My heart started fluttering when I realized I was going to get to have you and Albert in a room to myself for more than an hour. As I did with Penrose, and Greene. Your podcast is a (secular) godsend! I'm SO grateful!! ❤️
I wish, though, Albert could think ahead far enough to finish a clause before entering a qualifying sub-clause, and then a sub-sub clause, and then... My stack keeps overflowing.
I also learned about Sean on the JRE podcast, which I listen to daily. I have always loved science, especially physics, astronomy, cosmology, quantum physics, etc. I can't say I am an expert in any of said fields, but I love learning all I can about them. Thank you Sean for making difficult subjects interesting and easier to understand.
I love listening to David Albert and anything to do with the foundations of physics. The extra length was fine by me. Definitely bring him back some time and finish the discussion!
disagreements always confirm there's honesty to be had, i'm really skeptical of groups of people who talk about complex problems and always politely agree.
I can’t necessarily follow all of his philosophical reasonings, but I like Albert and it just gives me a reason to listen through the whole conversation multiple times. Rather than just clicking on another introductory lecture on quantum mechanics which will normally will only cover material I’ve already heard a million times.
Thanks for the content Mr Carroll - I work as a pianist, but I'm fascinated by physics. I'm grateful that people like you are out there providing these. Most TV documentaries think we're so stupid that we'd hate (or be hopelessly confused by) any REAL technical, philosophical, or mathematical discussion. So instead all we get are dumb animations, and inappropriate analogies. Podcasts like these satisfy my craving for genuine scientific information that isn't so dumbed-down that you don't actually learn anything!
Wow.. very good conversation. I will admit I'll have to listen to it a second time to really catch it all. Two great minds here having a great conversation.
Guest/Show request: "Nima Arkani-Hamed on Spacetime, SUSY, and building larger colliders" Also, I think this would be fun: "Leonard Susskind on Black Holes and no, I am not Mike from Breaking Bad" 😆
Leo hah… that would be "explain gauge symmetry" attempt #5 for poor Joe? If you start explaining hedge funds to a dog, the expression on that dog's face is the same expression Joe will have for the entire podcast if he has Nima on.
Thank you Sean and David for this beautiful talk. It strikes me that we may have to rethink our naming convention: what if quantum refers to the discrete event of the observation itself, in which the interpretation of the observed is an exclusion of one thing and an inclusion of the other. The quantum world then is not the world of discrete packets of energy or discrete, probabilistic outcomes of measurements, it is the absence of it and the Schrödinger equation describes the nature of reality in which there are no observations, that is, measurements, that is, exclusions caused by the nature of our intuitive reality which is discrete and dual by the concept of choice. Our reference frame is discrete, therefore we observe in a discrete manner. Our macroscopic worldview is evolving around differentiation. When we choose, when we measure, we entangle the observed reality to our quanta-based structure of reality. The so-called collapse of the wave function is there only because our reference frame in which we give meaning to observations is discrete. The idea is that our observational experiences live in a quantum world, but reality beyond that what is observed is continuous, described by the universal wave function. The microscopic world is without any quanta, and is fundamental to everything. We might want to rename classical physics into quantum physics, and quantum physics into fundamental physics, the theory in which everything is contained by an a priori logic principles, valid without any reference to observable results as opposed to empirical logic. I would like to know what other interested viewers and yourself as creator of these podcasts think of this.
I have close to 0 familiarity with this discussion (I'm a history graduate student and English is not the first language). Your comment helped me, I think, have some grasp of what they where talking about, and why a theory of decision-making is even relevant to the many worlds argument -- I was lost during most of the latter third of the podcast. I wish someone actually qualified would reply to your comment, but if I understand it you agree with the classic Copenhagen interpretation then? Meaning that there is a universal wave function, and discrete observations (interactions) create the reality we can experience? Edit: Thought about it some more and actually I don't know if the Copenhagen interpretation admits the idea of a universal wave-function or if it views the concept of a wave-function as something that only applies to particles that haven't yet interacted with anything.
"Sadly, we didn't get quite enough time to talk about what approaches to QM that he IS in favor of..." - Sean Carroll on a podcast that's 18 minutes shy of 2 hours. Alright, let's do this.
As an analytic philosopher, I really enjoy your podcasts. Thanks for bringing the disciplines of science and philosophy together for the rest of the public. I find the intersection of the two fascinating. We could all use some more education on social media too. Cheers.
semidemiurge has David made one precise point in almost 2 hours? If you are an interested amateur it is impossible to follow him and not because he uses math or jargon I don’t understand but because he actually doesn’t have a point.
@@robertchristiandau1090 He can be much better than he is in this conversation with Sean. I think he failed to recognize who his audience was in this conversation, ie. the listeners to the podcast and not Sean. The result of which is that the conversation was more between them and required their shared understanding of the physics, science history, terminology, etc. and the typical podcast listener struggled to follow. I also think David has considerable respect/admiration for Sean and was motivated to display his intellectual chops to him. David does have a serious predilection (not uncommon in philosophers) towards digressions and staying focused on the subject at hand is not one of his skills. I would recommend a couple of videos to see him more focused and succinct and insightful. ruclips.net/video/OcMj-_kgTL4/видео.html . ruclips.net/video/LmLYa9pbh6k/видео.html . ruclips.net/video/qok_3-z3URs/видео.html
@@semidemiurge His problem is he cannot finish a sentence - he sees what seems to be a better approach or analogy, and he thinks he will not remember it by the time the current sentence is finished (which would be the case), so he jumps to the 'improved' thought, which has its own tangents and he forgets the original point he was trying to make... this is where a good interviewer patches everything back together, if he was not thrown into confusion, too...
I have no idea what this had to do with quantum mechanics. All I heard was something about preferences and branches about ice cream and being fat. And who read or did not read what paper. I learnt nothing at all. To me it felt like a philosophical discussion between 2 very drunk people. But I know Sean is very intelligent so I am just going to assume the whole conversation was over my head. Maybe someone can explain what the preferences about branches stuff means in relation to science.
Albert is a heavy weight intellectual and thinker. But he lacks effective and cogent communication skills. Following his train of thought is not easy, he digresses and re-states things a different way repeatedly. He can’t seem to state things in a clear, crisp, direct and pointed way (at least in this discussion). Carroll on the hand always seems focused, sharp and direct in his points when he speaks.
@@miedzinshsmars8555, Did Satyendra Bose (after whome Bosons are named) win a Nobel? Did Freeman Dyson win a Nobel? Even Hugh Everett did not win a Nobel. Even John Stewart Bell did not win a Nobel. Nobel is not the measure of greatness or progress. But one thing is for sure i.e. you would not get beyond putting two-bit comments on youtube to make any contribution to the foundation of Quantum Mechanics. Smart alecs like you are utterly worthless.
He did make many points, but most of them probably went over your head. (And I don't mean to be rude. I too found this difficult to understand at first, but then did further research and came back a few weeks later.)
@@RAF71chingachgook that's a nice thing for you to say. It's a shame that it's not more positive comments like yours. Thanks you had the time to send such a nice comment 😊
@@klaasterpstra6119 Thank you for telling us about David Wallace, he seems extremely interesting and reasonable. Excited to watch and read more :) thanks!
Superb episode, as usual! I'm sure you have plenty of ideas for folks to co-host in future episodes, and I'd like to inspire you to maybe have someone that is pushing more "fringe" theories, like Erik Verlinde. I think you've already been through a MOND phase of your research and a conversation with both of you might be able to explain to us the gaps between MOND theories and observations. I'd also love to hear Edward Witten and Juan Maldacena. Thank you for helping to teach and inspire us!
Has Tim Maudlin never been a guest? I feel like he'd have some really great contributions to make on both quantum mechanics and relativity (especially shedding light on some remarkable things Bell said on both matters).
Really very good knowledge to learn.... You are the wealthist human in this world professor , you got a endless knowledge of physics , a wealth never ends.. Everyday I learn a lots from you... Thanks sir for your knowledge share....
Sean, I've listened to you and David in different venues over several years and am at a point where I'm grasping these ideas. Some. Great show. Look forward to reading your latest book. A book on the history quantum mechanics i.e. Bohr vs. Einstein would be fantastic.
Amazing podcast, but there should be at least one more with David on this subject. I'm also glad that Sean's pronunciation of Schrödinger much improved.
Shame the mice don't get to choose ice cream! Thoroughly enjoyed another thought provoking podcast. Thanks to both Sean and David for an enthralling discussion.
I know what uou mean. He can be frustrating. There are places where he does get more concrete. To try and be on his side for a second, he tries to only state what we know for sure as to not shut down paths to solutions.
Yay another physics episode!!! I like the other ones but this is really the only place i can find physics podcasts... anyone have a recommendation maybe?
Joe rogan has some long episodes with sean carroll, brian cox, neil degrasse tyson, and roger penrose among others. They arent all science based but the ones that are are great.
After an hour of limitlessly digressive discourse on "chancey universes", Albert finally gets the point of saying: "So, here's the germ of my objection [to MWI]." What follows is a limitlessly digressive discourse on "decision theory", which seems to revolve around how a rational person should best maximize returns among preferred investments in branching futures. While I'm not a fan of MWI (or Copenhagen for that matter), I much prefer Carroll's mercifully concise summaries of both Albert's lengthy dissertations and Carroll's own preferred interpretation of quantum mechanics.
Nobody asked you to be a fan of anything. One could expect from you and from these gentlemen that you have at least a cursory knowledge about the structure of Copenhagen, which is the correct interpretation. I am not holding my breath, though... I have rarely met anybody, serious physicists included, who actually understood what Copenhagen was trying to tell them about nature. That's a direct consequence of our horrible ways of teaching quantum mechanics.
