57,622 reputation
481195
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location New York City
age 41
visits member for 3 years, 2 months
seen 2 days ago

I do not participate on this site any longer, except to respond to comments regarding my own text, if that text is unavailable in another form. I do not accept the political moderation atmosphere here, it is not compatible with open science. I participate at physicsoverflow.org.


Jul
23
answered What happened to David John Candlin?
Jul
23
comment Is it true that an isolated fundamental particle does not decay?
@JerrySchirmer: The standard model predicts proton decay without GUT from 't Hooft's anomaly. Even without this, you can throw a proton and electron into a black hole and get photons out in the Hawking radiation, so gravity violates B. I didn't make it up, it's well known.
Jul
17
comment event horizons are untraversable by observers far from the collapse?
@lurscher: The white hole radiates exactly the same. I think you are assuming that things are going to get spat out by the singularity that's visible, but that's an assumption. The original argument is Hawking's--- a black hole with Hawking radiation is in equilibrium, and so time-reversal invariant, so a black hole and a white hole are physically identified. This argument is born out by AdS/CFT constructions, where either is a thermal background on the CFT.
Jul
16
awarded  Populist
Jul
15
awarded  Nice Answer
Jul
14
awarded  Necromancer
Jul
13
comment Reading the Feynman lectures in 2012
@SatwikPasani: There is nothing that I remember from that chapter that is incorrect, it was mostly qualitative. Perhaps the mechanism of charge separation is understood better today, I don't know. The only point of this thing is to explain why there is a voltage as you go up in the air, and this is due to lightning charging up the ground, and it's a fact, it's still true. I don't follow the atmospheric literature, unfortunately, I don't know if people know more about charge separation through air-droplet rubbing, this was the open mystery he talked about in that chapter.
Jul
13
awarded  Necromancer
Jul
7
comment Should linear algebra and vector calculus from traditional courses be replaced with `geometric algebra`?
The problem is that it is not simpler! Ask Dimension10 whether it was easier to learn Dirac matrices or Geometric Algebra. The matrices are concrete, computational, and you can immediately see how they work, and you can work with them with no prior intellectual labor. The abstraction is too abstract! The first rule of mathematics pedagogy: any student can easily reconstruct the abstract from the concrete, but it's much more difficult to go the other way. Dirac matrices are already pretty abstract, but a matrix representation is essential for when you get stuck, and students get stuck a lot.
Jul
7
awarded  Enlightened
Jul
7
awarded  Nice Answer
Jul
3
awarded  Nice Answer
Jul
1
awarded  Nice Question
Jun
29
comment Why Silver atoms were used in Stern-Gerlach experiment?
@hiki: They could have known, because the Sommerfeld quantization always gives odd numbers, so in addition to -1 and 1, you need 0.
Jun
29
comment On-shell symmetry from a path integral point of view
@drake: Oh, I see what you mean now about real part--- this is not necessarily fatal, you can transform the fields with an i too so that the determinant is non-real, but then perhaps there is a field redefinition in real and imaginary parts so that it becomes fields shifted by independent fields. Yes, it's an issue. The Lagrangian becomes imaginary. But perhaps one can see it as a smooth deformation to get to PT quantum mechanics? I don't know.
Jun
29
comment On-shell symmetry from a path integral point of view
@drake: Yes, I know about the reexponentiation, it doesn't matter for the case in the question where you are only shifting fields by other independent fields, since then the determinant is one. The problem with compensation is that the classical variation is order 1, while the determinant variation is order $\hbar$, so this only can happen if there is no classical limit. Maybe 2d Gross-Neveu type models (four Fermi), where Bosonization can reveal new symmetry, but in this case it's not going to be a determinant compensation either, but a symmetry which is only evident in one formulation.
Jun
29
awarded  Popular Question
Jun
28
comment On-shell symmetry from a path integral point of view
Yes, we agree... (I edited the comment too late, hence time travel!)
Jun
28
comment On-shell symmetry from a path integral point of view
Your Ward identity formula is not really general. what should the Ward identity be for the formal transformation on $S = \int |\partial \phi|^2 + |\partial \eta|^2 + \phi^2 \eta^2 d^4x$ under the formal non-symmetry transformation $\delta \phi = \epsilon \phi \eta$, $\delta \eta = - \epsilon \eta $? The reason I ask is because the general formula is false for this case, as the transformation is not a field shift, but a field-value dependent field shift. so there is a determinant in the measure.
Jun
28
comment On-shell symmetry from a path integral point of view
Excellent! Thanks. I got it. The formal current thing is the idea that I was missing.