Martin

most of the galaxies that we will ever be able to see with our telescopes are receding from us at FTL speeds and indeed were recedingfrom us FTL when they emitted the light that we are now receiving from them and Einstein's theory of special relativity does not contradict this because it has nothing to say about recession speeds (the rates at which distances are increasing) but only about speeds of local encounter but some people dont realize this and assume that the galaxies we are looking at cannot be receding from us FTL and they may also think the redshift is a doppler effect which you learn already in basic introductory astronomy courses it is not (a careful distinction is made between cosmological redshift and doppler shift) also think about this: the universe is full of CMB photons which have experienced a redshift of 1100 that is, each photon has lost 1100/1101 of its energy---it has lost over 99 percent of its original energy from the 'recombination' era when those photons originated where has this huge amount of energy gone? is there a global energy conservation rule in Gen Rel that says it has to have gone somewhere? probably several people at SFN are equipped to discuss this, fafalone's profile says he's interested in cosmology and my guess is a bunch more are too and these are basic cosmology topics

the moon's of jupiter have plenty of water ice and they have great scenery because jupiter is big in the sky and very beautiful and because of the gravity assist the delta-vee cost of getting there is not all that great like the galileo spacecraft did, you can do a close flyby of J and then slingshot with the other moons and arrive at Callisto with almost no fuel burned (after the main burn at earth for transfer orbit) mars is dry and does not have these advantages people on Callisto could use nuclear power to melt tunnels into the ice and create under-ice habitats and extract chemicals and stuff the jovian moons are a nice system--eventually a good place to live Bush was a turkey to say go to the moon and mars

also, to get back more to the Leonard Susskind topic, another reason I suspect AJL is a landmark result is that string theory (a competing attempt to get quantum gravity) is in such a mess confusion and even some despair about the "Landscape"----the roughly 10100 vacuum states----about the goal of background independence which Witten called for in 1992 and hasnt been achieved---about the cosmological constant ("dark energy" we keep hearing about)---the outward signs of a decline in the numbers of research papers and in citations----the move from HEP to astrophysics where Xray and gammaray astronomy and neutrino astronomy have gotten hot. in other words AJL comes at a moment when it looks like string may have been over-hyped and may have peaked

just a hunch really, Matt Visser (Washington State) provides some perspective: http://www.phys.lsu.edu/mog/mog19/node12.html John Baez comments on SPR after the May conference in Marseille especially his exchange of posts with Charles Strohmeyer and Thomas Larsson, samples here: http://www.scienceforums.net/forums/showthread.php?p=60096#post60094 the fact that it represents the culmination of a 20 year effort that many people had given up on---the simplicial approach (dynamical triangulations) has always looked like a direct sensible way to quantize gravity, to some people, and they started trying (you mentioned Regge but dynamical triangulations is slightly different and it was worked on starting I think in the 1980s) and the problem has always been that when you put a whole bunch of identical little simplices together randomly you get a crumpled self-impacted thing or a feathery fractal thing and it doesnt have the expected macroscopic 4D appearance----so this AJL paper is, I guess you could say, a breakthrough and also the sense that there are "firsts" here: as they say on page 3 "In what follows we will report on the outcome of the first ever Monte Carlo simulations of four-dimensional causal dynamical triangulation...We will present strong evidence that the Lorentzian framework produces a quantum geometry which is both extended and effectively four-dimensional. This is to our knowledge the first example of a theory of quantum gravity that generates a quantum spacetime with such properties dynamically." http://www.scienceforums.net/forums/showthread.php?p=60056#post60056 so for various reasons, a hunch that the paper signals a very interesting development that changes the Quantum Gravity picture (it has made a splash among people who know the field, it is a breakthrough in a 20-or-so-year line of investigation, it has firsts, also the approach is intrinsically simple and direct, if it works it has the quality of obviousness)

