Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: namaan on 21/01/2010 23:49:16
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Hey all,
Ok, I'm assuming there's a relatively straightforward answer to this since it directly involves the 2nd Law of Thermodynamics. When one typically thinks of an explosion, intuition tells us that something goes from a higher ordered state to a lower ordered state; so in an explosion, we intuitively think that entropy is increasing.
Now, I'm not sure what to call the big 'bang' anything but an explosion. So let's follow the logic: the universe starts with the big bang, thus it starts with high levels of entropy, and then goes on to form galaxies, stars, planets, etc., all the while going from a state of higher entropy to lower entropy.
Now, I'm sure that a good physicist can explain to us the technical reasoning behind why galaxies form, stars form, planets form, etc. But that's not the point. The point is that even though the whole universe has gone from a high entropy state to a lower entropy state, why then are we always told that in a trillion years or so from now, the universe will be filled by a 'soup of diffuse energy with which you can't do work'?
If we're only talking observation, I observe two contradictory tendencies: 1) Entropy is in fact the law of the universe. When I cut vegetables and put them in a bowl one after another in layers, and mix them, they will never go back to the ordered state. 2) Some 'law of ordering' does seem to be active in the universe. Not only is it active, but it is active to a greater extent than entropy; this is necessary for order to be created in a thoroughly disordered universe (as was the case following the big bang).
The point of all this is that the only way the case of entropy leading us to a universe with only diffuse energy that can't be used to do work is possible is if the universe had started with galaxies, stars, planets, etc.
Any thoughts?
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Yes, avoid thinking of entropy in terms of order. A Jackson Pollack painting is considered to be more ordered than the cans of paint he started with, whereas if someone accidentally spills those cans of paint and ends up with the same result it is considered to be less ordered.
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So your point is that entropy should be treated as purely a mathematical necessity, and should not reflect the ordered states of planets and galaxies etc?
You just put the lay-down on Mr. Pollack btw, lol
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I like this explanation (http://www.infidels.org/library/modern/richard_carrier/entropy.html) of entropy. And this one seems cool too (http://entropysimple.oxy.edu/)
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So your point is that entropy should be treated as purely a mathematical necessity, and should not reflect the ordered states of planets and galaxies etc?
Well, I wouldn't describe entropy as a 'mathematical necessity', but that entropy describes how energy tends to equalise by flowing from higher levels to lower levels.
When you refer to ordered states of planets and galaxies etc. what exactly do you mean? How is the degree of order of a planet or galaxy defined? Is a highly active body, such as Io, more or less ordered than an inactive body, such as Chiron? Is the trajectory of a body in a perfectly circular orbit more ordered than the trajectory of a body in an elliptical orbit? And how does that compare with the trajectory of a deep-space probe, which is not in orbit at all and which may have incorporated several very precisely calculated sling-shot encounters?
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I actually did read up on it and I see your point about energy flow from higher to lower potential states.
With regards to your question about order, well...I would say most of your questions involve differentiating nuanced examples. I couldn't answer them, but it doesn't really address the scale of order vs. disorder I'm thinking of. I was thinking more along the lines of a oft-related diffuse fluid-like future for matter and energy, versus planets, stars, cold places, hot places...order.
Rather than argue about what order/disorder refer to, let's sum it up with this: I've heard how Hawking explains that black holes aren't immortal beings, that they actually degrade slowly but surely over time. Even so, it's very difficult for me to imagine why black holes would continue to degrade in continuous fashion all the way to a few grams of stuff...and then nothing.
Why should this ever happen to completion? Granted I don't understand the actual mechanism behind black hole degradation. But the only reason I can imagine this happening, not that I find it sufficient, is due to the expansion of the universe and continuously less matter available per given volume for absorption; almost like a grand-scale application of Le Châtelier's principle (assuming of course that universe doesn't stop expanding in the future). Then again, I don't believe that the definitions of entropy (and thermodynamics) were meant to rely on the assumption of an ever expanding universe.
The point is, I can't make sense of, for example, why any tendency of the universe should lead to a continuous 'push' against the effects of gravity until nothing but a diffuse fluid of unusable energy would remain.
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What happens with Hawking radiation is that mass from the Black Hole appears as virtual particle pairs exactly on the Event Horizon boundary, with one particle just within the EH and the other just outside it, with the result that the particle outside the EH boundary becomes real and is lost to the BH, which has therefore lost mass as a consequence.
Whether two energy potentials can ever achieve 100% equalisation depends upon quantum factors. The rate at which the two energy potentials equalise depends upon the steepness of the gradient between them, so when the difference is great the rate of equalisation is correspondingly fast, but as the two states approach each other the gradient approaches flatness and the rate of equalisation slows. Without quantum factors, the two states can only ever approach each other ever more closely, at slower and slower rates, but never reach equalisation. With quantum effects though, it is as though there is an intrinsic 'noise' level and once the difference between the two states is less than the noise level they are effectively the same and have equalised.
I don't think you should be thinking of 'diffuse fluids' too literally here, and you need to remember that entropy can only be applied to closed and finite systems. In the case of the (potentially open and infinite) universe at large, galaxies are effectively isolated from each other and there's little flow of energy between them. Within each galaxy though, there is only a finite amount of energy, tied up one way or another, and stuff is close enough for the energy to eventually, given enough time, approach equalisation.