I am just reading "This Week's Finds in Mathematical Physics" number 208 by John Baez. Powerful stuff.
John Baez describes a conference at the Perimeter Institute. What was the topic? Well, it was obviously loop quantum gravity, but the organizers chose a pretty self-confident title that includes the words "Quantum Gravity in the Americas". Wow.
These conferences seem to be a spiritual continuation of the conferences decades ago, such as the 1962 general relativity conference in Warsaw. In a letter to his wife, Feynman wrote:
- I am not getting anything out of the meeting. I am learning nothing. Because there are no experiments, this field is not an active one, so few of the best men are doing work in it. The result is that there are hosts of dopes here (126) and it is not good for my blood pressure. Remind me not to come to any more gravity conferences!
Let's mention that Feynman himself derived the Feynman rules for general relativity in 1963. He also showed that the tree diagrams agree with the classical theory. Because of these and other contributions, it is clear that his opinion about quantum gravity mattered.
At the beginning of his essay, Baez describes the institute as a physicist's heaven, and explains that he gave the first talk because Abhay Ashtekar got lost in the new building. In his talk, Baez enumerated all recent papers about loop quantum gravity. I've heard about most of these papers, and there is probably nothing interesting to talk about. However it's already the "abstract" of John Baez's program that seems highly problematic - sort of shocking. One might say that it summarizes what Baez considers to be the main task for quantum gravity:
- The problem of dynamics in quantum gravity is still a big challenge. We don't know how to make spacetime into a truly dynamical entity with local degrees of freedom while taking quantum theory into account. Neither string theory, nor loop quantum gravity, nor the spin foam and causal dynamical triangulation approaches have yet found a background-free quantum theory with local degrees of freedom propagating causally. We sketch some avenues for making progress in this direction.
Wow. To understand dynamics in quantum gravity, according to John Baez, means to describe spacetime with local degrees of freedom that propagate causally. This is, according to Baez, what people interested in quantum gravity should work on.
It's not presented as a modest speculative proposal about a possible new description of quantum gravity, but rather as a universal key to judge the success of any enterprise in theoretical physics.
Well, there is a growing body of theoretical evidence that indicates that while the principles of quantum mechanics will survive in our future theories of quantum gravity without any modifications, the geometric concepts, including causality and locality, will not. As our understanding of quantum gravity deepens, it seems increasingly likely that geometry - as well as the related concept of locality - is a derived, approximate concept that follows from a much more rich theory, one that does not respect the naive ideas about geometry.
Non-locality or a subtle violation of causality is what seems to be an ingredient of the most likely resolution of the black hole information puzzle. String theory shows us a lot of new very specific phenomena that modify our ideas about geometry at very short distances, and many interrelations between the concepts that we usually associate with "geometry" and those that we usually associate with "matter". We've learned quite a lot about geometric transitions, dualities, and so forth.
Cargo cult science
OK, most of John Baez's assumptions what a theory of quantum gravity should look like seem relatively unlikely, and sort of obsolete. But even if we accept the idea that it is fine for a physicist involved with quantum gravity to be ignorant about string theory, including its very basic aspects, Baez's approach won't be really scientific because it is similar to the cargo cult.
There exist tribes at the Vanuatu archipelago, described by Feynman, that have chosen magicians with wooden "earphones", and they expect the airplanes to land, much like they were landing during the Second World War, and bring them a lot of nice stuff. Everything looks perfect but something must be missing because the airplanes do not land. It may be rather difficult to explain these tribes what's wrong with their science: they would prefer if you told them how should they modify the shape of their wooden earphones.
Baez's approach to quantum gravity is analogous. The starting point is easy because he already knows the answer to the main questions of quantum gravity: there should be exactly local and causal degrees of freedom that propagate through a "background free" spacetime. (These are the earphones.) Incidentally, the phrase "background free" does not really reflect anything reasonable, beautiful, or justified. According to the LQG ideology, string theory as such is not background free. If you discuss this topic with the LQG proponents, you won't learn what "background freedom" exactly is, but you will definitely end up with the same feeling as me - namely that it is some sort of Mach's principle, i.e. something that is known to be false.