@@QuicksilverSG There is no such thing as "wave function collapse". A wave function is the description of the quantum mechanical ensemble, which is the abstract idea that one can repeat the same experiment an infinite number of times to measure a probability distribution for its outcomes. No such thing is possible in reality (we can only count frequencies for a finite number of measurements), hence the wave function only exists in our minds and on paper in theory books. It does not exist in reality any more than the probability distribution of dice exists. Something that doesn't exist physically can't collapse physically. A single quantum measurement "lasts" for an infinite amount of time. Why? Because we demand that the measurement has to leave a permanent record, hence it has to be an irreversible physical process. Moreover, since we assume that the energy of the quantum that we are measuring is "precise", time-energy uncertainty (which is a classical uncertainty relation) allows us to pinpoint the beginning of the measurement, but not its end. All of this is trivial undergrad physics. If you ever meet a physicist who can't explain this to you in a few sentences, then you don't have to listen to them about QM any longer. They simply don't know what they are talking about. Like I said, you are the victim of horrible teaching methods relating to QM and that is all there is to it. Everything you think is a "mystery" is simply a teacher telling you nonsense or not telling you enough.
Sean, this was a little over my head (ok, a lot) but since you & David are smart guys, making good faith efforts to get at the truth, it would be interesting to hear your 5 minute summary of where exactly you & David differ on many worlds, what fact would bring you to David’s side & vice versa, if that’s possible.
yes I became a fan of Sean from Rogans show so I attempt 2 listen 2 the podcast but a lot of these episodes I'm lost don't know what's going on and theb my mind wonders. I'm fascinated by physics astro physics nuclear physics but I don't know I feel like an idiot listening 2 this podcast seems like its more 4 grad students then laymen's my pin interest explains things simpler maybe my dumb a$$ needs 2 stick 2 that.
None of this episode made any sense to me either. I have listened to a lot of quantum mechanics and philosophy and even when the math and concepts are complex and beyond my complete understanding, I still "understand" what they are talking about or why. But this episode just sounded like abstract nonsense to me, I have no idea why they were saying any of the things they were saying. There was no grounding to any of it, at least to me.
I might suggest that this is one episode where working through the *transcript* available on Sean Carroll's blog site at your own pace is what it's going to take to follow Albert's long nuanced argument against the Many Worlds interpretation with sufficient comprehension to gain much of a useful "takeaway" from it.
Thank you once again, Sean for your so articulate, and comprehensive, (yet verbally economical) (and unemotional) periodic summary explanations, laying out the logic of what your guest is actually talking about, and how it relates to the bigger picture. Great series of podcasts which compliment your carefully and so logically thought out, well written books! I admire your relatively non-judgmental way of examining problems from many (every?) different points of view.
My favorite podcast ever. I wish I were smart enough to posit a theory that David could then destroy using his philosophical contrarian sledgehammer of logic. A man can dream can't he.(and he can't, and can be both doing an not doing the dreaming till measured)
By the time David gets to his argument he completely looses me. What is the difference between a branching and a non branching future? Is he just talking about making a decision in a universe where many worlds is correct vs one that it isn't? If so, I don't get what that has to do with whether many worlds is correct.
Basically, all I got out of this ~2hr video beyond the history of QM interpretations is that Albert doesn't like the fact that MW basically makes probability meaningless in a sense. I can't help but wonder: so what? I don't even think he's right about that, though it might require a redefinition of what probability actually is. If every outcome happens, then we're not asking about the probability of it happening, but about how many versions of us will see any given outcome. It basically reduces probability to statistics in a sense.
Hi, Sean. Could you please consider running a live stream on youtube where we (fans) can interact directly with you through the chat by sending in questions and having discussions about different topics within your field?
Sean, someone that I think would make an excellent guest (outside of physics) is Bruce Schneier. If you're not familiar with him, just look up some his talks here on RUclips or read one of his books. His expertise is in cryptography, computer security and the psychology of security (not just computer security but all types of security from the national level to the personal level). He was one of the technical advisors for various media outlets during the Snowden leaks and frequently writes opeds for the NYT. I think a conversation between you two would be excellent. And I am sure you can book him as he is never shy about media or podcast appearances.
On the multiverse and probabilities. I really enjoyed the discussion and David Albert's analysis and objections to the "decision theoretic" approach (I am reading David Wallace's "The Emergent Universe"). I feel that this "problem of probabilities" hints at an understanding of the multiverse that misplaces its relation to QM, specifically to the epistemic formulation that constitutes "QM as we use it". I find that this misplacement comes out as astonishing (I mean to myself, of course) in the decision approach, but there are perhaps less blatant hints in other discussions as well (e.g. see the recent P. Tappenden's "Everett's multiverse and the World as Wavefunction" arXiv:1912.06821). The root of the problem I am seeing is that the multiverse is intended under an ontology that does not reflect the multiverse connection to the epistemic formulation of QM. I think that this understanding of the multiverse is in fact inconsistent to this relation. The probability problem seems to me more a symptom than the malady. Let's start by asking why and how we get to the notion of the multiverse. Everett had an amazing intuition, still that intuition was framed by the knowledge available at his time. We live in a different time, and I think that there are reasons today, that lead to the notion of the multiverse, that none had at his disposal when Everett wrote his thesis. I wrote in my other comment on the measurement problem that I deem that decoherence is at the core of the foundations of the interpretation of QM, what decoherence does that places it in this role is briefly written there. To the aim of the present argument, what is important to notice is that decoherence implies its occurrence within entanglement. The very nature (logic nature) of decoherence implies that alternatives are not excluded or selected, instead they are "combed" by the globalizing consistency enforced by interactions and encoded in entanglement. It is this consideration on the nature of decoherence that today shall be the concrete motivation for the notion of the multiverse. But it is the very same decoherence that in our epistemic formulation of QM yields that emergent classical logic that implies both our solely epistemic probabilities and the objective determination that renders legit a realistic intention of our physical experience. This shows that the multiverse is inseparable from probabilities already in its genesis. And this is one of the reasons why I emphasize a difference in our motivation respect to the possible motivations of Everett, we should just not forget from where we got to the multiverse: it stems from decoherence, from our epistemic probabilities, as a way to better understand the structure of QM. But it is also true that this does not completely excuses the problem. The multiverse is not a simple and direct analytical deduction, it is more like a model of the theory, a structure by which we hope to obtain terms and notions that would render the analytical narration of the structure of QM simpler and more intuitive, while reflecting it accurately. For this reason it is a valid question to ask if this model is satisfied by QM, and a way to disprove this is to show that the multiverse is inconsistent with the very probabilities that are its original motivation. I think that what is inconsistent with probabilities is the ontological intention used to narrate the multiverse, that does not corresponds to its origin and relation to decoherence and our probabilities. This results in an "ontological misplacement" consisting in the unawareness of the necessity of employing modal logic, which would instead reflect the multiverse constitutive relation with our epistemic formulation of QM. Modality here means that "being" "existing" can not be expressed without qualification, that it is necessary to specify the "modus essendi" when we predicate existence. The first kind of modality is the one that interests us more: "actuality", this is our way of being, and it applies to our experience and, thanks to QM's decoherence, to its implication of a determination satisfying realism. But the moment You move outside the determination scope, You can no longer talk in terms of actuality. The way decoherence, in our epistemic formulation of QM, provides us with solely epistemic probabilities is such that the fundamental workings of QM in the proximity of our emergent actuality shall be described as "propensity", i.e. as an objective feed to our epistemic probabilities. Or "potency", to use a classical term. But even this two connotations of the modus essendi of QM seem diminutive when they are regarded in the perspective of the multiverse. The only term I can find to describe it, reflecting its relation to us, conscious of how decoherence unfolds alternatives by mere consistency, is omni-potency. I don't use this term lightly, for its grandeur, and with much reluctance for something that may sound somehow religious (and it has nothing to do with that other notion, omnipotence), but I'm led to it simply because I can't find any better term to name what the fundamental logic of QM is about, and because this seems to me to reflect well the relation of the multiverse to us. What all this imports to our problem is that as soon as one realizes that the modus essendi of the multiverse is impossibly different from ours, notwithstanding the fact that ours is emergent still within that but crucially not the other way around, then that same person would never consider the possibility of even abstractly placing himself in the shoes of the multiverse and from that impossible perspective consider his expectations in the form of probabilities to argue the inconsistency of this perspective with ours. A reasonable modal narration of QM immediately disallows this perspective, as the way we can say that that exists has nothing to do with the way we say we exists. But note that the force of this argument apparently based on modality, in fact stems from what the narration is intended to reflect and represent, which is once again the relation of the general structure and logic of QM to our epistemic formulation and its interpretation, with the mediation between them relying on decoherence. It is the choice to follow an ontological narration of our analysis of this structure and logic that leads us to the form of these arguments, but then we have to choose very carefully the way we speak, somehow allegorically, of very formal notions, because one point is clear, that, if our narration is not an accurate representation of the structure of QM, we will be very easily led astray. Modality is a way to help this necessary accuracy, I think, and this is the motivation of its importance here. In conclusion, my advice is to remember that knowledge precedes ontology, thus set Your perspective accordingly and don't forget where You come from. Then, I think there won't be a probability problem for the multiverse.
At third listening i understood finally why there is a probability/Born rule problem in Everett and why Everett's theory is fully deterministic. Then my head nearly exploded when i started thinking about the meaning of the linear coefficients in wave functions in Everett. My god! These guys are great!
What is meant by “non-branching cases” and “branching cases”? I know that in the Everett analysis of quantum mechanics the wave function “branches”, but what then is a “case where the world is not going to branch”? What does that mean?
