[HN Gopher] A physicist who bets that gravity can't be quantized
___________________________________________________________________
A physicist who bets that gravity can't be quantized
Author : theafh
Score : 175 points
Date : 2023-07-10 14:48 UTC (8 hours ago)
(HTM) web link (www.quantamagazine.org)
(TXT) w3m dump (www.quantamagazine.org)
| tauwauwau wrote:
| Putting it here, as it seems relevant.
|
| Veritasium: Parallel World Probably Exist: Here's Why
|
| https://www.youtube.com/watch?v=kTXTPe3wahc
| light_hue_1 wrote:
| 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 wrote:
| This. So much this. And also https://xkcd.com/1240/
| goatlover wrote:
| 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 wrote:
| > 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.
| goatlover wrote:
| > 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.
|
| They do if they're going into foundations of physics.
| There's a sociological problem where the Copenhagen
| interpretation won out in the past and resulted in the
| shut up and calculate orthodoxy in teaching physics. But
| there are physicists like Sean Carol who think that's a
| historical mistake and based on failing to think about QM
| correctly.
|
| https://www.preposterousuniverse.com/podcast/2023/06/26/2
| 41-...
|
| > 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!
|
| Some make testable predictions like the collapse
| theories. For the others, it's more about the proper way
| to understanding what the mathematical formalism and
| experimental results are telling us about the world, and
| how we can use that to advance future physics.
|
| > What can I say to "should"? I can only report what
| physics is in the real world.
|
| And there have always been physicists like Einstein,
| Everett, Bohm and Bell who pushed back.
| tsimionescu wrote:
| > 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 how it's often presented, but this is wrong. In
| fact, it does add something to the theory, and that's a
| measure of how many "worlds" there are after a quantum
| measurement, which helps translate the wave function values
| into testable probabilities (the Born rule).
|
| In the CI, after a measurement, the wave function collapses
| into a single value, leaving the single world in a single
| classical state, with some probability that's equal to the
| modulus of the wave function amplitude of that state (or is
| there some squaring involved as well?).
|
| In the MWI, after a measurement, different versions of the
| observer observe different states, and the number of
| versions of the observer observing each state corresponds
| to the modulus of the wave function amplitude of that
| state. Then, via simple probability, we can say this count
| corresponds to the actual probability that any one version
| of the observer will notice one particular state, even
| though in the actual multiverse all of the states actually
| happen.
|
| As you can see, the two interpretations require the same
| amount of extra postulates above and beyond the wave
| function itself. Also, the MWI has to somehow define a
| formal notion of an observer/a classical world, which runs
| into questions of scale just as much as the measurement
| postulate of CI.
| goatlover wrote:
| > That's how it's often presented, but this is wrong. In
| fact, it does add something to the theory, and that's a
| measure of how many "worlds" there are after a quantum
| measurement, which helps translate the wave function
| values into testable probabilities (the Born rule).
|
| The distribution of worlds/branches is determined by the
| wave function. A more likely outcome means there are many
| more worlds with that outcome. You can calculate the
| percentage of worlds that have that outcome.
|
| > Also, the MWI has to somehow define a formal notion of
| an observer/a classical world, which runs into questions
| of scale just as much as the measurement postulate of CI.
|
| Measurement, observer and classical shouldn't be part of
| a physical theory. The answer as to why things appear
| that way to us is decoherence.
| tsimionescu wrote:
| > A more likely outcome means there are many more worlds
| with that outcome. You can calculate the percentage of
| worlds that have that outcome.
|
| First of all, this is a new postulate of QM, you can't
| derive it from the Schrodinger equation. It is perfectly
| equivalent with the measurement postulate.
|
| > Measurement, observer and classical shouldn't be part
| of a physical theory. The answer as to why things appear
| that way to us is decoherence.
|
| This contradicts the other part, where you were talking
| about a notion of worlds that can be counted. If they can
| be counted, they have to be defined as classical worlds.
| Decoherence only explains why worlds can't interact with
| each other, it doesn't help define what they are without
| appealing to measurements. Even the notion of "the
| environment" is somewhat ill defined if we go down to the
| philosophical level.
| consilient wrote:
| > and that's a measure of how many "worlds" there are
| after a quantum measurement, which helps translate the
| wave function values into testable probabilities (the
| Born rule).
|
| All interpretations need to induce a measure over
| observations ( _not_ "worlds") to produce meaningful
| results. Without that all you have is an abstract
| mathematical object.
|
| > As you can see, the two interpretations require the
| same amount of extra postulates above and beyond the wave
| function itself.
|
| CI isn't really a single thing. Some people use it to
| mean "shut up and calculate", which requires no
| postulates by virtue of making no meaningful claims. Some
| people use it to mean various sorts of subjective
| probability anti-realism, which is similarly not really
| competing for the same territory as MWI. And some people
| use it to mean objective collapse, which requires actual
| modifications to the formalism.
| Joker_vD wrote:
| > the MWI has to somehow define a formal notion of an
| observer/a classical world
|
| Yes: in MWI, those things don't exist. The world is
| quantum all the way up and all the way down, observers
| are simply (other) quantum system that get to interact
| with the quantum system under the consideration. An
| observation, then, is simply an interaction between two
| quantum systems and follows all the usual rules so
| instead of the wave-function collapse leaving you with
| the observed system in pure state X and the observer is
| in pure state Y, you get a huge superposition of "the
| observed system in pure state Xi, the observer is in pure
| state Yi" states in the end. Those substates, in a sense,
| are multiple worlds.
| tsimionescu wrote:
| You're missing the point I was highlighting. Quantum
| mechanics makes very specific, very precise predictions
| about the probabilities of an observer seeing any of
| those states, which we have confirmed are correct to
| extraordinary precision.
