The Universe from Scratch (new AJL paper)

[marcus]

"The Universe from Scratch" (new AJL paper)

http://www.arxiv.org/abs/hep-th/0509010

The Universe from Scratch

R. Loll, J. Ambjorn, J. Jurkiewicz
30 pages, 5 figures; review paper commissioned by Contemporary Physics and aimed at a wider physics audience
Report-no: SPIN-05/28, ITP-UU-05/34

"A fascinating and deep question about nature is what one would see if one could probe space and time at smaller and smaller distances. Already the 19th-century founders of modern geometry contemplated the possibility that a piece of empty space that looks completely smooth and structureless to the naked eye might have an intricate microstructure at a much smaller scale. Our vastly increased understanding of the physical world acquired during the 20th century has made this a certainty. The laws of quantum theory tell us that looking at spacetime at ever smaller scales requires ever larger energies, and, according to Einstein's theory of general relativity, this will alter spacetime itself: it will acquire structure in the form of "curvature". What we still lack is a definitive Theory of Quantum Gravity to give us a detailed and quantitative description of the highly curved and quantum-fluctuating geometry of spacetime at this so-called Planck scale. - This article outlines a particular approach to constructing such a theory, that of Causal Dynamical Triangulations, and its achievements so far in deriving from first principles why spacetime is what it is, from the tiniest realms of the quantum to the large-scale structure of the universe."

 

[Chronos]

I can already hear shell casings clicking against the breech of string artillery units.

 

[Kea]

I can already hear shell casings clicking against the breach of string artillery units.

Well put, Chronos! But I have my doubts. Have they bothered to say much about CDT?

 

[Chronos]

Lubos has dismissively waved off CDT as an offshoot of loops. My guess, since this paper again hits the hot button of background independence, it will draw fire. I expect Loll, et al will be compared [perturbatively] to Smolins BI paper. Smolin has a lot of nerve saying nice things about both camps... he should chose sides, don't you think... er, I don't. I greatly respect his efforts to combine the best of both worlds. Don't take me wrong on that, I respect Lubos. He is brilliant, and a good person.

 

[marcus]

What do you think is the key sentence or sentences that express the crux
of this paper?

The Universe from Scratch
http://www.arxiv.org/abs/hep-th/0509010

I would glad if anyone who has read it would copy and paste what they think is the key message.

what stands out for me is something at the top of page 17

-----quote Loll "Scratch" essay page 17-----
The art is then to come up with a path integral which allows for large shortscale fluctuations in curvature, but in such a way that the resulting large-scale geometry nevertheless does not degenerate completely, so that a sensible classical limit may exist. The method of causal dynamical triangulations has for the first time in the history of the nonperturbative gravitational path integral given us an explicit example of such a geometry.
-------end quote-------

Reading the essay, one sees immediately that more care is being taken about the impact on OTHER NONPERTURBATIVE lines of QG research. The impact on and reaction from string-thinkers is not an overall primary concern. String-thinking is on the margins of nonperturbative QG development and there is not a whole lot in the essay that even registers its existence.

But there is a serious message to OTHER NPQ people, here for instance:

----continuation on page 17-----
A related lesson that has been made explicit by the dynamical triangulations approach in general is the fact that once geometric excitations are “let loose” in a nonperturbative formulation of quantum gravity, just about anything can happen. Not even the dimensionality of (what we thought of as) the spacetime emerging from the quantum superposition has to come out right. At the same time one could therefore also worry that other nonperturbative quantum gravity approaches may suffer related pathologies, which have only gone undetected because one has not been able to determine expectation values like that of the Hausdorff dimension explicitly.
---end quote---

In other words the authors of "Scratch" are worried that there may be unrecognized troubles in, for instance, foams, which their foams colleagues may not have so-far detected because they have not yet progressed far enough to do the necessary computer simulations. In the case of the EUCLIDEAN DT research that went on for on the order of 10 years before CDT, they were struggling for much of the time with pathological dimension---they put in 4D simplex building blocks but got out largescale dimension that was either 2D or infinite-dimension. (the dimensionality of the building blocks does not determine that of the largescale outcome). In their discussion of earlier Euclidean DT results in the 1990s, the authors remark:

---quote page 14---
These structures persist also at large scales, and as a result the [Euclidean] DT path integral appears to have no meaningful classical limit, and therefore does not satisfy a necessary criterion for a theory of quantum gravity. (One can only wonder how long it may have taken to realize this, had one not been in a state to perform extensive simulations of the model.)
---end quote---

I think this message to other QG colleagues is one that, while not hostile at all, invites response. It is also balanced by a kind of inclusive or "universality" message in the conclusion section. The authors say encouragingly that CDT should be seen as providing an EXAMPLE THAT OTHER NONPERTURBATIVE APPROACHES CAN SUCCEED.

----quote conclusions page 25---
It is unlikely that the construction we have presented here will satisfy every-one’s prejudices of how a quantum theory of gravity should be constructed, be it through invoking this or that kind of fundamental discrete structure at the Planck scale or according to this or that favourite symmetry principle. This need not necessarily be a reason for concern: if we can find one way to Rome, we will be able to find many others...
---end quote---

This echoes something that Lee Smolin said when the April 2004 Loll et al paper came out. In response to the "Emergence of a 4D World" paper he said "It's exceedingly important...Now at least we know one way to do this." in other words: now that they've gotten this to work in one approach we will see how to make it work in other approaches. This was quoted by Adrian Cho in the American Physical Society Focus review of "Emergence of a 4D World". In itself the quote seems unexceptionable, but significant from Smolin so I will get the link
http://focus.aps.org/story/v14/st13

 

[Chronos]

I think you captured the key points, marcus. There are no new results in this paper that I see. It's mainly putting their research in perspective with recent discussions.

 

[marcus]

Yes, in fact it is what is called a "review paper"

you get them in many fields, periodically, and it is good to keep an eye out for them

A review paper (compared with one reporting new research) tends to be more widely accessible, and to summarize the results from a lot of different papers, and to have a more extensive bibliography

the journal Contemporary Physics saw the need for a review paper for CDT, and it actually commissioned A,J, and L to write one---kind of nice, I think. To get paid for writing an overview summary of one's field of research.