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Eyjafjallajökull has awakened!


cfgauss
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Originally Posted By: cfgauss
Well, more than a few percent (more like tens of percent). But this is a self-regulating thing since fires remove oxygen from the atmosphere, so if it's easier to start fires it's easier to remove oxygen. So you expect to see an equilibrium concentration about what we see.


I will freely surrender my last Piconewton of credibility to say that my source stating a 3-5% oxygen increase is none other that James Lovelock's book 'Gaia'. Any mainstream scientists among you may fire-when-ready...

Interesting that you suggest a self-regulating process Gauss - sounds a bit on the apocalyptic end of the scale don't you think?
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Originally Posted By: waterplant
I will freely surrender my last Piconewton of credibility to say that my source stating a 3-5% oxygen increase is none other that James Lovelock's book 'Gaia'. Any mainstream scientists among you may fire-when-ready..


(fires)

No really, Lovelock is not what I would think of as credible. Yes, the Earth-as-organism model is intriguing, but only if you redefine organism to the point where any complex system qualifies.
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Originally Posted By: tehpineapple
Warning: Large, beautiful image (of ash cloud)
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If I weren't so lazy, I would make this my desktop image.

Originally Posted By: waterplant
Interesting that you suggest a self-regulating process Gauss - sounds a bit on the apocalyptic end of the scale don't you think?

What's apocalyptic about equilibrium? Equilibria occur all the time in nature. Although universal equilibrium is the thermodynamic equivalent to apocalypse... (at least that's what the high school texts say).
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Originally Posted By: Master1
(at least that's what the high school texts say)


High school texts are prone to oversimplify things, because high schoolers are notorious for not understanding advanced concepts. To paraphrase Terry Prachett, it's one of those things where the analogy is excellent for making you understand, but is actually completely wrong. Or something.
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Equilibrium is actually a fascinating and mysterious thing, at least to me, and understanding it better is one of my main research topics. As far as I can see now, whether equilibrium is paradise or damnation depends entirely on one's perspective. Specifically, on what time scale one is considering, compared to the time scale on which the system fluctuates.

 

'Fluctuation' is really a critical part of what 'equilibrium' means. In statistical mechanics, but also in (even) more hand-wavy discussions about stability and change, equilibrium does not mean that literally nothing is happening. It means that things change within some kind of stable pattern, never going too far from a certain mean. It also means that the small changes that do keep happening are somehow random, without any detectable further pattern — because if there were a detectable pattern to those small changes, that would become part of the mean, and no longer count as a change. A symphony is not equilibrium, but a steady noise is. Small random changes around the mean are called fluctuations.

 

If you step back and consider a time frame long enough that the fluctuations have gone up and down many times, then for you they tend to average out over time, and you won't even notice them as changes at all. For you, on your long time scale, equilibrium really is as good as no change at all. And in this sense, equilibrium is death. It's like Doctor Who's answer to the question of what would happen if the Key to Time did fall into the villain's clutches: "Oh, nothing. Ever again."

 

[Explanation for non-native speakers of English who might have missed the Doctor's clever line: 'nothing would happen' is an idiom, meaning that nothing bad would happen, but there's no English idiom involving 'nothing would ever happen again', so adding 'ever again' suddenly makes it a literal statement.]

 

But if you're looking on a time scale comparable to that of the fluctuations themselves, then for you equilibrium is a constant parade of unpredictable changes, which fortunately never become large enough to amount to any radical alteration of the situation. So you have basic stability plus constant variety, and life is great.

 

When people talk about the 'heat death of the universe' as everything winds down to equilibrium, they're implicitly assuming that fluctuation time scales will be very short compared to any time frames relevant to conscious beings. When they talk about Gaian equilibrium as a good thing, they're assuming that the fluctuation time scales are at least years.

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That's correct enough, but I think it's a little of a misleading description. It seems to imply things out of equilibrium are just things that look like things which are in equilibrium, but changing, and that's definitely not true (well, it is in some cases, but not generically).

 

For example, if you do something like calculate the characteristic temperature of a process like a spark jumping from your finger to a door handle, you get an insanely gigantic number (IIRC; I don't feel like calculating this since it's almost bed-time!). This happens not because the temperature was actually that high, but because there was no well-defined temperature while this process was taking place.

 

I would really describe an equilibrium (at least, nontechnically) as a system that's "well-described by averages" (e.g., temperature, average concentration, average velocity, etc) and where deviations are "well-described by random differences." Which is almost exactly what SoT said.

 

So, in other words, the average is doing something reasonable--usually staying the same or changing slowly, and the "exact" value is (roughly) bounded by the average plus some typical fluctuation size.

 

Like you said, thinking about time scales is nice, too, but it's not really needed for a definition of equilibrium. Although to understand any particular equilibrium it is smile.

 

Although, to clarify, by "time scale" SoT really means in the usual physicist way for this to include "distance scale" in cases where time isn't what you're interested in (or X scale when X is what you're interested in). So, e.g., you don't notice very long ocean waves on your small boat, but they'll break your supertanker in half wink.

 

This is as technical as I'm willing to get right before bed.

 

And, for those interested, first thermodynamics (and statistical mechanics) as SOT mentioned, and then, non-equilibrium thermodynamics is the topic to look at to understand this kind of stuff the best. Although equilibria appear all over physics, statistical mechanics stuff is when you typically have to be the most careful about what they mean.

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Originally Posted By: Master1
Originally Posted By: waterplant
Interesting that you suggest a self-regulating process Gauss - sounds a bit on the apocalyptic end of the scale don't you think?

What's apocalyptic about equilibrium? Equilibria occur all the time in nature. Although universal equilibrium is the thermodynamic equivalent to apocalypse... (at least that's what the high school texts say).


Imagine out of control fires burning that consume 20% of the Earth's atmospheric oxygen.
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Originally Posted By: waterplant
Imagine out of control fires burning that consume 20% of the Earth's atmospheric oxygen.

But such a fire would rapidly consume oxygen and not last long. We are more than capable of living through such an event, however unpleasant it may be. Also, the thing with equilibria, as our physicists so nicely said, is that they tend not to deviate far (on whatever scale is being used) from the mean. So the level of oxygen would never get so high that a massive fire would consume the entire earth.
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