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bio website motls.blogspot.com
location Czech Republic
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visits member for 4 years, 3 months
seen 56 mins ago

Hi, I am a string theorist and a publicist.


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.
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.
Apr
1
awarded  Nice Answer
Mar
30
awarded  Nice Answer
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
answered What does 400kW(e) mean?
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
awarded  Nice Answer
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
29
answered Relativity asymmetry?
Mar
25
awarded  Good Answer