Martin

Failures to communicate due to terms like B.B. are not uncommon. You and I are experiencing a failure of that sort. To me it seems obvious that the Big Bang is generally agreed on. I can't understand how you can imagine that it is not, or that what I said implied that it is not. The question is not about whether there was a big bang. The question that many people are investigating these days is what conditions preceded it and what led up to it. Talk about "singularity" is, and always was bullshît. Nature does not have singularities (they are recognized to be breakdowns in manmade theory). There is no scientific reason to believe "time began" with the B.B. and it is difficult to imagine what that could mean. A good clear up-to-date exposition of Big Bang concepts and terminology is at the Einstein Online website. The link is in my sig. Please check out their essay titled "A Tale of Two Big Bangs"

I'm glad you had a look at Gerard 't Hooft's essay. It is chapter two of this new book: http://www.cambridge.org/uk/catalogue/catalogue.asp?isbn=9780521860451 This is an expensive (academic libraries market) book just published by Cambridge U. Press containing contributions by some 20 experts in the various approaches to defining quantum spacetime. (Gen Rel teaches us that the gravitational field is really a field describing the geometry of space and time, i.e. that gravity = geometry, and so quantum gravity must be a quantum theory of the geometry of space and time and how it interacts with matter.) The book's title is "Approaches to Quantum Gravity: Towards a New Understanding of Space, Time, and Matter" I have highlighted the second part of the title because it is more clearly descriptive of the subject matter. Free copies of some of the chapters are available online. (This is fortunate because as I say the book is quite expensive.) You have read Chapter 2. In case you might be interested, here is Chapter 1: http://arxiv.org/abs/gr-qc/0604045 (click on 'pdf' to get the full text) Here are some addtional links: The Nature of Time (first prize FQXi essay by Julian Barbour) http://arxiv.org/abs/0903.3489 "Forget Time" (first prize FQXi essay by Carlo Rovelli) http://arxiv.org/abs/0903.3832 Home page for the Foundational Questions Institute (long title: Foundational Questions in Physics and Cosmology, FQXi) http://www.fqxi.org/

Stars form out of gas clouds. Our galaxy still has some dense gas clouds where stars are forming. Not as fast and abundantly as in earlier times, but still. So you are right: there will be other stars replacing burned out ones, for a long long time. You are also right about other galaxies. Not many galaxies are even visible with naked eye. As long as there are stars in our own galaxy, and enough around us in our immediate neighborhood, the sky will continue to be beautiful. However after a very long time, after the star-forming gas clouds are depleted, and all the stars that have formed have burned out, this will present whatever life-forms remain with a considerable challenge. This is why I mentioned having to find some way to extract energy from dead stars.

BTW check out this recent essay by Gerard 't Hooft http://www.phys.uu.nl/~thooft/gthpub/QuantumGrav_06.pdf He grapples with the issue of the indeterminacy of Quantum Mechanics. How the collapse of the wavefunction is constantly (as you say it) injecting new decisions by nature or free will type stuff. "Initial conditions". I don't mean to be precise. He wants a deeper level of reality in which there is simple determinism. He wants the indeterminacy of QM to be just a superficial illusory appearance that emerges from this deeper deterministic mechanics. As we all know he is a Nobel theoretical physicist and one of a handful of the currently most influential in the world. Enormously respected guy. And he is very serious in this essay. He's been thinking about these things for some time and this is his latest and strongest statement on it. It is written for fellow professionals so it is technical and you wont understand much of it, if you take a look. But you might as well look. It will give some taste, some idea, of deep controversy going on among physicists themselves. You shouldn't have to rely entirely on popular books by Roger Penrose, or the commercial successes of Stephen Hawking from the 1980s. Try to get recent firsthand sources, post 2005 or at least post 2001.

