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A video on loop quantum gravity

Sabine Hossenfelder watched a 43-minute video on loop quantum gravity that was posted two days ago. She wasn't too impressed.

Well, I am always impressed by low-budget or no-budget teams that manage to shoot a semiprofessional video of this size but it's hard to avoid some criticisms. Most of them are really criticisms of the topic they chose to cover, loop quantum gravity, so they shouldn't be used against the creators of the video. And I will avoid detailed criticisms of the imperfect sound quality (noise filters were used too much at some places), the subtitles (and whether it makes sense to have a video if there exists a written form of it), and the speakers' limited rhetorical abilities.

00:00 It's strange that loop quantum gravity is being connected with the birth of stars etc. because it isn't really capable of explaining the particle spectrum and other parts of physics that are crucial for our understanding of the early Universe.

00:40 Loop quantum gravity is said to be "treating gravity quantum mechanically". While this phrase is omnipresent and seems like an innocent one, and it can even be interpreted innocently, in the strict form – which is probably one chosen by most of the speakers – it is a seed of lots of confusion. A theory of quantum gravity isn't just the same classical physics "treated" differently. It is new physics, a brand new theory that leads to different conclusions and predictions. The procedures needed to find a quantum theory with certain properties are, in general, more complicated and different than just "placing hats on the heads of all objects in classical physics". Loop quantum gravitists sort of understand that a quantum theory differs from a classical theory. But they live in the misconception that the transition from a classical theory to a quantum theory is straightforward and one-to-one. It's surely not. The space of quantum theories with certain properties is completely different from its classical counterpart. Their consistency conditions are different, and so on.

00:50 LQG is said to tell us that space has discrete units. Well, it pretty much assumes it so it's misleading to say that it's a prediction. And this assumption has indeed far-reaching consequences – far-reaching destructive consequences, to be precise. It makes the theory incompatible with the Lorentz invariance, other continuous symmetries, and it is a property that guarantees that the infinite indeterminacy seen in the non-renormalizable Einstein's theory can't be cured by this attempted theory. It's unfortunate to hype this property of LQG without asking whether it's actually a good thing or a bad thing and without making a credible attempt to answer this question. But this hyping is being done not only by filmmakers but also by most LQG proponents.

01:00 Ivan Agullo is said to be a "rising star" of the field. I've never heard of the name and INSPIRE seems to confirm it's not my fault. I don't quite get the concept of a "rising star" whose most famous paper has 52 citations shared by 5 authors. It's surely an inflation of words. What about an average young researcher in this subpar discipline?

01:10 Abhay Ashtekar is a pleasant guy. In some of his features of the smile, he resembles Cumrun Vafa (Vafa is Iranian while Ashtekar is Indian by his roots). He read a book by George Gamow as a kid. So did I. Agullo loved algebra and calculus. I don't quite understand why we're told these stories about the childhood of someone whose contribution to physics still remains completely unknown to me – and all other viewers.

02:20 Are you afraid that Einstein failed and you're trying to solve it? Agullo says Yes but it's OK because many people are working on it. Well, many people with much lower capabilities couldn't replace the genius of one individual. What allows modern physicists to go beyond Einstein is their much better mastery or quantum mechanics and related portions of physics. Unfortunately, this can't be said about the LQG researchers which is another reason why they should be afraid of their inevitable failure.

02:50 Ashtekar: Einstein's GR is geometric and predictive, QM is different, probabilistic and algebraic. There should be a unifying theory. These sound like innocent words for every "unifier" of gravity and QM. However, the vague formulation about the two limits – which are completely different and which impose very different restrictions on the unified theory – is a seed of confusion within LQG. In particular, the postulates of QM have to be exact and they affect every dynamical degree of freedom and interaction in physics. On the contrary, the foundations of GR are about one particular field only, the metric tensor or the field communicating gravity, and its interactions with other fields while most of the other degrees of freedom remain unconstrained. Moreover, the original equations of GR don't have a reason to be exact and aren't exact. LQG folks don't understand these differences.

