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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

#311

Post by eigenket »

cryptonector wrote: post #268
causality0 wrote: post #265

Out of curiosity, what's the quantum angle limit? If position is quantized and all fundamental particle are isotropic, there must be a minimum "angle of turn" for all objects, and by extension, a "maximum angular accuracy" for something like, say, orienting the face of a macroscopic object to face a particular direction. How many of those fit in a circle?

Between space quantization (Planck length) and the size of the visible universe, the minimum angle is fantastically small.

The Planck length does not mean anything like space quantization. It's just a length that pops out when you combine several physical constants. The values of those constants mean that the Planck scale is likely to be roughly where gravitational and quantum effects are both relevant but other than that it has no physical significance.

I have no idea where this idea that space is fundamental discrete, and divided up into chunks of Planck length voxels or something comes from, but there is absolutely no evidence for it.

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

#312

Post by raattgift »

And now for something not completely different: an introductory lecture in Perturbative Quantum Gravity (PQG) by Prof. John Donoghue. The lecture is one in the Basics of Quantum Gravity online school sponsored by https://isqg.org/ the International Society for Quantum Gravity. PQG stands in contrast to the approach in the fine article at the top ("TFA") and to semiclassical gravity.

By coincidence a retweet this morning re-surfaced a tweet from a couple of weeks ago by @BahramShakerin linking to these three 2023 lectures in Perturbative Quantum Gravity given a few weeks ago at École Polytechnique Fédérale de Lausanne. This material may be useful to anyone genuinely interested in constructing GR as a "second quantization" QFT, in contrast to the Oppenheim classical gravity - second-quantized quantum matter (CQ) approach whose surface was scratched in the quantamagazine link at the top.

As there are many comments which raised questions or made ... uh ... assertions about things like gravitons, there is probably some interest here, thus this comment. Additionally, problems in semiclassical gravity and in perturbative quantum gravity motivated the work discussed in TFA.

Lecture outline and links to materials:

https://blogs.umass.edu/grqft/

https://indico.cern.ch/event/1279592/ (pqg PDF links at lower right; links to the lecture notes of related lectures are a little above that).

Lecture notes

https://arxiv.org/abs/1702.00319

Three videos of zoom lectures

https://www.youtube.com/watch?v=ik0C-xTnK-k

https://www.youtube.com/watch?v=lYc6oicfo3U

https://www.youtube.com/watch?v=UOsRyYTwOgs

and finally

Professor Donoghue's page:

https://blogs.umass.edu/donoghue/

Extra credit, `t Hooft's PQG lecture notes: https://webspace.science.uu.nl/~hooft101/lectures/erice02.pd...

The twitter thread is:

https://twitter.com/BahramShakerin/status/166874044067626189...

PS: QG2023 (by ISQG) runs yesterday (10 July) until 14 July, see https://www.youtube.com/@isqg423 for live streams and starting here https://indico.imapp.ru.nl/event/106/ for info about the conference.

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

#313

Post by goatlover »

light_hue_1 wrote: post #293
goatlover wrote: post #124

Earlier quoted context omitted.

> 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…

> They do if they're going into foundations of physics People don't do PhDs on this topic because it's not productive. Famous/bored/tenured/rich people pitch in arguments for fun. Anyway, the vast majority of work in foundations of physics has nothing to do with these interpretations https://link.springer.com/journal/10701/volumes-and-issues This is a niche inside a niche in a mostly irrelevant field. No advance in p…

I'm just repeating arguments that physicists like David Deustch, Sean Carrol, David Wallace, Sabine Hossenfelder have made, because they sound like good arguments to me. Better than ignoring the issues you get with shut up and calculate. Maybe you should take the issue up with them? Perhaps you think throwing out the term antivaxer makes you smarter than Einstein, Schrodinger and Bell?

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

#314

Post by GoblinSlayer »

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…

Even for small particles superposition can span arbitrarily large distances. I think it will be difficult for classical gravity to gloss over such distances. And the rationale is strange: information paradox is pure math unrelated to reality, it doesn't warrant a new theory.

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

#315

Post by Gordonjcp »

cornel_io wrote: post #243
Gordonjcp wrote: post #236

Earlier quoted context omitted.

[flagged]

I think it didn't land because so many people are in the comments seriously arguing almost exactly what you were saying as a joke. Tough environment for parody to thrive in, alas...

