Is Quantum Mechanics or General Relativity More Fundamental?

alright so the job is to debate the question of whether or not general relativity or quantum mechanics is more fundamental it's not necessarily obvious what that means not obvious to me I take it to mean something like we all believe we both believe some of us believe that there is a correct theory of the world and when we find it that theory will reproduce the empirical successes of both general relativity and quantum mechanics and the appropriate regimes and the question of fundamental mistake Intuit firry sort of keep the fundamental structure of quantum mechanics exactly intact and general relativity will be emergent from that or will the basic principles of GR be right there front and center and quantum mechanics is somehow shunted off into a corner clearly we don't know the answer so a matter of taste isn't all but many matters of taste and we'll have to see unless a retro puzzle person in the room can tell us what the final answer is going to be but I'm on the side of quantum mechanics being more fundamental that's not because I don't like general relativity I wrote a book on it you can all buy it jeanna relativity is awesome I said in the book and I still think it is the most beautiful physical theory ever invented but in the contest between general relativity and quantum mechanics it's not a fair fight right my side is sort of the obvious straightforward choice in the debate which is not meters right I'm actually not making a value judgment are kumin yet I'm just saying that if you were that what happened in physics is there was everything that happened before Isaac Newton and then there was classical mechanics and then there was quantum mechanics these are the three phases of the history of physics and general relativity is a classical theory it's a classical field theory there is a degree of freedom there's the metric tensor field all over space-time it has you can write in a face based formulation with a conjugate momentum observables are every function of phase space you can imagine and all the apparatus of classical mechanics just with this particular Hamiltonian or Lagrangian whereas quantum mechanics says the role is the entirely different thing there's the uncertainty principle rather than face pace we have helper space and so when you were faced with any other classical theory of the world you would quantize it and it wouldn't be a battle between that classical theory and quantum mechanics it would be subsumed so the question is why would you not think that that was true about general relativity and the answer I think well for one thing there's two answers the the very short answers would be hard to see no sorry I need to add one more point to why it's obviously true the generality is less fundamental because it's hard to imagine how it could be more fundamental general relativity gives you an equation Einstein's equation with the Einstein tensor on the left hand side and the energy metric that's on the right hand side general attempt to be more fundamental with some how mean there was a classical space-time metric could still evade that equation and it's utterly unclear what the right-hand side should be what the source of gravity would be if the metric were left classical somehow if the metric were not straightforwardly quantized if you had a source that was in a superposition a macroscopic object misoo position of two different positions where's the gravitational field pointing is it in between is it to one or the other you would have to in order to make that work you would have to imagine some modification of quantum mechanics that made every macroscopic superposition collapse very quickly so there weren't any macroscopic superpositions of matter and energy and maybe you could do that but it's not at all obvious how would would okay so why would you never want to do something like that I think we all know the answer to that which is that when you try to quantize gravity all hell breaks loose Steve Karlin gave us a nice talk and how you can do it in this extraordinarily special case of two plus one dimensions without any matter or anything like that but more generally is hard and it's hard for three different sets of reasons number one the technical reasons it's not a renormalizable Theory everything is infinite it's hard to make predictions number two there are conceptual difficulties the nature of time and especially the nature of space is actually a big problem right the idea of a local observable doesn't seem to exist in general relativity because you imagine that the metric itself takes on different possible values let's say on different branches of the wavefunction reading language it's hard to save it to identify the same point in two different branches of the wavefunction so what is locality even supposed to mean and finally there are what we might call the visceral objections to generosity being quantized we think of space-time as the arena in which things happen this is clearly how Einstein thought you know Einstein really had trouble wrapping his mind around the idea that space-time was anything other than the arena and now you believe you quantized general relativity spacetimes not that fundamental he's the geometry of space-time is not that fundamental now I would argue that's not an especially good argument I would argue that any quantum theory devalues space-time as the arena in which place things take place because once you invention tangle man you know that it's more like configuration space or momentum space or some other polarization of space and I would further argue that if you really liked and honor the values of quantum mechanics you forget about space-time altogether quantum states live in hilbert space they're vectors in a vector space in Hilbert spaces where things happen and everything else is just a useful way of talking so this attitude is something that my collaborators and I have taken to extremes and something we call the mad dog version of ever-ready in quantum mechanics mad dog because many Eveready ins still help themselves to a notion of what the fundamental variables of the theory are you're quantizing the electromagnetic field in the drag field or something like that I think that's cheating I think we know plenty of examples where there's two different classical variables into different classical theories which you quantize and get the same quantum theory quantum theory should be thought of as a theory that lives describes vectors in Hilbert space and the right classical way to