Unfortunately (for clarity), I believe that in this context he meant the “non-branching case” to refer to the Everett picture, because no decisions are ever made. I think he was using “branching” to mean “choosing a definite outcome.”
I feel parts of my brain are in a super-position every time i cannot make up my mind about something (most of the time); perhaps our conscious thought process is quantum mechanical in nature. Another fantastic and thought provoking episode with a really brill guest. Keep up the great work Mr Carroll.
Allow me to offer an idea about “measuring” problem of electrons. Here it is: While a coin is tossing in the air, is it H or T? Well as the coin is in the air, it is both! The bias in the coin determines the probability of H or T. An unbiased coin is 50% H and 50% T. Therefore in the slit test, as we shoot electrons on to the screen through the slit, we are sending all the probable versions by wavelike thing which is reflected on the screen. QED!
Looks like Wheeler gave Everett the old "paradigm shaft," just imagine if he had supported him to the max, who knows what further contributions Everett might have made in a free form academic environment?
sean, that would be interesting to hear a talk on other interpretations, perhaps including interviews with the most prominent proponents of each of those.
I have watched Episodes 1 through 35 and all was well. This episode is different. Even though I have read books on Quantum Reality or various interpretations of the math of quantum physics I was not able to understand this guest's points. Roughly it seemed like he was talking to Sean, knowing what Sean knows about Quantum physics. It didn't seem that David Albert was considering the audience's understanding. That's unfortunate, since Sean's podcasts are not meant to only be understood by Sean. Cheers!
Ref. @ 15:00 ca., when David Albert describes the "measurement problem". I'm trying to synthesize this formulation of the measurement problem as: since determinism fails in QM and indeterminism is "contagious" (by entanglement) at all levels, how do we reconcile QM with our determined experience? Another form of the measurement problem is the question of the legitimacy of the projection rule for observations, if and how its discontinuity reconciles with the continuous linear evolution or QM, and if in general it is really consistent with locality, relativity, causality, etc.. The former is perhaps a wider formulation of measurement problem, and borders the whole interpretation problem. But the two are strictly connected. To see this consider quantum logic (of probabilities), the projection rule is the conditionalization of quantum logic. Conditionalization is very much a requirement for a logic of probabilities intended as expectations. Now, conditionalization is in principle non-problematic IF the probabilities that are conditionalized are solely epistemic, i.e. if their uncertainty is solely due to lack of knowledge. I say in principle because it still has to be consistent with all the rest of our physical theory (which I think turns out to be the case, for the projection rule and QM). The probabilities of QM are probabilities about the results of observations, moreover about determined results according to the nature of our experience (or naive understanding of it). (Note that we are starting from "QM as we use it", which prescribes us to use this naive understanding, deemed "classical" understanding by CI.) That is, we have solely epistemic probabilities IF uncertainty is entirely due to our lack of knowledge about the determination, more explicitly, the implication of solely epistemic probabilities is determination. Essentially, solely epistemic probabilities means a probability logic satisfying the ignorance interpretation of probabilities. So the two formulations of the measurement problem are linked: determination would both correspond to our experience and allow an un-problematic conditionalization. The general problem then stems from the failure of determinism in QM (or, QM does not satisfy determinism): uncertainty is not solely epistemic, it has an objective content in the sense that it is not that we don't know the determined value of the physical quantity, it is that the physical quantity is not determined beyond some level. It is the impossibility of the infinite removal of uncertainty that constitutes the failure of determinism in QM. We know that determinism is satisfied by classical mechanics, and more specifically by probabilities that satisfy classical logic, because we can get to a non probabilistic formulation of the mechanics as the limit of infinite removal of uncertainty. This assure us (analytically!) that classical probability logic satisfies the ignorance interpretation. In this case we are ok: the determined quantities of the theory match our experience and conditionalization is solely epistemic: it does not affect in any way a system if I peep or not. The same criterion fails for QM: the limit of infinite removal of uncertainty does not exist, does not converge within the theory. BUT, even if CM was right, we never experience that limit: all the determinations that we experience have a limit of resolution (we don't distinguish pi from all other real numbers in our observations). Determinism is an infinite, and moreover it is metaphysical. So, we don't actually need that classical logic remains valid to the infinite and metaphysical limit to match our physical experience, we only need that it is valid within a scope, and then we can let it break down when we reach some (hopefully objective) resolution limit. Both formulations of the measurement problem can be solved if QM gave us a scope of determination that matches our physical experience, and within that scope conditionalization would also be solely epistemic and for this reason unproblematic. But determination is not fundamental, i.e. determinism fails, so how can we get to the needed determination if we can't get it directly as a formulation of the mechanics that avoids probabilities, which again would be the determinism that we can't get to? We get the necessary scope of determination by implication from our epistemic and probabilistic formulation of QM, we don't need any more than this to solve the problem. And the way we get this implication is already sketched above: determination is implied by the ignorance interpretation of probabilities, which in turn is analytically implied by classical logic. If quantum logic has a feature by which, in our epistemic formulation of QM, classical logic is emergent (still within quantum logic), we have the implied scope of determination that is all we need to solve the two versions of the measurement problem. It has it, and it's decoherence. I think this is all the reasoning required to give the foundations of the interpretation of QM. The very non trivial rest (a modal logic narration, the implied realism, the multiverse "ontology", how the orthodox formulation and even the quantum-classic complementarity of CI are both justified as practices and revolutionized in their fundamental meaning, possible implications for an emergent space-time extension, ....) is the analysis of the consequences of this interpretation.
I would love to hear some good arguments against everett. Sadly I didn't understand a thing David was saying, especially how it relates to everett. Maybe if Sean would be willing to summarize and steelman it and perhaps offer a counterargument?
Do not listen to this while driving. It’s not boring but it is soothing. It’s like being a kid and hearing adults talk in the other room. You are not the audience here, you’re eavesdropping on two really smart guys talking. My guess is they are pretty good friends.
In the many-worlds theory wouldn't the universe be splitting at every nanosecond at every subatomic region. Do they address the question of what causes the universe to split? And how often? It would just seen that every second of our life involves and unfathomable Infinity of subatomic interactions how could there be universes being created at each of these subatomic levels?
I don’t quite understand the philosophical concern with the concept of superposition: an electron could be in both states at once to be determined only at time and action of measurement. That philosophically speaking so to speak is just saying the future exists only as “potentials” up until the time you take an action, at which point that potential has turned into a realized state. You now have the information and are no longer taking a chance-whether or not the electron is there! (I loved the bit about the 50-50 probability as the portal to thermodynamics of heat flow: the exact experience I had in the my first (and the last) statistical mechanics course and the feeling I had of having missed a whole prerequisite course prior to this one.)
One electron can't be in superposition and it isn't. An electron is a quantum of energy that has been irreversibly exchanged between the free field and the "detector". If you want to compare this with dice, then "the electron" is the equivalent of the outcome of a dice throw. It's not the dice itself, but only the result that happens after the dice have come to rest. Just like there is no dice value as long as the dice are still rolling, there is no "electron" while the free field hasn't been detected, yet. However, we can characterize the "state" of rolling dice with a superposition of all possible outcomes one to six. That, however, is a statement about an infinite number of dice throws, usually called "the statistical ensemble". In the same way we can talk about the quantum mechanical ensemble of a single electron system, which allows us to predict the frequency of outcomes of the detections of many repeats of the same single electron experiment.
EXCELLENT interview . . Here's my opinion : like Einsteins "Blunder' of adding the cosmological constant to TWEAK his theory, the Copenhagen interpretation seeks to UNNECESSARILY modify what the math would otherwise suggest, the many worlds Theory.. for that reason ALONE I agree with Sean Carroll..
Really enjoyed this one also. Would love to hear a great chat between yourself and David Kaplan and also another with Leonard Susskind. This is such an interesting set of podcasts. Fascinating. Oh, how about Steven Weinberg. And Freeman Dyson and and... You ever watched those “Web of Stories” videos on RUclips?
To "measure" a cosmological "probability" would inherently include a length or amount of space and time... No? On the one side of the "equation"? If you exclude the "infinity" of space and time... The "geometry" or gravity of it... Then perhaps that's why "infinity" is spat back out ? Imagine "measuring" the very first particle in the universe. How would you measure it? What 'scale' would you use? What would you "relate" the particle to?
Defining and discussion the actual problems is as important (if not more) than the possible solutions. What I gather from this discourse though is that one cannot experience a difference between branching and probabilistic theory; by all appearances the single branch you experience would feel the same as having been chancy whether or not the other branches have a physical representation somewhere. We can put something back into a superposition in an experiment (or in a quantum computer), but then there must be some universes where we didn't do this; can the branches come back together? I think the cases where they can are by definition indistinguishable by experiment due to the nature of superposition.
Superb Content ! as usual. DA is a pretty damn BIG thinker thats for sure ! imo the rather simple concept of "Rendering"(as per ANY flavor Digital Physics/VR model) is the ONLY "sensible"(NOT that it "has to" or should be) interp as of yet. Though it really doesn't "say" any more than the Cope as is, except that one can easily wrap their head around the "why" of the matter. "Many worlds" was/is an extremely "out of the box" idea, especially at the time. But regardless of the fact that an infinite # of imaginary universes surely has poor Occam rolling over in his razor grave, Dave's argument is a much more significant reason(s) to be extremely skeptical of the notion. But at least they(these imaginary universes) would be "physical", besides just being entirely & permanently non-observable/un-testable/un-falsifiable. Its funny af that such mostly otherwise smart people still favor it. VR can "do" ALL of the same things + MUCH more & "do it" with zero infinites.
big fan of both of these people and I really think it shows a certain amount of maturity you dont always see in these types of arguments that it was very civil.Alot of times when you hear these discussions theres alot of sarcasm in the conversation when the people involved dont agree.that being said I fall on alberts side of this.It may seem like one of the more silly arguments on its face but if you look at history any "centric" i.e. anthro geo centric models have always been wrong this in of itself I admit is noy a good argument but (and I could be wrong here) it does not seem to explain why we are the universe from which these others branch and to me that is one of the few areas in the field of physics where a why question almost demands a plausible answer. again I could be making an invalid argument so if anyone reads this and can tell me why i am wrong I would welcome a response
Essentially the probability part of quantum mechanics is pure invention and superfluous. The truth is when you look at an electron, it has 100% chance of being exactly where it is.
