Naturalness and the Landscape, by Leonard Susskind

[Martin]

http://arxiv.org/hep-ph/0406197

 

this recent 10-page paper by Susskind is descriptive of

the current situation in string research

 

and may be worth printing off, for anyone interested in stringy theories

 

Naturalness and the Landscape

(it appeared yesterday and is available in PDF format at the above link)

 

Although Susskind is highly optimistic and says that a new "paradigm" of string theory has now "emerged from the ashes" of presentday string theory, he does touch on several possible reasons for concern.

 

the business about a new string theory rising from the ashes is on page 1, in the introduction.

 

Susskind is at Stanford, one of the leaders along with Ed Witten, Tom Banks, Michael Douglas, David Gross, ...

Of the elder statesmen/founding father types he has been the most

vocal in arguing for the Anthropic Principle as a way out of current string difficulties. Witten and some others have been reluctant to go that way.

 

Witten just had a piece in the journal Nature. has anyone seen it? I regret to say I have not, my understanding is that the article was talking about the main issues in theoretical high energy physics, the scale of EW symmetry breaking, what to expect in the next few years, how to get back HEP back on track. It is possible that Witten's piece in Nature did not have much of anything to say about string theory. Hopefully someone who has seen it can fill in on that.

[admiral_ju00]

so uhm, how many string theories are out there?

 

It is possible that Witten's piece in Nature did not have much of anything to say about string theory.

 

Since the Nature Journal is a highly respectable scientific journal, I kind of doubt the paper turned in will be published(taking the above quote in consideration and as an example) if it does not meet their strict criterion.

[Martin]

so uhm' date=' how many string theories are out there?

 

 

 

Since the Nature Journal is a highly respectable scientific journal, I kind of doubt the paper turned in will be published(taking the above quote in consideration and as an example) if it does not meet their strict criterion.[/quote']

 

It is unusual that Ed Witten published his recent essay in Nature

which is not normal for theoretical physics----more normal would

be Physical Review Letters----or Physics Today----APS journals

(but he writes a style good enough for anybody except maybe the New Yorker, certainly good enough for Nature)

 

I dont understand what is with Susskind. he is one of the founders of string theory and M-theory in particular. I will get a link with his fascinating reminiscences about the early developments of it.

 

But now he seems to be in crisis mode. I personally have no opinion.

Also i dont want to try to say how many varieties of stringy theories there are. there is a huge lexicon that you probably know as much of as I do.

So much work by so many hundreds of people over the past 20 years have made a great plethora and hodge-podge.

 

Susskind in the article is talking about the 10¹⁰⁰ different versions---the "stupendous Landscape" of distinct possible string theory vacuum states---and the difficulty of finding the one which has the right cosmological constant, how do you choose? it sounds so incredible i can only suggest you read this 10-page article. There is a longer article by Tom Banks called "Is there a string theory Landscape?" that discusses the same concerns.

 

A quote which would show that Susskind's article is not a good stylistic match with Nature journal:

 

Page 1: "During the last couple of years an entirely new paradigm has emerged from the ashes of a more traditional view of string theory. The basis of the new paradigm is the stupendous Landscape of string theory vacua [1][2][3][4]---especially the non-supersymmetric vacua [5]. These vacua appear to be so numerous taht the word Discretuum [1] is used to describe the spectrum of possible values of the cosmological constant...."

[admiral_ju00]

Also i dont want to try to say how many varieties of stringy theories there are. there is a huge lexicon that you probably know as much of as I do.

So much work by so many hundreds of people over the past 20 years have made a great plethora and hodge-podge.

 

Unfortunately, Physics are NOT my primary interest so no, I'm not equipped with this lexicon, nor perhaps do I want to know the fine details. I was kind of hoping for a cliff-notes version

 

{edit}

 

By the way, I did print, although haven't had the chance to read those two 10 page pdf's you've provided, but I will.

[Martin]

 

By the way' date=' I did print, although haven't had the chance to read those two 10 page pdf's you've provided, but I will.[/quote']

 

I'm excited that someone else at SFN has printed out that other

ten-pager----I think you probably mean

 

"Emergence of a 4D World from Causal Quantum Gravity"

 

http://arxiv.org/hep-th/0404156

 

from my perspective that's the good one. I suspect it is a landmark paper and someone who knows more about it and is better equipped to judge (Baez) has been making confirming noises on sci.physics.research.

 

the AJL paper "Emergence of a 4D world" is something I want to learn more about. also they have a followup paper in the works

 

the Leonard Susskind paper is a window on a nightmare muddle that most of the time I just want to ignore. People should know about it though, it is happening inside string research community and it has some bearing on a substantial decline in publication and in the fewness of recent highly-cited "top-forty hit" papers.

[admiral_ju00]

Yes, that's the other one.

 

And yes, please post some more, especially on the M-theory...

[jana]

"Emergence of a 4D World from Causal Quantum Gravity"

 

http://arxiv.org/hep-th/0404156

 

I suspect it is a landmark paper

 

What is the basis for your "suspicion"?

[Martin]

What is the basis for your "suspicion"?

 

just a hunch really, Matt Visser (Washington State) provides some perspective:

http://www.phys.lsu.edu/mog/mog19/node12.html

 

John Baez comments on SPR after the May conference in Marseille

especially his exchange of posts with Charles Strohmeyer and Thomas Larsson, samples here:

 

http://www.scienceforums.net/forums/showthread.php?p=60096#post60094

 

 

the fact that it represents the culmination of a 20 year effort that

many people had given up on---the simplicial approach (dynamical triangulations) has always looked like a direct sensible way to quantize gravity, to some people, and they started trying (you mentioned Regge but dynamical triangulations is slightly different and it was worked on starting I think in the 1980s) and the problem has always been that when you put a whole bunch of identical little simplices together randomly you get a crumpled self-impacted thing or a feathery fractal thing and it doesnt have the expected macroscopic 4D appearance----so this AJL paper is, I guess you could say, a breakthrough

 

and also the sense that there are "firsts" here:

as they say on page 3

 

"In what follows we will report on the outcome of the first ever Monte Carlo simulations of four-dimensional causal dynamical triangulation...We will present strong evidence that the Lorentzian framework produces a quantum geometry which is both extended and effectively four-dimensional. This is to our knowledge the first example of a theory of quantum gravity that generates a quantum spacetime with such properties dynamically."

 

http://www.scienceforums.net/forums/showthread.php?p=60056#post60056

 

so for various reasons, a hunch that the paper signals a very interesting development that changes the Quantum Gravity picture

(it has made a splash among people who know the field, it is a breakthrough in a 20-or-so-year line of investigation, it has firsts, also the

approach is intrinsically simple and direct, if it works it has the quality of obviousness)

[Martin]

also, to get back more to the Leonard Susskind topic, another reason I suspect AJL is a landmark result is that string theory (a competing attempt to get quantum gravity) is in such a mess

 

confusion and even some despair about the "Landscape"----the roughly 10¹⁰⁰ vacuum states----about the goal of background independence which Witten called for in 1992 and hasnt been achieved---about the cosmological constant ("dark energy" we keep hearing about)---the outward signs of a decline in the numbers of research papers and in citations----the move from HEP to astrophysics where Xray and gammaray astronomy and neutrino astronomy have gotten hot.

 

in other words AJL comes at a moment when it looks like string may have been over-hyped and may have peaked