describe it needs to emerge so that's what we're working on so even space-time has to emerge and the good news is you can kind of see the outline of that how that happens for a generic Hamiltonian you wouldn't expect any local space-time description but for very special Hamiltonians there is a local space-time description and when there is is more or less unique and you can take that space-time description just based on the Hamiltonian just based on the spectrum the eigen use of the Hamiltonian and you can say that if I had a state in that Hamiltonian ol eyeing state near the vacuum I can use the entanglement structure of that state to define a metric on space and perhaps on space times and I can even argue that there's an equation that that metric obeys and that equation looks exactly like Einsteins equation so this is very speculative we're nowhere near filling in all the blanks but it seems at least plausible that rather than quantizing gravity you can find gravity within quantum mechanics given the right Hamiltonian and the right state you see that at the level of the semi-classical approximation what is being described or some fields propagating on a background metric I think that's a very satisfying way of doing things there's plenty of problems with this the biggest single problem is that we deal with locally finite dimensional hilbert spaces which makes it very difficult to implement things like Lorentz invariance and diffeomorphism invariance etc so we say guess what those are not fundamental they are emergent that may or may not work this is what we're trying to do so basically we are making all the visceral problems of quantizing gravity harder we're facing up to them by saying they weren't problems at all you gonna have to suck it up okay so the and this goes very much hand-in-hand with what I think is just to sort of finish up this little rant why do some people think quantum mechanics is more fundamental and why other people think general relativity is more fundamental there is just as much of a matter of taste here as a matter of physics because you have to give up something and the question is what are you gonna give up right what what what at which of your cherished beliefs or your fundamental principles are going to have to go and I remember vividly Lee Smolin pointing out to me that there's a strong correlation between people who believed in the Everett interpretation of quantum mechanics and people who thought that computers could someday be conscious and you wouldn't expect these are very two very different kinds of questions right but it's the same people who light up and you know if you believe in Everett you also believe that AI can become conscious why is that well I think that there's a there's an explanation it depends on what you value if you're never ready and you've learned to accept what you value are very very simple fundamental principles and what you give up on is a very short direct connection to the underlying fundamental theory and the world of our experience there might be many layers and many different kinds of emergence and a lot of project and work full employment to be done in relating the simple underlying principles to the world that we see and that same kind of cast of mind lets you think I don't really know what consciousness is but I don't see why the computer can't do it just as well as a person where there's another cast of mind that says consciousness is something that I'm feeling in my brain I can't even imagine a computer doing it tables and chairs are something I see they're located in space-time I don't want to give up on them as fundamental things I don't know which one is right but I thank for the very same reasons that I think that effort is a good formulation of quantum mechanics that in the battle between general relativity and quantum mechanics general relativity is going to be the one that ends up more fundamental making his case for quantum mechanics licensed quantum mechanic okay well first thing I want to say is that before I get into trying to defend do you think that they kind of work but I think that the kind of project that Shawn is talking about is the most valuable kind of project that we have in the foundations of quantum mechanics which is I don't think that we can resolve the problem of the interpretation of quantum mechanics without trying to push these ideas into new physics right so if if mad dog have already it isn't turns out to be right and Shawn has the correct account of the emergence of space-time and the emergence of gravity then it will be very difficult to argue that their variety and point of view is wrong and so this is precisely the kind of work we should be doing however what they disagree on is how this is going to play out so the first thing is as Shawn mentioned we have to interpret what the question which is more fundamental quantum more general relativity means so the way I interpret this question is possibly a bit different I'm not thinking that we're going to end up with a theory which has a class called metric well I what we all agree on is that the fundamental theory whatever it is has to in the appropriate limit give us general relativity and give us quantum field theory in the standard model right so if you want to call that theory quantum gravity fine it may not turn out to be a quantum theory or recognizable as a as as a straightforward gravity theory but it's some theory which has those things as limit so the way I interpret the question is in that theory will I recognize principles that come from general relativity as being its foundation or will I recognize principles that come from quantum mechanics as being as foundation which is which are more likely to be reliable okay so I want to give two arguments for thinking of principles from relativity as being more fundamental the first one is I think a fairly solid argument as somebody who spends time thinking about quantum foundations are definitely state right in the second argument I'm going to go way out of my comfort zone and and be very speculative and talk about things they didn't know very much about so let's start with that so the first thing is when I think about all this fundamental theory what what kinds of principles will it have will they have principles from quantum mechanics or principles from doing relativity well both when I when I think about possibility event having principles from general relativity I think it's very clear what those are at least you can list some things that are important in general relativity for example the