Related to GR: lots of folks I know including Nima, Brian, Robbert, Juan etc. organize a 100th birthday party of General Relativity at Princeton from today to Friday. Hat tip: Willie Soon
Originally, I didn't want to run a story on the interview with Ashtekar
First, I must say that I've been sort of brought up to appreciate Ashtekar's name when I was an undergraduate at the Charles University in Prague (or at least, they have tried hard). There were no strings in Prague, of course. But there was some community of quantum field theorists; and a decoupled group of general relativists. The guru of the latter was and still is Prof Jiří Bičák [George Whipper, if you like to translate names], a top Czech relativistic astrophysicist. Already during communism, he had lots of contacts in the West. As far as I know, be behaved in a decent enough way during communism as well. He became my GR instructor later. A charismatic man.
By the end of my first year in Prague, however, I have read tons of original articles on string theory and my certainty that it was the only right approach to quantum gravity was very high. By that time, I have also been exposed to loop quantum gravity a little bit and I was sure that it was on a completely wrong track. Nevertheless, Prof Bičák made it clear that he wasn't a string enthusiast and sometimes praised the "Ashtekar program" and I never found it appropriate to publicly protest against that. ;-)
I want to stress that unlike Lee Smolin, Abhay Ashtekar has never been a focus of any war I would participate in. We've probably never met but on December 24th, 2002, I received this message from him:
Dear Dr. Motl:We exchanged about 5 friendly mails in total. It was clear from the beginning why I got this selective enthusiasm from Abhay Ashtekar. He was the main father of loop quantum gravity and my result was arguably the only analytical result in the history that had some reasons to be considered a "successful nontrivial test" of loop quantum gravity (and its opinions about the numerical factor in the black hole entropy). This optimistic interpretation didn't really work as we became certain rather quickly but in a field where nothing really works, something that "almost" works may have easily become the most cited loop quantum gravity paper of 2002, like my paper.
your analytical result is very impressive! I am wondering if your methods can say anything about the Reissner-Nordstorm case. In particular, how does the smallest non-zero real part of the frequency depend on the charge? If it is too much to repeat the whole calculation, can one say anything about the charge dependence near \(Q=0\) and near \(Q=M\)?
Merry Xmas and Happy new year!
(In those days, I received a comparably excited mail from Jacob Bekenstein, caused by similar reasons: Bekenstein was also invested in some misguided theories about the discrete spectrum of black holes, although not directly LQG-compatible theories.)
Incidentally, within three weeks, Andy Neitzke and myself crystallized a much more geometric (monodromy) method to get the same analytic result (the original method was using continued fractions). That new method was more flexible and we could also answer Ashtekar's question what happens when you add the charge etc. These answers refuted the hypothesis that was optimistic for loop quantum gravity.
But none of these friendly exchanges and accidentally correct isolated analytical results have changed anything about the fact that loop quantum gravity is a textbook example of the inconsistency of all non-stringy proposed theories of quantum gravity.
In 1986 and 1987, Ashtekar proposed that the problems of quantized GR go away if you rewrite the metric field in an unusual way using the gauge field and loop quantum gravity was born. His two original papers have 1,000 citations each – which is OK but one can find lots of groundbreaking papers in string theory that safely beat that. It's sort of crazy to present LQG as a competitor of all of string theory – when it comes to the impact, it may be at most compared to some out-of-top-ten papers defining "subfields" of string theory.
The change of the variables is unnatural and it is not one-to-one. Because the gauge field's Wilson lines are "periodic" in some sense, one gets a quantization rule for their complementary variables – which happen to be the proper areas. So the spectra of the "proper area operators" in loop quantum gravity are discrete. This is of course a reflection of the fact that the change of the variables wasn't correct globally, just locally. And this discreteness of the spectrum is a reason why LQG unavoidably breaks the Lorentz symmetry.
Even Sabine Hossenfelder began to understand why every theory with the discrete spectra of areas unavoidably picks a preferred frame and breaks the Lorentz symmetry. The "spin foam" composed of the Planckian areas simply fills the spacetime by something effectively indistinguishable from the aether and the aether only looks the same in one particular frame. The Lorentz violation has been used by LQG folks to make the (safely falsified) robust prediction of the dependence of the speed of light on frequency. Be sure that Sheldon was right when he mocked Leslie because of this particular prediction in this first episode of TBBT I had watched in my life.