OK, so we have the earphones. The only task that remains is to find the right variables that express the data about the metric, and the right rules how to deal with these variables. (This is the shape of the wooden earphones.) Today, this idea may look even more naive than Einstein's rather naive ideas about the form of the unified theory.
Baez does not find it important to verify experimentally - or at least by a more detailed, quantitative theoretical calculations or arguments - whether his assumptions are correct and sufficient, or at least predictive. The important answers are known a priori, and the task for a physicist is to accept these assumptions as dogmas and try to find evidence. Sorry, but this attitude is analogous to creationism, and it is pseudoscientific, especially if someone continues with this approach after more than 40 years of failed attempts to obtain anything from this approach that goes beyond classical GR, and could be compared with experiments.
Particles as wormholes
When I was 15, I thought I had a perfect theory of elementary particles. First of all, at that time I believed that there were only three truly elementary particles - the electron, the neutrino, and the proton, and perhaps their antiparticles. All of them were modeled as topological defects in spacetime.
Imagine that you remove two balls from the 3-dimensional space, and identify the boundaries of these two balls. Well, then you obtain the electron. If you remove two "solid doughnuts" and identify the 2-toroidal boundaries, you will end up with a neutrino. If you remove two "solid genus 2 Riemann surfaces" and identify them, you will obtain a proton. Moreover, the genus 2 Riemann surface may be chosen to have a S3 symmetry, which - I thought - should explain the fact that it seems that there are three quarks inside the proton.
I did not know what the higher genus wormholes meant. Probably new particles? ;-)
Well, it would probably be quite difficult for me to advocate this theory today. The immediate reason why I discarded the theory at that time was my inability to describe the annihilation as a smooth process - today, I would have other reasons to be suspicious. Nevertheless, it seems to me that John Baez is trying to do more or less exactly this thing. What's wrong with these theories? Naively, they can be attractive, but after 5 minutes, if you try to reconcile them with any "details" we know, you will see that no detail about them can work - certainly not quantitatively. Moreover, once we have quantum mechanics, there is a plenty of new ways how new physics emerges (new massless states arising from geometries that look singular classically, which is a common phenomenon in string theory), and the idea that all known elementary particles must be associated with a particular smooth geometry is much less attractive in a quantum mechanical world than it would be in a classical world.
The difference between the string theoretical approach and this loop quantum gravity approach can be summarized by another observation: string theory is focusing on the mathematical structures that can lead (and do lead) to predictable and sufficiently unique physically (semi-)realistic outcomes that morally resemble the other things we know must be there in a physical theory, to say the least. Loop quantum gravity and similar approaches, on the other hand, emphasize naive classical pictures of reality (such as LEGO, spin foams, particles as wormholes, cellular automata), and it tries to "prove" that they are relevant for the Universe.
You might think that a string is just another object, much like a wormhole, a cell in a cellular automaton, or a piece of spin network. But we don't study string theory because we like the shape of the string or because it looks simple to us. We study it because the two-dimensional theory describing stringy worldsheets is conformal, which allows us to eliminate all local metric degrees of freedom on the worldsheet (it's therefore a renormalizable 2D theory of gravity), and reduce the path integral over the Riemann surfaces (histories of strings) into a finite-dimensional convergent integral whose results moreover give us a unitary S-matrix that reproduces all qualitative phenomena that we know from GR and gauge theories.
If the strings could not do it, we would definitely avoid their investigation.
In other words, string theorists start with the question - only a scientific one that can a priori have several different answers - and then try to find calculations or solid arguments, and these calculations eventually support one particular answer - which often forces them to enrich their mathematical toolkit, the ideas about "what is natural to expect", and the warehouse of nice ideas. The result, happily, happens to be a unique theory that unifies all these ideas, namely string/M-theory, and we are learning new stuff from it.
Loop quantum gravity guys start with a constant collection of ideas and an answer - that has been revealed to them by a divine power - and then try to show that the answer must be correct even though most of these answers are quite obviously incorrect. More generally, the "discrete people" (see the sci.physics.discrete newsgroup or Wolfram's book) start with the dogma - the most important insight about the Universe is that it is discrete. The only task that remains, according to them, is to fill in the details of their ridiculous model of the Universe.