idea that gravity reduces to geometry the idea that the equivalence principle that grabbed that grant is equivalent to acceleration in certain sense I wrote down a few more here and for government what they were let's see yes equipments of different coordinate systems that the laws of position depends on new coordinate system etc so you can write down these things and it's sort of clear what they mean and it's clear what they mean independent of the mathematical framework - well of course if I'm going to talk about gravity being geometry I have to be talking about some sort of geometry but it doesn't have to be Romanian pseudo Romanian geometry right if I formulate a discrete geometry I can talk about what it would mean to have a coordinate independent theory in that geometry or if I even formulate a noncommutative geometry I can talk about what those principles would mean for that geometry they are principles which have broader than the specific mathematical framework of differential geometry that general relativity is usually formulated it so although the details the details of doing these things are indeed extremely tricky right to talk about what do you mean by the retsoor diffeomorphism invariants in the discrete geometry is a very difficult thing but nonetheless I can think about what those things mean and it seems to make sense on the other hand what principles quantum theory based on well there are things that we tell undergraduates they're based on or we say the uncertainty principle is very fundamental but I can't get any way in the air deriving the mathematical formalism of quantum theory from that and also when you talk to different physicists they will give you a different list of things that are the principles of quantum theory so some might say oh it's the canonical commutation relation it was always the training require that we knows or it's the structure of hilbert space all of these things posited as fundamental principles of quantum theory but they are number one very different depending on who you talk to and number two usually very closely tied to the mathematical formalism right so if I say this fundamental theory is gonna be so how quantum like does that mean it has to be formulated in Hilbert space it's difficult to say so since possibly I mean there may be there's more agreement in this room than in general but generally speaking you know I find it hard to understand what even the principles of quantum theory that is supposed to keep preserved in this theory are right so I think if I'm asking myself which of those two sets of principles are more reliable as a guide to constructing this theory I'm going to choose the ones which I can I can understand what they mean in a broader framework so that's the first argument so for those reasons I think general turistic principles are more reliable guides to very construction in the second argument I'm going to get a bit speculative I'm going basically I'm willing to defend the idea or make the prediction quantum theory will be long gone before we get to the scale of course of gravity and so I have to give some reason even recorded so say that there are of course people who believe that quantum theory will break down for various reasons usually the reason is they won't solve the measurement problem so if you think about the spontaneous collapse theory for instance there will be some size of macroscopic system at which the rules of quantum theory break down and we'll have a different theory now I don't think that's the reason that quantum theory will break down I think that it may make it replace by a theory that's even weirder than corn some Theory I don't think it's going to break down in order to solve the measurement problem but what I do think is that there's an awfully large number of orders of magnitude between the scale at which we know quantum theory and quantum field theory works and the Planck scale right so it's quite to think that our physical framework won't break down when we get there it's quite as quite an extrapolation specifically there are problems in quantum field theories in high energy physics that occur before we even get to the Planck scale so this I have to start looking at minus 4 so for instance the grand unification scale is believed to be about 3 orders of magnitude smaller than the Planck scale so about 10 to 25 electron volts verses 10 to 28 for the Planck scale as of now the best bet for unification grand unified theories of the three forces have been supersymmetric theories in which the coupling constants are supposed to run together at that kind of value all evidence that we have so far suggests that those theories aren't the right theories like the LHC has not found any evidence for supersymmetry at this point so right now we're in a limbo where you know even though really how grand unification is going to work for the other three forces let alone gravity now three orders of magnitude might not seem that much so it might be that there are effects that come from the Planck scale that to thank that scale of physics but there's still quite a quite a large number there's quite a large difference there could be new physics that comes like you know that scale that doesn't have anything to do with Planck scale physics and so high-energy physicists have got really great model building trying different kinds of symmetry groups trying different kinds of ideas and but they're all formulated within quantum field theory like nobody has posited a model that isn't quantum theoretical order to try and solve these problems so what I'm suggesting is that maybe there there will be a theory that you know reduces to quantum theory appropriately but is somehow more general that comes in somewhere around this scale and that by the time we get to quantize it gravity maybe we don't have something that's recognizable as as quantum theory after all so I let me give a very very because you know I'm not an expert in this area but I'll give you a an idea for an idea so in the foundations of quantum mechanics we have studied there is at a more general than quantum theory specifically in the framework of generalized probabilistic theories within the last few decades one of the things that you find in those theories is that the structure of the state spaces can be very different from the helmet space and in particular the way the way in the relationships between numbers of parameters work can be different so you can talk about the number of states that can be perfectly distinguished in a theory that's some number let's call it D