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A physicist who bets that gravity can’t be quantized

• btilly 3y ago
  If you believe in the Everett Interpretation, then he is wrong. The Everett Interpretation, aka Many Worlds, holds that both observer and observed are quantum mechanical systems. From this it follows that the act of observation does not produce a "collapse", but it does separate the observer into multiple observers that can't interact with each other again. So Schrödinger's cat is in a superposition of alive and dead before the box is opened, and after it is opened the observer is in a superposition of one who saw the cat alive, and the other who saw the cat dead. It feels bizarre, but there are no contradictions. And it is what quantum mechanics predicts. For those who believe in this interpretation, there is a simple test of quantum gravity. Set up 2 Cavendish experiments to measure gravity. Based on whether there is a click in a Geiger counter, choose which one to put a cannon ball next to. Measure both. This has been done, and we only see the gravity from the cannon ball that we placed, and not the alternate location it might have been placed at. But believers in other interpretations of quantum mechanics will disagree that this experiment tests anything at all.
  • fallingfrog 3y ago
    This is a good summary. I like to put it the following way as an alternative: instead of imagining that you are the researcher, imagine that you are the cat.
  • treeman79 3y ago
    Where would energy come from for creating each universe?
    • arethuza 3y ago
      Maybe the implementation uses copy-on-write to minimise the amount of work required? ;-)
    • tgv 3y ago
      Among other problems. It's just bollocks on the level of "but you didn't say it couldn't be this, nanananana."
      • captainclam 3y ago
        I think because of the abuse of "many worlds" in science fiction media (especially as of late) as a convenient plot device, people develop this idea that it's an unserious proposal wrt the foundations of physics. As far as I have read, it strikes me as perhaps the most parsimonious interpretation of QM out there. Genuinely curious, what do you mean by "but you didn't say it couldn't be this, nanananana...?" The Everett interpretation isn't some fantastical notion spun up by a scifi writer...it is simply the consequence of removing collapse of the wavefunction as an objective event from the picture. And if it turns out our observations wouldn't be changed by removing this feature, then perhaps it was an extraneous feature in the first place!
        • kgwgk 3y ago
          > it is simply the consequence of removing collapse of the wavefunction as an objective event from the picture. And if it turns out our observations wouldn't be changed by removing this feature, then perhaps it was an extraneous feature in the first place! > What we observe as the collapse of a function is simply an artifact of being a conscious being that can only observe one value of the of the wavefunction at any particular moment. If we simply assume that our observations are as if a wave function happened we can indeed have our Schrödinger cake and eat it too.
        • tgv 3y ago
          Assuming many worlds, and many here means: enormous amounts, far exceeding the number of particles in the universe, is everything but parsimonious. There happens to be a model that fits some data, but that's it. It's a grotesque assumption to avoid a conflict in a man-made theory. It's a funny thought, but no more than that. There's also nothing special about observing. Our consciousness isn't super-natural, so the idea is in desperate need of some other underpinning. And probabilities: if this is one of many, many worlds, the distribution of events as we can observe them is heavily skewed. The next observations should follow a radically different pattern, unless you also assume that each split influences the probabilities of future events.
    • MattPalmer1086 3y ago
      Why would it necessarily require any energy? Energy is what we have to expend to effect change within a universe. There may be no requirement for energy to split into a branching multiverse.
    • lavelganzu 3y ago
      The question is a misunderstanding. In the pure wave (Everett) interpretation, there are no universes being created when the wave function branches. Rather, different regions of the wave function become separated (decohered) from each other. https://physics.stackexchange.com/questions/41588/many-worlds-where-does-the-energy-come-from https://physics.stackexchange.com/questions/41588/many-world...
    • captainclam 3y ago
      I think this question stems from a fundamental misunderstanding of the Everett interpretation...the idea of "many worlds" (a phrase I do not favor specifically because of this very misunderstanding!) evokes this imagery of an entirely new universe BURSTING forth dramatically upon the collapse of the wave function, and as an entirely new universe is "created," the amount of extent energy is literally doubled (or multiplied by many times more). Indeed, where does this energy come from? What the Everett interpretation suggests is that in the same way you are comfortable with the function f(x)=y having (infinitely) many values (you don't HAVE TO choose an x, ie the function doesn't collapse), the wavefunction simply is what it is and doesn't collapse either. What we observe as the collapse of a function is simply an artifact of being a conscious being that can only observe one value of the of the wavefunction at any particular moment. None of these unique values (observables) of the wave function can interact with each other (in the same way the value of x=2 doesn't "interact" with the value of x=8 for f(x)=y), so energy cannot be gathered or duplicated at any particular point of the wave function, so this energy creation/duplication is no issue.
    • FollowingTheDao 3y ago
      "Where would energy come from for creating each universe?" Why does it have to come from somewhere? What if it just is?
    • ly3xqhl8g9 3y ago
      Sean Carroll, following Everett, puts it in the most concise form: (i) systems are described by wave functions, (ii) wave functions obey the Schrödinger equation [1]. "Many-worlds", universes, observations, observers, and so on become just entia multiplicanda [2], superfluities. [1] https://www.youtube.com/watch?v=nOgalPdfHxM https://www.youtube.com/watch?v=nOgalPdfHxM, one set of rules in quantum mechanics at 36:00; at 1:13:14 where does the energy come from? energy of the set of all universes is conserved. [2] https://en.wikipedia.org/wiki/Occam%27s_razor https://en.wikipedia.org/wiki/Occam%27s_razor
    • Mizza 3y ago
      Where do the eggs come from when you cut a cake?