McGarr, I hope I'm not being dense or impertinent. What do you get by arguing with any of us? I'm not saying you shouldn't! We like to argue and we do a lot and it can be beneficial. But what does it do for you specifically? Would it be fair to say that you have some kind of vision that causes you to rejoice in the universe? And this vision has absorbed a fair amount of post-1950 quantum physics and cosmology----either as serious premise or as metaphor? You say in one of your posts that you come here (to sfn) to learn. I wonder what you can learn. Is there anything you can learn from me, I wonder. You could also try to teach---you could be posting links to writings and recorded lectures by Roger Penrose, for instance. You could be trying to get other people here to look at them. This might fail of course. Argument is the most sure-fire mode of engagement. There is always that. BTW it is not settled that the universe began with the big bang. Until around 2005 the orthodox view was that "before the bb" was meaningless because there was no time. but since then there's been a shift in the research community. just hasn't been popularized yet. links if you want

Hello McGarr, I just saw your post #1 here and looked at your blog first two essays* and i will tell you what I think, since you ask. I think literature has a serious/vital purpose of refreshing the spirit and preventing the soul from drying out and getting stiff-like. We actually need it. We pursue science for the honor of the human mind and for evolutionary reasons, but probably we don't need it the way we need inspired literature. I think your two blog essays are literature of a quite alive kicking sort and I'm pleased with you for trying. *Well it turns out that the two essays I saw were these: http://phoenix-from-the-wasteland.com/blog1.php/Philosophy/ which means the "Initial Conditions" essay and the "Any Belief at All" essay. I don't know how I got to the second one about belief. I actually liked that one best. **Shucks! I just realized that the essay I liked about belief wasn't your writing, but was by somebody named Gaiman. You make that clear in the first sentence but for some reason I didn't register the first time.

Sure does! Big chill (if I understand what you mean by it) is the standard picture of the longterm future. Our galaxy will be bigger then (due to merging with other galaxies like Andromeda). Stars will burn out*. Dark and cold. Other darkening galaxies drifting away gradually fading out of sight. Surprises could of course happen. This dark cold picture just happens to be what is predicted by the best model so far devised. Why couldn't future discoveries change the model? and thereby change the forecast? *If we are still around, in some form or other, we will have to figure out how to eat dead stars. There will be a lot of dead stars in our vicinity. According to the standard model, expansion does not affect individual galaxies, so the galaxy won't drift apart (it will in fact merge with some others) so there will be billions of stars around.

This is what you get from the standard mainstream cosmic model. I haven't seen a professional research paper about "Big Rip" for several years. The idea is either dead or taking a nap. Some new discovery could conceivably revive it. Same for "Big Crunch". Barring some new discovery it doesn't seem to be in the cards. It might appeal to the imagination. I don't think you report the "Big Bounce" idea accurately. The idea is not cyclic. It is not about the future. There has been an explosion of interest in the idea that the big bang could have been a rebound from an earlier collapse. So many new researchers have gotten into this and there are many more papers appearing about it, than say 2 oer 3 years ago. But when people talk about Bounce cosmology or non-singular cosmology they are normally not talking about the future. They are just talking about mathematical models of what happened 13-some billion years ago. There could be a bounce in future but that is conjectural and would depend on dark energy decaying (something we have no evidence of) or some other surprising aspect of the universe that we have no clue of as yet. So the focus of attention is on the possible past Bounce, where we may be able to observe aftereffects in the cosmic microwave background. A quick way to get current research literature on Bounce cosmology models is just do a keyword search on "quantum cosmology" recent papers, like this: http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=FIND+DK+QUANTUM+COSMOLOGY+AND+DATE+%3E+2006&FORMAT=www&SEQUENCE=citecount%28d%29

I don't know of any scientist who believes this. It is not the common opinion of cosmologists, or any kind of astronomer. In the usual model that is used professionally, the expansion decelerates for at least the first 6 or 7 billion years. Then it gradually starts accelerating. So it is not believed that expansion has been speeding up the whole time. Much of the time it has been slowing. Also there is no reason to imagine that a singularity actually existed. All the model can say is that when you project back in time you find conditions of very high density. We can't say that the density was ever *infinite* however. Some recent models show a bounce----a prior phase of the universe contracts down to very high (but not infinite) density and then rebounds. This issue is not yet resolved. All one can say is that there is no scientific reason to suppose that time "began" at the big bang. The universe can very well have been there before, evolving according to its laws. You talk about some kind of cycle: contracting---then rebounding---then expanding for a while and then starting to contract again. There are some conjectured mechanisms that would cause that, but it's too complicated for me to find interesting right now. What interests me is whether or not a model with one single rebound might be right. That would have to be tested by detailed study of the cosmic microwave background and any other relevant data, to see if it agrees with observation.