and you can talk about the number of parameters required to specify a state of a theory let's call that number n in quantum theory you have the relationship N equals d squared but you can there is in which this is very different now n is basically the dimension of the vector space in which your your theorem is so the thing is one of the one of the things that you need when you're building a grand unified theory is you need space in which a representation of york grand unified symmetry group is going to act if you assume standard Hilbert space courts of mechanics you have you are restricted to talking about unitary representations on the hilbert space if we started talking about representations of groups on these more generous spaces we could have indeed more parameters to play with more parameters more different kinds of representations that we could try and look at and maybe this is extremely speculative but maybe you could get some kind of unification without needing supersymmetry or maybe you could make a prediction which could be verified in the near term so that's just that's just one speculative idea extremely speculative but I think the general point is that there's a large number of order of magnitude still to go the idea of modifying quantum theory in order to solve these kinds of problems hasn't really been tried because the people who've been trying to understand these kinds of theories haven't haven't commented from a sort of foundational background we're modifying quantum theory is even on the table so if I'm going to make a prediction I think the principles of general relativity will survive up until that scale but the principles of quantum theory if we can even agree what they are won't necessarily survive I don't think there are reliable games for building like we briefly you spawn before we open it up so um I actually think in your second point where you point out that there are things we don't understand yet and exploring the possibility modifying quantum mechanics might be a sensible strategy I agree with the motivation I think that modifying quantum mechanics is probably a longshot strategy but that's some extent a matter of taste I do think that modifying on a field theory makes perfect sense because many of these problems are up near where gravity becomes important and quantum mechanical and if I'm right about the finiteness of the hilbert space that describes local regions of space-time in quantum gravity then quantum field theory is going to give out they might give out long before you reach the Planck scale even though it's still fully QFT if I make you a glass of wine yeah quantum field theory QFT will give out but quantum mechanics I think could still be perfectly valid to address some of these questions to your first point yes and no I mean certainly at face value the principles of general relativity which is part of the fact that is the most beautiful physical theory are crystal clear we all agree on them they're not big debates on the interpretations of general relativity the principles of quantum mechanics as you point out there's not a consensus on what they are but there are options for what they might be and I would say that if any one of those options whether it is some bohmian view or many worlds or TRW or something epistemic if that's what turns out to be right in the ultimate unification of quantum mechanics and gravity I would still declare victory in the debate today certainly if it doesn't turn out to be I'm ready in point of view then I would declare victory but I would ask the real point in getting to is that you say correctly that in gr we have a geometry you could imagine that a better theory of gravity it might not be the pseudo Romani in geometry that we have in general relativity 100% opined that that's very interesting I've tried it myself but do I take from that the implication that you think there will be a geometry rather than a wavefunction of geometries or a superposition of geometries well I mean I don't know what they'll be absolutely I mean I mean I am a scientist M assists so I don't think that but that the wavefunction Hilbert space quantum states on the Hilbert space is fundamental in many sense right so this is one sense in which I think you know if if this kind of view is right and I develop this a scientist a mikvah you into a fully interpretation of quantum mechanics and then I say okay what does that mean for what does that mean for gravity then the result of that I think might not be recognizable as something that you'd want to call the quantum theory because I'm going to say the fundamental degrees of freedom I mean I may end up using more or less none of the mathematical structure of quantum theory in their theory so I'm not I mean I guess it's a question of technology but I'm not sure that that if I accomplish that goal you really would say that thing that you've derived is recognizable as a quantum theory in some sense so it's not clear I would also say to the other point you know we do have you know at least half a dozen interpretations of quantum theory that work I don't think we have half a dozen different sets of principles underlying quantum theory don't work because the way that most interpretations group work is that they take the map there's a quantum theory of some place values represent at least most realistic interpretation taking us at face value is representing no we don't need to have another derivation of the Schrodinger equation from some principles residence we take that mathematical formalism as face value and then we add things to it or we try to interpret it as of Tears we don't try to in general foundation the foundations of quantum mechanics in terms of physical principles like the ones in terms of general relativity so even if bohmian say bohmian mechanics turns out to be right I think it's still being sort of hard pressed to say exactly what the principles of quantum theory are in a way that everybody would would agree in the way that would be non-controversial to the same extent that you went in in general relativity so it's it's not sort of a question of maybe one interpretations right and the other ones wrong well I'm trying to point to is that you know they don't seem to be these these founded principles that we could get any kind of consensus it's gonna be on the record of saying that makes me very sad that I am on the stuffy conservative side of the debate I would much rather be the radical bomb thrower but conscious forbids me in this particular [Applause] okay in some sense general relativity is