  • x3n0ph3n3 3y ago
    > This has been done, and we only see the gravity from the cannon ball that we placed, and not the alternate location it might have been placed at. Doesn't this presume that your brain deciding where to place the cannon ball was an uncertain quantum event? Doesn't it also presume quantum gravity does not exist?
    • lavelganzu 3y ago
      No: > Based on whether there is a click in a Geiger counter, choose which one to put a cannon ball next to.
    • FollowingTheDao 3y ago
      Your brain is not deciding where the cannon ball is, it is predicting where it is and them making that prediction a certainty. That is the collapse of the wave function, turning a probability into a certainty.
  • AnimalMuppet 3y ago
    > If you believe in the Everett Interpretation, then he is wrong. If. > And it is what quantum mechanics predicts. It is an interpretation of what the equations predict. It is very much not the only possible interpretation, so it is not "what quantum mechanics predicts". > But believers in other interpretations of quantum mechanics will disagree that this experiment tests anything at all. Within other interpretations of quantum mechanics, this does test nothing at all.
  • edgyquant 3y ago
    Why would anyone believe such a thing?
    • bowsamic 3y ago
      Because it’s very straightforward and has less assumptions and weird features than most other interpretations, and it has seen the most advancements in quantum fundamentals research
      • Scarblac 3y ago
        Complete layman here -- does that theory say that every time a quantum effect does or does not occur, in fact both possibilities happen but in different worlds? The entire universe instantly splits up in two for every quantum event anywhere in it? Edit: I read more comments and no, it's not literally that.
        • bowsamic 3y ago
          Yes, it does mean that
      • edgyquant 3y ago
        These are just mathematical tools we use to make predictions. Assuming they represent objective reality and somehow prove an infinite number of universes doesn’t seem very scientific to me.
        • bowsamic 3y ago
          I mean, that's obviously a huge matter of debate, but most of us physicists at least hope that our theories reflect objective reality in some way. Also, there are of course explicitly ontological aspects of quantum mechanics, such as PBR theorem and Bell's inequalities
        • goatlover 3y ago
          That's way too instrumentalist. The math also describes how atoms work and many other things. Modern physical understanding is based on those descriptions. Cosmology and nuclear physics would be useless without an understanding. How do the predictions work if they aren't in some way modeling the way reality is? Why does the technology based on them work? Instrumentalism gives no answers to those questions. Science is about understanding the world, and using that to make predictions.
  • rvcdbn 3y ago
    I’m a “believer” in the Everett Interpretation (on the grounds that it’s the simplest interpretation) but I can’t see how such an experiment could prove anything. This experiment seems to assume we can keep the whole apparatus including the Geiger counter and the mechanism to move the cannon ball from becoming entangled with the environment (decohering). This seems very far outside the realm of current technology.
    • btilly 3y ago
      It shows that when there is a superposition of quantum mechanical states, you also get a superposition of two different space-time structures. The reason why the Everett Interpretation is needed for the experiment to be valid is that according to it decoherence results is a superposition of quantum mechanical states, and not a collapse. If you believe in another interpretation, there has been a collapse, and so the experiment demonstrated nothing.
      • rvcdbn 3y ago
        Once decoherence has occurred I don’t think it’s meaningful to say that the system exists in a superposition of states. The multiple decohered branches of the wave function can no longer interfere with each other and so there’s no way for them to affect each other, which is why we call them separate worlds. Within each decohered branch there’s an experimental apparatus and a human that sees one specific outcome. This was Everett’s main insight that each observer in each branch experiences what looks like a collapse but is really just their brain becoming entangled with one or other of the measurements.
        • btilly 3y ago
          According to quantum mechanics, they can no longer interfere with each other. But we are bringing gravity into the mix. Gravity works differently than the other forces. Do space-time structures also exist in a superposition of states? Or does QM work against an essentially classical gravitational space-time background? Without a theory combining quantum mechanics and gravity, the answer is not obvious. The fact that the superpositions do NOT interfere gravitationally means that either there was no quantum mechanical superposition (collapse actually happened) or that gravity can also exist in superimposed states. If you believe in the Everett interpretation, then there must have been a quantum mechanical superposition, and this tells us something non-trivial about a quantum theory of gravity.
  • slowmovintarget 3y ago
    In Everettian Mechanics, decoherence occurs long before the box is opened. The photons in the box interact with the cat. That entanglement with the environment which causes the branching of the wave function occurs then. The observer that opens the box is already either in a branch where the cat is asleep (to borrow from Sean Carroll) or awake. The interesting thing to come to understand is that probability doesn't actually seem to exist as a real feature in the universe. Rather, probability is a measure of what observers believe they will or are seeing. Probability, in Everettian Mechanics, is about perception of the universe, not a feature of the universe itself.
    • VirusNewbie 3y ago
      But the observer is in all “probabilities” no?
      • goatlover 3y ago
        There are observers in all probabilities of the experiment, but each observer only observes their world because decoherence interferes with the superposition of the other worlds. Worlds being the entanglement of the entire experimental setup spreading to everything else.
    • fallingfrog 3y ago
      The cat is all entangled with the other stuff in the box, but by assumption the stuff inside the box is not entangled with the stuff outside the box.