The answer is no! (one or two perhaps, almost none), but you have to be careful with the concept of "major physics guru". For example, some people think of Stephen Hawking as a major figure. He is major in the media, and he was major in research back in the 1980s. Here's how to do a preliminary rough spot check on someone's standing in the research community. See if they have written any highly cited papers in the past 5 or 10 years. Citations measure how interesting the other experts think someone's research is. If it makes valuable progress they will cite it in their own papers as a reference, and build on it. Here is how to do that for Hawking. http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=FIND+A+HAWKING+AND+DATE+%3E+2005&FORMAT=www&SEQUENCE=citecount%28d%29 You will see 5 papers and the most cited one got 18 cites, the rest got 10 or less. Change the name to Ashtekar. You can do that manually, just backspace over Hawking and type Ashtekar, then click search. http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=FIND+A+ASHTEKAR+AND+DATE+%3E+2005&FORMAT=www&SEQUENCE=citecount%28d%29 You will get 22 papers with several citation counts in the 100s. Explanation: the letter A after the word "Find" stands for Author. The search engine looks for scholarly research papers by that author, and it sorts them by citation count (assuming you have selected that type of sort.) ================================ Another concept to be careful with is "acceptance". The author of a scientific theory himself does not necessarily accept it. He puts a conjecture out there under the spotlight to be tested. It might be something he would like to see observation or experiment rule it out! The important thing is that it be interesting---there should be consequences whether it is true or false---and that it be testable. A theory is not necessarily a pet, not for everybody anyway. It's a step along the way. Hans Bethe was a Nobel laureate who I guess qualified as a guru in various senses. He collaborated on several neutron star papers that helped to get settled on ways to test CNS. At least he was that interested. George Ellis co-authored with Hawking around 1970s and has stayed productive. He has shown considerable interest in CNS, written positive things about it. A recognized top authority on cosmology, but on the old side. Lee Smolin might be considered a physics guru in his own right. He seems to have a lively interest in CNS. I would not imagine that he believes it, though. Theories are meant to be tested, and used to make predictions.

Sure, a whole book was written about the idea, and how we might detect signs of it having happened and thus get some observational evidence to back it up. The book is called The Life of the Cosmos, by Lee Smolin. Look it up on web, if you want. The extra idea he put in, that you may not have thought of, is evolution. Each time a black hole buds off a new expanding region, the daughter can be just slightly different from the mother. The difference would be that some of the physical constants, the proportions in the laws of physics, might be slightly different. Slightly different masses of particles, slightly different strength of electromagnetic force, etc. The daughter region might thereby be more able to produce black holes (and have more offspring) or less able, and have less offspring. This would lead to natural selection favoring "reproductive fitness" and a population that had physical constants fine-tuned to make lots of stars, especially stars able to collapse to form black holes etc. So we can ask if the fundamental physics constants are well-adapted to star-formation, etc. or are they not. It turns out that several of the basic constants have an effect on the rate of star-formation and black hole production. So there are good prospects for testing the idea. The whole idea is sometimes called Cosmological Natural Selection or CNS, and there is an empirically testable CNS conjecture. The book, Life of the Cosmos, came out in 1999. A more recent paper on CNS was http://arxiv.org/abs/hep-th/0612185 The status of cosmological natural selection Lee Smolin 25 pages (Submitted on 18 Dec 2006) "The problem of making predictions from theories that have landscapes of possible low energy parameters is reviewed. Conditions for such a theory to yield falsifiable predictions for doable experiments are given. It is shown that the hypothesis of cosmological natural selection satisfies these conditions, thus showing that it is possible to continue to do physics on a landscape without invoking the anthropic principle. In particular, this is true whether or not the ensemble of universes generated by black holes bouncing is a sub-ensemble of a larger ensemble that might be generated by a random process such as eternal inflation. A recent criticism of cosmological natural selection made by Vilenkin in hep-th/0610051 is discussed. It is shown to rely on assumptions about both the infrared and ultraviolet behavior of quantum gravity that are very unlikely to be true." This is probably more technical than you want. But it is free for download, while the book takes a trip to the public library or ordering from amazon. There will be a chapter devoted to CNS in a new book coming out later this year, called Beyond the Big Bang: Prospects for an Eternal Universe, edited by R. Vaas. If you are interested, keep asking about it now and then and some more free online stuff will probably turn up.

