Yarn theory seems like a good way to go...
I have been plowing through the comments and thought I'd do a meta-response, especially since no one is likely to be reading that far down any more.
I should note that my comments on apples and physics hirings has in some ways been taken the wrong way, in particular, I am not advocating the hiring of people who are beyond the scientific fringe and pushing theories that are "not even wrong", but I do genuinely worry that physics in particular, and academia in general is too "faddish" - there is too much chasing after the latest greatest flash in the pan sub-field and too little long term perspective on balancing within departments and sustaining diverse research programs - this is always a problem, since departments are finite sized and each hire is precious, and there is always tension between reinforcing existing research strength vs opening up new research directions.
In apple terms, the problem is not just the propensity to buy Red Delicious, it is that getting Gala apples instead doesn't really help, because everyone suddenly starts buying Gala (well, they are tastier...). We need Stayman Winesap and Honey Crisp and Epicure and Cox's and Braeburn. etc and so forth.
In theoretical physics, the problem is subtlish - most theory is either trivial or wrong: by "trivial" I don't mean easy, I mean that the work is either parametric extension of known theory, or a technical solution of a well formulated problem - both can be very hard to do, and for much of the 20th century both were very productive endeavours, although they are becoming increasingly more trivial; the rest of theory is mostly wrong, in that it consists of people making exploratorive assumptions and then working out the consequences of the assumptions - most of the assumptions are wrong, more rarely the calculations are in error (and on interesting occasions the assumptions are right and the calculations are wrong).
We know that most of this theory is wrong, because it is mostly contradictory.
It is interesting, in that someone may be right, and observations and experiments may test the theories and show this. Or they may all be wrong.
One could take a cynical view on this, and support diversity to make sure someone stumbles on The Truth, or one could bet all on the "best theory", but neither is entirely correct - we want to think about what the best approach is, and why, but we also want to hedge our bets, just a bit.
This then becomes a market problem, who is going to take a chance on alternative approaches with apparently low(er) probability of success but possibly higher payoff, vs who makes the safe bet.
And then a market failure occurs, because there is pressure to go with "safe bets", people working on well established mainstream theories which will make incremental progress in some direction, because even if it is wrong, it will at least lead to good solid stuff, and associated grant income, in the mean time.
Now some details: I deeply do not care if the landscape predicts 10^500 or 10^1000 discrete vacua, I've stood on that precipe, looked in and backed away.
It is also not interesting that there may be infinite number of states, that just tells us that there are sectors of the theory that are continuously parametrised as opposed to discretely parametrised (hm, interesting analogy with bound vs free states in atomic theory compared with positive vs negative cosmological constant vacua pops into the mind...)
I've been reading anthropic arguments since Tipler and Barrow, they are fascinating, possibly correct and frustrating.
The issue of Doubly Special Relativity is interesting - at some conceptual level it is comprehensible that it should fall out of 2+1 Quantum Loop Gravity (and I do NOT want to hear a string theorist complain of calculations done in the "wrong" number of dimensions, ok?) - and conceptually there is a possibility it fails in 3+1 dimensions, if area is the fundamental geometric quantum then there may be an extra degree of freedom to let the length (and hence energy scale) transform to keep Lorentz invariance. Although there ought to be something testable to higher order, maybe an additional term in the transverse Doppler effect (in classical terms).
As a testable proposition, this is certainly closer to falsification then anything in string theory, the main quibble might be that QLG could wiggle out of Lorentz invariance in a broader theory, and, well, we can't have that. Not rigorous y'know.
Having said all that, I still think string theory is onto something and has capture some essential truths about the universe... and it is just so cute.
Especially when it is angry.
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The thing about 2+1D gravity is that there are no propogating degrees of freedom. It really is fundamentally different than higher dimensional gravity. To be honest, it's never been clear to me that DSR in 2+1D is anything more than just stating stuff we already know in terms of new variables. Every time I've looked at papers on the subject, I've found it very difficult to isolate the physical quantities of interest. Unfortunately, a better understanding of it on my part demands more time that I have to devote to it at the moment.
Still, I just want to say again: DSR and Lorentz violation just aren't in any way predictions of LQG. (The DSR paper in 2+1D, for example, doesn't use any LQG at all as best I can tell.) I don't know where these quibbles and broader theories you're talking about are. The prediction simply doesn't exist.
(And what's with the QLG as opposed to LQG?)