04:05 Ashtekar: Gravity is encoded in the very geometry of spacetime. Well, the same is true for every theory that respects the equivalence principle, e.g. string theory. However, in string theory, one may derive such conclusions from a more fundamental starting point. Ashtekar says things like "there is no background and everything is interacting with everything else". These two statements aren't equivalent. The background is just a condensate that must be allowed to develop and exist. So if you can't get background from your theory, it's doomed. You can't mask this doom by demagogic identifications of the lethal flaw "there is no background" (bad) with the principle that "everything is interacting with everything else" (true, it does in pretty much every theory).

05:44 Comparison with string theory. LQG folks from the GR background, ST came from particle physics. So we're told by Ashtekar that string theorists are focused on the unification of forces and not the peculiar properties of the spacetime itself. That's just untrue. String theory has discovered and still studies lots of new remarkable properties of the quantum spacetime – T-duality, mirror symmetry, other dualities, holography, entanglement is a glue or wormhole, emergence of spacetime from other degrees of freedom, topology change – flops, conifolds, others. It's just a lie that string theory isn't fascinated and fascinating us with new insights about the peculiar properties of the spacetime in the quantum theory.

06:20 In string theory, SUSY and extra dimensions are essential, in LQG, they're not. Friendly speculations about the cooperation of LQG and ST. Not really realistic but OK.

07:30 Agullo about the Big Bang singularity. GR fails over there. He uses "loop quantum gravity" and "loop quantum cosmology" in the same sentence. It's pretty bad that they don't even explain enough to explain the viewer that these are two things and what the relationship between them is. Sabine has already complained about it. LQC isn't really a limit or special case of LQG; it is a simplified toy model of it. It suffers from all the problems of LQG and more. The "cure" of the singularity is presented as an extra quantum-caused repulsive force that creates a bounce. I find this 1) a wishful thinking, 2) a naive strategy to deal with the problem. Such a bounce only moves all the problems to (even more serious problems with) the pre-big-bang cosmology and it doesn't seem one has made any progress at all.

08:50 Sponge has a maximum capacity to hold water, and space has similarly a maximum density. That's just stupid. Even semiclassical GR is enough to see that the density is maximized for the minimum-size i.e. Planckian black hole when it's Planckian, too. But this is really just a transitory regime, not an extreme one. Lighter black holes are like elementary particles and their density isn't too well-defined or isn't finite because they're pointlike in some very specific sense. Larger black holes have a much lower than the Planckian density, and so do all other celestial bodies. There's really no way to make a whole "filled sponge", a macroscopic piece of the maximum-density "material" of space, and if a theory predicts otherwise, it's immediately dead. It's dead because the final stage of the collapse of localized matter is a large black hole whose density (mass/volume) is much lower.

09:50 Big bounce instead of Big Bang. Agullo says the naive things about the bounce, without really solving anything. One could really embed this "solution" even to classical GR as long as one adds some (non-local) interaction. But we really do know there was a Big Bang and by the second law, the entropy "before it" if there was any "before" couldn't have been higher than during the Big Bang. Agullo explains terms in a high-school-level equation but it's both incomprehensible and inadequate for analyses of extreme physical conditions such as the Big Bang.

12:00 Ashtekar says that this bounce force comes out automatically blah blah blah. He's happy about it. It's hard to see where this satisfaction comes from. Every new theory predicts some new effects. A generic effect is bound to get corrections from the new physics. This is not yet a reason to be happy. It is not a consistency check analogous to those in string theory. There are no successful consistency checks in LQG.