a very special it's a period of special the spatial representation unitary principle of superposition representing properties in terms basis and so there's a cluster of jets shared needs of parents so I think we are on the track I don't know what the next generation I agree so I mean one thing I want to say is that I mean it's not I mean okay so the idea here is that the way look at it quantum quantum mechanics or quantum theory is a frame you look for variety of theories general relativity is one very specific theory I do agree with that but it's not and it's not quite true you can talk about framework you can talk about theories though - a bit different from general relativity and satisfy similar principles you can talk about some of the general framework of differential geometric like theories that satisfy some of these theories and some of these entire alternatives have been investigated but indeed have been investigated but there's a framework for things that are possible alternatives to general relativity and there's an understanding of what kinds of evidence it takes to rule them out there as you see the situation in quantum theory is much less than that it is more - well I mean it's like the range the range of theories that's a quantum light not exactly quantum mechanics but quantum like is completely you know I mean there are various frameworks talking about it but in terms of pushing them to the point where we can say okay here's the experiment where this one would make a different prediction from that one and this is where we expect it to happen it's much less well well explored that than in the case of general relativity in general relativity there was a concerted program starting in the nineteen fifties to try and experimentally determine whether this theory was really true and some people really the idea of these alternatives and we haven't had that same degree of rigor and understanding this in quantum mechanics so in the sense to me what you're saying seems precisely the wrong way around we have territory we have extremely understand nearly as well as we understand it might as well as we understand key CD or something we can calculate in it so I'm so sure it's not that we have it's not we have in between at just the level of those dudes themselves we have - implants shovel but there's a perfectly happy peaceful coexistence between general relativity if we decide it's not direct evidence but if we were to decide upon best features then it seems the need to modify quantum mechanics knowledge education scales scales of I mean if we just picked up something essentially if we if we what if you want to say how it is that I've got an ordinary get a process whereby articles trying criterion enzyme position this time hole if we try to put for instance the expected values like areas so I even modify quantum mechanics so as to collapse the wave function which is an honorable starting to play but it's a race to the states pretty dramatically all we say we need to guess where this comes into the babies I don't I don't think but if we want to modify or mechanics so as the whole octopus copy to captivity this is again if you're dumb yes this is a Christian but you were saying what you didn't think we should exactly so I guess it takes as a question to matter guess how we do how we think about morality of the ordinary macroscopic situations and instead of decadence classical chaos context it deliberately didn't argue that I think gravity should be under quantized they deliberately didn't argue that I think we the theory in which you have a definite space-time with a definite metric tensor and was fundamental I was rather making a different argument as to the principles of which theory are more reliable going to very building so I totally agree with with with what you're saying of course you know being somebody who's somewhat place to people who are trying to build quantum computers I thoroughly believe in the idea that you can get an arbitrary large system into coherent macroscopic superposition and once you're at that position and you think that you can have super positions of mass at least mesoscopic scale and the idea that gravity has quantum features is obviously true so when I when I really arguing is gravity which is exact going to nature I mean to say yeah I agree that could happen and that would be very happy about this isn't Shawn's position and it's also the mainstream position in in physics people who are taking like string theory based approaches they also think that gr is essentially an emergent it's because Quan gravity's hard and we have clues from things like black hole information that seem to be pointing us in a direction that says that at the deepest level general relativity is not there it could be wrong I was showing you when you were saying whenever interpretation of quantum mechanics turns out to be right I still could win so I just it's not your scientists team at the home state is talking about I guess the way I was thinking that it was more along the lines would some of the things we currently think about these theories need to be abandoned in this more fundamental theory so I think that math is very young bravely says that he's actually imagining that when we try to solve problems like the hierarchy problem grand unification maybe the cosmos the constant problem quantum mechanics is really gonna have to be modified from any of its current formulation and so I would include an epistemic version in that thing that would need to be modified is that fair or do you where's your epistemic it's very easy for me to say these things by the way because I know next think about these problems that's a quote slavery yeah if I you know I may at some point like looking more detail into these things it's not - its program right now and I'm sure I will find very big problems with everything I've suggested but nonetheless that's the proposal yeah I mean to me in quantum gravity in the standard ways I can think about it locality is not fundamental whereas in general relativity or even in quantum field theory it is so in my view how gravity wall Tinley be quantized this important feature of general relativity is going to have to go and I take the important features of quantum mechanics library to quantum mechanics to be was a state in the hilbert space that evolves unitarily and everything else is emergent with that and that will be exactly true in the ultimate theory is my guess so that's the sense in which I'm giving up on something from do you are I mean I would say I mean I think that