Episode 23: Many Worlds Quantum Mechanics
- Links to this episode: Spotify / Apple Podcasts
- This transcript was generated with AI using PodcastTranscriptor.
- Unofficial AI-generated transcripts. These may contain mistakes. Please check against the actual podcast.
- Speakers are denoted as color names.
Transcript
[00:00:07] Blue: Welcome to the theory of anything podcast. We’ve got a new co -host today. This is Tracy, who is a friend of mine. Actually, we’ve been friends since high school. And interesting story. She was actually the one I originally started this podcast with. And it was just hard to find time that she could do it. And so just by chance, cameo asked me to do a podcast with her too. And so we ended up starting with cameo, but I’ve been trying to get Tracy on the show for forever. Tracy, say hello.
[00:00:37] Red: Hello. I’m just a, you know, basic working person who’s interested in all sorts of topics. I guess I’d classify myself as somewhat of a nerd. So I like to read books such as David Deutch’s books, Fabric of Reality and anything that’s interesting about the world and reality and science and learning. And that’s all that I’ve got to say. If you’re interested in getting me up, I’m, I’m all about learning about it. And
[00:01:08] Blue: that’s certainly true. You ask a lot of really good questions. I appreciate that. And then we have our guest today. Sam Kipers. Did I say that right, Sam?
[00:01:21] Green: Yes.
[00:01:21] Blue: He and David Deutsch recently did a paper called Everettian Relative States in the Heisenberg picture. So Sam and David Deutsch did this paper and I had noticed it and that was why I asked him on the show was to ask him questions about his paper and get him to kind of describe it. It is a bit of a rough go mathematically, but Sam and I have been working on also some YouTube videos that describe the mathematics for someone who’s an interested layman who’s willing to put a little bit of effort in to understand the mathematics and by the time you hear this episode those should be up on edited and up on YouTube. The first one anyhow we may do some additional ones and it’s the first one was about explaining Q numbers which is a big part of this paper. So Sam can you introduce yourself tell us a little bit about yourself.
[00:02:15] Green: Yeah so I am a D. Phil student at Oxford where I research the foundations of quantum theory and that usually means well in my case it means I’m interested in the many worlds interpretation of quantum theory and that’s the topic of the paper. So the paper is about a kind of reformulating Everett’s relative state construction which is the foundations of the many worlds interpretation of quantum theory.
[00:02:45] Blue: Yes so Sam I got lots of questions about this but I think my first question for you though is how many people actually accept many worlds interpretation of quantum physics as someone who’s in the field and really doing this in real life. How much of a minority are you?
[00:03:05] Green: I’m not clear on how big of a minority we are at the moment. I remember reading a blog post by Sean Carroll in which if I remember correctly it’s roughly 30 percent of the physics community who are studying these topics are studying kind of foundations of quantum theory believe that the many worlds interpretation is the way to go. Of course in Oxford there’s kind of the gang of Everettians reside here so usually if you read for example the book by Peter Bern about Everett’s life then there’s a section on the the Oxfordian Everettians and so here it’s a big deal. Yes that’s something.
[00:03:53] Blue: In fact you got a conference coming up that you guys are doing right that is it Oxford that’s putting that on or is it just a conference in general the virtual conference?
[00:04:01] Green: That is just a virtual conference that I’m putting up with a friend of mine a friend and colleague Shrel Bidar. Yeah it isn’t really connected to the university in any direct way other than that I’m a default student here and I wanted to organize a conference and luckily doing that online means that there’s there’s not a lot of administrative work so you’re very free in just inviting people and having them speak and yeah so it’s something that we didn’t organize with the university specifically.
[00:04:35] Blue: But you’ve got like a lot of really big names coming to the conference from what I could see so I actually think people should check it out. What is the name of the conference?
[00:04:44] Green: The name of the conference is on the shoulders of Everett and yeah we have a bunch of really interesting speakers we have Chiara Mileto, David Wallace, we have Carlo Revelli, yeah Flutco Vettero, my supervisor is supposed to also be speaking and I think it will be Zurich there’s another one there’s another speaker that I am very much looking forward to hearing present at the conference. Yeah we have a nice list of people attending so it should be fun.
[00:05:16] Blue: Yeah absolutely so you just mentioned that like 30 % or so of those looking into the foundations of quantum physics except many worlds at this point but I’ll bet you if we went back a decade or two even just maybe even just a decade it would be nowhere near that large would I be right or am I misinformed here?
[00:05:38] Green: I don’t actually sure so I think that the poll that Carol writes about on the block is quite old okay but I could also be misremembering the statistics here. I think it is still something like that I imagine well so it’s very strange because I think many students are becoming ever more interested in the topic but the doesn’t necessarily translate to there being more people in high positions in universities who actually research it and I think there’s a distinction but I’m hopeful that it is a growing community of people. I guess the reason I don’t have the statistic in mind is because I tend to just be convinced that it is the right way of doing things and if it’s the right way of doing things then if you make the best case where that you can you’ll hopefully convince others and it will increase in popularity over time. Yes and also it’s the thing worth working on if it’s true.
[00:06:32] Blue: Give us a little bit of history about many worlds how did that interpretation come about and like where did it come from and what was the why was it necessary?
[00:06:43] Green: So it came from Everett’s thesis under the supervision of Wheeler. The main problem that Everett tried to address was this idea that there’s two ways in which quantum systems appear to evolve. One of them is the way systems evolve in general which is kind of if it’s not being observed then there’s a set of rules like for example the Schrodinger equation that determines how systems evolve over time and whenever they’re being measured they apparently collapse what they call collapse and it’s a different kind of dynamical law that is used when systems are observed and the two can’t really be unified in any nice way and the situations in which we apply the collapse rule are rather parochial there when for example some kind of what you think of as a classical measure or a person or someone who’s conscious or something measures a system and that brings into fundamental physics all kinds of things that I don’t think belong in fundamental physics in the way that they’re being presented as. So for example I don’t think that consciousness is part of quantum theory that is a separate interesting topic that should be treated differently and it’s very unsatisfactory to have as a rule in quantum theory that something evolves differently when it’s being consciously observed so that’s that’s kind of one criticism of the collapse postulate.
