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Post by Progenitor A on Mar 9, 2011 21:55:46 GMT 1
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Post by buckleymanor1 on Mar 10, 2011 1:50:26 GMT 1
But wouldn't necessarily mean gravity was reversed if the universe contracted after expanding -- and cosmological models with a positive curvature do just that -- expand, then contract. I am not sure if you watched the program if you did you would have noticed tha several tons of the ice that broke of the glacier into the lake, then clearly jumped out of the water reversing gravity and restuck itself onto the glacier. Giveing the impression that time and gravity were reversed. So if the universe contracted after expansion and gravity did not reverse would this mean that contraction is not symetrical and Brian Coxs film analogy is not likely.
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Post by speakertoanimals on Mar 10, 2011 2:02:05 GMT 1
Yes, the contraction wouldn't be just the expansion in reverse, entropy would still increase.
If we have a cosmological arrow of time that points in the direction of expansion, then contraction would then see the cosmological and thermodynamic arrows disagreeing, unless the cosmological arrow switched to now point in the direction of contraction rather than expansion.
One if the reasons why it is useful to think about several different arrows of time, and what would happen if they didn't all agree.
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Post by Progenitor A on Mar 10, 2011 8:24:32 GMT 1
But wouldn't necessarily mean gravity was reversed if the universe contracted after expanding -- and cosmological models with a positive curvature do just that -- expand, then contract. I am not sure if you watched the program if you did you would have noticed tha several tons of the ice that broke of the glacier into the lake, then clearly jumped out of the water reversing gravity and restuck itself onto the glacier. Giveing the impression that time and gravity were reversed. So if the universe contracted after expansion and gravity did not reverse would this mean that contraction is not symetrical and Brian Coxs film analogy is not likely. The universe cannot contract if gravity becomes a repulsive force, unless there is another force whose magnitude is greater than the sum of the (now missing) attractive force of gravity plus the new repulsive force of gravity. Professor Cox was just attempting to illustrate time reversal that results in the disorder reverting to the previous order It would appear as if the universe contracting back into a singularity is such a reversal from disorder to order, IF, as Professor Cox intimated the expansion of the universe is entropy in action - then the entropy would be decreasing during contraction by definition. That does not mean that things will revert back to exactly what they were before (such as your ice jumping back onto a glacier), but it does by definition (if Professor Cox is right) mean that the universe is approaching a more ordered state as the contraction proceeds. Begs the question really does it not? Was the BB a more ordered state than the current state of the universe? It was certainly a highly improbable event and as Professor Cox said, the degree of entropy is the degree of probability of a thing or event - a disorganised pile of sand can occur in millions of ways and hence its probability is high; a sand castle of a particular form has only limited possibilities of arrangements of sand particles and hence is highly improbable- low probability So if we take the probability of an event as an indicator of the entropy or disorder of a system, then the BB is a more ordered state than we have today, and as the contraction approaches the conditions near to the BB then order is increasing. BUT If we look at London today it is a far more ordered place than it was 300million years ago for example. Therefore London has become more ordered since the BB. Does this confute the arrow of time if we define the arrow of time by entropy - the movement from order to disorder? To my mind there is an observable trend toward disorder and Professor Cox outlined what the universe might look like as we approach total entropy - photons moving randomly in a temperature of 0K with no events to measure time by, so time itself ceases to exist. However this trend toward increasing entropy is not absolute and we can also have order arising out of disorder as time progresses - e.g. London. Unfortunately Professor Cox's definition of order arising from disorder defining the direction of time means we have time reversal locally (although the general trend toward disorder continues.) That is one reason why the programme was unsatisfactory - it leaves outstanding questions about time and entropy
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Post by marchesarosa on Mar 10, 2011 10:01:54 GMT 1
I did not see the prog but it seems it also missed out discussion of organic change on our home planet which over time has gone from simple to complex - from single cell life to highly complicated organised life-forms like us. Over time should we expect to revert to single cell creatures? What a bore this entropy is!
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Post by Progenitor A on Mar 10, 2011 11:30:20 GMT 1
I did not see the prog but it seems it also missed out discussion of organic change on our home planet which over time has gone from simple to complex - from single cell life to highly complicated organised life-forms like us. Over time should we expect to revert to single cell creatures? What a bore this entropy is! Quite Many unanswered questions on Professor Cox's definition of entropy
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Post by abacus9900 on Mar 10, 2011 12:45:37 GMT 1
I did not see the prog but it seems it also missed out discussion of organic change on our home planet which over time has gone from simple to complex - from single cell life to highly complicated organised life-forms like us. Over time should we expect to revert to single cell creatures? What a bore this entropy is! Well, I think the point is that entropy at the atomic level acts very, very, gradually, so that despite complex life forms being able to form from simpler ones the the underlying disorder of matter (atoms and their constituents) is relentless so that matter will continue to be able to make complex organism for a very long time but in the end matter itself will become so disordered that nothing of any complexity will be able to exist. In the programme Brian Cox gave an example of a sandcastle. Just next to the sandcastle was a pile of sand and the point he made was that when you shift the sand around in the pile you are not really disorganizing anything since it was very disorganized in the first place, i.e. high entropy. But if you knocked a bit of sand off the sandcastle you would be disorganizing it quite a bit since it is a highly ordered construction and thus very sensitive to entropy, i.e. low entropy. It is the same principle when we age; we are very highly organized organisms and therefore possess very low entropy so that it doesn't take very much for us to become disordered (higher entropy), for example, if we were to become victim to some life-threatening disease. So, we have entropy acting on two levels. The atomic level and the level of chemical reactions that form the world around us, including all life on earth. The latter tends to be more sensitive to entropy as it is involved in complex lifeforms.