• sandworm101 3y ago
  >> Jonathan Oppenheim, who runs a program exploring post-quantum alternatives at University College London, suspects that’s because gravity simply can’t be squeezed into a quantum box. So... he is still part of the Orthodoxy. He is challenging ideas, but as an employee of University College London he is hardly any sort of outsider to the physics community.
  • outlace 3y ago
    Genuine question here, when was the last time a total outsider made a significant contribution to physics?
    • tonycoco 3y ago
      When was the last time an insider made one?
    • sandworm101 3y ago
      https://en.wikipedia.org/wiki/STEVE https://en.wikipedia.org/wiki/STEVE https://en.wikipedia.org/wiki/Subauroral_ion_drift https://en.wikipedia.org/wiki/Subauroral_ion_drift Not exactly the discovery of a new quark, but it triggered realworld research into the cause of the phenomena. That qualifies as contribution imho.
    • FollowingTheDao 3y ago
      Ask yourself; why are they outsiders in the first place? It's like asking why hasn't anyone who is not a player in major league baseball ever won a world series?
      • outlace 3y ago
        Those are disanalogous in the case of theoretical physics where you don’t need access to any expensive equipment. There’s nothing in principle preventing someone from learning math and physics on their own or from a traditional university and then coming up with new theories alone. And in fact this happens all the time and most of those people get called crackpots.
• eterevsky 3y ago
  (Not a quantum physicist. Please correct me if I am misunderstanding.) From what this article says, his assumption is that gravity is classical, but "fuzzy" or probabilistic: you can't precisely measure the gravitational field of a sufficiently small object. In the last years we've seen progressively bigger objects being put in quantum superposition. The theory from this article is incompatible with this process continuing indefinitely. When and if we create a sufficiently big object that we are able to entangle it with something else via gravitational interaction, this would immediately disprove this theory. So the good news is that this theory is clearly falsifiable, possibly even without creating a particle accelerator the size of the solar system.
  • bowsamic 3y ago
    I’m a quantum physicist and yeah you’re totally right. It’s already obvious that a quantum theory of gravity is needed because we need a way to talk about superpositions of spacetime curvatures. Either QM is wrong in general or we need a way to treat spacetime that is in a quantum superposition Also my old supervisor actually suggested an experiment to observe this gravity induced entanglement but it would require extremely low temperatures to not degrade due to thermal noise
    • phkahler 3y ago
      >> It’s already obvious that a quantum theory of gravity is needed because we need a way to talk about superpositions of spacetime curvatures. Is that because of things like the double slit experiment they mention? A particles could be monitored via its gravitational effect on spacetime to determine which slit it went through. What if the particles mass behaves as a mass distribution in such an experiment? Does that save classical gravity?
      • bowsamic 3y ago
        The problem is that it doesn't act like a mass distribution, it acts as two non spatially overlapping possibilities. I think that the only way to save classical gravity would be superdeterminism. If quantum states correspond to anything other than our ignorance, i.e. if superposition states are actual physical states of reality, then gravity will need to be quantum.
• tauwauwau 3y ago
  Putting it here, as it seems relevant. Veritasium: Parallel World Probably Exist: Here's Why https://www.youtube.com/watch?v=kTXTPe3wahc https://www.youtube.com/watch?v=kTXTPe3wahc
  • light_hue_1 3y ago
    The word "probably" there is completely meaningless. There are many interpretations of QM and there is no compelling reason to choose one over the others. In particular the many worlds interpretation makes no testable predictions, it's just a story. There's absolutely nothing "elegant" about it. There's no math, no experiment, no explanation. A story like you read in some religious text or science fiction novel. It has nothing more going for it than that. Practically, the most "likely" situation, if you want to define likely as what most researchers think and how they behave, is that most people are fans of "shut up and calculate". QM is what it is. That's it. We don't need crazy interpretations, particularly ones that don't contribute anything to our understanding.
    • semi-extrinsic 3y ago
      This. So much this. And also https://xkcd.com/1240/ https://xkcd.com/1240/
    • goatlover 3y ago
      Shut and calculate is putting your head in the sand and avoiding that physics should be telling us what the world is. You’re wrong about MWI in that it’s a more elegant interpretation because it adds nothing extra to the wave equation and treats the universe as fundamentally quantum with no arbitrary dividing lines for classically scaled objects.
      • light_hue_1 3y ago
        > Shut and calculate is putting your head in the sand and avoiding that physics should be telling us what the world is. It's how physics is done. PhD students don't sit in physics departments getting ideas from their interpretation of QM. The interpretations people have are so meaningless and useless they never appear in physics publications. You don't like it, but it's what physics is. > You’re wrong about MWI in that it’s a more elegant interpretation because it adds nothing extra to the wave equation and treats the universe as fundamentally quantum with no arbitrary dividing lines for classically scaled objects. That's the thing about most QM interpretations, and in particular the many worlds interpretation, and why they aren't science just stories: you're right, they add nothing. So they are untestable! What does untestable mean? It means that they make no predictions about our world at all. They are stories. Their impact on physics or the universe we live in is the same as that of Winnie-the-Pooh. They're a waste of time. > avoiding that physics should be telling us what the world is. What can I say to "should"? I can only report what physics is in the real world. That's like saying, biologists "should" be working on building Jurassic Park because that's what you feel is the goal of the field. It's not. That's not what they do.