[00:08:19] Blue: So when I looked into quantum physics I was immediately struck by the fact that there were many things that Canada has an observation that caused this collapse and it didn’t require anything like consciousness it could be a detector it could be a photo plate so it’s interesting that I mean I agree with you when you bring this up but even just with the collapse function even with the way they were teaching quantum physics it’s not immediately obvious at all that you have to insert consciousness that seems almost forced that people started inserting it in it it wasn’t part of even that interpretation.
[00:08:56] Green: Yeah well that kind of brings us to a paradox known as Wigner’s Friends in which you have two observers and one of the observers is isolated in the room and and they and their task is to look at a quantum system and to note down what kinds of measurement outcomes they
[00:09:23] Green: detected and there’s a second person who is usually known as Bob and the first person is usually known as Alice who is outside of the room that Alice finds herself in and the the room is completely isolated and no one knows what’s going on inside the room except for Alice and the the strange thing is that if you apply the rules according to how they’re prescribed then you would say okay well Alice is performing a measurement and therefore something collapses but there is a there’s a larger system which is consists of Alice and the measurement device that you know are contained in this room and and they’re isolated and therefore they evolve according to the Schrodinger equation and there’s this person outside of the room called Bob he he concludes well as long as I haven’t measured this this room that contains Alice she doesn’t actually have anything like objective existence right she’s in a super position right and I think you could say the same thing of a measurement device you could say well there’s a measurement device in the room forget Alice it’s just the measurement device it has recorded something but it doesn’t actually mean anything for it to have recorded anything until we see it because then we that’s when we apply the the collapse positive it sure so uh that that kind of I think people were forced to um start talking about consciousness because of this nonsensical rule or well it’s not completely nonsensical it’s I think it’s too harsh to to say it’s entirely nonsensical because systems do appear to collapse but the the logic of it is unclear like there’s no explanation for why the collapse happens
[00:11:08] Blue: so this is the measurement problem yes
[00:11:10] Green: yes so the
[00:11:12] Blue: idea that Alice has made a measurement and from a certain point of view you could say the system is now collapsed the wave function is now collapsed and from another point of view you could say well she’s now caught up in the the wave function and so for bob she doesn’t she is in a superposition herself at this point
[00:11:31] Green: yeah so in the usual interpretation you would say something like well she doesn’t have objective existence or something just like a particle in a superposition doesn’t have or isn’t assigned anything like objective existence until it’s measured you could then say the same thing about Alice and then all kinds of interesting questions come up from that like well is if we are in a in a kind of a larger room in the sense of if we are part of the larger system that’s not yet being observed by some other observer charlie then how are we to know that our existence is objective I think this is actually one of the the kind of questions that everd asked in his thesis where he raised the possibility that you know Alice of course has a history before bob measures the room that she’s in and she for her nothing changes when bob walks into the room and when bob starts talking about how he gave her objective existence because he measured her in the room he can then be countered with the argument that well you know from her point of view nothing changed how about if they’re still in a kind of a larger room that is yet to be measured by char by some other observer charlie how are they to know that they already have objective existence or not and so that gets all that gets into trouble very quickly yeah
[00:13:07] Blue: for that matter why are we going in one direction I mean couldn’t alice also claim that bob had no objective existence until he walked in the room yeah and very true the other thing that I found interesting when I was studying this years ago was the epr paradox if I recall I don’t remember much I remember working it out once on paper so that I make sure I understood it but then it like started slipping out of my mind almost immediately but it was a paper that einstein wrote um with two other people p in the r he was trying to demonstrate that there was a problem with quantum physics because it violated non locality um of his theory of general relativity is that correct do I understand that correctly uh
[00:13:54] Green: yeah so it violated locality locality sorry yes so einstein has what is known as einstein’s criterion of locality which which states that whenever you have two subsystems that are spatially separated from one another like for example if um if if you have the earth and the sun that’s spatially separated in the sense that there you know there’s some distance between them and you you do something on earth then that operation that you perform an earth film that that has no immediate effect on the sun it’s only when people when uh objects come into contact with one another that they can exert some kind of effect on one another so operations that you perform on earth don’t necessarily affect distance stars the sun other systems etc maybe an even nicer example is that you know if the the light from the sun has to travel to the earth and before it arrives on the earth it’s it can’t have any physical effect on us that’s part of the kind of theory of relativity that the speed of light is the speed limit of the universe and that means that before light reaches the earth the the sun cannot have any effect on us and that that is another way of viewing locality that systems need to be in in contact with one another before they can physically affect one another
[00:15:19] Blue: yeah so einstein’s purpose then for coming up with the epr paradox was to show that quantum physics must be wrong because if it were right then there would be this non -local aspect to it that was his point right the problem was that the experiment he laid out actually works in real life there thereby leading to this idea that quantum physics is non -local and how many to what degree do physicists currently accept accept the idea that quantum physics is non -local
[00:15:53] Green: it’s it’s that’s a very controversial topic most people most physicists think that quantum theory is non -local in other words most physicists think that if you have a quantum systems and they can exert effect when they can affect one another even when the space is separated so if you have a quantum system on on the earth and one on the sun then they can they can affect each other faster than the speed of light in some sense and this is why einstein thought of that thought experiment because he was convinced that is not how physics is supposed to or how physical systems are supposed to behave and it seems from the usual way of formulating quantum theory that that is in fact what’s going on that that these systems have effects on one another even though they aren’t supposed to even though they’re very far apart and should not in any way be able to change each other’s behavior
[00:17:01] Blue: so in your paper you mentioned that Everett formulated his picture of quantum physics in the schrodinger picture instead of the heisenberg picture and that’s like kind of the main reason why you’re writing this paper is because you’re saying that the schrodinger picture is non -local and therefore it seems like if we are taking those equations seriously then we would have to say that quantum physics is is non -local is that correct