|
| The problem is explaining what is the relationship
| between these observed probabilities and the wave
| function amplitudes. If we say that all possible quantum
| states are realized in the universal wave function, we
| the need to explain why different states have different
| probabilities to an observer. The only way to do that in
| the deterministic world of MWI is to add a new postulate:
| one that says that for any state Xi, there are N
| observers in state Yi, where N = |psi(Xi)|, so that we
| can compute regular frequentist probabilities of an
| observer seeing a particular state over the amount of
| observers.
|
| This is perfectly reasonable, but it is just as much an
| extra postulate as the measurement postulate.
| tauwauwau wrote:
| I watched it a long time ago. I just put it here because it
| presents the idea visually, not because I support or even
| understand it :)
| TillE wrote:
| The "popular science" takes on QM have generally been an
| enormous failure, focused on irrelevant fluff like this. I
| mean, I get it, it's a subject that you can't seriously
| understand without a degree in physics or chemistry. I had
| plenty of trouble with the math even at the undergrad level.
|
| But surely there's more interesting stories to tell of eg
| particle accelerator research, rather than fantasies.
| consilient wrote:
| > In particular the many worlds interpretation makes no
| testable predictions, it's just a story.
|
| Yes, that's what "interpretation" means. If it made
| predictions that distinguished it from other interpretations,
| it would be a competing theory to quantum mechanics - and so
| far all such attempts have failed.
| inciampati wrote:
| > 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.
| omnicognate wrote:
| But as far as we know there is just such a singularity inside
| every black hole. (We don't know there really is, but unlike
| the ultraviolet catastrophe we have no evidence there isn't.)
| urinotherapist wrote:
| So, it's like a bubble in a pressured bottle of water.
| [deleted]
| peterfirefly wrote:
| Wouldn't random noise work instead?
| eigenket wrote:
| I think this entire comment thread is using the wrong
| definition of the word "quantized". There are two relevant
| meanings of this word
|
| 1. Something split up into discrete chunks (like integers vs
| real numbers)
|
| 2. Something which acts quantum mechanically (i.e. with
| superposition and entanglement and all that stuff)
|
| In this context "space-time is also quantized" means space-time
| is acting like a quantum system.
|
| Whether space-time is discrete or continuous is a wholly
| seperate question. There is no particular evidence that it is
| discrete, but if you make the discretization scale small enough
| it theoretically could be. Most things work more nicely if it
| is continuous though.
| marcosdumay wrote:
| If space is not quantized, but everything that applies into it
| is, there is no reason for a catastrophe but the behavior is
| still different from a quantized space.
|
| Just like the ultraviolet catastrophe was solved by quantizing
| the photons, not the energy levels.
|
| (What I don't know is if there is some space-space interaction
| that can't be quantized at the "interaction" level instead of
| the "space" one.)
| CuriouslyC wrote:
| People talk about how space time can't be quantized as if the
| universe was made up of tiny little cells like minecraft
| because there is some evidence against that hypothesis.
|
| An alternate hypothesis, that a particle is "virtual" and its
| position/momentum "vector" contains a finite amount of
| information, is actually both extremely plausible and explains
| odd paradoxes like the Heisenberg uncertainty principle. That
| would result in "quantized" spacetime for the same reason
| floating point numbers are imprecise.
| guerrilla wrote:
| I was told that this is exactly what the Bekenstein bound
| ultimately means. It seems reasonable to me, but then why is
| that not conclusive?
|
| Also if anyone who knows wants to explain why the Bekenstein
| bound is even a thing, I'd love to hear that too.
| consilient wrote:
| Exceeding the Bekenstein bound would mean having less-than-
| equilibrium free energy. See
| https://arxiv.org/abs/1802.07184
| H8crilA wrote:
| For the layman, what exactly is going to theoretically
| "explode" if spacetime is continuous?
| naasking wrote:
| 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 wrote:
| 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
|
| 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 wrote:
| > 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
| NoMoreNicksLeft wrote:
| Zeno would win. And though he is thousands of years dead at
| this point, I'll be damned if I'm going to let that happen.
| Gordonjcp wrote:
| But space is quantised, that's what Planck's Constant is.
|
| The oddity we see with things at relativistic energies is
| simply because - as with all discretised representations of
| continuous-time systems - things get a bit fucky close to
| Nyquist, and the maths breaks down.
| jfengel wrote:
| That's not what Planck's Constant is. Planck's Constant
| connects energy to length (in space or time). It's just a way
| of shifting units around.
|
| You're thinking of the Planck Length, which is the result of
| combining the Planck Constant with the speed of light. It
| also happens to be the wavelength of a photon so short that
| its energy makes it a black hole, connecting it to gravity.
|
| It's often presented as if it were the fundamental unit of
| length, but that's a guess at best. There's no specific
| reason to believe it, other than the fact that it happens to
| be a way to connect length, charge, and gravity. That's
| evocative, but hardly proof. There's neither evidence for it
| nor a well-supported theory behind it.
| fsh wrote:
| There is zero evidence that space is quantized, and this
| would contradict the Standard Model, as well as General
| Relativity. The Planck constant gives the relation between
| the energy of a photon (or more generally a harmonic
| oscillator) and its frequency. It has nothing to do with the
| structure of space.
| Gordonjcp wrote:
| [flagged]
| zerodensity wrote:
| How exactly does quantized space-time contradict the
| Standard Model and General Relativity?
| consilient wrote:
| It would break Lorentz symmetry, for one.
| edgyquant wrote:
| You're thinking of Planck scale which is just a mathematical
| abstraction not some physical thing we've observed.
| srlowe wrote:
| Doesn't the plank distance imply quantization of space? At
| least in relative terms.
| jacquesm wrote:
| Planck.
| srlowe wrote:
| Right, sorry.
| xeonmc wrote:
| [flagged]
| consilient wrote:
| No, Planck units have no intrinsic physical significance.
| They're just convenient to work in because they set the
| numeric values of several different physical constants to 1.