The lethal problems of LQG are numerous and mutually related. The theory doesn't really predict any nearly smooth space in the "classical" limit at all. It makes no progress in solving the non-renormalizability of the naively quantized GR because there are still infinitely many parameters that remain undetermined in the theory. Nicolai et al. have written very meaningful critiques. Years ago, I wrote tons of blog posts, see e.g. this list, explaining why LQG is dead wrong. My most frequently discussed topics have changed – e.g. to the fight against anti-quantum zealots – because thankfully, I haven't been exposed to much LQG hype in the recent 5 years (unlike the anti-Copenhagen hype).
In the interview, Ashtekar talks about his encounters with Chandrasekhar and Roger Penrose and gravity essays – the usual uncontroversial personal stories you could expect. Then he offers one section of weird hype about LQG – including truly crazy assertions that LQG is expecting some experiments (Planck?) that will confirm its predictions. This is of course complete nonsense. There's no realistic chance to reconcile LQG with any experiments that can be easily done, let alone with the future ones.
But I want to focus on Part IV of the interview on "arrogance of string theory".
He's asked about claims on the uniqueness of string theory – which were made on the annual string conference in Bangalore. Ashtekar avoids the immediate disagreement concerning the uniqueness and says that string theory has done some cool stuff, especially extending the applicability of gravity to non-gravitational systems via the AdS/CFT correspondence.
However, things get sour immediately after the first paragraph:
Ashtekar: So there is no question that string theory has enriched us. What is unfortunate is that they are extremely intolerant, in my opinion. It’s everywhere. There is no need to be so intolerant. Because in science there should be a competition of ideas. Let it be a free competition of ideas rather than declarations. It’s not faith; and somehow when you say this is the only true thing, I don’t see much difference between that and some guru saying that his is the only true path.Perhaps the most important "act" underlying the scientific method is the falsification or the elimination of ideas that have been proven wrong. This may look cruel or intolerant but science wouldn't be science without that. LQG is a great example of ideas or opinions that one must abandon as long as he has anything to do with science.
The competition of ideas continues but it is also important not to overlook the results of such competition – because the results and answers are the ultimate motivation why the research is done at all. And the results are clear: string theory keeps on producing valid insights about fundamental physics – and also other branches of physics – while nothing works in LQG. This is a fact and it's just plain dishonest to try to hide this fact.
When I say a fact, it doesn't refer to any "guru". Every careful observer sees that string theorists generally don't refer to gurus and authorities. They refer to papers and evidence. In many cases, the evidence speaks a totally clear language. In other cases, the evidence is ambiguous and people are correspondingly open-minded. But if someone wants to be called a guru, it's people like Smolin and, sadly, also Ashtekar.
But the support from ill-informed journalists and laymen isn't what matters in science. What matters is whether some ideas – e.g. their ideas – have passed the tests. And be sure that the LQG ideas haven't.
One of them even made a claim that alternative approaches have been incorporated to string theory and, therefore, it’s the only true theory.Sorry but Polchinski didn't just "say" it. In this semi-technical 1998 paper, Polchinski analyzed a quantitative thought experiment and using some recent crucial discoveries in the field, he concluded that all roads led to string theory. This is very different from a LQG champion's hype about LQG – because it's just nonsensical talk supported by no scientific results. Polchinski's conclusions are backed by some of the most important results.
Ashtekar: Joe Polchinski, a very prominent string theorist, he did say explicitly that – this was some years ago, at KITP [Kavli Institute of Theoretical Physics] Santa Barbara at its 25th anniversary. He said, well, string theory has incorporated everything.
I will mention my evidence that Ashtekar hasn't even tried to study Polchinski's paper in detail because he's totally unfamiliar with the candidate "alternative" theory that Polchinski considered only to find out that it is actually string/M-theory, too.
Ashtekar: String theory is a little like Microsoft because at that time Microsoft was incorporating everything. He said loop quantum gravity was more like Apple. I thought it was a great complement! [Laughs] He explicitly said so. I said somehow I could accept Apple; at least Microsoft wouldn’t gobble us up!This is surely funny and I am laughing just like Ashtekar. I hope that Polchinski hasn't shorted the Apple stocks too much. It could have been costly now when Apple is the world's most expensive company whose capitalization is around $0.8 trillion. Microsoft is at the second place around $0.5 trillion.
That was a huge compliment.
Well, yes. In retrospect, it was a huge compliment, exactly!