• AnimalMuppet 3y ago
  Somewhat off topic, and somewhat related: I don't think there actually is such a particle as a graviton. General relativity says that you can't tell the difference between being unaccelerated, and being in free fall. But in one case you have no gravitons coming in, and in the other you have gravitons. I can change whether gravitons are there or not by a (general) relativistic transformation. But that's not possible. Either the gravitons are there, or they aren't. There's not two sets of reality of what particles exist for two different coordinate systems. Therefore gravitons don't exist. The bending of spacetime is what's really going on, and there is no quantum version of the gravitational field. (Except possibly that the bending of spacetime could be quantized, but that's not what we mean by a graviton.) I am very open to being shown to be wrong here. Can anyone do so?
  • 317070 3y ago
    > There's not two sets of reality of what particles exist for two different coordinate systems. Look up Unruh radiation for an example of such particles. Gravitons wouldn't be the first. https://en.m.wikipedia.org/wiki/Unruh_effect https://en.m.wikipedia.org/wiki/Unruh_effect
  • LatteLazy 3y ago
    In relatively gravity is not a force like Electromagneticism etc. Instead, space time is curved and as a result the object itself is travelling in a straight, in accelerated line and only looks like it is accelerating to an observer outside of the curved space. So relativity has no gravitons and instead curves space. In quantum physics the opposite is true: space is flat, and gravity is like other forces, the result of particle exchange. This is one way that the two systems are incompatible. Again, happy to be corrected...
    • eigenspace 3y ago
      This is incorrect. A classical limit of the graviton picture fully reproduces general relativity. The incompatibility lies in the non-renormalizability of people's attempts and quantization of the Einstein Hilbert action, meaning that all the quantum loop corrections have unfixed coupling constants which can't be derived from the classical theory (unlike things like electromagnetism).
• FollowingTheDao 3y ago
  The basic misunderstanding is that "things" exist as particles, and the Planck limit, IMHO, proves that everything is a wave and the Planck length is the smallest wavelength, or resolution, that any particle (certainty) can exist. Fundamentally, all matter is uncertain. The problem physicists have is that they are trying to align classical psychics with quantum physics when it is quantum physics all the way down. You cannot use quantum physics to describe classical physics because classical physics is only our minds collapsing the quantum universe so we can, well, exist. To me, gravity is a side effect of quantifying. A byproduct of our brains collapsing wave functions. A wave is a probability of a particle. Our minds create certainty out of a probability, that is, our minds collapse the wave function. We need to be certain of gravity, since it is a risk to our survival is we do not. Space-time itself is a collapse of the quantum field and it is through this collapse that gravity is "observed" or "felt". With out particles, no gravity, with out wave collapse, no particles. Therefor wave collapse creates gravity.
  • FollowingTheDao 3y ago
    I wish any of you with the down vtes would at least have a conversation with me...
    • eigenket 3y ago
      There is absolutely no evidence that the Planck length is anything like the smallest possible length or wavelength. Its "just" a length scale that pops out when you combine several physical constants. That combination of constants means that at the Planck scale quantum effects and gravitational effects are likely to both be relevant, which is cool, but other than that it doesn't really have any fundamental meaning.
      • FollowingTheDao 3y ago
        I’m giving you a hypothesis about the reason why the Planck length exists as a constant. Below Planck length there is no length, time, mass, or temperature. All of those things are given “reality” by our mental process.
• slowmovintarget 3y ago
  The "Maybe Bigfoot is Fuzzy" interpretation of gravity?
  • api 3y ago
    Fun fact: Bigfoot has no gravity because he doesn't exist in this slice of the multiverse.
• javajosh 3y ago
  I think it would be funny if we discover that refraction is caused by the slowing of light in close proximity to mass. That is, one of the most common and observable phenomena in physics is a quantum gravity phenonema! (The usual explanation for refraction is that light as an EM wave causes sympathetic vibration in the electrons (and the protons, a little) which slows it down. But what if light's proximity to protons were caused a multitude of miniature Shapiro delays [0]?) 0 https://en.wikipedia.org/wiki/Shapiro_time_delay https://en.wikipedia.org/wiki/Shapiro_time_delay
  • archibaldJ 3y ago
    Now I'm curious: does the refraction that happen within our eyes has anything to do with the processing and the consequent experience of sight in a way that it utilizes quantum mechanics that we don't understand?
    • jacquesm 3y ago
      In theory it could but in practice there isn't much about the optical portion of the eye that we do not understand, the boundary of that understanding is well behind the optical nerve. What sort of an effect is it that you are getting at?
  • amelius 3y ago
    Refraction is well understood. It is caused by interactions of the incoming wave and electron clouds of atoms. https://en.wikipedia.org/wiki/Ewald%E2%80%93Oseen_extinction_theorem https://en.wikipedia.org/wiki/Ewald%E2%80%93Oseen_extinction...
    • javajosh 3y ago
      Ah, perhaps you are inclined to stop reading when you come across open parenthesis?
      • amelius 3y ago
        Ok, I confess that I didn't read the entire comment. But you have a second problem, as you should explain why the conventional explanation does not hold or is canceled by your theory.
• sharikous 3y ago
  Isn't this similar to the view of Penrose?
  • antognini 3y ago
    My understanding is that Penrose does believe that gravity is fundamentally quantum in nature. But his proposal is that gravity is connected to the collapse of the wavefunction. In his view, it is the exchange of a graviton that precipitates the wavefunction collapse. But this is still a fundamentally quantum theory because it posits that the gravitational field is quantized (and hence gravitons exist).