13:00 The female narrator starts to talk about cosmic inflation. What does LQG say? Ashtekar says confusing things about the probability of inflation that is small at some moments and so on. Bizarre. Inflation is a generic process that arises in theories with scalar fields and viable enough potentials that are surely not "infinitely unlikely". It arises whenever a region of space sees a scalar field sitting near the maximum of the slow-roll potential. It even arises without the slow-roll potential. Once the small region obeys this condition, which must be clearly allowed because it's an allowed configuration, exponentially large volumes of inflating space will be created. All the "inflation is unlikely" criticism trying to argue otherwise is complete unscientific gibberish, and so is Ashtekar's story about this topic.

14:40 Ashtekar also praises LQC for giving us a preferred instant, the bounce, while the Planck time after the Big Bang isn't special. Those comments are extremely special, too. The Planck scale surely is special in a theory of quantum gravity. Special things tend to occur there or its vicinity by dimensional analysis. On the other hand, the bounce in LQC isn't special, at least not the exact moment, because the evolution isn't time-reversal-symmetric (e.g. because the second law of thermodynamics) so the volume-minimizing instant is just a convention to choose a moment in the middle, and not the only convention. So the situations are equally special or non-special.

16:00 She asks a surprisingly sensible question – doesn't the observed positive cosmological constant rule out the recollapse, namely the bounce? Agullo says that LQC doesn't predict that there will be a recollapse. That's nice but it does say t – or at least they do say - that there was a bounce so it was a collapse of an earlier Universe. One has problems with this in a positive C.C. world, too, especially because the pre-big-bang universe had to be more or less maximally symmetric due to its low entropy.

17:40 Why was the entropy low at the beginning – which is not the beginning in the bounce models? A good question. Agullo says that the entropy was "reset". Wow. I suppose he means that an agent reduced a high positive value to zero. Jesus Christ? This is a flagrant violation of the second law and it is enough to kill any model with this feature. He talks this rubbish to a laywoman for many minutes and she has no way to learn that it is rubbish contradicting basic physics, even physics taught to undergraduates.

19:30 Ashtekar correctly says that we usually count matter entropy and not horizon entropy. But he doesn't elaborate upon this good point. One has to be a bit careful about double-counting here. Well, the matter entropy is always parameterically lower than the horizon entropy so if we count the horizon entropy, it beats the matter entropy, anyway. Ashtekar is trying to solve the same problem as Agullo by equally if not more childish comments. The entropy is "infinite" at the bounce and it drops to zero because the horizon disappears. But this is just an oxymoron. The event horizon is always defined relatively to the causal structure of the whole future spacetime. So if it's somewhere, it just can't disappear. Ashtekar's comments about the disappearance make no sense and they're added on top of Agullo's nonsense about the "reset entropy". Moreover, the idea that one may just ignore the decreasing matter entropy is also wrong. One may only ignore the disappearing matter entropy in GR if the matter ends in a singularity. But if it doesn't, the matter density separately can't decrease, either. Those folks are a community of sloppy folks who just support each other when they talk vague physical nonsense.

21:50 Resolution of black hole singularities. There are no precise calculations, we hear. They hype a Pullin-Gambini paper that happens to be about a similar question. The singularity is replaced by a new large spacetime. You know, that's a big problem, something that would revive all the information loss puzzles after they were solved.

22:50 She asks about Alan Guth who dared to publish a paper that bounces are prohibited. Agullo says that Guth assumed classical GR near the bounce while LQG violates it. Ashtekar agrees. She also asks about other problems of contracting universes. Agullo just says that there are "papers" and they imply that singularities don't appear. But this really means that the theory disagrees with GR in rather mundane contexts, a point that isn't noticed at all.

25:00 Ashtekar is worshiping the repulsive force. I can't get rid of the feeling that these people still live somewhere in the 17th century and they're adding terms to Newton's equations. Physics no longer operates like that and similar extra terms aren't considered fundamental or interesting. Ashtekar says that unlike previous attempts, LQC has a way to make a "cube of size 10 Planck lengths" uniform. I suppose that the mechanism he refers to is the Jesus Christ who resets the entropy and flattens the region. That's nice but the Jesuits already had the same mechanism (Jesus Christ) many centuries ago. All this stuff is ludicrous. Inflation and things equivalent to it is the only known actually operational mechanism to produce flattened, nearly empty large regions of space out of a different environment.

