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bio website lightandmatter.com
location Fullerton, California
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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.


Sep
16
comment What are the forms of energy at fundamental level?
The classification scheme in that chapter of my book is meant for students who are only a few weeks into a freshman course on mechanics. Re photons, quantization doesn't affect the list of types of energy, so let's talk about a classical electromagnetic wave. It has an electric energy density $\propto E^2$ and a magnetic one $\propto B^2$. I would call these potential energies. You can consider them relative rather than absolute, since they're energies that are relative to the energy that would be present if we just had a vacuum with no EM fields.
Sep
16
comment Is there a definition of force?
@AnthonyX: Force can exist without acceleration The F in F=ma is the total force.
Sep
16
revised Is there a definition of force?
edited body
Sep
16
comment How the Lorentz transformation affects the metric tensor?
A Lorentz transformation is just a change of coordinates, and although it does happen to leave the components of the metric invariant, it's not really a problem if a change of coordinates doesn't do that. All that really matters is the signature of the metric (the signs of its eigenvalues), and that's guaranteed not to change (assuming the transformation is nonsingular) because of Sylvester's law of inertia. For example, there is nothing wrong with a transformation in which you double all the coordinates, so that $g=\operatorname{diag}(1/4,-1/4,-1/4,-1/4)$.
Sep
16
answered Is there a definition of force?
Sep
16
answered How to know if something is a primitive concept, a law, a definition or a theorem
Sep
16
revised Equilibrium for a rope hanging in a Schwarzschild spacetime
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Sep
15
comment Equilibrium for a rope hanging in a Schwarzschild spacetime
I think I may understand the final issue. There is a version of the calculation in an appendix of Fouxon, arxiv.org/abs/0710.1429 . The rope's fibers are taken to be radial, so it's a cone, not a cylinder. The actual tension found by integrating across the cone's cross-section is not $T$ but $y=Tr^2f$. With the change of variables I almost get the $-(2/r)T$ term to go away, except that I'm still getting confused by signs.
Sep
15
revised Equilibrium for a rope hanging in a Schwarzschild spacetime
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Sep
15
comment Equilibrium for a rope hanging in a Schwarzschild spacetime
So taking into account MBN's correction about the additional covariant derivative terms, and Trimok's correction about the $r^{-2}$ factor, I get the following: $0=-(2/r)T+T'+(f'/f)(T-\mu)$. This differs from Brown's result by the $-(2/r)T$ term, and I think that term is clearly physically wrong, since in the Newtonian limit I get $-(2/r)T+T'-\mu g=0$. It should be $T'-\mu g=0$, without the first term.
Sep
15
comment Can Maxwell's equations be derived from Coulomb's Law and Special Relativity?
Cool! A copy is available here: scribd.com/doc/168392117/… . May be illegal, depending on the laws on copyright and fair use in a given country.
Sep
15
answered Why frequency doesn't change during refraction?
Sep
15
comment Why frequency doesn't change during refraction?
I'm not sure I quite buy this answer. The things that have to be continuous at the boundary are $D_\perp$, $E_\parallel$, $B_\perp$, and $H_\parallel$. On the other hand, there can be discontinuities in $D_\parallel$, $E_\perp$, $B_\parallel$, and $H_\perp$. So I think there is really more that needs to be filled in to make this a valid argument.
Sep
15
comment When studying electrodynamics do we assume Maxwell's Equations or derive them?
Axiomatizations are not unique. For example, you can take Euclid's postulates and prove the Pythagorean theorem. But you can also take Euclid's first four postulates, get rid of the parallel postulate, and add in the Pythagorean "theorem" as a postulate. Then the parallel postulate becomes a theorem.
Sep
15
comment Equilibrium for a rope hanging in a Schwarzschild spacetime
Ah, I see. I'd been neglecting $\nabla_t T^t_r$, since $T^t_r=0$. It hadn't occurred to me that the covariant derivative of something that vanishes identically could still be nonzero! I think the calculation really gives a condition for static equilibrium because the stress-energy tensor is taken to be diagonal, so there is no flux of energy-momentum.
Sep
15
comment Equilibrium for a rope hanging in a Schwarzschild spacetime
@Trimok: He does a treatment that becomes Schwarzschild when $\chi=f$, which is the special case I present above.
Sep
15
revised Equilibrium for a rope hanging in a Schwarzschild spacetime
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Sep
15
revised Equilibrium for a rope hanging in a Schwarzschild spacetime
added 51 characters in body
Sep
15
asked Equilibrium for a rope hanging in a Schwarzschild spacetime
Sep
15
comment Intuitively, why are bundles so important in Physics?
This is an interesting question, and I'd like to read a good answer. However, I'm not sure I buy the premise that bundles are very important in physics. The impression I get (which may be totally wrong) is that they're an optional tool, and that physics could get along fine without them. Cf. categories in mathematics.