Well, I have nothing useful to say about loops vs. strings, but it seems like the simplest way to encourage useful crazy ideas is to start a prize or endowment that specifically funds efforts that are high risk. The harder part is finding a group of people who can accurately pick the useful crazy ideas.
I keep getting this feeling that within the next five years or so, we'll find out something bizarre and outrageous, which completely upsets all of our pretty philosophizing. The two sides of the AdS/CFT correspondence are gonna turn out to be really just two members of a universality class, and LQG will appear as a new limiting case of M-theory. Something like that.
I do not believe that it is the case that generic quantum gravity models (ie quantum field theories including a graviton) in 2+1 dimensions show violation of Lorentz invariance.
I'd be interested if that were the case.
There are already a couple of good prizes for finding these things... part of why so many smart people obsessively seek them ;-)
My general sense is close to what Blake said, with some additional speculation, which I'm sure I'll get to blogging Real Soon Now. Although I think it may be close to M-theory being a limiting case to LQG then the other way around (I wrote QLG rather than LQG because it was Really Late at the end of a Long Day!)
I do not believe that it is the case that generic quantum gravity models (ie quantum field theories including a graviton) in 2+1 dimensions show violation of Lorentz invariance.
I'd be interested if that were the case.
There is no graviton in 2+1D. And, I'm not sure the Friedel-Livine model violates Lorentz invariance. I'd guess that if you isolate the physical parameters, one just recovers a nice (locally) Lorentz invariant theory.
duh, can't accommodate two orthogonal polarization states in 2+1...
haven't read Livine's thesis, from what little I have heard it wouldn't be surprising if it were Lorentz invariant
I was actually gonna write an April Fools paper "proving" that LQG was a limiting case of M-theory, but I haven't had the time to construct a sufficiently dense packing of n-category jargon. (This is what I get for having to earn a living.) I figure you'd just have to explain the current situation as clearly as possible, referencing the right papers, then introduce a couple equations relating to quantized areas and information bounds. Next, you say, "Eq. (1.7) has a natural interpretation in terms of n-monoids," and you're off and running!
People who were around for the Bogdanov incident will remember how far this sort of nonsense can take you. . . .
:-)
Welcome to the ranks of those who wonder about Lorentz Invariance. Roger Blandford talked about it in his GLAST summary talk last month.
Just to get back to Aaron's comments on DSR - my understanding is that Gambini and Pullin (took me forever to pull that ref from my hindbrain) argued that LQG in general admits Lorentz violating dispersion relations and of the DSR form, which inspired the Magueijo and Smolin papers.
My understanding is also that Bojowald and others have argued that this may not be intrinsic to LQG, but rather a feature of certain approximate solutions only and that it may go away in the full theory.
You can presumably get DSR like Lorentz violating terms in other theories, but they ought not to appear at all in other theories.
In so far as there is no full theory of Loop Quantum Gravity, it is therefore not a prediction of the theory, but it is found in LQG inspired semi-classical approximations, which is something.
It is at least as much as the "string inspired" calculations being claimed recently as testing string theory.
I'm not sure which Gambini and Pullin paper you're referring to. Could you be more specific maybe? Is there an example of an LQG-inspired semi-classical theory that has DSR as a physical symmetry? Note that there's a change of variables that takes DSR to ordinary relativity and vice versa. The trick is understanding what is the physical quantity that one would measure in an experiment.
"Still, I just want to say again: DSR and Lorentz violation just aren't in any way predictions of LQG." - Aaron Bergman.
I want to explain why Aaron is so wrong. LQG does quantize spacetime. Smolin makes the point clearly in "The Trouble with Physics" that whatever the spin network grain size in LQG, the grains will have an absolute size scale (such as Planck scale, or whatever).
This fixed grain size contradicts the Lorentz invariance, and so you have to modify special relativity to make it compatible with LQG. Hence, DSR in some form (there are several ways of predicting Lorentz violation at small scales while preserving SR at large scales) is a general prediction of LQG.
The physical dynamics here are apparently far too difficult to be understood by string theorists who publish on the internet disinformation and the worst propaganda:
... the rules of the so-called "doubly special relativity" (DSR) to transform the energy-momentum vectors are nothing else than the ordinary rules of special relativity translated to awkward variables that parameterize the energy and the momentum. - http://motls.blogspot.com/2006/02/doubly-special-relativity-is-just.html
However, this is LM's usual plain nonsense, because doubly special relativity was applied by Giovanni Amelino-Camelia in 2001 to explain why some cosmic rays have been detected with energies exceeding the limit of special relativity, 5 x 10^19 eV = 3 J (the Greisen-Zatsepin-Kuzmin limit). It's not just LQG which makes a speculative prediction, because there's experimental evidence validating it!