When Ashtekar says "in retrospect", he doesn't realize that sometimes, there exist true seers ;-) and gurus in the society who can actually say such things in advance. In a 2005 blog post "Strings and Microsoft", I said that I was not particularly insulted by that comparison but I also mentioned the following:
Lumo 2005: On the other hand, our loop quantum gravity colleagues should definitely feel flattered if someone compared them to the Apple or Linux of quantum gravity. The reason is that the Apple computers more or less work, and sometimes the same thing holds even for the Linux computers.As the price of the Apple stock was exploding while LQG remained the same semi-crackpot failed fringe attempt to construct an alternative theory of quantum gravity, they should have felt increasingly flattered, of course. ;-)
Ashtekar: String theory has achieved a lot. I don’t know why science needs such statements; indeed, scientists should not make such statements.Sorry but since Galileo's trials, science has won the "right" to openly announce the results of its research. We are living in the 21st century now; why should scientists be told that they "should not" make certain statements? Even far-reaching conclusions such as the uniqueness of a theory are scientific and not sociological statements and it's very important that they're analyzed by scientists who evaluate the evidence in favor and against such statements, too.
In the case of the uniqueness of string theory as a consistent theory of quantum gravity, the amount of positive evidence is found overwhelming by most of the leaders of the field which is why they sometimes explicitly talk about the uniqueness. It's a pretty important questions so why should it be a taboo?
Ashtekar: Let the evidence prove that it’s the only theory. Let the evidence prove that it is better than other theories or let its predictions be reproduced more than those of others. Science should not become theology. And, somehow such statements have a strong smell of theology, which I don’t like.On the contrary, science would be reduced to theology if one insisted that people "should not" publicize certain inconvenient conclusions that contradict some prior assumptions – dogmas – such as the previously believed possibility to construct an Ashtekar-like theory of quantum gravity – are shown to be incorrect.
Let the evidence speak. But indeed, because the evidence has already spoken on some questions, physicists are making certain claims such as the claim that string theory is the only game in town. Thirty years ago, "the only game in town" could have been just a slogan summarizing the unmatched excitement among string theorists. But we're in 2015 and the claim is much more than a slogan. By now, there exist lots of papers with more or less rigorous proofs of inevitability of string theory under certain (increasingly mild) assumptions.
There’s even been talk of “post-empirical science”.I won't discuss the "post-empirical science" here. Instead, let me recommend Richard Dawid's recent guest blog instead of dozens of my older ones. But let me say something about the cosmological constant.
Ashtekar: Yes. There was another Strings conference in India at TIFR, in 2001. I happened to be in India at that time – people had just discovered that the universe is going through accelerated expansion, so that the cosmological constant may be positive. And I saw in newspapers that Tom Banks and Edward Witten had said that, no, the cosmological constant cannot be positive because it is incompatible with string theory. It has to be negative, they said. And that these observations are premature. They were completely wrong. The fact is that nobody goes back to these things and says, well, let us be a little more modest about it.
Minkowski and AdS vacua are compatible with supersymmetry, de Sitter vacua are not, and even beyond SUSY, it just seems much harder to establish the existence of de Sitter solutions in string theory. Those were intuitive reasons why Edward Witten and my PhD adviser Tom Banks have made the negative statement about the positive cosmological constant. I think that this conclusion isn't included as a conclusion of any full-fledged research paper they have done. You may check the most relevant Witten's 2000 paper (where he points out how hard it is to get a positive cosmological constant in string theory) and see that it was largely an informal and speculative lecture posted on hep-ph, not one of his genuine technical papers on hep-th. In this sense, they were informal remarks that were blown out of proportion. No one really knew whether a nonzero cosmological constant would be discovered and what its sign would be. Some people guessed correctly, others guessed incorrectly (I haven't dared to make any guess but I think that I would have preferred a vanishing cosmological constant in the 1990s), and I think that Weinberg's "anthropic" argument was the only argument that was more than a "guess" (before the hints of observations started to arrive).
In spite of that, I find it obvious that Banks and especially Witten became much more silent and decided to avoid any prophesies etc. You can hear this claim "I am not good at prophesies" from Witten's mouth very often. I think that you can hear it much more often than what would be helpful because despite Witten's failed prophesy about the sign of the cosmological constant, Witten's guesses are still much more likely to be sensible than the opinions of many people who are much louder. Wise people are learning from their failed predictions, however.
The status of stringy vacua with positive cosmological constants remains a controversial topic. As far as I can say, each major class of them – especially the KKLT class – may turn out to be completely wrong. This issue has a possible potential to even disprove string theory. I only believe that this is extremely unlikely because of the amazing properties of string theory in other respects. But the "falsified string theory via the C.C." is not yet a conclusion of any robust calculation. There are theorems that de Sitter vacua of certain kinds can't exist but none of them is a full-fledged no-go theorem that would apply in all descriptions of string/M-theory.