| The planck distance also happens to be _roughly_ the length
| scale at which we expect quantum-gravitational effects to
| become significant, but roughly here means within a few
| orders of magnitude.
| srlowe wrote:
| Oh, really? I totally misunderstood then. I thought I knew
| physics a little better than that! Thanks.
|
| EDIT: Hmm, so doesn't the fact that this particular value
| makes the math simplified not also imply some kind of
| meaning?
| consilient wrote:
| > Hmm, so doesn't the fact that this particular value
| makes the math simplified not also imply some kind of
| meaning?
|
| No. There are plenty of other sets of constants you can
| choose to set to 1, inducing other length scales. Only
| dimensionless constants have physical meaning in
| isolation; dimensioned quantities are meaningful only
| with respect to each other.
| srlowe wrote:
| Thanks for the explanation. I had to google
| "dimensionless constant" lol. Maybe it is all just math
| at the foundation after all!
| eterevsky wrote:
| (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.
| contravariant wrote:
| > possibly even without creating a particle accelerator the
| size of the solar system.
|
| Sure, you just need to entangle two objects big enough to exert
| a noticeable amount of gravity on one another but somehow do
| not interact gravitationally with the rest of the set up.
|
| Anyway, let's try a cat sized object first, then we'll finally
| know if Schroedinger had a point.
| amluto wrote:
| I wonder if LIGO's mirrors would be useful for testing this
| type of theory. They're big, and if they couple classically
| to the massive objects around them, I would expect an effect
| that could be detectable.
| eterevsky wrote:
| If they are close enough to each other and far enough from
| everything else, then why not.
|
| The smallest objects for which we measured their
| gravitational interaction weighed just 90 mg
| (https://arstechnica.com/science/2021/03/researchers-
| measure-...).
|
| The biggest object put in quantum superposition weighed
| around 1 mcg (https://physics.aps.org/articles/v16/s45).
| feoren wrote:
| So 90,000 times smaller. At first glance you made it look
| 90x smaller because I couldn't tell if "mcg" was a typo for
| "mg" or not (without clicking the link). I've never seen
| anyone use "mcg" to mean "microgram" before and I feel like
| it's misleading. Use "ug", or "mg" if you're feeling fancy.
| twic wrote:
| Use of "mcg" for micrograms is common practice in
| medicine. I always assumed it was because it's easier to
| type than mg, but it seems a medical body recommends it
| because mg is too easily mistaken for mg:
|
| https://www.ismp.org/sites/default/files/attachments/2017
| -11...
| marcosdumay wrote:
| > mg is too easily mistaken for mg
|
| Is that some joke about medical writing?
|
| Anyway, I just noticed that table doesn't have rules for
| nanogram. There is also no mega-anything.
|
| (On a serious parenthesis, I think I actually understand
| their rationale; but changing the abbreviation of only
| one of them is still confusing.)
| tlb wrote:
| Before Unicode, 'm' was typically entered as an 'm' in a
| Greek font. This can go wrong in several ways, like if
| you converted the document to plain text, or if your
| laser printer didn't have the font and it substituted a
| regular font, you're suddenly off by a factor of 1000.
| 'mc' is ugly but safe.
| marcosdumay wrote:
| Interesting. There's that too.
|
| I assumed it was because of the phonetic similarity.
| eterevsky wrote:
| Yes, I meant micrograms.
|
| 90'000 times sounds a lot, but just a few decades ago the
| biggest objects in superposition were individual
| particles and atoms, and the crystal from the new study
| is 10^16 times bigger than that.
| _a_a_a_ wrote:
| 90 mg... that is beyond gobsmacking
| lamontcg wrote:
| I honestly can't tell if you're impressed we've measured
| the gravitational interaction of something that small, or
| staggered by our inability to measure a feature of things
| so large that you can easily hold them in your hand.
| ithkuil wrote:
| > somehow do not interact gravitationally with the rest of
| the set up.
|
| And how would you do that?
|
| If we had a way to shield a region of space from
| gravitational influence from something else we'd have a very
| useful technology
| jacquesm wrote:
| Assume a point sized spherical cat...
| brianpan wrote:
| I think if it's point sized, you're allowed to imagine any
| shape of cat. :D
| jacquesm wrote:
| For a uniform gravity field it is essential that the cat
| is spherical.
| TeMPOraL wrote:
| It's also crucial that the point-sized spherical cat is
| not spinning, because that's a quick way to get a ring-
| shaped cat.
| bowsamic wrote:
| 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
| H8crilA wrote:
| What would such an experiment look like? If it makes it
| easier, perhaps an experiment that's even harder (or much,
| much harder) to realize but easier to explain.
| bowsamic wrote:
| It's pretty simple. You basically have two suspended
| mirrors on wires and you shoot lasers at each mirror. Due
| to gravity there will be an interaction between the
| mirrors. There are then two things you can probe. First,
| the correlation between the two reflected lasers, and if
| you can achieve the extreme temperature requirements,
| entanglement between the two reflected lasers caused by the
| gravity. Now, the former doesn't really imply that gravity
| is quantum, it just shows that quantum correlations can be
| mediated by gravity, but this may also be true classically,
| however, the much more difficult to show entanglement would
| definitely require quantum gravity. The first step is still
| good though because no one has yet managed to demonstrate
| quantum correlations via gravity. The experiment would also
| have two mirrors two turn the laser beams into cavities,
| increasing the interaction strength. The setup would look a
| bit like this: _ _
| ! ! ---|-----| |-----|---
|
| where | are mirrors, ! are suspension wires, and - is the
| laser beam
| H8crilA wrote:
| Those reflected beams would at best be very very weakly
| entangled, right? I'm not sure what's the name for it,
| but if you arranged the quantum state of the two beams
| into a 2x2 matrix then the determinant would be just a
| tiny bit non-zero.
| bowsamic wrote:
| Yes
| phkahler wrote:
| >> 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 wrote:
| 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.
