What caused the shift of interest in quantum cosmology?

marcus

This raises an interesting physics question. What physics reason could there be for the shift of researcher interest in quantum cosmology which this Inspire search illustrates?
Here is the Inspire top ten quantum cosmo list for 1996-1998.

http://inspirebeta.net/search?ln=en&...=10&sc=0&of=hb

If I used it correctly, Inspire search says there were 20 String papers in quantum cosmology during that time period and THREE made it to the top ten. Three out of twenty is doing well, as I see it. They were numbers 5, 7, and 9 in the top ten list. Click on the link to see what the three stringy QC papers were about. There were no Loop papers that made the list.

By contrast, the same list for the period 2009-2011 shows no stringy QC papers but five are LQG. A sixth is part LQG and part some other approach. Again there were 20 String papers classified as QC, but this time none made the top ten.

http://inspirebeta.net/search?ln=en&...=10&sc=0&of=hb

You can see by clicking what sorts of quantum cosmology papers WERE favored by researchers. Horava gravity shows up. Verlinde entropic gravity also. Horava and Verlinde are former string folks whose current ideas do not require extra dimensions.

If you wish you can also enlarge the list to show the topcited 25 instead of the top 10. Same general impression.

So why did the quantum cosmology research community's interest shift in this pronounced way over the course of a dozen years? What physical reasons do you think could underly this change in focus?
Physics considerations might involve factors such as compatibility with inflation (generic in LQG), lack of evidence for supersymmetry, doubt about extra dimensions, the String Landscape, observations confirming a positive cosmological constant: in other words deSitter rather than AdS universe.

marcus

I also think we can learn some physics by asking what caused the sizeable drop in researcher interest in string over the past 10 years. Those who remember the confidence and excitement back around 2001-2003 must realize there has been a huge decline. We don't need statistics to prove this, it's frankly obvious. But I'll give an illustration--one of quite a few available.

It used to be that as many as twelve recent string papers would make the annual Spires top 50 list---the most cited papers during a particular year.

Here are top 50 lists for some past years with number of recent string papers making the list shown in parentheses.

http://www.slac.stanford.edu/spires/...1/annual.shtml (twelve)
http://www.slac.stanford.edu/spires/...3/annual.shtml (six)
http://www.slac.stanford.edu/spires/...5/annual.shtml (two)
http://www.slac.stanford.edu/spires/...7/annual.shtml (one)
http://www.slac.stanford.edu/spires/...9/annual.shtml (one)
http://www.slac.stanford.edu/spires/...0/annual.shtml (zero)

In this tally, papers are counted as recent if they appeared during the past five years. For instance in 2001 (recent meaning 1997-2001) twelve of the most highly cited fifty were recent string . Their ranks were 2,3,4,5,6,13,14,17,22,39,49, and 50.

By contrast in 2009 (recent being 2005-2009) only one of the fifty top-cited papers was recent string . It was number 33 on the list.

There are many kinds of evidence all pointing to the same disappointing fact. Recent string papers simply are valued less by other researchers and attract less attention (and citations) than they used to.

What is of interest is not this or that piece of evidence, most of us probably realize this has happened and do not require proof at this point. The interesting thing is the concrete physics reasons. What theoretical features and results correlate with this decline and may have contributed to it?

What do you think are the most important reasons?

Here are some possible physics causes you might wish to consider, I would be glad to have other possibilities suggested.

Supersymmetry not confirmed.
The String Landscape (the KKLT paper of 2003, so far no way to choose among 10⁵⁰⁰ versions of physics)
Positive cosmological constant (universe is not AdS) measured in 1998 but took a while to sink in
Seeming awkwardness accommodating cosmic inflation (search for alternatives to it)
Many parts of program dependent on a "fixed prior geometry" (Wheeler's term)

Any other ideas of physics circumstances that contributed? Which causes do you think are the most important?