[00:17:31] Green: yes yeah in the so everything I just said about quantum systems affecting one another even when they’re not at the same location it appears to be true in the schrodinger picture and the reason for this is that if whenever you have multiple systems that you describe in the schrodinger picture so whenever you have say perhaps you have two three or more particles that you’re describing in the schrodinger picture the the way that you describe those particles depends on how you describe the other particles so there’s no way of describing the particles individually separate from the other particles present in the system in the composite system and this seems to suggest that there is no such thing as the individual particle
[00:18:17] Green: this kind of weighs around this what’s known as the density matrix formalism in which you can talk about the state of an individual particle but that formalism is incomplete so in this schrodinger picture if you have if someone gives you what’s known as the state vector then in some sense you know everything about the system you know all of the possible expectation values of every possible measurement on the system and if you use this different formalism the so -called density matrix formalism then although you are able to describe subsystems independently of the other systems so you’re able to describe say a single particle in this multi -particle composite system the density matrix formalism isn’t complete so you’re not able to actually compute all of the possible expectation values you lose some information by going to this local description and consequently people have concluded that quantum theory really isn’t local that it isn’t really meaningful to describe subsystems individually because they they always have effects on one another when they are part of a larger system
[00:19:28] Blue: and you mentioned in your paper that even some really big name physicists even ones that believe in many worlds see it as non -local like vaidman vaidman you mentioned
[00:19:39] Green: yes yeah vaidman has a very nice entry on stanford encyclopedia of philosophy in which he writes about this and he explicitly states that his his notion of a world of the universe is a non -local in other words so in in kind of our terminology we would say that the the universe is all of classical mechanics so it’s all is kind of the classical world that we see around us and it is but one of many such universes which exist in in the multiverse so the multiverse is a larger object than just the universe and in vaidman’s terminology he says that the universe is a non -local thing that whenever a split happens it’s really the entire universe that splits all at once
[00:20:33] Blue: which which follows from the schrodinger picture correct
[00:20:37] Green: yes yes that’s right so if you’re working in the schrodinger picture then that i think that’s indeed what’s going on because again there’s not really a way of describing systems separately from one another so whenever one system splits this has to also have an effect on the other systems in the schrodinger picture because because there is no way of describing them separately so
[00:21:01] Blue: let’s take just a moment to kind of explain what we mean by non -local the kind of the standard experiment would be like you’ve got a particle that’s in a spin up and a spin down superposition and it splits into two particles which are now in a superposition do i have this correct and then you like separate them take one light years away and leave one on earth as long as they don’t become entangled with their environment they should still be entangled with each other
[00:21:31] Green: yes yeah so it’s a it’s a what’s known as a bell state where you have well i said well Einstein and uh Rosen Podolski they they didn’t actually know about bell states but the way in which typically it’s formulated today is basically the way that you formulate it now namely you have these small quantum systems namely qubits that are entangled with one another and they’re entangled in such a way that if you for example measure up in one of the qubits then the other qubits will also be in an upstate and if you measure down on one of the qubits then the other qubit will also be in a down state for example that’s there’s one of the possible ways doesn’t don’t they have
[00:22:11] Blue: to be reversed i thought like one had to be up and one had to be down because of
[00:22:15] Green: um i think i think it’s both are fine it all that matters is that they are are entangled in the sense that the the joint measurement it gives you some certainty so if you if you know that one of them is up they know with uh with with certainty what the other qubit will be when you measure it and it doesn’t matter if it’s sorry go ahead
[00:22:37] Blue: well that’s why it seems non -local then right is because we just we just measured this one particle and light years away we now know something about the other particle that wasn’t known prior to that point
[00:22:48] Green: yes and part of the reason why that isn’t exactly true is for example that you can still perform operations on the other particle and you you know a conditional truth which is that if for example so is you take the state in which uh the particle both particles in superposition in such a way that if you measure one particle uh in the upstate then the other one is also in the upstate and you measure one particle in the downstate then the other one’s also in the downstate and now you you separate those two states you again keep on on earth go and and bring the other one to another galaxy for for purposes of this thought experiment and perform a measurement on the qubit here on earth then you immediately know you think you immediately know what’s going on with the qubit in in the other galaxy but that’s not exactly true because you know that only if nothing happened to that qubit right uh you’re able to determine its state but it could be that it rotated or that something happened to it that you can’t know about because physical systems actually adhere to the Einstein’s criterion so in other words maybe some observer decided to perform an operation on its qubit and you won’t know about that operation because it as it turns out it has no physical effect on you
[00:24:04] Blue: right
[00:24:05] Green: if you’re spaced like separated yeah
[00:24:07] Blue: so that makes that’s a very good example i’ve never actually heard explain that well before um the reason why i guess they see it as non -local is because at least in the idealized circumstance you had two particles in superposition of up and down and one of them resolves and in their minds they’re thinking that resolved the other one too yes so it was light years away it was in superposition now it’s not in superposition because you resolved one light years away um yeah
[00:24:40] Green: and that’s all because of the Schrodinger picture i think that in so in physical reality the the particles cannot exert any effect on one another when they’re not uh near to one another unless something propagates in between but we’re considering the case in which that nothing is propagating in between them that’s that’s kind of what makes it so uh bizarre is that without anything physically propagating from one particle to the other they still seem to have some effect on one another and the resolution is basically to say well that that’s all just an attribute of the Schrodinger picture and there’s other ways of describing quantum theory so in particular there is the Heisenberg picture and it has all of the properties of locality that we could desire from a physical theory in other words whereas in the Schrodinger picture there is no way of describing subsystems independently of other systems so there’s no way of describing the uh two qubit system as the wave function of two separate qubits you always you always have to consider the wave function of the combined system for it to give you full information about the state that the system is in in the Heisenberg picture you can have a complete description in which it’s perfectly possible to talk about subsystems which evolve locally in the sense that if you do something to the one particle doesn’t have any effect on the description of the other particle and it’s also a complete description in the sense that anything you could possibly want to compute is computable in the Heisenberg picture so you have complete information about the state
