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LQG calculates the correct black hole entropy

Not really but the stupidity of the LQG proponents has reached levels that are utterly comical

Just a week ago, I discussed the incompatibility between the black hole thermodynamics and loop quantum gravity. In a new paper, Ashoke Sen updated the list of black holes whose logarithmic corrections to their entropy are calculable.

For the Schwarzschild black hole in \(d=4\), the right formula contains terms such as \((212/45-3)\ln(a)\) while the LQG folks have confidently claimed that their theory predicts the coefficient equal to \(-2\) or \(-3\). Sorry, guys, that didn't work well. ;-)

But a "slightly less intelligent" paper was pointed out by Backreaction and Physics Forums:

Entropy of non-extremal black holes from loop gravity
The stupidity of this paper by a young Gentleman called Eugenio Bianchi – no, he is not Luigi Bianchi (1856-1928) of the Bianchi identities fame – has reached such celestial levels so that even Sabine Hossenfelder was able to notice. Let me extrapolate some of these lessons (including her justified yet not really novel arguments against Lee Smolin that the nonlocality implied by "doubly special relativity" means an inconsistency) in a more respectful way: I think that her IQ could be some 10-20 points above the IQ of an average LQG researcher such as Lee Smolin.

The paper goes approximately like this.




We want to calculate the right entropy of black holes from loop quantum gravity, with the ultimate goal to show that loop quantum gravity is a competitor of string theory as a theory of quantum gravity. How do we do that?

Take a few tons of spin foam and construct a little model of Stephen Hawking, Jacob Bekenstein, and Bill Unruh. The pieces look like this:



The 6,000-views video above is the most valuable contribution to the humanity that was at least inspired by loop quantum gravity. The structure has nothing to do with physics but it is cool.

Now, make the spin foam model of Bill Unruh accelerate with a constant acceleration. Because loop quantum gravity has "quantum gravity" in its name, his observations will match those in the real world which contains quantum mechanics as well as gravity. In particular, this spin foam model of Bill Unruh must be able to calculate the temperature he experiences. In the same way, the spin foam models of Stephen Hawking and Jacob Bekenstein will do the same calculations as their real-world counterpart and they will obtain the result\[

S = \frac{A}{4G}

\] for the black hole entropy. It's great because we have just proved that loop quantum gravity obtains the right result for the black hole entropy, regardless of the value of the Barbero-Immirzi parameter that was promoted to fix factors in previous calculations of the entropy – calculations whose results were proportional to this parameter.

Or have we? ;-)

I often find it frustrating to read similar papers because there's just so much stupidity everywhere in the world that it no longer makes me laugh in most cases; I usually feel physically threatened by the combined brute power of all the idiots in the world. But this paper was different; it made me roll on the floor because its stupidity was transparent enough even for Sabine Hossenfelder.

Obviously, the paper has nothing to do with a microscopic calculation of the black hole entropy which requires that you count microstates. Instead, it is just a rewriting of the usual macroscopic or thermodynamic derivation – equivalent to the semiclassical derivations that emerged from the work by Bekenstein and Hawking – into a language that tries to sound as much loopy as possible. But the actual microscopic theory, the loop quantum gravity, isn't used anywhere. So the paper obviously doesn't do anything beyond the work done in the 1970s.

As Sabine has equally noticed, the situation is actually much worse than that. By rephrasing the semiclassical arguments – which are not quite correct but they're close enough to be called a spin foam caricature of the right calculation – in the loopy language, one is forced to say many things about the spin foams and their number of states that clearly and brutally contradict statements about the same things that exist in the loop quantum gravity literature.

In particular, the entropy of the black hole should depend on the Barbero-Immirzi parameter but Bianchi derives that it doesn't. So one gets two different values predicted "by the same theory" for the same object. That's clearly a contradiction. Obviously, the contradiction doesn't imply that mathematics is inconsistent. Instead, what it implies is that some of the assumptions had to be incorrect. The assumption that was incorrect was that the structures that are constructed from the spin foam toys ever resemble physics in a nearly flat space. They never do. That's why you can never construct an accelerated loopy Bill Unruh in a Rindler space and that's why you can't use this impossible Bill Unruh to prove a contradiction in mathematics.

Even though the author himself works hard to keep his head in the sand and not to see the flagrantly obvious fact, Bianchi's paper is a proof that loop quantum gravity is inconsistent with physics in a nearly flat space. We don't even have to discuss any curvature – the Rindler space doesn't have and doesn't require any. Even in the flat space, we may derive a contradiction. That's because the flat space, as an approximation, isn't among the environments that are allowed by loop quantum gravity.

