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Post by Progenitor A on Sept 30, 2010 8:17:38 GMT 1
Some cosmologisrts talk of the Big Bang as an expansion of space-time, not an explosion Other Cosmologists talk of a Big Bang explosion Theses questions focus on the expansion os S-T. What causes space to expand? We are told (unreliably radio4scienceboards.proboards.com/index.cgi?action=gotopost&board=talk&thread=223&post=2585) that no force/energy is required to cause space-time to expand. Some cosmologists speculate that gravity is causing the expansion rate of space-time to decrease Does this not indicate that a counter-force (gravity) is opposing the force that is causing the expansion of S-T? And as the forces of gravity were very very large at t=0, then why did not gravity completely inhibit the expansion of S-T at that moment? Could it be (it must be surely?) that at time t=0 the forces causing expansion of S-T were greater than the inhibiting force of gravity.
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Post by abacus9900 on Sept 30, 2010 15:15:07 GMT 1
It must have been greater, yes. People used to wonder whether the attractive force of gravity or of expansion would win out but things have been complicated since the discovery of dark matter which seems to be pulling matter against gravity so perhaps the universe will eventually become a wispy distribution of matter. Perhaps future discoveries about the universe might makes things clearer because our knowledge to date is limited.
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Post by speakertoanimals on Sept 30, 2010 15:46:26 GMT 1
Not really -- the idea there is that the negative pressure of dark energy that is causing the expansion to accelerate.
The problem with this question is that you are trying to look at the universe based on analogies. So, if we had some stuff expanding outwards from a point, then obviously as things move outwards, they gain gravitational potential energy, hence slow down, right? So if they keep going, then something must be putting energy in?
Except this is the wrong picture -- stuff getting further apart because space is expanding is not the same as stuff expanding outwards from a point because stuff is moving outwards.
Gravity opposing the expansion is also wrong, in the sense of gravity as an attractive force between masses. Why? We take a universe filled with matter, and make it homogenous (same everywhere), and isotropic (same in every direction). Hence it follows from symmetry that the gravitational force on any one bit of matter due to the rest of the universe is ZERO. Hence it opposes nothing.
Except in relativity, gravity isn't just gravitational forces (and isn't even gravitational forces), it is curved and expanding spacetime. So, when it comes to how the mass in the expanding universe effects the expansionj, we don't have simple gravitational forces between matter particles reel things back in, that's plain wrong.
Nor we do we have a simple picture of space as some sort of fabric, that requires a force to stretch it -- that is a picture appropriate for materials, and space isn't a material!
Okay, you say, but there must be SOME energy in spacetime? Yes there is, but think carefully here. Mass likes to attract other mass, how else are planets formed. But matter all collected in a planet gives a BIGGER gravitational field outside that with matter spread out. That matter falls fast as it collects, so the earlye arth was hot, where did that energy come from? What else is changing here apart from the way the matter is distributed?
In terms of spacetime, larger mass means more curved, and from the abiove, we can hence conclude that MORE curved spacetime has LESS energy. Less because the difference is where the energy to make stuff fall faster and faster comes from.
SO, energy in curved spacetime is NEGATIVE, and the more curved, the more negative! Which is totally different to what most people would expect, and is a complicated way of saying that gravity attracts, since matter tries to arrange itself to give the MOST curved spacetime it can (black holes eventually).
So, all this stuff about MORE gravity at t=0 is just plain wrong, based on some mistaken ideas about gravitational forces between matter particles.
The point being, you can't get an intuitive grasp of what is going on here, where more curved means less energy, and pressure reduces expansion of the universe, and negative pressure accelerates it -- because space doesn't behave like a thing, so saying a force MUST be needed to expand it is just wrong in several ways. First because gravity in GR isn't forces anyway. Second, because the way spacetime and the matter in it interact is totally non-intuitive anyway (negative pressure accelerates expansion remember!).
SO, we come yet again to the point that you are asking the wrong questions and making the wrong assumptions because you are trying to use analogies based on matter and materials, and trying to apply them to spacetime.
