metacristi said:
Heh, I find Motl's comments a bit too 'arrogant', he's too 'orthodox'...Whilst I do not deny his competences, he has still to prove that there is a single path toward...
The main thing wrong with Motl comments I've found (despite his frequent charm and wit) is unreliability: part of the time he doesn't know what he's talking about. But this does not hinder him from making pronouncements. Here is an example from the recent 12-page post re:Smolin's paper.
This comes at the conclusion of the section "Background Independence of GR". If you are scrolling down looking for it, it is right before the section "Background independence in string/M theory":
---quote Motl---
On the other hand, there are no successes whatsoever of the approaches that Lee wants to call "non-perturbative approaches". The main problem is that they don't care about physics, experiments, and the new principles that are revealed by them; they prefer philosophical dogmas from the 16th century. It is a waste of time to discuss these "non-perturbative" speculations in detail. In all cases (causal set theory, loop quantum gravity, triangulation models), the speculations are based on the naive picture of space as being composed of infinitely sharp points - like in the classical theory - which are moreover exactly discrete. All these approaches make incredibly strong assumptions about the physics at the Planck scale whose probability to be incorrect safely exceeds 99.9999999999%; all of them belong to the discredited category of "gravitational aether theories" and no 16th century philosophical principle is strong enough to transform this intellectual waste into a topic for a meaningful physical debate in the 21st century.
---end quote---
I can't find any true statement here. I don't know much about Causal Sets and something he says might apply to that. But he is making blanket statements including the triangulations approach CDT that Lee was talking about, indeed has a section on.
The developers of CDT explicitly say they have found no evidence of spacetime discreteness or a minimal distance scale. So Motl is attributing to them what they neither postulate nor find as a result.
Here is a May 2005 CDT paper hep-th/0505113. This from the introductory section, page 2 (the first page of text.)
"The alternative we will advance here is based on new results from an analysis of the properties of quantum universes generated in the nonperturbative and background-independent CDT (causal dynamical triangulations) approach to quantum gravity. As shown in [5, 6], they have a number of appealing macroscopic properties: firstly, their scaling behaviour as function of the spacetime volume is that of genuine isotropic and homogeneous four-dimensional worlds. Secondly, after integrating out all dynamical variables but the scale factor a(\tau ) in the full quantum theory, the correlation function between scale factors at different (proper) times \tau is described by the simplest minisuperspace model used in quantum cosmology. We have recently begun an analysis of the microscopic properties of these quantum spacetimes. As in previous work, their geometry can be probed in a rather direct manner through Monte Carlo simulations and measurements.
At small scales, it exhibits neither fundamental discreteness nor indication of a minimal length scale."
There is nothing I can see in CDT about spacetime being composed of infinitely sharp points, or according to 16th Century dogma, or an "aether theory". As far as I can see that is just Motl foaming at the mouth. The CDT spacetime is a 4D topological continuum which for analytical purposes is triangulated and then the scale of the triangulation is allowed to shrink to zero. It certainly is not made of triangles, it just happens not to have a prior designated metric which makes it different from the arena in which strings operate.
And the CDT approach is not rigidly linked to one particular version of the action. They can freely modify the microscopic dynamical principle that is used to generate random spacetimes and indeed HAVE explored different microscopic actions in Monte Carlo simulations.
This contradicts what Motl suggests here:
"All these approaches make incredibly strong assumptions about the physics at the Planck scale whose probability to be incorrect safely exceeds 99.9999999999%"
What he says here is either exaggerated handwaving or is intended to give some definite impression---such as that he knows, somehow, that all these approaches depend rigidly on some fixed choice of the Einstein-Hilbert action to which they cannot add additional terms. And that is simply not true.
I have to conclude from this and other Motl critiques of QG that he actually does not know very much about the approaches to nonperturbative quantum gravity that Smolin was discussing in his paper.
====ON THE OTHER HAND I definitely LIKE THIS=====
I have to acknowledge that a lot of what Lubos says is charming and appealing in its own right. Like this passage really speaks to me. It just is not effective as blanket criticism of nonperturbative QG.
---quote of nice passage from Motl---
Coordinates: synthetic vs. analytical geometry
Also, as a kid, I was very impressed by coordinates and the possibility to analyze geometrical questions analytically, by looking at the equations involving coordinates. I don't know whether some of you have had the same feelings, but the mathematical tasks to solve a geometrical problem by "synthetic geometry" without the coordinates always looked like a useless childish exercise to me (which does not mean that one can't get good at it); it was recreational mathematics for children. If you can find the truth by using the coordinates, why shouldn't you use them? Coordinates are great and they are, to some extent, real. They are real modulo translations, rotations, and the Galilean (or Lorentzian) boosts. But there are only 10 parameters for this Poincare group in the whole Universe while the coordinates of the objects you want to study may be counted in thousands. No doubt, most of them are physical. We can't live without them or something more or less equivalent. The Cartesian coordinates, for example, look more fundamental than the angle between a bucket, Mercury, and a Mercedes, so why shouldn't we use them?
The relationist approach seemed to be an attempt to fight against the concept of coordinates; an attempt to pretend that they don't exist or they are unphysical; an attempt that must fail unless the coordinates are replaced by some assumptions that are equally powerful and essentially equivalent (but perhaps more awkward than the coordinates) because the space simply exists, to some extent, and you can't hide it. Also, the relationist approach did not look like a mathematical constraint on the possible form of physical laws. Instead, it was a way to make the questions quantitatively ill-defined. The relationist principles never looked like well-defined symmetries of a physical system; they were a method to show that no choice of the degrees of freedom is good enough for a sufficiently dogmatic person. We may summarize the situation: there was nothing that I would naturally like about the relationist approach.
Don't get me wrong: self-contained "bootstrap" systems that look non-quantitative and uncalculable at the beginning may be fine and very deep in physics but only if they're temporarily uncalculable. Relationism seemed to be a direction that wanted to make things permanently uncalculable.
---end quote---
The trouble with this charming passage is that the nonpert. QG I watch definitely does NOT go in a direction that "wanted to make things permanently uncalculable"! Very much to the contrary.
There is a lot of calculation already. Areas, volumes, angles, bending of lightrays. spreading of congruences of lightrays. diffusion processes. dimensionality. I will get some examples.
Lubos seems to be wavering here and allowing that there could be "good" nonperturbative and "bad" nonperturbative. It is bad if it makes things permanently uncalculable. But then this is a straw man argument.
Calculation is a big part of the nonpert. QG program. The idea is that it may be harder to get started (without a prior fixed choice of background) but once you get started you just might calculate BETTER: with fewer infinities blowing up and more uniqueness in the results.
