Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

A physicist who bets that gravity can’t be quantized

quantamagazine.org

Post reply on HN
Print view View original

111–120 of 378 posts

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

#111

Post by consilient »

light_hue_1 wrote: post #41
tauwauwau wrote: post #20

Putting it here, as it seems relevant. Veritasium: Parallel World Probably Exist: Here's Why https://www.youtube.com/watch?v=kTXTPe3wahc

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 h…

> 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.

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

#112

Post by tlb »

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…

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.

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

#113

Post by FollowingTheDao »

FollowingTheDao wrote: post #29

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…

I wish any of you with the down vtes would at least have a conversation with me...

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

#114

Post by rapjr9 »

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.

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

#115

Post by naasking »

CuriouslyC wrote: post #94
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…

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 fooli…

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

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

#117

Post by srlowe »

consilient wrote: post #99
srlowe wrote: post #91

Earlier quoted context omitted.

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

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.

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?

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

#118

Post by consilient »

tsimionescu wrote: post #65
goatlover wrote: post #47

Earlier quoted context omitted.

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.

> 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 he…

> 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.

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

#119

Post by naasking »

H8crilA wrote: post #82
naasking wrote: post #73

Earlier quoted context omitted.

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.

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 mod…

> 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

Post reply on HN