I don't have an intuitive grasp of what spacetime does, I just have to rely on the equations, because we don't have an intuitive description, analogies such as balloons or rubber sheets are WRONG in essential aspects, like the energy stored in spacetime for starters.
Going to the equations, things get even worse, because although conservation of energy holds locally in GR (because of symmetry in time), same is not true for the entire universe, because same symmetries no longer hold. Hence energy conservation is something that strictly speaking is usefully locally and in special cases globally, but not in all cases.
As to WHY the universe is expanding, in a sense a partial answer is provided by inflation, where a real change in the vacuum energy caused exponential expansion early on, and after that stopped, the universe just kept expanding at a more leisurely rate.
How to look at the expanding universe? Everyday analogies lead you astray, that is not the way to go. The equations are there, just you have to do the maths, no comforting mental pictures, you just have to first admit that there can be things in the universe (like space itself) which behave DIFFERENTLY to the way that the everyday objects around us behave. Unless you can admit that, then you'll be stuck asking the wrong questions and getting answers that sound ludicrous.
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Post by abacus9900 on Sept 30, 2010 16:21:18 GMT 1
All descriptions are wrong ultimately, it is level of detail that is required which is important. How can you expect the average person to look-up the equations in order to get a handle on inflation? Analogies are necessary to allow at least a conceptual representation of phenomena to be grasped and to also act as a kind of framework on which to hang further ideas. The study of any subject requires analogies to begin with, otherwise, you have nowhere as a starting point. The 'plumb pudding' model of an atom, for example, is now defunct because it has been corrected by the scientific method, which is how science works - by refining crude models in light of further research. STA, it seems to me that you expect people to begin as experts instead of working towards that goal.
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Post by Progenitor A on Sept 30, 2010 18:00:59 GMT 1
All descriptions are wrong ultimately, it is level of detail that is required which is important. How can you expect the average person to look-up the equations in order to get a handle on inflation? Analogies are necessary to allow at least a conceptual representation of phenomena to be grasped and to also act as a kind of framework on which to hang further ideas. The study of any subject requires analogies to begin with, otherwise, you have nowhere as a starting point. The 'plumb pudding' model of an atom, for example, is now defunct because it has been corrected by the scientific method, which is how science works - by refining crude models in light of further research. STA, it seems to me that you expect people to begin as experts instead of working towards that goal. I agree with this If analogies are difficult to finnd, if parallels are futilely searched for, if looked-for similies fail us, then we are on the cusp of new information, information that is not understood, rather as child plays with the quadratic formula , gets the right answers but cannot explain what he is doing. It is the refuge of the scoundrel to pretend that they do understand when others cannot see.
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Post by abacus9900 on Sept 30, 2010 18:17:23 GMT 1
naymissus, are we expected to use maths symbols every time we discuss this subject? Ridiculous idea, isn't it?
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Post by Progenitor A on Sept 30, 2010 18:21:23 GMT 1
Well, congratulations on avoiding insults and disparaging dismissal! Perhaps there is hope for physics on this board! Not really -- the idea there is that the negative pressure of dark energy that is causing the expansion to accelerate. Many physicists, including the Cambridge University Cosmology Dept. postulate that gravity is decelerating the expansion of S-T. The fact that dark energy opposes deceleration that is not really to the point The problem with this question is that you are trying to look at the universe based on analogies. Looking for analogies is not the problem. Finding them is the problem. Understanding cannot be expressed without suitable language. The language of understanding is invariably analogy, simile, parallel, logical, understandable models Without these understanding is Not present. The language of maths is fine, but unless mathematical ideas can be expressed in analogy, simile, parallel, it is quite simply not understood. That is what Feynman was talking about You have said elsewhere that you do not understand what is happening outside the mathematical equations OK, that's fine, you cannot explain in English language what is going on That was Feynman's point My analogies probably are faulty , but you have no replacement analogies that hold water. Fine , you/we are struggling to understand. I don't mind that; just don't call people that admit they do not understand stupid, for th fact is, as Feynman has repeatedly stated, no-one understands
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Post by abacus9900 on Sept 30, 2010 18:48:24 GMT 1
STA likes to pretend that maths is the 'Holy Grail' of scientific understanding. The fact is, even maths, no matter how useful in describing difficult concepts, is just another human-based analogy. Maths is never the thing itself but a descriptive model of it. As an illustration, take M-theory. M-theory has been criticized for being a mathematical philosophy of something that cannot, as yet at least, be tested. No matter how elegant the maths of M-theory and its internal consistency, it is not considered a scientific theory until such time as it can be tested. This shows that maths alone is not enough to describe any scientific theory, and other means must be employed to check the mathematical ideas. Now, take this a bit further and think about the kinds of instruments that have to be used to test scientific ideas. Such instruments themselves have evolved from designs that relied on analogies in order to conceive and build them, so this shows it is impossible to completely get away from analogies in order to conduct scientific enquiry.
