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


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.
May
10
comment The meaning of action
Of course, the only difference is that non-physicists don't learn to use the action at all. If the laymen's experience with a concept measures whether something is "intuitive", then the action simply is less intuitive and there is no reason to pretend otherwise. However, physicists learn that it's in some sense more fundamental than the energy. Well, the Hamiltonian is the key formula defining time evolution in the Hamiltonian picture while the action is the key formula to define the evolution in the nicer, covariant, "spacetime" picture, which is why HEP physicists use it all the time.
May
10
comment The meaning of action
This is a strange stream of consciousness, as Stephen Hawking would say, motls.blogspot.com/2005/10/briefer-history-of-time.html - the statement that the Hamiltonian corresponds to energy is a vacuous tautology because the Hamiltonian is a technical synonym for energy. In the same way, you may say that the action intuitively corresponds to Wirkung because it's the same thing, too, and blame the energy for having "unnatural" units of action per unit time. Well, energy is intuitive because it's conserved, and the action is intuitive because it's minimized - what's the difference?
May
10
answered Noether's theorem and “translations” of the Hamiltonian function
May
10
comment equivalence principle and nontrivial compactifications
Dear @lurscher, the difference between attraction and repulsion is just in the sign of the force, and the sign of the force is given by $q_1 q_2$. In the Kaluza-Klein theory, the charges $q_1,q_2$ may have both signs (each of them) because they correspond to a component of the momentum and momentum can have both signs; $q_1/R=p^5_1$, $q_2/R=p^5_2$. Energy/mass is however positive - it's the time component of the same vector that must be timelike and future-directed, much like velocities of material objects ($p^\mu=m_0 c v^\mu$).
May
10
comment Is it pions or gluons that mediate the strong force between nucleons?
Thanks, @anna and everyone. And a big Yes to genneth, too. @Qftme: if the student wants the most accurate picture, well, she must learn about the strings being exchanged and calculated via path integrals over Riemann surfaces, and that these strings are just the perturbative approximation of more general physical objects interpolating between strings, branes, and generic black hole microstates. ;-) However, be sure that string/M-theory is not just these words - it's the formulae. It's nice to ask for the most accurate picture, but without maths, one doesn't really get it.
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
reviewed Approve suggested edit on What gauge is used in the Lagrangian for a non-relativistic point particle in an electromagnetic potential
May
10
revised What gauge is used in the Lagrangian for a non-relativistic point particle in an electromagnetic potential
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