25:45 Ashtekar is asked about the proposal by Smolin – whom he doesn't like – and a physicist with a Polish name that black hole singularities are big bangs of other universes. Asthtekar thinks that this can't be the case because the singularities are different.

26:50 Ashtekar is asked whether inflation may be eternal according to LQC. He doesn't seem to have an answer. He has no intuition, we hear, and asks others to do a strange calculation.

28:35 Agullo is asked why the previous Universe contracted. He says we don't have observational access to this question. That's a fun excuse because if you claim to have a theory replacing the Big Bang singularity, it should give you a mechanism for that process, too. After all, we don't have the observational access to the Big Bang singularity which doesn't mean that we can sweep it under the rug. He's using different standards for his pet theories and the (more viable) conventional theories. Agullo says that quantum effects wash out the information about the previous phase. Great. But the theory should have the information about the overall evolution, otherwise it says nothing about the early Universe.

29:10 The female narrator complains that a paper by Guth et al. (Borde, Vilenkin, Guth) didn't refer to LQG. Well, Alan Guth rarely cites crackpots in general. Agullo meaningfully explains the paper, saying that it claims that inflation can't be eternal in the past. At the end, he says that the theorem may be ignored but there is no justification except for foggy claims that we don't have the final theory yet. A great power of the theorem is that they may prove many things even before we know everything else, a point that Agullo clearly fails to grasp. Ashtekar says that the theorem doesn't apply to LQC because LQC has a contracting phase, something assumed to be absent in the theorem.

32:25 It's hard to test quantum gravity experimentally; interesting that no one dares to criticize these folks for this fact of Nature. Random wrong papers about the Lorentz violation by gamma-ray bursts, wrong ways in which black holes should evaporate, and so on are hyped. The CMB is most promising for Agullo except that his LQG has nothing to say about it.

34:00 Ashtekar says that everything we observe is compatible with LQC. Wow. An unexpected conclusion after half-an-hour of hyping the difference between LQC and the theories that actually do agree with the data.

35:40 Violations of Lorentz invariance due to the discreteness space were experimentally falsified "while other experiments have confirmed it" (the latter is bullshit, all these positive signals have been debunked or explained as being due to a more mundane effect). Ashtekar says that this violation of relativity is a robust prediction of LQG, in analogy with crystals in condensed matter physics. Very true: LQG imagines that the world literally is a sort of a crystal. So why doesn't he see that LQG has been ruled out? He wants to believe that the correction only contribute at the second order. But even the (experimentally falsified) first-order correction is a wild underestimate of the huge violations of relativity that a fundamentally Lorentz-violating theory predicts!

37:10 Detection of gravitational waves, polarization of CMB, etc. What does LQG have to say about these matters? The only honest answer, nothing, isn't heard.

38:50 Small black holes at the LHC. They would be great, Agullo says. All this stuff is incompatible with the LHC because black holes may only appear at the LHC with the extra Randall-Sundrum warped dimensions which aren't a part of LQG. Agullo doesn't explain this fundamental thing – either he doesn't understand it or he finds it beneficial to hide the fact.

39:50 The narrator talks about a Planck press conference, anomalies, differences from WMAP. The video doesn't look honest concerning the question who authored these comments. They're not hers, are they? Agullo talks about some perturbations from the pre-big-bang cosmology and while the gap between the conventional physics and LQC is deep, he blurs the boundaries so that it is often completely unclear whether he is talking about the LQC research or proper physics. Ashtekar admits he doesn't understand these anomalies so he offers general wishful thinking about learning things about the initial state instead. We're told that Planck data are compatible with LQC except that LQC predicts nothing that looks like the real world, and talking about details of Planck data in this context is way over the edge.

The narrator wraps the video by comments about an entirely new vista that the bounce may open.

