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I used to check every question here but not any more. Physics Overflow, which was built from the archives of a defunct "Theoretical Physics Stack Exchange", seems more promising as a place for advanced physics Q&A.


Dec
10
awarded  Announcer
Dec
5
awarded  Caucus
Dec
4
comment How quarks converted into leptons
It's the detailed interaction of the particles with the electromagnetic field which suggest fractionally charged constituents, e.g. physics.ohio-state.edu/~kass/P780_L8_sp03.ppt slides 13-14 (decay of vector mesons) or arxiv.org/abs/hep-ph/0210054 (various other processes).
Dec
4
comment Do unoriented strings possess asymptotic states?
... a baryon, a collection of N strings terminating on a compact brane. A non-gauge-invariant object like a diquark might be part of a baryon, e.g. you would think of a brane with 3 strings attached, and two of the strings very short, so that the third, long string can then be conceived as connecting a quark and a "diquark object". But all this would be just one way that gauge theory embeds in string theory. A truly comprehensive account might list half a dozen ways to get one from the other.
Dec
4
comment Do unoriented strings possess asymptotic states?
The right way to visualize perturbative string theory is as defined by Riemann surfaces (possibly with boundaries) with marked points corresponding to the asymptotic states. (A conformal rescaling of the worldsheet is used to map the infinite string history onto the finite Riemann surface.) An unoriented string could be conceived as an equal superposition of oppositely directed oriented strings... Concerning the relation with gauge theory: an individual "quark" would be a string between a color brane and a flavor brane; a meson, a string between two flavor branes...
Dec
2
comment How quarks converted into leptons
Since this is the direction of your thinking I will point out arxiv.org/abs/hep-ph/0408305 which is a paper trying to unify "iquarks" (author's name for quarks with integer charge) with leptons. That is a paper with no citations and it's probably making a mathematically wrong argument somewhere. If you want to understand what physicists already think, you need to understand fractionally charged quarks, and the SU(5) theory might help too. But people follow their own ideas right or wrong, and maybe you can learn something else from this professional paper even if it is wrong.
Dec
2
comment How quarks converted into leptons
Normally by "lepton" we mean particles that don't interact via QCD. You seem to be thinking of "integer charged quarks"? That is an old idea, but there are some (difficult) experiments which do seem to falsify it directly. Also, the fractional charges start to make sense in a unified theory like SU(5) grand unified theory.
Dec
2
comment How quarks converted into leptons
Quarks aren't observed because of en.wikipedia.org/wiki/Color_confinement ... someone will post about this. Like anna said, in your diagram, the quark and antiquark confined in the pion are annihilated, but that is a different thing, it is not the reason why you don't get single free quarks (except at very high temperatures).
Nov
24
answered Help an aspiring physicists what to self-study
Nov
18
awarded  Revival
Nov
16
comment Have I discovered how to calculate the proton's mass using only integers?
Fred, the bottom line here is that a kilogram is an arbitrary unit of mass, and if you used a different unit, you would have a different number to explain. The actual numbers in physics that people seek to explain are quantities like "ratio of proton mass to electron mass", which don't depend on units.
Nov
14
comment Is physics rigorous in the mathematical sense?
The ideal of rigor for science is the hypothetico-deductive method. You make a hypothesis, deduce its consequences, and then test them against reality. The mathematical formalization of this process would be something like the AIXI algorithm in computer science, which uses data to make causal models. Also see the whole field of statistics, and its methods for establishing the likelihood of a hypothesis. The difference between mathematics and physics is that in physics you use empirical data as an input. But you can still be rigorous in your methods.
Nov
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comment Does the Higgs Mechanism contradict Entropic Gravity?
Entropic gravity has other problems arxiv.org/abs/1108.4161 arxiv.org/abs/1108.5240.
Nov
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awarded  Announcer
Oct
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comment Reference-request: Computational science and physics
... or exactly how mathematics relates to reality. Galileo's remark is a poetic statement that there is some form of connection but states no details. Unless you adopt the pythagorean position that "reality is mathematics", you need some broader ontological idea in which "number" is just one "aspect" of reality, but that is a nature-of-reality question. It's metaphysics, it's about whether reality consists of "things with properties" or something else entirely.
Oct
30
comment Reference-request: Computational science and physics
UGPhysics, I don't think you know what you're asking. Computability has a specific technical meaning, see "Turing computability". You could hypothetically have a theory of physics that was technically noncomputable, but the only part that could be tested would be a computable truncation of the theory. Insisting that a theory includes an algorithm for making quantitative predictions is just the bare minimum requirement for it to be testable, and doesn't say anything about whether reality contains anything noncomputable in the technical sense, whether reality is "mathematical" ...
Oct
29
comment What does the wind speed have to be to blow away a person?
Sort of similar question: physics.stackexchange.com/q/36439
Oct
28
comment Reference-request: Computational science and physics
But what is the relation between being computable and being mathematical? And why would this be relevant for a TOE, but not for an incomplete theory?
Oct
26
comment Which arguments for $m_u \approx 0$ are still in the market?
arxiv.org/abs/hep-ph/9403203 is the deepest theoretical discussion I have found.
Oct
23
comment Is there perfect vacuum 10000 billion lightyears away?
The second reason is harder to convey, but it has to do with the holographic principle and the existence of a "dual description" of a theory with quantum gravity, on the boundary of its space. The main counterargument here is simply that there is no working example of holographic duality in which the cosmological horizon is the boundary. He's just guessing how the duality works for an expanding universe, and the illogic of the answer suggests he is guessing wrongly.