The discovery of the tiny cosmological constant has caused lots of mess in the field. The expansion of the anthropic lack of principles followed from that "harmful" discovery, too. Even though I think it's fair to say that string theory hasn't produced anything like a settled final answer to any major question linked to the cosmological constant in the real world, we must realize that it's the only framework we have where we see the clear potential to actually "derive" something about these matters at all. String theory is truly a theory of everything. Once you specify the discrete data needed to pinpoint a solution to string theory's equations (a vacuum), all of its properties including the cosmological constant become calculable in principle. "Everything" is a huge percentage of predictable quantities, indeed, and the potential for falsification is huge even if many things remained uncalculated.
In effective quantum field theories (with no or broken supersymmetry), any cosmological constant may be explicitly added. This is largely true even in frameworks like LQG – if we generously overlook that the likes of LQG fail to work at many other levels. So I think that it's fair to say that while the discovery of the positive C.C. is an example of string theory's failure to answer all outstanding questions so far, it is also a topic in which the superiority of string theory relatively to all other proposed approaches is self-evident.
It’s like shifting the goal post.As I said, it is not true that the wrong "prediction" by Witten was ignored. Witten hasn't ignored it at all and started to avoid all "prophesies", probably too much. But the field didn't ignore it, either. It tried to radically change lots of things in order to fully appreciate the newly measured positive cosmological constant. The anthropic paradigm shift – an unfortunate paradigm shift, I think – emerged as an overreaction to the surprising piece of data about the cosmological constant.
Ashtekar: Exactly! There is nothing wrong with making the statement. But then ignoring completely that you made that statement – that is wrong. And then to say that this is the only theory. It has not had hard experimental/observational success, and it has not made that much progress in quantum gravity. It does not tell us, for example, what really happens when a black hole evaporates. There is some dual description of it but there is no space-time description of it. It doesn’t tell us what happens to the singularities of general relativity. All the hard questions that are there in quantum gravity, they are not answered by string theory.
Collectively, string theorists have at least overreacted to the discovery of the positive cosmological constant. Other self-described quantum gravity theorists have pretty much ignored the discovery – largely because they know that their theories obviously have no muscles to address questions such as a "derivation of an unnaturally small positive cosmological constant from the first principles" at all.
Around 2000, string theorists focused on the difficult problem to calculate a viable cosmological constant. Around the same time, they have also learned lots of things about the singularities. The amount of deep conceptual stuff that we know about singularities is amazing. It's fair to say that it becomes modest when you focus on singularities in the real world such as the Schwarzschild black hole or Big Bang singularities. Ashtekar is completely wrong that the result of the evaporation is incalculable. String theory may be used to derive both the thermal character of the Hawking radiation and the temperature – even in practice; and all of its corrections, in principle. What's really a bit open-minded and a "problem for theorists per excellence" is some complete description of the black hole interior, something that the infinitely lived observer can never observe.
And his summary that "no hard questions are answered by string theory" is just an outright lie.
But I want to protest against another assumption. It's wrong to "move the goal posts", we are told. You know, changing the expectations, goals, and strategies is a part of the learning, a reason why the scientific research is done at all. When someone sees a surprising observation, e.g. the observation of a positive cosmological constant, he doesn't immediately commit suicide. He tries to explain the observation. He may invent new ideas or modify and recycle and combine the old ones. People try as hard as they can. "Not to move the goal posts" would mean to "completely deny the evidence". The surprising sign of the cosmological constant was probably an event that marginally lowered people's confidence in string theory as a theory of cosmology; but it's still true that the confidence remained incomparably higher than the confidence in the competing theories. Goal posts have been moved but string theory remained the only promising framework where similar answers may be derived.
After 1.5 additional paragraphs praising AdS/CFT (plus a comment about a panel discussion of him with Polchinski and Maldacena), Ashtekar says:
However, the AdS/CFT conjecture is the only definition of non-perturbative string theory one has – and it’s a definition, it’s not a proof of anything.Sorry but this statement is totally wrong. At least, Matrix theory is another (and, in fact, older) validated nonperturbative definition of string/M-theory (in Minkowski-like vacua, in the discrete light cone gauge). The BFSS paper has 2,400 citations by now – more than Ashtekar's papers founding LQG combined. The original BFSS model had nonperturbatively defined M-theory in 11 dimensions.