http://arxiv.org/hep-ph/0406197 this recent 10-page paper by Susskind is descriptive of the current situation in string research and may be worth printing off, for anyone interested in stringy theories Naturalness and the Landscape (it appeared yesterday and is available in PDF format at the above link) Although Susskind is highly optimistic and says that a new "paradigm" of string theory has now "emerged from the ashes" of presentday string theory, he does touch on several possible reasons for concern. the business about a new string theory rising from the ashes is on page 1, in the introduction. Susskind is at Stanford, one of the leaders along with Ed Witten, Tom Banks, Michael Douglas, David Gross, ... Of the elder statesmen/founding father types he has been the most vocal in arguing for the Anthropic Principle as a way out of current string difficulties. Witten and some others have been reluctant to go that way. Witten just had a piece in the journal Nature. has anyone seen it? I regret to say I have not, my understanding is that the article was talking about the main issues in theoretical high energy physics, the scale of EW symmetry breaking, what to expect in the next few years, how to get back HEP back on track. It is possible that Witten's piece in Nature did not have much of anything to say about string theory. Hopefully someone who has seen it can fill in on that.

some more comment on AJL paper, from a conversation on sci.physics.research between John Baez and Thomas Larsson: ---------quote from Baez 6 June post, SPR---- In article <24a23f36.0405170344.69e74067@posting.google.com>, Thomas Larsson <thomas_larsson_01@hotmail.com> wrote: >1. Is the AJL model really quantum? Yes! It has a Hilbert space of states, observables described as noncommuting self-adjoint operators on this Hilbert space, and discrete time evolution described by unitary operators on this Hilbert space. >Some time ago, Urs >Schreiber argued that LQG, or at least the LQG string, >fails to be a true quantum theory, and I tend to agree. I disagree, but it's not really relevant here: we're not talking about those other theories. >However, the AJL model can be viewed as a statistical >lattice model, and if such a model has a good continuum >limit, it is AFAIK always described by some kind of QFT. >What else could it be? Right! >2. Is the AJL model really gravity. The action is a rather >straightforward discretization of the Einstein action with >a cosmological term: > > sum over (d-2)-simplices > > det g = volume => sum over d-simplices. > >What is perhaps somewhat unusual is that all edges have >the same length, which is different from Regge calculus. >Nevertheless, I don't think that this really matters, but >one could check if the results look different if you >allow for variable edge lengths. Right! But, the test of whether the model "is really gravity" is to carefully examine its behavior in the limit of large distance scales (i.e. lots of 4-simplices). One can't easily guess this from looking at the action. Nonperturbative effects are too important! So, in the absence of good analytical techniques, one really needs to run computer simulations - as AJL are doing. >3. Is the measure right? Here is the place where AJL differ >significantly from previous simulations. AFAIU, the crux is >that AJL insist on a strict form of causality: they exclude >spacetimes where the metric is singular, even at isolated >points. This may seem like an innoscent restriction, but it >rules out things like topology change and baby universes, >which require that the metric be singular somewhere. > >It is not obvious to me whether one should insist on such a >strong form of causality or not, but this assumption leads >at least to better results, e.g. a reasonably smooth 4D >spacetime. Thus, I believe that it is a fair chance that >AJL have indeed succeeded in quantizing gravity. The issue of the "right measure" is very tricky, so tricky in fact that I again think the most efficient way to begin tackling it is to run computer simulations and see if the AJL model acts like general relativity at large length scales. >They do so not by assuming a lot of experimentally unconfirmed >new physics, but rather by strictly implementing the >time-honored principles of old physics, especially >causality. That is cool. Yes! Very cool! ----end quote, I have emphasized parts with boldface----