I don't think we're interested in social, or political/economic, explanations in this thread---mainly because they don't appear to be very important in this case. The decline in string citations began by 2003, long before any public news or discussion (at least that I recall.) And I think the physics reasons are in any case much stronger and more decisive than any social ones could be. So hopefully we can focus on physics explanations. Potentially far more instructive.

mitchell porter

Big discoveries in string theory were made in the period 1995-2000, above all AdS/CFT, and the field is still dominated by the study of their implications. In 2010, the three most cited theory papers (#3, #8, #9 in the list) - not just string theory papers, but any theory - were still the three founding papers of AdS/CFT. So the story is that none of the subsequent developments within the subfield of AdS/CFT (or any of the newer string discoveries, such as the ABJM model) have attained to the same central significance as those three founding papers, which get cited in almost every paper on the subject. But I repeat: those were the most cited theory papers of 2010 (all the others around them are cosmological observations) - not just the most cited string theory papers. So string theory is still dominating theoretical research in general.

atyy

Maybe string has declined because it has become accepted physics. It's taught to undergraduates nowadays. That doesn't mean there isn't still a lot to be done, it's just become harder and harder.

http://en.wikipedia.org/wiki/Hype_cycle

As you can see, a decline does not indicate that there was not real progress!

ensabah6

A scary thought given that string theory remains unverified. Do undergraduates understand that string theory is speculative?

atyy

Well, they already learn false theories like Newtonian mechanics, so what's the harm

ensabah6

:)

Would you object to Universities follow Penn state in creating LQG-specific departments within physics, and hiring LQG trained professors, and teaching undergraduates LQG?

atyy

Is there really an LQG specific department at Penn State?

ensabah6

I should say they have several professors who are LQG-focused :)

smoit

Dear Marcus,
Once again, your analysis is completely misleading. First of all, note that there are only 3 theory papers in that 2010 list, which came out in 2009! By the way, one of them is on AdS/CMT by Sean Hartnoll that uses strings in AdS directly so your zero is wrong. To get the real feel of what's going on it would be more productive to compare citations for the papers which all came out in 2008 and 2009.
The list of the most highly cited papers written in 2008 and 2009:

http://www.slac.stanford.edu/spires/...e=citecount(d)
A theory paper with the highest number of citations on that list is:
N=6 superconformal Chern-Simons-matter theories, M2-branes and their gravity duals
by Ofer Aharony, Oren Bergman, Daniel Louis Jafferis and Juan Maldacena.
http://arxiv.org/abs/0806.1218

There are plenty of string theory papers in the top 100 papers on the list, mostly on AdS/CFT applications but also on F-theory as well as the M2-branes. Note also that there are exactly zero papers on LQG in the top 100 list of the most highly cited papers written in 2008 and 2009.

marcus

Right. Most of the young researchers in Loop are either Ashtekar PhDs (from Penn State) or Rovelli PhDs (from Marseille). Employment opportunities seem currently good because more university physics department now want a stake in Loop. And the new centers (Lyon, Sydney, Beijing, Erlangen, Tours, Morelia, University of Florida, Louisiana State,...) are still getting up to speed.

A typical path would be you get an Ashtekar PhD then go postdoc at Marseille. Or vice versa. Then possibly job. So far it is looking like a fairly good choice of specialty, for a theoretical physics PhD.

Undergrads normally do not get exposure (they have plenty of standard physics they should be learning!). But John Barrett at Nottingham has set up a two-year QG Masters program. I think this may be the first of its kind.

marcus

I checked my numbers for 2009 (one) and 2010 (zero) and they were correct. The main thing is apply the same measure consistently over time and see what the trend is.

Does anyone have ideas about physics explanation for the decline in interest?

What people have offered so far doesn't come to grips with any of the technical issues I mentioned. We are talking about a broad phenomenon. Top people getting other interests, or getting out of String altogether. Decline in the quality of the annual Strings conference (look at Strings 2010 in College Station, Texas and the advance material on Strings 2011 in Upsalla, Sweden.) Former friends like Weinberg and Gell-Mann expressing disappointment and impatience.

It is possible to make excuses based on socioeconomic circumstances, or quibble with specific pieces of evidence, or hurl epithets at the messenger (like "misleading!" and "obfuscation!" ) but the overall picture of decline is clear. It is the elephant in the room and I'm pretty sure there has to be some physics basis.

