jana

Hi severian, You’ll be happy to know that the known low-energy effective field theories of string theory are in fact various supergravity theories. However, supergravity doesn’t on it’s own make sense as a QG theory. Hi martin, Progress in physics doesn’t pivot on dotting and crossing every mathematical ‘i’ and ‘t’. Concentration on mathematical rigor indicates we’re stuck.

Interestingly, wolfgang pauli at one time turned to the kabbalah in attempts to gain insight into the value of the fine structure constant.

Okay, I'm trying to figure it out what's really going on here and will get back to you as soon as I can. As always, sorry about the delay.

Yeah, your right: If someone says "hawking's path-integral approach to quantum gravity", everyone knows that they mean "Euclidean Path Integral". But it doesn't hurt to add that the path-integral's "euclideaness" is the whole point of hawking's approach. It's what led to his famous no-boundary proposal.

There have been many arguments put forward to resolve this issue. Hawking is simply going to discuss his within the description of quantum gravity he knows best, euclidean quantum gravity.

Oops. Then the condition should be kappa < E. Try any review of LQG ever written. In fact, my remark is true of all canonical approaches to quantizing gravity, whether the phase space variables are the spatial metric and extrinsic curvature of earlier approaches, or ashtekar's variables as in LQG: The hilbert action with higher order contributions would determine a different and more complicated hamiltonian and therefore different canonical variables.

I'm going into my final year. But since canadians go to university one year later than americans, many senior courses are really at the graduate level. In fact it's common to see both undergrads, and graduate students coming from universities outside of canada taking the same courses.

I meant that I just took delivery of it. It's called "A first course in string theory" and is aimed at undergrads.

Ahh!!! Barton zwiebach's book just showed up! I'm so excited! Sorry, just had to share. Edit: The book is beautifully bound, smells nice, and uses that heavy, slightly shiny high grade paper cambridge uses for many of their textbooks. I wish they would do the same for their monograph series since they charge even more for them.

Firstly, keep in mind that unless I explicitly qualify something I post by saying "I know for a fact that..." or "I'm absolutely certain that..." you shouldn't take what I say too seriously since I'm just guessing or giving a preliminary comment, and in fact when I’m discussing papers at this level, I expect that much of what I say will probably turn out to be badly wrong. I guess most of what I post will be sort of "blog-like". Nevertheless, you should of course continue to challenge anything and everything I say whenever you please since for me this is the most stimulating way to learn. However, I know that just because this works for me doesn't mean it makes sense for everyone, and in particular I think that you don't need people to be constantly challenging what you post to have fun, which is of course the whole point of being here: Your an enthusiast who just enjoys following things which for whatever reasons happen to pique your interest at any given time. So unless you say otherwise, I'm not going to spend time trying to find ways to challenge your posts. I hope all of this is okay with you. Okay, so back to your question about what I meant by the remark: My outburst had to do with an impression I got from the last two paragraphs on page 11. It looks like expression (12) depends on the conformal properties of the hilbert action. However, it’s believed to be highly unlikely that this action is exact and should instead be viewed as only an effective theory that is a good approximation at sufficiently low energies. At higher energies higher order interactions consisting of various derivatives of the curvature can be expected to contribute and these will ruin the simple multiplicative scaling behaviour of the action under conformal transformations which the argument for (12) seems to rely on. So consider the energy scale E at which the lowest order such correction becomes important. Now, in geometrized units kappa has units of inverse energy. So (12) seems to require that 1/kappa < E. I guess even if this is true, I’m not sure that this is a problem. I’ll think about it some more. Edit: On a related matter, something I do know is that LQG must assume that the hilbert action truly is exact, which from what I understand is the main reason why people don't believe in it. In other words, the elegance of LQG is a result of viewing the gravitational interaction as being simpler than it actually is.

Okay, I'm ready to make one comment on the "Deformed Special Relativity as an effective flat limit of quantum gravity" paper. It seems to me that for this whole scheme to have any chance of working the energy scale given by kappa must lie below the energy scale at which the first correction to the hilbert action of GR becomes important.

They cancelled it. But I'm going to audit the graduate course which I believe will be taught by amanda peet.

Thanks.

I'll be sure to provide this info in the future. This paper looks very interesting. I'm going to read it. Don't sweat it. I've made my email address available. Note the ".ca" part.

I could be wrong (and probably am), but from Introduction to Doubly Special Relativity I’m getting that it’s not in fact a matter of certain approaches to quantum gravity requiring something like DSR. However the motivation of DSR by the "cosmic ray paradox" seems easy enough to understand. But from a paper entitled Doubly Special Relativity I’m getting that even the people directly involved in this research admit that it’s all quite speculative. I'm going to ask john moffat what he thinks about this stuff since this kind of thing is right up his alley, as I guess you must already know.

I'm pretty sure that the site administrators can verify that jeffrey winkler is connecting from hartford conneticut as he claims and that I'm connecting from toronto ontario' date=' or at least that I'm connecting from canada and that winkler is connecting from the states. In fact, under the circumstances, I would appreciate very much if they would do so and let you know what they find. I emailed winkler to ask him what the deal was with him and you, but he hasn't responded. Interesting. I'm going to look into this right and try to get back to you as quickly as I can. Edit: I forgot to ask what papers you suggest I look at.

Maxwell discovered that his equations describe electric and magnetic fields as propagating at the speed of light, from which he guessed that these fields are light. In fact, maxwell's equations are relativistic to begin with, i.e. lorentz-invariant. Special relativity extends the physical requirement of lorentz-invariance to mechanical systems. To do so however required more than just the postulate that light travels at the same speed at all times in all inertial frames. It required that we postulate that physics be the same in all inertial frames, and it is in fact the combination of these two postulates that sometimes makes special relativity seem a bit odd to people when they first encounter it.

I noticed that you give good answers on the subject of astrophysics at the level of freshman physics courses. I think you do so because you understand this material well enough to answer in a way that the kinds of people who populate this site can understand. If this is true, then I think you must know that hardly anyone here has any chance of understanding any of the papers to which you are so fond of giving links. But there is nothing in this or any of your other posts at SFN to indicate you understand these papers yourself. So why should anyone here take seriously your opinion that the above paper on doubly special relativity (which btw is a highly speculative idea which few physicists, and certainly not the top ones, take seriously) is important? I've very politely asked you more than once to backup your claims, and all I've ended up getting from you are insults. So I'll give you yet another chance. Why should we take the work on doubly special relativity seriously? For example, does it solve or provide a better approach to some outstanding problem?

Time is money.

Better (but not necessarily best) might be TIME IS A CONVENTION

Hi, Is there anyone here who understands any of the approaches to QG?

And pathetic.

Any decent text on GR. I use "General Relativity" by Wald. If the physical quantities you're after depend on M and R only through their ratio M/R, then yes, it doesn't matter. The functional dependence on M of the analytic formula (which I don't know off hand) for deflection differs from that on R such that the approximations in terms of limits of M or of R (with other variables held fixed) will be inequivalent.

Hi Atheist, What you've called R is actually the impact parameter, not the point of closest approach, and is usually written as b. If b < 33/2M the photon is captured. Note that the approximation you begin with is valid only for M<<1.