some comment on AJL paper from John Baez ----6 June post, sci.physics.research---- --------Baez post Sunday, quote---- In article <61773ed7.0405240822.1c7108de@posting.google.com>, Charlie Stromeyer Jr. <cstromey@hotmail.com> wrote: >Here are three other reasons to be skeptical of discretized approaches >to gravity: .......(part of Strohmeyer post omitted).... Nobody has yet constructed a background-free quantum theory that has general relativity as its limit at large distance scales. The Ambjorn- Jurkiewicz-Loll model is the closest anyone has come. If they succeed, this will be of interest regardless of whether their model displays mathematical features that appear in certain other theories! >2) How are such approaches to be made compatible with Bell-like >correlations, non-locality and non-causality which are each present in >the experiment described in this brief four page paper [2]. As a quantum theory, the Ambjorn-Jurkiewicz-Loll model automatically has Bell-like "entanglement" and all that jazz. >3) To paraphrase a sentence that Stephen Hawking once wrote, to not >believe in the beauty and unity of the dualities of M-theory is like >believing that evolution did not occur because instead God placed by >hand all the fossils in the Earth just to play a joke on the >paleontologists We resort to theological arguments in physics only when better arguments are lacking. If a scintilla of experimental evidence for M-theory is ever found, people will instantly stop making arguments of the sort you mention here. Please understand what I'm saying: I'm not saying that M-theory is "wrong" or that the Ambjorn-Jurkiewicz-Loll model is "right". M-theory makes too few definite predictions to be wrong. The AJL model does not include matter, so it cannot be right. But the AJL model is *interesting*, because it represents the best attempt so far to find a background-free quantum theory that reduces to general relativity in the large-scale limit! ----------end quote, my emphasis----

I hope you will expand on these points whenever you have time. It's not my intent to rebut them (I'm not a debater really) I'd rather learn from them---especially the thinking underyling them. it is the macroscopically apparent, or Hausdorff, or "effective" dimension which is allowed to vary. so perhaps it does not govern the number of degrees of freedom and the model is unitary after all. John Baez was saying on SPR just the other day that it was unitary. I will try to find the link. this is in partial response to your point 1) maybe the Hausdorff dimension is just something one observes in the result after one runs the model---it is an observable rather than a determinant of the degrees of freedom but in any case your point 1) and also the related point 2) are quite intriguing your point 3) is entirely valid!----even I ( just a fascinated watcher of the Quantum Gravity scene) can see that you are right! they put in the Einstein-Hilbert action (a simplicial version of it) "by hand" because they think GR has worked well and is probably about right and they want their quantum model to match it in the large-scale limit. as a debating point you might come back with "but look string theory pulls the einstein equation out of the hat" or midair, or from behind your ear or something. which is definitely impressive but I'm not debating about who's best right now so much as mulling over your points----I'm actually not worried by the fact that they use a quantum version of einstein-hilbert to build the model. they need some action formula and they thought that was the right action there is a lot of experimental evidence that GR is right, so it figures the question foremost in my mind is will the AJL approach work, will it have the gravity we know as largescale limit? they have another paper in the works it is a barebones "minimalist" theory----quantum gravity with the least imaginable extra assumptions and structure, it seems---and if it works it should be a real breakthrough

Some quotes from the AJL paper "Emergence of a 4D world from Causal Quantum Gravity" http://arxiv.org/hep-th/0404156 ---- page 2---- ...This may be seen as a particular case of a more general truth, not always appreciated, that in any nonperturbative theory of quantum gravity "dimension" will become a dynamical quantity, along with other aspects of geometry. ----end quote---- ---- page 3---- In what follows we will report on the outcome of the first ever Monte Carlo simulations of four-dimensional causal dynamical triangulation...We will present strong evidence that the Lorentzian framework produces a quantum geometry which is both extended and effectively four-dimensional. This is to our knowledge the first example of a theory of quantum gravity that generates a quantum spacetime with such properties dynamically. ----end quote---- AJL use a simplicial quantum gravity model which they implement in a computer to generate universes with various spacetime geometries and the universes have been tending to come out macroscopically appearing 4 dimensional as they report in their paper. Because the effective dimension is "dynamical quantity" or outcome of the model means you don't pick what the apparent dimension of spacetime (observed at macroscopic scale) at the outset, instead the effective number of spacetime dimensions is one of those aspects of geometry that emerge by running the model, either by solving the equations explicitly (which has sofar not been possible) or running a computer simulation.

