that wave pattern. ⁓ So if you try and think, well, what does this mean about the wave particle behavior? think the best way to think of it is that you can describe your extra kind of a single particle detector is also, it has like a wave function, it's described in this wave-like way by quantum mechanics. And overall, if you zoom out, everything is still behaving in this kind of wave-like way where it can interfere with itself. But what's happening is that the detector is, because it's becoming entangled with the photon, it is preventing it from interfering with itself, which leads to a particle-like behavior. But really it's described by the same type of quantum wave that describes it. when you don't introduce that detector. you can kind of have this consistent story for both scenarios where the real difference between the two is whether entanglement is happening or not, rather than the photon changing the way that it's behaving in a different depending on who looks at it. ⁓ Yeah, there's this kind of ambiguity of what counts as an observer because you can go all the way from, yeah, you can imagine these kind of single particles kind of acting as a detector in that it moves one way depending on which way the photon goes and another way if it goes the other way. ⁓ But that's not what we kind of use as a measurement device in practice because for us to look at anything It's going to have to be kind of macroscopic. It's going to have to be a large scale where it's kind of in a definite state and we're not, it's not in this regime where we can kind of control the micro quantum effects. ⁓ And so yeah, there's not really ⁓ a universal definition of what an observer is and people tend to use it, a human as like the the kind of upper limit of like, we know that a human is an observer. we know that one is an observer. A single particle, like most people are not convinced that that counts as an observer because it's not like something that, yeah, it seems too small, not complex enough, not large enough to really record a result in like a proper sense. so ⁓ I think often the one I hear people happy where there's like a good... level to describe it at is a photo detector. the kind of the component that you use if you're in a lab when you're detecting a photon and you want to register the click. ⁓ There's a kind of process that happens in that device where the single photon ⁓ creates triggers this kind of cascade of like a load of photons and that gets the signal big enough that you can measure it on this device. And I think the community tends to be happy with calling that kind of a, at least an example of a minimal unit of what we should think of as an observer in the sense of like a measurement device, something that counts as doing a measurement in a proper sense. ⁓ I don't think I've heard it before, so maybe if I try and kind of recap what you said to check, I, ⁓ yeah, processed the idea. So the idea is that, in a similar way to how you can, there are some parts of a video game that aren't rendered until you kind of reach that point. When you... do an observation, you're rendering part of a reality that wasn't there before. that, yeah, with maybe a kind of similar underlying principle of like trying to space that wasn't needed to be rendered until it needs to be kind of thing. I guess I can try maybe... So the view that... ⁓ that I find most satisfying of how observation is working, which is a contentious issue, but I find the many worlds interpretation the most satisfying one. in that scenario, kind of... ⁓ You've kind of got this, you start off with your... ⁓ particular state, is kind of, so if you imagine that you stop, everything's kind of the same and then it's going to split. So everything's the same. And then maybe you have this quantum system that's in a superposition of zero and one, and then you measure it. And then according to this interpretation, you become entangled with it. And now you've kind of split from a version of you that ⁓ hadn't seen the outcome yet to two different versions of you, one that saw outcome zero, one that saw outcome one, and they've got this kind of 50-50 weighting in the multibus. So there's this kind of weighting applied to the wave function. ⁓ In that sense, there is something kind of, There's something extra after the measurement that wasn't there before the measurement in the sense that after the measurement, there's now two versions of you that are different, but they've each got this kind 50-50 weighting, which is what brings the kind of, conserves the total amount of you in the universe, or in the multiverse. ⁓ So, yeah, I guess that's how I'd see the measurement processes kind of that you are, there is an aspect of creation in that there's an aspect of differentiation of observers of whatever the measurement device is, whether that's an observer or something else is kind of getting differentiated by these interactions. ⁓ But I suppose the ⁓ Yeah, I guess the simulation idea is kind of saying that there's a... that there's something, some kind of blank register and then you're like creating something, some information into that blank register that wasn't there before. ⁓ But I think, yeah, I'd see it more in this multiverse view of like that what was there before is getting differentiated ⁓ rather than something new being added on the level of the dynamics of that measurement. I think ⁓ most of the time in practice, even if we're not actively measuring something that happens, it's still getting measured automatically by the environment. if you imagine, yeah, you have some quantum event instead of the tree falling and then you're asking, did that happen if no one... is observing it since we only get these measurement outcomes. We get this probabilistic collapse into a certain measurement outcome when you measure it. So you might think, okay, if I don't measure it, well, it's going to be in this superposition state. I've not interfered with it yet. so it doesn't count as having been in a particular state yet. ⁓ And in practice, if... we get a very quick process of a quantum event interacting with