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Post by speakertoanimals on Sept 30, 2010 19:35:01 GMT 1
This is just utter rubbish, because it is totally obvious that for certain phenomena (such as curved space, or the expansion of the entire universe), there are and never can be any suitable analogies. Because any analogies we build are based on our experience, which just doesn't encompass the necessary objects. Just as there are no analogies for quantum objects.
There is a suitable language, however, and it isn't analogies, it is maths. And maths doesn't have analogies either, for the same reason.
Complaining that we don't really understand something because we have no analogies is just piffle -- we DO understand, just not in the analogical way that some would like to restrict the meaning of understand.
So, what needs broadening here is what YOU mean by understanding, because looking for suitable analogies for the expanding universe is totally fuitile and doomed to failure. Complaining that the universe keeps telling us that maths is the only way to understand stuff is futile as well, and just says that you should just plain GIVE UP, you'll never understand the relevant physics if you chose to restrict yourself in that way.
Wrong. Physics so far tells us that rather than being wrong, out mathematical descriptions of the universe fit exactly, as far as we are able to judge. Postulating some philosophical mismatch, just for the sake of it, is pointless and unscientific. Some might wonder why the maths that we are able to discover fits the universe so well, but ultimately we just have to accept that we are damn lucky, and that the mathematical approach works.
I don't expect the average person to get a handle on inflation, why should I? There are some analogies that can give them some idea of what is going, but those analogies are wrong in some essential aspects. Which is just saying that the average person will only ever be able to have a limited understanding of scientific and mathematical subjects, unless they go off and do the relevant study to an appropriate level. And sensible person ought to be able to just accept that, and admit that some people are capable of understanding some things at a level they will never be able to.
Seems to me here that the important step that people keep complaining about, is the ability to go from the physical to the abstract -- the first stage in maths as soon as you get beyond kiddy stuff like arithmetic or plane geometry. If you can't make that step of abstraction, then you'll only ever have a limited and distorted view of the whole rest of maths, and much of physics. Why some people can't just accept that, I don't know. Instead they seem to prefer to believe that all those things beyond that point are just a great con, erected by physicists and mathematicians to keep the great unwashed out.
Rubbish, it shows nothing of the sort! The limitation here is not in the maths (which is a complete description of the physical theory), but our physical ability to construct physical experiments to test the theory. A purely physical limitation, which would still be there even if the ultimate correct theory WAS string theory or M-theory. Basically, a limitation imposed by our evolved size and shape and limited ability to manipulate the physical world around us.
Really, I have to say that what are appearing on here are some of the stupidest supposed arguments that I have had the misfortune to ever come across. I'd like to think that it's all a giant wind-up, because I'd hate to think that someone actually believed all this tosh.............
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Post by abacus9900 on Sept 30, 2010 20:41:48 GMT 1
In what way is maths not just an anology, STA?