| eigenket wrote:
| I attended a talk by Jonathan Oppenheim a while back on
| this subject, and my understanding based on that is that
| in his model something (very) roughly like this happens.
| You put your massive object in superposition, it
| interacts with space-time and "tries" to put space-time
| in superposition but since space-time is fundamentally
| classical (in his model) what happens is space-time ends
| up in a probabalistic mixture of the different states
| rather than a superposition. Then the interaction between
| space-time and the massive object end up pushing the
| object from the superposition state you tried to produce
| into a mixed state just like the space-time.
|
| Essentially from the point of view of the massive object,
| the interaction with space-time acts as some decoherence
| process.
| bowsamic wrote:
| > the interaction with space-time acts as some
| decoherence process.
|
| Well, the question is, can we use gravity in the same way
| that we use quantum processes, i.e., in a coherent way?
| If we can, e.g. using it to establish entanglement, then
| the theory you describe cannot hold.
| eigenket wrote:
| Thats definitely a good question, and one that people
| have thought about before. I think its _amazingly_
| difficult to test it experimentally, even compared to
| something like directly measuring to see whether a
| massive object in superposition is forced to decohere
| faster than we expect, which is already incredibly
| difficult.
| skinner_ wrote:
| I was taught that choosing a QM interpretation is a
| matter of taste. Am I understanding correctly that we do
| have proposed quantum gravity experiments that can
| falsify various QM interpretations, it's just that they
| are all very hard to execute?
| consilient wrote:
| Seems like this would show up as an equivalence principle
| violation, no?
| eigenket wrote:
| That isn't obvious to me - the equivalence principle
| works fine in classical GR, I'm not sure why it would
| break in Jonathan's model, but I'm very far from
| qualified to speak authoratively about his work!
| nocoolnametom wrote:
| When you say superdeterminism are you referring to
| something like Pilot Wave theory, where what appear to
| our measurements as probabilistic yet random interactions
| are merely expressions of a more complex yet non-random
| underlying system that we cannot, as yet, measure? (I
| don't even know if that's the proper description of the
| hypothesis.)
| meroes wrote:
| Do you know if Bohmian mechanics with a particle ontology
| (still with non-local "hidden" variables ofc) or GRWf has
| potential to save classical gravity as well?
| AnimalMuppet wrote:
| 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?
| LatteLazy wrote:
| 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 wrote:
| 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).
| thechao wrote:
| > General relativity says that you can't tell the difference
| between being unaccelerated, and being in free fall.
|
| Just to be clear, GR only says this for a very specific case,
| and a very specific observer. In fact, one of the easier
| exercises when first learning how to work with metrics is to
| show how to measure the difference.
| snowwrestler wrote:
| Saying one theory must be wrong because the other must be right
| is begging the question. Where general relativity and quantum
| theory disagree, we don't know which one is correct (or maybe
| neither is).
|
| Highlighting a point of disagreement between the two doesn't,
| by itself, resolve the disagreement.
| light_hue_1 wrote:
| The question of how can the Equivalence Principle work in the
| presence of gravitons comes up surprisingly often!
|
| You will enjoy this discussion
| https://physics.stackexchange.com/questions/589074/why-can-t...
|
| They go over several reasons why gravitons don't break the
| equivalence principle. In particular the answer about virtual
| particles is very important.
| eigenspace wrote:
| > 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.
|
| No, this is completely false. Particle number is coordinate
| system and frame dependent.
|
| Quantum particles aren't little billiard balls bouncing around,
| and it's really best to not think of them as 'particles' at
| all. They're just a calculational tool to describe excitations
| in quantum fields.
| noslenwerdna wrote:
| Can you provide an example of a Lorentz boost that changes
| the number of particles?
| eigenspace wrote:
| It's pretty general. Just draw the worldlines for some
| particles bouncing off eachother, and then perform a
| Lorentz boost, which means choosing a new tilted spatial
| surface to intersect the worldlines. If you perform a
| boost, then the spatial surface intersect fewer or more
| worldlines, and worldlines which were for particles in one
| frame can become antiparticles in another.
|
| Consider an electron absorbing a photon at spacetime point
| x, and then emitting a photon at spacetime point y, where
| (y0 - x0) > 0.
|
| An observer in another lorenzt boosted frame would then say
| that they see in their boosted coordinated system (x' and
| y' with rapidity v), that (y'0 - x'0) = cosh(v)(y0 - x0) -
| sinh(v) (y1 - x1)
|
| For large enough v, then you can have y'0 < x'0, so the
| observer in the boosted frame would see the events in a
| different order. One observer sees a negatively charged
| electron moving from x to y, and the other observer sees a
| positively charged positron moving from y to x.
| AnimalMuppet wrote:
| If the events in frame 1 are causally connected, then to
| see the events in frame 2 in reverse order takes a boost
| of velocity greater than c, which is not a valid boost.
|
| Sure, you can do all kinds of things with a boost like
| that. It's not physically realizable, though.
|
| Can you show me an actual experiment to the contrary?
| eigenspace wrote:
| Compton scattering with a space-like separation for the
| absorption and emission points are a well known
| phenomenon in quantum field theory. Classically these
| events would be causally disconnected, but in a QFT the
| propagator is non-zero and instead has an exponential
| suppression in the space-like interval.
|
| This is a phenomenon with experimental consequences and
| uses:https://arxiv.org/abs/1301.3819
| https://www.nature.com/articles/s41567-019-0774-3
| 317070 wrote:
| > 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
| codethief wrote:
| > 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
| nomel wrote:
| > it's just a field like any other -- and therefore we should
| quantize it.
|
| So, trade dogma for tradition!
| skinner_ wrote:
| I believe you are misparsing that sentence. The dogma and the
| tradition are the same thing.
| hughesjj wrote:
| 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/6OpAreb779U
|
| - https://youtu.be/9crggox5rbc
|
| - https://youtu.be/OBPpRqxY8Uw
|
| - 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.
