110,735 reputation
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bio website motls.blogspot.com
location Czech Republic
age 41
visits member for 4 years, 3 months
seen 1 min ago

Hi, I am a string theorist and a publicist.


11h
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.
23h
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/…
2d
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.
2d
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.
2d
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.
2d
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
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
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
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.
Apr
3
comment Prove that $e^{\frac{i\lambda}{\hbar} S_x}S_ze^{-\frac{i\lambda}{\hbar}S_x}=S_z\cos(\lambda)+S_y\sin(\lambda)$
@Paradox101 - it is often useful (but not necessary, as Valter shows) to consider power series for the functions. But if you use them for the exponential, you should probably know what these series (and the factorials and signs in them) look like for the sine and cosine, too.
Apr
2
comment Prove that $e^{\frac{i\lambda}{\hbar} S_x}S_ze^{-\frac{i\lambda}{\hbar}S_x}=S_z\cos(\lambda)+S_y\sin(\lambda)$
You already have it in the form if you know the Taylor expansion of cos and sin! Just put the Sz terms together and Sy terms as well.
Mar
30
comment What does 400kW(e) mean?
OK, what was meant was 33.3%.
Mar
30
comment Wave on a guitar string, differential equation
It is not, Valter, due to locality. The energy moving through a point is a local quantity related to other local quantities by purely differential operations - so whenever there is an integral sign, it gets cancelled by a derivative.
Mar
30
comment What experiment would disprove string theory?
Yes, we can calculate almost every physical observable in string theory that is well-defined, including many of those that were out of reach of the previous approximate theories. Tens of thousands of papers are full of such calculations.
Mar
29
comment Why are the proper time and the proper length not defined in the same frame of reference?
The proper time of a process P (that the object O, e.g. a train, undergoes) and the proper length of object O are defined in the same frame, namely the rest frame of O (in your case the train).
Mar
17
comment Do photons with a frequency of less than 1 Hz exist?
Sorry, the second half of the answer is silly. There is absolutely no problem in producing photons of less-than-one-hertz frequency. Just oscillate with a charge at a frequency less than one hertz and be sure that lots of electromagnetic waves are produced. The waves are composed of many photons - at such low frequencies, we actually produce a huge number of photons. What is hard is to "count" them or experimentally prove that the number is integer - it is not hard to produce them.
Mar
13
comment Why the CMB has not been dispersed so far?
Or alternatively, imagine that the box is as huge as the whole Universe and you're inside it - imagine somewhere near the center. The radiation gets diluted as the box grows but you're still inside it and the radiation is still coming from all directions.
Mar
1
comment Can Maxwell's equations be derived from Coulomb's Law and Special Relativity?
Dear @Stan, I am confident that most experts with 5,000+ cits on quantum gravity papers will agree with me that magnetic monopoles are unavoidable in quantum gravity. It's really because you may create black holes with a confined magnetic flux - imagine two end points of a big bar magnet separately collapse into black holes. So these objects exist in the form of black holes and even if and when the black hole evaporates all the particles with $Q_m=0$, something with $Q_m\neq 0$ has to be left, anyway. The magnetic monopole elementary particles may be identified with the lightest BH microstates
Feb
20
comment Is the wave-particle duality a real duality?
@StanShunpike - the wave-particle duality has never been a "problem", it's been a defining feature and virtue of quantum mechanics (or "quantum theory") since the beginning. Even non-relativistic quantum mechanics describes the objects so that they have both wave-like and particle-like properties. Quantum field theory does the same - while it's also easier to describe all the collections of particles as a "quantization of a classical field" (or "of classical waves"). But when it comes to basic conceptual properties like the co-existence of wave-like and par-like properties, QFT changes nothing