Comments on: Is the second law of thermodynamics connected to the expansion of the universe? http://jellymatter.com/2011/05/09/is-the-second-law-of-thermodynamics-connected-to-the-expansion-of-the-universe/ The blog that is not afraid of equations... or bees Sun, 17 Mar 2013 00:21:41 +0000 hourly 1 http://wordpress.com/ By: Nathaniel Virgo http://jellymatter.com/2011/05/09/is-the-second-law-of-thermodynamics-connected-to-the-expansion-of-the-universe/#comment-75 Mon, 09 May 2011 16:18:25 +0000 http://jellymatter.com/?p=826#comment-75 I’ve updated the post to point out that estimates of the total entropy of the universe vary dramatically; they’re all less than the upper bound, obviously.

]]> By: Nathaniel Virgo http://jellymatter.com/2011/05/09/is-the-second-law-of-thermodynamics-connected-to-the-expansion-of-the-universe/#comment-74 Mon, 09 May 2011 16:05:08 +0000 http://jellymatter.com/?p=826#comment-74 Thanks, I hadn’t seen it. Max Tegmark can always be relied upon to say something interesting. I disagree about the multiverse though. It’s interesting that the figure he quotes for the observed entropy is so much lower than Penrose’s (10^{89} bits versus 10^{123}) – if they’re both calculating the same thing (which they might not be), I wonder which is correct.

]]> By: Joshaniel Cooper http://jellymatter.com/2011/05/09/is-the-second-law-of-thermodynamics-connected-to-the-expansion-of-the-universe/#comment-73 Mon, 09 May 2011 15:54:13 +0000 http://jellymatter.com/?p=826#comment-73 Have you seen this: http://www.scribd.com/doc/2136072/entropy-in-cosmology
Makes a similar arguement(ish)

]]> By: Nathaniel Virgo http://jellymatter.com/2011/05/09/is-the-second-law-of-thermodynamics-connected-to-the-expansion-of-the-universe/#comment-72 Mon, 09 May 2011 12:22:34 +0000 http://jellymatter.com/?p=826#comment-72 Here’s another interesting point that relates to this post: there’s an argument in cosmology about whether time would appear to run backwards if the Universe were shrinking instead of expanding. In other words, would reversing the expansion of the Universe also reverse the second law? As far as I know, most cosmologists currently think that it wouldn’t. According to the book ‘Time’s Arrow and Archimedes’ Point’ by Huw Price, Stephen Hawking used to think it would, but he later changed his mind. (Of course, this debate had more practical relevance when we thought the expansion of the Universe might one day reverse, ending in a Big Crunch. This isn’t thought to be the case anymore, because of “dark energy”, but it’s still an interesting thing to think about.)

What does the argument in my post say about this? Well, if the size of the Universe provides an upper bound on its entropy then it’s impossible for the size to shrink unless the entropy is less than the upper bound. If the cosmic horizon is going to shrink down to zero size then, necessarily, the entropy also has to shrink to zero, and it has to do it at least a bit faster than the shrinking of the horizon. So if this upper bound idea is correct then reversing the expansion of the universe would have to involve reversing the arrow of time after all.

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