• inciampati 3y ago
  > So if gravity is quantized, that means space-time is also quantized. But that doesn’t work, But... How could space-time not be quantized? That would imply the existence of infinities in the structure of the universe. It is like the ultraviolet catastrophe but in space-time.
  • peterfirefly 3y ago
    Wouldn't random noise work instead?
  • H8crilA 3y ago
    For the layman, what exactly is going to theoretically "explode" if spacetime is continuous?
    • naasking 3y ago
      The article discusses this question actually, about the apparent incompatibility between classical and quantum systems. Basically, there's a fundamental inconsistency where you can detect a particle's position gravitationally as it passes through a slit in the double slit experiment, which destroys the quantum properties of "passing through both slits" that leads to interference patterns.
      • H8crilA 3y ago
        Oh no I get that, but two comments above inciampati was suggesting that we will end up in a situation where some variable will have to have an infinite value. Like, famously, with the ultraviolet crisis: https://en.wikipedia.org/wiki/Ultraviolet_catastrophe https://en.wikipedia.org/wiki/Ultraviolet_catastrophe Detecting (something about) the particle through gravity would at best remove the interference, not create an \infty somewhere in the model, implying that the model is self inconsistent.
        • naasking 3y ago
          > Oh no I get that, but two comments above inciampati was suggesting that we will end up in a situation where some variable will have to have an infinite value The black hole information paradox from that article presumably fits. The conclusion from GR is that no information can escape, which is ultimately incompatible with QM, and that conclusion ultimately depends on the infinite density of the singularity. I think the more charitable reading is that we'll find situations where either no sensible calculation can be done, or the sensible calculations we do churn out nonsense. Divergence in the UV catastrophe was an example of that, and Baez covered more here: Struggles with the Continuum, https://arxiv.org/abs/1609.01421 https://arxiv.org/abs/1609.01421
• naasking 3y ago
  > But when they tried to quantize gravity, they ran into unnatural infinities that had to be sidestepped with clumsy mathematical tricks. Maybe they run into unnatural infinities because all of our formalisms in physics are still fundamentally continuous rather than discrete. Uncountable infinities are baked right into the foundations of how we use reason about these systems, so infinities will naturally result. Physics has repeatedly had to tame infinities by elaborate tricks, or by eliminating them entirely [1]. Some people are increasingly looking to discrete formalisms, and I think this is a promising way forward, both for mathematics and physics. [1] Struggles with the Continuum, https://arxiv.org/abs/1609.01421 https://arxiv.org/abs/1609.01421
  • qazpot 3y ago
    > Physics has repeatedly had to tame infinities by elaborate tricks, or by eliminating them entirely or In case of black holes by actually interpreting infinity as a real place in the universe.
    • CuriouslyC 3y ago
      If I were a betting man I'd say black holes have no singularity, but rather a core of extremely dense exotic matter (probably formed from top/bottom quarks) which we haven't detected because it decays quickly under less extreme circumstances.
      • mr_toad 3y ago
        I think it’s true that once an event horizon forms no physical force (known or hypothetical) can stop a singularity forming. There may be some form of very dense matter that stops large stars from collapsing to the point where an event horizon forms in the first place, but that doesn’t seem to apply to super massive black holes. For super massive black holes the event horizon grows too fast.
        • naasking 3y ago
          Singularities are a deficiency in GR, they don't really exist.
• codethief 3y ago
  > It’s become dogma. All the other fields in nature are quantized. There’s a sense that there’s nothing special about gravity — it’s just a field like any other — and therefore we should quantize it. I keep on citing Stephen Hawking here on HN, but it again seems very appropriate: > It would be rather boring if this were the case. Gravity would be just like any other field. But I believe it is distinctively different, because it shapes the arena in which it acts, unlike other fields which act in a fixed spacetime background.[0] [0]: https://arxiv.org/abs/hep-th/9409195v1 https://arxiv.org/abs/hep-th/9409195v1
  • hughesjj 3y ago
    I'm definitely into the "it's an emergent phenomena" camp. I think it's inherently relational, as that's a more efficient way to encode geometry rather than space itself being a quantized "thing". Afaik this theory is the "leading"/imho most promising theory of "quantum gravity", that being the "gravity = entanglement" conjecture and the related ideas of "Complexity= action/volume/whatever" that susskind and many others have been developing for the past 20 so years. All that said I'm nowhere near a physicist and am probably just spewing a total misunderstanding of the situation from my armchair. That said, I've been incessantly watching lectures in this space to try to beat an understanding into my dumb dumb brain because it's super, super fucking cool. - https://youtu.be/6_7aKoEx_kk https://youtu.be/6_7aKoEx_kk - https://youtu.be/6OpAreb779U https://youtu.be/6OpAreb779U - https://youtu.be/9crggox5rbc https://youtu.be/9crggox5rbc - https://youtu.be/OBPpRqxY8Uw https://youtu.be/OBPpRqxY8Uw - https://youtube.com/@isqg423 https://youtube.com/@isqg423 I'm also fascinated by the idea of phase transitions, which seems to be how the laws of physics have "evolved" so far. It's crazy how much quantum computation is coming into play with this stuff, ex last year's Nobel prize with the bell inequality. That said I'm sure being a programmer I'm biased to think the universe is inherently computation/math.
    • EvgeniyZh 3y ago
      There are some pretty strict limitations on emergent phenomena, like Weinberg–Witten theorem. It doesn't rule out emergence of gravity but it makes it less likely
• LonelyTree 3y ago
  Then it seems like gravity isn't one field, but a conglomeration of fields yet to be discovered.
