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bio website motls.blogspot.com
location Czech Republic
age 40
visits member for 3 years, 9 months
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Hi, I am a string theorist and a publicist.


Dec
24
comment Where does the hidden supersymmetric sector of the MSSM come from?
Good, @Matt, will look at those as well.
Dec
23
awarded  Enlightened
Dec
23
awarded  Nice Answer
Dec
23
comment Is decoherence even possible in anti de Sitter space?
Thanks, @Ron. I still think that there are many confusions, however. AdS is a near-horizon geometry of a black hole geometry but AdS itself doesn't have any horizon. Also, the reversibility may require the system to be closed and finite, and AdS isn't one. More generally, I don't understand the logic of the original question because it mixes cosmology with local quantum prcesses, but that's not really your fault.
Dec
23
comment Gauge redundancies and global symmetries
4) whenever an action exists, a symmetry is really a transformation (rule) that keeps the action invariant, 5) keeping the Lagrangian invariant at each point means to have a symmetry that doesn't mix any points, a very "internal one", and e.g. diffeomorphisms and translations don't count, 6) non-Abelian symmetries are equally redundancies as U(1) gauge symmetries but U(1) gauge symmetries are easier to be gauge-fixed at the quantum level, e.g. no $bc$ ghosts are required because the functional determinants in gauge-fixing are constant etc.
Dec
23
comment Gauge redundancies and global symmetries
It's been answered before but 1) physical states have to be invariant under gauge symmetries, so all of them are singlets and there are no nontrivial representations, 2) this claim may be relaxed for gauge symmetry transformations that remain nontrivial even at infinity, so this "global part" of the gauge symmetries can admit nonzero charges and produce "global symmetries" out of the local one, 3) in general relativity one generally expects that all global symmetries arise from local symmetries in this way, so only local ones are fundamentally possible
Dec
23
comment Plausible explanations for 3 local space dimensions
Great to see you here, John! I hope you are, with smile and USD 500 in an envelope, celebrating the arrival of the Higgs to be discovered in Spring. My $500 is ready for you! ;-)
Dec
23
comment “gauge fixed world-sheet action”
good answer, +1. User: your first comment got invalidated because you wrote a quotation mark instead of a dollar in front of the first $T_{++}$ in it. So all maths and non-maths got exactly reverted afterwards. You still had time to edit it and fix it, a few minutes!
Dec
22
comment Where does the hidden supersymmetric sector of the MSSM come from?
Dear @Silent_Lurker, thanks for your excitement. My emotional relationship to "fudge factor" in general is similar but it's how effective theories always have to work – and the parameters are found by fitting/measuring even though fitting is another portion of the things we probably don't like in general. ;-) Of course, finding a more accurate explanation that needs no fudging is a key driver of theoretical research in HEP physics but one must also understands that finding an unfudged explanation is often tough, and it's often useless in practice.
Dec
22
revised How to derive Fermi-Dirac and Bose-Einstein distribution using canonical ensemble?
added 822 characters in body
Dec
22
comment Deterministic quantum mechanics
Nonlocality isn't the problem of these theories; the "realism" or "classicality" is the problem. Nonlocal realism is approximately as excluded as local realism, see e.g. arxiv.org/abs/0704.2529 - Strings were rightfully abandoned when people thought they were anomalous (and a wrong theory of QCD), and they were only resuscitated in 1984 when it was shown that the expectation was wrong and anomalies canceled. Now, we're "before 1984" when we know that 't Hooft's theory is wrong, so I will kindly change my opinion only when I see a 1984-like discovery and not because of "analogies", OK?
Dec
22
comment Why are differential equations for fields in physics of order two?
Dear @Nikolaj, unfortunately, you haven't started to understand the answer at all. The point is that $A,B$ have different units so statements like "$A$ is much smaller than $B$" are meaningless. Which of the terms is more important and which of them may be neglected depends on the situation, on the particular scale $R$ of the problem you're solving. The estimate is that $B\sim AL^2$ always holds where $L$ is a microscopic scale associated with the "fundamental physics of $\phi$", so the $B$ term produces negligible effects $B/R^2\sim A L^2/R^2 \ll A$ at long distances $R\gg L$.
Dec
22
comment Is decoherence even possible in anti de Sitter space?
There are no Poincare recurrences in the AdS space or any other non-integrable system with infinitely many degrees of freedom (dS space is the only exception with a "finite maximum entropy" where it may occur), so your whole answer is totally wrong. Moreover, the very idea that decoherence depends on some big-scale cosmological properties of the spacetime is entirely misguided. One may say that you copied both of these misconceptions from the OP but that doesn't change the fact that your answer lacks any positive value.
Dec
22
comment Is decoherence even possible in anti de Sitter space?
OK, I thought the question was about dS rather than AdS, because the OP mentioned Poincare recurrence and it only exists in dS, not AdS, because AdS has an infinite number of degrees of freedom. So the question was internally inconsistent but that changes absolutely nothing about my answer, namely about the fact that decoherence doesn't care whether the surrounding space is dS, AdS, or flat. Decoherence doesn't depend on cosmology in any way.
Dec
22
answered Variation of delayed choice quantum eraser
Dec
22
comment Variation of delayed choice quantum eraser
Dear Lukáši, sorry for Czechifying the weird Polish accents. ;-) See motls.blogspot.com/2010/11/delayed-choice-quantum-eraser.html for an alternative explanation of the very same experiment (the very same picture).
Dec
22
answered Where does the hidden supersymmetric sector of the MSSM come from?
Dec
21
comment Differentiating the ideal gas law
@Ron, your criticism of this answer is completely incoherent gibberish. "Differential" (noun) is the correct word for ${\rm d}f$ which, in calculus (and physics), always means exactly the same thing that you terminologically incorrectly represent by the (noun?) "infinitesimal". See the first line under "mathematics" at en.wikipedia.org/wiki/Differential - All these differentials (and this is the only correct word!) are always and by definition infinitesimal (adjective) quantities so it's nonsensical to say that the agreement holds only to the leading order: there's no other order.
Dec
21
comment Is decoherence even possible in anti de Sitter space?
In large enough de Sitter space, like our Universe, the local physics is surely de facto indistinguishable from that of a flat space or a large AdS space, isn't it? Why do you find the cosmic horizon relevant for decoherence? Macroscopic things decohere within $10^{-50}$ seconds, long before things reach the cosmic horizon. And yes, dS space eventually thermalizes which in its case means that it gets empty, except for thermal quanta of wavelength comparable to the dS radius. What's the problem? Recurrence is just a very low-probability effect of a dropping entropy. Is this problematic?
Dec
21
revised How to derive Fermi-Dirac and Bose-Einstein distribution using canonical ensemble?
added 150 characters in body