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Below are the 11 most recent journal entries recorded in Current research in theoretical physics by members' LiveJournal:

Thursday, June 10th, 2010
6:09 pm
Naïve quantization, anyone?
So as to maintain the *current* of the community, I thought I'd best post at least once this year.

I'm trying several very plain and extremely dumb ways to quantize gravity, including simply plugging several Dirac-style energy-momentum tensors into the field equation. I'm looking to find an untried way of tying the field equation up with field theory, including several harmonic approaches. For instance, coming up with a simple way of inverting the vacuum equation (that is, in treating the Riemannian side of the EFE as an operator).

Yes, this has been difficult, trivial in some places, and a partial waste of time; but it's certainly broadened my way of thinking about these things.

What I was most wondering is if anyone here has any insight into the Wheeler-Dewitt equation, which is actually what I've been studying for a few months. It's one area of my education which most professors entirely glossed over, which has long frustrated me.

I'm shooting in the dark in many ways, but as with some ventures, it's profited my understanding greatly, so if you just have an interesting calculation to suggest, that'd be fun.
Wednesday, June 3rd, 2009
5:42 pm
Hi all, I just recently found this community after a spirited in the physics community. I'll be moving to a post-doc in Numerical Relativity in about a month, but my main area of research is discrete spacetime for use in both quantum and classical gravity.

So hello, and I hope I'm able to contribute to some of the future posts.
Friday, September 12th, 2008
4:09 pm
Relativity from absoluteness

The shortening of bodies in the direction of motion, Lorentz contraction, follows from the solution of Maxwell's equations. Moving light clocks will tick slower than those at rest because the speed of light does not depend on a source of the light. The latter and Lorentz contraction imply the relativistic time dilation. The invariance of the light speed defined as the round-trip value follows from the time dilation and Lorentz contraction. An observer is incognizant about his motion relative to the absolute frame of reference. So, in order to synchronize spaced clocks in a moving reference frame he uses the same procedure as in the absolute frame. We deduce the Lorentz transformation from the Lorentz contraction, time dilation, invariance of the light speed and synchronization procedure. Lorentz transformations constitute a symmetry group of Maxwell's equations. That is the reason why the absolute frame can not be distinguished among other inertial reference frames.

Wednesday, July 9th, 2008
3:57 pm
the unknown strong coupling constant
So, I've been pulling my hair out over the question of what the right value of the strong coupling constant at the Z pole mass to use is.

Some experiments say it's one thing, and others say it's another... and they just don't seem to be very compatible with each other.

To resolve it, I checked in the latest Particle Data Book which they have online:

But incredibly, they even report two different overall values for it (each averages from a lot of different kinds of experiments), which are not within 1-sigma of each other.

If you look in the "Electroweak Model and Constraints on New Physics" document, they go over lots of different values for it, and then give an overall "global fit" value of 0.1217 +- 0.0017.

But then if you look in the "Quantum Chromodynamics" document, they give a whole lot of other values for it from a mostly different set of experiments (QCD-stuff) and get an overall weighted averaged value of 0.1176 +- 0.002. They mention that the smallest errorbars are on the lattice qcd results which may be pulling it down a bit... if you remove that and look at everything else, it goes up to 0.1185.

If you look on the table of "physical constants" they provide, it gives the 0.1176 number. I seem to see more papers that assume it's 0.118, but the odd thing is that the error bars on that are not quite as good. So are we just supposed to *ignore* all of the precision electroweak data?

The really strange thing is that they make no attempt to reconcile these two reports, contained within the same website and updated the same year, or average them together.

The most straightforward MSSM prediction for alpha_3, assuming an SU(5) GUT theory is 0.130. If you allow for the unknown threshold effects, this could vary by +- 0.01... agreement with either of them is really not as good as some people like to say, although the GUT-scale threshold effects could conceivably fix it. At least it's a hell of a lot better than the Standard Model SU(5) prediction of 0.073 +- 0.001! If you believe in any kind of GUT group up there, you at least have to have new physics somewhere, whether or not it's SUSY.
Friday, March 24th, 2006
6:27 pm
Cosmology - discussion on WMAP data?
Just wondering if there are any cosmologists/cosmology grad students/undergrads here who would like to discuss the recent WMAP data release.....
Tuesday, March 21st, 2006
9:17 am

I've just joined the group, so thought I'd quickly write an entry saying "hi". I work in (low-energy) nuclear physics theory, so essentially many-body quantum mechanics. At the moment, I'm concentrating on time-dependent approaches to nuclear collisions and giant resonances.

I've collected a list of papers which are broadly in my area of research at del.icio.us (which is a great way of organizing a personal database of research papers). See: http://del.icio.us/pstevenson/physics-papers
Tuesday, January 24th, 2006
1:39 pm
Hello people in theory_research. I joined recently and thought I might introduce myself.

I'm a post-doc at ICTP, Trieste in condensed matter theory. I think most of you are in high energy physics, but we might still have a lot in common as I like playing with so called 'effective low-energy field theories', particularly of one dimensional systems so I use tools such as conformal field theory, integrability, etc...