• rapjr9 3y ago
  I wonder if the recent news about the background gravitational hum of the universe has any bearing on this: https://www.sciencealert.com/breaking-news-physicists-have-detected-the-background-hum-of-the-universe https://www.sciencealert.com/breaking-news-physicists-have-d... It essentially means that gravity is always in flux everywhere, at all frequencies of gravity waves, so there is noise in gravity. This is in addition to the background EM radiation in the universe. Gravitational and EM fields are infinite are they not? Maybe this is the source of gravitational randomness that Oppenheim is looking for. Although he seems to be positing that gravity itself has some inherent randomness.
  • bdamm 3y ago
    Do we actually know that the fields are infinite? I thought it was just the easiest way to model what we know and can observe.
    • rapjr9 3y ago
      I think the question would be, what happens at the edge of the universe? If the universe is finite, though expanding, then the waves can not be infinite unless they reflect at the edge, although one definition of the size of the universe might be that it extends as far as gravity and EM waves have traveled since the big bang. In which case I think the answer would be that if waves define fields then fields are not currently infinite nor will they ever be infinite, but they will always define the edge of the universe and keep traveling forever and over infinite time they will keep traveling infinitely. Which means they are essentially infinite, but not actually currently infinite, but that difference might not be much of a difference, even mathematically. It would suggest that EM radiation and gravity become more dilute in any one place over time though, as the universe gets bigger and the waves spread out more. There is also the speed of light problem. If EM radiation and gravity can only travel at the speed of light, then when new fields are created they do not propagate to infinity instantly, it takes time. So there is a kind of localness to EM/gravity but given the age of the universe a huge number of waves have propagated very far, which may be close enough to infinite we can't tell the difference. Matter can be created and destroyed so there can be new gravity fields created? Although I believe current thinking is that energy also exhibits gravity so maybe all the gravity that will ever exist is already here. That would be a difference in the nature of gravity versus EM waves since new EM radiation can be created while perhaps new gravity can not. Is a wave a perturbance that travels at the speed of light in a field that propagates instantly? Or is a field the propagation of a wave that travels at the speed of light? The idea of infinite fields suggests instant propagation, but gravity waves and EM waves certainly do not seem to propagate instantly. Maybe the idea of fields makes no sense and there are only waves?
• djmips 3y ago
  Article is 404 right now, for me at least.
  • gpvos 3y ago
    > Error 404. This page doesn't exist. At least not in this universe. I like the error message though.
  • redtexture 3y ago
    Lose the V1 at the end of the URL.
  • deleted 3y ago
    [deleted]
• tzs 3y ago
  Page is 404 at the moment. Here's an archive.org link that works [1]. [1] https://web.archive.org/web/20230710160259/https://www.quantamagazine.org/the-physicist-who-bets-that-gravity-cant-be-quantized-20230710/ https://web.archive.org/web/20230710160259/https://www.quant...
• sedatk 3y ago
  > J. Oppenheim I wonder what influenced him to become a physicist.
  • booleandilemma 3y ago
    I'm just waiting for a person with the surname Oppenheimest now.
• andrewflnr 3y ago
  IANAP, but... > if these hybrid theories are true, there must be some minimal amount of gravitational noise. ... this already sounds suspiciously Heisenbergian, reminiscent of quantum foam, the Casimir effect and all that jazz.
• padjo 3y ago
  What if gravity is the creator’s v2? She finally managed to figure out an elegant solution without all the weird quantum edge cases but never got around to applying the pattern everywhere.
• kromem 3y ago
  In the decades since the establishment of these theories, both the continuous (classical) spacetime of general relativity and discrete matter of quantum mechanics, the world has changed in rather significant ways. One of those has brought forth advances in technology which led to creating virtual world geometry with continuous function derivation which then gets converted into discrete voxels in order to track state around free agent interactions. Often, to save memory these systems only convert to voxels when the free agent is observing or has interacted with the relevant geometry. We sit in a massive universe that we can observe but cannot interact with over 99% of because it's expanding away from us faster than the speed limit of local information from us to it. Within the local area where we can interact with things, they behave as if continuous until free agents interact with them when they appear to collapse to discrete units, but if the information relating to those interactions is erased, they go back to behaving as if continuous. Maybe the relationship between apparently continuous spacetime and quantized matter is much simpler than it seems to those ignoring what's currently being built within the world they are studying so closely.
  • eigenket 3y ago
    > In the decades since the establishment of these theories, both the continuous (classical) spacetime of general relativity and discrete matter of quantum mechanics, the world has changed in rather significant ways. It was essentially a historical accident that the first systems where quantum mechanics was studied extensively (black-body radiation and atomic spectra) were ones in which quantum effects ended up discretizing something which was continuous in classical mechanics. Quantum mechanics does not generally impose, require or even match having things be discrete. You can very easily have quantum systems where the relevant quantities are continuous, rather than discrete (wiki link: https://en.wikipedia.org/wiki/Continuous-variable_quantum_information https://en.wikipedia.org/wiki/Continuous-variable_quantum_in...). Simple theoretical examples like the particle living on a 1d line or in 3d space are easy to understand, there are many (many many many) more complicated examples. TLDR: there is no fundamental link between things being quantum-mechanical and things being discrete. Quantum mechanics makes some things discrete, but not everything.