correctly says that we had this program for decades now of modifying general relativity and testing it empirically in a way that we don't have an analogous program for quantum mechanics I would suggest it's because quantum mechanics is in some sense very fragile if you change it at all it breaks immediately and is ruled out and that is a feature of Ponemah case it's a good feature general relativity is all loosey goosey some fields and you write down some Agron gian and anything can happen and why it's this particular thing why there isn't some massless dilettante field we all know there's all sorts of other things that could have been whereas quantum mechanics it like all the theories ever it seems to be that the fundamental framework of it is not optional like here it is the Schrodinger's equation when there's hilbert space this is like this I totally disagree I mean I think there definitely is a tradition of multiplying quantum mechanics and things like that where you seem to show as soon as you make it tiny changed right all hell breaks loose again superluminal signaling and things like this but this is because these these modifications were done by people who were not particularly sophisticated on the meaning of the various terms in quantum theory in particular you know if you list the axioms of quantum theory one of the things when the I think the biggest lesson that we've learned from the foundations of quantum theory is that those mathematically independent axioms are not in fact physically independent right so you can't just go and change the an equation without also changing the measurement crossness is inconsistent and a lot of this generalization work has been done in not thinking about these things in a very sophisticated way and you know we've shown that in the last decades that in principle it's possible to have probabilistic frameworks which are more general than either classical or quantum they exist we don't know how to formulate real physical theories in them but it's possible to do it in a way that doesn't violate their signaling and things like that I mean that seems to say he's wait to change abstract framework but here's how you determine such as this is yeah we don't even have much more basic things like that such as how to what's really the full full understanding of the way to represent symmetry groups and this kind of thing how do you represent time translation and general and things like that so this isn't it this is not well understood and it may be that all of these theories are complete but of course there may also be much more radical modifications of quantum theory that have to happen other than that like it could be that our most fundamental theory won't predict precisely miracle probabilities for instance that's something there isn't usually on the table or you know you can think of many other kinds of structures that we usually think of as it's just obviously having to be true might not be so so I think there is there's a lot of scope there there's maybe not a great physical reason for thinking that doing any of this will solve any specific one problem of I mean the only one that there's any real way to the patient for is sort of discretization as a whole and there's some motivation for doing that coming from quantum gravity but all I'm fighting out is that there is at least a large space of logically consistent possibilities that don't necessarily lead to pathologies that could potentially dissolve some of these problems it's worth thinking about and we haven't thought about it because the people who they're looking at these things are very sophisticated about quantum field theory and the people who are yeah quantum field theory aren't sophisticated about this stuff so there's a little bit of a culture gap there but there's no real reason not to try and push this good I just want to perspect that I'm Matt's claim we don't have as fundamental principles for quantum mechanics as we do for Chiara or hope to so two examples I wanted to put out so I'll agree the fact we have this workshop is an example that of course it's controversial so not resolved but being an optimist at least two examples that I hope that we converge on is some resolution around the gauge principle which does a whole lot of work in theory building when you want to talk about like a single principle that like unifies like why we have horses does a whole lot of work so you want to talk about want to talk about a principle it's gonna survive theory change gauge principle is way out there representation theory and just groups in general which definitely play a big role in gr but like yeah they play unique role in quantum mechanics that's going to survive in a way out I think we fully said as a group choose things that I would hope that we may clarify better thank you I have to say I you largely agree with you yeah I mean there is sort of pushing some quarters to move away from symmetries and representations as being fundamental but with his obviously wooden base successful programs in all of physics and I think I think that it's it's not obvi Smith each principal will suppose but it is a candidate so I'm generally sympathetic with Shawn's viewpoint here although I have to admit it's partly because I don't know very much about modifications of quantum mechanics that don't break it but something I should learn about but I have a couple of reservations one has to do with exactly what you mean by quantum mechanics or quantum theory survivin I think that there are actually very good arguments that the Hilbert space picture is not going to survive that come from these results of Torian ver aren't Florida Roger that even for a flat space-time free scalar field that different choices of time slicing give you unitarily in equivalent hilbert spaces you can fit that all in what I would still call quantum mechanics but in an algebraic framework and not a Hilbert space framework so depending on which parts of quantum mechanics you're willing to give up in which parts you aren't I think the answers might differ the other thing is that it scares me to death to here to T there you say that you think that diffeomorphism invariance is going to be an approximate than emergent symmetry because without diffeomorphism invariance you don't have the right degrees of freedom and there's no sensible way to enforce to keep the graviton massless as soon as you give up diffeomorphism invariance it's equivalent to giving up gauge invariance in about your magnetism any perturbations would give you mass to the photon keeping it