Study validates general relativity on cosmic scale, existence of dark matter Princeton scientists say Einstein's theory applies beyond the solar system Confirmation of general relativity on large scales from weak lensing and galaxy velocities I think a news site I frequemnt summed it up nicely when they headed this news release under the title "Seriously, this is getting boring". On a related note, a nice summary of experimental tests of GR: The Confrontation between General Relativity and Experiment
It's always half good and half bad when yet another round of experiments validate a theory. The Standard Model and GR have had 30 years and 95 years of experimental testing and to the limits of our measurements they are accurate. On the one hand this is an enormous testament to the brilliance of the people who came up with them but it makes further development somewhat difficult. With no 'beyond the SM' physics currently known its been difficult for people to work on unification models because we don't have an guiding data and instead have to use mathematical elegance as a guide, which will only get you so far. No doubt the relativity nay sayers (Bishadi, Mac, Geist, Jack_, Uno) will argue there's some kind of conspiracy but they fail to accept physicists are desperate for experiments which the SM or GR can't explain because it'll give a direction to current theoretical physics research. None of this "They will maintain the status quo to keep their jobs" nonsense is right because killing old theories means more work.
This is so well put and I had not even thought of it this way... I wonder what Garret Lisi would say? Please Register or Log in to view the hidden image! If the LHC yields the Higgs Boson, I wonder, too, what will develop. I don't envy Unification Theorists.
Well you might be at least a little enthused to hear that the project I'm working with has a good chance of punching holes in the Standard Model, especially in the area of CP violation. The old project found strong evidence that the Standard Model electroweak parameters known from existing meson decays weren't sufficient for describing the decays of other mesons. From what I've been told so far, the new evidence will help explain the degree of CP violation in the universe (i.e. the degree to which matter dominates antimatter in the known universe) due to processes thought to have occurred during the Big Bang. The project I'm working with is a vast improvement on the luminosity achieved by the previous experiment, so, provided we get the necessary funding which still isn't 100% guaranteed, we'll be able to get vastly higher statistics on B-meson CP violation and open the door to some new physics. There's a lot of talk about possibly predicting a new 4th generation of quarks and leptons, although it doesn't sound like that would help the search for quantum gravity very much. The experiment will be measuring all kinds of things in very large quantities, and there's a lot of talk about tests for things like Supersymmetry, dark matter and even the Higgs boson through loop effects and missing energies.
Bork---are you on LHC-B? Anyway there is lots of interesting physics that is still up in the air. A good example can be found by looking at the electroweak precision data. There's a huge tension between the leptonic data and the hadronic data---the leptonic data wants the higgs mass to be smaller, and the hadronic data wants the higgs mass to be larger. And you can look at the ``most likely'' value of the higgs mass and see that it's already excluded to something like 1 sigma: Please Register or Log in to view the hidden image! Another mystery, from what I understand, is the D meson decays. I think they don't agree with the standard model calculation by quite a bit (2-3 sigma), but since the decay constants have to come from the lattice, people are taking these numbers with a grain of salt. Anyway they have important consequences for new physics. The same is true for the B meson decays---processes like \(b \rightarrow s\gamma\) and \(B_s \rightarrow s \mu^+ \mu^-\) are very important in SUSY models. The former process has been measured, and there is a slight disagreement (<2 sigma) with the SM prediction. As for the fourth generation, I'd be very surprised to see that happen. If we DO have a fourth generation, then it can't look like the first three generations. For example, we know how many massless neutrinos there are by looking at Z decays, and by looking at neutrino mixing---so we know there can't be a lepton doublet, or it has to have some weird new quantum numbers which prevent it from coupling to the Z. There was a guy giving a talk here from England (Christopher Hill) who was looking for vector-like exotics, which are kind of like a new generation of quarks with slightly different quantum numbers. These experiments are very interesting, and will give some very important results if he finds anything. So yes---it IS an exciting time to be a physicist. It's a pity I'm leaving so soon Please Register or Log in to view the hidden image!
I think I heard about this somewhere- or something very similar to it anyway - plans to improve the luminosity at first the RHIC and then later (if successful) the LHC...