  • strogonoff 3y ago
• winterismute 3y ago
  I have always been fascinated by the problem of quantum gravity but, well, it somehow happened "too late", after I got also very deeply into "computing" (mostly HPC, GPGPU, rendering). Does anybody know if there is a way for somebody with my background to actually help/contribute in advancing this field?
  • rvba 3y ago
    I always wonder if we could draw some parallels between an universe and a simulation. Time -> reason why universe is expanding Lots of quantum mechanic problems -> something that actually in theory could be simulated - but "lots of stuff is happening at the same time" (tons of fields and particles flying around, so things are "unpredictable") Particles are not particles but manifolds Time slows down around heavy objects like black holes - the same way servers slow down when they have this massive battle of thousands of ships in Eve online -> it is difficult to run a simulation So the question would be, can we escape one layer up -> to the virtual machine what is simulating our reality? Same way some program can escalate privilege and see what is inside the system. What would be a good way to escalate privileges and break free from a virtual machine? And by this, I mean the theoretical machine that runs our reality?
  • gilbetron 3y ago
    Go work for a university in the physics department, they're always looking for help writing software ... of course it only works if you're willing to work cheaply ;)
  • tim333 3y ago
    I have a personal theory that we have not been able to crack quantum gravity and similar because it's too much for human brains in the same way that we have a job visualising 5d space and dogs don't understand relativity. In which case AI going beyond human brain limits might be the way to crack it. Go build it!
• mjfl 3y ago
  does this mean there's an ultraviolet catastrophe for gravitational waves?
• raydiatian 3y ago
  The one question that matters: is it testable
• jiggawatts 3y ago
  Fields aren't quantised, that's the problem, that's why gravity can't be quantized. It's a field! Immediately, dozens of people are about hit the reply button and say something like: "But all of modern physics is based on quantization!" or something to that effect. The issue is that there's two ways to think of quantization, and for almost all practical experiments, there's no way to distinguish between the two. Essentially, you can think of either the fields being quantized, or the interactions with fields being quantized. Unfortunately almost all experiments measure only interactions with fields, so there's few practical ways to distinguish between the two. The mathematics is largely equivalent as well, so physicists "picked one" of the two options, and forgot about the other, equally valid option. This is similar to the Veritasium video titled "Why No One Has Measured The Speed Of Light".[1] From inside the universe, it's basically impossible to measure the one-way speed of light, all experiments measure the two-way speed of light. So... we just assume that it's the same speed both ways. Right now, that works well enough. If it stops working, then we need to revisit that assumption instead of devising ever more complex mathematics to explain away our faulty assumption. One difference between the "fields are quantized or not" issue and the "one-way speed of light" issue is that the former is testable. It's just that most experiments don't happen to test it. Any experiment that uses atomic orbitals is conflating the quantization of atomic orbital levels with field quantization. There are experiments that don't involve orbitals, such as free electron lasers or radio waves. All such experiments show no quantization of fields. Unfortunately, those experimental results are cheerfully ignored and hand-waved away. But stop and think about it: how exactly would you model a five kilometre long radio wave quanta as a point? How could something like that be instantaneously absorbed? How would the "rest of the wave" know that the tiny detector had made it vanish? It's madness, clearly, but that hasn't stopped thousands of physicists using this flawed model of quanta buzzing around in free space, because for short wavelengths like UV light you can make the mathematics work without paradoxes. The channel Huygens Optics has some great videos[2] on the topic. Another aspect of quantization is that for any wave in a continuum, you can model its behaviour in several ways mathematically that all arrive at the same numerical result, but "paint a different picture" in the imagination. For example, rendering something like light waves or sound waves bouncing around in a room can be done in two distinct ways: Either using local simulations with little oscillators at each volumetric point interacting only with their neighbours, OR as points moving around the space, bouncing around like particles, and carrying properties around with them such as intensity, wavelength, and phase. The former is the "wave pool" approach, the latter is the "Monte Carlo" approach. All of modern computer raytracing graphics uses the latter, not the former. Why? Because it requires less memory. The former scales a x^3 as the side-length of the volume 'x' goes up, the latter requires x^2 memory to accumulate the rays on the surfaces of the simulated volume, ignoring the intermediate states in the middle. Richard Feyman's QED is famously successful, and in large part it is practical because it uses the more efficient Monte Carlo simulation approach, treating fields as little particles bouncing around. This has entrenched the "fields are made up of little quanta" in the minds of entire generations of physicists. It's just a mathematical trick! An efficient way to do integration! It's not the One True Path, an insight into the truth of the universe. We all need to take a step back and revisit our assumptions, and try to get away from thinking of integration tricks as having explanatory power in and of themselves. [1] https://www.youtube.com/watch?v=pTn6Ewhb27k https://www.youtube.com/watch?v=pTn6Ewhb27k [2] https://youtu.be/ExhSqq1jysg?t=283 https://youtu.be/ExhSqq1jysg?t=283
• NiloCK 3y ago
  High integrity move for quanta magazine to allow space for this opinion.
• causality0 3y ago
  Out of curiosity, what's the quantum angle limit? If position is quantized and all fundamental particle are isotropic, there must be a minimum "angle of turn" for all objects, and by extension, a "maximum angular accuracy" for something like, say, orienting the face of a macroscopic object to face a particular direction. How many of those fit in a circle?
  • cryptonector 3y ago
    Between space quantization (Planck length) and the size of the visible universe, the minimum angle is fantastically small.
