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Hi, I am a string theorist and a publicist.


May
11
answered The meaning of action
May
11
comment The meaning of action
OK, I will strip the "dialog" form and promote it to an answer, @Approximist.
May
11
comment Why is the Yang–Mills existence and mass gap problem so fundamental?
And so does the definition of rigor in theoretical physics. ;-)
May
11
comment Why is $\frac{dx}{dt}=0$ in this average momentum calculation?
+1, Marek. Interestingly, the question contains the first formula for $\langle p \rangle$ which shows perfectly what the objects depend upon. It says that $\psi$ and $\psi^*$ depend on $x,t$ while $x$ doesn't depend on anything, especially not on $t$. So despite this detailed notation, the dependence was ignored by the OP. Moreover, to claim that $x$ depends on $t$ in those formulae would be totally preposterous because $x$ is being integrated over in the formula - it takes all real values simultaneously and "all real values" (the real axis) clearly can't depend on $t$.
May
11
comment Is fire matter or energy?
Richard Feynman has answered a similar question asked by young rabis: myspace.com/richard_feynman/blog/332429588 The question was: "Is electricity fire?" His answer may be used for your question, too. So read the chapter of his book carefully to catch the answer which may be more important than the detailed question. ;-)
May
11
comment What is the value of the fine structure constant at Planck energy?
BTW one can't even say that it's possible to find an electron-positron-photon QED that agrees with the real world up to the electron mass scale. It's because the real world also has neutrinos and gravitons that are lighter, and maybe axions.
May
11
comment What is the value of the fine structure constant at Planck energy?
Dear Claude, what do you exactly mean by QED? Just photons, electrons, and positrons? Or do you include muons, tau, and quarks? When you do, why don't you also include W-bosons which will destroy your "QED" at the electroweak scale, anyway? The incorporation or not incorporation of muon - and all other things - has a profound impact on all the numbers and running. It's straightforward to compute the running in any of these (wrong) toy models, as long as they are consistent, but why would you do it?
May
11
answered Why is the Yang–Mills existence and mass gap problem so fundamental?
May
11
comment A lightning protection device physics
Dear Martin, if you think that you cannot draw energy from an electrostatic field, how do you explain that some people claim that they use batteries in their cell phones or even laptops? ;-) Is that unimaginable that the electrostatic fields allow one to lift charged objects from one place to another, doing work along the way, and reducing the strength of the electrostatic field (which is why batteries get discharged)?
May
11
answered What is the value of the fine structure constant at Planck energy?
May
11
revised How is perturbation theory applied to the Bunch-Davies state for an interacting quantum field theory?
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May
11
comment How is perturbation theory applied to the Bunch-Davies state for an interacting quantum field theory?
And by the way, the de Sitter space's radius was still much larger than the Planck length even during inflation. That's why the non-uniformities of the CMB temperature today are of order $10^{-5}$ only. These non-uniformities are predicted from the correlation functions. Whether the number of particles in a state is conserved doesn't play any role; the number of particles in the states relevant for inflation can't even be sharply well-defined because they're thermal from all real observers' viewpoint.
May
11
revised How is perturbation theory applied to the Bunch-Davies state for an interacting quantum field theory?
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May
11
comment How is perturbation theory applied to the Bunch-Davies state for an interacting quantum field theory?
I wrote it clearly. It's a thermal state and there exist standard rules how to compute all correlators using Feynman diagrams that are conceptually on par with Feynman diagrams in a thermal ensemble in the Minkowski space. BTW this comment of yours should have been posted as a comment, not as an answer. Because it answers nothing about the question above, I rated it -1.
May
11
revised How is perturbation theory applied to the Bunch-Davies state for an interacting quantum field theory?
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May
11
answered How is perturbation theory applied to the Bunch-Davies state for an interacting quantum field theory?
May
11
answered What is the risk from radiation on imported food from Japan
May
11
comment What gauge is used in the Lagrangian for a non-relativistic point particle in an electromagnetic potential
Thanks, Qmechanic, but nope. $A_\mu$ may be treated as a background but even with this Lagrangian, it may be perfectly dynamical as well, so that the charged fields influence the electromagnetic field and vice versa. It's exactly how physics was supposed to work throughout the 19th century. The Lagrangian is that of ordinary electrodynamics so why should the key field be non-dynamical? Of course, there should also be $(E^2-B^2)/2$ in the Lagrangian which is gauge-invariant, too. Again, there is no need to gauge-fix it - don't get confused. Gauge symmetry is a virtue not vice.
May
10
revised What gauge is used in the Lagrangian for a non-relativistic point particle in an electromagnetic potential
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May
10
comment The meaning of action
Otherwise, the main raison d'etre for the action is the principle of least action, en.wikipedia.org/wiki/Principle_of_least_action , which is what everyone should learn if he wants to know anything about the action itself. It makes no sense to learn about a quantity without learning about the defining "application" that makes it important in physics. Energy is defined so that it's conserved whenever the laws of Nature are time-translational symmetric; and action is defined as whatever is minimized by the history that the system ultimately takes to obey the same laws.