112,135 reputation
7164327
bio website motls.blogspot.com
location Czech Republic
age 41
visits member for 4 years, 4 months
seen 1 hour ago

Hi, I am a string theorist and a publicist.


Apr
23
comment Does the path integral measure have dimension?
In principle, one can imagine the logarithm of dimensionful things as well - but they produce things like bizarre additive log(meter) terms. But in quantum mechanics, there is a reason why you are right: one computes the path integral over a spacetime with a thermal circle. And the Euler character of $A\times B$ is $\chi_A\times \chi_B$, and because $\chi$ of a (thermal or other) circle $S^1$ is zero, $\chi=0$. In classical statistical physics, one must be more careful about the normalization factors in front of $Z$ when going in the continuum limit.
Apr
23
answered Does the path integral measure have dimension?
Apr
18
comment Double slit experiment in the Heisenberg picture
Try to read > motls.blogspot.com/2015/04/… for my treatment of the double slit experiment in the Heisenberg picture.
Apr
18
comment 273 + degree Celsius = Kelvin. Why 273?
Particle physicists do, and they usually express energy - and therefore also temperature - in electronvolts and its power-of-ten multiples. Room temperature is about 0.025 electronvolts: wolframalpha.com/input/…
Apr
16
comment Making a bright beam of artificial white light
@fffred, fair enough. But you're asking people to solve a very technical engineering problem. Physics is really about the individual mechanisms that may make something work. One may discuss the useful components - small holes, strong sources, lenses, mirrors, reduction of size of the source of light etc. It's pretty clear that there is no miraculous way to make ordinary light laser-like without combining the things above. Why don't you buy a laser? They are cheap. You may combine several colors of laser beam, too.
Apr
16
comment Making a bright beam of artificial white light
Dear @fffred, you mentioned my name but I think that you have ignored 100% of my comment. I was talking about a parabolic mirror in the flashlight, not a pair of holes.
Apr
16
comment Light clocks measure conformal time - detailed argument
Sorry, this equation is completely wrong. If you have $r$ as a function of $\tau$, the correct formula is still $r(\tau)=r(\tau(t))$, i.e. $r=r$. There is no $dt/d\tau$ here. The latter factor only appears if you express the derivative $r'$ - $dr/dt$ and $dr/d\tau$ differ by this factor.
Apr
16
comment Ratio of electrons and protons in Universe
Quantum field theory shows that particles may be born out of nothing as long as a few general conservation laws - energy, momentum, electric charge, angular momentum - are conserved. So yes, they did arise from "nothing" corpuscular (at the end of inflation).
Apr
15
answered Ratio of electrons and protons in Universe
Apr
15
comment Making a bright beam of artificial white light
@fffred, I am not sure whether you realize the power of Emilio's tool. A proper flashlight contains a parabolic mirror and this parabolic mirror guarantees that - in the approximation in which the glowing wolfram is located at one point - all reflected photons are moving exactly in the same direction (the wolfram is in the focal point of the parabola). The photons that get out of the flashlight "directly" have all conceivable directions, but those disappear at a big distance. Only the reflected ones are collimated, and therefore don't fade away at a big distance (in the right direction).
Apr
15
comment Deterministic universe for dummies
Right, with this if. But this question asks about our Universe and in our Universe, MWI or Bohm's theory or GRW collapse theory or... isn't realized. Our world is described by quantum mechanics.
Apr
15
answered Universality classes
Apr
15
answered Deterministic universe for dummies
Apr
15
answered Light clocks measure conformal time - detailed argument
Apr
15
answered Arrow of time and entropy?
Apr
15
answered Do electrons oscillate into muons just like electron-neutrinos into muon-neutrinos?
Apr
11
comment Particle physics: Why is J^P called spin parity if J is the total angular momentum?
All these discussions about spin-parity talk about elementary particles, so the orbital angular momentum is tautologically zero. The total internal angular momentum of the elementary particle is called the spin. And if one imagines the particle as a bound state of quarks or something else, the orbital angular momentum of these quarks is included to the angular momentum $J=S$ which is called the spin, too. It's simply impossible to "separate" the orbital part and people don't do it. You're mixing particle and nuclear physics with atomic physics where the formulae like $J=L+S$ are omnipresent.
Apr
9
awarded  Enlightened
Apr
9
awarded  Nice Answer
Apr
4
comment Total energy of the Universe
It just means that the fields describing the geometry of the spacetime, $g_{\mu\nu}(x^\alpha)$, are not predetermined constants but rather space- and time-dependent observables analogous to other fields (or coordinates of particles) such as the electromagnetic fields. "Dynamical" is something that nontrivially evolves with time according to the laws of physics and that can be measured.