Also thanks to you for what seems a pretty cogent reponse in the other thread. It quickly got buried, so I will copy it here so i dont have to go scrolling for it. this is post #23 in the thread "What are the other dimensions for?"

Vague, I agree with part of what you say:

my wife and I are certified divers last month we were diving in Bonaire dutch antilles, near venezuela coast, all coral, very lush colorful underwater life the people speak a strange language called 'papiamentu' which is a mixture of dutch spanish and a bunch of others wild donkeys roam the island and there are thousands of flamingos one can dive off the shore from hundreds of spots the good diving is near shore many people just rent a car and tanks and go on their own but we went on boat dives from the small sort of divers "rooming house" where we stayed the boat ride is usually only about 20 minutes because the good diving is so close the water is so warm one does not really need a wetsuit but should have at least a diveskin for protection against sunburn and scratches since you are in Florida I guess you can dive many places near home what's it like

some animations of some earlier computer simulations http://www.nbi.dk/~ambjorn/lqg2/'>http://www.nbi.dk/~ambjorn/lqg2/ Ambjorn's homepage http://www.nbi.dk/~ambjorn/ Matt Visser's commentary in the newsletter "Matters of Gravity" http://www.phys.lsu.edu/mog/mog19/node12.html A picture of Renate http://www1.phys.uu.nl/wwwitf/fotopagina's/Medewerkers/Renate.htm http://phys.uu.nl/wwwitf/fotopagina's/Medewerkers/Renate.htm Ambjorn and Loll's work is very closely related to the "spin foam" approach to quantum gravity developed by John Baez and others. Baez is also involved in this approach now. Here's a picture of him http://www.edge.org/3rd_culture/bios/baez.html that's enough pictures for now

this is a landmark paper that represents several firsts essentially they programmed the emergence of the universe in a computer simulation and ran it over and over again to get averages the preprint was posted at arXiv in April (arXiv is where you read things first before they have been published in the journals) here's the link: http://arxiv.org/hep-th/0404156 ----- I guess there are several observations to be made about this. Gravity is the geometry of spacetime so a theory of gravity should generate the geometry. You should not have to pick a fixed geometry ahead of time (as is done in string theory---it's the main shortcoming in fact) instead, the geometry of the universe should emerge from the model. so they set up a quantum version of the 1915 Einstein equation of Gen Rel, and turned it into a computer model and ran it and for the first time they got an extended 4D world now some things have to be checked, they have to make sure that their quantized model really does give standard 1915 Gen Rel in the largescale limit, and they have to do much bigger runs their biggest run so far had around a third of a million "pixels" that is it used some 300,000 little 4D simplex blocks BTW people have been trying to do this since the 1980s just because you start with a bunch of 4D simplex blocks doesnt guarantee that the overall thing you get when they all stick together is macroscopically 4D-----fractal stuff happens and other bad things. so like after 20 years Ambjorn Jurkiewicz Loll finally got it to work. I am going to post links to photos of them.

first question would be "Are they there?" the theories (string...) that need D > 4 to work have not made predictions allowing them to be checked empirically no evidence they're right and growing indications they aren't field in something of a muddle right now around the huge number of possible vacuum states and the "anthropic principle" So it's probably safest to assume the world is just plain 4D. A satisfactory theory of gravity (gravity = geometry of spacetime) should predict, among other things, this four-dimensionality. You should be able to put the equations of the model into a computer and run a simulation of (at least the broad outlines of ) the universe and have that dimensionality emerge from the model. this was done for the first time this year. the work is still very new. (it has no relation to string theory but it does relate to some other approaches to quantum gravity which people are researching) two of the people working on this are Jan Ambjorn and Renate Loll http://arxiv.org/hep-th/0404156

