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bio website lightandmatter.com
location Fullerton, California
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visits member for 3 years, 3 months
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I teach physics at Fullerton College, a community college in Southern California. I have an undergrad degree in math and physics from Berkeley and a PhD in physics from Yale. Back when I was doing research, my field was experimental low-energy nuclear physics.


2d
answered Why is Earth's gravity stronger at the poles?
2d
answered Infinite space or finite space?
Oct
17
comment Infinite space or finite space?
it can't have an edge because then you'd have to ask what's beyond the edge. I don't think this is a valid argument. After all, we do have geodesic incompleteness in GR. To explain why it's OK to have geodesic incompleteness but not OK to have a model that's on a manifold-with-boundary, you need something more than this argument.
Oct
17
comment Why is Earth's gravity stronger at the poles?
Nice. Although the answer never uses the term "centrifugal force," that's implicit in the argument, because the equipotential is an equipotential in the rotating frame.
Oct
17
comment Is there a null incomplete spacetime which is spacelike and timelike complete?
@yess: I see. Sorry, my comments were not that helpful then. I'll delete them.
Oct
17
comment Is manufacturing roughness really the only reason we don't see optical interference in thick dielectrics like windows?
@Floris: I was referring to the monochromatic case. Colored light == short coherence length of light. Huh? For example, red laser light is colored, and has a long coherence length.
Oct
16
comment Numerical Error Propagation
I have the analytic expression for the uncertainty What is it?
Oct
16
revised Are Lockheed Martin nuclear fusion claims realistic?
add link to claims in media
Oct
16
comment How much energy does a photon need to form a black hole?
@JerrySchirmer: No, LQG does not have to violate Lorentz invariance. See Rovelli, 2010, “Loop quantum gravity: the first twenty-five years,” arxiv.org/abs/1012.4707 . "I want to stress the fact that loop gravity does not imply a violation of Lorentz invariance. In particular, the naive argument, often heard, that a minimal length is incompatible with Lorentz invariance is wrong, because it disregards quantum theory."
Oct
16
comment Diffraction by small holes
related: skeptics.stackexchange.com/questions/4483/…
Oct
16
revised Diffraction by small holes
added 956 characters in body
Oct
16
revised Diffraction by small holes
added 360 characters in body
Oct
16
comment What happens to waves when they hit smaller apertures than their wavelenghts?
I think the difference between water and EM waves is that you do end up getting a different exponent in the equation for thetransmission. I discussed this briefly at the end of this answer: physics.stackexchange.com/a/141713/4552
Oct
16
comment What happens to waves when they hit smaller apertures than their wavelenghts?
I think I understand the $(a/\lambda)^4$ result now. I wrote out an explanation here: physics.stackexchange.com/a/141713/4552
Oct
16
answered Diffraction by small holes
Oct
16
comment What happens to waves when they hit smaller apertures than their wavelenghts?
@LubošMotl: A second thing about your argument also doesn't make sense to me, which is that it seems to be wrong dimensionally, unless the transmission is defined as I described in my preceding comment.
Oct
16
revised Does velocity determine a geodesic?
delete introductory comment
Oct
16
comment What happens to waves when they hit smaller apertures than their wavelenghts?
@LubošMotl: From online discussions I found by googling, it looked to me like the transmission was defined as the ratio of the diffracted power to the power incident on the hole. But I don't have access to the Bethe paper, and maybe the online sources were wrong or I was misinterpreting them. Doesn't Huygens' principle not even work in odd dimensions?
Oct
16
comment Is there a null incomplete spacetime which is spacelike and timelike complete?
[...] create the definition of a naked singularity, which is pretty intricate and doesn't just refer to what types of geodesics hit it.
Oct
16
comment Is there a null incomplete spacetime which is spacelike and timelike complete?
Discussions I've seen of this kind of thing actually talk about the characteristics of the singularity, not the characteristics of the incomplete geodesics. The main cases of interest are spacelike singularities (e.g., a black hole's singularity) and timelike singularities (e.g., cosmological ones, or naked singularities). Googling does turn up discussion of null singularities as well. I'm not convinced that the classification in terms of the characteristics of the complete and incomplete geodesics is actually useful or consistent. If so, then it would seem to have been unnecessary to [...]