Layman understanding - Isn't quantum mechanics simply saying that every time you measure an electron, it's position in spacetime has been defined, ie everything is in a constant state of "definition," each of these definitions, one after the other, creates the feeling of time, but each moment comes irrevocably from the last, and the electron's other possibilities don't exist once it has been defined in spacetime in that precise moment. Don't know how anyone get's "Many Worlds" from this, except they're afraid to lose freewill.
The irreducible uncertainty comes from the 1st person perspective, "which future "me" will I be?" There is no perspective, not even the Universal Wave Function's perspective that can know which future you, you will be. I will see one thing the other me will see something else, this is only meaningful from the 1st person perspective. This is why MWI is compatible with irreducible chanciness, even though the UWF is completely deterministic and fully exhaustive of all possibilities.
Jung actually talked a lot about quantum mechanics and thought it might be an explanation for his observation of 'synchronicity'. Jung had some pretty extreme ideas but he's intellect was incredible so i wouldn't denote him as a pseudo scientist.
While David Albert is fun to listen to, I have a real problem grasping the core of his argument against the many worlds interpretation, or against the various approaches / theories supporting it. Can anyone enlighten me? What I did get, in very crude terms, is that his problem with the decision-theoretical arguments is that we imagine being able to infer facts about our preferences in the branching futures from our preferences in non-branching futures. He objects to the idea that "on pain of pure irrationality" we'd obtain these facts or probabilities about our preferences in branching futures only due to our preferences in non branching futures, as if the former would leave a trace constraining our choices in the latter, which he thinks is silly. While this may have merit, I don't understand how it objects to the Everettian standpoints in general (maybe it doesn't). His other problem with the many worlds interpretation seems to hit more at the core of the idea. It seems we can imagine a state where we, (or a state of particles in an experiment) are about to split, and we can momentarily hide the result from our knowledge. And then rightly wonder "did I (or the particle) end up on the left or on the right (metaphorically speaking) even though the event is over. Though the question is at core simply indexical, i.e depending on the circumstances in which the question is asked. The facts are already established, and moreover all the possible outcomes have always been known beforehand, deterministically. And he seems to find a problem in this, in that chance becomes an illusion or chance only exists subjectively. And that's where i'm losing the plot, and certainly misinterpreting him.....
47:40 IMO it's more than just our mental state can only be one way or another. It's that the information cannot pass between the two states anymore due to decoherence.
Fascinating. I have to read Becker's book and any others that delve into the historical interplay with Bohr, Wheeler, Everett, McCarthyism, the Cold War and beyond. Like John Nash's life, Everett's seems made for a movie. Sad ending to his many whirls life and I have got to believe the negative atmosphere relating to his theories was harmful.
@1:03:00 Sean is wrong about those "obvious" decision theory axioms. Transitivity does not hold. There are plenty of examples where a completely rational person can prefer A to B and prefer B to C but will prefer C to A. One pure logic example is rolling a set of three dice, probability player A gets higher score than B (A>B) and B >C C but C>A. Just make the dice sides A={2, 2, 4, 4, 9, 9} B={1, 1, 6, 6, 8, 8} C={3, 3, 5, 5, 7, 7} for example. Although contrived it is realistic - just make a high stakes game of it! But in economics there are also oodles of examples where consumer choice transitivity fails, and examples from several other fields of study (war, conquest, ... too numerous to mention). So I hope (for the sake of the MWI stans) the probability "just so" stories do not hinge on that particular postulate of decision theory! If course I think David is correct in this criticism of MWI. But I also have an Occam's razor argument against MWI, which naturally like all Occam arguments cannot be a proof MWI is wrong, but is just a good argument for why MWI is merely _not the best interpretation_ given you subjectively might desire greater _parsimony_ (which is a mathematically precise notion, see the work of Elliot Sober).
"Collapse" of the wave function? The math of QM seems a bit firmer than many of our concepts. Perhaps we could look from its point of view and consider it the wave function becoming briefly hyper dimensional. So in effect we may not be measuring its position but establishing it and thus we are a macro conglomeration of many worlds.
The contrasts of nature * to be written by many hands * from the stiffness of the stones to the smoothness of the heights, of the fluidity of the water to the rigor of the ice, from the sharpened sharpness of the thorns to the trampolines of the drops that make the petals, from the horror of darkness to the gleams of the nights, from bright stars to frosted skin, from the heat of the fire to the freshness of the breeze, abyssal dread of the depth of the ocean the delightful surface of the gliding of the waves, from the ignominious bestial insanity of beasts to matrimony, from the stealing of the erosion to the way of the rivers, The contrasts of humanity from the odious odor of sweat from slave labor to the sweet refined sweetened kiss of love, from the evil lie to the fulfilled promise, from joy to contagious boredom from the solitary cry inconsolable to the cry of collective passion from eternal longing to the heart kept in remembrance of the plea for forgiveness from sentence to piety of punitive human rational justice to divine mercy
@28:52 "Wigner has this idea that it was precisely conscious agency that caused departures from the standard quantum mechanical equations of motion that caused the collapse of the wave function" is, I think, partly incorrect respect to Wigner's "Remarks on the Mind-Body question", though it could describe other parts of Wigner's thinking, and it doesn't change the general discussion which is about how some physicists (Wigner was not alone) used to invoke "minds & stuff" to make up for the inability to figure out QM. ... which perhaps contributed to the lull on the reasoning on the interpretational foundations of QM?!? I mean, it really makes sense to react with a "shut up and compute!" if on the opposite side of Your desk they are debating if a mice can collapse the wavefunction... This was the article of the famous "Wigner's friend" scenario. Wigner's observation is that the statistics that are correct for him to describe his friend making an observation of which Wigner himself ignore the result are not achievable by the pure state formulation of QM, they require a mixture, a statistical operator. The departure was not so much that of the projection rule from unitary evolution, as that of the mixed state formulation from the pure state formulation of QM (that he calls "orthodox QM"). He puzzles about this, and it is this that he proposes to be due to his friend's consciousness (with which we can't really agree). I find interesting a couple of observations he makes in the article. That the pure sate description of his friend plus object won't give statistics that are physically correct, the mixture does. That in the statistical operation formulation of QM, the evolution from a pure state to the required mixture can be continuous. It is not completely clear to me why the statistical operator formulation was deemed puzzling. Obviously, though, Wigner could not rely on the work done in later years on decoherence. The Wigner's friend problem is, at least in part, connected to the problem of the correct statistics for an un-observed measurement: how do You represent Your knowledge that a (specific) measurement has occurred of which You don't know the result? The reasoning about the physically correct statistics are the same as those made by Wigner, but of course consciousness can't apply here, since the measurement device has none.
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David Albert is my favorite guest thus far. Definitely have him on again.
My heart started fluttering when I realized I was going to get to have you and Albert in a room to myself for more than an hour. As I did with Penrose, and Greene. Your podcast is a (secular) godsend! I'm SO grateful!! ❤️
Now get Deutsch too! :D
@@sirilandgren Definitely Deutsch! That'd be amazing.
I wish, though, Albert could think ahead far enough to finish a clause before entering a qualifying sub-clause, and then a sub-sub clause, and then...
My stack keeps overflowing.
I have no idea what Albert just trying to say But I think he just wants to disagree
Didnt know who you were Sean, until i saw you on Joe Rogans, now im hooked on your podcast, your such a good guy, scientist and podcaster
I also learned about Sean on the JRE podcast, which I listen to daily. I have always loved science, especially physics, astronomy, cosmology, quantum physics, etc. I can't say I am an expert in any of said fields, but I love learning all I can about them. Thank you Sean for making difficult subjects interesting and easier to understand.
same here haha.
@@yotty97 thanks for correcting me .
@Oners82 Thanks man i'll check it out straight away. Cheers from Sweden
Same here, I am now a follower of Dr. Carroll.
I love listening to David Albert and anything to do with the foundations of physics. The extra length was fine by me. Definitely bring him back some time and finish the discussion!
Smart people who disagree are excellent guests to have conversations with! Thanks for the great podcast Sean.
disagreements always confirm there's honesty to be had, i'm really skeptical of groups of people who talk about complex problems and always politely agree.
I really wish there was another, longer episode of you two talking about these topics. Fascinating, time just flies by so fast listening to it.
I can’t necessarily follow all of his philosophical reasonings, but I like Albert and it just gives me a reason to listen through the whole conversation multiple times. Rather than just clicking on another introductory lecture on quantum mechanics which will normally will only cover material I’ve already heard a million times.
Loved the length of this one. Longer is better.
YES
With a few years' retrospective viewpoint, I think this may be one the finest interviews Carroll has done..
Thanks for the content Mr Carroll - I work as a pianist, but I'm fascinated by physics. I'm grateful that people like you are out there providing these. Most TV documentaries think we're so stupid that we'd hate (or be hopelessly confused by) any REAL technical, philosophical, or mathematical discussion. So instead all we get are dumb animations, and inappropriate analogies. Podcasts like these satisfy my craving for genuine scientific information that isn't so dumbed-down that you don't actually learn anything!