everything around it and ending up getting collapsed. ⁓ It's, yeah, I'd say typically for general events that happen, ⁓ the quantum superpositions that get created will end up kind of interacting with other stuff around them so quickly on such a short time scale that they decahere, which means they kind of effectively end up collapsing to one of the states with some probability. And then that decoherence kind of spreads out as a domino effect to the whole environment, reaching everything else. so even if you're not trying to measure that thing and you don't care about measuring that thing, you're still going to kind of end up, it's going to be decohered into kind of your reality very quickly. So in that sense, we're kind of measuring everything even if we don't actively try to. But that's what we're trying to do the opposite of in quantum technology and in quantum computers, for example. The whole challenge is that the universe is all fighting to measure your quantum bits, your quantum systems that you're trying to keep in their superposition states. And you're having to fight off those interactions, which are essentially unwanted measurements from the environment. And so the goal with building a large-scale quantum computer, for example, is exactly to kind of push out these unwanted measurements and ⁓ kind of to create the scenario of having a tree that can fall in the forest without anyone being able to hear it. But in the quantum version of your quantum computer, you can have... ⁓ things happen in the isolated ⁓ space and they're isolated from their environment. So ⁓ until it's the right time to look at them and get your outcome, you can just let it in isolation do its own thing. And in that scenario where you've done the very hard work of isolating it, I would say that you do get this kind of a new type of ⁓ phenomenon. possible with quantum systems where you can have events happening in the isolated quantum system that are separate in quite a fundamental way from the events happening outside them. We're not in the same branch of reality yet until we interact. Yeah, what you said all sounds quite right. Really, until we've got the hardware to play around with at large scales at low error rates, we're not going to know the full scope of what quantum computers can do because it's very difficult to prove what they can do theoretically and what they can do faster than a classical computer theoretically. These risks to encryption, the algorithms that can attack encryption are some of the few that you can actually work out theoretically, the kind of advantage over classical computers. But one particularly interesting area for, there's lots of different areas where they can be applied that are much more positive ones, in particular, for example, in drug discovery, because as I mentioned in the beginning, the equations of chemistry and how molecules interact, you need quantum mechanics to describe how those interactions happen. And it was one of the origins of quantum computing was a lecture by Richard Feynman where he was explaining kind of the idea that what you'd expect the best hardware for simulating quantum systems that underlie chemistry would be actual quantum systems. And so Looking at quantum algorithms for ⁓ simulating molecules for things like chemistry and drug discovery are one of the main applications of quantum computers. ⁓ There's also applications to material science, for example, people looking into things like discovering new fertilizers, for example, again, with this idea of simulating chemistry. ⁓ And then there's a whole other category where people look at accelerating certain aspects of machine learning processes. that's quantum machine learning. then that would have very a range of applications ⁓ itself because of the range of ways that you can apply that. yeah, as you said, it's kind of these, there's... specific algorithms that it can speed up, but they can be significant in the sectors that they do provide a speed up in. Then there's the kind of more long-term fun quantum foundations type experiments that you can imagine doing on quantum computers like simulating observers and try to actually test out some of these interpretational questions, which are a long way off because they would need a lot more development than just having a quantum computer available. ⁓ that's kind of where I'd see the far future of this technology feeding back into the fundamental science. Where we're at is that we have quantum computers running. The company I work for builds quantum computers out of superconductors. The same type of hardware that Google and IBM build them out of. All of these organizations have quantum computers running. ⁓ Ours have 32 quantum bits, for example, which you can actually run stuff on those qubits and run algorithms. But the problem is that when you just purely have these quantum bits built out of a quantum system, you've got errors. These are these unwanted measurements from the environment that are constantly happening. And that limits how far you can, how accurately you can run something. And so... ⁓ We're at the stage where some people, especially like IBM, are going for trying to squeeze out what they can from these types of devices that have a lot of these physical ⁓ qubits with some errors that they've got pretty low. ⁓ But to really kind of unlock the major scope of a large range of applications, you need to do a process called error correction. where you do a kind of encoding where you use lots of ⁓ noisy qubits that have errors to represent one clean qubit that's got low errors by encoding that one qubit on all these noisy ones and then doing ⁓ a correction process continuously. ⁓ Usually the last few years when we've been hearing about big breakthroughs in quantum computing, A lot of them have actually been experiments demonstrating that this error correction process works. ⁓ And so in a way, it seems like you've gone backwards because we've maybe gone from saying like, ⁓ look, you can get a hundred qubits and do this simulation with them and like, that's cool. To going, look, you can get one qubit