BenTheMan

At least, I'm glad you can see why one would be confused by these statements. It is also odd that Loop Quantum Gravity's main proponent is disconnected from the field. Let me ask a direct question, which may not have an answer. Do momenta larger than the Planck scale make sense in Loop Quantum Gravity? If yes, then how could one ever describe such a thing, when your operators are required to give discrete answers? If no, then how does this NOT violate Lorentz Invariance? ============= Please go easy on me, I am but a simple phenomenologist! I also want to make it clear that I am probably misquoting Nima. (If he or one of his students is reading this, then apologies!)

Martin--- I don't want to misrepresent Nima's position, and it is clear that I must have misunderstood his argument. I haven't had a chance to ask him about it, so as to clarify my understanding. The comments he made were in regards to taking a weakly coupled theory, that is defined in the IR, and trying to run it up to a strongly coupled, UV regime. The example he used was QED, which has been studied on the lattice at strong coupling. Either way, this is the Loops program, no? Take a well defined theory (i.e. Einstein-Hilbert action), which is weakly coupled in the IR, and try to extrapolate back to the UV completion. In regards to LQG being formulated on a lattice, this is from Lee Smolin's ``Invitation to Loop Quantum Gravity'', http://arxiv.org/abs/hep-th/0408048: The area and volume operators can be promoted to genuine physical observables, by gauge fixing the time gauge so that at least locally time is measured by a physical field. The discrete spectra remain for such physical observables, hence the spectra of area and volume constitute genuine physical predictions of the quantum theory of gravity. Is this no longer the state of the art? It seems that this is a (Lorentz violating'' hard cutoff, a la Lattice QCD.

I think we might be talking past each other. The point of the argument (probably obfuscated by me in the original post) is, very specifically, that discretizing a path integral to complete the theory in the UV hasn't worked so well in the other example that we have, specifically QED. QCD doesn't really apply because we have a well-defined theory in the UV, and we are running it down to strong coupling. Sure---we never SEE these problems until we try to work it out. This was the hope behind the lattice formulation of QED, I guess. As long as you're ok with sacrificing Lorentz Invariance (which most Loop people don't mind), then fine---make your cuts. This is probably a matter of taste, right? It depends on if you admit arbitrarily high energies into the theory. The cutoff isn't anything physical, it's just a place holder. You won't find any arguments here! Again, no complaints. But he is the type of fellow who either ends up being spectacularly correct (and changing the field) or famously wrong. I mean---he's not even forty yet and he's got a permanant position at the IAS.

Well, QED breaks down at the weak scale, which is what we expect---the Landau pole is just a sign that something is wrong with the theory somewhere, maybe 10^38 GeV or something ridiculous like that. But it is a generic (and real) feature of the theory. The argument is that we have one example of trying to fix the UV limit of a strongly coupled gauge theory by latticizing it---QED. And discretizing the path integral in that context fails miserably. In some regime we expect gravity to look like a gauge theory, so it makes sense to compare it to other gauge theories, and see what we can learn. You may wish to point out that latticizing QCD has produced very beautiful results. And you would be 100% correct. The problem is, of course, that when we do this, we are looking for the IR completion of a well-defined UV theory. The point is not so much the appearance of the landau pole, but the general failure of trying to build a UV completion of a theory by latticizing it---perhaps I was a bit unclear in the original post. The big difference is the starting point. In Loops, one starts with an effective field theory (GR), and tries to complete the UV. In String Theory, one starts with a well-defined UV theory, and tries to find an IR realization. This is most likely why the landscape is an inevitible feature of string theory untill we understand fully how to compactify it---we don't know HOW to run the theory down exactly, in other words. If we did it wouldn't be a problem. But this doesn't change the fact that string theory is very well defined in the UV, and has at least one (or 10^500) consistent realizations. Ahh yes. So you know him personally? I will say that if you can't get excited listening to Nima talk about physics (even if you don't agree with him), then you can't get excited about physics.

He's usually surrounded by an entourage of graduate students, but I'll try to ask him sometime about this argument. And to be fair, I don't know that Nima would call himself a string theorist. Also, at TASI in Bouder, I got Joe Polchinski to drink a toast to Loop Quantum Gravity. His argument agains LQG is that it is less ``natural'' than string theory. For example, in strings, you only assume that the fundamental particle is not a point but a string, and go from there. By making normal assumptions (anomaly cancellation, unitarity, etc.) one is led to a rich low energy phenomenology, a prediction for the number of space-time dimensions (which is absent in GR and loops), etc. In loops, however, one has to invent many more tihngs to make the theory work. Nima's argument makes much more sense to me, especially if we expect gravity to look like a gauge theory at very high energies.