[00:26:25] Blue: so it can compute the same things as the as the Schrodinger picture but it does so in a way that makes that is transparent about where the locality is is that correct
[00:26:37] Green: yes yeah so and this is um a discovery made by Deutsch and Hayden and the way that they kind of summarize it is by saying well in a sense the the Schrodinger picture is like a highly condensed way of formulating quantum theory it’s like having a compressed file on your computer and you can kind of see that you’re losing out on information because of the compression and this this as a result that sometimes you can’t describe systems as being independent from one another for example when they’re entangled you cannot write down their state as the state of two independent wave functions but they have to be somehow the state of the combined wave function of both particles or both systems but that’s that’s a an artifact of the Schrodinger picture being a very condensed description and the Heisenberg picture does not have this feature so
[00:27:34] Blue: is this the motivation for your paper then is to make this clear are there other motivations um for why you are writing this paper and you’re trying to do this in the Heisenberg picture
[00:27:43] Green: well I think this is the main one I mean the the way that I kind of view this is that the Heisenberg picture is the more fundamental description of quantum theory that the Heisenberg picture has the property of locality because it is superior in in some sense like there it should be stressed that both the Heisenberg picture and the Schrodinger picture are mathematically equivalent but they’re nonetheless very different descriptions and I suspect that the Heisenberg picture is the fundamental description of quantum theory maybe I mean these are all unknown but perhaps the Heisenberg picture perhaps in a future theory of quantum mechanics or a successive theory rather there will be no Schrodinger picture so Durak actually wrote a nice paper about this kind of arguing that well maybe there’s field theories which allow solutions in the Heisenberg picture but which don’t have solutions in the Schrodinger picture so maybe there’s an asymmetry between the two
[00:28:42] Blue: yeah
[00:28:42] Green: and that’s all kind of the motivation for thinking that well maybe the Heisenberg picture is just the way to go in general and we should leave behind the Schrodinger picture as just a useful tool for calculations because of that and because of the fact that before the the paper there wasn’t really a good work on how to formulate Everett’s construction in the Schrodinger in the Heisenberg picture so in the Heisenberg picture that it seems useful to us to to provide that to give a description of of Everett’s construction which I think is the solution to the measurement problem in the Heisenberg picture which is the the solution to to the problem of locality in a sense I guess it’s not usually called that but I guess we could call it that for for the podcast the the problem of locality that Einstein noticed in his EPR paper
[00:29:31] Blue: okay why did two questions here why did Everett choose to formulate this in the Schrodinger picture instead of the Heisenberg picture and is there any advantages to the Schrodinger picture like is it like easier to compute in some cases or something like that
[00:29:49] Green: yes well the the answer the first question is rather simple it’s just that it’s the most widely used description of quantum theory and the reason for that is that the Schrodinger picture is just as you put it easier to compute with yeah so the Heisenberg picture isn’t the picture that people traditionally use it’s it’s it’s invented around the same time as Schrodinger but I think that researchers very quickly adapted the Schrodinger picture because it’s just so much easier to calculate with and also at that time it wasn’t known that there was this benefit with the Heisenberg picture because those issues with locality were still being kind of uncovered I
[00:30:38] Blue: see
[00:30:39] Green: and I think well it’s only been since Deutsch and Hayden which is in the late 90s that we have discovered the these attributes of the Heisenberg picture that I think make it preferable over the Schrodinger picture
[00:30:56] Blue: so as you have Everett come up with many worlds to try to explain the measurement problem it’s my understanding that initially he was largely just dismissed as crackpot science with with the exception of his advisor who was his advisor again
[00:31:14] Green: so his advisor his supervisor was Wheeler and Wheeler had a very strange reaction to Everett’s thesis I think initially he was rather optimistic about it and he felt like Everett’s solution to the measurement problem could be used to gain a deeper insight into quantum gravity because there could be such a thing as say the way function for the universe but because Wheeler was educated by Bohr I think in turn that Wheeler supervisor was Bohr whom he greatly admired and Bohr was very unfavorable he to the many worlds interpretation and consequently I think that Wheeler changed his mind and was of the opinion that the many worlds interpretation was the wrong way of viewing quantum theory there’s a nice interview with Wheeler on YouTube in which he summarizes Everett’s interpretation as you couldn’t get quantum theory more wrong so that was his opinion about the theory for most of his life I think after it had been presented to Bohr and it went to the extent where Wheeler really found it necessary to edit to severely edit Everett’s thesis so they sat together over the course of several evenings after Everett had presented his work to other researchers and they edited out many of the words and much of the core of the theory that Everett presented in his what’s now known as his longer thesis which is available online for free and that kind of shows that he was a bit hostile towards the whole idea of many worlds.
[00:33:05] Blue: Yeah, is there a connection between Wheeler and David Deutsch?
[00:33:12] Green: Yes, so Wheeler also supervised David, it’s David Deutsch. Yes, there’s an interesting history where Deutsch was supervised by Wheeler at the time when Everett’s work was kind of being rediscovered by other researchers like Bryce DeWitt. There’s actually a nice documentary in which they talk about this I think it’s called Parallel Lives and it’s about the son of Everett who is now a famous rock star of the eels and they kind of follow him, they follow the son of Everett while he discovers about his dad’s work and about kind of notoriety that he is acquired in the physics community and there’s a nice segment where David talks about meeting Everett and how that meeting came about because I think it was due to Bryce DeWitt and Wheeler having invited him back for a conference and a series of talks on the topics of many worlds.
[00:34:40] Blue: Interesting and David Deutsch, I guess I perceive him as being kind of the premier bearer of the Everett torch today. I know there’s some other famous names like Max Tigmark who believes in many worlds also but there seems to be a bit of a difference. Max Tigmark is just interested in interesting weird ideas and he latches on to a lot of them and he’s very open -minded which is good, I don’t mean that in a negative way, where Deutsch seems to have come to it based on epistemology and the realization that this is the only actual explanation of quantum physics that’s available to us and then was compelled to believe it based on that, is that correct?