My home page tells you more about my research. Just now, I'm working on a project to do with correlations in carbon nanotubes, but this is just one of many things. I'll gladly tell you about various things going on nowadays in condensed matter theory, when I get time. I'll also be happy to discuss life as a post-doc at ICTP, for any of you who might be interested in coming here.

I'd enjoy discussing differences in approaches to field theory between high-energy physics and condensed matter - we certainly use many of the same tools but have very different interpretations of them. For example, there is renormalization, which we use in the theory of phase transitions, or supersymmetry which we use in the theory of disordered systems.

Another thing very interesting to me is solitons - both quantum and classical. I'm very familiar with the sine-Gordon model - which is integrable (in one spacial-dimension) and supports solitons, etc... These solitons are topological in that they extrapolate between two different ground states, which leads to many topolgical conservation laws that in some sense define what a soliton is. Now go to another integrable model, the non-linear Schrodinger equation (NLSE) which I'm beginning to hear a lot more about in the context of non-linear optics. This also supports solitons - it is easy to write down a solution of the differential equation that corresponds to something moving without changing shape, and then see that solitons scatter of each other without losing identity, etc.. However in this case, there is a unique ground-state of the model (unless I am mistaken), so these are not topological solitons, at least not in the way I know it. The question that interests me then is this: is there another way of viewing the NLSE such that the solitons are topological, and so the solitonic behavour follows naturally from topological conservation laws like in the sine-Gordon model? Is this known? Is this interesting to anybody?

Well, that's all for now. I like the idea of this community, and I hope we can have some interesting discussions on it.
Monday, October 17th, 2005
6:21 pm
Simulating a CMB map
Has anyone of you ever simulated a CMB map? I am trying an approach that requires me to take the FFT of a map in fourier space - ever encounter this? Please let me know if you have.
Thanks very much.

x-posted to _scientists_, physics and astronomy
Sunday, September 11th, 2005
12:28 pm
Quantum Filed Theory
I'm sorry if this is irrelevant in this community, but I am a graduate student in Cosmology in dire need of a QFT text. Which one do you guys use? (I have had one intro to QFT class where we were taught how to work with Feynman diagrams for tree-level QED calculations). From the library, I have a choice of Peskin/Schroeder, Roman, Zee and Bjorken/Drell.

Any suggestions will be welcome.

Again, if you think these posts are irrelevant here, please let me know.
Tuesday, August 16th, 2005
1:53 pm
On information content in a relativistic quantum mechanics setting
This recent paper on loss of entanglement in non-inertial frames, together with this one on quantum information had me thinking.

The first article states that entanglement is observer-dependent in non-inertial frames, the second article links entanglement and the available information. What I'm wondering is:
Is it non-trivial that the information available to an observer in a given space-time point is dependent on the observers acceleration?

It seems like an important question if you wonder about the information content of a spacetime volume, which indicates how much it takes to describe that volume which in turn indicates the complexity of the visible world or what is actually there. (barring "hidden variables" ala the EPR paradox)
Monday, July 4th, 2005
11:42 pm
ping... first post!
I know this will barely be read given the veritable logjam of traffic that flows through this community, but since I'm finally to the point where I have something to legitimately post here I figure why not give it a try.

I've just started working on my first particle theory project in graduate school, and I'm finding it very exciting and challenging. Having just recently finished my first year of QFT, I still have a lot more to learn. But it's already sinking in pretty fast.

I'm working with BFSS Matrix Theory, the discrete light cone quantized 11-dimensional theory of interacting D0-branes which in the large N limit might be the full non-perturbative description of M-Theory. (N here is the size of the matrices, which replace coordinates leading to a non-commutative geometry at short distances). It seems very promising to me so far, but everything I hear about it is from Tom Banks who of course has a lot invested. One thing I'd be curious about, if anyone knows more about the history behind this, is if it's my imagination or if work on this and excitement for this dropped off a bit after around '99 or so. If it did, I'd be interested in hearing why. Are there any good reasons to be skeptical of it, or is it just a matter of people getting stuck and spending time on other things?

I'm trying to get quantum corrections to a classical solution for Matrix Theory which appears to have the proper spectrum for supergravity. Hopefully the corrections don't screw anything up. I'm doing everything at finite temperature, which has caused me to have to quickly learn "QFT at finite temp" which we never covered in my first year of QFT. It's simpler than I'd expected though, and seems pretty similar to how regular Green functions work aside from the periodic time variable which "threw me for a loop" at first. The Matrix Theory Lagrangian itself is also surprisingly simple, and works essentially like non-relativistic quantum mechanics, but with a variable mass given by the discrete momentum in the longitudinal direction of the compactified light-cone dimension.

If anyone is more curious about this, or wants a quick run-down on how Matrix Theory works, I'd love to discuss it... to the extent that I understand it (which is not that much yet, but quickly improving!) I'm very much enjoying this so far, but I'd be curious to hear any positive or negative comments about this direction of research. Since I'll soon have to figure out whether I want to stick with this or look into something else.

Current Mood: hopeful
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