First of all, this is not my argument, it is due to Nima Arkani-Hamed. And I have probably misinterpretted it, or misunderstood it. Either way I will sumarize it as best I remember, and hopefully some of the smart people here can help me understand what is actually going on. I am at a summer school in Princeton this week, and Nima is giving a series of lectures on phenomenology beyond the SM. This argument came up in the recitation section that Nima gave after his first lecture. I have discussed it some with other grad students here, but still have some gaps in my understandings. I want to write it down here, so that I can get everything straight in my head, and maybe have some intelligent discussion with you people. Ok. First take QED at weak coupling. The theory is very well defined---we can compute scattering amplitudes and runnings of beta functions, etc. What one quickly notices is that QED has a Landau pole, at a place where the (inverse) coupling runs to zero. When the inverse coupling runs to one, the theory ceases to have a well-defined perturbative limit. So let's imagine a universe where we actually WANTED a UV completion of QED. The best way we know how to treat a non-perturbative gauge theory is to discretize the space-time, put the theory on a lattice, solve for something, and pray that the answer we get is independant of the lattice spacing. We have had great success in this vein, in the field of Lattice QCD. The problem is that the UV completion of the theory is very ill-defined. UNLIKE in Lattice QCD, we are trying to understand the UV completion of a theory, as opposed to the IR limit of the theory---while there is a unique IR limit of a theory, the UV completion is far from unique. Either way, when we put QED on a lattice, it becomes something that is no longer QED. Indeed, when we look at the IR behavior of the new theory, we have lost all of the predictions of QED, as well as the beautiful IR limit---in short, the IR limit of the theory we found by discretizing QED is not QED. Nima then argues that this is why LQG is bound to fail. While it is quite obvious how to deal with the low energy limit of a well-defined theory in the UV, it is nowhere near as obvious how to extrapolate a well-defined theory in the IR back up to the UV---this is easy to understand, because if it were not true, then we would have no problems understanding the UV completion of the standard model. The only way we could possibly begin to understand the problem is to discretize the path integral in a lattice-like formulation, and search for a theory that is independant of the lattice spacing. But this is exactly what fails in the case of QED, and is exactly what is attempted by the LQG program. Things like ``background independance'' are just our attempts to describe a physical system, and don't have much to do with the actual physics of the situation. (His analogy was gauge invariance, which is more or less a tool we use to calculate things, and not anything physical.) The counterargument is, of course, that gravity is not QED. But I think this argument might be a bit vacuous, because we are trying to understand gravity as a gauge theory, and the theory certainly doesn't care about the way in which it is parameterized. So, I hope that I haven't mangled this argument too much. I will try to think of this some more, and possibly talk to Nima. What do you guys think?

Please show me an equation that gives such predictions.

Yeah---I think science is a long way from understanding this. The general concesus is that, even if we did completely understand string theory, or some other theory of everything, actually predicting anything with it (along the lines of what you are talking about) would be too difficult, becase of the complexity involved.

Look---I must be working with a scientist's definition. If you go to arxiv.org or slac.stanford.edu/spires, and type in a search for ``exotic matter'', this is what you'll find... ---Heavier versions of the matter we already have, quarks, leptons and neutrinos. New families of standard model particles. ---fractionally charged states that we haven't seen before. These are sometimes common in string models. My advisor calls them ``scalar exotica'', or, as he likes to say ``sexotica''. ---oddball fermions that don't live in GUT multiplets. These are called chiral exotics. ---Dark matter candidates. These are electrically neutral, heavy, stable, weakly interracting particles. Sometimes called WIMPs (weakly interacting massive particles). These can come from supersymmetry (lightest neutralino or gravitino, usually), or can be in a ``hidden sector''. ``Hidden Sector'' matter will only interract with our matter via gravity, so it can't possible anihilate any of the atoms in your body. What you seem to be referring to is some sort of tachyonic matter, which is generally a sign that the theory you're working with is sick... Among other things, you violate Lorentz Invariance, which would be troube. And for the record, anti-matter has completely opposite quantum numbers as regular matter, not just opposite charge. EDIT=========== Maybe I should have read the post above...

I'm not familiar with any matter which couples to gravity like this. You're thinking of anti-matter. Anti-matter anihilates matter. Exotic matter can be just very weakly interracting matter (i.e. dark matter, or fractionally charged fermions, or extra scalar particles with large masses).

Hmm. On 1 I think quantum fluctuations keep the temperature above zero. For 2, energy isn't created---it's just changed form. Think of potential energy being changed into kinetic energy. There is some energy due to the rest mass of the particles (E=mc^2) that gets turned into high energy photons.

It's never too early. Do the work you need to do for your classes, and try to really understand the physics. It may help to write a set of notes as if you were actually giving the courses that you're taking. You can always pick up the books from your next year's classes and start reading them---deriving every equation, and understanding every statement in the text.

