# Nice Landscape Paper

1. Feb 13, 2006

### duke_nemmerle

This is an easy to read paper by Hawking and Hertog that takes a top-down look at the landscape. It uses the path integral approach to look at different histories with certain constraints. I spose I like it because it reminds of the CDT paper I so enjoy. It's worth a look at least.

http://arxiv.org/PS_cache/hep-th/pdf/0602/0602091.pdf [Broken]

Last edited by a moderator: May 2, 2017
2. Feb 13, 2006

### marcus

I remember you liked the CDT paper "The Universe from Scratch" (I think that was the one, it could have been "Reconstructing...").
There is a connection with Hawking's Euclidean Path Integral.
the CDT approach is something like a grandchild descendant, and Loll papers often cite Hawking. I am not certain of the dates but it's something like this.

Hawking worked out his approach to quantizing gravity in the 1980s.
It was a path integral---meaning a weighted average (roughly speaking) of all possible spacetimes that begin and end some specified way---analogous to a Feynman path integral weighted average of all possible paths a particle can take to travel from some specified here to there.

Around 1990 some people, including Jan Ambjorn, started researching a Dynamical Triangulations path integral. It wasn't the "causal" version---it was an earlier version. Ambjorn's was a close relative of Hawking's path integral.

Again a weighted average of all possible spacetimes that begin and end some specified way. The 1990s Ambjorn papers cite Hawking. Then in 1998 Loll got together with Ambjorn and they worked out the CDT approach. This was "Lorentzian" rather than "Euclidean" in that it used simplices that were chunks of Minkowski special relativity space---and stacked the simplices up in a way that respected causal ordering.

But despite the emphasis on Lorentzian/causal instead of Euclidean, the Loll CDT approach is still a direct descendant (by way of Ambjorn's earlier DT) of Hawking's. And the 2004 CDT papers cite Hawking quite a bit, if I rember correctly.

I believe you are referring to a kind of FAMILY RESEMBLANCE, basically the fact that both are path integrals (applied to spacetime) and this is something that Hawking pioneered.

But you may be picking up on something else, which I am missing.

I looked at the recent Hawking Hertog paper a few days ago, but not at length. I will take another look at it in light of your comments. Thanks.

Last edited by a moderator: May 2, 2017
3. Feb 13, 2006