Two weeks ago, the Strings 2015 annual conference began in Bengalúru, India. Numerous people often suggest that string theory has alternatives that may also be the right theories of quantum gravity. At the level of science, this widely spread belief is a laymen's misconception. But one may discuss this statement at the level of sociology – look at the people who are making similar claims. These people may be counted. And they often suggest that these alternatives are being "discriminated against".
Loop quantum gravity (LQG) is often quoted as the "first competitor" of string theory. Yesterday, a biennial conference on that subject began in Erlangen, Bavaria. You may check its website; superficially, it is completely analogous to the Strings 2015 website. The similarities don't stop there. The key number announced in this blog post is that the LQG conference hosts 192 participants.
The analogous figure I counted for the Strings conference was 283 participants (although one has to admit that the conferences at "more accessible places" for the Westerners have had a slightly higher number of participants). The LQG population is about 2/3 of the ST population. What about the results?
Well, the gap becomes much larger when you look at the science. One may see dozens of very interesting and novel talks at the stringy conference. A similar analysis of the loopy conference shows that the number of valuable ones is approximately or exactly zero.
One problem with the idea that LQG is science is that it doesn't have any internally consistent foundations. Another problem is that there is no progress that would be "transforming the character of the field". These two problems are closely interrelated. One really cannot build any meaningful scientific discipline if it is based on assumptions or foundations that are shaky and/or mathematically inconsistent. And one is unlikely to find better foundations if he's too stubborn, dogmatic, and lacks the creativity and the desire to learn new things.
To be specific, the loopy conference listed the following 9 subtopics.
Classical and/or Quantum non-LQG Cosmology: theory and/or experimentYou may see that they're the same that similar conferences would have 10 or 20 years ago; there has been no progress. And each of them is still "stillborn" for the same reasons that have been known for 10 or 20 years, too.
Asymptotic Safety Approach and Renormalisation Techniques
Homogeneous and Hybrid Loop Quantum Cosmology (LQC)
Isolated Horizons and Quantum Black Holes
Foundations of Canonical LQG
Foundations of Covariant LQG (Spin Foams)
Group Field Theory and Tensor Models
Quantum Gravity Phenomenology
Other Related Topics including NCG, CDT, Causal Sets
LQG in general can't produce a smooth spacetime or Newton's law, let alone answer more advanced questions about quantum gravity such as the black hole information puzzle. Loop quantum cosmology is even worse because it isn't a valid limit of LQG; all of its predictions are known to be wrong, too. "Homogeneous" and "hybrid" versions of loop quantum cosmology are just "differently wrong" subparadigms.
Asymptotic safety isn't a part of LQG – so this topic is a "guest" at this conference – but it is almost certainly wrong, too. The scaling-invariant field theory on whose existence the asymptotic safety approach depends probably doesn't exist and there is still no non-vacuous evidence that it does exist. Moreover, the approach is as inconsistent with recent insights (last 25 years) about the black hole dynamics as LQG.
"Isolated horizons" are the LQG fans' attempt to say something about black holes. They want to isolate the black hole horizons and study them as pieces of LEGO. But the truth is that horizons are not isolated at all. The information they carry is dramatically entangled and related to the information outside the black hole and the information inside the black hole; some nonlocal considerations are almost certainly necessary to understand the nature of horizons in quantum mechanics. To assume that the event horizons are "isolated" means to miss all important ideas and insights concerning the black hole information puzzles, to throw the baby out with the bath water.
It's always like that in pseudoscientific enterprises such as LQG. One simply starts with a very naive, childish dogma, and then he tries to rationalize it. There is no rational justification or evidence so he keeps on repeating the same slogans and primitive caricatures of calculations for many years. Nothing works and there's no progress – the papers in 2015 are no better than papers 10 or 20 years ago. No guesses from the past are ever shown wrong, no guesses are shown to be right, no questions are ever replaced by more sensible and more well-defined questions that would lead to a better understanding of the issues. People aren't learning anything. They're just oscillating around some vague mathematical ideas where there's nothing interesting to be found. The people must know it. But it's their job – to constantly masturbate with half-baked and physically meaningless pseudo-mathematical equations all the time – so they prefer not to emphasize this crucial point.