I don't plan to proofread this text.

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snail feedback (35) :

reader Dilaton said...

Dear Lumo,

thanks for this nice discussion of what is in the video.

I had almost no clue about how these guys would describe cosmological issues, but to me it seem neither they themself have a clue quite often ..

I once wanted to know how they explain the expansion of space microscopically here,

but as David Zaslavsky summerized, the same no clue answer seems to apply, at least back then ...

reader Giotis said...

“02:20 Are you afraid that Einstein failed and you're trying to solve it?”

I think there is confusion here. Einstein was not trying to quantize Gravity but to unify in a classical manner Electromagnetism and Gravity i.e. the known forces back then. LQG on the other hand is not trying to unify the fundamental forces (they have nothing to say on this issue) but to quantize Gravity.

reader buzzyoung said...

this is what happens when the west takes on massive debt to fund smolinfied "physics" and the smolinifcation of physics.

handwaving hype, unclear thoughts, no testable experiments proposed, insider cronyism, atheistic regimes where smolin reigns supreme, arrogance, and hatred of truth, beauty, and eternity.

if you would like to see the nature of the indecipherable, nasaly, holier-than-thou sophist a**hole from where LQG sputtered out like watery poop and diarreaha, watch this video:

WTF is he saying? never before in the history of science has someone recieved so much $$$$ for failure after failure after poooopy failure.

reader Giotis said...

Also around 3:20 Ashtekar says that QM and GR are special cases of QG.

This is the wrong way to see things I believe. QM is not a special case of anything. QM describes how the nature works on its fundamental level. GR on the other hand is indeed an effective theory, a classical limit of a QM theory…

reader Euclides said...

Clearly the world is properly described quantum mechanicaly. But, I
simply don't buy the idea that we need QM as the only way out of
gravitational singularities. For instance, torsion can easily avoid this
messy topic, without loops or strings, while keeping compatibility with
GR at well stablished experimental level ( ).

reader NikFromNYC said...

Where/why does time overlap math?

reader anna v said...

Thanks for this resume . Ammunition for discussions with LQG proponents.

reader Vladimir Vrhovski said...

Thanx for the video . The female narrator is hot . I'll send her an email .

reader Dimension10 (Abhimanyu PS) said...

If there were LaTeX in the comments, the "$$$$" would have disappeared .

reader Dimension10 (Abhimanyu PS) said...

Is LQG dead? Sorry for the perimetre institute link, but :

reader Phil Jones said...

The download time is really slow, but if you're patient enough to download Dimension10's link, you see that Martin Bojowald's final slide says, in brief:

"LQC has failed (in an interesting way)....Some problems shared with full theory (meaning LQG)....
What remains is too formal, improvised, overhyped."

This is MARTIN BOJOWALD, one of the champions of LQC, talking. Respect to him for being up front and honest.


Phil (twistor, not skydive!)

reader Gordon said...

Awhile back, I remember on Physics SE, Carlo Rovelli saying that LQG did NOT violate Lorentz Invariance, and that he had written papers about this.
IMO what it boiled down to was his use of the Bellman's rule of three ("Anything I say three times is true"--L Carrol, The Hunting of the Snark) :)
WRT Smolin:
"Ashtekar is asked about the proposal by Smolin – whom he doesn't like "...aside from publishers and assorted magazine editors, and himself, does anyone really?

reader smoitz said...

When most people take a dump, they flush the toilet.

Smolin picks up the turd in his hand, puts it on a golden platter, and serves it up as "physics."

Thus we have gthe fundamental Smolin equation:

Smolin Shit = Physics

And thus we have hype, failure, deceit, failure, hype, failure, deceit, lies, failure, LQG funded by millions, failure, mass media nuclear media firestorms, failure, lies, failure, time failure, "Garrett Lisi is the next Einstein," failure, as long story short, Lee Smolin's shit is not physics.