Any ideas?

smoit

In fact, at Penn State a Center for Gravitation & the Cosmos was created recently and they just held the Inaugural workshop last September. Guess what the theme of the workshop at the new center at Penn State was, this will make Mr. Marcus really happy , -
"Black Holes in Supergravity and M/Superstring Theory" !!!
Here is the link: http://www.gravity.psu.edu/events/bl...ty/index.shtml

atyy

I don't think it is true that Weinberg and Gell-Mann have expressed disappointment in strings.

Weinberg said asymptotic safety is not ruled out and has shown progress, which is true.

Gell-Mann basically said people should work harder on the fundamental symmetries of string theory, which is a call for increased research in string!

http://arxiv.org/abs/gr-qc/9508064 "it seems that any acceptable quantum theory of gravity, whatever its ultimate formulation, is likely to reduce to a perturbative string theory in the appropriate limit."

http://arxiv.org/abs/gr-qc/0210094 "If there is any valid perturbative description of quantum gravity then it is almost certainly string theory."

So Gell-Mann, Smolin '96 and Thiemann '02, Gross etc all agree that a major research direction should be to find out what a non-perturbative definition of string theory is.

marcus

Weinberg used the word "disappointed" talking to the national conference of science writers I think in 2009. That was the theme of what he had to say about String. There is a video. It is interesting. He's usually (talking to physicists) more gentle and careful not to hurt their feelings. Nice guy. Do you want me to hunt for the link?

Gell-Mann, as I recall, expressed impatience (translation: why don't you guys go ahead and tackle the hard roadblock problems, don't just keep diddling around with the same old stuff).

I should have made it clearer. Weinberg expressed the disappointment. Gell-Mann the impatience

Hermann Nicolai, a longterm and influential string theorist also had some sharp words I quote: "this is another way that string theory has failed" and "string phenomenology is increasingly Baroque, if you follow the literature" That was summer 2009.

atyy

Weinberg is the only one you can make an at least plausible case for.

As you say, Gell-Mann expressed more impatience.

And Nicolai, whatever he may say, has turned out
http://arxiv.org/abs/0906.1987
http://arxiv.org/abs/0907.3048
http://arxiv.org/abs/0912.3491
http://arxiv.org/abs/1007.5472
http://arxiv.org/abs/1010.2212

and even the apparently non-string http://arxiv.org/abs/0907.3298 is motivated by "Our construction is inspired by a recent re-derivation from gauged supergravities in three dimensions [20, 21] of the conformally invariant and globally supersymmetric models thought to describe multiple M2 branes."

marcus

Random collection of misinformation plus failed sarcasm. The Institute for Gravitation and the Cosmos was created in 2007. The Inaugural Conference was not "last September" which would have been September 2010. It was August 9-11, 2007.
I reported on it here at Beyond forum:
http://physicsforums.com/showthread.php?t=177711

The complete program (containing Loop and String people together with observational cosmologists, Roger Penrose etc etc) is here:
http://igc.psu.edu/events/conference...m_complete.pdf

I also gave a link here:
http://physicsforums.com/showpost.ph...&postcount=250
to the listing of IGC workshops including the September 9-11 one on Black Holes in Supergravity, M-theory.
http://www.gravity.psu.edu/events/workshops.shtml
That was NOT inaugurating IGC, it was simply the first of a planned series of workshops to be held in several different places. So it was titled "Inaugural" merely because it began that series of workshops.

The IGC mixes theory observation and in the theory department it mixes Loop and String--I approve of this. I like to see institutes, departments, and conferences mix a bunch of different active approaches and get people talking to each other. I think String has to some extent stalled (at least in US institutions) because the community got too securely entrenched and was just talking to itself.

The best QG conference of 2011 will pretty clearly be the June Zurich "Quantum Theory and Gravitation" which is organized by Barrett, Nicolai(string) and Rovelli(loop) and will have about equal Loop and String, plus several other contingents (CDT, NCG, GFT, QEG, NC-QFT...) Thirty plenary speakers. Historical conference. Exciting.