    • eigenket 3y ago
      The Planck length does not mean anything like space quantization. It's just a length that pops out when you combine several physical constants. The values of those constants mean that the Planck scale is likely to be roughly where gravitational and quantum effects are both relevant but other than that it has no physical significance. I have no idea where this idea that space is fundamental discrete, and divided up into chunks of Planck length voxels or something comes from, but there is absolutely no evidence for it.
      • mrguyorama 3y ago
        >I have no idea where this idea that space is fundamental discrete, and divided up into chunks of Planck length voxels or something comes from Like everything else in quantum physics, this comes from various people who don't even know algebra listening to people who work all day in algebraic fields and number theory try to explain concepts that don't make sense without those mathematical foundations and actual experience in physics papers. No, a quantum computer isn't "trying all combinations at once" No, the planck units aren't fundamental measurements of the universe No, quantum entanglement doesn't allow you to transmit information faster than light etc. I'm plenty guilty of this, I'm no physicist and I graduated after linear algebra, which I didn't do amazing in. The examples I reference are just the ones I was lucky enough to learn from people who managed to demonstrate and describe the underlying math enough to convey meaning. Plenty of educational and ostensibly knowledgeable physics and science youtube channels for example still run with those misconceptions.
• ummonk 3y ago
  It certainly feels wrong for spacetime to be quantized, because it’s fundamentally no linear and geometric in a way that other things aren’t, and quantized theories like string theory tend not to be inherently background independent, which seems really wrong. At the same time, we know that a basic semiclassical + Everettian model of the world does not correctly model reality - if we run a quantum-linked cavendish experiment, we don’t observe the gravitational effects of superposition. I.e. each branch of a quantum superposition behaves as if it is in its own spacetime. We can even run a Bell experiment linked to gravitational detectors and observe that the results are consistent with gravity being quantized.
• societyis 3y ago
  Physicist best humans can’t find some way to write something down. This isn’t saying physicist beta Moon will explode unleashing a kaiju. Physicist cannot visualize what has yet to visualized. Blah blah blah. The primates are so full of themselves.
• nologic01 3y ago
  Surely gravity must be quantized at Planck scale. The question is whether that remote scale is the only relevant one and whether there might not be interesting phenomena governing the interplay of the quantum universe with gravity and geometry at larger scales, lower energies. The footprint of fundamental forces across scales is definitely not uniform. Strong and weak forces are confined to the quantum regime: no long range manifestations. Electromagnetism differs in that it has both a quantum regime and a classical field limit. Gravity may exhibit its own quirky behavior across scales. The enormous intellectual effort to unify all fundamental forces into a grant scheme may have been a case of premature optimisation. The most exciting would be if we could break with endless paper universes and device experiments probing the interplay of gravity with quantum.
• raattgift 3y ago
  And now for something not completely different: an introductory lecture in Perturbative Quantum Gravity (PQG) by Prof. John Donoghue. The lecture is one in the Basics of Quantum Gravity online school sponsored by https://isqg.org/ https://isqg.org/ the International Society for Quantum Gravity. PQG stands in contrast to the approach in the fine article at the top ("TFA") and to semiclassical gravity. By coincidence a retweet this morning re-surfaced a tweet from a couple of weeks ago by @BahramShakerin linking to these three 2023 lectures in Perturbative Quantum Gravity given a few weeks ago at École Polytechnique Fédérale de Lausanne. This material may be useful to anyone genuinely interested in constructing GR as a "second quantization" QFT, in contrast to the Oppenheim classical gravity - second-quantized quantum matter (CQ) approach whose surface was scratched in the quantamagazine link at the top. As there are many comments which raised questions or made ... uh ... assertions about things like gravitons, there is probably some interest here, thus this comment. Additionally, problems in semiclassical gravity and in perturbative quantum gravity motivated the work discussed in TFA. Lecture outline and links to materials: https://blogs.umass.edu/grqft/ https://blogs.umass.edu/grqft/ https://indico.cern.ch/event/1279592/ https://indico.cern.ch/event/1279592/ (pqg PDF links at lower right; links to the lecture notes of related lectures are a little above that). Lecture notes https://arxiv.org/abs/1702.00319 https://arxiv.org/abs/1702.00319 Three videos of zoom lectures https://www.youtube.com/watch?v=ik0C-xTnK-k https://www.youtube.com/watch?v=ik0C-xTnK-k https://www.youtube.com/watch?v=lYc6oicfo3U https://www.youtube.com/watch?v=lYc6oicfo3U https://www.youtube.com/watch?v=UOsRyYTwOgs https://www.youtube.com/watch?v=UOsRyYTwOgs and finally Professor Donoghue's page: https://blogs.umass.edu/donoghue/ https://blogs.umass.edu/donoghue/ Extra credit, `t Hooft's PQG lecture notes: https://webspace.science.uu.nl/~hooft101/lectures/erice02.pdf https://webspace.science.uu.nl/~hooft101/lectures/erice02.pd... The twitter thread is: https://twitter.com/BahramShakerin/status/1668740440676261891 https://twitter.com/BahramShakerin/status/166874044067626189... PS: QG2023 (by ISQG) runs yesterday (10 July) until 14 July, see https://www.youtube.com/@isqg423 https://www.youtube.com/@isqg423 for live streams and starting here https://indico.imapp.ru.nl/event/106/ https://indico.imapp.ru.nl/event/106/ for info about the conference.