and this experiment has been demonstrated. But it's quite different because that one qubit is kind of one of these ⁓ clean logical qubits that's built out of lots of physical noisy qubits. And that's demonstrating that this process of error correction works, which was just theoretical before. so ⁓ these, I think most of the, the excitement in the community recently has been around these error correction experiments, demonstrating that the theory all checks out and then then you can think, okay, well, if the error question works, then. There's nothing stopping us building the large scale, low error quantum computers. So yeah, I'd say the stage the industry is at now is... trying to scale up the error corrected approach and start with some early versions of doing the error correction. Then everyone has these roadmaps for pushing to lower errors, larger devices with more of these clean qubits. That's the current trajectory is getting to the next stage of these These devices which will each kind of unlock a larger and larger space of possible applications that you could run on that regime of device. Yeah, it's a very interesting question. ⁓ well-known approach to considering that is from Roger Penrose, is a Nobel Prize winner in physics and has this theory that there are quantum effects underlying consciousness. And I think he uses that to solve the measurement problem as well in thinking that. ⁓ Yeah, it's involved in causing the collapse maybe. ⁓ So yeah, according to that, to his theory, ⁓ quantum effects play a role in consciousness. ⁓ Yeah, I'd say probably the... their community view on it is that they're like, well, Roger Penrose is working on it, so we should not dismiss it. But most people will probably expect that quantum effects aren't needed for consciousness. And there's maybe a small number of people kind of pursuing the Penrose line of research, but it's kind of a... I'd say maybe a fringe but respected avenue to explore as a possibility. ⁓ Then there's maybe a kind of broader area of research that's related, which is in quantum biology. More generally looking at is the quantum effects that affect things in biology. ⁓ The common one that we maybe have the most signatures might actually be happening is quantum effects being involved in photosynthesis and you get this weird situation in quantum biology where it seems that even though biology is an environment that's warm and wet and it's not where you would expect these quantum effects to be appearing, it seems that in certain cases like phage synthesis, the eyes of birds for migrating, that's kind of another one of these signature quantum biology areas. ⁓ The sense of smell is another one that people look into. ⁓ In these particular areas, it seems like there might be some quantum effects kind of surviving in a warm, wet environment. so, ⁓ yeah, various... Researchers are looking into that and kind of coming up with models of how that physics could work. And yeah, they may ultimately kind of relate to the investigations about quantum effects in the brain. If it works out that we can understand by what mechanism quantum effects could be sustained in these warm and wet. environments. Hmm, ⁓ Yeah, I'd say what comes to mind when you ask about a Holy Grail is quantum gravity. think if we had, so we've got this problem where you have Einstein's theory of general relativity is kind of the other theory that's seen as one of the most successful theories that we have. And that kind of explains the universe on a large scale in the sense of explaining cosmological aspects and things. But we have this discrepancy where when you try and combine Einstein's theory of general relativity to describe gravity with quantum mechanics, we end up with various problems emerging and it's kind of the probably the biggest problem people think of when they think of what's the big problem in theoretical physics at the moment. It's what's the theory of quantum gravity to reconcile quantum theory and general relativity. ⁓ And a lot of the, I think when you consider these kind of different interpretations of quantum mechanics, often that's kind of part of the motivation ⁓ is which direction is going to kind of lead us to the theory of quantum gravity. Like it seems like it might be significant in enabling that. ⁓ But more broadly, I'd kind of split our understanding of quantum mechanics into maybe three categories. I'd say there's an axis along, you could say we understand it really, really well because it's like so well understood in a sense that it predicts things so precisely and we really, we understand how it manipulates matter and we understand a lot of aspects of it in extreme detail and people are using it to build our mobile phones and And things, so there's definitely a kind of a very broad sense of which it's understood really well. And then there's another axis along which there's kind of controversy and debates today. And I think different people might think, ⁓ this thing is actually well understood or we've got satisfying explanations for it, but another physicist will disagree with them. it's like, so I'm I might say like, I think we've got satisfying explanations for the double slurred for entanglement and so on, another physicist will say, no, actually we don't. And then you've got this kind of a, third layer, which is the one I mentioned of like the theory of quantum gravity and that type of unknowns of where quantum sits with the or other fundamental theory of physics. Mm-hmm, sure. So in training as a cat, the thought experiment is that you've got a cat trapped in a box with a radioactive atom and the atom has some probability of decaying and some probability of not decaying. And according to quantum theory, it can be in a superposition of decaying and not decaying at the same time. And then you've got this atom hooked up to a system so that If it decays, it triggers a hammer that smashes open a vial of poison that kills the cat in the box. If it doesn't decay, then none of this happens. The cat stays alive. And then if you describe everything that happens in this thought experiment