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A physicist who bets that gravity can’t be quantized

quantamagazine.org

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

#101

Post by Gordonjcp »

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 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.

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

#102

Post by guerrilla »

CuriouslyC wrote: post #89
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.

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 wou…

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.

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

#103

Post by AnimalMuppet »

eigenspace wrote: post #98
noslenwerdna wrote: post #71

Earlier quoted context omitted.

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

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…

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?

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

#104

Post by consilient »

tsimionescu wrote: post #83
CraftingLinks wrote: post #22

Earlier quoted context omitted.

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.

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.

> 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).

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

#105

Post by codethief »

> 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

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

#106

Post by consilient »

tgv wrote: post #84
captainclam wrote: post #38

Earlier quoted context omitted.

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…

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 i…

> 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.

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

#107

Post by tlb »

marcosdumay wrote: post #77
twic wrote: post #64

Earlier quoted context omitted.

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.)

Before Unicode, 'μ' 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.

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

#109

Post by fsh »

Gordonjcp wrote: post #101
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 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.

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.

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

#110

Post by eigenspace »

AnimalMuppet wrote: post #103
eigenspace wrote: post #98

Earlier quoted context omitted.

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…

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?

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

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