Actually, there are quite a lot of indications of this non-Lorentzian behaviour in quantum field theory, even at lower energies, where space does not look quite the same to all observers due to pair production phenomena. For example, on page 85 of their online Introductory Lectures on Quantum Field Theory, Professors Luis Alvarez-Gaume and Miguel A. Vazquez-Mozo explain, http://arxiv.org/abs/hep-th/0510040 :
In Quantum Field Theory in Minkowski space-time the vacuum state is invariant under the Poincare group and this, together with the covariance of the theory under Lorentz transformations, implies that all inertial observers agree on the number of particles contained in a quantum state. The breaking of such invariance, as happened in the case of coupling to a time-varying source analyzed above, implies that it is not possible anymore to define a state which would be recognized as the vacuum by all observers.
This is precisely the situation when fields are quantized on curved backgrounds. In particular, if the background is time-dependent (as it happens in a cosmological setup or for a collapsing star) different observers will identify different vacuum states. As a consequence what one observer call the vacuum will be full of particles for a different observer. This is precisely what is behind the phenomenon of Hawking radiation. [Emphasis added.]
... Still, I just want to say again: DSR and Lorentz violation just aren't in any way predictions of LQG. - Aaron Bergman.
LQG does quantize spacetime. Smolin makes the point clearly in "The Trouble with Physics" that whatever the spin network grain size in LQG, the grains will have an absolute size scale (such as Planck scale, or whatever).
This fixed grain size contradicts the Lorentz invariance, and so you have to modify special relativity to make it compatible with LQG. Hence, DSR in some form (there are several ways of predicting Lorentz violation at small scales while preserving SR at large scales) is a general prediction of LQG.
The physical dynamics here are apparently far too difficult to be understood by string theorists who publish on the internet disinformation and the worst propaganda:
... the rules of the so-called "doubly special relativity" (DSR) to transform the energy-momentum vectors are nothing else than the ordinary rules of special relativity translated to awkward variables that parameterize the energy and the momentum. -http://motls.blogspot.com/2006/02/doubly-special-relativity-is-just.html
However, this is LM's usual plain nonsense, because doubly special relativity was applied by Giovanni Amelino-Camelia in 2001 to explain why some cosmic rays have been detected with energies exceeding the limit of special relativity, 5 x 10^19 eV = 3 J (the Greisen-Zatsepin-Kuzmin limit). It's not just LQG which makes a speculative prediction, because there's experimental evidence validating it!
Actually, there are quite a lot of indications of this non-Lorentzian behaviour in quantum field theory, even at lower energies, where space does not look quite the same to all observers due to pair production phenomena. For example, on page 85 of their online Introductory Lectures on Quantum Field Theory, Professors Luis Alvarez-Gaume and Miguel A. Vazquez-Mozo explain, hep-th/0510040:
In Quantum Field Theory in Minkowski space-time the vacuum state is invariant under the Poincare group and this, together with the covariance of the theory under Lorentz transformations, implies that all inertial observers agree on the number of particles contained in a quantum state. The breaking of such invariance, as happened in the case of coupling to a time-varying source analyzed above, implies that it is not possible anymore to define a state which would be recognized as the vacuum by all observers.
This is precisely the situation when fields are quantized on curved backgrounds. In particular, if the background is time-dependent (as it happens in a cosmological setup or for a collapsing star) different observers will identify different vacuum states. As a consequence what one observer call the vacuum will be full of particles for a different observer. This is precisely what is behind the phenomenon of Hawking radiation.
On this blog page, there is a command
meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"
and then the contradicting
meta http-equiv="Content-Type" content="text/html; charset=utf-8"
This has the result, that the page does not display correctly on Safari.
Cheers
Frank
Indeed there is - the header specifies ISO and then switches to Unicode
But it displays just fine on my Safari browser, except that Nigel's quote
symbol's " " don't display correctly - which suggests to me they were cut and pasted from IE7, which does not correctly code a couple of extended ASCII symbols
Almost completely off topic, but "yarn theory" appears in today's (Sunday March 11) Doonesbury.
Cool, but... am I correct in thinking that this is a re-run strip?
In which case I got the "yarn theory" from Trudeau rather than vica versa.
Be nice if it was original to me, but the strip looks very familiar.
Yeah, it's a rerun of the 25 June 2006 strip.