The crackpot paper got endorsed by a large portion of the "loop quantum gravity community" that hasn't prevented Bianchi from submitting this embarrassing preprint. The stupidity that these people indirectly endorsed is so breathtaking that you may want to remember the names of some of these – if you allow me an euphemism – staggering idiots:
Thanks to A. Perez, C. Rovelli, H. Haggard, J. Russo, L. Freidel, L. Smolin, L. Modesto, N. Afshordi, R. Sorkin, T. Jacobson, W. Donnelly, and W. Wieland for many conversations on the problem of black hole entropy. I am especially grateful to C. Rovelli for precious comments and suggestions during the last stage of this work.
A nice gang, indeed. ;-)

The discussion at the Physics Forums is hilarious, too. There may be hundreds of rather detailed comments including formulae in which the participants try to make sense out of the loop quantum gravity literature on black hole thermodynamics. Obviously, certain technical yet elementary questions arise such as:
  1. Does the leading LQG result of the entropy of a large black hole depend on the Barbero-Immirzi parameter?
  2. May the degenerated faces with the spin foamy spin \(j\gt 1/2\) be neglected?
  3. Can the correlations between groups of faces be neglected when one computes the entropy at the leading order?
  4. Do the leading and subleading terms depend on physics beyond the semiclassical approximation?
  5. Is the paper XY in the LQG literature – where XY is pretty much any element of a large set – correct?
  6. Does the Barbero-Immirzi parameter run?
  7. If it runs, is this fact consistent with the discreteness of the level of the Chern-Simons theory?
  8. Should the Barbero-Immirzi parameter which was inserted as a fudge factor to mask a wrong result be correct by another fudge factor?
and many others. You may see that the answer to absolutely every elementary question of this type becomes totally open by the end of the Physics Forums discussion. The situation is totally fuzzy when it comes to absolutely everything and absolutely anything in LQG. It's clear what the right answers to these questions (except for those that depend on the nonsensical LQG magic toolkit) are in the correct theory – most of them follow from approximations to quantum gravity and every consistent full theory of quantum gravity must respect these answers.

It's clear that the analogous qualitative questions in a functional theory of quantum gravity would be instantly answered – otherwise a sensible physicist wouldn't dare to suggest that he has any theory of these phenomena at all. The term \(S=A/4G\) is universal and derivable from semiclassical gravity, the logarithmic corrections also depend on the low-energy spectrum only, some higher-order corrections may need you to know more, and so on. String theory has calculated the entropy for lots of diverse multi-parameter classes of black holes – with different dimensions, different topologies, different charges, different dual descriptions, different levels of deviations from the extremality, and so on – and whatever had to agree with the macroscopic calculations has always exactly agreed although the agreement has always looked miraculous. Despite these initial feelings of a miracle, no one is surprised anymore. Those things have to hold in a consistent theory of quantum gravity and string/M-theory is one (and quite certainly, the only one).

But none of them get answered over there, in loop quantum gravity. There aren't any answers because there isn't any fixed theory here.

Instead, what they have are just some vague ideas about mathematical tools that could be used in the desired theory – and there is a lot of wishful thinking that such a theory could be found. But every, arbitrarily basic question about the "right way" to combine the proposed "building blocks" and how to choose the right assumptions remains unresolved. Whatever combination of building blocks and assumptions is chosen always leads to some immediate contradictions. In fact, it is trivial to show that any and every combination of the assumptions and building blocks used by the LQG folks is inconsistent with physics of gravity.

So whenever these people find enough contradictions in some way of thinking, they make it fashionable to say completely different things, adopt completely different additional assumptions, and totally change all the mathematical formulae for basic things.

If you want to know the current answers to some questions in LQG, you're not studying science. Instead, this research is a part of humanities because it is all about random fads in a community of some totally irrational and mostly dishonest folks.



The amount of useless details that the people at the Physics Forums are willing to write before they become able to figure out that the theory doesn't work – and maybe they will never be able to make this trivial statement – reminds me of the following situation. They find a new, fantastic, fundamental aircraft – namely the aircraft at 8:35 constructed by a civilization that was amazingly sophisticated relatively to the LQG criteria. It's surely more advanced, faster, and more carbon-neutral than any Boeing or Airbus you can think of, these folks tell us. Now, the goal is to figure out how this bamboo aircraft was used to fly and stay in the air.

So just like the constructors of the aircraft were attempting to exploit various types of wooden airphones, their LQG descendants are trying tons of possible explanations that try to determine which part of the bamboo aircraft was lifting the mass, moving it, keeping it, and sending signals to the folks at the airport. Every combination of assumptions they try implies that the bamboo aircraft couldn't work. Every single consistency check fails. But they don't give up because it's a dogma for them that something like that has to work and no finite amount of evidence – even proofs that they're exactly at the state zero where they don't know anything at all, not even answers to the most elementary questions about the structure of their very theory so at least all the research in the last 30 years was a waste of time – can ever help them to understand that the "theory" doesn't make any sense, that it doesn't exist in any useful sense.

The zeal of these "LQG researchers" makes Osama bin Laden look like a moderate and sensible agnostic in comparison. Their inability to draw lessons out of totally elementary contradictions knows no limits. I find it unbelievable but I think that many of them actually know that LQG is just an inconsistent pile of worthless feces. They have just spent enough time and connected themselves with this stuff so tightly that their courage is simply not enough for them to admit that they have wasted years with feces – something that an honest scientist must always have the courage to admit (primarily to himself, but then also to others which is the easier part) if he wants to save his integrity and research potential for the future.

Meanwhile, the arrogance of some of these stunningly hopeless crackpots is so breathtaking that they will come to a journalist and tell him once again that they have surely discovered something that could be compared to string theory – if it's not even better! That's the case despite the fact that they demonstrably don't have the slightest clue what they're talking about even if we talk about complete basics of their "theory". They always find some journalists and some laymen who are eager to devour these feces and loudly smack their lips – because unlike the Universe, the human stupidity has been demonstrated to have no limits.

And that's the memo.

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