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bio website motls.blogspot.com
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Hi, I am a string theorist and a publicist.


Oct
6
answered Relative Minus signs from different Feynman Diagrams
Oct
6
comment Relative Minus signs from different Feynman Diagrams
Thanks, @DavidZ - I wasn't quite satisfied with my remark and wanted at least one more hint what is the stumbling block... Maybe I see it now.
Oct
6
comment Relative Minus signs from different Feynman Diagrams
Do you understand why, for anticommuting $c_i$ variables, $12 c_1 c_2 + 5 c_2 c_1 = 12 c_1 c_2 - 5 c_1 c_2 = 7 c_1 c_2$? If you do, then I can't understand how you could misunderstand the thing you are asking about.
Oct
6
comment Relative Minus signs from different Feynman Diagrams
You need to bring the states to the same order because the two contributions to the amplitude must be standardized in the same way, otherwise you would be adding apples with oranges. One may calculate the amplitude for a fixed well-defined (including the sign) initial state and final state. If your other calculation calculates a different term but the final and/or initial state differ by an odd number of exchanges of fermions, then the initial and final states have to be changed to the same form as the previous term which produces a minus sign.
Oct
4
comment why non orthogonal states are indistinguishable?
You probably meant that non-orthogonal states are not mutually exclusive - motls.blogspot.com/2014/07/… - for two different but non-orthogonal states in quantum mechanics, there is always some probability given by the squared inner product (in absolute value) that one state will fully emulate the other, and some probability that it will not. You can't "prove" such things by pure linear algebra - it is a claim about physics so you need some postulates of physics (Born's rule).
Oct
2
comment What exactly does $S$ represent in the CHSH inequality $-2\leq S\leq 2$?
There are systematic errors and statistical errors, the latter (relative sttistical errors) typically decrease as $1/\sqrt{N}$ with the number of measurements $N$, and you should learn how all these things work. These are sort of 101 issues of an experimenter's life.
Oct
2
comment What exactly does $S$ represent in the CHSH inequality $-2\leq S\leq 2$?
dear @dk14, if you want to violate (rule out) an inequality by an experiment, you have to keep track of the inevitable error margin of the experiment. With the distribution of the "true" value of $S$ indicated by your measured mean value and error margin, you must calculate the probability that you got such a high value just by chance. And only if this probability is tiny, like smaller than $10^{-6}$ (the usual 5-sigma criterion), you may say that the experiment has established something. 1,000 measurements yielding $S=2.21\pm \epsilon$ is probably more than enough to falsify the $S\lt 2$ law.
Oct
1
comment What exactly does $S$ represent in the CHSH inequality $-2\leq S\leq 2$?
Dear dk, local realist theories imply that $|S|\leq 2$ is true, i.e. it always hold. The negation of this statement is that $|S|\gt 2$ holds at least sometimes. So it's enough to find one example where $|S|\gt 2$ and all local realist theories are ruled out.
Sep
30
awarded  Explainer
Sep
28
awarded  Enlightened
Sep
28
awarded  Nice Answer
Sep
28
awarded  Enlightened
Sep
28
awarded  Nice Answer
Sep
24
awarded  Enlightened
Sep
21
awarded  Enlightened
Sep
21
awarded  Nice Answer
Sep
21
comment Anti-neutrons, anti-quarks, isospin: What is observed and what is derived?
Well, yes, no, even Majorana neutrinos are in principle different from the antineutrinos because the words "neutrinos" and "antineutrions" get correlated with the helicity. It's the same species but if you see a quickly moving particle, you may say whether it's what is called "neutrino" or an "antineutrino" - the Majorana means that the Lorentz-invariant multiplet must contain both of them.
Sep
21
answered Anti-neutrons, anti-quarks, isospin: What is observed and what is derived?
Sep
21
awarded  Nice Answer
Sep
20
answered Why does Energy-Momentum have a special case?