But even string theory with some actual fundamental strings (M-theory has none), most easily type IIA and heterotic \(E_8\times E_8\) string theory, may be nonperturbatively defined by a related model, the so-called screwing or matrix string theory. My original paper that established the subject is appropriately called Proposals on nonperturbative superstring interactions. These were the first proposals in the field to define string theory in a way that works for every \(g_s\), beyond the perturbative power laws, and the 289 followups agree that the proposal works well. I won't hide that I consider just this paper of mine to be more valuable than all Ashtekar's and his followers' papers on LQG combined (despite the neverending media brainwashing campaign that tries to suggest something entirely different) – because it works and it conveys an important lesson about the ways in which quantum gravity actually works, how an interesting and important approximation such as the perturbative string theory may emerge from something that is defined more universally than by the power laws.
Ashtekar's ignorance of Matrix theory's very existence is flabbergasting for another reason I have already mentioned. He criticized Polchinski's statement that "all roads lead to string theory". But Polchinski had actually used an example of an alternative theory of quantum gravity that someone could propose, a matrix model (the BFSS matrix model), before he showed that it was another road that led to string theory. Because Ashtekar said that the AdS/CFT is the only nonperturbative definition of string theory, we know that Ashtekar couldn't have successfully read that Polchinski's paper that he criticized because he would at least be aware of the existence of Matrix theory. He has criticized something he has never read.
Ashtekar: It talks about duality, but there’s no proof of duality. To have a duality, A should be well defined, B should be well defined and then you say that A is dual to B. Since we don’t have another definition of string theory, we cannot hope to prove that string theory is dual to its conformal field theory. You can define string theory to be the conformal field theory. You have to construct a dictionary relating string theory in the bulk and conformal field theory on the boundary. That dictionary has not been constructed in complete detail.The boundary CFT may be defined e.g. by adjusting a theory on the lattice etc. The bulk stringy theory of gravity in the AdS space doesn't have an independent full definition but it may be defined e.g. by string perturbative expansions. Even this approximate definition is totally sufficient for the duality to imply infinitely many extremely nontrivial mathematical predictions and all of them seem to work.
Ashtekar's suggestion that the duality is vacuous proves that he doesn't have the slightest clue what he is talking about. It's also untrue that we can't prove the duality. Lots of nontrivial evidence has been given to show that it works – it's the normal approach to decide in natural sciences. But even when it comes to an explicit mathematical proof, there are papers that probably contain the proof or proofs or at least their sketch.
Ashtekar: Again, nobody is taking anything away from the successes that the AdS/CFT duality has had; but there is a big gap between the successes and the rhetoric. The rhetoric is at a much higher level than the successes. So, for example, in this conjecture, first of all the space-time is 10 dimensional. The physical space-time is supposed to be asymptotically anti-de Sitter, which has a negative cosmological constant. But we look around us, and we find a positive cosmological constant.The total dimensionality of the bulk spaces in all supersymmetric examples of AdS/CFT is 10- or 11-dimensional – all of them are compactifications of string theory or M-theory. This is not a "failure" in any sense. On the contrary, it is an unbelievably impressive piece of evidence that all quantum gravity descriptions have to be a part of string/M-theory. Everyone, including Edward Witten, tends to understate the importance of all these patterns we have seen.
You must realize that the boundary CFTs – let's talk about the supersymmetric ones only – come in many diverse families. For example, you have the most famous example of the \(\NNN=4\) gauge theory in \(d=4\). You could think that the bulk dimension is \(d+1=5\) because there's the extra one holographic radial dimension. But you discover that the theory has exactly 6 scalars which carry the \(SO(6)\) R-symmetry, the isometry of the sphere \(S^5\), and you may actually see that there are extra 5 dimensions compactified on this sphere. Their curvature radius is the same as the curvature radius of the \(AdS_5\) factor of the spacetime. The total bulk space dimension is \(D=5+5=10\), just like what you need in string theory. And indeed, it's not just the dimensionality: you may check that all the information about the bulk agrees with the claim that it's occupied by type IIB string theory.
Take an \(\NNN=1\) superconformal CFT in \(d=4\). You get the same result but the \(S^5\) is replaced by a more complicated five-dimensional manifold. Take \(d=2\) boundary CFTs on symmetric orbifolds. A completely different starting dimension. When you do things right, you will see that the bulk space is something like \(AdS_3\times S^3\times K3\). The total dimension is \(D=3+3+4=10\) again. There is type IIB string theory in it. Sometimes you get M-theory but it's known to be dual to string theory, too.
