The Interpretations of Quantum Mechanics

okay this video is on the interpretations of quantum physics which i've been down a little bit of a rabbit hole over the last couple of months if i'm honest because the interpretations of quantum physics aren't easy to understand and i thought hey i'll do a video on those that'll be fun and then i did my research and looked into it and then kind of my brain melted so anyway this is my best effort at the interpretations of quantum physics i hope you enjoy so what are they well first of all there's a hole in the bottom of physics when you're down at the level of atoms and subatomic particles you're in the realm of quantum physics and there's some things in quantum physics that simply just don't make sense physicists don't like things not making sense and so the interpretations of quantum physics an an attempt by many different physicists to come up with ways of making quantum physics make sense now if you're not familiar with the terminology of quantum physics i recommend you watch my last video because it's kind of an introduction to quantum physics and this video is a sort of part two to that so interpretation number one is the copenhagen interpretation and that's kind of the standard way we're talk quantum physics when we learn it in university and it's also kind of the place where i can illustrate where some of the problems start happening so in the copenhagen interpretation the standard description is is that subatomic particles like an electron is described by a wave function and this wave function obeys the schrodinger equation which tells us how that wave sort of smoothly evolves over time and features of this wave gives us all the properties of quantum physics that people talk about like superposition entanglement energy quantization decoherence quantum tunneling all of those things are phenomena that appear because of the wave-like nature of sub-atomic particles so the conceptual problems begin when we do a measurement on these waves so the copenhagen picture of quantum physics is you have a wave function moving along smoothly according to the schrodinger equation until it hits an object like a detector and then what we see what we measure as a human is a single particle in a specific position which means that the wave function which is like spread out over space when it hits the detector it suddenly collapses it's called localization so the wave localizes from a spread out wave to a very sharply defined wave where we see the particle the trouble is there's no actual physics in quantum mechanics that describes how that collapse happens and this is known as the measurement problem so this measurement is sort of a barrier which shields the quantum realm we can never see those wave functions or we can ever see a particles and in the copenhagen interpretation of quantum physics what they say is like we're us humans we'll never see those waves there's nothing we can do to access that realm so don't worry wondering whether they're real or not real the important thing are the measurements reality is in the observations which is why it's been termed the shut up and calculate interpretation of quantum physics but people have not stopped there they don't like this discontinuous collapse of the wave functions they've come up with other ways of trying to interpret what's going on and this brings us to the second interpretation i'm going to cover the many worlds interpretation of quantum physics so the many worlds interpretation quantum physics takes the wave function to be physically real and says if the schrodinger equation is just a description of reality what does reality look like so what it says is that when you do a measurement on a particle that's in a superposition of many different places at once actually that particle turns up at all of those different places just in different versions of reality another way to think of it is if a particle is in a superposition of two places at once and it hits a detector it now puts that detector in a superposition of measuring the particle in one place or another and if you look at the results from that detector then it puts you in a superposition of seeing the particle in one place or seeing a particle in another place but because the results of one place or another are mutually exclusive it means that now your universe between the two u's have decohered from each other are now split into separate branches so a superposition is described by a wave function so really there's just one giant mega wave function that describes all of the possibilities that could ever have happened in the entire history of the universe so if there's an interpretation quantum physics that you've heard about it'll be this one the many worlds interpretation because it seems to capture public consciousness and i think it's because people like the idea that if you've made like a really bad decision that you regret in the past you can kind of take comfort in the belief that there's another universe out there where you made a better decision and so there's a version of you who's happy right now it also seems to be a very useful storytelling technique in films that want to have different sort of realities like it was just in the into the spider-man into the spider-verse for the different kinds of well i shouldn't ruin the plot anyway it's a good film it also seems to be pretty popular amongst physicists although i'm not such a huge fan of it because one of the problems is it breaks probability so imagine if you've got a particle that's in a 30 probability of being here and a 70 probability of being here when it hits the detector it splits into two universes where it definitely is with a hundred percent probability it's in one place and a hundred percent probability it's in the other place so if everything all happens what do those original probabilities actually mean this problem is addressed in a interpretation called the cosmological interpretation which says that many worlds theory is trivially true if the universe is infinitely big because there's an infinite number of views doing that experiment and those that infinite number just splits in proportion so thirty percent of those infinite u's will see one thing and seventy percent of those infinite u's will see the other thing unfortunately there's no experiments that we can actually run to see if this interpretation is more valid than the copenhagen interpretation and that's actually a general feature of all of these interpretations is that we can't pick them apart with any experiments and so until we have experiments it's all kind of like physics-y storytelling let's now look at another aspect of quantum physics that doesn't quite make sense to us which is called non-locality otherwise known as spooky action at a distance and this can be summarized by the epr experiment which is where you take two particles say electrons and you entangle them together to have equal and opposite spins say and then you separate them by a very very large distance because they are now inextricably linked the wave function that describes them has to describe both particles and what this means in reality is when you do a measurement on one of these particles it instantaneously influences the other one which can be you know billions of miles away and it does that instantaneously so it sort of feels like faster than light action that's the action at a distance which einstein was really creeped out by so non-locality means that one electron its properties are not local to its physical place its properties are dependent on something very very far away and this is an alien concept in physics because if i throw a ball the ball's position or velocity or color or anything physical to do with that ball they're all localized to where that ball physically is whereas in quantum physics you