Why don't you spend time learning the physics you are studying, before trying to understand things more deeply? Don't get me wrong, it's great that you're interested in things like htis. But if you spend all of your time pondering the fundamentals, you'll never actually understand anything. Like Pink Floyd says---``How can you have your pudding if you haven't had your meat?'' Or like Feynman said---``Shut up and calculate.'' You have to be able to CALCULATE things before you can understand them at any deeper level---it's like trying to go to the Olympics without actually training. It's never too early to start learning these things, because no matter who you are, or how much you study, or how much you want it, there is another person who wants it more, who studies harder, and who is trying to get the same job or PhD slot that you are.

Well...maybe you can describe what you mean by exotic matter?

Well, it's called ``dark'' because it only interacts with things gravitationaly. So, for example, there are probably very many dark matter particles streaming through your body this instant, but they don't interact with you, so you don't even know they're there.

Easy. Grits. Oh. Damn. Wikipedia says that grits IS porridge. http://en.wikipedia.org/wiki/Grits

What do you mean by ``dangerous''?

You're just arguing over words. There are no ``anti-photons''---the opposite of ``light'' is ``no light''.

The situation may be slightly different in the states, ajb. Maybe not at the top top tier places, though---I got my master's degree at Baylor (pretty much bottom tier) and amd at Ohio State for my PhD, which is top 25 or so.

Out of curiosity, why? I think cosmology is the place to be if you want a job. Well, that and biophysics. I think the next 30 years or so are going to be HUGE for biophysics. Probably some Nobel Prizes to be won...

This is, of course, classic. Soccer is a terrible game. I've never played it, and I've never seen a full match, but nonetheless it's a terrible sport. Sigh. Snail---ajb may be a bit of a pessimist when it comes to these things, but he is right---string theory is a highly competitive field. You will have much time to decide what kinds of things interest you, and during those discoveries, you will decide what you want to do. If you DO decide on strings (it is much more than a fad, I can assure you), then keep an open mind as to which programs you go to. If your only desire is to study string theory, then you may be better off going to a lesser known program---there will be less competition for spots in the theory group, and your advisor may not be as well-known. This will mean that it may be difficult or impossible for you to find a post-doc, and will probably mean that you won't be doing physics past your PhD. This is ok, because even if you go to a very good grad school (like Princeton or Stanford in the US), you still probably will end up leaving physics in the long run---I have friends in finance, AI R&D, and software engineering, all who have PhD's in string theory. Another good option is to do particle phenomenology or cosmology. There will be very much interest in these fields in the coming decade, with the LHC opening up in Switzerland. You could also try to do cosmology---the Planck satellite is the next world-wide collaboration, and there will be many new and interesting insights in to things like inflation and such. I think that there may be less competition in these things because many students choose to do string theory instead. This is all assuming you want to do theory. You will know that you want to do theory when you take your first advanced mechanics course---this is the trend that I've noticed, at least...if you really like classical mechanics, then chances are you'll like theory. Good luck! So after rereading your original post, I have realized that very little of what I said actually applies to it. Strings vs. LQG. Got it. Do strings. Like it or not physics is trendy---that is, people tend to follow trends. The theory departments at most universities do not have any LQG researchers, but they do have one or two string theorists. This means that there are more opportunities to DO string theory, both in grad school, and afterwards when you look for a post doc. And by no means does doing one mean you can't do the other---this is a misconception about grad school. You are not locked to your thesis topic for the rest of your life. If you do strings, there's no reason why you can't read LQG papers if you're interested in them---who knows, you may find something that everyone else has missed.

So you're arguing semantics?

I think you're reading too much into this thing you've heard---the point they (surely) were making is that there are no ``anti-photons'', in the way we have positrons, which are anti-electrons.

Well, I have been at OSU for a year and I haven't seen Tseytlin once. He is a very good physicist, but one of the graduate students here told me that he likes his graduate students to be like TV dinners---he doesn't want to have to train them that much. As far as Samir's track record---it depends on what you think of his fuzzballs. If you agree with his interpretation of stringy black holes, then he has explained some of the biggest problems in that field. Aside from that, he's probably the best teacher that I've ever had---I am very much looking forward to his String Theory lectures next fall. Cosmology is very interesting, but it is not really my bag---I am much more interested in the particle physics side of things (and my advisor is Stuart Raby). Right now I am fighting with C++, teaching it to do calculations that I could do myself, but which would take me too long. You are right, though---if I were REALLY looking for a faculty position, I'd find someone who was on the Planck Explorer experiment. Most of the WMAP grad students got post docs partly because they had access to the data (this is a rumor, to be clear!). Either that, or do particle phenomenology for LHC or ILC. And I am a first year grad student, but in name only. I have a Master's degree from Baylor University, where I worked with Gerry Cleaver, also doing stringy model building.

I am a ``stringer''. I'm a grad student at OSU. Also, are you counting journal articles or preprints (i.w. arxiv.org or spires)? I was thinking of preprints earlier, when I made my comments about a surge around September. This would make sense, as well, because it usually takes a few months to go from preprint to refereed article.