Wow.. very good conversation. I will admit I'll have to listen to it a second time to really catch it all. Two great minds here having a great conversation.
Fun to listen to these two people. So in harmony and respectful to each other.
This conversation needed like two more hours. Please have David back!
Oh my lord, Sean Carroll has a podcast. Where has this channel been all my life?
Guest/Show request:
"Nima Arkani-Hamed on Spacetime, SUSY, and building larger colliders"
Also, I think this would be fun:
"Leonard Susskind on Black Holes and no, I am not Mike from Breaking Bad" 😆
Leo hah… that would be "explain gauge symmetry" attempt #5 for poor Joe? If you start explaining hedge funds to a dog, the expression on that dog's face is the same expression Joe will have for the entire podcast if he has Nima on.
Thank you Sean and David for this beautiful talk. It strikes me that we may have to rethink our naming convention: what if quantum refers to the discrete event of the observation itself, in which the interpretation of the observed is an exclusion of one thing and an inclusion of the other. The quantum world then is not the world of discrete packets of energy or discrete, probabilistic outcomes of measurements, it is the absence of it and the Schrödinger equation describes the nature of reality in which there are no observations, that is, measurements, that is, exclusions caused by the nature of our intuitive reality which is discrete and dual by the concept of choice. Our reference frame is discrete, therefore we observe in a discrete manner. Our macroscopic worldview is evolving around differentiation. When we choose, when we measure, we entangle the observed reality to our quanta-based structure of reality. The so-called collapse of the wave function is there only because our reference frame in which we give meaning to observations is discrete.
The idea is that our observational experiences live in a quantum world, but reality beyond that what is observed is continuous, described by the universal wave function. The microscopic world is without any quanta, and is fundamental to everything. We might want to rename classical physics into quantum physics, and quantum physics into fundamental physics, the theory in which everything is contained by an a priori logic principles, valid without any reference to observable results as opposed to empirical logic.
I would like to know what other interested viewers and yourself as creator of these podcasts think of this.
I have close to 0 familiarity with this discussion (I'm a history graduate student and English is not the first language). Your comment helped me, I think, have some grasp of what they where talking about, and why a theory of decision-making is even relevant to the many worlds argument -- I was lost during most of the latter third of the podcast. I wish someone actually qualified would reply to your comment, but if I understand it you agree with the classic Copenhagen interpretation then? Meaning that there is a universal wave function, and discrete observations (interactions) create the reality we can experience?
Edit: Thought about it some more and actually I don't know if the Copenhagen interpretation admits the idea of a universal wave-function or if it views the concept of a wave-function as something that only applies to particles that haven't yet interacted with anything.
I simply love this talk and have listened to it many times. Often as a bedtime story.
"Sadly, we didn't get quite enough time to talk about what approaches to QM that he IS in favor of..." - Sean Carroll on a podcast that's 18 minutes shy of 2 hours.
Alright, let's do this.
As an analytic philosopher, I really enjoy your podcasts. Thanks for bringing the disciplines of science and philosophy together for the rest of the public. I find the intersection of the two fascinating. We could all use some more education on social media too. Cheers.
Listening to David Albert is a challenge. His answer/point is only reached after a long random walk of excursions into tangentially related minutea.
semidemiurge has David made one precise point in almost 2 hours? If you are an interested amateur it is impossible to follow him and not because he uses math or jargon I don’t understand but because he actually doesn’t have a point.
@@robertchristiandau1090 He can be much better than he is in this conversation with Sean. I think he failed to recognize who his audience was in this conversation, ie. the listeners to the podcast and not Sean. The result of which is that the conversation was more between them and required their shared understanding of the physics, science history, terminology, etc. and the typical podcast listener struggled to follow. I also think David has considerable respect/admiration for Sean and was motivated to display his intellectual chops to him. David does have a serious predilection (not uncommon in philosophers) towards digressions and staying focused on the subject at hand is not one of his skills. I would recommend a couple of videos to see him more focused and succinct and insightful. ruclips.net/video/OcMj-_kgTL4/видео.html . ruclips.net/video/LmLYa9pbh6k/видео.html . ruclips.net/video/qok_3-z3URs/видео.html
@@semidemiurge His problem is he cannot finish a sentence - he sees what seems to be a better approach or analogy, and he thinks he will not remember it by the time the current sentence is finished (which would be the case), so he jumps to the 'improved' thought, which has its own tangents and he forgets the original point he was trying to make... this is where a good interviewer patches everything back together, if he was not thrown into confusion, too...
I have no idea what this had to do with quantum mechanics. All I heard was something about preferences and branches about ice cream and being fat. And who read or did not read what paper. I learnt nothing at all. To me it felt like a philosophical discussion between 2 very drunk people. But I know Sean is very intelligent so I am just going to assume the whole conversation was over my head. Maybe someone can explain what the preferences about branches stuff means in relation to science.
Albert is a heavy weight intellectual and thinker. But he lacks effective and cogent communication skills. Following his train of thought is not easy, he digresses and re-states things a different way repeatedly. He can’t seem to state things in a clear, crisp, direct and pointed way (at least in this discussion). Carroll on the hand always seems focused, sharp and direct in his points when he speaks.
Absolutely loved this episode! Would be awesome to have David back on again.
Albert never actually gets around to making a point. He has a soothing voice though. So does Carroll.
LOL
Because if he could, he could claim a Nobel prize in physics. Making progress in QM foundations is not *easy*.
Yes, I did not mean to put Albert down in any way, just a 1st-take observation.
@@miedzinshsmars8555, Did Satyendra Bose (after whome Bosons are named) win a Nobel? Did Freeman Dyson win a Nobel? Even Hugh Everett did not win a Nobel. Even John Stewart Bell did not win a Nobel. Nobel is not the measure of greatness or progress. But one thing is for sure i.e. you would not get beyond putting two-bit comments on youtube to make any contribution to the foundation of Quantum Mechanics. Smart alecs like you are utterly worthless.
He did make many points, but most of them probably went over your head. (And I don't mean to be rude. I too found this difficult to understand at first, but then did further research and came back a few weeks later.)
Forget the wave function. My brain has collapsed.
Ljttle Bro that is the wave function tho
Ljttle Bro, thanks for that, made me laugh!
But did it though or did it just split into multiple branches?
@@RAF71chingachgook that's a nice thing for you to say. It's a shame that it's not more positive comments like yours. Thanks you had the time to send such a nice comment 😊
Good to see sean carroll again.
Please get *David Deutsch* on the podcast!!!
Yup, or David Wallace from Oxford...also a smart cookie and interesting guest. Both of them "Everettians"
@@klaasterpstra6119 Thank you for telling us about David Wallace, he seems extremely interesting and reasonable. Excited to watch and read more :) thanks!
wow... this episode was worth every penny I've ever given on patreon. so good.
Superb episode, as usual! I'm sure you have plenty of ideas for folks to co-host in future episodes, and I'd like to inspire you to maybe have someone that is pushing more "fringe" theories, like Erik Verlinde. I think you've already been through a MOND phase of your research and a conversation with both of you might be able to explain to us the gaps between MOND theories and observations. I'd also love to hear Edward Witten and Juan Maldacena. Thank you for helping to teach and inspire us!
Has Tim Maudlin never been a guest? I feel like he'd have some really great contributions to make on both quantum mechanics and relativity (especially shedding light on some remarkable things Bell said on both matters).
Sean did a show with Maudilin recently
A great podcast. I'll have to give it another 10 listens to really grasp what he was talking about at core :D
My favourite guest so far. Classy conversation.
Really very good knowledge to learn....
You are the wealthist human in this world professor , you got a endless knowledge of physics , a wealth never ends..
Everyday I learn a lots from you...
Thanks sir for your knowledge share....
Thoroughly enjoyed this session-3 years later.
Why are you enjoying technically false videos? :-)
Sean, I've listened to you and David in different venues over several years and am at a point where I'm grasping these ideas. Some. Great show. Look forward to reading your latest book. A book on the history quantum mechanics i.e. Bohr vs. Einstein would be fantastic.
What a treat was to listen to the conversation of these bright minds. Sean, you are a great pair with David, please invite him more.
Amazing podcast, but there should be at least one more with David on this subject. I'm also glad that Sean's pronunciation of Schrödinger much improved.
Shame the mice don't get to choose ice cream! Thoroughly enjoyed another thought provoking podcast. Thanks to both Sean and David for an enthralling discussion.
It always puzzles me that there are time limits imposed on such conversations. How dare you, Sean?
Sorry I struggle with Albert he talks in riddles and around points never getting to an answer or cementing his view
I know what uou mean. He can be frustrating. There are places where he does get more concrete. To try and be on his side for a second, he tries to only state what we know for sure as to not shut down paths to solutions.
Yay another physics episode!!! I like the other ones but this is really the only place i can find physics podcasts... anyone have a recommendation maybe?
Joe rogan has some long episodes with sean carroll, brian cox, neil degrasse tyson, and roger penrose among others. They arent all science based but the ones that are are great.
After an hour of limitlessly digressive discourse on "chancey universes", Albert finally gets the point of saying: "So, here's the germ of my objection [to MWI]." What follows is a limitlessly digressive discourse on "decision theory", which seems to revolve around how a rational person should best maximize returns among preferred investments in branching futures. While I'm not a fan of MWI (or Copenhagen for that matter), I much prefer Carroll's mercifully concise summaries of both Albert's lengthy dissertations and Carroll's own preferred interpretation of quantum mechanics.
