Thus, the holographic correspondence is not just a wild new possibility for a quantum theory of gravity. Rather, in a fundamental way, it connects string theory, the most studied approach to quantum gravity, with theories of quarks and gluons, which are the cornerstone of particle physics.

— The Illusion of Gravity

— Juan Maldacena

2012.08.31 Friday ACHK

The accuracy of the measured value of G has increased only modestly since the original Cavendish experiment. G is quite difficult to measure, as gravity is much weaker than other fundamental forces, and an experimental apparatus cannot be separated from the gravitational influence of other bodies. Furthermore, gravity has no established relation to other fundamental forces, so it does not appear possible to calculate it indirectly from other constants that can be measured more accurately, as is done in some other areas of physics.

— Wikipedia on Gravitational constant

2012.08.30 Thursday ACHK

Charge (physics), the susceptibility of a body to one of the fundamental forces

— Wikipedia on Charge

2012.08.29 Wednesday ACHK

An important technical difference between decoherence and the collapse of the wave function is that decoherence actually doesn’t decide which result will be measured. It just diagonalizes the density matrix to diag(p1,p2,…,pN) where p_i then play the role of the probabilities that the individual preferred basis vectors will be detected. Because the information about the relative phases is getting quickly lost, the off-diagonal elements rapidly converge to zero. But decoherence never transforms the density matrix with the many p’s to something like (0,0,1,0,…,0). Never, ever. Quantum mechanics remains the only player here and its predictions remain and will always stay probabilistic. One can never and one will never restore determinism and decoherence doesn’t try to do anything of the sort.

— Lubos Motl

2012.08.28 Tuesday ACHK

Such QFTs arise naturally in generalizations of QCD from three colors to a larger number of colors, N.

— Solving quantum field theories via curved spacetimes

— Igor R. Klebanov and Juan M. Maldacena

2012.08.27 Monday ACHK

Compactification quantizes momentum.

— String Theory Demystified, p.218

— David McMahon

2012.08.24 Friday ACHK 

Like most individuals, and probably, most mathematicians outside his field, I do not deeply understand much of Thurston’s work. But he did have a rather large impact on my life insofar as a paragraph of his has stayed with me several years (not many do, I’m afraid), and has been my reminder to slow down whenever I find myself saying “psh, that was easy, all I had to do was browse through the documentation”:

I prided myself in reading quickly. I was really amazed by my first encounters with serious mathematics textbooks. I was very interested and impressed by the quality of the reasoning, but it was quite hard to stay alert and focused. After a few experiences of reading a few pages only to discover that I really had no idea what I’d just read, I learned to drink lots of coffee, slow way down, and accept that I needed to read these books at 1/10th or 1/50th standard reading speed, pay attention to every single word and backtrack to look up all the obscure numbers of equations and theorems in order to follow the arguments.

If a Fields medalist needed to slow down to read some maths, I can slow down to really understand whatever it is that I’m doing. And when I tell myself “I’ve learned that already!” I stop and ask whether I learned it at a “1/50th pace.”

— kevinalexbrown 1 day ago

— Hacker News

2012.08.24 Friday ACHK

One must distinguish gauge symmetries and global symmetries. A gauge symmetry is a redundancy of a description, not a property of a physical system.

— Lubos Motl

2012.08.23 Thursday ACHK