Wrong or Unfalsifiable? String Theory's Biggest Competitor in Trouble

I was recently alerted to a video by my friend and colleague Brian Keating that claims Loop Quantum Gravity (LQG), string theory’s biggest competitor, has been disproven. I was somewhat surprised by this because I was pretty convinced it is for all practical purposes untestable -- much like string theory. I had a look at what he is talking about.

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#science #sciencenews #physics

[SabineHossenfelder]

This video comes with a quiz which you can take here:


You can now also create your own quizzes on my website! Just set up an account and a creator profile. ChatGPT will help you: Click on "Create Quiz With AI"

[DataIsBeautifulOfficial]

If Loop Quantum Gravity had a dollar for every time it got disproven, it could fund a real experiment.

[parrotraiser]

In short, physicists have tied themselves into knots with infinitesimally small strings. Have any passing topologists jumped into the fray?

[glypheye]

There’s nothing wrong with conceiving/believing theories which prove themselves wrong eventually. The key is to not go down with the ship, so to speak. Ego’s cling to arguments. They may also cling to the notion that every facet of reality is testable.

[dahlia]

My unprovable theory beat up your unprovable theory

[DrBrianKeating]

Don’t shoot the messenger- I’m just explaining what the paper authors told me and has been reported for years.

[malavoy]

I remember the first time they measured one of these gamma ray bursts around 10-12 years ago. However, they never mentioned Loop Quantum Gravity. What they were measuring was the 'smoothness' of space. Their results, within measurement precision and accuracy limits, was that space was smooth to 13 orders of magnitude smaller than the Planck length. The article did mention, however, that this invalidated string theories, as they rely on quantizing spacetime to the Planck length to avoid singularities. That was the last time I paid attention to string theory.

[BenGras]

The paper title..

Stringent Tests of Lorentz Invariance Violation from LHAASO Observations of GRB 221009A

Is that first word (STRINGent) a little poke at string theory perhaps :-) ?

[claudio-roma]

More than ten years ago (2002), Rovelli and Speziale argued that LQG does not necessarily imply violation of Lorentz invariance in the paper:

Carlo Rovelli, Simone Speziale, Reconcile Planck-scale discreteness and the Lorentz-Fitzgerald contraction, Physical Review D 67, 064019 (2003) DOI: 10.1103/PhysRevD.67.064019

In short:
1. The minimum length is not a fixed property of space-time, but a minimum value of a quantum observable. This means that there is no fixed “grid” in space-time, but rather a set of possible discrete values ​​that can be observed.
2. The eigenvalues ​​of observables, such as area, remain unchanged under Lorentz transformations. A moving observer would see the same spectrum of discrete values, with the same minimum area.
3. What changes with the Lorentz transformation is the probability distribution of observing one of these discrete values. This means that the probability of measuring a certain area may change, but the possible measurable values ​​remain the same.

[matheusduarte]

Well, to be fair, this paper just CONSTRAINED the breaking parameter of Lorentz invariance violation (LIV), and it's not even a stronger bound than previous ones. Hard to say that this disproves LQG, but I'm not an expert on the strength of LIV predicted by this model to affirm that is still viable.
(Limits on LIV is my area of research)

[tim]

Sabine, I would like to hear what you have to say about Turok and Boyle's idea about explaining the Big Bang as being a mirror and having very little more than the Standard Model. No inflation. Gravity problems are decreased by having some large number of particle-free fields that somehow remove the singularities. Dark matter is right handed neutrinos.

[BackwardsJohnsCreations]

Omg perfect! Saw the first half of the video yesterday before I fell asleep again, very excited about sabines opinion on that.

[itsawonderfullife]

Rovelli has already somewhere answered this as being a "misunderstanding" of loop quantum gravity; LQG does not claim to be not Lorentz invariant according to him.

[Robocop-qele]

Well at least Smolin et al. proposed something when string theory was the only game in town.

[GrahamChristie-jgsw]

If someone comes up with an idea that has the possibility for explaining quantum gravity, how would you know it might have merit today?, Is it a popularity contest and people work on the popular thing to find proof, or wait 20/30 years and say well he/she was on the right track and apologies for putting them in the funny farm 20 years ago. Honestly how do you tell if something has a glimpse of merit.

[francescocannistra]

Telling that LQG predicts violations of Lorentz invariance or an energy-dependent speed of light is incorrect. Therefore, the experimental evidence showing that light of different wavelengths propagates at the same speed does not impact the validity of LQG.

Lee Smolin's original suggestion was aimed at exploring how quantum gravitational effects might be tested through astronomical observations, allowing for a distinction between LQG and other theories. This was never a core statement about LQG nor a necessary prediction of the theory.

Smolin’s suggestion was, essentially: "If LQG predicted that different wavelengths of light propagate at different speeds, we could have a way to either verify or falsify the theory." However, this was a speculative idea, not a theoretical assertion or an experimental prediction of LQG.

This possibility was later clarified by Carlo Rovelli and others, who showed that this was not the case. Rovelli’s argument demonstrates that the minimal fixed length of space is a result of possible measurements, but spacetime must be understood within a quantum mechanical framework. In this framework, the discreteness of measurement outcomes is not linked to the speed of light propagation. The fixed length of space quanta would only affect the speed of light if spacetime were treated classically, which is not the case in LQG.

See also: Rovelli & Speziale's 2003 paper: "Reconcile Planck-scale discreteness and the Lorentz-Fitzgerald contraction":

[David-lcw]

Thank you for your amazingly understandable summary of loop quantum gravity and the issue regarding light speed invariance. You have a great talent for conveying the meaning of the physics instead of just throwing mathematical gobblygook at us folks of just barely above average intelligence.

[RFC]

4:02 - OMG! I just realised that my 2 year old niece is a string theorist!

[OrdenJust]

"It is possible that the Planck length is the shortest physically measurable distance, since any attempt to investigate the possible existence of shorter distances, by performing higher-energy collisions, would result in black hole production. Higher-energy collisions, rather than splitting matter into finer pieces, would simply produce bigger black holes." --from Wikipedia article on Planck Length.

So if, as Sabine says, having a smallest area is not compatible with Einstein's theory, can the same argument be deployed against the notion of a smallest possible length, i.e., the Planck length?

[Thomas-gk]

I was waiting for this since I watched Brian Keatings vid some days ago! Outstanding summary again. So good that Sabine couldn´t resist, such a lovely smile.