Feeds:
Posts
Comments

Archive for the ‘string theory’ Category

After complaining bitterly about my lack of free time, at least now I have finished writting the first paper of the year, so now I can show some of the work I’ve been doing. I’ve been feeling so overworked that at least I can celebrate the small accomplishments of the day. This is why, if I write a paper, I tell my family about it and have little celebrations. These are like drinking a nice glass of wine with dinner, or doing other silly little things to show my good mood.

It feels good to let go! There is still the apprehension of what will people say. But after writing so many papers, you kind of get used to it. Considering that from gestation of ideas, to calculations, to a paper it took more than 9 months, it’s not that different in time scale from delivering a baby. Seeing that it is only 4 pages, it doesn’t seem like it would take so much effort.

 

(more…)

Advertisements

Read Full Post »

We now have a few working examples of a microscopic theory of quantum gravity, all come with specific boundary conditions (like any other equation in physics or mathematics), but otherwise full background independence. In particular, all those theories include quantum black holes, and we can ask all kinds of puzzling questions about those fascinating objects. Starting with, what is exactly a black hole?

(more…)

Read Full Post »

Back in 2001, in a truly beautiful paper, Juan Maldacena formulated a version of Hawking’s information paradox, which has the added advantage that it could be discussed and analyzed in the context of a complete background independent theory of quantum gravity, namely that of the AdS/CFT correspondence.

This variant is similar to the original paradox, formulated for black holes surrounded by flat space, in that it displays a sharp conflict between properties of black holes in classical General Relativity, and basic postulates of quantum mechanics. Alas, it is also different in many crucial ways from the original paradox. Despite that, Juan’s proposed resolution to his paradox seems to have led to Hawking’s arguments, who managed to convince himself (though I think it is fair to say not too many others, unless they were already convinced) that information is not lost after all in the process of black hole formation and evaporation.

(more…)

Read Full Post »

I have been a bit tied up with the beginning of classes here at UCSB, which began this week. I will be/am teaching a course on Advanced Quantum Field Theory where we will describe the Standard Model of Particle Physics to various degrees of precision and we will study various observables for collider collisions. I will post my lecture notes online as I move along the course, and they might be useful to a larger community, but they are not anywhere near ready for a formal publication and they will be removed from the website at the end of the year. As usual, there are no guarantees to the reliability of the formulae (especially minus signs, factors of 2 and factors of pi), but hopefully some people might find it useful and some of you might even point to typos in the notes (I would appreciate that kind of feedback).

Secondly, a new program on the fundamentals of string theory started this week at the KITP. This week has been very intense (many talks) and I have been busy with various other things that have prevented me from attending most of the talks so far.

Finally, we have a faculty search (on)going in particle physics phenomenology. This was just posted to SPIRES and the particle rumor mill. The deadline is somewhat short due to how the academic schedule works for these. The position only became available again very late last year. If you know of someone who would be a good match for our search, please encourage them to apply.

Read Full Post »

Comments by Giotis on my previous post, about emergent gravity, reminded me about one of the tricky points in gauge-gravity dualities such as the AdS/CFT correspondence. This is the understanding of them as dualities between one theory that lives in the “bulk” spacetime (say quantum gravity on five dimensional AdS space) and another that lives on the boundary of that spacetime (four dimensional supersymmetric gauge theory in that case). This is kind of correct, if you know precisely what you mean, but it can also lead you to scratching your head if you don’t.

The tricky part in thinking about dualities is internalizing the idea that the two sides are actually two descriptions of one and the same object.  So, the gauge theory has the same properties as quantum gravity in five dimensional space, it is a five dimensional quantum gravity theory. Like any other description of this quantum theory, it has a bulk and a boundary, it has gravitational forces and propagating gravitons, it has black holes forming and evaporating, etc., etc. … All of those are realized in a somewhat unfamiliar language, more suited for the quantum rather than the classical theory, but all the same they are still there.  It’s a good mental exercise to phrase gravitational properties of the theory in terms of the gauge theory variables, it forces you to distinguish physics from language. So, I’ll do that here by discussing bulk and boundary of spacetime in the gauge theory language.

(more…)

Read Full Post »

Alright, time to discuss some physics again. A while back I outlined the basic dichotomy of quantum gravity. In brief: we have classical general relativity as an excellent description of all observed gravitational phenomena, but when we go to extremely short distances we need to have a good description of quantum gravity. There are two possibilities: either the description of gravitational phenomena by the machinery of general relativity (metric, the principle of equivalence, etc.) holds all the way down to those extremely short distances, or it doesn’t. If it doesn’t, which is my own prejudice, classical general relativity and its variables are never to be quantized, there is no range of energies (or distance scales) which is described by quantum metrics obeying a quantum version of Einstein’s equations.

This situation is similar to the hydrodynamical description of fluid motion: it is a classical effective field theory, which breaks down long before quantum mechanics is needed. We can observe granularity in the fluid on distance scales much larger than those relevant to quantum mechanics. From that perspective, quantizing Einstein’s equations makes as much sense as quantizing the Navier-Stokes equations, which is to say not too much sense.

(more…)

Read Full Post »

This post in “Science After Sunclipse” reminded me of one of the most beautiful ideas to come out of the continuing attempts to combine String Theory and Cosmology. The idea of Robert Brandenberger and Cumrun Vafa, dubbed more recently as “String Gas Cosmology”, is a wonderfully creative attempt to explain why our world has three spatial directions. There is no other theory on the market where the dimensionality of space could be determined dynamically, or at least come out as a result of a calculation, rather than being put in as an input, so in String Theory this is a natural question to ask. The idea for an answer,  provided by String Gas Cosmology, could be stated naturally and simply, which is what I try doing in this post. As usual, one has to remember that the devil is in the details, and those at the moment provide a real challenge for the idea.

One of the beautiful aspects of this approach is even asking the question: why four dimensions?  famously, the dimension of spacetime in string theory is larger than the observed one, in the simplest scenarios 10 or 11 spacetime dimensions, depending on details. We of course only see 3 spatial directions to move in, and can only make sense out of one time direction. Isn’t that a clean Popperian falsification of String Theory?

(more…)

Read Full Post »

« Newer Posts - Older Posts »