I'm currently spending a lot of time in La-La-Laplacian Land, so maybe my sense of humour is currently a bit warped. A bit like my filter responses, really.

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

#316

Post by pyinstallwoes »

codethief wrote: post #105

> 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…

Perhaps it’s a fictitious force just like centrifugal motion and otherwise. :)

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

#317

Post by GoblinSlayer »

zmgsabst wrote: post #271
xmcqdpt2 wrote: post #260

Earlier quoted context omitted.

Superdeterminism is simply the idea that all quantum experiments results could have been known before performing them, assuming a perfect knowledge of the state of the universe. It's one of those things that could be true and explain all of QM but also is kind of a cop out. "Of course your two detectors are giving correlated results, they were tightly coupled 13.8 billion years ago and now they are forever linked lik…

How is it a cop out? We know two things: - universe was small enough everything was tightly coupled - non-local phenomena occur Insisting that our beliefs reflect an ideology (eg, you can segregate off portions of reality to study in isolation) which seem contrary to observed reality (eg, the points above) is religion — not a scientific investigation of the universe.

>- non-local phenomena occur

No. Also superdeterminism doesn't have non-local phenomena. The idea is that measurements are predetermined to look like quantum.

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

#318

Post by raattgift »

amluto wrote: post #78
contravariant wrote: post #39

Earlier quoted context omitted.

> 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.

I don't know but I doubt it. That said, just now at the QG2023 conference a member of the Aspelmeyer group at IQOQI Univ. Vienna is presenting their work on an experiment where a mirror is mounted on a Cavendish torsion pendulum, and in order to do calibration and exclude sources of noises they have borrowed some protocols from LIGO. (Their lab is in central Vienna, near a couple of tram lines, and a 90-tonne tram passing 70m away induces a gravitational attraction on the same order as their test mass! When they test at night, they can also see the night busses in their spectral data.)

So there are similar issues with noise, but the mirror they're using in Vienna is small and light enough to suspend from a thin wire.

There are different things being detected: LIGO is interested in gravitational waves from the quadrupole moment of fairly-quickly-mutually-orbiting astrophysical binaries with many-light-second separations; this experimentalist (who just disclaimed deep theoretical familiarity as opposed to focusing on developing experiments) is looking for measuring the gravitational influence of very small masses (~microgram, with a move to a less-busy test location) and across very short distances (and his collaborators are interested in putting ever larger (but still PS: the speaker was Hans Hepach, a PhD student in the Aspelmeyer Group.

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

#319

Post by tsimionescu »

goatlover wrote: post #291
tsimionescu wrote: post #139

Earlier quoted context omitted.

> 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…

Measurements are interactions that result in entanglement. Atoms radioactively decaying in your body are interactions. The environment is the entire universe as it becomes entangled with a quantum event.

Entanglement is not enough to give you the properties of measurements. While it is often presented as "when a spin-0 particle decays into two particles, one must have spin +1 and the other spin -1", that is not what the math actually says.

What the math actually says is that the spin of each particle is a linear combination of |+1> and |-1>, and that their sum must be 0. For example, one of the entangled particles could have state 1/3|+1> + 2/3|-1>, and the other would have 2/3|+1> + 1/3|-1> (or something similar, I'm a bit fuzzy on the exact math). There is nothing that says that particles must be in pure states from their own perspective. And yet, we only ever observe pure states (with some probability) in our own perspectives. So, there must be some additional law that favors these states.

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

#320

Post by GoblinSlayer »

zmgsabst wrote: post #302
drdeca wrote: post #275

Earlier quoted context omitted.

First, as a religious person: no, contrary to what you seem to be saying, dismissing superdeterminism isn't "religion". Second, dismissing superdeterminism is not, as you seem to say, "contrary to observed reality". The belief that one is capable of forming ideas that bear any resemblance to reality, is justified on account of, in order for one's beliefs or lack-thereof to have any use, it would have to be true. Thin…

Except that, we know such macro correlations exist — for the two reasons I outlined. We know that the universe was once highly correlated, due to its size, and we know that such correlations form super-macro structures, from galaxies to filaments. We further know that knots in the wave equation are a non-local phenomena, such as with anyons. The minimal assumption is that any particle we encounter is part of such a n…

Superdeterminism is fine tuning, not a mere correlation. A mere correlation is hidden variables theory.
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