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Post by marchesarosa on Mar 10, 2011 13:41:53 GMT 1
"So, we have entropy acting on two levels. The atomic level and the level of chemical reactions that form the world around us, including all life on earth. The latter tends to be more sensitive to entropy as it is involved in complex lifeforms."
I don't get that bit about "the latter", abacus. Looks to me as if 'life' is one of the exception to the rule of everything "running down".
And what about stars and planets forming as I speak from cosmic dust clouds? That is change in the direction of greater not less organisation isn't it?
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Post by speakertoanimals on Mar 10, 2011 13:55:44 GMT 1
This is NONSENSE. The point about the second law and the thermodynamuic arrow of time is that ity talks about the entropy OF A CLOSED SYSTEM. London isn't.
Plus hand-waving definitions of entropy in terms of order or disorder can mislead.
As regards life, it is perfectly SIMPLE -- it's NOT a closed system, but can only continue by taking large amounts of energy out of the surrounding environment. If you then add up supposed increase in order of living matter, and change in order of environrment (to get a closed system), then entropy and disorder has net increase, as second law says.
When it comes to planets, that too involves a net INCREASE in entropy. Why do some people think otherwise? Because they have forgotten the bit about a closed system.
Take some lumps of rock floating in a cloud. Gravity pulls them together, and as they fall their gravitational potential energy is converted to kinetic energy. When they collide in a lump, this kinetic energy is now converted into heat. Which is why earth originally molten.
As the heat is lost we finally end up with a cold planet. LOOKS more ordered than floating lumps of rock, but only if we forget the large amount of entropy associated with the heat that has now been radiated off. When we take that into consideration as well, we see that we have gone from floating lumps of rock (whose GPE could have been turned into useful work), to just plain ole waste heat. Hence net entropy INCREASE due to gravitational collapse.
This is utter nonsense! As I said, the ONLY reason we can have seemingly highly-ordered forms of matter such as life is because they have to take in a large amount of energy to keep going. When you look at the entropy increase, in effect, involved in going from food to poo, this more than offsets the entropy decrease by seemingly ordered living matter.
So, the second law saved by excreta.......................
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Post by abacus9900 on Mar 10, 2011 14:09:51 GMT 1
"So, we have entropy acting on two levels. The atomic level and the level of chemical reactions that form the world around us, including all life on earth. The latter tends to be more sensitive to entropy as it is involved in complex lifeforms." I don't get that bit about "the latter", abacus. Looks to me as if 'life' is one of the exception to the rule of everything "running down". And what about stars and planets forming as I speak from cosmic dust clouds? That is change in the direction of greater not less organisation isn't it? Well, what you are looking at there is entropy at a much lower level, i.e. complex structures, as opposed to entropy at the atomic level (very high entropy). You see, to go back to the example of a pile of sand and the sandcastle, they are both made of sand which is, in turn, made of bits of matter which as you know is made of atoms but despite the sandcastle being extremely sensitive to disorder, it does not affect the atoms that compose it. You could destroy the sandcastle and its atoms would be exactly the same as those in the pile of sand. Stars are still forming, yes, but in doing so an infinitesimal amount of disorganization takes place, as with all other processes in the universe, so that the universe will still go on for a very long time but very gradually the matter in it is tending towards disorder. Eventually, in an unimaginable amount of time, there will be no more matter in the universe, just a sea of photons at absolute zero.
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Post by abacus9900 on Mar 10, 2011 14:12:29 GMT 1
Why don't people live forever then?
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Post by speakertoanimals on Mar 10, 2011 14:21:43 GMT 1
Actually a LARGE amount of disorganisation, since the heat produced by theb gravitational collapse is enough to start nuclear fusion. So we have the conversion of gravitational potential energy into heat by the initial collapse, then it gets even worse as the heat and light pumped out by the star, generated by mucking about with the nuclear organisation of the matter from which it was made. This isn't just a teeny change, but the most exothermic (i.e. rate of heat generation per unit mass), hence most entropy-increasing physical process we have!
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Post by abacus9900 on Mar 10, 2011 15:03:12 GMT 1
Actually a LARGE amount of disorganisation, since the heat produced by theb gravitational collapse is enough to start nuclear fusion. So we have the conversion of gravitational potential energy into heat by the initial collapse, then it gets even worse as the heat and light pumped out by the star, generated by mucking about with the nuclear organisation of the matter from which it was made. This isn't just a teeny change, but the most exothermic (i.e. rate of heat generation per unit mass), hence most entropy-increasing physical process we have! Yes, but the atoms don't suddenly disappear do they. You're talking about entropy at a much higher level.
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Post by speakertoanimals on Mar 10, 2011 15:43:42 GMT 1
Meaningless drivel.
Entropy is either THE entropy of a system, or it isn't, there is no higher/lower level.
Plus thses repeated mistakes as regards entropy (like ignoring nuclear fusion, produces a teeny bit of energy there!), just shows that you have no idea about the basic physics you are attempting to discuss.
Hence all this bollcks about life seemingly going against the second law, which is BASIC stuff, and the usual ploy of creationists. Its total bollocks when they bring it up, and its total bollocks here..............
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Post by abacus9900 on Mar 10, 2011 18:43:35 GMT 1
Now what the f*** does that mean? Be clearer.
Entropy works at different levels because it is defined as disorder and many things, both on the atomic scale and in the ordinary world, become disordered. You are probably using the term in some specialized sense, therefore obfuscating its general meaning to the good people here who seek to gain a basic understanding of the concept.
This is your whole problem, STA. You always seem to neglect making basic definitions and then build on them, which is the correct way to teach. What you do is to plough straight into complexities that most people here have little idea about and then expect them to know what you are talking about. Doesn't work that way, but I do not honestly think you will ever grasp it. You're no Brian Cox!
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