Simultaneity and the existence of "universe time" in cosmology is no big deal. It comes straight out of the existence of the CMB. An observer is at rest relative to CMB if he sees no doppler hotspot (only the usual fluctuations in temperature which are like 1/1000 of one percent). Aside from gravitational effects (which can be made small) two stationary observers can agree---synchronize clocks and all that. Stationary observers are called "comoving" (some cosmo jargon). Before the CMB was discovered cosmologists already had the idea of universal rest---being at rest with respect to the expansion process or "Hubble flow". If you are at rest then the expansion looks symmetric. Same thing. The point is that using universe time and comoving distance (based on the simultaneity seen by observers at rest) is optional. It just happens that the statement of Hubble Law uses this idea in its statement. And the Friedman model (a couple of simple equations) uses it. It would be a lot more complicated to state the basic facts of Hubble Law and Friedman model if you used some other coordinate system, some other idea of distance, etc. and pretended you didn't have this natural idea of simultaneity. Ultimately using those things is optional and based on convenience. In pure GR you don't have the CMB and life is harder.

I tried to say thanks with the button but it said spread some around first. So I'll just say thanks, NowThat, in traditional buttonless fashion. Lakmilis, probably the most common distance measure in cosmology is the "comoving distance" which is the same as the "proper" or actual physical distrance if it could be measured today. (By a pre-arranged sequence of observers with radar devices stretched out between here and there all measuring simultaneously according to the universal timescale.) The Friedman equation model basic to all cosmology, and the Hubble Law are both stated in terms of that distance and depend on the same criterion of rest and simultaneity. Cosmology is slightly different from GR in having those features, it is a specialized and simplified form of GR. So the usual measure is the distance to the object now. It is not the light travel time or any of the other various distrance scales, iike luminosity and angular size, that occasionally get into the act. NowThat suggested one of Ned Wright's calculators. You might also take a look at another one of his calculators (if you haven't already): http://www.astro.ucla.edu/~wright/CosmoCalc.html For some reason I would have guessed that you were thoroughly familiar with Ned Wright's cosmology tutorial website. But just in case you are not, please do check it out.

Lot of questions, maybe we can share them. I'll take this one: what is Sgr A*? http://en.wikipedia.org/wiki/Sagittarius_A* Officially it is the radio source associated with the supermassive black hole. Some of the signal is in millimeter band. I don't know what the overall spectrum is like but we could probably find out. Black holes often have processes going on round them that radiate, like the hot accretion ring, and polar jets. The way I think of it, it's all one object. So I think of the 4 million solarmass black hole, and all its accoutrements including the radio source as a single object called Sgr A*. You may have had something different in mind, like what processes might be causing the radiofrequency radiation? If so keep asking. Ask more detailed questions. It's better to be definite even if it's something I or the others can't answer.