[00:35:26] Green: Well I don’t know about how Tigmark came to discover many worlds, I think he’s pretty sound on the topic and yeah he seems sound enough but yeah I’m not sure, I imagine that if you’re interested in many worlds you have to be a realist, you have to think that quantum theory really describes the physical world, it’s a theory that isn’t just the parochial description of very small things but it applies to all physical systems in some sense and those are attributes of Evoretians in general, I think those are the kinds of things that lead people to being an Evoretian in general so I imagine that Tigmark is much much the same that he was interested in similar issues of just describing the real world but yeah I’m not sure how he came to theory.
[00:36:26] Blue: I’ve read some of his books and I think you’re correct that he buys into it quite strongly but he also has advanced other types of many worlds theories that are a lot more there’s no evidence for and he’ll tell you that he’s not like trying to hide that but he’s the one who came up with like the four different kinds of multiverses and things like that, are you familiar with those?
[00:36:53] Green: Right, yes, I’m only somewhat familiar with those ideas, it’s like you have multiverses that are really bubbles in space, right, there, yes. Like
[00:37:04] Blue: one and two, right.
[00:37:06] Green: Yes and yeah I’m not too familiar with the topic, I basically just about know what the different universes are according to Tigmark or what the different types of multiverses are but yeah other than that I’m not very familiar.
[00:37:23] Blue: The ultimate multiverse in his mind which is clearly not something there’s any evidence for would be that reality is mathematical and so any mathematical system that’s consistent exists so he has this kind of concept of a mathematical multiverse which clearly is very speculative, very different than the quantum multiverse where it’s a requirement to understand the explanation of quantum physics, he’s definitely more willing to dip into really speculative things.
[00:37:56] Green: Right, yes, well that’s definitely different from Everett’s construction and I think it’s solving a different problem, it sounds like that’s trying to solve the issue of why does mathematics describe the real world so well.
[00:38:12] Blue: Yeah, that’s the problem he’s trying to solve, he mentions that. Yeah so
[00:38:15] Green: that’s a different issue,
[00:38:16] Blue: yeah
[00:38:16] Green: it’s a related one in some sense because it’s about why the universe is turned complete which has something to do with quantum computation, well I say the universe is turned complete but what I mean with that is a rather inexact statement, what I mean is that the universe allows for physical systems to be simulated with arbitrary accuracy with finite means so if you have a computer running on finite physical means then it can simulate any other physical system but that’s a related thing in the sense that we have a theory of computation which is the quantum theory of computation and that kind of gets you back to quantum theory but the motivations for Techmark’s multiverse seem to be very different in that Everett was solely concerned with an issue in quantum theory.
[00:39:14] Blue: Yeah, so let’s maybe let’s describe what the, I mean in layman’s terms, without using math and I know with quantum physics it’s really hard to describe things without the math but let’s describe what the Heisenberg picture looks like and what the multiverse looks like and what does a local version of the multiverse look like and what is the Heisenberg picture telling us about it?
[00:39:41] Green: There’s perhaps this thing that we should discuss before we get into this which is entanglement which we
[00:39:46] Blue: already
[00:39:46] Green: mentioned and it’s kind of at the heart of what quantum theory is about so there’s this property of quantum systems known as entanglement where as we said if you say have two particles or two qubits I think qubits are probably the easier example if you have two qubits then they each qubit individually can be in a superposition meaning that they can have value say one or minus one but they can also be in in both of those states simultaneously they can be in the state one and minus one simultaneously.
[00:40:31] Blue: Just to clarify a qubit is a generalization of the concept of a bit just like with a computer so this would be a quantum bit. Yes
[00:40:39] Green: so it’s a thing that has two definite states it has it can be in a state one or minus one or as it turns out in the superposition of one and minus one.
[00:40:51] Blue: So this is something that always bothered me a bit about the way people describe quantum computation is they’ll say like the media just butchers science a lot I found but they’ll often say that you can do more with a quantum computer because you can have this bit as a one or as they usually use one and zero because of bits instead of one and negative one but just a matter of how you’re interpreting them so it can be one or zero or it can be both and therefore it’s more powerful and the first time I ever read that I thought that would be so easy to emulate on a classical computer so that can’t possibly be by itself the full explanation so entanglements the real answer entanglement is more than that.