Smolin funds entire legions of useful idiots with his money that puts the west in debt:

In the comments is another Smolin sockpuppet, claiming:

"His E8 theory was not "debunked" as you call it. It is in development and he has published several peer reviewed papers about it. Science is a process, it takes a while and you got twists and turns. Einstein was wrong most of the time while he was deducing relativity."


Smolin's greatest achievement has been the creation of the Garrett Lisi wikipeida pages (Lisipedia) pages.

Smolin would crap out a piece of poop, sell the poop as physics to the trusting journalists, and create a vast nuclear media firestorm "exalting Garrett Lisi as the next Einstein." Then Lisi/Smolin/Woit would harvest the nuclear fallout, repeat it on their blogs, and place it on a wikipeida page. Then, when it was all shown to be just another turd from Smolin, they let the wikipedia lisipedia page stand as an everlasting monument to the Smolin equation:

Smolin Shit = Physics

Smolin is doing the exact same thing with LQG:

LQG = Smolin Shit = Physics

Smolin is a one-trick pony with shit, after shit, after shit bolstered by ambitious arrogance and a hate for god and physics.

reader smwoitbeelisi said...

Is it not great how the hallmark of Woit, Smolin, and Sabine Hossenfender (Bee) attack all the crackpots who remind them too much of themselves? For instance Woit is violently attacking Alexander Unzicker for saying the *EXACT* same thing that Woit says. After first hyping Lisi as the next einstein to the press, Smolin then attacked Garrett Lisi and threw him under the bus once Lisi started reminding Smolin of himself too much, with Lisi's lust for the spotlight for pure, unpublished, unsceintific bullshit surpassing Smolin's lust for media attention for the shit he craps and tries to pass off as physics. And now Sabine Hossenfender (Bee) is violently attacking those who merely express the true nature of LQG, which Sabine Hossenfender (Bee) blames on her socks/a bad mood/that time of the month like feminists and Lee Smolin oft do.

The Smolin/Sabine/Woit/Lisi team are bankrupt losers incapable of physics. It is hilarious and fun to watch their empire of lies, deceit, and hatred of physics crumbling, as their glaring hypocrisy and true self-hatred shines forth. They are imploding under the weight of all the lies and hype they have spat forth, as the towering stack of Smolin turds are too much for even Smolin/Sabine/Woit/Lisi to bear anymore.

It was just yesterday Sabine was hyping Smolin's/Lisi's lies as "science:"

Sabine wrote, "I think Garrett's paper has the potential to become a very important contribution, and his approach is worth further examination."

What an idiot! And not only that, but an immoral coward too, as she never retracted her idiotic statemnt fater the SMolin meida firestorm was debunked by SMolin himself!

reader Dilaton said...

Too bad, skydive would be even more awesome for journalists than surfer or biker dude ... ;-)

reader NumCracker said...

Dear Lubos, if these loop guys have any reason in their researches, shouldn't "Loop Quantum Supergravity" be the UV completion of SUGRA, and so, reproduce all results of Matrix-models of M-theory? In this same vein, could not be this a way to prove they are really doing no real physics? I mean, ST people would compare predictions from LQSUGRA to BFSS model and see what happens. Thanks

reader Giotis said...

Well they could start by reproducing GR at some limit and then move to more elaborate cases :-)

reader francis said...

think about how much damage smolin/bee/woit have done to physics over the past four decades.

failure, after failure, after poopy failure, after failure hyped to the press as "THE NEXT EINSTEIN" and "SMOLIN-STYLE FABULOUS PHYSIKS!" and "GOD SUX! ATHEIST LISI IS GREATER THAN GODZ! LOOP LOOP LOOP RHYMES WITH POOP POOP POOP!"

Yes perhaps it was a typo when Smolin crapped and meant to send out to his useful idiot media contacts that he had just come up with Poop Quantum Gravity (PQG) but one of the useful idiots wrote it as Loop Quantum Gravity (LQG) and Smolin stuck with the name, even though it smelled like a turd.

the amazing thing is how much funding smolin has received and continues to receive.

who funds bee to fret out her favorite pair of socks/sockpuppets on a "physics" blog? why does she get so much cashola to hype lisi's lame lies?

reader Luboš Motl said...

