91,966 reputation
7121246
bio website motls.blogspot.com
location Czech Republic
age 40
visits member for 3 years, 3 months
seen 17 hours ago

Hi, I am a string theorist and a publicist.


Mar
17
comment What was the major discovery on gravitational waves made March 17th, 2014, in the BICEP2 experiment?
A more advanced essay on what the discovery means, by Prof Liam McAllister of Cornell: motls.blogspot.com/2014/03/…
Mar
13
comment Heisenberg's uncertainty principle - Planck's (reduced) constant divided by two or not?
Well, in these formulae, $\Delta x$ means the standard deviation $\sigma_x$ or whatever is your preferred other notation.
Mar
11
answered Can a D-brane be closed and contractible?
Mar
11
comment Isometries and Kaluza Klein theories
Not really. If you pick an "irrational" combination of two natural generators, $t_a+K t_b$ where $K$ is irrational, the group that this combined generator generates is isomorphic to $R^+$ and not $U(1)$ because you will never return quite to the same place - the periodicity disappears. But it's always possible to choose a Cartan (maximal commuting) subalgebra of commuting and compact generators isomorphic to $U(1)^\ell$. Any generator of a compact group may be written as a combination of some generators in some $U(1)^\ell$ subalgebra.
Mar
10
answered Isometries and Kaluza Klein theories
Mar
5
comment Does limit $\hbar \rightarrow 0$ in Quantum Mechanics mean anything?
Raskolnikov, you meant $c\to\infty$, not $c\to 0$.
Mar
1
answered Majorana mass vs Dirac Mass
Feb
23
awarded  Nice Answer
Feb
21
awarded  Guru
Feb
19
awarded  Enlightened
Feb
19
awarded  Nice Answer
Feb
19
awarded  Enlightened
Feb
19
awarded  Nice Answer
Feb
19
comment How can a photon have no mass and still travel at the speed of light?
Dear @clicky-hacky-clicky-hacky, sorry for using a transliterated version of your name via Czech (orig: klikyháky, klikykáky). :-) I don't have the right keyboard to write your name and copy-and-paste would be mindless and disrespectful... If individual claims in a set of claims in physics imply each other so that you may find a loop that you call "circular reasoning", it surely doesn't mean that any of the claims is wrong. Instead, everything I write and everything I have written in my life, more or less, is true. In physics, some claims have to be assumed/extracted from observations.
Feb
18
answered Did CDMS identify dark matter?
Feb
18
comment How can a photon have no mass and still travel at the speed of light?
Dear @clicky-hacky-clicky-hacky, I wasn't trying to claim that one may derive the identity out of "nothing". Some formulae or principles always have to be assumed - and defended by a successful comparison of the results with the experiments. That's why criticisms of "circularity" in physics completely miss the epistemical status of natural sciences in general.
Feb
18
comment Why can't photons have a mass
Because the strong force is confining, so only color-neutral ("uncharged") particles are allowed to exist in isolation. Correspondingly, the force among such neutral particles decreases quickly with the distance. In fact, the decreases is faster than the power law because the gluons self-interact so they are confined, too. The mass of colored objects is a subtle thing - it depends on the RG scale and the masslessness is only relevant at very short distances, much shorter than the proton radius (QCD scale) where the confinement starts to matter.
Feb
18
answered Why consider only direction cosines?
Feb
17
comment Why some particles interact with the Higgs field and others don't?
Hi, in a quantum field theory, it is an undetermined parameter - perhaps constrained by some inequalities (at each scale, due to RG running). In string theory, more precisely in each string vacuum, the Yukawa couplings are calculable. In the leading order, they may arise from various intersection numbers between branes and other geometric quantities. We may reverse-engineering determine the Yukawa couplings from the fermion masses.
Feb
15
answered If nature exhibits symmetry, why don't up and down quarks have equal magnitude of electric charge?