From: B.C. ® 31/01/2001 9:38:19 Subject: Quantum Gravity post id: 217851 Chris ...you still around? From past posts of yours you don't seem to have much time for a quantum gravity theory that quanticises gravity. Have you any other ideas as to what a theory combining the four forces should be? What about string theory and it's derivitives,another area where you don't seem to be too enthralled about..any specific reason? Just trying to pry into that mind of yours. From: Chris (Avatar) 31/01/2001 10:55:08 Subject: re: Quantum Gravity post id: 217881 Hi BC My feeling is that quantum gravity won't look like the other quantum theories, eg QED and QCD. If you recall, QED explains the interaction between electrons and photons extremely well. It also incorporates weak bosons well. So we try and cast the strong force in the same style of theory - enter QCD. This is not as successful as QED - there are many more degrees of freedom (8 instead of 3 for the weak force or 1 for e/m) due to the "colour charge" on both gluon and quark. Plus the strong force is not as well understood as the other forces. Still, it works pretty well. All 6 quarks have been discovered. And you get the feeling we're on the verge of manufacturing the quark-gluon plasma which will further support the theory. But gravity's different. All the attempts to squeeze gravity into the same mould as QED have failed. The most important failure so far is that quantum gravity resists renormalisation. What does this mean? Remember your Feynman diagrams? Consider two electrons interacting - there are literally an infinite number of possible interaction paths between them corresponding to first, second, third (etc) generation virtual particles all spawning further generations and interacting with themselves. These infinities carry over into the theory - infinite paths over increasingly infinite trajectories blow the field potentials to infinity. Renormalisation is a technique which allows us to cancel out these infinities to get a workable finite theory which matches experiment. It works for e/m and the weak theory. It can be applied in QCD as well. But it doesn't work for gravity (for a number of reasons). I don't doubt that we will find a quantum theory of gravity, but I don't think it will look like QED, and this may be a problem for a TOE. I mean, what sense can you make saying that gravity behaves like a spin 2 massless boson on a quantum scale (graviton) but behaves like a curved geometry on a macroscopic scale? How can you quantise geometry? And does this mean space and time are (a) quantisable (b) behave like particles on a quantum scale (c) you can have virtual packets of space-time? To be blunt, I think GR is too good to throw out. Despite the recent throngs who've come in here suggesting it's tosh, it explains too many things too well to be completely wrong, so I'd say it's here to stay. Similarly QED is too precise to be wrong - it's here to stay. So what I think is that quantum gravity will require a new physics (as Karl often suggests) which will contain GR in the classical limit, and QED in the free-field limit. Such a theory would provide a deeper understanding for both GR and our current quantum theory. Now it may well be that this grand new theory will be a variation of super string theory, although string theory needs to start making some concrete testable predictions to be taken seriously. (Also needs to settle down and decide on its format, imo). Or it may be that the new theory will be something completely different. Just my ideas Chris From: B.C. ® 31/01/2001 11:18:40 Subject: re: Quantum Gravity post id: 217889 Thanks Chris I completly aqgree with you about GR,but it just amazes me the amount of posts we get on this forum that continually throw doubt on it,despite how it fits in so readily with our universe....anyway thats another matter. Interesting point about quanticising space,but hav'nt we already quanticised time,at Plancks time of 10 to the minus 43 sec. This is the time that quantum gravity is supposed to come into play.I suppose if you hav'nt got one you hav'nt got the other. With string theory,I read Hyperspace and was impressed with the argument that Kaku put up and also Greene in "Elegant Universe". I will try to revise relevant sections and get back to you with a possible viable argument for it's validity. From: Dropbear ® 31/01/2001 11:20:52 Subject: re: Quantum Gravity post id: 217892 perhaps a stupud question: why does there NEED to be a theory of Quantum Gravity? Surely gravity is such a weak force at atomic levels that it can be fairly well discarded? From: B.C. ® 31/01/2001 11:24:24 Subject: re: Quantum Gravity