| miga wrote:
| Why would gravity behave different than Higgs field?
| krastanov wrote:
| Any reason you are specifically picking the Higgs field in
| your question? I am asking, because this sounds a bit like
| you are riffing on a common misconception that the Higgs
| field has something to do with gravity, which is not the
| case. The interaction with the Higgs field is the reason some
| (only some) of the particles have a mass, but explaining
| gravity does not need to have anything to do with the Higgs
| field.
|
| But yeah, it is fair to ask why gravity should behave any
| differently than any other quantum field -- in the context of
| Quantum Field Theory (one of the two incredibly successful
| theories of physics) that is a great question. One handwavy
| reason it seems different is that in General Relativity (the
| other incredibly success theory of physics), gravity has to
| do with the geometry of space and time, not with what other
| fields exist in that space and time (and as such is
| explicitly different than the other fields).
| pravus wrote:
| My understanding is that the Higgs field should be simpler
| than gravity because it's just a static value whereas
| gravity is SU(1)? At least to me it seems logical that if
| the Higgs is quantized, surely more complicated fields
| would be as well?
| kromem wrote:
| 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.
| strogonoff wrote:
| A simulation produces philosophical zombies, yet you are not
| one.
| eigenket wrote:
| > 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_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.
| naasking wrote:
| > 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
| qazpot wrote:
| > 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 wrote:
| 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.
| CuriouslyC wrote:
| Digital physics seems so obviously the correct approach from a
| philosophical perspective, it's a shame it blows up the math.
|
| Real numbers are the most ironically named thing ever, and
| infinity is a thought experiment - not a real thing. Any model
| of the universe based on such constructs should be heavily
| suspect outside the range of established observations, and
| taking predicted limit behavior seriously is just foolish.
| naasking wrote:
| I think naive approaches to digital physics have been
| inadequate, but newer works have made good progress on
| important questions. Arguably one of the biggest tools in the
| physicists' toolbox are symmetries, and there's now a good
| account for those:
|
| A Noether Theorem for discrete Covariant Mechanics,
| https://arxiv.org/abs/1902.08997
| eigenket wrote:
| Speaking as a physicist I think its very far from obvious
| that digital physics (or anything else) is the correct
| approach from a philosophical perspective.
|
| Real numbers are so ubiquitous in physics because space-time
| (and other quantities) look _really_ continuous.
| CuriouslyC wrote:
| Rational numbers can produce "continuous" values to a
| precision way beyond what any equipment you could get your
| hands on could differentiate without physically ludicrous
| postulates.
| eigenket wrote:
| That is certainly true. In fact you don't even need
| rational numbers, it is entirely possible that there are
| a finite number of positions on the universe, for
| example.
|
| Nevertheless physics seems to work very nicely when
| expressed in the language of calculus. Everything from
| Schrodinger's equation to the Einstein field equations,
| and from classical mechanics to the standard model of
| particle physics are expressed in the language of
| calculus. This all looks like a wild and strange
| coincidence if fundamentally we are living in a relm like
| the rationals, where calculus doesn't really make sense.
| consilient wrote:
| Agreed that digital physics is far from obvious, but the
| use of real numbers as our default model of the continuum
| is at least in part historical accident. We could have for
| instance easily ended up with locale-theoretic foundations
| instead, though I doubt the finitist crowd would find that
| any more satisfying.
| abdullahkhalids wrote:
| I can't help but suggest "Information, physics, quantum: The
| search for links" [1] by the one and only John Wheeler. This
| philosophical paper is bold, bolder than most physicists ever
| would be.
|
| > Abstract: This report reviews what quantum physics and
| information theory have to tell us about the age-old question,
| How come existence? No escape is evident from four conclusions:
| (1) The world cannot be a giant machine, ruled by any
| preestablished continuum physical law. (2) There is no such
| thing at the microscopic level as space or time or spacetime
| continuum. (3) The familiar probability function or functional,
| and wave equation or functional wave equation, of standard
| quantum theory provide mere continuum idealizations and by
| reason of this circumstance conceal the information-theoretic
| source from which they derive. (4) No element in the
| description of physics shows itself as closer to primordial
| than the elementary quantum phenomenon, that is, the elementary
| device-intermediated act of posing a yes-no physical question
| and eliciting an answer or, in brief, the elementary act of
| observer-participancy. Otherwise stated, every physical
| quantity, every it, derives its ultimate significance from
| bits, binary yes-or-no indications, a conclusion which we
| epitomize in the phrase, it from bit.
|
| [1] https://philarchive.org/archive/WHEIPQ
| tzs wrote:
| Page is 404 at the moment.
|
| Here's an archive.org link that works [1].
|
| [1]
| https://web.archive.org/web/20230710160259/https://www.quant...
| javajosh wrote:
| 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
| amelius wrote:
| 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...
| javajosh wrote:
| Ah, perhaps you are inclined to stop reading when you come
| across open parenthesis?
| amelius wrote:
| 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.
| javajosh wrote:
| I'm already convinced[0] the effect is small, if it
| exists at all. However I thought of another experiment
| that might be easier to perform, to see if there is any
| effect at all: take two crystals, as similar in depth as
| one can make them, with two different isotopes, and
| measure the difference in index-of-refraction. Silicon
| (28 and 30?) would probably be good for this, as would
| laser interferometry. (Maybe the Gravity Probe B people
| have some extra pure isotope wafers they'd be willing to
| lend?) If I'm right then the Si-30 sample will have a
| slightly larger index of refraction than the Si-28
| sample. Someone needs to do the math though because if
| the effect is far less than like one layer of atoms, or a
| few impurities here and there, then the experiment isn't
| worth doiong.
|
| 0 - https://news.ycombinator.com/item?id=36671605
| archibaldJ wrote:
| 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 wrote:
| 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?
