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> 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 th

> 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
      - GoblinSlayer 3y ago
        Problems 1-3 there are caused by point particles, not by space. They mention ultraviolet catastrophe was caused by continuous space, but it was caused by non-quantum radiation, so there's no classical electrodynamics with discreet space, instead there's quantum electrodynamics with continuous space.
        naasking 3y ago
        I'm not sure what "problems 1-3" are referring to here, but all of these problems fundamentally trace their roots to issues with the foundational assumption of continuity.
        GoblinSlayer 3y ago
        Per numbered titles: 1 Newtonian gravity 2 Quantum mechanics of charged particles 3 Classical electrodynamics of point particles They explicitly mention they use point particles. Problem in 1 Newtonian gravity is especially egregious example. They model planets(!) as material points and see how they come infinitely close to each other. The author does mention these problems are pure math unrelated to reality.
- NoMoreNicksLeft 3y ago
  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.
marcosdumay 3y ago
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.)
omnicognate 3y ago
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 3y ago
  So, it's like a bubble in a pressured bottle of water.
CuriouslyC 3y ago
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 3y ago
  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 3y ago
    Exceeding the Bekenstein bound would mean having less-than-equilibrium free energy. See https://arxiv.org/abs/1802.07184 https://arxiv.org/abs/1802.07184
    - guerrilla 3y ago
      Hmm so that would be a system being out of equilibrium but in a "negative" direction, which is nonsense since any amount of non-equilibrium is a positive amount of free energy. So, I don't understand any of the math in that paper but is there any easy/intuitive way to explain why a higher information density would require negative free energy? I guess I need to understand the relationship between information density and free energy first. Hmm.
      - consilient 3y ago
        > So, I don't understand any of the math in that paper but is there any easy/intuitive way to explain why a higher information density would require negative free energy? Not really, no. If you don't have the equivalent of a good undergraduate education in physics very little about QFT is going to be accessible to you.
srlowe 3y ago
Doesn't the plank distance imply quantization of space? At least in relative terms.
- jacquesm 3y ago
  Planck.
  - srlowe 3y ago
    Right, sorry.
    - xeonmc 3y ago
      [flagged]
- consilient 3y ago
  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 3y ago
    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 3y ago
      > 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.
Gordonjcp 3y ago
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.
- fsh 3y ago
  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 3y ago
    [flagged]
    - dang 3y ago
      Could you please stop posting unsubstantive comments and flamebait? You've unfortunately been doing it repeatedly. It's against what this site is for, so we have to ban accounts that keep doing it, and we've already had to ask you this more than once: https://news.ycombinator.com/item?id=36449159 https://news.ycombinator.com/item?id=36449159 (June 2023) https://news.ycombinator.com/item?id=34194236 https://news.ycombinator.com/item?id=34194236 (Dec 2022) https://news.ycombinator.com/item?id=33501516 https://news.ycombinator.com/item?id=33501516 (Nov 2022) Nothing Phone (1) - https://news.ycombinator.com/item?id=32086641 https://news.ycombinator.com/item?id=32086641 - July 2022 (97 comments) If you wouldn't mind reviewing https://news.ycombinator.com/newsguidelines.html https://news.ycombinator.com/newsguidelines.html and taking the intended spirit of the site more to heart, we'd be grateful.
  - zerodensity 3y ago
    How exactly does quantized space-time contradict the Standard Model and General Relativity?
eigenket 3y ago
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.
deleted 3y ago
[deleted]

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
    - GoblinSlayer 3y ago
      Problems 1-3 there are caused by point particles, not by space. They mention ultraviolet catastrophe was caused by continuous space, but it was caused by non-quantum radiation, so there's no classical electrodynamics with discreet space, instead there's quantum electrodynamics with continuous space.
      - naasking 3y ago
        I'm not sure what "problems 1-3" are referring to here, but all of these problems fundamentally trace their roots to issues with the foundational assumption of continuity.
        GoblinSlayer 3y ago
        Per numbered titles: 1 Newtonian gravity 2 Quantum mechanics of charged particles 3 Classical electrodynamics of point particles They explicitly mention they use point particles. Problem in 1 Newtonian gravity is especially egregious example. They model planets(!) as material points and see how they come infinitely close to each other. The author does mention these problems are pure math unrelated to reality.
NoMoreNicksLeft 3y ago
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.

marcosdumay 3y ago

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.)

omnicognate 3y ago

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 3y ago
So, it's like a bubble in a pressured bottle of water.

CuriouslyC 3y ago

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 3y ago
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 3y ago
  Exceeding the Bekenstein bound would mean having less-than-equilibrium free energy. See https://arxiv.org/abs/1802.07184 https://arxiv.org/abs/1802.07184
  - guerrilla 3y ago
    Hmm so that would be a system being out of equilibrium but in a "negative" direction, which is nonsense since any amount of non-equilibrium is a positive amount of free energy. So, I don't understand any of the math in that paper but is there any easy/intuitive way to explain why a higher information density would require negative free energy? I guess I need to understand the relationship between information density and free energy first. Hmm.
    - consilient 3y ago
      > So, I don't understand any of the math in that paper but is there any easy/intuitive way to explain why a higher information density would require negative free energy? Not really, no. If you don't have the equivalent of a good undergraduate education in physics very little about QFT is going to be accessible to you.

srlowe 3y ago

Doesn't the plank distance imply quantization of space? At least in relative terms.

jacquesm 3y ago
Planck.
- srlowe 3y ago
  Right, sorry.
  - xeonmc 3y ago
    [flagged]
consilient 3y ago
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 3y ago
  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 3y ago
    > 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.

Gordonjcp 3y ago

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.

fsh 3y ago
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 3y ago
  [flagged]
  - dang 3y ago
    Could you please stop posting unsubstantive comments and flamebait? You've unfortunately been doing it repeatedly. It's against what this site is for, so we have to ban accounts that keep doing it, and we've already had to ask you this more than once: https://news.ycombinator.com/item?id=36449159 https://news.ycombinator.com/item?id=36449159 (June 2023) https://news.ycombinator.com/item?id=34194236 https://news.ycombinator.com/item?id=34194236 (Dec 2022) https://news.ycombinator.com/item?id=33501516 https://news.ycombinator.com/item?id=33501516 (Nov 2022) Nothing Phone (1) - https://news.ycombinator.com/item?id=32086641 https://news.ycombinator.com/item?id=32086641 - July 2022 (97 comments) If you wouldn't mind reviewing https://news.ycombinator.com/newsguidelines.html https://news.ycombinator.com/newsguidelines.html and taking the intended spirit of the site more to heart, we'd be grateful.
- zerodensity 3y ago
  How exactly does quantized space-time contradict the Standard Model and General Relativity?

eigenket 3y ago

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.

deleted 3y ago

[deleted]