Even though you start from some totally different theories and theories were once believed to have no relationship to string/M-theory – non-gravitational boundary CFTs of very many types – you always find out that whenever a theory of quantum gravity emerges in any way (e.g. the holographic AdS/CFT way), it's one of the theories that follow from string/M-theory, in the right dimensionality of the spacetime and with all the right objects, properties, transitions, interactions, and processes that string theorists may also derive in many seemingly totally inequivalent ways.
If Mr Ashtekar thinks that this is a sign of a failure of string theory, he needs to increase his dosage.
None of these vacua is the vacuum we live in but no one claims such a thing. String/M-theory obviously has many solutions or vacua. Every viable enough theory would surely have to have the same property. After all, even the very Einstein's equations published exactly 100 years ago this month have many solutions, especially once you allow some dimensions to be compactified. Only someone who hasn't ever come close to cracking some questions of quantum gravity could present the existence of numerous solutions of a theory of quantum gravity as a failure.
AdS/CFT isn't studied because it's directly relevant for the particle content in our Universe. But it's studied because it's a totally new sharp tool to approach some other solutions to the same equations that govern our Universe as well.
Ashtekar: Secondly, the internal dimensions in the conjecture, or this definition, are macroscopic. The Kaluza-Klein idea is that there are higher dimensions but because they are all wrapped up and microscopic, say, at Planck scale, we don’t see them. That’s plausible. But here, in AdS/CFT duality, they need the radius of the internal dimensions to be the same as the cosmological radius. If so, if I try to look up I should see these ten dimensions; I don’t. So, it can’t have much to do with the real world that we actually live in. These are elephants in the room which are not being addressed.The AdS bulk vacua are clearly not describing the real world around us. But they are solutions of string theory. At low curvature, the same Einstein-like field equations with some structure of the matter fields and interactions are obeyed. And the same Einstein-like field equations (or their more accurate stringy generalizations at higher curvatures or other more extreme conditions) may be seen to have other solutions, and some of those other solutions are remarkably close to the real world around us, including the spectrum of gauge fields and fermions.
These facts may be called elephants but all of them are friendly. None of them contains an epsilon of evidence that there is something discouraging about string theory. All these things work as well as they a priori could. String theory hasn't answered all the questions we want to be answered. But it has passed all the tests.
Ashtekar: There’s a subsection called “Elephants in the Room” and I say explicitly that there are these obvious issues and practitioners just pretend that they don’t exist. And that to me is unconscionable; I feel that that’s not good science. I don’t mean to say string theory is not good science, but publicizing it the way it’s done is not good science. I think one should say what it has done, rather than this hyperbole.Mr Ashtekar, no string theorist is responding to your comments about elephants because those who know your essays at all think that you have lost it. They are way too polite to tell you this into your face but I happily will. I agree with you it is not right to remain silent about such important facts. You clearly don't have a clue about the current status of the research of quantum gravity – your ignorance of the very existence of Matrix theory is a particularly shocking example of that fact.
At the end of the interview, Ashtekar says that he didn't read Lee Smolin's "The Trouble With Physics" (you haven't missed anything important or anything true, Mr Ashtekar, the book is just a pile of feces and lies), he thinks it's right when LQG folks like Smolin spread their hype, but Ashtekar's own work is more important because he prefers research over popular books. Well, I surely have lots of understanding for this position of Ashtekar's. The real problem is that he is associated with the same invalid research program as Lee Smolin. And he also associated himself with some of the very same bogus criticism of the legitimate research in the field. And that's too bad, Mr Ashtekar.
The November 2015 issue of Physics Today published a "mini-textbook" by Edward Witten, What every physicist should know about string theory. The URL contains the full article right now, including the PDF formatted copy.
It's an introduction to the first quantized treatment of particles and quantum fields via world lines; explanation why the stringy 2D counterpart of that produces gravity in the target space; and why it eliminates the infinities and other problems etc. Similar to the first chapters of Polchinski's book and similar sources. He has given talks with this title at least since 2013, including a talk at Strings 2015.
I totally agree that every physicist – including Abhay Ashtekar – should know these matters, at least to some extent. For a general physicist, not knowing is like not knowing that there are superconductors.
Nautil.US and later the Guardian hyped Hogan's misconceptions about the "holographic noise" – with a "happy end" in the form of a straight pißing on all modern physics provided by the professional crackpot Lee Smolin.