Obviously, this approach is very unlikely to work in science. It's not surprising that this approach prevents its advocates from seeing the truly interesting ideas - those that only emerge if one is ready to admit that he or she does not know everything from the very beginning, and he or she is ready to learn new things, either from the experiments, or from unbiased, mathematically deep theoretical research of some well-defined and interesting theories, or from others. Ideas such as renormalization group, confinement, holography, geometric transitions, enhanced gauge symmetries, dualities, and many others.
Contact with observable physics
Let's return to Baez's week. Baez continues with a discussion of some random papers - it seems that he must have a policy that he would never choose a paper that has led to some progress, e.g. a paper with at least 10 citations. The main reason why I find these attempts hopeless is undoubtedly the same reason as why Feynman did not like this stuff. They just don't care about making a contact with observable physics.
If we study gravity, it seems likely that classical general relativity is sufficient for our understanding of all phenomena at pretty long distances (it may, conceivably, break down at cosmological distances or tiny accelerations, but let's not discuss this possibility here). If we want to reveal the quantum influences on gravity, we are more or less inevitably thinking about physics at short distances, which are usually tested by high-energy particle physics (e.g. accelerators). The language of effective field theory, cross sections, amplitudes, and so forth is unavoidable.
Particle physicists and string theorists study many different ideas - some of them are combinatorial and "discrete", most of them are not - but in all these cases, they must be equivalent or continuously connected to the "ordinary" types of physical theories - those that can be verified in actual experiments. These connections are necessary for any research to be called "physics". If someone is inspired by a symphony, by a tiger's skin, a principle of the philosopher Ernst Mach, LEGO, or anything else, and she or he tries to claim that these ideas underlie all of physics, such a statement is not yet physics and most likely, it never will.
Progress since 1960s
John Baez describes various random speculations about the ideas that might be relevant for quantum gravity sometime in the future - such as "gravity as perturbation of a topological field theory" or "treating 4D gravity much like 3D quantum gravity, even though the latter has no local excitations". He also tries to explain "dark matter using quantum gravity", before he makes any attempt to explain the known matter (such as quarks, leptons, and gluons). It seems pretty clear that most of these ideas could have been proposed 50 years ago, and they are not really affected by any solid insights in physics made in the last 50 years (and there have been very many!). In the most optimistic cases, the authors may speculate that Newton's laws from the 17th century might possibly emerge from some crazy structure that needed to be fine-tuned and modified by several unjustified procedures.
It looks obvious that this type of research is stuck in a closed loop and the rate of progress equals zero. I am amazed by the nerves that the people must have to study physics in this way. The only reason why physics research is meaningful even today is that we are doing things that go beyond the things that were known to the physicists 50 or 250 years ago - at least sometimes we are doing such things. The progress in the 1920s may still have been more profound than in the 1990s, but the recent progress is not negligible.
But some dogmatic ideas about the discrete structure of spacetime - whether it's A New Kind of Science or Loop Quantum Gravity just seem like randomly imported ideas from the era of aether, or from the contemporaries of Democritus. They clearly constitute a very negative progress if we compare it to the Standard Model or General Relativity: they don't even seem to have a chance to describe the known phenomena from the SM and GR - and their research is even more disconnected from the cutting-edge experimental physics that will give us new information which paths beyond the SM and GR - e.g. which scenarios in string theory - are realistic.
The fact that a nonzero fraction of the theoretical physicists is working on something that is so obviously less interesting than science studied by the generation of physicists 35 years ago makes me a bit depressed, but I can imagine that physics always looked like that.
At least, the rest of us is constrained by all the known experimental facts about GR and the Standard Model, and those of us who try to look for new phenomena, new dual descriptions, and new language for string theory are also constrained by the large body of knowledge we have about string theory - a theory that is connected to all physics of the previous theories. But I just don't understand how someone can try to construct a theory of physics from the scratch, without looking at the known detailed facts about physics (that certainly go much beyond the clichés about background freedom), and how should we distinguish this research from the research of other people who try to start from the "scratch", those that are usually called crackpots.