LQG vs. String Theory... FIGHT!

[bascule]

So hey, yeah, I'm a layman, but I'm interested in the debate amongst the scientific community which I am far too tangential to be a part of.

 

So let me ask the following questions:

 

Which theory has put out the most papers YTD?

 

Which theory is presently the most progressive?

 

Which theory is presently the most predictive in terms of real-world phenomena, or presently on its way to publishing a paper which may make it predictive?

[Martin]

So hey, yeah, I'm a layman, but I'm interested in the debate amongst the scientific community which I am far too tangential to be a part of.

 

So let me ask the following questions:

 

Which theory has put out the most papers YTD?

 

String theory by far. On a world wide basis String community has roughly 10 times more people and puts out substantially more papers. It is hard to get exact numbers because if you say "string" to a search engine you get papers from half a dozen extraneous---cryptography (string of characters) materials science genetics etc.

 

Which theory is presently the most progressive?

As objective measures of activity (mistrust objective measures )

look at the two big conferences Strings '07 had around 450 participants and Loops '07 had around 150.

 

The spring 2007 American Physical Society meeting had 5 invited speakers from String and 3 invited speakers from the Loop community (this was the FIRST YEAR there was anything approaching PARITY)

 

The situation is radically different if you compare inside USA with rest of world. In US there is ONLY ONE university, Penn State, that has a research group doing non-string QG----i.e. more than one faculty member.

 

Outside the US there are a number of world-class groups

 

If you look from an inside USA perspective, nonstring QG almost doesnt exist---the numbers are like 20-to-1, or maybe 100-to-1 depending on how you count it----huge perponderance of researchers in string and support money going to string

 

If you look from a European-UK-Canada perspective (not to mention Latin America, India etc...) then you see more like a 10-to-1, roughly speaking, and also you see people and NEW MONEY flowing into the LQG community.

 

The flow of new MONEY is quite interesting. In 2005 megabucks to (Renate Loll's) Utrecht-based network which supports non-string QG.

In 2006 megabucks to (John Barrett's) new arm of the ESF (European Science Foundation) which supports non-string QG. A French collaborative network involving four campuses has appeared (Marseille, Lyon, Montpellier, Tour). I don't know the funding situation. More German universities are involved than a couple of years ago. I keep seeing new names, based in new places.

 

In Canada there is now not just Perimeter (Smolin's group) but also a couple of other places supporting non-string QG work---notably a group at Uni Western Ontario that does supercomputer Spinfoam calculations. Simulations on supercomputer clusters are also going on at Max Planck Potsdam and somewhere in the UK.

 

What we are seeing is a sudden upsurge, outside the US, in the variety of QG research (e.g. new use of supercomputers, new spinfoam models, Loll's stuff at Utrecht...) and in the willingness of administrators to bet their money on nonstring QG

and in the willingness of young PhD and postdocs to bet career-life on it.

 

Which theory is presently the most predictive in terms of real-world phenomena, or presently on its way to publishing a paper which may make it predictive?

 

The short answer is that there are NO CONCRETE DEFINITE PREDICTIONS. to be testable, a theory cannot merely "post-dict", it cant merely be compatible with past observations. it must bet its credibility on the outcome of a future observation which could go either way, i.e. might go against it.

 

Your question is phrased in a way to elicit SUBJECTIVE PERSONAL assessment. different people could talk hopefully about a whole range of things, but I will tell you my personal focus. I think that QG is basically about quantizing GENERAL RELATIVITY which is the interaction of spacetime geometry with matter----and discovering the fundamental descriptors ("degrees of freedom") of geometry and matter.

 

To a large extent, the testing arena of Gen Rel has always been OBSERVATIONAL COSMOLOGY and other kinds of astronomy. That is, if you want to check to see how well GR works and where it breaks down, you look at cosmological and other astro data. You look at the CMB, you catalog supernovae, you time binary pulsars, you study black holes etc etc. You think about how GR breaks down at bigbang or blackhole and how it leads to seeming paradoxes etc etc. there are a lot of avenues to checking GR.

 

Therefore I expect the same sort of situation to emerge in TESTING QG. I expect it will focus on studying high energy cosmic rays, gammaray bursts (like the GLAST satellite nearing launch will observe), CMB, supernovae, galaxy surveys mapping structure formation.

 

the evidence for successful QG will be more or less AS CIRCUMSTANTIAL AS THE EVIDENCE FOR GENERAL RELATIVITY and it will center on observational cosmology and other related astronomy.

 

So when you ask about EMERGING PREDICTIVENESS the particular areas I focus my attention on (and this is a personal choice) are:

 

1) natural or Occam-friendly explanations of the early universe horizon problem, flatness and structure problems

2) Occam-friendly explanations of accelerated expansion

 

Vintage 1980 inflation scenarios, devised to resolve horizon problem etc., invoke EXOTIC MATTER which does not obey energy conservation. the inflaton field hugely violates energy conditions because energy is roughly constant throughout space so as space expands you get more and more energy, essentially for free.

 

That is also what the presumed DARK ENERGY field does for you. It is also often pictured as a kind of exotic matter that violates energy conditions.

 

So people have this habit of inventing exotic stuff, if they can't think of any natural way to explain something, like accelerated expansion or the temperature uniformity of the CMB.

 

Occam's rule is Don't Invent Unnecessary Stuff. So if you can ever come up with an economical explanation that explains it just using what you have already---without inventing special faeries and unicorns to explain it---then the "inflaton" exotic matter goes out the window.

 

So this is my assessment of WHERE THE ARENA of action is going to be. There will be a lot of people passionately defending inflatons. And there will be people trying to see if we can do without them.

 

So I can point you to two or three papers that give some indications to me of directions in which to look for future action.

http://arxiv.org/abs/0705.4398

(LQG can probably give a natural explanation of accelerated expansion without "extras" like a dark energy field, and it leads to a distinctively different history of acceleration--this needs to be worked out in detail to make it testable)

http://arxiv.org/abs/0707.2548

(LQG provides a natural explanation of the horizon problem, without assuming exotic matter---this needs to be extended to cover more irregular cases, to make the conclusion more robust: this particular paper includes anisotropies)

http://arxiv.org/abs/astro-ph/0703566

(outlines what has to be done in order to address the structure formation problem and get specific checkable cosmology predictions from LQG)