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-   -   Susskind on the cosmological constant. (http://www.physicsforums.com/showthread.php?t=341428)

Freeman Dyson Sep29-09 03:43 PM

Susskind on the cosmological constant.
 
I am watching a lecture by Susskind on his book The Cosmic Landscape. And he is going on about how "tuned" and unlikely it is. Largely based on work done by Weinberg. He said it was one of the biggest surprises in science and can't be ignored. How fine tuned the cosmological constant is. Is this true? I have read a little on here and people do not seem to be too big on him or string theory. And seem to side with Smolin. What do you guys think of this? Is this fine tuning really the problem that Susskind says it is?

turbo Sep29-09 04:57 PM

Re: Susskind on the cosmological constant.
 
Go to the Perimeter Institute site and download Penrose's inaugural lecture (it is one that he has given in various forms, so you'll probably recognize some or all of it). He also addresses the subject in Chapter 2 of the Philosophy of Vacuum, edited by Saunders and Brown. Expensive book, but highly recommended if you can borrow it or find a used copy.

http://streamer.perimeterinstitute.ca/mediasite/viewer/

Freeman Dyson Sep29-09 05:37 PM

Re: Susskind on the cosmological constant.
 
Quote:

Quote by turbo-1 (Post 2370924)
Go to the Perimeter Institute site and download Penrose's inaugural lecture (it is one that he has given in various forms, so you'll probably recognize some or all of it). He also addresses the subject in Chapter 2 of the Philosophy of Vacuum, edited by Saunders and Brown. Expensive book, but highly recommended if you can borrow it or find a used copy.

http://streamer.perimeterinstitute.ca/mediasite/viewer/

Thanks. I will look at that. Susskind almost seems to be setting up a false dichotomy between his string theory multiverse and intelligent design. The fine tuning HAS to be explained and his string theory multiverse are the only explanation short of a benevolent creator that tuned the universe for life.

turbo Sep29-09 05:56 PM

Re: Susskind on the cosmological constant.
 
Quote:

Quote by Freeman Dyson (Post 2370974)
Thanks. I will look at that. Susskind almost seems to be setting up a false dichotomy between his string theory multiverse and intelligent design. The fine tuning HAS to be explained and his string theory multiverse are the only explanation short of a benevolent creator that tuned the universe for life.

If you look at Penrose's work, he poses some questions relating to unification quite clearly. If quantum theory predicts one thing, and classical physics (including GR) predicts another, how can divergences in observations be understood and reconciled? Penrose uses the metaphor of a mermaid who lives both above and below the water (a Universe that inhabits both quantum and classical domains). That may or may not be helpful to you (or to me), but it is a nice visualization - a way to frame the question that is inclusive of both domains - which is probably a good way to figure out which scientific domain may have to "give" a little or a lot before they can be reconciled, so a TOE might be viable.

Freeman Dyson Sep29-09 11:51 PM

Re: Susskind on the cosmological constant.
 
I also came across this from Weinberg:

A Priori Probability Distribution of the Cosmological Constant

Abstract

"In calculations of the probability distribution for the cosmological constant, it has been previously assumed that the a priori probability distribution is essentially constant in the very narrow range that is anthropically allowed. This assumption has recently been challenged. Here we identify large classes of theories in which this assumption is justified."

http://arxiv.org/PS_cache/astro-ph/p.../0002387v1.pdf

To be honest, the article is a little over my head. Anyone dumb it down a little?

Wallace Sep30-09 02:59 AM

Re: Susskind on the cosmological constant.
 
My short version of this problem is that we are dealing with one point statistics, in that we have one Universe, and the conditions are what they are. If you want to argue about how likely those conditions were before you knew about our Universe, the answer you get depends completely on the assumptions you make. In the language of Bayesian statistics, the answer is dominated by the prior (which is the probability you assume before you observe the data).

There is a similiar never ending bun fight in the literature around the probability of life forming. Some argue that the one data point we have (Earth) implies life will form very commonly in the Universe, some argue that the evidence of the timescales of evolution on Earth tell you it will be very rare. Others say that the one data point tells you very little. Again it all depends on the prior that is assumed (either implicitally or explicitally).

My own view is that the cosmological constant problem can't sensibly be answered using these kinds of arguements, and will require some other yet to be realised insight, which hopefully also points to new ways of verifying this new idea (e.g. predictions about experiments or observations that can be made but have not yet been done).

Freeman Dyson Oct2-09 12:45 PM

Re: Susskind on the cosmological constant.
 
Ok, I watched the Penrose lecture, which was very good. Penrose is one of my favorites. But this only compounds my confusion. Penrose is talking about the "fine tuning" of the initial entropy state of the universe. How "special" it is. How unlikely it is to happen by chance. So what are the implications of this? What would the universe be like if it were any different? Could life still arise?

This goes back to the Einstein question. Did the universe have any choice when doing this? What caused the initial entropy state of the universe?

Duderonimous Apr9-11 02:05 PM

Re: Susskind on the cosmological constant.
 
Are you really Freeman Dyson?!


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