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marcus

Science Advisor

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## Main Question or Discussion Point

we are familiar with the "running" of coupling constants, as for instance in QCD or even in QED where alpha (the electrodynamic coupling) is about 1/137 at long distances and low energies but increases to about 1/128 as energies increase and distances diminish.

Who would have imagined that the spacetime dimension could be 4 at large scale (as we observe) but decrease to around 2 at very small scale.

Ambjorn Jurkiewicz Loll have been doing computer experiments with their universe model and have found a

AJL tell us to expect a long article soon with a lot of detail about their CDT model (Causal Dynamical Triangulations----universe assembled from 4-simplices----no discreteness, no minimal length, "fractal" in character they say)

AJL say their long article is called

What they posted today is just a short article which has this strange new result of the spacetime dimension going down gradually as you get into ultra small scale.

http://arxiv.org/abs/hep-th/0505113

J. Ambjorn (NBI Copenhagen and U. Utrecht), J. Jurkiewicz (U. Krakow), R. Loll (U. Utrecht)

10 pages, 1 figure

SPIN-05/05, ITP-UU-05/07

"We measure the spectral dimension of universes emerging from nonperturbative quantum gravity, defined through state sums of causal triangulated geometries. While four-dimensional on large scales, the quantum universe appears two-dimensional at short distances. We conclude that quantum gravity may be "self-renormalizing" at the Planck scale, by virtue of a mechanism of dynamical dimensional reduction."

I :!!) Renate Loll.

Utrecht is a good place. They had Gerardus 't Hooft already. Then they got Renate Loll. Then, just this year, they have gotten Jan Ambjorn to come part time from Copenhagen and be part time at Utrecht. AJL is a kickass QG team.

CDT (causal dynamical triangulations) is one of or the most exciting thing going in QG. When you can model the emergence of the universe in your computer with like a quarter of a million simplexes and watch it evolve over and over again and you find that it has the right 4D dimensionality at large scale etc.

2005 is looking like another "annus mirabilis" (wonder year) like the other year of wonders that by coincidence occurred 100 years ago.

Who would have imagined that the spacetime dimension could be 4 at large scale (as we observe) but decrease to around 2 at very small scale.

Ambjorn Jurkiewicz Loll have been doing computer experiments with their universe model and have found a

*running of the spacetime dimension*.AJL tell us to expect a long article soon with a lot of detail about their CDT model (Causal Dynamical Triangulations----universe assembled from 4-simplices----no discreteness, no minimal length, "fractal" in character they say)

AJL say their long article is called

**Reconstructing the Universe**and it is dated May 2005 from the University of Utrecht, the Netherlands. But it is not yet out on archiv.What they posted today is just a short article which has this strange new result of the spacetime dimension going down gradually as you get into ultra small scale.

http://arxiv.org/abs/hep-th/0505113

**Spectral Dimension of the Universe**J. Ambjorn (NBI Copenhagen and U. Utrecht), J. Jurkiewicz (U. Krakow), R. Loll (U. Utrecht)

10 pages, 1 figure

SPIN-05/05, ITP-UU-05/07

"We measure the spectral dimension of universes emerging from nonperturbative quantum gravity, defined through state sums of causal triangulated geometries. While four-dimensional on large scales, the quantum universe appears two-dimensional at short distances. We conclude that quantum gravity may be "self-renormalizing" at the Planck scale, by virtue of a mechanism of dynamical dimensional reduction."

I :!!) Renate Loll.

Utrecht is a good place. They had Gerardus 't Hooft already. Then they got Renate Loll. Then, just this year, they have gotten Jan Ambjorn to come part time from Copenhagen and be part time at Utrecht. AJL is a kickass QG team.

CDT (causal dynamical triangulations) is one of or the most exciting thing going in QG. When you can model the emergence of the universe in your computer with like a quarter of a million simplexes and watch it evolve over and over again and you find that it has the right 4D dimensionality at large scale etc.

2005 is looking like another "annus mirabilis" (wonder year) like the other year of wonders that by coincidence occurred 100 years ago.

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