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Post by Progenitor A on Sept 30, 2010 20:59:34 GMT 1
In what way is maths not just an anology, STA? Indeed, unless maths is the reality, then it surely only modelsphenomena. Who will say that a few scribbled equations on a scrap of paper is the reality of an expanding ubiverse? No-one in their right mind! Maths is simply an attempt to model reality. Models are never the real thing Hence maths is an analogy of reality - and not a good one as far as understanding the reality is concerned
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Post by olmy on Oct 1, 2010 8:38:36 GMT 1
Well, congratulations on avoiding insults and disparaging dismissal! Shame you couldn't reciprocate. What do you mean " only models"? Nobody interacts with or understands (external) reality directly. Every single way in which we interact with reality and all of our understanding is done via models - from the one that is auto-generated by our senses and lower-level brain functions onwards. Hence maths is an analogy of reality - and not a good one as far as understanding the reality is concerned 'Analogy' and 'model' are not synonyms. How do you know that a mathematical model is not (in general) good for understanding? Quite obviously it isn't for you but you seem to be assuming that this implies that it can't be for anybody. This smacks of arrogance. The fact remains that the mathematics of relativity and quantum mechanics match reality to the limit of our ability to do experiments. The fact also remains that there are no very satisfactory analogies that can be used to explain these things in other ways. You appear to have an article of blind faith that says that just because you cannot use the mathematics for understanding, nobody else can either.....
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Post by abacus9900 on Oct 1, 2010 10:06:44 GMT 1
This is indisputable and I can't see how anyone can deny it.
So, this would include all higher brain functions using symbolic representations of our experiences, such as language and maths. All mental representations are models of reality of one complexity or another so that when we talk of scientific theories we are really talking about ideas that have a robust causal correlation with observations. We have to remember, however, that we are inevitably forced to interpret such observations according to our innate predispositions which are rooted firmly in the evolutionary history of the species. This is something we are tied to and cannot avoid in any scientific investigations and it is a tribute to the adaptability of human ingenuity that we have been able to produce mental models of our experiences of reality (not necessarily reality itself) that have proved to be consistent and, therefore, predictable and useful to mankind. Mathematics, no matter how abstract, is an emergent property of human brain function and therefore is the product of our four-dimensional world, which means whatever phenomena we apply ourselves to will be 'tailored' to such restraints. No matter how abstract we think we are being in studying some phenomenon or other, what we are really doing is 'tailoring' our experiences of the external world to the 'inner' world of our mental 'constructs'. Perhaps this is the reason why quantum mechanics has proved to be so puzzling to human beings, i.e. it forces us to confront human-based ideas about reality with reality itself; at this level things do not seem to behave in ways our evolutionary heritage has prepared us for.
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Post by principled on Oct 1, 2010 11:31:18 GMT 1
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Post by Progenitor A on Oct 1, 2010 12:04:00 GMT 1
No matter how abstract we think we are being in studying some phenomenon or other, what we are really doing is 'tailoring' our experiences of the external world to the 'inner' world of our mental 'constructs'. Perhaps this is the reason why quantum mechanics has proved to be so puzzling to human beings, i.e. it forces us to confront human-based ideas about reality with reality itself; at this level things do not seem to behave in ways our evolutionary heritage has prepared us for. Another important aspect of constructing models of reality is the language that is used in describing those models. Mathematics is a language that has evolved and become extremely useful, but it is only that, a language that helps some people to 'understand' reality. Now all mathematicians speak another language in addition to the language of mathematics, and indeed many mathematicians are more fluent in their native language than in mathematics (not always true of course). Languages should be translatable, and the fact that it is not possible to construct models of what is happening in Cosmology and QM in the mathematicians native language would indicate to me that those that claim to understand in the language of mathematics do not really understand what is going on. It does not help at all when some of these attempted explanations run counter to other, long established models of physics. It is certainly not a valid option to call people who do not understand the mathematics stupid, especially when some of the supposed assertions have not bee observed experimentally and cannot in principle be observed experimentally. That is not to say th emodels should be abandoned, but some of th estranger claims should be viewed with a healthy scepticism I keep referring to Feynman who was brilliant and above all open-minded Here he is again on understanding physical models : I think it is safe to say that no one understands Quantum Mechanics. ….. One does not, by knowing all the physical laws as we know them today, immediately obtain an understanding of anything much. The more you see how strangely Nature behaves, the harder it is to make a model that explains how even the simplest phenomena actually work. So theoretical physics has given up on that.
What I am going to tell you about is what we teach our physics students in the third or fourth year of graduate school... It is my task to convince you not to turn away because you don't understand it. You see my physics students don't understand it. ... That is because I don't understand it. Nobody does.
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