February 24, 2006

Entanglement at High Temperature

Aires Ferreira, Ariel Guerreiro, and Vlatko Vedral have published interesting results on entanglement in Phys. Rev. Lett. 96 060407. I quote the authors:

" ... the entanglement between macroscopic mirror and a cavity mode field can arise due to radiation pressure at arbitrarily high temperatures as the system evolves in time. This is very surprising because it is commonly believed that high temperature completely destroys entanglement. We will study entanglement in the time domain using a discrete variable method and identify its dependence on the relevant physical parameters, such as the strength of the radiation pressure coupling and temperature."

February 23, 2006

Quarks, Strings, Branes, and Gravity

Nick Evans, Southampton Univ., has written an article in Nature 439, 921-923 (23 February 2006) titled Quarks on a gravitational string. Short and readable for non-specialists, one figure, gives references to technical papers.

Gary Horowitz and Joseph Polchinski have written a contribution to "Towards Quantum Gravity" titled Gauge/Gravity Duality. Excellent coverage of this central topic. References given to the classical papers.

Joshua Erlich, Graham D. Kribs, and Ian Low wrote a paper Emerging Holography. The authors rederive the AdS/CFT predictions by a four dimensional approach.

February 13, 2006

Refining MOND: This is NOT Newton's Apple

H.S. Zhao and B. Famaey have published their arXiv paper Refining MOND interpolating function and TeVeS Lagrangian in The Astrophysical Journal, 638:L9–L12, 2006 February 10. A soft introduction to this paper can be read at Interactions.org.

February 10, 2006

Hawking and no boundary initial conditions in the landscape

As if the authors knew of each other, the previous post was about Hartle, and today it is Hawking together with Hertog! The title is Populating the Landscape: A Top Down Approach. The paper is very clearly written. The top-down approach is compared to bottom-up way. Predictivity of the approach is discussed. Being not able to write anything of it better, I give a few quotes:

From their abstract: "We put forward a framework for cosmology that combines the string landscape with no boundary initial conditions. In this framework, amplitudes for alternative histories for the universe are calculated with final boundary conditions only. This leads to a top down approach to cosmology, in which the histories of the universe depend on the precise question asked."

Later " ... the quantum state of the universe at late times is likely to be independent of the state on the initial surface. This is because there are geometries in which the initial surface is in one universe and the final surface in a separate disconnected universe. Such metrics exist in the Euclidean regime, and correspond to the quantum annihilation of one universe and the quantum creation of another. Moreover, because there are so many different possible universes, these geometries dominate the path integral. Therefore even if the path integral had an initial boundary in the infinite past, the state on a surface S at late times would be independent of the state on the initial surface. It would be given by a path integral over all metric and matter field configurations whose only boundary is the final surface S. But this is precisely the no boundary quantum state."

"In the bottom-up picture it is thought that the universe begins with a grand unified symmetry, such as E8 ×E8. As the universe expands and cools the symmetry breaks to the Standard Model, perhaps through intermediate stages. The idea is that string theory predicts the pattern of breaking, and the masses, couplings and mixing angles of the Standard Model. However, as with the dimension of space, there seems to be no particular reason why the universe should evolve precisely to the internal space that gives the Standard Model. It is therefore more useful to compute no boundary amplitudes for a spacelike surface S with a given internal space. This is the top down approach, where one sums only over the subclass of histories which end up on S with the internal space for the Standard Model."

From conclusions: "A central idea that underlies the top down approach is the interplay between the fundamental laws of nature and the operation of chance in a quantum universe. In top down cosmology, the structure and complexity of alternative universes in the landscape is predictable from first principles to some extent, but also determined by the outcome of quantum accidents over the course of their histories."

February 07, 2006

Hartle on Quantum Spacetime

James B. Hartle has written a contribution to the 23rd Solvay Conference titled Generalizing Quantum Mechanics for Quantum Spacetime. It was delivered there by Murray Gell-Mann and David Gross in the absence of the author (I wonder how many people would have such eminent proxies ;-). The paper is very pedagogically written, starting practically from textbook QM which Hartle developes into a more general form.

"The key idea is that the fine-grained histories do not have to represent evolution in spacetime. Rather they can be histories of spacetime." In a sentence, branch state vectors can be constructed for each coarse-grained history by summing over the metrics and fields in the corresponding class of fine-grained histories. Considerations in signature lead to the conclusion "... if we live in a large universe it must have one time and three space dimensions." Now, if you are not very familiar with Hartle's work it is up to you to read the rest. -- At least try to avoid the two famous bloggers on the right concentrating at the moment in cartoons and movies ... ;-)

February 06, 2006

Dark Matter Temperature Measurement

BBC News science reporter Jonathan Amos interviews (February 5, 2006, 23:25 GMT) Gerry Gilmore, IoA, Cambridge, on first measurements of properties of dark matter:

"It looks like you cannot ever pack it smaller than about 300 parsecs - 1,000 light-years; this stuff will not let you. That tells you a speed actually - about 9km/s - at which the dark matter particles are moving because they are moving too fast to be compressed into a smaller scale."

"The speed is a big surprise. Current theory had predicted dark matter particles would be extremely cold, moving at a few millimetres per second; but these observations prove the particles must actually be quite warm (in cosmic terms) at 10,000 degrees."

February 01, 2006

Emergent Spacetime

Nathan Seiberg's review Emergent Spacetime (hep-th 0601234) is most interesting, at least to non-stringy people (with some exceptions, of course). Being at the moment out of town (AdS) near the boundary of the Baltic Sea, and having limited knowledge in these questions I just give some quotes from the conclusions. Seiberg gives no equations, apart from short definitions within the text, so the review is in principle layman readable ;-).

"One challenge is to have emergent spacetime, while preserving some locality - at least macroscopic locality, causality, analyticity, etc. Particularly challenging are the obstacles to formulating physics without time. It is clear that in order to resolve them many of our standard ideas about physics will have to be revolutionized."

"Understanding how time emerges will also have other implications. It will address deep issues like the cosmological singularity and the origin of the Universe."

"With so many different definitions we are tempted to conclude that we should not ask the question: "What is string theory?" Instead, we should ask: "Which string theories have macroscopic dimensions?" Although we do not have an answer to this question, it seems that large N will play an important role in the answer." (Sent as text by wireless webmail.)