| tlb wrote:
| This can be refuted by observing that the refractive index
| isn't proportional to density, but is accurately predicted by
| charge mobility. Also, how refractive index changes with
| wavelength is related to the resonant frequencies of electrons.
| javajosh wrote:
| Yeah, I was thinking about it and also lasers wouldn't work
| if it was 100% mini-Shapiro delays. I was thinking that maybe
| there's a small GR component to refraction, but the
| experiment would be tricky. You'd need to pin the electrons
| in the refractor down in a strong magentic field and then
| shoot some high energy photons at it, probably at least
| X-rays, and see if they still refract. (The high energy
| photons would be necessary to bring their wavelength closer
| to the diameter of the nucleus).
| LonelyTree wrote:
| Then it seems like gravity isn't one field, but a conglomeration
| of fields yet to be discovered.
| djmips wrote:
| Article is 404 right now, for me at least.
| [deleted]
| redtexture wrote:
| Lose the V1 at the end of the URL.
| gpvos wrote:
| _> Error 404. This page doesn 't exist. At least not in this
| universe._
|
| I like the error message though.
| sedatk wrote:
| > J. Oppenheim
|
| I wonder what influenced him to become a physicist.
| booleandilemma wrote:
| I'm just waiting for a person with the surname Oppenheimest
| now.
| sharikous wrote:
| Isn't this similar to the view of Penrose?
| antognini wrote:
| 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).
| btilly wrote:
| 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 Schrodinger'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.
| AnimalMuppet wrote:
| > 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 wrote:
| Why would anyone believe such a thing?
| CraftingLinks wrote:
| Because that's what quantum mechanics in it's purest form
| tells us. Avoiding the many worlds requires tagging on extra
| stuff not in the equations.
| tsimionescu wrote:
| Quantum mechanics tells us that, in order to predict the
| outcome of a measurement, we have to compute a specific
| probability based on the amplitude of the wavefunction.
|
| We can explain this probability as some kind of collapse,
| or we can explain it as some measure of the number of
| observers making the measurement in parallel "worlds".
| Neither is inherently closer to the math.
| edgyquant wrote:
| Why should we not just take it as a probability based
| abstraction for something we don't have a true
| understanding of and move on? Anything else seems like
| weird theology to me.
| meroes wrote:
| 1) Because physics has had an ontology, what there really
| is, since like Aristotle. Even if ontology changes,
| posing one has worked for roughly 2500 years to partially
| guide science. It really does inform all kinds of
| experiments, even thought experiments. BUT, you might be
| right the trade off you propose is more worth it
|
| 2) Reichenbach's principle says any correlation must have
| a cause. Even without an idea of an ontology like in 1)
| this looser principle is hard to give up as well. It
| explains the mystery behind Simpson's for example. And
| why without the causal model behind correlations, you
| could easily take the wrong drug for you specifically.
|
| Again though, these are guiding principles and trade offs
| I am curious to see how we could progress without them
| consilient wrote:
| > Quantum mechanics tells us that, in order to predict
| the outcome of a measurement, we have to compute a
| specific probability based on the amplitude of the
| wavefunction
|
| Even this is already wading into interpretational waters.
| The math says nothing about whether a given POVM should
| be thought of as a measurement or an interaction (or
| both, or neither).
| TheOtherHobbes wrote:
| As opposed to tagging on a literally incalculable number of
| extra universes for every possible quantum interaction.
| bowsamic wrote:
| 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 wrote:
| 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.
| edgyquant wrote:
| 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 wrote:
| 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 wrote:
| 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.
| edgyquant wrote:
| They work because they use human logic which has evolved
| to understand the universe just enough to "work." Doesn't
| mean it in anyway reflects some objective truth. It
| breaks things down into small enough parts you can then
| model those parts and make predictions. This does not
| mean those small parts are "real" just that they are
| convenient ways to describe the flowing of energy for our
| purposes.
| slowmovintarget wrote:
| 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 wrote:
| But the observer is in all "probabilities" no?
| rvcdbn wrote:
| 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.
| treeman79 wrote:
| Where would energy come from for creating each universe?
| FollowingTheDao wrote:
| "Where would energy come from for creating each universe?"
|
| Why does it have to come from somewhere? What if it just is?
| MattPalmer1086 wrote:
| 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.
| arethuza wrote:
| Maybe the implementation uses copy-on-write to minimise the
| amount of work required? ;-)
| tgv wrote:
| Among other problems. It's just bollocks on the level of "but
| you didn't say it couldn't be this, nanananana."
| captainclam wrote:
| 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!
| tgv wrote:
| 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.
| captainclam wrote:
| Correct, it is an enormous amount...Under Everett, the
| "number of worlds" practically exists on a continuous
| spectrum, so basically an infinitude of worlds. But this
| doesn't violate parsimony in terms of building a model of
| reality. I don't have the greatest comprehension of the
| history of science, but I imagine the behavior of water
| and gas was not initially explained as being the
| collective behavior of moles (6.02x10^23) and moles of
| individual molecules. The parsimony of thermodynamics is
| measured by the simplicity of the underlying equations
| that describe their behavior, not the staggering number
| of atoms involved.
|
| As for observation, I didn't suggest there was anything
| special about observing. In fact, this is part of what
| makes Everett among the most parsimonious
| interpretations: While some (not all) other
| interpretations are tasked with explaining the nature of
| observation (what counts as observation, how quickly does
| collapse propagate, etc), under Everett, there is no
| notion of observation at all. My point about
| consciousness is that, being conscious, we are forced to
| have a point of view which depends on where we exist
| in/on the wave function. I wasn't suggesting that
| consciousness has any actual effect whatsoever on the
| wavefunction.
|
| I admit that the nature of probability is among the most
| difficult parts of Everett to wrestle with, though I'm
| not sure I understand why subsequent observations should
| follow a radically different pattern? It's still the same
| wavefunction with the same distributions as
| before...Schrodinger's equation is the same wherever you
| are on the wavefunction.
