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## Main Question or Discussion Point

What is spacetime? Where does it come from? Oft asked questions here.

I recently posted the summary of a Scientific American article (found by Marcus) under the thread title, Emergent Quantum Spacetime,

https://www.physicsforums.com/showthread.php?t=294666

Here is another view of how space time that appears to match our universe MIGHT emerge, this time from string theories rather than quantum theory. It is UNPROVEN!!!!! These excerpts are from THE TROUBLE WITH PHYSICS, Lee Smolin, 2007, pages 153 to 160, and offers fascinating insights on what it might take to "create spacetime" from string theory.

An interesting feature is that electric and magnetic flux (of branes) stabilizes extra dimensions!!!!!

I apologize for the length, but the theoretical insights are worth the read. For one thing, this is the next step past M Theory, from five or so string theories to Maybe 10

(parenthetical comments are my own for clarity)

This explanation has attributes similar to the Emergent Quantum Spacetime in that both seem rather complicated with rather extensive requirments; I'd hope eventually we find a simpler set of emergent conditions from which spacetime can evolve....

I recently posted the summary of a Scientific American article (found by Marcus) under the thread title, Emergent Quantum Spacetime,

https://www.physicsforums.com/showthread.php?t=294666

Here is another view of how space time that appears to match our universe MIGHT emerge, this time from string theories rather than quantum theory. It is UNPROVEN!!!!! These excerpts are from THE TROUBLE WITH PHYSICS, Lee Smolin, 2007, pages 153 to 160, and offers fascinating insights on what it might take to "create spacetime" from string theory.

An interesting feature is that electric and magnetic flux (of branes) stabilizes extra dimensions!!!!!

I apologize for the length, but the theoretical insights are worth the read. For one thing, this is the next step past M Theory, from five or so string theories to Maybe 10

^{500}or so!!!(parenthetical comments are my own for clarity)

...from string theory (through M theory, I think) one of the few things we could conclude was that the cosmological constant was zero or negative...imagine the surprise,then in 1998 when...expansion of the universe was accelerating meaning the cosmological constant must be a positive number...(How to get string to match this?) ...moduli are the constants that denote the properties of (small) extra dimensions (beyond our four) ....certain aspects of the geometry of the higher dimensional spaces have to be frozen...(moduli stabilization)...otherwise the geometry starts to evolve..to a singularity or fast (unstable) expansion....in time....one way (to freeze extra dimensions) was to find string theories for which every change is a discrete step....Polchinski told us there were such objects in string theory: branes...branes come in discrete units and there are string backgrounds in which branes wrap around surfaces in extra dimensions....such branes carry (discrete) units of electric and magnetic flux...in the 1990's Polchinski and Bousso were able to get theories in which some parameters could no longer vary continuously (with this concept)...

# Renata Kallosh, Andrei Linde, Shamit Kachru and Sandip TrivediThe crucial breakthrough came in early 2003 by a group of scientists from Stanford# ..starting with flat four dfimensional spacetime with a small six dimensional geometry over each point....they wrapped large numbers of electric and magnetic fluxes around the six dimensional (calabi yau) spaces over each point...the discrete units tend to freeze out instabilities....you have to call on certain additional quantum effects....to get all the moduli stable....wrapping huge numbers of fluxes (branes) gets a cosmological constant that is small but still negative....by wrapping antibranes (analogous to antiparticles) energy can be added so as to make the cosmological constant small and positive....at the same time the tendency of string theories to flow into one another is suppressed, because any changes require a discrete step....if we ant a negative or zero cosmological constant, there are an infinite number of string theories...for a positive value there is evidence for

10^{500}or so...each will give different predictions for the physics of elementary particles and different predictions for the values of the parameters of the standard model...so this LANDSCAPE idea has taken hold....the evidence is based on indirect arguments...we do not know how to describe strings moving in these backgrounds....there is no proof that strings exist in these backgrounds...and Horowitz ( a discoverer of Calabi Yau spaces) and collaborators have recently discovered possible instabilities...in all solutions that involve six dimensional Calabi Yau spaces....

This explanation has attributes similar to the Emergent Quantum Spacetime in that both seem rather complicated with rather extensive requirments; I'd hope eventually we find a simpler set of emergent conditions from which spacetime can evolve....