A physicist who bets that gravity can’t be quantized • Hacker News Board

Re: A physicist who bets that gravity can’t be quantized

#71

Post by noslenwerdna » Mon, Jul 10, 2023, 5:37 PM UTC

eigenspace wrote: post #54

AnimalMuppet wrote: post #23

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.…

> 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…

Can you provide an example of a Lorentz boost that changes the number of particles?

Re: A physicist who bets that gravity can’t be quantized

#72

Post by eterevsky » Mon, Jul 10, 2023, 5:39 PM UTC

feoren wrote: post #57

eterevsky wrote: post #48

Earlier quoted context omitted.

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 ).

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 "μg" if you're feeling fancy.

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.

Re: A physicist who bets that gravity can’t be quantized

#73

Post by naasking » Mon, Jul 10, 2023, 5:40 PM UTC

H8crilA wrote: post #66

inciampati wrote: post #61

> 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.

For the layman, what exactly is going to theoretically "explode" if spacetime is continuous?

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.

Re: A physicist who bets that gravity can’t be quantized

#74

Post by amelius » Mon, Jul 10, 2023, 5:40 PM UTC

javajosh wrote: post #40

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…

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...

Re: A physicist who bets that gravity can’t be quantized

#75

Post by omnicognate » Mon, Jul 10, 2023, 5:42 PM UTC

inciampati wrote: post #61

> 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.

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.)

Re: A physicist who bets that gravity can’t be quantized

#76

Post by qazpot » Mon, Jul 10, 2023, 5:42 PM UTC

naasking wrote: post #69

> 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. Phys…

> 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.

Re: A physicist who bets that gravity can’t be quantized

#77

Post by marcosdumay » Mon, Jul 10, 2023, 5:43 PM UTC

twic wrote: post #64

feoren wrote: post #57

Earlier quoted context omitted.

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 "μg" if you're feeling fancy.

Use of "mcg" for micrograms is common practice in medicine. I always assumed it was because it's easier to type than μg, but it seems a medical body recommends it because μg is too easily mistaken for mg: https://www.ismp.org/sites/default/files/attachments/2017-11...

> μg 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.)

Re: A physicist who bets that gravity can’t be quantized

#78

Post by amluto » Mon, Jul 10, 2023, 5:44 PM UTC

contravariant wrote: post #39

eterevsky wrote: post #8

(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 cont…

> 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.

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.

Re: A physicist who bets that gravity can’t be quantized

#79

Post by bowsamic » Mon, Jul 10, 2023, 5:45 PM UTC

H8crilA wrote: post #68

bowsamic wrote: post #14

Earlier quoted context omitted.

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 ex…

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.

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

Re: A physicist who bets that gravity can’t be quantized

#80

Post by kgwgk » Mon, Jul 10, 2023, 5:48 PM UTC

captainclam wrote: post #38

tgv wrote: post #10

Earlier quoted context omitted.

Among other problems. It's just bollocks on the level of "but you didn't say it couldn't be this, nanananana."

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 Evere…

> it is simply the consequence of removing collapse of the wavefunction as an objective event from the picture. And if it turns out our observations wouldn't be changed by removing this feature, then perhaps it was an extraneous feature in the first place!

> What we observe as the collapse of a function is simply an artifact of being a conscious being that can only observe one value of the of the wavefunction at any particular moment.

If we simply assume that our observations are as if a wave function happened we can indeed have our Schrödinger cake and eat it too.