Dear NumCracker, I agree with Giotis. It would be great - and the natural goal - to reproduce GR. To ask them to reproduce things like SUGRA or Matrix theory is clearly way too much.

To be more precise, it is spectacularly clear that LQG not only fails to reproduce the details of SUGRA or BFSS or anything else that is well-defined in quantum gravity. It directly contradicts some of the very fundamental general features of space and time as we have learned it.

Of course, a possible answer is that the totally different picture painted by LQG is the right one and the picture seen in various approaches to string theory are all wrong. The internal coherence of the string theory descriptions as well as the internal leprosy of the LQG ideas are two reasons among many to think that these two competitors aren't created equal.

reader warnerheisenberg said...

Funny how Smolin, Woit, and Bee are all horrified when they look in a mirror and see themselves. Funny how much they all hate their very own reflections.

When Lee Smolin and Peter Woit beheld Lisi soaking up all the media spotlight for his crackpottery/wikipedia sockpuppetry they had to throw him under the bus as his unholy lust for the spotlight and crackpottery reminded them too much of themselves.

When Bee beheld the creators of the LQG (or PQG=Poop Quantum Gravity) crackpottery, she had to throw them under the bus as the young whippersnappers reminded her too much of herself.

When Peter Woit witnessed a German (and heaven forbid a woman!) parroting Peter Woit and echoing his crackpottery, he had to throw them under the bus, probably so Columbia wouldn't cancel his contract as a computer administrator.

This is a classic case of symmetry-breaking--Woit/Smolin/Bee must forever seek to break and destroy their very own reflections. For truly, these scummy cums create nothing, but have only ever gotten ahead by hype, PR subterfuge, sockpuppeting tom-foolery, and tearing others down.

reader Brooke Anstey said...

In string theory, SUSY and extra dimensions are essential, in LQG,
they're not. Friendly speculations about the cooperation of LQG and ST.
Not really realistic but OK.

como funciona spybubble

reader Dilaton said...

... hm, to me it looks like Alexander Unzicker is a (very bad!) guy though ... ;-)

reader Dilaton said...

Hi Lumo,

sorry for the off topic but can you say something about this?

It seems to be a nice question and nobody seems to be able to give him an answer, even though the asker (he came from MathOverflow) has given the shirt off his back to offer a non-negligible bounty because he really wants to know...
It is a pitty that such question do no longer find answers, on TP.SE it would probably have workd better...

Maybe you can help him a bit ;-) ?

reader Luboš Motl said...

Dear Dilaton, a very serious question, indeed. ;-) No, I don't know the answer or the way to answer it.

reader Dilaton said...

Ok thanks Lumo for having a look at it, if you can not help it is really difficult to find somebod who can I guess ... ;-).

By the way did you receive my recent email message from about two days ago? I am not sure if my darn webmailer has swallowed it, or what is more likely that you have obtained it but it was a bit over the top ...

If you just did not feel like replying because the second is true, I just apply the "no answer IS an answer too" interpretation and apologize for bothering you. It just is that I had a very nice chat

with a nice young theoretical physicist about the issues I mailed about...


reader Dimension10 (Abhimanyu PS) said... ???

reader Dimension10 (Abhimanyu PS) said...

Yup, I remember that too . When I checked the paper, it seemed to use holography which LQG, for all I know, doesn't satisfy .

reader Dilaton said...

Hi Dimension10,

I have seen that you asked about Neo's suspension, and I think it is just horrible how Manishearth very actively prevents any mentioning of such things, which are with a very high probability very unjust. Concerning such things as dishing out draconic punishments for disagreeing or political reasons to silence people, sweeping all traces that something bad happend under the rug, etc strongly reminds me of how the regimes of non-democratic dictatorial states of all kinds in the real world behave ...