strictly massive us either either either diffeomorphism in variances there or something magical happens that somehow remarkably exactly reproduces the results of differing orphism invariance to arbitrarily high orders and I just don't believe yeah Weinberg Wickham theorem and all sorts of well I don't think someone into the Weinberg wind theorem but it's because you've already done something worse which is you discretized and have a five dimensional hilbert space so you don't have perfect Lorentz invariants of you know the white rabbit okay yeah but still you're right that getting things to be massless whether the photons of gravitons seem miraculous from this point of view the fact that we don't have easily phenomenologically noticeable violations of Lorentz invariants seems of very surprising from this point of view so and that's why I always emphasize that this is the big looming problem in the whole program I don't know how to get things like black hole entropy and de sitter entropy right if I have an infinite number of degrees of freedom which is the only way I know how to get special relativity exactly right so something has to go I don't think in either case it's quantum mechanics that has to go but I think many people have been pushing on keeping diffeomorphism invariants and hoping the black holes in the center space work out okay it's worth some plucking minority of us pushing on the other possibility okay I hoped argue with that and my heart goes more with but just think and they get these very suggestive things though but you know two considerations one is I mean this is really a thigh man the stress-energy tensor and again this is late evening sort of thing you just seem so phenomenological the you know equation but maybe it's just screaming out its phenomenological Ness it doesn't seem like I sort of candidate to be the fundamental thing and then I was wondering whether it was useful to think about you know any particular particular experiments that particularly cases that would you know we suggested one way rather than the other and the one that was thinking it was the cow experiments so we have the neutron that's broke that your parameters and then the wave function except the wave function then depends on the mass and whether that and I subsisted awkwardly with this sort of idea that the wave function would so look freely from the plate if I need to talk about both of those things I do understand the second thing at all what do you want a cow expert yeah what is your but what about it well well they are lend well they're thinking about those lends support one way rather than the other so there you have some sense that you have you know there's a need for moving in a gravitational field right quantum gravity what it's not quantum grando yeah so the question is whether dis advised some breaking of the principle of equivalence because the wavefunction the face of the wave function is not mean I think the principle Combs is very overrated it was a great way to be inspired to invention a relativity at the end of the day general relativity is the Einstein Hilbert action plus some matter terms and everything comes out of it so you just do that and you get the answer whatever the answer is is consistent with what you've been doing [Laughter] [Music] so what was exactly the claim it's it's a neutron interferometry experiment in which the two paths move through occasional field at different heights that's that depends on the gravitational field so that demonstrates gravity gravitational classical gravitational field does have an observable effect on the quantum behavior of neutrons but to people that the thing that people sometimes worry about is that the phase shift and therefore this quantum effect would be different if you do it's different if you do it with neutrons or hydrogen atoms saying depends on the mass there are claims that that shows a violation it shows a violation I mean so I can imagine that you say there are other things that go along the lines of if you have a free electron and proton they move in certain ways and when you put them together to make a hydrogen atom now there's binding energy or there's a difference in binding energy and the principle of equivalence would have that binding energy couple the same as any other energy to the gravitational field is that the kind of arguments you know it's more that I mean there's a jinx it but there's a control of interference that says look the way feelings behave in gravitational fields is the TPMS so with the interference of the neutron the gravitation electro gravitational field is a mass term in on that particular very crude conception will be feelings principle is really very defensible okay that seems to me the correct equivalence principle is number one I can choose coordinates in which the metric number two there's no there's no definitely definitely definitely sort of I mean at the corner of 50 there seems to be okay so as a scientist emesis I have to kind of make an objection here which is like the people tend to think that either there's the wavefunction which in an interferometer spreads out along two paths or there's not the wavefunction there's a particle which goes one way or the other but that's that's not so you can still even if you're a scientist emesis you can have that the ontological degrees of freedom are spread out and in fact if you want to reproduce interference as it precisely what you have is just that these degrees of freedom aren't enough to reconstruct what the wave function is so the idea that you get a different interference pattern depending on the mass you know doesn't really bother me because that mess whatever it is can be spread out some of those two paths and we produce that apparent I mean I'm not bothered by that any more than I'm bothered by a usual mach-zehnder interferometer which I know how to reproduce in you you said that you were said that you couldn't I guess I guess my heart is with my - here's the thing I mean I wanted asking for Sean's original comments which was about the status lines and two other patients so where do you see those going the teacher you talk about some principles of gr that you want to hold on to what's going to happen to incentive I mean I I mean I don't know what the final Theory looks like what mathematical framework is formulated in so exactly what happens to those I don't know I am fond of well I mean this sort of comes from my affinity with some of the things that can like to talk about as well I am fond of thinking about the field equations as opposed