[00:41:35] Green: Yeah I think that’s a nice point it’s because there’s loads of superpositions that exist in nature the superposition is this idea that you can have that if you have a bunch of solutions for a set of equations then a some of those solutions is also a solution and perhaps the simplest case of that is just if you have light being in superposition you can imagine that you have a ray of light pointing to the west and a ray of light pointing to the north and those are separately allowed rays of light and also if you add them together so if there’s if there are two light rays traveling north and west or whatever directions I chose just now then those are also allowed so you can have rays of light traveling in different directions at the same time and that’s because light is a kind of a wave in the electromagnetic field and light rays each individual light ray is a solution to to like Maxwell’s equations and Maxwell’s equations allow you to superpose rays of light so rays of light traveling in different directions are allowed provided that each light ray individually is an allowed solution and and in the same way a quantum theory says that it well if what if you have two solutions then the sum of their solutions is also a solution to the Schrodinger equation um I don’t know if that clarifies it I think I feel like I just dropped a whole lot of terminology on you I
[00:43:07] Blue: think I can actually explain this um in in quantum computation it’s true that you have the bits in a superposition but they also have a relationship to each other and if you were to try to emulate that on a classical computer that relationship that they have to each other the amount of memory you need grows exponentially and even after like I started actually programming a quantum computation simulator on a computer I couldn’t even get to nine qubits before the amount of memory I needed to be able to track all the relationships that exist between each of the qubits was more than anything I was going to be able to afford um and in fact I doubt I mean like the reason why that you you couldn’t even get to like 30 qubits you would have to use um actual quantum computer to be able to get that far I know some of the quantum computers that exist use like 30 qubits um or equivalent to 30 qubits that’s probably well beyond I like to try to emulate that on a computer I would I don’t have the math worked out in my head but I suspect it would require a computer the size the universe or something to be able to uh track 30 qubits so it very quickly becomes completely intractable for a classical computer to be able to even just keep track of all the entanglements that are going on and at least that was how I came to understand oh now I can see why a quantum computer would be more powerful than a classical computer is because it’s actually capable of computing things um that are would be physically impossible for a classical computer without a universe sized computer
[00:44:54] Green: yes uh that’s certainly part of it it’s the the possible states that quantum systems can be in is just so much larger than physical than classical systems that they one of them skills exponentially in the other one uh doesn’t meaning that the quantum quantum systems skill exponentially in the uh state space yeah
[00:45:23] Blue: and but that’s also why it can sometimes do these a quantum computer can sometimes do these exponentially faster computations is because it can actually compute across all the qubits which would be like doing a computation that is exponentially large um but in just a few steps
[00:45:47] Green: yeah so that that’s the argument though which usually gives for why quantum computation is like a reason to believe in the multiverse it’s because you are performing these computations somewhere in physical reality but they’re not in our universe they’re happening somewhere else yeah and they’re happening simultaneously in different universes that isn’t the whole answer because also you need to be able to kind of obtain the right result from those parallel computations happening in different universes yeah that that is where like quantum interference comes in and you you have to use clever methods of yeah selecting the right results
[00:46:28] Blue: it’s almost it’s almost like you’re tricking the system right you set up you set up the the system so that you’re doing this quantum experiment you want to do a computation and so you set up a almost like a quantum experiment and you say okay if i get this result that’s going to tell me that you know maybe i’m doing a search problem that my answer is in the top half of the search and then you can continue to do that and it gives you a piece of information that in physical reality in classical physical reality shouldn’t be available to you that allows you to run at a much faster much faster in one step than a classical classical turning machine would be able to
[00:47:10] Green: yes but i feel like we so we haven’t really addressed yet well in tangland is we we went down an interesting path
[00:47:17] Blue: yeah
[00:47:18] Green: yes uh so to to kind of come back to the example of the qubits so first of all this is notion of superposition which is that something can be in two states at the same time so the qubit can be in state one and zero at the same time and then there’s an interesting feature where the the qubits can also be in these indefinite states at the same time in such a way that they are what we call entangled so you have say a particle like we described earlier where or a qubit where if you measure the qubit to be in the opposition or in the in in the up state then you know the other qubit is also in the up state and if it’s in the down state then the other qubit is also in the down state even though separate measurements on the qubits like or individual measurements on them are uncertain so an individual measurement on the qubit will not tell you with certainty whether you’re not you’re going to see up or down but you know that you’re going to see if you’re going to see up on one of the qubits then you’re also going to see up on the other qubit so a joint measurement on them gives you some certainty about about the results about the joint results but the individual measurements are uncertain and that’s what we call entanglement
[00:48:53] Blue: yeah so and my original question was how do we relate this to locality so maybe let’s try to work that in with locality now because entanglement certainly feels non -local
[00:49:08] Green: yeah so usually in the surrounding picture it seems like the when things become entangled they can’t be described separately from one another because your description of the different systems doesn’t allow you to describe each system individually without also losing information about what the state of the system is in and that seems to suggest that if you from these measurements on the qubits individually you’re also affecting the other qubit because again you can’t really separate them from on from on another if they’re entangled yeah and that’s and that in turn suggests that if you if in Edward’s picture some branching event happens where the universe suddenly has two histories then that must affect the entirety of the universe because there is no way of really describing what’s happening in the universe separately from what’s happening to everything else in the universe so if one system branches then everything branches almost necessarily so in but as it turns out that is a
[00:50:22] Green: feature of the surrounding your picture that that is not a feature of physical reality it’s a it’s a feature of a way of describing physical reality and it’s a an attribute of the surrounding your picture that makes it less fundamental in the Heisenberg picture and if you describe things in the Heisenberg picture then it turns out that you have these local branching events where for example so you imagine you can you’re in a lab you perform a measurement on a qubit well when you perform a measurement on the qubit then you branch into two versions of yourself one of whom sees one particular measurement outcome and the other one sees another measurement outcome and that is actually a completely isolated event in the sense that it’s just you and the particle or the qubit that branched and before the environment starts to obtain information about the branching event that happened it is not yet in any one of those universes so it’s in some sense there’s this bubble that contains you and the qubit where within that bubble you can talk about two different histories resulting from the measurement that occurred but outside of the bubble there’s not yet any information about which branch you are in so that kind of spreads out to the environment so maybe you have this bet with a friend where if the if you measure a qubit in the upstate
[00:51:55] Green: then you will buy your friend a beer and in the other states if you measure the other state then he will buy you a beer okay well that means that you know you you measure the particle and it’s an up you buy you go to the bar and buy a beer well that’s a very different history from you and your friend going to the bar and him buying you a beer and systems behave very differently in those two universes but it has to kind of so that that’s one way in which the measurement can have effects on the environment and you you know now by knowing who is buying the beer you know what the outcome of the measurement was and so information about the measurement outcome has leaked into the environment and many things in these different universes will be different as a result of the measurements and you can kind of imagine a sphere of influence that’s growing over time where the measurement result has had some effect on the environment and this is the view of the many worlds in which interactions happen locally so it’s not that the whole universe splits old ones is that systems kind of split or they branch I think branching is a term that I prefer they branch only because they have interacted with something that has interacted with the measurement results so again you holding the beer rather than your friend is a sign of what the measurement result was and that means that the the bar that you’re at has also split in as part of this history that you are in so that is a local picture of what’s going on in the many worlds.