Nobody asked you to be a fan of anything. One could expect from you and from these gentlemen that you have at least a cursory knowledge about the structure of Copenhagen, which is the correct interpretation. I am not holding my breath, though... I have rarely met anybody, serious physicists included, who actually understood what Copenhagen was trying to tell them about nature. That's a direct consequence of our horrible ways of teaching quantum mechanics.
@@schmetterling4477 - OK, perhaps you can resolve this issue: According to Copenhagen, how long does it take the wave function to collapse?
@@QuicksilverSG There is no such thing as "wave function collapse". A wave function is the description of the quantum mechanical ensemble, which is the abstract idea that one can repeat the same experiment an infinite number of times to measure a probability distribution for its outcomes. No such thing is possible in reality (we can only count frequencies for a finite number of measurements), hence the wave function only exists in our minds and on paper in theory books. It does not exist in reality any more than the probability distribution of dice exists. Something that doesn't exist physically can't collapse physically.
A single quantum measurement "lasts" for an infinite amount of time. Why? Because we demand that the measurement has to leave a permanent record, hence it has to be an irreversible physical process. Moreover, since we assume that the energy of the quantum that we are measuring is "precise", time-energy uncertainty (which is a classical uncertainty relation) allows us to pinpoint the beginning of the measurement, but not its end. All of this is trivial undergrad physics. If you ever meet a physicist who can't explain this to you in a few sentences, then you don't have to listen to them about QM any longer. They simply don't know what they are talking about. Like I said, you are the victim of horrible teaching methods relating to QM and that is all there is to it. Everything you think is a "mystery" is simply a teacher telling you nonsense or not telling you enough.
Sean, this was a little over my head (ok, a lot) but since you & David are smart guys, making good faith efforts to get at the truth, it would be interesting to hear your 5 minute summary of where exactly you & David differ on many worlds, what fact would bring you to David’s side & vice versa, if that’s possible.
yes I became a fan of Sean from Rogans show so I attempt 2 listen 2 the podcast but a lot of these episodes I'm lost don't know what's going on and theb my mind wonders. I'm fascinated by physics astro physics nuclear physics but I don't know I feel like an idiot listening 2 this podcast seems like its more 4 grad students then laymen's my pin interest explains things simpler maybe my dumb a$$ needs 2 stick 2 that.
None of this episode made any sense to me either. I have listened to a lot of quantum mechanics and philosophy and even when the math and concepts are complex and beyond my complete understanding, I still "understand" what they are talking about or why. But this episode just sounded like abstract nonsense to me, I have no idea why they were saying any of the things they were saying. There was no grounding to any of it, at least to me.
I might suggest that this is one episode where working through the *transcript* available on Sean Carroll's blog site at your own pace is what it's going to take to follow Albert's long nuanced argument against the Many Worlds interpretation with sufficient comprehension to gain much of a useful "takeaway" from it.
so much respect for your approach to all this Sean
Thank you once again, Sean for your so articulate, and comprehensive, (yet verbally economical) (and unemotional) periodic summary explanations, laying out the logic of what your guest is actually talking about, and how it relates to the bigger picture. Great series of podcasts which compliment your carefully and so logically thought out, well written books! I admire your relatively non-judgmental way of examining problems from many (every?) different points of view.
My favorite podcast ever. I wish I were smart enough to posit a theory that David could then destroy using his philosophical contrarian sledgehammer of logic. A man can dream can't he.(and he can't, and can be both doing an not doing the dreaming till measured)
There is nothing to destroy about quantum field theory, kid. It just works. :-)
Stimulating and informative as always, SC. Thank you both.
(-I suggest an all-David podcast. Albert, Deutsch, Wallace..)
I've been waiting for this episode for a long time.
By the time David gets to his argument he completely looses me. What is the difference between a branching and a non branching future? Is he just talking about making a decision in a universe where many worlds is correct vs one that it isn't? If so, I don't get what that has to do with whether many worlds is correct.
I was hoping for something more like a debate
Basically, all I got out of this ~2hr video beyond the history of QM interpretations is that Albert doesn't like the fact that MW basically makes probability meaningless in a sense. I can't help but wonder: so what? I don't even think he's right about that, though it might require a redefinition of what probability actually is. If every outcome happens, then we're not asking about the probability of it happening, but about how many versions of us will see any given outcome. It basically reduces probability to statistics in a sense.
Hi, Sean. Could you please consider running a live stream on youtube where we (fans) can interact directly with you through the chat by sending in questions and having discussions about different topics within your field?
I am so glad you started this series! Thank you!
from 1:37 "I think that there always will be people who draw the wrong conclusions in an Everettian Multiverse..." Exactly what I thought also
David Albert is a G
Sean, someone that I think would make an excellent guest (outside of physics) is Bruce Schneier. If you're not familiar with him, just look up some his talks here on RUclips or read one of his books. His expertise is in cryptography, computer security and the psychology of security (not just computer security but all types of security from the national level to the personal level). He was one of the technical advisors for various media outlets during the Snowden leaks and frequently writes opeds for the NYT. I think a conversation between you two would be excellent. And I am sure you can book him as he is never shy about media or podcast appearances.
On the multiverse and probabilities.
I really enjoyed the discussion and David Albert's analysis and objections to the "decision theoretic" approach (I am reading David Wallace's "The Emergent Universe").
I feel that this "problem of probabilities" hints at an understanding of the multiverse that misplaces its relation to QM, specifically to the epistemic formulation that constitutes "QM as we use it".
I find that this misplacement comes out as astonishing (I mean to myself, of course) in the decision approach, but there are perhaps less blatant hints in other discussions as well (e.g. see the recent P. Tappenden's "Everett's multiverse and the World as Wavefunction" arXiv:1912.06821).
The root of the problem I am seeing is that the multiverse is intended under an ontology that does not reflect the multiverse connection to the epistemic formulation of QM. I think that this understanding of the multiverse is in fact inconsistent to this relation. The probability problem seems to me more a symptom than the malady.
Let's start by asking why and how we get to the notion of the multiverse.
Everett had an amazing intuition, still that intuition was framed by the knowledge available at his time. We live in a different time, and I think that there are reasons today, that lead to the notion of the multiverse, that none had at his disposal when Everett wrote his thesis.
I wrote in my other comment on the measurement problem that I deem that decoherence is at the core of the foundations of the interpretation of QM, what decoherence does that places it in this role is briefly written there. To the aim of the present argument, what is important to notice is that decoherence implies its occurrence within entanglement. The very nature (logic nature) of decoherence implies that alternatives are not excluded or selected, instead they are "combed" by the globalizing consistency enforced by interactions and encoded in entanglement. It is this consideration on the nature of decoherence that today shall be the concrete motivation for the notion of the multiverse. But it is the very same decoherence that in our epistemic formulation of QM yields that emergent classical logic that implies both our solely epistemic probabilities and the objective determination that renders legit a realistic intention of our physical experience.
This shows that the multiverse is inseparable from probabilities already in its genesis. And this is one of the reasons why I emphasize a difference in our motivation respect to the possible motivations of Everett, we should just not forget from where we got to the multiverse: it stems from decoherence, from our epistemic probabilities, as a way to better understand the structure of QM.
But it is also true that this does not completely excuses the problem. The multiverse is not a simple and direct analytical deduction, it is more like a model of the theory, a structure by which we hope to obtain terms and notions that would render the analytical narration of the structure of QM simpler and more intuitive, while reflecting it accurately. For this reason it is a valid question to ask if this model is satisfied by QM, and a way to disprove this is to show that the multiverse is inconsistent with the very probabilities that are its original motivation.
I think that what is inconsistent with probabilities is the ontological intention used to narrate the multiverse, that does not corresponds to its origin and relation to decoherence and our probabilities. This results in an "ontological misplacement" consisting in the unawareness of the necessity of employing modal logic, which would instead reflect the multiverse constitutive relation with our epistemic formulation of QM.
Modality here means that "being" "existing" can not be expressed without qualification, that it is necessary to specify the "modus essendi" when we predicate existence.
The first kind of modality is the one that interests us more: "actuality", this is our way of being, and it applies to our experience and, thanks to QM's decoherence, to its implication of a determination satisfying realism. But the moment You move outside the determination scope, You can no longer talk in terms of actuality. The way decoherence, in our epistemic formulation of QM, provides us with solely epistemic probabilities is such that the fundamental workings of QM in the proximity of our emergent actuality shall be described as "propensity", i.e. as an objective feed to our epistemic probabilities. Or "potency", to use a classical term. But even this two connotations of the modus essendi of QM seem diminutive when they are regarded in the perspective of the multiverse. The only term I can find to describe it, reflecting its relation to us, conscious of how decoherence unfolds alternatives by mere consistency, is omni-potency. I don't use this term lightly, for its grandeur, and with much reluctance for something that may sound somehow religious (and it has nothing to do with that other notion, omnipotence), but I'm led to it simply because I can't find any better term to name what the fundamental logic of QM is about, and because this seems to me to reflect well the relation of the multiverse to us.
What all this imports to our problem is that as soon as one realizes that the modus essendi of the multiverse is impossibly different from ours, notwithstanding the fact that ours is emergent still within that but crucially not the other way around, then that same person would never consider the possibility of even abstractly placing himself in the shoes of the multiverse and from that impossible perspective consider his expectations in the form of probabilities to argue the inconsistency of this perspective with ours. A reasonable modal narration of QM immediately disallows this perspective, as the way we can say that that exists has nothing to do with the way we say we exists.
But note that the force of this argument apparently based on modality, in fact stems from what the narration is intended to reflect and represent, which is once again the relation of the general structure and logic of QM to our epistemic formulation and its interpretation, with the mediation between them relying on decoherence.