to thinking about ER in terms of actual principles right they tend to think of as being what general relativity maybe is including the solutions with place timelike curves and things like I consider that to be part of general relativity so I think thinking about the Einstein field equations is probably more in line with where I think the theory is going to go in and thinking about the Einstein Hill protection principles I mean people they say it's sort of thank you really understand this point about the marble versus wood business it's like people concerned that F equals MA that M is horrible or something I mean what what's wrong with this I mean it's it's the thing that generates the we have very beautiful clean descriptions general relativity where the right hand side is generating familiar branches and that appeal is very beautiful in the regime where we actually test so the claim is that you would like a cleaner derivation is please just saying if you believe if you think as Matt doesn't want to do general relativity is coming from the action principle the niceness of the left-hand side is simply reflection of there aren't that many scalar functions you can write down for the metric there's just one that lowest order right and then it's pretty determined at first order at most order that's right yes whereas there seems to be a lot more arbitrariness and lagrangians you can write down on the right-hand side which is okay but it's still there is this mathematical operation I've learned where I can take one side of an equation and move it to the other by subtraction so they're both on the same side and they're both the functional derivatives back to the metric I don't really see the difference so you're right Chandan just about every piece given what we really shouldn't know better we really should have known better to keep time separate from space-time space-time we really should know better talk about States on instantaneous surfaces we should have known better talk about operators operating on the state science and pain services we should have known better to go back to the original motivation of the Turner equation which was to do things it took they took the hamilton-jacobi action which doesn't leave us response it's an integral and it's integral to the point that is now assuming that you measure it there and then you can compact all that and say there's my state even though if you don't measure it there it goes on and that wasn't the action at all we should know better than to go to configuration space from stop map which we knew was not a real thing and say this is the framework when we can clearly see where we're shooting or stolen from Stephanie it was obviously and so we took all these things that we should know better to put them together and now we have this tiny box and sure enough but it seems to me that what we do is alternate without success that's where our best matches to data Oh what the shorter equation gives us these probabilities that we've dumbed down when the thing is a good prediction when I think you talking very historically about where we bought but but I think that you're then doing disservice to the final product which regardless of the frankensteining away which was assembled it's not actually a bunch of different pieces fit together it's a quite compact sleep me machine of each burger equation acting on vectors in hilbert space and it's hard to mess with it and I would further say in the gr versus quantum mechanics thing we haven't even mentioned the word singularities and one problem with all classical mechanics classical mechanics is enormous ly more complicated than quantum mechanics because quantum mechanics is linear so classical mechanics you can run into singularities at the boundary of configuration space in quantum mechanics h sy equals i D by DT sy and if you think of it the way I want to think of it as you know you're a vector in hilbert space with a Hamiltonian right sy in the energy eigenbasis and all that ever happens in quantum mechanical evolution is the face of every little energy eigenstate rotates so all of your evolution no matter how complicated your system is is moving on a straight line in a little torus and it will never end it will go on forever it's mediate and it's pretty and there's no singularities and everything is great and you guys want to give up on that I don't know because that's because if you're if you're if you're really if you feel hilbert space in your bones you do not think of it as a set of square root of all functions of configurations you think of it as a vector that's doing something and so that classical probability distribution is necessarily publius tribution on classical things that could go singular whereas the quantum vector is just moving around in a little circle it's much nice well I think I think all that emerges that's no longer conversation but yeah okay [Music] [Music] yeah I want to say first of all the causality is an interesting program I think it's an interesting program I have some sympathy for it but I think that your historical argument there was misguided because you could say the same thing about almost any physical theory in the history of science yeah I mean look good look at Newton's inspirations for coming up with classical mechanics he was you know a magpie you pick things up from you know all sorts of strange places and it shouldn't work you know I mean he was beautiful calmly the last alchemists of the last magician I did not get bashed classical mechanics because you know you think that from all sorts of places and he decided you know I'm gonna like I'd say it would be crazy adversity but you know something proportional to B squared is conserved and something proportional to B is also conserved ready to give them different pans like their back that wasn't perfectly I know but what I'm saying is that you know that attack all these things together right so this is a fun conversation there's wandering away but I move that we stop the video and declare victory and then gather around

Channel: Sean Carroll

Views: 46,069

Rating: 4.882472 out of 5

Keywords: physics, quantum mechanics, general relativity

Id: 9OGfvwt3H74

Length: 71min 23sec (4283 seconds)

Published: Mon Jun 04 2018

Reddit Comments

[u/atnorman]

Does anyone have references to the alternative theories Leifer talks about near the beginning of his segment?

[u/938h25olw548slt47oy8]

If I was doing that much sideways hand movement with a wine glass in front of me, I promise the result is wine all over the table. Sean is a master.