| consilient wrote:
| > Assuming many worlds, and many here means: enormous
| amounts, far exceeding the number of particles in the
| universe, is everything but parsimonious.
|
| "Many worlds" is a misnomer. MWI is just wavefunction
| realism + unitary evolution. There's only one world, and
| really only one dynamical object: the wavefunction. It
| evolves according to some unitary operator, and that's
| the whole story. No splitting, no collapse, no objective
| classical transition, just quantum mechanics taken at
| face value.
|
| > There's also nothing special about observing.
|
| Yes, that's the MWI position.
| kgwgk wrote:
| Is it possible to observe anything at all in MWI? If yes,
| what does it mean to "observe something" in MWI? If not,
| is there any physical content in MWI?
| consilient wrote:
| Observations in MWI are just ordinary physical
| interactions. Specifically they're perturbative-regime
| interactions between the thing being observed and large
| thermalized systems (e.g. humans). From the perspective
| of the thermal bath, you get exponential suppression of
| everything but the eigenstates of the interaction
| Hamiltonian, which is why our observations "look
| classical".
| kgwgk wrote:
| Will other large thermalised system (e.g. rocks) also
| experience observations that "look classical"?
|
| Another MWI proponent here is talking about how the "look
| classical" thing is "simply an artifact of being a
| conscious being that can only observe one value".
|
| Is there something special about "large thermalised
| systems" (and/or humans)? How large have they to be to
| allow for "our observations"? Where is the boundary
| between the thing being observed and the observing thing?
|
| MWI seems to still face most of the difficult questions -
| it not all.
| consilient wrote:
| > Will other large thermalised system (e.g. rocks) also
| experience observations that "look classical"?
|
| The inner life of rocks is somewhat beyond our reach, I'm
| afraid. But they'll induce decoherence in the same way as
| a human, yes.
|
| > Is there something special about "large thermalised
| systems" (and/or humans)?
|
| Aside from being large and thermalized? No.
|
| > How large have they to be to allow for "our
| observations"?
|
| It's a continuum. The more internal degrees of freedom
| you have, and the more thoroughly they're mixed, the
| faster you'll decohere things.
|
| > Where is the boundary between the thing being observed
| and the observing thing?
|
| At the level of fundamental physics, there isn't one.
| "Observation" is an approximate and thermodynamic notion.
| kgwgk wrote:
| > But they'll induce decoherence in the same way as a
| human, yes.
|
| From the perspective (?) of the (non-human) thermal bath,
| will that decoherence result in a single (diagonal,
| mixture) state or in a particular state of those N
| separate states that would "look classical"?
|
| Decoherence doesn't make things "look classical" by
| itself - at least until you define what "looking" is.
| goatlover wrote:
| Many worlds just falls out of superposition and
| entanglement. Those happen in quantum systems, and we are
| made of quantum systems. To avoid that you have to
| stipulate something additional that avoids putting
| macroscopic objects like devices, cats and brains into
| superpositions.
| kgwgk wrote:
| > 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 Schrodinger
| cake and eat it too.
| lavelganzu wrote:
| 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-
| world...
| captainclam wrote:
| 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.
| ly3xqhl8g9 wrote:
| Sean Carroll, following Everett, puts it in the most concise
| form: (i) systems are described by wave functions, (ii) wave
| functions obey the Schrodinger equation [1]. "Many-worlds",
| universes, observations, observers, and so on become just
| _entia multiplicanda_ [2], superfluities.
|
| [1] 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
| x3n0ph3n3 wrote:
| > 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 wrote:
| No:
|
| > Based on whether there is a click in a Geiger counter,
| choose which one to put a cannon ball next to.
| FollowingTheDao wrote:
| 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.
| FollowingTheDao wrote:
| 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.
| msla wrote:
| I posted a response to this. It went through, but the board
| said I was posting too quickly. Bug dang about making it
| visible.
| FollowingTheDao wrote:
| I wish any of you with the down vtes would at least have a
| conversation with me...
| eigenket wrote:
| 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 wrote:
| 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.
| consilient wrote:
| It's not a constant, any more than the meter or the foot.
| It's just part of a unit system that happens to be
| convenient for carrying out certain calculations.
| FollowingTheDao wrote:
| I have questions about your understanding. Or maybe it is
| a problem with my communication. There is a planck
| constant.
|
| https://en.m.wikipedia.org/wiki/Planck_constant
| consilient wrote:
| Planck's constant is not the Planck length.
| eigenket wrote:
| The Planck length and Planck's constans are both named
| after Max Planck. That doesn't mean they're the same
| thing. The Max Planck Institutes are also named after
| Planck, but they are neither lengths nor constants.
| eigenket wrote:
| That is certainly a hypothesis, but there is no evidence
| for it.
| FollowingTheDao wrote:
| Hey, I'm no Einstein, but even he had hypothesis that
| took decades to be shown evidence to prove they were
| true.
|
| I don't know how someone is supposed to come up with
| evidence before a hypothesis, but if you have a new
| understanding of the scientific process Please let me
| know.
| eigenket wrote:
| > I don't know how someone is supposed to come up with
| evidence before a hypothesis
|
| Sure, this is usually how science works. A good example
| is the famous Michelson-Morley experiment which was
| performed in 1887 and was a key piece of evidence
| directly inspiring Einstein's theory of special
| relativity in 1905.
|
| The way science usually progresses is that there is some
| piece of evidence which current theories can't explain or
| deal with. That prompts the development of new theories,
| which make new predictions, which are then tested in
| further experiments.
| 317070 wrote:
| > our minds collapse the wave function
|
| While I know there are many sources available of people
| saying this exact thing, this is a common misunderstanding of
| the Copenhagen interpretation of quantum physics. There is
| nothing special about our minds.
| goatlover wrote:
| It was Wigner who speculated that consciousness causes
| collapse. But Neils Bohr had a Kantian approach to science,
| so he did think it mattered how our minds understood the
| world, and what we can and can't say about physical
| formalisms.