can have things that are non-local and that seems weird to a physicist because you're like all physics is like this except for this one bit here in quantum physics so this has spawned other interpretations of quantum physics like trying to explain away this one and one of these is called hidden variable theories so the idea in a hidden variable theory is that when you do this entanglement there's some secret label which captures the actual state of these two particles so it's like if i flip a coin and i hold it in my hands even though i don't know whether it's heads or tails the coin actually is heads or tails when it's in my hand and i just reveal it when i open up my hands so that's kind of what these hidden variable theories are the particles are in a definite position we just don't know what it is until we measure it now this interpretation was actually killed by an experiment called bell's theorem i'm not going to go into the details of this because i don't have time but basically the hidden variable theory and quantum physics predict different probabilities of you observing certain things under bell's experiment and when they did the experiment they actually found the results match quantum physics and not hidden variable theories so hidden variables are out now it's not completely game over for the hidden variable theories because bell's theorem is built on some axioms some things that you assume to be true and one of these is locality and so you can have a successful non-local hidden variable theory also known as bohmian mechanics also known as pilot wave theory now the idea in this interpretation is that the particles are always real but they're kind of surfing about on an underlying wave and so when you see the probability of particles appearing in different places it's because they've been nudged in different directions by this underlying wave but still we can't see those waves and the advantage of this is that it kind of returns quantum physics to a sort of determinism which none of the other interpretations of quantum physics do but the trouble again is there's no testable hypothesis so there's no way of us doing an experiment to see if it's true so another thread in the interpretations of quantum physics is to instead focus on the collapse of the wave function and try and make that bit make sense so there are interpretations called alternative collapse theories that try and add some extra physics into that collapse either explain why it happens or adds like describe the dynamics of exactly how the wave function localizes one idea called spontaneous collapse theory is that the wave function has got a probability of collapsing at any time a bit like sort of radioactive decay so for a small particle there's a very low probability that its wave function collapses maybe like once in a hundred million years whereas for big groups of particles together in like a football there would be way more collapses because if any one of those collapses it has a knock-on effect to collapse the wave functions of all the others and so there'd be maybe 100 million collapses per second which kind of goes some way to describe why the world we experience is all kind of solid and deterministic and the quantum realm isn't so it describes kind of the boundary between those things and the advantage of this interpretation is it's actually making a testable prediction and so at some stage in the future we could design an experiment that could maybe see if it's true or not so you know it doesn't say that it's any more likely to be true than any of the others but at least it's got a testable prediction which is an experimental physicist i like so those are the main interpretations of quantum physics but there's many many more i'm just going to cover a few more here but this isn't an exhaustive list also word of warning this is where i started to get really rather confused so let's proceed with some amount of caution first up is quantum bayesianism also known as cubism which takes the ideas of bayesian probability and applies it to quantum physics so sort of like a informational theory of quantum physics and the idea as i understand it is that when you get new information about a state it updates the probabilities of the things that you will measure the consistent history is interpretation quantum physics apparently the mathematics is somewhat of a hybrid between hidden variable theories and spontaneous collapse theories it includes spontaneous collapses of the wave function but different aspects not just position can collapse it also says that those collapses aren't physical events but just a way of you picking through a history of that quantum object that makes sense that's consistent the next one's quantum darwinism which is where when the quantum system interacts with the environment certain things are killed off so interactions between particles in the environment are kind of like natural selection for the properties of that quantum object another interpretation is called the transactional interpretation which in the actual mathematics of quantum physics it allows solutions that travel backwards in time as well as forwards in time but we normally just throw away the backwards in time ones because you know you're gonna travel backwards in time but like i don't know maybe you can so that's what the transactional interpretation it keeps those and says that the properties that you have may be dependent on things that happen to you in the future and this is attractive because it can actually get around bell's theorem another one is called the relational interpretation which doesn't focus on the properties of quantum objects but everything's defined by the relations between them so those are the main interpretations of quantum physics there's a few others but those are the main ones i sometimes get asked what is my favorite interpretation and i don't really have one my thoughts about this uh because there's so many different interpretations of quantum physics it's kind of a sign that we might be missing something kind of fundamental so maybe one way to attack this problem is to really go back to first principles and back to the fundamental assumptions of quantum physics because there's some interesting assumptions that were made like when you have a wave function which predicts probabilities of things happen that was just a guess and it works really really well but there was no real reason for that other than nice guesswork so yeah quantum physics is going to [Music] well that's the end thanks for sticking through all of that i hope it was of interest i think you've earned yourself a nice sit down and maybe even a biscuit so thanks for watching and thanks also to the sponsor of this video skillshare i always get loads of comments asking me how i produce my videos and what software i use and the truth is it's a combination of all the adobe software illustrator photoshop after effects and premiere and skillshare is a fantastic website if you want to learn any of these programs for your own creative projects as well as covering the adobe software they've got more than 18 000 classes covering everything from drawing writing through to film production their premium subscription service unlocks all of these lessons which are delivered by trained professionals in their fields but if you want to get a head start you can get a two month free trial for the first 500 people to click on this link the link is also in the description below well that's it from me thanks for watching and i'll see you next time you

Channel: Domain of Science

Views: 205,344

Keywords: quantum, physics, quantum mechanics, science, explainer, animation, many worlds, entanglement, superposition, quantum theory, copenhagen, pilot wave theory

Id: mqofuYCz9gs

Length: 17min 11sec (1031 seconds)

Published: Wed Apr 03 2019