A nail in the coffin of Loop Quantum Gravity, or just a tack in its rubber sole?

[joigus]

LQG (loop quantum gravity) predicts the minutest dependence of the speed of light on frequency, which would be best detectable on large populations of high-energy photons with very long astrophysical paths. A good candidate to test this would be a very far away (=> very early) gamma ray burst. GRB 221009A stepped forward some years ago.

Quote

Very recently, the Large High Altitude Air Shower Observatory (LHAASO) reported the observation of the very early TeV afterglow of the brightest-of-all-time GRB 221009A, recording the highest photon statistics in the TeV band ever from a gamma-ray burst. We use this unique observation to place stringent constraints on an energy dependence of the speed of light in vacuum, a manifestation of Lorentz invariance violation (LIV) predicted by some quantum gravity (QG) theories. Our results show that the 95% confidence level lower limits on the QG energy scales are EQG,1>10 times of the Planck energy EPl for the linear, and EQG,2>6×10^-8 EPl for the quadratic LIV effects, respectively. Our limits on the quadratic LIV case improve previous best bounds by factors of 5--7.

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

Although this doesn't totally do away with LQG, it seems to rule out a vast landscape of the LQG parameter space. 

The somewhat less hyped version of these news is that we are a tad surer that LIV does not occur in Nature.

[MigL]

Way to go, Joigus.
When Super String Theory failed to meet requirements, I had some hope Loop Quantum Gravity might still come through.
Now you've dashed those hopes also ...

Seriously, it was my understanding, and it's mentioned in your link also. that SoL dependence on frequency was a characteristic of some flavors of LQG, not all.
Just exactly how large is the 'LQG landscape' ?
Surely it isn't as large as that of String Theory ?

[joigus]

23 minutes ago, MigL said:

Way to go, Joigus.
When Super String Theory failed to meet requirements, I had some hope Loop Quantum Gravity might still come through.
Now you've dashed those hopes also ...

Seriously, it was my understanding, and it's mentioned in your link also. that SoL dependence on frequency was a characteristic of some flavors of LQG, not all.
Just exactly how large is the 'LQG landscape' ?
Surely it isn't as large as that of String Theory ?

LOL, for a while I was thinking of writing "I'm sorry, MigL". But I was sure you were going to comment anyway.

No, I agree that maybe it's just a tack in the sole, as suggested in the headline. I'm no expert on LQG, I must say. But I've observed that some high-profile champions of the theory have spent some time thinking hard about extensions of the principle of SR (like doubly-special relativity, and such). There must be a compelling reason why these people have found worth their while thinking about it. Sometimes it's about how natural a hypothesis seems to be.

I think it's been observed in the past that you could reasonably try to save any crazy idea (and I'm not saying LQG is crazy) by introducing more and more ancillary hypotheses.

As to "landscape", that's just a word I loaned from an experimentalist talking about this. I think it woud be better to say "the region of parameters". 

Very shrewd counter-criticism, btw.

Edited September 6 by joigus
minor correction+minor addition

[Markus Hanke]

13 hours ago, joigus said:

Although this doesn't totally do away with LQG, it seems to rule out a vast landscape of the LQG parameter space.

Shame, that…I had some hopes for LQG. But this is just how science works.

[joigus]

4 hours ago, Markus Hanke said:

Shame, that…I had some hopes for LQG. But this is just how science works.

I take it that you see this as a nail in the coffin then?

[iNow]

While I’m sorry to see LQG further minimized as an idea, I appreciate the threads reference to what is IMO the best Beatles album 🎶 

(homonym jokes work better verbally than in text 😂)

[Mordred]

I wouldn't go quite so far as to state the article conclusively shows the Lorentz invariance violations of LQC as being inaccurate it certainly supplies strong constraints on any Lorentz invariance violations. Which does include LQC. I would think there will be subsequent rebuttals in defense of quantum gravity models with inherent LIV being published so for myself I will wait and see the rebuttals which I fully expect.

though this certainly isn't the first attempt to find evidence of spin foam using cepheids. All other attempts have also failed AFIAK.

In one of the earlier examinations it was argued that the spin-foam lattices were too miniscule to have an measurable effects in signal propagation. This was in regards to one of the earlier tests resulting from a supernova event.

Edited September 7 by Mordred

[joigus]

4 hours ago, iNow said:

While I’m sorry to see LQG further minimized as an idea, I appreciate the threads reference to what is IMO the best Beatles album 🎶 

(homonym jokes work better verbally than in text 😂)

You should take a look at exchemist's beautiful picture here then. Isn't it gneiss?

17 minutes ago, Mordred said:

I would think there will be subsequent rebuttals in defense of quantum gravity models with inherent LIV being published so for myself I will wait and see the rebuttals which I fully expect.

Oh I certainly would. Gone are the times of just a couple of fellows defending their idea against everybody else. Theorists today enlist in armies, complete with headquarters and all. I'm personally neutral in all this, btw. 

[geordief]

2 hours ago, joigus said:

7 hours ago, iNow said:

 

You should take a look at exchemist's beautiful picture here then. Isn't it gneiss

Would'n it  be gneiss if the Boy found it in the Beach?

Edited September 7 by geordief

[MigL]

I imagine L Smolin and C Rovelli are a lot more disappointed than any of us.

[Markus Hanke]

17 hours ago, joigus said:

I take it that you see this as a nail in the coffin then?

I wouldn’t put it so strongly, it just means that the data places strong constraints on which models might be viable or not.

On the other hand though we have good reason to believe that there is entropy associated with the horizon of BH’s - and since the concept of entropy only really makes sense for a system that exhibits discrete microstates in some form or anither, the interior region cannot be smooth and continuous empty space everywhere. So I’d still bet my money on some deeper structure that underlies classical spacetime, even if that turns out to not have anything to do with spin foams.

[joigus]

On 9/7/2024 at 10:41 PM, geordief said:

Would'n it  be gneiss if the Boy found it in the Beach?

I'm relieved that someone appreciated the pun!

On 9/8/2024 at 6:43 AM, Markus Hanke said:

I wouldn’t put it so strongly, it just means that the data places strong constraints on which models might be viable or not.

On the other hand though we have good reason to believe that there is entropy associated with the horizon of BH’s - and since the concept of entropy only really makes sense for a system that exhibits discrete microstates in some form or anither, the interior region cannot be smooth and continuous empty space everywhere. So I’d still bet my money on some deeper structure that underlies classical spacetime, even if that turns out to not have anything to do with spin foams.

Yes, it seems the most sensible idea that some kind of cutoff mechanism has to be applied at short distances/large momenta. For some reason GR cannot be taken as is at infinitely short space-time scales. I hear lots of noise in the direction of complexity and gravity. I wonder if there's something to it or it's just more fuss.

[Markus Hanke]

7 hours ago, joigus said:

I hear lots of noise in the direction of complexity and gravity. I wonder if there's something to it or it's just more fuss.

I’d bet there’s something to it. In fact, I’d go so far as to say that complexity, chaos and emergence in general are seriously underrated and under-utilised in modern physics. 

Just my opinion though