Moffatt's ideas are out of date. The beautiful thing is that in many papers that have been coming out, including the cluster collisions ones, they map the irregular clouds of DM by weak gravitational lensing. They make these contour maps showing the varying density of the clouds. I gather the computer software needed to do the mapping has only recently been developed. I've only been seeing it the DM cloud maps since around 2006-2007. At least the ones with stunning detail. ================= Back in 2004-2006 I used to think the Mog people might be right. Not necessarily Moffatt and Browstein, but there were some other proposals. Bekenstein had one. I was never convinced by Bekenstein's version of Mog, but I did think it had a chance and I was sometimes secretly "rooting" for some type of modification to do away with the need for DM and DE. Then in 2006 the Bullet Cluster stuff came out. Moffatt's response was weak, I thought. Moffatt and Browstein talked as if they could account for it but they never had much case after that. The other people like Bekenstein stopped doing Mog and went away. Moffatt is the die-hard but even he hasn't published very much about it since 2007. Except that popularization book that apparently makes him the central figure . I don't count a pop-sci book like that, I mean peer-review research papers. Heard very little from him about Mog after 2007. Then later there was another cluster collision----like Bullet only different enough so if Moffatt fudged around and made it look like Bullet was OK then he couldn't make his Mog work for this other one. So the interest in Mog died after 2007. ========================== The decisive 2006 paper was this http://arXiv.org/abs/astro-ph/0608407 A direct empirical proof of the existence of dark matter If you want to discuss it, read the abstract and then click on PDF and have a glance at the paper. This paper has been cited in the professional literature 205 times. Citations are a measure of the interest and importance of research as judged by other experts. Moffatt papers about Mog written after 2006 tend to get, like, 2 citations. They could well be just Moffatt citing his own earlier work. The 100 to 1 ratio in that objective expression of interest can't tell the whole story, but it is suggestive. Another thing is the way other researchers moved on to other topics, which happened but would be more involved to document. Personally I still think it is possible that someone (probably not Moffatt since he shows signs of being a bit inflexible) might come up with a modification of General Relativity that could explain dark energy. That would be nice. Maybe some of the new approaches to quantum GR will turn up something. It would probably take a much younger person with completely fresh insight. Might happen though.

Those are good questions, Inquisitor. Easier to think about when cleanly separated from theological issues. Physicists are undecided about time. There was just an essay contest on The Nature of Time and several world-class physicists contributed. For some purposes it is useful to geometrize time---think of the universe as a 4-dimensional block---treat time as if it were another spatial dimension---a block containing the track of every particle back into the past and forward into the future. That can be a useful effective model for analyzing some things but it seems to have problems at a fundamental level. So what you are raising are, in fact, open questions. Physicists haven't settled on a single idea of time. They discuss it and argue, take different positions on it. You may have heard of the current difficulty they're having putting the two main 20th century theories together (General Relativity and Quantum Mechanics). Both are fine accurate theories that have given us a lot correct precise predictions and a lot of progress etc, but they actually have incompatible ideas of time and that is the key reason they don't fit together nicely.

Airbrush, I haven't reviewed the numbers since I saw them a while ago, but I think you have it right. To sum up, this is a "cost effective" way to look for earthlike planets and also get statistical estimates of how common they are. It is expensive in time and resources to measure the wobble of a star because you have to do very precise doppler shift measurements on various spectral (color) lines in the starlight from different chemical elements and you need to commit precise instrument time over several years. The first exoplanets were (at least mostly) found by wobble, measured at groundbased observatories. So this Kepler mission approach is going to look at a patch of 100,000 stars all at once. All it needs to keep track of the light intensity of one star is one pixel. A cheap commitment of resource. It can report back inside of 2 or 3 years if there was a periodic dimming. A quick return on investment. The trade-off is that only 1/200 of planet systems would be oriented right so that the planet transits the disk of the star and briefly cuts down the light. So you do a large sample like 100,000 and you figure that you will only get 1/200 of the planets that are really there. But that's OK. It is a simple practical method. I think that's the idea (and also I'm just repeating what I think you already figured out.) We don't actually know how it will work! I think the idea makes sense, so I am looking forward to see what success they have. BTW do you know how they estimate the mass of a star starting from things like the color of the light (first using a curve called Hertzsprung-Russell, or "the main sequence"). Then using a mass-luminosity relation calibrated by observing binaries etc. It's rather neat. You might look it up. Once you know the mass of the star, and the orbit period (days or years) of the planet then one of Kepler Laws tells you the distance from the star to the planet. And then knowing brightness of the star and the distance you can infer the temperature etc. so the keystone to inferring what the planet is like is to estimate the mass of the star (and determine the length of the "year", which is what this spacecraft will do). The method is fundamentally so elegant that it is hard not to suspect that humans were evolved by nature specifically to find habitable planets. What other purpose could evolution possibly have had in mind?