It is the choice to follow an ontological narration of our analysis of this structure and logic that leads us to the form of these arguments, but then we have to choose very carefully the way we speak, somehow allegorically, of very formal notions, because one point is clear, that, if our narration is not an accurate representation of the structure of QM, we will be very easily led astray. Modality is a way to help this necessary accuracy, I think, and this is the motivation of its importance here.
In conclusion, my advice is to remember that knowledge precedes ontology, thus set Your perspective accordingly and don't forget where You come from.
Then, I think there won't be a probability problem for the multiverse.
31:08 dog collapsing wave function
I wish this kept going for another hour or two.
At third listening i understood finally why there is a probability/Born rule problem in Everett and why Everett's theory is fully deterministic. Then my head nearly exploded when i started thinking about the meaning of the linear coefficients in wave functions in Everett. My god!
These guys are great!
I don't understand Albert's explanation at 1:11:00
What is meant by “non-branching cases” and “branching cases”? I know that in the Everett analysis of quantum mechanics the wave function “branches”, but what then is a “case where the world is not going to branch”? What does that mean?
It means you have been listening to bullshit all of the time. :-)
Unfortunately (for clarity), I believe that in this context he meant the “non-branching case” to refer to the Everett picture, because no decisions are ever made. I think he was using “branching” to mean “choosing a definite outcome.”
This is what I've been looking for! More plz!
I feel parts of my brain are in a super-position every time i cannot make up my mind about something (most of the time); perhaps our conscious thought process is quantum mechanical in nature. Another fantastic and thought provoking episode with a really brill guest. Keep up the great work Mr Carroll.
Excellent podcast Sean! It is nice how at 3:22 you "open your umbrella" for your support to the many worlds theory! LOL ;-)
always relevant and educational!
Allow me to offer an idea about “measuring” problem of electrons. Here it is:
While a coin is tossing in the air, is it H or T? Well as the coin is in the air, it is both! The bias in the coin determines the probability of H or T. An unbiased coin is 50% H and 50% T.
Therefore in the slit test, as we shoot electrons on to the screen through the slit, we are sending all the probable versions by wavelike thing which is reflected on the screen. QED!
Looks like Wheeler gave Everett the old "paradigm shaft," just imagine if he had supported him to the max, who knows what further contributions Everett might have made in a free form academic environment?
sean, that would be interesting to hear a talk on other interpretations, perhaps including interviews with the most prominent proponents of each of those.
I have watched Episodes 1 through 35 and all was well. This episode is different. Even though I have read books on Quantum Reality or various interpretations of the math of quantum physics I was not able to understand this guest's points. Roughly it seemed like he was talking to Sean, knowing what Sean knows about Quantum physics. It didn't seem that David Albert was considering the audience's understanding. That's unfortunate, since Sean's podcasts are not meant to only be understood by Sean.
Cheers!
The many minds of david
Ref. @ 15:00 ca., when David Albert describes the "measurement problem".
I'm trying to synthesize this formulation of the measurement problem as: since determinism fails in QM and indeterminism is "contagious" (by entanglement) at all levels, how do we reconcile QM with our determined experience?
Another form of the measurement problem is the question of the legitimacy of the projection rule for observations, if and how its discontinuity reconciles with the continuous linear evolution or QM, and if in general it is really consistent with locality, relativity, causality, etc..
The former is perhaps a wider formulation of measurement problem, and borders the whole interpretation problem. But the two are strictly connected.
To see this consider quantum logic (of probabilities), the projection rule is the conditionalization of quantum logic. Conditionalization is very much a requirement for a logic of probabilities intended as expectations. Now, conditionalization is in principle non-problematic IF the probabilities that are conditionalized are solely epistemic, i.e. if their uncertainty is solely due to lack of knowledge. I say in principle because it still has to be consistent with all the rest of our physical theory (which I think turns out to be the case, for the projection rule and QM).
The probabilities of QM are probabilities about the results of observations, moreover about determined results according to the nature of our experience (or naive understanding of it).
(Note that we are starting from "QM as we use it", which prescribes us to use this naive understanding, deemed "classical" understanding by CI.)
That is, we have solely epistemic probabilities IF uncertainty is entirely due to our lack of knowledge about the determination, more explicitly, the implication of solely epistemic probabilities is determination. Essentially, solely epistemic probabilities means a probability logic satisfying the ignorance interpretation of probabilities.
So the two formulations of the measurement problem are linked: determination would both correspond to our experience and allow an un-problematic conditionalization.
The general problem then stems from the failure of determinism in QM (or, QM does not satisfy determinism): uncertainty is not solely epistemic, it has an objective content in the sense that it is not that we don't know the determined value of the physical quantity, it is that the physical quantity is not determined beyond some level. It is the impossibility of the infinite removal of uncertainty that constitutes the failure of determinism in QM.
We know that determinism is satisfied by classical mechanics, and more specifically by probabilities that satisfy classical logic, because we can get to a non probabilistic formulation of the mechanics as the limit of infinite removal of uncertainty. This assure us (analytically!) that classical probability logic satisfies the ignorance interpretation. In this case we are ok: the determined quantities of the theory match our experience and conditionalization is solely epistemic: it does not affect in any way a system if I peep or not.
The same criterion fails for QM: the limit of infinite removal of uncertainty does not exist, does not converge within the theory.
BUT, even if CM was right, we never experience that limit: all the determinations that we experience have a limit of resolution (we don't distinguish pi from all other real numbers in our observations). Determinism is an infinite, and moreover it is metaphysical. So, we don't actually need that classical logic remains valid to the infinite and metaphysical limit to match our physical experience, we only need that it is valid within a scope, and then we can let it break down when we reach some (hopefully objective) resolution limit.
Both formulations of the measurement problem can be solved if QM gave us a scope of determination that matches our physical experience, and within that scope conditionalization would also be solely epistemic and for this reason unproblematic.
But determination is not fundamental, i.e. determinism fails, so how can we get to the needed determination if we can't get it directly as a formulation of the mechanics that avoids probabilities, which again would be the determinism that we can't get to?
We get the necessary scope of determination by implication from our epistemic and probabilistic formulation of QM, we don't need any more than this to solve the problem.
And the way we get this implication is already sketched above: determination is implied by the ignorance interpretation of probabilities, which in turn is analytically implied by classical logic.
If quantum logic has a feature by which, in our epistemic formulation of QM, classical logic is emergent (still within quantum logic), we have the implied scope of determination that is all we need to solve the two versions of the measurement problem.
It has it, and it's decoherence.
I think this is all the reasoning required to give the foundations of the interpretation of QM.
The very non trivial rest (a modal logic narration, the implied realism, the multiverse "ontology", how the orthodox formulation and even the quantum-classic complementarity of CI are both justified as practices and revolutionized in their fundamental meaning, possible implications for an emergent space-time extension, ....) is the analysis of the consequences of this interpretation.
I would love to hear some good arguments against everett. Sadly I didn't understand a thing David was saying, especially how it relates to everett. Maybe if Sean would be willing to summarize and steelman it and perhaps offer a counterargument?
To sum up what I've learned from this:
There is no point for the fact of the matter.
"whatever it means"
or
"what everett means"
?
Do not listen to this while driving. It’s not boring but it is soothing. It’s like being a kid and hearing adults talk in the other room. You are not the audience here, you’re eavesdropping on two really smart guys talking. My guess is they are pretty good friends.
Thought it was David Deutsch for a second there, got a bit excited. But this David's cool too, though. :)
In the many-worlds theory wouldn't the universe be splitting at every nanosecond at every subatomic region. Do they address the question of what causes the universe to split? And how often? It would just seen that every second of our life involves and unfathomable Infinity of subatomic interactions how could there be universes being created at each of these subatomic levels?
In another world this comment is replaced by a concise, obviously true interpretation of quantum mechanics.
mrloop Most underrated comment in this thread.
I don’t quite understand the philosophical concern with the concept of superposition: an electron could be in both states at once to be determined only at time and action of measurement. That philosophically speaking so to speak is just saying the future exists only as “potentials” up until the time you take an action, at which point that potential has turned into a realized state. You now have the information and are no longer taking a chance-whether or not the electron is there! (I loved the bit about the 50-50 probability as the portal to thermodynamics of heat flow: the exact experience I had in the my first (and the last) statistical mechanics course and the feeling I had of having missed a whole prerequisite course prior to this one.)
One electron can't be in superposition and it isn't. An electron is a quantum of energy that has been irreversibly exchanged between the free field and the "detector". If you want to compare this with dice, then "the electron" is the equivalent of the outcome of a dice throw. It's not the dice itself, but only the result that happens after the dice have come to rest. Just like there is no dice value as long as the dice are still rolling, there is no "electron" while the free field hasn't been detected, yet. However, we can characterize the "state" of rolling dice with a superposition of all possible outcomes one to six. That, however, is a statement about an infinite number of dice throws, usually called "the statistical ensemble". In the same way we can talk about the quantum mechanical ensemble of a single electron system, which allows us to predict the frequency of outcomes of the detections of many repeats of the same single electron experiment.
18:30 So, is 5 seeing anyone at the moment?
EXCELLENT interview . . Here's my opinion : like Einsteins "Blunder' of adding the cosmological constant to TWEAK his theory, the Copenhagen interpretation seeks to UNNECESSARILY modify what the math would otherwise suggest, the many worlds Theory.. for that reason ALONE I agree with Sean Carroll..
Ps.. The flavor of David's OVERALL belief biases needs MORE scientific materialism and less of the woo-woo seasoning, but I loved the guy.
Sean, please have Niels Bohr on. He seems mighty interesting.