[00:53:47] Blue: So relating it back to the Alice and Bob example it solves the measurement problem because first you have the instrument that gets entangled with the particles so you’ve got one instrument in one universe that measures up and one that measures down then Alice looks at it and she becomes entangled into it and so now her brain contains information about it either being an up particle or down particle then Bob enters the room and she tells him what it was and so now that he becomes entangled in into this then they decide that they’re going to get the beer and because it was up she’s buying so they go out to the bar now that bar becomes entangled into it and it just continues to kind of move out from there is that correct?
[00:54:33] Green: Yes that’s how it works and that’s how you can kind of imagine there being a sphere of influence where the effects that the measurement can have had on its environment have to grow with time so if you’re very far away from where the measurement happened then it has to then the information about what the measurement outcome was has to travel some distance before it can reach you and before you can become part of that history and until then you cannot be in any one of these histories yet.
[00:55:09] Blue: I’ve got a weird question for you that I’ve always wondered about in beginning of infinity David Deutsch talks about how the an article in a newspaper could be part fiction have a fictional history and a true history at the same time and I never really understood what he was getting at but I later read a book by Frank Tipler where he explained it a little differently and he was saying that once universes there is nothing different between them then you can think of them as the same universe again and it could in theory be that that history of say you know the pharaoh or something that there was a fictional version that created exactly the same state as the real version and that those would both have some feed into our history is that correct that’s always seemed so weird to me.
[00:56:05] Green: Oh I’m not sure um I guess what’s being said there is that certain certain fictional well certain works of fiction have counterparts in the multiverse that are very close to that fiction so maybe there’s a story about two people falling in love or I don’t know what’s what’s the good fiction book maybe it can’t be too outrageous. It still has to follow physical reality right. Yes yeah it still has to obey our laws of physics but maybe somewhere in the multiverse something like Breaking Bad happened and like not just a little bit like Breaking Bad very much like Breaking Bad like maybe there really was a guy called Walter White who lived in Albuquerque who was dealing drugs as a high school teacher and who almost won a Nobel Prize in chemistry and that will be true somewhere in the multiverse.
[00:57:01] Blue: Yeah
[00:57:01] Green: but here in our part of the multiverse it’s a fictional story because it’s this idea is related to that but it takes it just a little bit further where it says okay let’s see let’s use King Arthur as an example we have these stories of King Arthur and there’s this debate over whether King Arthur actually existed or not so we can imagine one set of universes where King Arthur actually existed and he was the inspiration for the stories that we tell today but everything else about him has been lost there’s no history of him other than these stories then you have another universe where you have the exact same stories happen to come into being but there was never a King Arthur he was a totally fictional character other than other both these universes in their current state today are identical they both produced the exact same stories about King Arthur so you have this history one’s fictional and one’s not and both would be have fed into the universe that we see today and so it would be there would be an open question the question did King Arthur exist may not even be a valid question in that universe that was really what Frank Tipler was getting at that you could actually maybe that one’s far fed far more into it into the other perhaps the one where King Arthur did exist is a much more realistic more probable circumstance so you end up with that one feeding into the present much stronger than the fictional version but he was suggesting that many worlds suggested that you would actually have both as part of your history yeah that seems I mean that’s true in principle but unlikely in practice because the the
[00:58:53] Green: ways in which those universes differ from one another is not just about whether King Arthur really existed in those universes so for example there if King Arthur really existed then there will be certain historical artifacts that are that will be here today and then certain people’s lives will have been different in relevant ways and it is true that if all that information was lost then perhaps we could re -merge with that universe in which King Arthur was alive the
[00:59:31] Blue: way you’re describing this actually is exactly what Tipler was getting at that they would have the identical it would have to be that there’s no other differences between these universes except there are no differences between these universes they both have exactly the same stories that for whatever reason they happened to get into the exact same state but one King Arthur was real one he wasn’t and that’s our universe today we don’t know if King Arthur is real or not today there’s people who say he was people that say he wasn’t they’re trying to look at the quote evidence to determine if he’s real or not but the the idea would be that in theory because we don’t have any really anything that really shows for certain that he was did or didn’t exist that it could be that both those histories are valid histories of our universe today I don’t know how realistic that is this is obviously they’re trying to trying to give a kind of out there example of how you could have a history that’s both fictional and isn’t and it would it would require some really very special things to happen where all the information got completely deleted so there was no other differences but yes and also the
[01:00:48] Green: stories are completely the same so they’re imagine
[01:00:50] Blue: completely the same correct
[01:00:52] Green: yes so that those things are very unlikely because even if there’s some historical evidence that we’ve yet to uncover that could point to there having been a king Arthur right that word I made that that is a distinction between the universe where king Arthur is was a real historical figure and the one in which he wasn’t that’s right and there’s just stories about him
[01:01:13] Blue: that’s right so you even even if this idea were true it seems like it would only it would be some little infinitesimal part of the multiverse for whichever for one of them and the other one would be
[01:01:24] Green: yes
[01:01:25] Blue: a much stronger feed into the history
[01:01:27] Green: yes but the principle is true the principle that universes rejoin or can rejoin because they lose some information is true so for example if you have a particle that goes along two paths because it’s hit a beam splitter and then you make it re -merge again there’s then the particle at the end of the experiment which is in a definite like sharp state there’s no there’s not really an answer to the question which trajectory did it take because it took both at the same time and then it re -merged right so the information about which trajectory it took are it’s completely lost to everyone it’s we all let me know is that it took well if we know enough then we know that it took both at the same time and but there’s no way of saying oh well those instances of the particle took the one path and the others took the other path because they those instances that have taken different paths have become indistinguishable from another again upon re -merging that
[01:02:32] Blue: that makes sense so they’re taking that idea to an extreme when they suggest something like this to show how in theory it could happen but like in practice it doesn’t seem like even slightly probable
[01:02:46] Green: yes
[01:02:47] Blue: all right make sense could you maybe just briefly if possible describe what decoherence is your paper talks about a couple things that i don’t know i have kind of a vague notion of decoherence which is you know why is it it is somehow related to why is it that we can’t see both universes at the same time and then you also talk about quasi classical systems and is that related to the same question yes