To answer Ben and Trippy, no I'm not personally involved with the LHC, at least not at this point. I do have lots and lots of friends involved with the LHC that I see on a daily basis, especially with the ATLAS detector. My supervisor happens to be involved with ATLAS and it was an option when I was starting my Master's in 2008, but I think I've found a really nice niche for myself with the project I'm in now. I'm involved with the BaBar B-factory experiment, using Monte Carlo studies to help design its proposed high luminosity successor (SuperB). The idea of a B-factory is to produce, as my supervisor puts it, "copious amounts of B-mesons" whose decays give you all kinds of indirect statistical info about physics at LHC energies and even beyond. SuperB will up the production rate by a factor of ~100 using a new breakthrough in beam collision design. For my Ph.D., if things go well enough, I might be directly involved in the actual construction phase too, with robots and all kinds of crazy stuff, but that's a bit too far in the future to say with certainty just now. As for the fourth generation of quarks, I don't know much about the scheme because it's my supervisor who mentioned it to me in the first place. This is one reason I'm taking another course in Quantum Field Theory this year, so I can learn how to do things like loop corrections, QCD calculations and renormalization, and get a better understanding of where the cutting edge models in physics are coming from and how they're developed. It's been almost a year since I completed my Standard Model course, so I can't remember all the details, but it seems from my personal recollection that it would change a lot of things if you were to add a fourth generation of quarks. Wouldn't you have to switch to a 4-dimensional representation of the color SU(3) group? Well anyhow, if you guys know anything about what a 4th generation of quarks and/or leptons would do, I'd love to hear it.
Well, no---the 3 dimensional rep of SU(3) comes because there are three colors. I think that a fourth generation of quarks and leptons is something that is possible but not likely. For example, I've already mentioned the main problem: we know from cosmology (supernovae, I think) how many neutrinos there should be. And we know from looking at Z decays, too. So if there is a fourth generation, it shouldn't look like the first three---in that sense, it's not really a new generation, but just new matter.
I should look all this stuff up again. I was thinking about the QCD sector of the Lagrangian and how quarks comes into the picture, which made me think of SU(3), but yeah I keep forgetting that only acts on colour indices. Let's see, you start with a bunch of quark fields and then make linear combinations of them to "diagonalize" the fields into the standard form. Maybe I'm thinking of electroweak SU(2)xU(1) then, perhaps the generators of this symmetry group are the ones that mix different fields in the Lagrangian and need to change to 4d representation? Whatever it is, I imagine the whole process of adding the Higgs in, diagonalizing the fields, making the photon massless, etc. and getting the CKM matrix is what needs to be altered. I think what my supervisor is talking about focuses on adding another row and column to the CKM matrix along with a fourth lepton/quark generation to account for the extra CP violation. As long as we find things that disagree sharply with the Standard Model, that's the most important issue as I see it. In my opinion, the more tweaking required to account for new evidence, the better, because we're going to need a whole lot of tweaking if we ever want to find that holy grail of unification.
I might not be following properly since I skim read but I feel I should remind people of the difference between SU(3) flavour, which is the number of families, approximate SU(3) mass symmetries and then SU(3) charge, which is the number of colours. The SU(3) colour symmetry is the gauge symmetry of the strong force and is exact. SU(3) mass symmetries are approximate because they are the symmetries between the up, down and strange and since the strange is much much larger its a pretty poor symmetry, SU(2) between up and down is isospin. You get that symmetry by rejigging the QCD Lagrangian. 3 families and 3 colours means lots of SU(3)s but not the same ones and I think people might be mixing them up. I know I did a few times.
I think you're mistaken here: SU(3) flavor (note the spelling!) relates up, down and strange quarks, and not the three families.
Yes, this is probably what he means. But realize that if you do that, you can't have a new lepton doublet, for the reasons that I've already mentioned. This means that you kill things like grand unification, which is fine, but not particularly well motivated. Anyway, we could go around in circles here, and this is the typical argument between theorists and experimentalists Please Register or Log in to view the hidden image! I hope you DO find something new, but I also hope that it agrees with all of my preconceived notions Please Register or Log in to view the hidden image!
Yeah, I half realised that a few moments ago. Still, my point that its important not to confuse the SU(3) of colour with the SU(3) of flavour is the relevant bit. As for the spelling we invented the English language. Though I will admit we originally used 'aluminum' and changed to 'aluminium'. Probably just to spite you Please Register or Log in to view the hidden image!
loop effects. You have access to a supercollider? Sumbitch. Cool. If this is OT I apologize. But have you read Carlo Rovelli's text on QLG? If so, have you also read that paper from a couple years ago "toward a general relativistic QLG" of something. (I forget the tiotle offhand) If yes to both, can you understand what he is calculating in the second matrix in section 4? Please Register or Log in to view the hidden image! I guess this is Off Topic. But dang. I cannot make heads or tails of what he is actually calculating. Anyway. Good luck.