Has anyone ever seen a presentation of the Planck units in which the Planck charge is defined to be precisely equal to the elementary charge e? This would mean that the coulomb force constant would be derived (rather than being a unit itself.) Using the fine structure constant, alpha, approximately 1/137, the coulomb force constant would be [math]\alpha \hbar c/ e^2[/math] It would also mean that the Planck charge value does show up in nature (all the time! ) I think it's optional. You have a choice when you set up Planck units. Wikipedia is not always the best source. http://en.wikipedia.org/wiki/Planck_units The authors there are pushing the version of Planck units with the coulomb force constant used as unit. But I've seen different versions.

The Rovelli essay came out on arxiv yesterday. The font is larger so it is a more readable version. Minor addition(s). http://arxiv.org/abs/0903.3832 "Forget time" Carlo Rovelli 'First Community Prize' of the FQXi 'The Nature of Time' Essay Contest (Submitted on 23 Mar 2009) "Following a line of research that I have developed for several years, I argue that the best strategy for understanding quantum gravity is to build a picture of the physical world where the notion of time plays no role. I summarize here this point of view, explaining why I think that in a fundamental description of nature we must 'forget time', and how this can be done in the classical and in the quantum theory. The idea is to develop a formalism that treats dependent and independent variables on the same footing. In short, I propose to interpret mechanics as a theory of relations between variables, rather than the theory of the evolution of variables in time." Using the standard-format Arxiv link rather than having to go into the FQXi website makes the essay more conveniently available and easier to cite. Rovelli's lecture series at the September 2009 Corfu school can be expected to mark a new stage for LQG. Here's the summary: Title: Covariant loop quantum gravity and its low-energy limit "I present a new look on Loop Quantum Gravity, aimed at giving a better grasp on its dynamics and its low-energy limit. Following the highly succesfull model of QCD, general relativity is quantized by discretizing it on a finite lattice, quantizing, and then studying the continuous limit of expectation values. The quantization can be completed, and two remarkable theorems follow. The first gives the equivalence with the kinematics of canonical Loop Quantum Gravity. This amounts to an independent re-derivation of all well known Loop Quantum gravity kinematical results. The second the equivalence of the theory with the Feynman expansion of an auxiliary field theory. Observable quantities in the discretized theory can be identifies with general relativity n-point functions in appropriate regimes. The continuous limit turns out to be subtly different than that of QCD, for reasons that can be traced to the general covariance of the theory. I discuss this limit, the scaling properties of the theory, and I pose the problem of a renormalization group analysis of its large distance behavior." This link gives descriptions of several of the other mini-courses, or lecture series, being given at the school. http://www.maths.nottingham.ac.uk/qg/CorfuSS.html

Good! That's the kind of thing I should have said when Moo asked for help. I want to deputize you. Instead of just yakking you injected relevant quantitative fact. Thanks also to C.R. for the links.

You are right! I was thinking of electric oscillator or atomic clocks. That was a keen observation Mr. S. The pendulum clock at the top of a tall building would run slower than his brother on the ground floor because of less pull on his pendulum.