If a paper is going to get published it is typically a year or so after the preprint is posted on arXiv. so the 13% decline in publications, 2002 to 2003, should have been foreshadowed by a similar decline at arXiv from 2001 to 2002, the numbers measure different things and cannot be compared but the trend or percentage change can be. At the moment I see a 9 percent decline in stringy preprints here: Year 2001: http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+OR+string+brane+abs:+OR+braneworld+D-brane+abs:+OR+M-theory+p-brane/0/1/0/2001/0/1 Year 2002: http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+OR+string+brane+abs:+OR+braneworld+D-brane+abs:+OR+M-theory+p-brane/0/1/0/2002/0/1 We dont have a final publication count for 2004, so the NASA ADS system doesnt help. But we can estimate what the decline in publication from 2003 to 2004 may be by comparing preprints in the preceding year. At the moment I get a 13 percent decline in stringy preprints between 2002 and 2003 by comparing these two numbers: Year 2002: http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+OR+string+brane+abs:+OR+braneworld+D-brane+abs:+OR+M-theory+p-brane/0/1/0/2002/0/1 Year 2003: http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+OR+string+brane+abs:+OR+braneworld+D-brane+abs:+OR+M-theory+p-brane/0/1/0/2003/0/1 So while we already know a decline in stringy research publication of roughly 13 percent, 2002 to 2003, we can forecast a decline of about the same size from 2003 to 2004 It is also possible to project somewhat into 2005 because the preprints are known for the first half of 2004. These continue the downwards trend. Last twelve months (e.g. 15 June 2003 to 15 June 2004): http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+OR+string+brane+abs:+OR+braneworld+D-brane+abs:+OR+M-theory+p-brane/0/1/0/past/0/1

this thread raises a good issue why are there just this number of dimensions? Does anyone know of any evidence that there are more than 4 dimensions? (various theories require more than four in order to work, but as far as I know there is no experimental confirmation of any of those theories) a theory of how spacetime arises should probably be able to explain why it appears to be 4D and it looks like the Ambjorn paper, with their computer simulation of the universe where 4D emerges dynamically, is making a start

Jan Ambjorn (copenhagen U) and Renate Loll (utrecht U) "We present evidence that a macroscopic four-dimensional world emerges from this theory dynamically." http://arxiv.org/hep-th/0404156 they construct a quantum model of spacetime where the development of spacetime geometry follows a quantum version of einstein's GR and they implement this quantum model in a computer-----four dimensional worlds result. Ambjorn and Loll began their collaboration when they were both at the MPI Potsdam. they (and Jurkiewicz) have taken a step towards explaining why D = 4 the paper is called "Emergence of a 4D world from causal Quantum Gravity"

yeah Im back swansont was beginning to talk about the fourthpower law sigma is what tells how brightly hot things glow to understand sigma you need the metric system of units----know what a square meter of surface area is, and what a watt of glow is, to measure the radiant power: watts per square meter. the law is you take the temp in kelvin. like say 1000 kelvin and raise it to the fourth 1,000,000,000,000 kelvin4 and multiply by sigma and that gives the watts per sq. meter! sigma = 5.67 x 10-8 watts per sq. meter per kelvin4 so if you have 1012 kelvin4 and you multiply by sigma you get 56,700 watts per sq. meter but when something cools down to 100 kelvin then it sheds energy much more slowly, like 5.67 watts per sq. meter---by the same rule. so cooling things taper off even tho night lasts two weeks they dont have time to get colder than whatever you said

losfomot, swansont said it but i can expand on that there is a limit to how fast something can radiate its heat away in a vacuum (stefan-boltzmann fourthpower law with magic number sigma) if it is hot it glows brightly and radiates lots of watts and gives up heat quickly but as it cools it can only radiate fewer watts and the cooling process slows maybe the moon rocks cool all night and are coldest just before sunrise and then they get another shot of heat all day and then they have to cool off again, by radiating into the darkness so temp follows a sawtooth pattern have to go, more another time