using quantum mechanics, so you treat the hammer as a quantum system, the ⁓ poison as a quantum system, the cat as a quantum system, then it leads you to say that this superposition that the radioactive atom is in kind of spreads out to include the hammer and the poison and the cat. So what you end up with is all of these things in this big entangled superposition where one version of the cat is entangled with the hammer that has smashed open the poison and the atom that's decayed. And another version of the cat is just alive. ⁓ had the poison smashed and the atom has not decayed. So that's what applying quantum theory to what's inside the box kind of gives you. And then you've got this rule of, well, an observer looking at a system will collapse it. ⁓ But when you see it this way, you can see that there's not really a difference between the cat or the poison or the hammer in terms of what counts as an observer. They all undergo the same process of once the hammer interacts with the atom, they become entangled and then the hammer kind of splits into these two versions. And then the hammer interacts with the poison. Same thing, poison splits into these two versions. And then the cat interacts with, the poison interacts with the cat. Same thing again, the cat splits into these two versions. And then if you open the box, then the same thing happens again, that you split into these two versions once the cat interacts with you. So we have this kind of chain where a measurement of some sort is happening at each step, that the hammer measures the atom, the poison measures the hammer, the cat measures the poison, you measure the cat. ⁓ And all of you equally count as an observer. If you just describe everything by a quantum system and then they're all getting entangled with each other as they interact. ⁓ So that's kind of ultimately what leads to the many worlds view of the thought experiment is that you end up with these different branches that equally exist of the different versions of what can happen with the cat being dead or alive. The more standard narrative is the Copenhagen interpretation, which says that observation causes collapse. So at some point in this chain, you have an observer that caused a collapse, but the controversy or the unanswered question by that theory is at which point in the chain. So it doesn't give an answer of, is it when the human looks at it? Is it when the cat looks at it? Is it when... the poison interacts, is it when the hammer interacts? Because the hammer is a large object. It doesn't give any indication of which point in the chain you draw that line. that's what put me off the interpretation is that it doesn't give you an answer. So yeah, that's my take on it. Yeah, it kind of depends on what you want to use it for, but I think that, yeah, my take would be that we don't really need to distinguish ⁓ the role of the human or the cat, the poison, the hammer in this experiment. And it's just as valid to consider that kind of single particle version that we talked about earlier. like single particle detector is going to do the same thing ⁓ that these kind of larger observers will. But I think where it becomes important is if you want to test out two different theories against each other and one theory says there are interpretations of quantum mechanics that say an observer is like a system that's large or has a lot of mass or there's like these conditions that they think will cause a collapse. So if you want to test those theories out, then you use that theory to figure out what your condition for an observer is. Maybe it's something over a certain mass, and then you do an experiment with that being in the place of your observer. Or maybe it's something with a certain complexity, and so you build an observer with that type of complexity. I'd say it depends on which specific theory you want to consider or test. Yeah, I mean, the Copenhagen interpretation just, think doesn't count. you have to... There are objective collapse theories that give a condition for the collapse and they've got their own open problems, but they've not been reconciled with relativity, for example. ⁓ But I think you have to commit to an interpretation like that rather than Copenhagen, which doesn't really give an answer to even compare to. ⁓ But you can test the many worlds interpretation in theory against a collapse theory by ⁓ imagining a false experiment where you reverse a measurement. And if you can reverse the measurement successfully, then that's evidence for the many worlds interpretation. If you can't reverse it, then that's evidence for a collapse interpretation where a measurement caused an irreversible collapse. Yeah, there was this thought experiment that David Deutsch came up with in 1985. He imagined essentially running an observer, simulating an observer on a quantum computer that makes a measurement and then you reverse the observer making the measurement and then test out whether the reversal happened successfully or not. And that would be a way of testing the many worlds interpretation against a theory where the observer caused measurement to collapse and that's one of the ways that along with the Feynman origin story that we mentioned earlier, that's the other origin story of quantum computing is that he thought of the type of device you'd need to simulate someone in order to run this experiment would be a quantum computer because you have to be able to sustain superpositions in the hardware to be able to actually run it. ⁓ So that's one of the ways you could test it. There's a range of other wacky ideas that you can imagine running. Something I'd like to make a content series about at some point is all the different ways that you can test the many worlds interpretation. yeah, it's yet to exist. Sure, yeah, my YouTube channel is Dr. Maria Villaris. That's just my name. I do short videos on there as well as my Quantum Foundations podcast. And the podcast is also on all the audio platforms, Spotify, Apple, and so on. People can also find me on LinkedIn, Twitter, and X, and Instagram. ⁓ Yeah, I think that's the main places.