It's really hard to tell what XxFar is saying, and so to figure out what the real issue is here. XxFar, your post really needs grammatical sentences with punctuation. It isn't fair to the others to make it so confusing. Anyway, in my opinion the big issue here is gravitational time-slowdown. This is a really practical thing that engineers have to allow for and correct for. Clocks really do run faster at the top of a building than at the bottom, the same identical clock. So the GPS satellites each have a very accurate clock on board---atomic clock actually---and that helps them send out signals that help someone on the ground locate themselves. And those GPS satellite clocks actually run a little bit FASTER than the same clocks would on the ground. It had to be allowed for in designing the system. Because being deeper down in a gravity well slows your time. And we down here on the surface are down in a potential energy well, so we are slowed, our chemical reactions and everything, compared with people living out in space. And this is actually very simple to visualize and not hard to understand if you first think about the gravitational redshift. If someone at the top of a building shines a green light down at someone on the ground the light will gain a little energy and be imperceptibly blue shifted by the time it gets down. If the groundster shines the same green light up at the topster, the light will be a little redshifted by the time it gets to him or her. It will have lost a little energy. This isn't perceptible but it's real. And it is measurable if the gravitational field is strong and the difference in altitude is large. Well the ticking of a clock is analogous to the vibrations of a lightwave. So if you are high up in a gravity field and you look down at some groundster and his clock, you have to see his clock moving slower! Because it is just like he sent you light of a certain frequency and it was redshifted to a lower frequency when it got to you. the factor of gravitational redshifting is the same factor as that for gravitational time-slowing (technically called gravit'l time-dilation). DH, Swansont, Atheist and I forget who else are all very good on this. It might be good to call on them sometime just to share the fun, and as insurance (in case I'm mistaken I appreciate getting straightened out). BTW that Cylon Six is dazzlingly beautiful.

You're OK.:cool: That's not as wacky as it sounds. Probably both those ideas are something physicists have explored at one time or another. BTW people look at the world differently. the way i look at the world a planck unit doesn't need to BE anything in particular. There doesn't have to be an applied interpretation or use for every planck quantity. And factors like 2 pi and 8 pi don't matter. I don't feel I have to interpret a planck quantity although it is nice when an interpretation or application shows up. so I'm not opposed to looking for them. ============================= here's a problem for you. Calculate planck density and say approximately how many times denser than water is it. What would you guess? A million times? A billion times denser? 10100 times denser? In quantum cosmology, the currently most studied model is LQC (loop quantum cosmology). That approach currently has the most highly cited papers and is getting the most researchers started working in it. In that approach the way the big bang works is there is a collapse of a universe similar in its basics to ours, leading to a moment of very high density. And quantum corrections to gravity become dominant at that density and cause gravity to reverse briefly and repel rather than attract; so there is a rebound, that causes the expansion we see and all the stars and galaxies have to condense and form all over again. Now the question is, what do you suppose that critical density is, that is reached when the so-called Big Bounce happens? The density at which quantum gravity effects become dominant, in the LQC model, and cause the collapse to rebound. How many kilograms per cubic meter, or what is slightly different how many grams per cc?

buttacup is unquestionably the most exciting person on Scienceforums today. The solution to the naturally flying human problem is to do two things. Migrate some humans to a colony where there is gentler gravity like Ceres or one of the moons of Jupiter. And there, where gentle gravity makes flying easy, genetically engineer a human offshoot species with small, tastefully designed wings. A large artificially-lighted ice-cave on Ceres filled with ordinary earth-like air would be a satisfactory venue, at least for starters. Earth gravity is unforgiving. With this kind of gravity wings would need to be awkwardly large and moved by unattractively bulky muscles. In mild gravity the additional muscle could be graceful and the wings wouldn't get in the way of other activities. In any case there need to be some new human species and this could be one.

gre, do you want to move on, with your discussion of Planck units? We got some help from Swansont and it seems like that resonant frequency issue is under control now. Or? Earlier you were "exploring" in a sense by calculating some Planck quantities and comparing natural quantities. Do have any more of those random calculator explorations to show us or that you want to talk about? If not that's fine---you may have more purposeful things to do. I'll ask you a question which you don't have to answer (just a playing around type question in case you want) What type of physical quantity is this?: c4/G I mean, is it a length? A volume? A mass? What sort of quantity is it? Or is it some nameless type of quantity that nobody has a use for? I ask because the main equation of General Relativity uses the reciprocal of that quantity as its key coefficient. The lefthand side has units of curvature (reciprocal area, like 1/square meter) and the righthand side has units of pressure (or equivalently of energy density, a pascal is a unit of either one.) curvature is related to energydensity (or equivalently to stress) and the proportion that mediates between them is [math]8 \pi G/c^4[/math] So there is the reciprocal of c4/G what type of physical entity is it? Does it play a role in the Planck system of units? If so, then is it the Planck unit of what? You may already know this, but I mention it in case you dont because it's kind of nice.