In the Cosmology forum a reference by Marcus references an article in Scientific American I think might be of interest to Quantum Physics readers:(adsbygoogle = window.adsbygoogle || []).push({});

(What are the (quantum) building blocks of space and time?)

http://www.signallake.com/innovation/SelfOrganizingQuantumJul08.pdf [Broken]

(by Ambjorn, Jurkiewicz and Loll)

Here's a series of excerpts which provides an overview of the paper, a quantum gravity model:

(Italic scripts are my additions for clarity from other portions of the text )

(The) dynamical emergence of a four dimensional universe of essentially correct physical shape from first principles is the central achievement of our approach If we think of empty spacetime as consisting of a very large number of minute structureless pieces (triangles which approximate curvature) and if we then let these interact with one another according to simple rules dictated by gravity and quantyum theory they will spontaneously arrange themselves into a whole that in many ways looks like the observed universe....(four dimensions with sufficient time) Similar mechanisms of self assembly and self organization occur across physics, biology and other fields of science...Casual Dynamic Triangulations approximates spacetime as a mosiac of triangles.....Computer simulations dashed the (Stephen Hawking) hope that casuality would emerge as a large scale property from microscopic quantum fluctuations.....Eucledean geometry indicates that space and time are treated equally....and does not build in a notion of causality...we must enforce causal gluing rules...an arrow of of time....for our model to work we needed to include from the outset a so-called cosmological constant (energy)....It is truly remarkable that by assembling microscopic building blocks (triangles) in an essentially random manner we end up with spacetime that on large scales has the highly symmetric shape of the de Sitter Universe....All of which sounds like some really interesting work. The number of dimensions depends on the scale...spacetime appears to have a different number of dimensions (over time)....evidently a small object experiences spacetime in a profoundly different way than a large object does.

Now my simple minded questions:

Inputs: simple triangles, gravity, quantum theory, causality, time, and a cosmological constant (vacuum energy);

Output: spacetime...

(1) Seems like a possible model for today, with expanding space....but how did it start???? Were all those input quantities ALWAYS present?? Seems like an awful lot of "stuff" to have sponaneously emerged all at once!!!!! Seems too complicated..

(2) If you put in causal structure, do you automatically get out some kind of Lorentzian manifold of which de Sitter space is one? I think de Sitter space is slightly curved even in the absence of matter or energy. Is this a significant result??

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# Emergent quantum spacetime

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