| FollowingTheDao wrote:
| First, thank you for responding.
|
| I'm not saying there's anything special about our mind,
| it's just a function of our mind.
|
| And is it misinterpretation or is it my interpretation that
| you disagree with?
|
| How can one rationalize the fact that a photon can be a
| wave and a particle at the same time?
|
| I do not believe, for example, that the moon exist, only
| because I look at it. I think it exists in a probabilistic
| state in that probabilistic state turns to certainty when I
| look at it. But that is only my certainty. How do I share
| my mind's interpretation of what the moon looks like with
| someone else? Exactly? I can't, it's impossible. The moon
| both exist, and does not exist at the same time. And what I
| mean it does not exist. I mean it does not exist in any
| sense of certainty. Hence the moon is a wave function. The
| fact that my brain in your brain can collapse the way
| function in an apparent 100% similarity provides no proof
| that the moon exists in a single state outside of our own
| minds.
| consilient wrote:
| > How can one rationalize the fact that a photon can be a
| wave and a particle at the same time?
|
| Photons are not particles or waves. They are similar to
| classical particles in certain respects and similar to
| classical waves in other respects, but what they're
| really like is a localized excitation of the
| electromagnetic field. Because that's what they are.
|
| Physics by analogy simply does not work. If you
| understand the math, analogies can occasionally be a
| helpful signpost. If you don't, they're worse than
| useless.
| kromem wrote:
| I'm curious why you think the commenter is interpreting
| according to the Copenhagen interpretation?
|
| It sounds more like they are working within a view similar
| to the Von Neumann-Wigner interpretation.
| FollowingTheDao wrote:
| Yes, and I feel that it will be proved to be right.
|
| But I will disagree with them on one point, that I don't
| think that consciousness is a thing that causes wave
| collapse, rather, the brain has a function that
| calculates the highest probability of a particle, and
| provides that to consciousness.
| slowmovintarget wrote:
| The "Maybe Bigfoot is Fuzzy" interpretation of gravity?
| api wrote:
| Fun fact: Bigfoot has no gravity because he doesn't exist in
| this slice of the multiverse.
| sandworm101 wrote:
| >> 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.
| srejk wrote:
| You have to be trained in physics to theorize about it.
| Thinking otherwise is the realm of the crackpot.
| https://www.youtube.com/watch?v=11lPhMSulSU
| weatherlight wrote:
| I love Dr. Collier's videos. Her method of story
| telling/video essay is extremely unconventional but very
| effective.
| srejk wrote:
| Me too bestie, I haven't taken physics since undergrad but
| I appreciate that it's a layer of understanding above
| popular science (that never covers any of the math) while
| remaining accessible.
| bowsamic wrote:
| I'm a quantum physicist and you wouldn't believe how many
| people try and sell me on their theory that solves everything
| that is literally just an idea in their head and can't even
| produce any quantitative predictions. The levels of self
| delusion that these people display is astounding and we need
| to mock it more openly imo
| light_hue_1 wrote:
| The newage people are the worst at this.
|
| Beware, you may want to gouge your own eyes out:
| https://www.watkinsmagazine.com/quantum-life-of-healing-
| crys...
| jacquesm wrote:
| There is this thing that I like to call 'the cult of
| stupid' where people are somehow proud of their lack of
| knowledge and they are not afraid of putting it out
| there. They tend to laugh when asked pointed questions
| because in their world 'truth' has no value, it's all
| about belief. One family member of mine wears healing
| crystals because they keep her cancer free. It's really
| sad and it irks me that given the excellent educational
| opportunities we have that so many people choose
| willingly not to avail themselves of those opportunities.
| But they _do_ have opinions on all that stuff that you
| need to work at to learn.
| rvcdbn wrote:
| if i have a crackpot theory and i'd like a real physicist
| to look it over and am willing to pay for their time where
| would i find one? asking for a friend
| binarycoffee wrote:
| Sabine Hossenfelder apparently does this [1, 2] for
| $50/20min.
|
| [1] http://backreaction.blogspot.com/p/talk-to-
| physicist_27.html
|
| [2] https://aeon.co/ideas/what-i-learned-as-a-hired-
| consultant-f...
| legel wrote:
| Agreed. Those who confuse "astronomy" and "astrology" are
| confused.
| orblivion wrote:
| What do you intend to achieve by mocking these people?
| bowsamic wrote:
| To make them feel bad (I am salty)
| dang wrote:
| This sort of objection only arises because titles say baity
| things like "challenging orthodoxy". There's no information
| there--not in the bait and not in the objections the bait
| triggers.
|
| The solution is to have a more substantive title. Fortunately
| the article supplies one of those in its HTML doc title, so
| we've switched to that (plus the s/the/a/ debaiting trick).
|
| Let's focus on the article content now please.
| outlace wrote:
| Genuine question here, when was the last time a total outsider
| made a significant contribution to physics?
| sandworm101 wrote:
| https://en.wikipedia.org/wiki/STEVE
|
| 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 wrote:
| 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 wrote:
| 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.
| 1270018080 wrote:
| A better analogy is asking why someone who is bad at
| baseball isn't on an MLB team
| tonycoco wrote:
| When was the last time an insider made one?
| andrewflnr wrote:
| 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.
| rapjr9 wrote:
| 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-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.
| dawnofdusk wrote:
| Yeah the "gravitational hum" would not seem to be the same as
| gravity itself having some inherent randomness. On the
| flipside, in a very real sense if the cosmological history of
| our universe is such that we exist in a universe with an always
| present indeterministic hum, then for our intents and purposes
| this is the same as gravity itself being random... unless we
| can devise experiments to study gravity in other universes with
| different cosmological initial conditions.
| bdamm wrote:
| 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 wrote:
| 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?
| winterismute wrote:
| 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?
| padjo wrote:
| 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.
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