Really enjoyed this one also. Would love to hear a great chat between yourself and David Kaplan and also another with Leonard Susskind. This is such an interesting set of podcasts. Fascinating. Oh, how about Steven Weinberg. And Freeman Dyson and and... You ever watched those “Web of Stories” videos on RUclips?
Good one SC & DA-tks.
To "measure" a cosmological "probability" would inherently include a length or amount of space and time... No? On the one side of the "equation"?
If you exclude the "infinity" of space and time... The "geometry" or gravity of it... Then perhaps that's why "infinity" is spat back out ?
Imagine "measuring" the very first particle in the universe. How would you measure it? What 'scale' would you use? What would you "relate" the particle to?
Defining and discussion the actual problems is as important (if not more) than the possible solutions. What I gather from this discourse though is that one cannot experience a difference between branching and probabilistic theory; by all appearances the single branch you experience would feel the same as having been chancy whether or not the other branches have a physical representation somewhere. We can put something back into a superposition in an experiment (or in a quantum computer), but then there must be some universes where we didn't do this; can the branches come back together? I think the cases where they can are by definition indistinguishable by experiment due to the nature of superposition.
Love it! REAL Science! How does one eventually prove a theory to be correct? ...Try proving yourself wrong!
Superb Content ! as usual. DA is a pretty damn BIG thinker thats for sure ! imo the rather simple concept of "Rendering"(as per ANY flavor Digital Physics/VR model) is the ONLY "sensible"(NOT that it "has to" or should be) interp as of yet. Though it really doesn't "say" any more than the Cope as is, except that one can easily wrap their head around the "why" of the matter. "Many worlds" was/is an extremely "out of the box" idea, especially at the time. But regardless of the fact that an infinite # of imaginary universes surely has poor Occam rolling over in his razor grave, Dave's argument is a much more significant reason(s) to be extremely skeptical of the notion. But at least they(these imaginary universes) would be "physical", besides just being entirely & permanently non-observable/un-testable/un-falsifiable. Its funny af that such mostly otherwise smart people still favor it. VR can "do" ALL of the same things + MUCH more & "do it" with zero infinites.
big fan of both of these people and I really think it shows a certain amount of maturity you dont always see in these types of arguments that it was very civil.Alot of times when you hear these discussions theres alot of sarcasm in the conversation when the people involved dont agree.that being said I fall on alberts side of this.It may seem like one of the more silly arguments on its face but if you look at history any "centric" i.e. anthro geo centric models have always been wrong this in of itself I admit is noy a good argument but (and I could be wrong here) it does not seem to explain why we are the universe from which these others branch and to me that is one of the few areas in the field of physics where a why question almost demands a plausible answer. again I could be making an invalid argument so if anyone reads this and can tell me why i am wrong I would welcome a response
Essentially the probability part of quantum mechanics is pure invention and superfluous. The truth is when you look at an electron, it has 100% chance of being exactly where it is.
Layman understanding - Isn't quantum mechanics simply saying that every time you measure an electron, it's position in spacetime has been defined, ie everything is in a constant state of "definition," each of these definitions, one after the other, creates the feeling of time, but each moment comes irrevocably from the last, and the electron's other possibilities don't exist once it has been defined in spacetime in that precise moment. Don't know how anyone get's "Many Worlds" from this, except they're afraid to lose freewill.
The irreducible uncertainty comes from the 1st person perspective, "which future "me" will I be?" There is no perspective, not even the Universal Wave Function's perspective that can know which future you, you will be. I will see one thing the other me will see something else, this is only meaningful from the 1st person perspective. This is why MWI is compatible with irreducible chanciness, even though the UWF is completely deterministic and fully exhaustive of all possibilities.
Well, you clearly drank the break fluid. ;-)
I had no idea that physics was such a blood sport!
Jung actually talked a lot about quantum mechanics and thought it might be an explanation for his observation of 'synchronicity'. Jung had some pretty extreme ideas but he's intellect was incredible so i wouldn't denote him as a pseudo scientist.
While David Albert is fun to listen to, I have a real problem grasping the core of his argument against the many worlds interpretation, or against the various approaches / theories supporting it. Can anyone enlighten me?
What I did get, in very crude terms, is that his problem with the decision-theoretical arguments is that we imagine being able to infer facts about our preferences in the branching futures from our preferences in non-branching futures. He objects to the idea that "on pain of pure irrationality" we'd obtain these facts or probabilities about our preferences in branching futures only due to our preferences in non branching futures, as if the former would leave a trace constraining our choices in the latter, which he thinks is silly. While this may have merit, I don't understand how it objects to the Everettian standpoints in general (maybe it doesn't).
His other problem with the many worlds interpretation seems to hit more at the core of the idea. It seems we can imagine a state where we, (or a state of particles in an experiment) are about to split, and we can momentarily hide the result from our knowledge. And then rightly wonder "did I (or the particle) end up on the left or on the right (metaphorically speaking) even though the event is over. Though the question is at core simply indexical, i.e depending on the circumstances in which the question is asked. The facts are already established, and moreover all the possible outcomes have always been known beforehand, deterministically. And he seems to find a problem in this, in that chance becomes an illusion or chance only exists subjectively. And that's where i'm losing the plot, and certainly misinterpreting him.....
47:40 IMO it's more than just our mental state can only be one way or another. It's that the information cannot pass between the two states anymore due to decoherence.
49:33 and you would observer true randomness of measurement outcome.
Fascinating. I have to read Becker's book and any others that delve into the historical interplay with Bohr, Wheeler, Everett, McCarthyism, the Cold War and beyond. Like John Nash's life, Everett's seems made for a movie. Sad ending to his many whirls life and I have got to believe the negative atmosphere relating to his theories was harmful.
@1:03:00 Sean is wrong about those "obvious" decision theory axioms. Transitivity does not hold. There are plenty of examples where a completely rational person can prefer A to B and prefer B to C but will prefer C to A. One pure logic example is rolling a set of three dice, probability player A gets higher score than B (A>B) and B >C C but C>A. Just make the dice sides A={2, 2, 4, 4, 9, 9} B={1, 1, 6, 6, 8, 8} C={3, 3, 5, 5, 7, 7} for example. Although contrived it is realistic - just make a high stakes game of it! But in economics there are also oodles of examples where consumer choice transitivity fails, and examples from several other fields of study (war, conquest, ... too numerous to mention). So I hope (for the sake of the MWI stans) the probability "just so" stories do not hinge on that particular postulate of decision theory! If course I think David is correct in this criticism of MWI. But I also have an Occam's razor argument against MWI, which naturally like all Occam arguments cannot be a proof MWI is wrong, but is just a good argument for why MWI is merely _not the best interpretation_ given you subjectively might desire greater _parsimony_ (which is a mathematically precise notion, see the work of Elliot Sober).
"Collapse" of the wave function? The math of QM seems a bit firmer than many of our concepts. Perhaps we could look from its point of view and consider it the wave function becoming briefly hyper dimensional. So in effect we may not be measuring its position but establishing it and thus we are a macro conglomeration of many worlds.
The contrasts of nature * to be written by many hands *
from the stiffness of the stones to the smoothness of the heights,
of the fluidity of the water to the rigor of the ice,
from the sharpened sharpness of the thorns to the trampolines of the drops that make the petals,
from the horror of darkness to the gleams of the nights,
from bright stars to frosted skin,
from the heat of the fire to the freshness of the breeze,
abyssal dread of the depth of the ocean the delightful surface of the gliding of the waves,
from the ignominious bestial insanity of beasts to matrimony,
from the stealing of the erosion to the way of the rivers,
The contrasts of humanity
from the odious odor of sweat from slave labor to the sweet refined sweetened kiss of love,
from the evil lie to the fulfilled promise,
from joy to contagious boredom
from the solitary cry inconsolable to the cry of collective passion
from eternal longing to the heart kept in remembrance
of the plea for forgiveness
from sentence to piety
of punitive human rational justice
to divine mercy
What the hell is going on with all of that background noise? So distracting.
@28:52 "Wigner has this idea that it was precisely conscious agency that caused departures from the standard quantum mechanical equations of motion that caused the collapse of the wave function" is, I think, partly incorrect respect to Wigner's "Remarks on the Mind-Body question", though it could describe other parts of Wigner's thinking, and it doesn't change the general discussion which is about how some physicists (Wigner was not alone) used to invoke "minds & stuff" to make up for the inability to figure out QM.
... which perhaps contributed to the lull on the reasoning on the interpretational foundations of QM?!? I mean, it really makes sense to react with a "shut up and compute!" if on the opposite side of Your desk they are debating if a mice can collapse the wavefunction...
This was the article of the famous "Wigner's friend" scenario.
Wigner's observation is that the statistics that are correct for him to describe his friend making an observation of which Wigner himself ignore the result are not achievable by the pure state formulation of QM, they require a mixture, a statistical operator.
The departure was not so much that of the projection rule from unitary evolution, as that of the mixed state formulation from the pure state formulation of QM (that he calls "orthodox QM").
He puzzles about this, and it is this that he proposes to be due to his friend's consciousness (with which we can't really agree).
I find interesting a couple of observations he makes in the article.
That the pure sate description of his friend plus object won't give statistics that are physically correct, the mixture does.
That in the statistical operation formulation of QM, the evolution from a pure state to the required mixture can be continuous.
It is not completely clear to me why the statistical operator formulation was deemed puzzling.
Obviously, though, Wigner could not rely on the work done in later years on decoherence.
The Wigner's friend problem is, at least in part, connected to the problem of the correct statistics for an un-observed measurement: how do You represent Your knowledge that a (specific) measurement has occurred of which You don't know the result? The reasoning about the physically correct statistics are the same as those made by Wigner, but of course consciousness can't apply here, since the measurement device has none.