[01:03:18] Green: so okay if you’re if you’re a mini world sir and you think that everything is described in quantum theory then a very natural question to ask is well why why do things even seem classical in that case like i think i live in a classical universe where things have definite values and chairs and tables and other things are not in super positions as far as i can tell and and they behave roughly as as though newton’s equations are true so why is that why why is it the case that this quantum world somehow behaves classically at all and it turns out that when systems when quantum systems evolve generically then due to the interactions between all of the quantum systems they start to behave somewhat classically so um if you have some environment that’s constantly interacting with i i’m just gonna take a kind of random object you know imagine you have a chair you have a quantum chair and the quantum chair is in a superposition of being in all kinds of different locations well okay that chair isn’t isolated it’s it’s in a room and it’s interacting with the light in that room interacting with like several trillions of photons that are hitting it at every point in time and or every couple of seconds i think i should say and um so that that chair is providing information to the photons about where what position it’s at the the photons are getting entangled with the chair and it’s not just the photons it’s everything else in the room that’s getting entangled with it so the photons are hitting off the chair they’re then
[01:05:09] Green: bouncing off the room and and or the the door or the door or the walls in the room and then the photon hits your eye and you see where the chair is and in in that process like the the photons the chair the the wall and you and you have all become entangled with one another and that’s happening very very often so it seems like everything has a definite position uh because everything is constantly interacting with almost everything else at least within the near vicinity so everything in the room is interacting with everything else in the room and the the result of that is that things behave roughly classically so that that kind of suppresses the typical quantum effect so the chair can’t interfere with the other versions of itself because of all these different interactions that are occurring and that’s there’s roughly why the chair is is like a classical system it’s not because it has only a single history it’s that the different histories of the chair can’t interfere with one another because of all this noise because of all the different interactions that the chair is partaking in and
[01:06:18] Blue: that’s and that’s what decoherence is yes
[01:06:21] Green: so the coherence is what occurs because of all these different interactions with the environment so you could say like the chair is this some physical system it’s interacting with an environment in this case say the air molecules in the room the photons that are hitting it and basically anything that interacts with the chair at all and these are decohering the chair so they’re the chair roughly has a single position and it will it will or rather every the chair in the multiverse the chair has multiple positions but these positions but these different instances of the chair will never interfere with one another because of the interactions that are taking place with all the other systems and and what is a quasi classical system then um is that just mean no it effectively we can treat it as if it’s classical or is quasi classical something that’s actually more in between well so the the reason why we use the term quasi classical is because the chair is never really a classical system I see like for example the if you were to say that the chair is a classical system it would only have one history and many things about the chair would be false that many attributes that the chair has in reality it could not have it was a classical system like the very fact that it is stable is is an attribute of it being a quantum system because only quantum theory allows us to explain why atoms are stable and the so these attributes of the chair are are attributes that it couldn’t have it was if it was a classical system
[01:08:08] Green: and yet it behaves roughly as if it’s a classical system in the sense that the chairs appear to only have a single definite position somewhere in space and they don’t interfere with other instances of itself they’re always like unless we interact with them they’re wherever we leave them
[01:08:30] Blue: interesting so
[01:08:33] Green: it’s yeah it’s a way of saying well things seem to behave as as if they as we thought they would before discovering quantum theory and we have to explain the behavior of particles of chairs of everyday objects as a result of quantum theory but those things never really are classical systems
[01:08:54] Blue: okay so that’s what a quasi classical system is it just it’s the explanation for why things seem classical to us but they never really are classical yes
[01:09:04] Green: okay so my favorite example of this is that say refrigerator magnets are they they can’t be classical systems this is nice proof by Bohr where he showed that magnets as as in materials with magnetic properties ferro magnets are cannot exist according to classical physics in classical physics any material that has some magnetization can only have that magnetization because it’s in a magnetic field but it can’t have any intrinsic magnetization and we actually like quantum theory is part of the explanation for why things like magnets refrigerator magnets ferro magnets can exist and in that sense those magnets are not classical objects they they’re quantum objects just like everything else in the world there’s a quantum object but they behave roughly according to our classical intuitions so we never see these magnets interfere with other instances of the magnet right in daily life
[01:10:12] Blue: right interesting I think you’ve given us all a lot to think about Sam this is a this is a very interesting paper I hope that this overview will kind of help people either gain interest in it or understand it a little bit better but and we will continue to do some YouTube videos I’ll put those up that will also help explain the mathematical side of it so Sam thank you very much for coming on the show I really appreciate you doing that I think this is just been fascinating
[01:10:48] Green: cool thanks so much for having me it was great fun chatting with you
[01:10:51] Blue: yes thank you and Tracy thank you for joining us also
[01:10:55] Red: oh thank you both that’s great
[01:10:58] Blue: glad
[01:10:58] Green: you liked it yeah thanks so much for having me again and talk to you soon yes thank you bye bye
[01:11:05] Blue: the theory of anything podcast could use your help we have a small but loyal audience and we’d like to get the word out about the podcast to others so others can enjoy it as well to the best of our knowledge we’re the only podcast that covers all four strands of David Deutch’s philosophy as well as other interesting subjects if you’re enjoying this podcast please give us a five star rating on apple podcasts this can usually be done right inside your podcast player or you can google the theory of anything podcast apple or something like that some players have their own rating system and giving us a five star rating on any rating system would be helpful if you enjoy a particular episode please consider tweeting about us or linking to us on facebook or other social media to help get the word out if you are interested in financially supporting the podcast we have two ways to do that the first is via our podcast host site anchor just go to anchor.fm slash four dash strands f o u r dash s t r a n d s there’s a support button available that allows you to do reoccurring donations if you want to make a one -time donation go to our blog which is four strands dot org there is a donation button there that uses paypal thank you
Links to this episode: Spotify / Apple Podcasts
Generated with AI using PodcastTranscriptor. Unofficial AI-generated transcripts. These may contain mistakes; please verify against the actual podcast.