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Previous Next Up Topic Cosmology / Alternative Cosmology / The Arrow of Time, Entropy, and the Big Bang (5343 hits)
By Mike Petersen Date 2011-11-25 21:59
Just got done reading Brian Greene's "Fabric of the Cosmos" and was struck by his comments about time's arrow.   Basically he says that time has a directional arrow that leads from one of low entropy to high entropy.  That's why we see spilled milk fall to the floor but never fall "up" from the floor...milk on the floor is higher entropy.

He then goes on to say (really the point of this post) that somehow the universe in the past had a lot less entropy, and the Big Bang was the lowest entropy state of the universe...thus he is a BB proponent AND a String Theory proponent.

I must admit, it was hard to argue the logic of entropy.

Can anyone here give an argument for entropy that does not involve a beginning?
By Jade Annand Date 2011-11-25 23:59
In an infinite-in-time universe, entropy would have to be "recycled" somehow.

"Big Crunch" cosmologies have entropy resetting during the crunch.

Quite frankly, it seems a bit weird to have entropy being low during the Big Bang in the first place. If it had remained perfectly isotropic, would entropy not be at an absolute maximum? It seems to me that in such a system, expansion can only lower the entropy if irregularities happen so that isotropy is destroyed, but the assertion is rather often a stark "no, entropy started low and never did anything but rise".

Some of the explanations for why the Big Bang is minimally entropic seem a bit weird to me. This posting, for example, has a "yes, the matter was in thermal equilibrium, but the gravity degrees of freedom are not [in thermal equilibrium?]". The explanation felt a little flat to me.

Someone else had similar thoughts, but the discussion petered out before it got really interesting.
By Mike Petersen Date 2011-11-26 12:43 Edited 2011-11-26 13:19
I'm not quite sure about entropy always increasing in the universe.  It could be that gravity will prove to be the ultimate free lunch.  It's a force that never seems to go away, so it's possible that it will just go right on pulling stuff together forever, thereby recycling everything: stars, galaxies, entropy, us. 

The trouble with entropy always increasing is that it practically begs for a beginning of some sort.  I have to admit it all fits together nicely.  Inflationary Big Bang, low entropy early condition, symmetry breaking, expansion, dark energy.  Even after reading all the compelling arguments, I still don't buy it.  If it ultimately turns out that "cosmological redshift" is not an indicator of an expanding spacetime but rather something intrinsic, we will be in for some very interesting times.  In that case I think, as has been discussed before, that what goes on in the center of galaxies will become of the utmost importance, and entropy will end up being a recyclable condition refreshed by whatever is going on in galactic centers.

Maybe it's like this.  Vacuum energy and the quantum foam conjur up some particle pairs, and from prior reading I recall that particle pairs are more likely to form near mass.  Perhaps this produces a runaway process tending to form these massive objects (black holes?) that ultimately become the center of galaxies.  If this is the case, it might not even matter if the expansion of spacetime is real, as matter keeps getting created in a bunch of mini-bangs, so to speak.
By Jade Annand Date 2011-11-28 18:25
Well, gravity is not typically involved in entropy calculations, though you do see it involved in black hole calculations mostly to ensure that entropy cannot decrease simply by going over an event horizon.

Well, unless you want to have some fun - I'm sure it's been torn apart in the meantime, but you can have a gander at Dr. Verlinde's expression of gravity as an emergent property of entropy :)

It does seem to beg for a beginning of some sort, but it seems hard to reconcile low-entropy conditions with Big Bang conditions. I haven't read deeply enough into the literature to see how they modify the description of entropy - but it seems like it must be modified, because without asymmetry in the expansion, none of that initial energy would be usable to do work, and that's the very essence of a highly entropic state. I can see expansion plus asymmetry pushing entropy lower, but to the purist for whom entropy can never reverse itself, I am sure that the "potential for expansion plus asymmetry" would have to be jammed into a more encompassing description of entropy. I wonder if anyone's managed it properly.

Intrinsic redshift certainly would gum up the works. It certainly seems as though the center of galaxies would come into play one way or the other, whether the variable mass hypothesis (I wonder if Narlikar has done entropy calculations on that) or otherwise.

Just a quick aside on particle creation - from various things I've read, it seems that particle pairs form most readily in the presence of energy and charge in particular more so than just mass. The E-M fields in light are sometimes sufficient enough that aiming energetic light beams directly at one another is enough to generate particles, even though otherwise, gamma rays can travel from far-away bursts.

Mike Petersen said:

Maybe it's like this.  Vacuum energy and the quantum foam conjur up some particle pairs, and from prior reading I recall that particle pairs are more likely to form near mass.  Perhaps this produces a runaway process tending to form these massive objects (black holes?) that ultimately become the center of galaxies.  If this is the case, it might not even matter if the expansion of spacetime is real, as matter keeps getting created in a bunch of mini-bangs, so to speak.


*laugh* That actually sounds a lot like Hoyle, Burbridge and Narlikar's QSSC. It's quite bizarre - it involves a "C-field" very reminiscent of the mainstream "inflaton field" that accumulate in gravity centers but with negative pressure that "mini-bang" or radiate away when it overcomes gravity - but it has the exact features to which you refer :)

Happy Birthday, by the way!
By Eduffy80911 Date 2011-12-16 01:58
I suppose you could have a minimal entropy state with respect to our universe under the following scenario. The stuff of our universe was a single homogenous mass, but it wasn't stationary. One fine day it suddenly entered an environment of far lower pressure than that it had been traversing. Kablooey! That supposes that our universe is a pocket of low pressure surrounded by high pressure. Simple, yes, but I've heard sillier.
By Jade Annand Date 2011-12-24 09:04
How about gravity instead of pressure?

(Otherwise, I might just have to ask, "high pressure what?" :) )
By Eduffy80911 Date 2011-12-30 03:39
i think relative pressure is a better descriptor if you have a mass perhaps ejected into it, then exploding. As for relative pressure of what, let me take the popular way out and dub it "dark pressure". in other words, I don't know.
Previous Next Up Topic Cosmology / Alternative Cosmology / The Arrow of Time, Entropy, and the Big Bang (5343 hits)

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