My guess about what happend to Neo,

who sometimes was capable of asking rather interesting questions, is that he too strongly disagreed with his plasma physics question (which I still say should not be closed and it even had 3 or 4 reopen votes!)

was scapegoated in the course of the controversial recommendation policy discussions, that can hopefully soon be resolved for good. You can see how he disagreed by looking at the edit history of this question.

I strongly disagree with dishing out such draconic punisments, for doing (almost) nothing wrong, shrugging everything under the rug etc...

Recently, from some "constructive" (tongue in check) discussions with Shog9 (even though I still disagree with him for banning Ron and having done nothing to deescalate the situation then), that not the SE network pers se is what is evil. The SE rules etc are not that stringent as the people in power, for example on our Physics site often claim, they never explicitely forbid book/reference/list question etc... They give the individual sites and communitie quite some freedom to determine there own rule in principle.

So the really bad things happen, if the people in power persue enforcing their own personal opinions or do otherwise bad things ... :-/. With good reasonable moderators that take the experts and more generally the community serious, there would be no (or much less problems) ...

reader Dilaton said...

BTW I had a nice chat with Logan M, who has written an answer to the popular-science burnicate request too

No Manishearth is reading this room which is scary, Logan should not have mad it readable for everybody ... :-/

reader Dimension10 (Abhimanyu PS) said...

That's automatic, it's not possible to make it invisible . If you see, the privelege to create a gallery chat room is awarded at 1000 rep , but it's not possible to make invisible chat - rooms (except to moderators, probably.) .

reader Dilaton said...

I see, now I have once again the bad feeling of being hunted or persecuted ... :-/

I think we both have to be careful, since getting virtually killed by mouse click does not improve the freedom and level of the site for people who want to learn and enjoy Physics ...

May strategy is to enjoy physics there myself and do helpful things that improve the site, prevent bad things, etc as I can.

For example I have pinged Shog9 to have a look at the popular-science issue. Of course it is not guaranteed that this will lead to the by us favored outcome. But from recent discussions with him a have slight hopes, and I think every reasonable unbiased outsider looking at the issue will see that Manishearth tries to enforce his personal opinion (which is not sheared by the majority of the community) way to much. So I think things can not get worse either ...

But the last thing I mentioned may be dangerous sometimes ...

reader Dimension10 (Abhimanyu PS) said...

I don't exactly understand what you mean about the "It pretty much assumes it.", though. According to most LQG sources, they obtain the volume operator and find the volume spectrum and show it's discrete.

Do you mean that they assume the quantum geometry (using volume operators) and that' is what leads to the discrete spacetime results? .

reader Luboš Motl said...

Dear Dimension10, LQG assumes the discreteness of the areas (which are more well-defined than the volume operators) at the moment when it says that the (complementary variable to the 3D spatial metric) is given by a gauge field.

The integral of a gauge field is a Wilson line and it's a periodic variable spanning a compact target space, unlike the noncompact configuration space of the metric. Consequently, the switch from the metric to the "new variables of LQG" by Ashtekar is not a one-to-one map. The compactness of the Wilson lines is equivalent to the discreteness of the dual observables which turn out to be areas.

All of this is sort of trivial where the discreteness comes from. In proper quantum gravity, the areas are of course not discrete - they're really ill-defined operators. The discreteness in LQG is a consequence of the map to "new variables" that isn't one-to-one - the same failure of the field redefinition to be one-to-one also makes it inevitable that LQG violates the Lorentz invariance. After all, one may show independently that a Lorentz-invariant theory of the spacetime simply can't impose any discreteness of areas (or other geometric quantities) because in a Lorentz-invariant theory, they must be allowed to continuously Lorentz contract.

reader Dimension10 (Abhimanyu PS) said...

Thank you for the wonderful explanation!

reader John Panuccio said...

Nice Article thanks for sharing this information ..

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