Whether one starts with canonical (Hamiltonian quantization) or covariant (Lagrangian, spacetime quantization) incarnation of these ideas, he ends up with a theory that unavoidably breaks the Lorentz invariance, probably can't produce a smooth space at all, fails to agree with entropy bounds, Newton's law, Bekenstein-Hawking law, existence of fermions and non-gravitational forces, and everything else. Not an iota has changed about any of these matters. The "theory" is just completely hopeless.
In the case of the string conference, one can't have 9 subdisciplines that would be the same all the time. The foci of the research keep on changing as certain problems are fully solved, new problems have to be looked for, and they constantly emerge at interfaces of various previous stringy disciplines because the theory really makes sense and is "unified". Important individual insights regularly create a new "subfield". That differs from LQG which is a loose conglomerate of 9 mutually similar but inequivalent hunches, none of them really works, none of them generates any progress, none of them may be combined or connected with other insights because they aren't really compatible and their unions are both inconsistent with the empirical data and unpredictive.
But that doesn't prevent the people from organizing large LQG conferences. The reason is that there are lots of idiots around who fund this garbage. It's enough for subpar researchers to say "give me the money" and they get the money.
Even if you were incapable of verifying that the content of the LQG conference talks is worthless garbage, you may check certain things that don't require any genuine physics expertise. Compare string theory and loop quantum gravity at Google Trends. You will see that the searches for string theory beat their LQG counterparts by a factor of 50.
Even more dramatically, look for string theory and loop quantum gravity at Google Scholar. You find 240,000 and 8,660 hits, respectively. String theory wins by a factor of almost 30 by the number of papers but this hugely understates the gap between the two values of the content. That is more sensibly estimated by the total number of citations. You may check that the string-theory-related papers have a citation advantage over LQG-like papers by another order of magnitude, to say the least. So by this counting, string theory's "GDP" is about 300 times larger than LQG's GDP.
And this ratio is still almost certainly a huge underestimate of the relative value of LQG and ST – because most of the papers and their citations in LQG are just wrong. Even if you're uncertain about any particular paper, the probability that it is wrong is much higher for papers that are stuck with low citations counts and this is the case for a much higher percentage of LQG papers.
I want to remind you that these dramatically different "GDPs" are produced by communities whose sizes are almost the same – 192 vs 283 participants of the big conference. The ratio of the GDPs per capita of string theory and LQG that I suggested above, 300, is about the same as the ratio of Qatar's GDP per capita (wealthiest country), above $140,000, and that of the Central African Republic, the poorest country, which has $600 per capita.
The difference between the LQG and ST communities isn't about the quantity; it is almost entirely about the quality.
Can this huge gap in papers and citations be explained by something else than the absence of content in the "LQG research"? It is not hard to see that the string researchers work at much more prestigious places – top universities etc. – while the LQG fans are affiliated with second-class, third class, and random rural universities. You might think that it is a matter of discrimination. But the problem with this interpretation is that if the same theorist is moved to a less famous place, he can still do the same thing he would be doing elsewhere. Someone's "more prestigious place" is vaguely a consequence of his previous achievements, not so much a cause of his future achievements. Despite the possibility to do research anywhere, there are still no meaningful results coming from the LQG activities.
LQG is clearly an example of a pseudoscientific activity. It's not as bad as the women's or blacks' studies which have their absolutely redundant departments at various universities and whose only purpose is to make the privileged "minorities" look stronger in the Academia than they actually are according to the meritocratic criteria. But LQG is bad enough, too. The fact that there may be big conferences with 192 participants who claim to study a subject that hasn't had any results at all for 30 years is a testimony of the wastefulness of the governments across the world, the ease with which one may become a freeloader in the scholarly institutions.
Aren't you good enough a theorist to produce actual valuable science and succeed in the tough job market in the serious sciences? It doesn't matter. Just become a researcher of a bogus scientific discipline such as loop quantum gravity, be hired by a less famous place. You will be getting a comparable salary (perhaps 30% lower but not dramatically lower), you won't be expected to ever produce any valuable results because no one in LQG has ever done such a thing, either (and everyone knows that the 300 times lower output per capita is still OK), but you will be able to participate at "analogous" conferences etc.