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


Dec
20
comment Assumptions in Heisenberg's 1925 paper
But the knowledge of the actual behavior of the real world of atoms (and emission of light) was very important for Heisenberg to find what he found. He was extremely good at it and he also knew what was wrong about various classical models one may have proposed. In turn, he could guess the right formulae that should replace the classical ones in those simple enough cases, and he was also able to "derive" these new formulae from a detailed formalism that was surprisingly new but mathematically natural. And most importantly, it was completely right.
Dec
20
comment Assumptions in Heisenberg's 1925 paper
There's no reason why certain things in atomic physics should be nicely analogous to a completely different context. Relativity was just mentioned because it showed that one may easily get confined his reasoning by invalid assumptions, by talking about things that aren't really real or objective (such as simultaneity of events). So Heisenberg, avoiding this mistake, made a new look at the microscopic world, and this led him to reformulate the dynamics of atoms in the new quantum way using (observable quantity) energy eigenstates and matrices - needed to get a qualitatively good theory.
Dec
20
comment Assumptions in Heisenberg's 1925 paper
Thanks for your interest, @Hsirihs. I am not sure what to do with similar questions. All reasoning in physics ultimately does boil down to experimental resuts although one is often forced to process it by a long chain of reasoning. Lots of qualitative (and some quantitative) properties of the atoms were known in the mid 1920s and it was enough for Heisenberg to make his conclusions "otherwise by pure thought" and he was shown right. The analogy to special relativity is just a philosophy.
Dec
20
revised Assumptions in Heisenberg's 1925 paper
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Dec
20
revised Assumptions in Heisenberg's 1925 paper
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Dec
20
answered Assumptions in Heisenberg's 1925 paper
Dec
19
comment Are proton, antiproton, electron, positron the only observed subatomic particles that can freely exist and don't decay, i.e. are stable?
Electron, positron, all neutrinos, photons, gravitons, and possibly the lightest supersymmetric particle (LSP) if the R-parity is broken are exactly stable. However, protons and antiprotons aren't "subatomic particles". Proton is really a hydrogen nucleus, so you should also include hundreds of stable nuclides (isotopes) as well. In GUT theories, proton (and nuclei) are long-lived but ultimately unstable. They decay. Similarly, there is a risk that there exists a tiny decay of heavier neutrino flavors to lighter ones and photons/gravitons.
Dec
19
comment Are proton, antiproton, electron, positron the only observed subatomic particles that can freely exist and don't decay, i.e. are stable?
These new supersymmetric particles (and top dark matter candidates) are called "neutralinos", not "neutrinos", they're completely different particles although both of them are neutral spin-1/2 fermions.
Dec
19
comment Are proton, antiproton, electron, positron the only observed subatomic particles that can freely exist and don't decay, i.e. are stable?
Right, @dmckee. User: "decay" is linked to the function $\exp(-t)$, oscillations (caused by mixing) are linked to the function $\sin(t)$. These are different functions. Only the first one converges to zero, so only the first one deserves to be called "decay" and only the first one is linked to an instability. Incidentally, one may prepare any superposition of flavor states of neutrinos as an energy eigenstate. It won't be a momentum eigenstate at the same moment, however.
Dec
19
awarded  Nice Answer
Dec
19
awarded  Enlightened
Dec
17
comment Quantum gravity at D = 3
But I think that what we're learned is that when we study the problem nonperturbatively and accurately, the simplicity really goes away, the simplified formulae and equivalences to field theory don't work, and there's still a whole emerging structure that requires a theory of the "same degree of complexity" as string theory, if I want to avoid questions whether e.g. the CFT duals of pure 3D gravity are a part of string theory or not.
Dec
17
comment Quantum gravity at D = 3
Yes, this question has many layers and I wouldn't be able to cover all of them. But 3D quantum gravity has been attacked in many ways: by AdS3/CFT2 which has some special features because of the Virasoro symmetry of CFT2 (Witten's CFT dual for pure 3D gravity involves monster symmetry); by equivalences with other topological theories i.e. Chern-Simons theory (which only holds perturbatively etc.), and others. The dynamics of pure gravity in 3D is seemingly trivial.
Dec
17
revised Which collision energy at LHC is better for hunting 125 GeV Higgs, 7 TeV, 8 TeV or 14 TeV?
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Dec
17
answered Do Positrons Bend The Same Direction As Electrons In A Magnetic Field?
Dec
17
answered Dimensional Regularization Integral Formula
Dec
16
comment Can the Heisenberg interpretation or path integrals apply to open quantum systems?
Some people celebrating the Heisenberg picture and path integrals were right! motls.blogspot.com/2011/12/celebrating-heisenberg-picture.html
Dec
16
comment What is a single word that describes the idea of the second time derivative of energy?
Within the full systems studied by proper physics, the single word you're looking for is "zero". The reason is called the energy conservation law. ;-) Well, a more accurate term has several words: "a silly awkward way to write zero as a time derivative of yet another zero". :-)
Dec
16
answered Why is there extra minus sign in Feynman's rules for every closed fermionic loop?
Dec
15
comment Interplay between the cosmological constant and “microscopic” properties of string vacua
Dear @Squark, I don't know. I am not aware of an example of such an obstruction. Don't forget that the cosmological constant is just some vacuum energy (and momentum). The presence of generic nonzero stress-energy tensor can't invalidate the existence of hidden dimensions with a topology. If this were the case, no matter could ever propagate in the hidden dimensions because matter carries some stress-energy tensor, too, not just the vacuum energy. The background just "backreacts" and adjusts itself to whatever you insert, and if the inserted stress-energy tensor is tiny, it's not a problem.