post id: 217894 I think the point is dropbear that the further we go back to the BB the more united the four forces become.The problem is as Chris put that we hav'nt been able to reconsile it with the other three yet. Whatever form it takes I think we may have it in 20 years,thats according to Hawking. From: Dropbear ® 31/01/2001 11:25:49 Subject: re: Quantum Gravity post id: 217895 havnt we already decided that gravity isnt a force though?? From: B.C. ® 31/01/2001 11:34:19 Subject: re: Quantum Gravity post id: 217902 I should wait for Chris to answer this but yes according to GR gravity is a warping of the space time continuum,but we have quanticised the other forces so why not gravity.I suppose that the warping of space is a successful picture that Einstein gave us to imagine gravity more easily,but be that as it may there may still be a particle attributed to it that arises from the reaction of a body with the higgs field. From: Dropbear ® 31/01/2001 11:36:52 Subject: re: Quantum Gravity post id: 217910 the quanticising (is that a word) of EM is a reality, its the photon. You can show that EM can be quanticised, and even Bohr did that. There is no evidence yet that gravity can be quanticised is there? From: B.C. ® 31/01/2001 11:42:00 Subject: re: Quantum Gravity post id: 217913 No none yet,but before the 20th century who would have thought that light could be quanticised? From: tritium ® 31/01/2001 12:33:16 Subject: re: Quantum Gravity post id: 217946 it's more to do with the fact that all three other forces are quantised so it would seem logical that gravity could be quantised too. it's possible Dropbear that you are right and it isn't, however one of the discoveries of the 20th century was that everything else is (except time and space, i'm not convinced these are quantised, could be wrong though). why should gravity be fundamentally different in not being quantised? From: Dropbear ® 31/01/2001 12:38:17 Subject: re: Quantum Gravity post id: 217948 why should gravity be fundamentally different in not being quantised? Well I guess it comes down to 'what is a force', but if Gravity is the result of a curvature of space time, not some 'pervasive field', then why it should be the same as the others?? (of course I know nothing about this and am throwing wil speculation around until someone comes and jumps on me) From: B.C. ® 31/01/2001 13:12:53 Subject: re: Quantum Gravity post id: 217973 Dropbear Well I'll tell you something,it's obvious to me that you have picked up a fair bit on this forum or you have been reading or both. That stands out by the standard of questions you are asking and also replies. From: Alan McLachlan ® 31/01/2001 17:59:11 Subject: re: Quantum Gravity post id: 218255 BC, To quote Haisch, Reuda and Dobyns: "Even when the Higgs particle is finally detected, it will continue to be a legitimate question to ask whether the inertia of matter as a reaction force opposing acceleration is an intrinsic or extrinsic property of matter. General relativity specifies which geodesic path a free particle will follow, but geometrodynamics has no mechanism for generating a reaction force for deviation from geodesic motion." I.e. if it turns out that inertial mass is a reaction force countering acceleration through the vacuum then attempts to find a theory of quantum gravity are really seeking a way of unifying frames of reference in a meaningful way that can describe gravitational effects in a manner consistent with GR. Chris, this doesn't mean throwing GR out, it simply means that some physicists are seeking a physical mechanism for generation of apparantly geometrodynamic effects.(yes, we've discussed this before.... :0) ) There is a new paper (pub. October 2000) on this very subject by Giovanni Modanese, titled "Zero-point field induced mass vs. QED mass renormalization" His abstract: "Haisch and Rueda have recently proposed a model in which the inertia of charged particles is a consequence of their interaction with the electromagnetic zero-point field. This model is based on the observation that in an accelerated frame the momentum distribution of vacuum fluctuations is not isotropic. We analyze this issue through standard techniques of relativistic field theory, first by regarding the field A_mu as a classical random field, and then by making reference to the mass renormalization procedure in Quantum Electrodynamics and scalar-QED" The full paper is at: http://xxx.lanl.gov/abs/hep-th/0011250 if you're interested. This forum is un-moderated. The views and opinions expressed are those of the individual poster and not the ABC. The ABC reserves the right to remove offensive or inappropriate messages.