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bio website lightandmatter.com
location Fullerton, California
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visits member for 3 years, 1 month
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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
17
comment Does spacetime in general relativity contain holes?
@dcgeorge: Basically yes, although I would resist boiling down my complex answer to a sound bite.
Sep
17
comment How do we know we've unified two interactions?
@jinawee: That means that if you see an E field other observer could see just B, right? A field that's pure E in one frame can't be pure B in another. But, e.g., a field that's pure E in one frame would be a mixture of E and B in another.
Sep
17
comment Do Maxwell's equations independently impose constraints on the speed of light?
@2physics: What do you mean by "individually?" Do you mean, e.g., whether Gauss's law all by itself forces $c$ to be constant or to be some value?
Sep
17
comment Depolarisation of Light in Optical Fibre
Even if the conditions in the first sentence are satisfied, I would think you could lose polarization due to the fact that different parts of the wave are being reflected from portions of the surface that have different orientations.
Sep
17
comment Dark energy and dark matter
Our galaxies are speeding apart from each other and that is because of dark energy No, dark energy is not needed in order to explain the expansion of the universe. It's only needed in order to explain why the expansion is accelerating.
Sep
17
comment What is the universe expanding into?
@dj_mummy: Some very loosely bound systems are dispersing slowly due to inflation. Do you really mean inflation, or did you just mean to write "expansion?" I think Schlomo's answer is accurate, since it states the case correctly for galaxies (with the appropriate qualifier "mostly"), and he does note that expansion does affect structures on larger scales.
Sep
17
comment Is the number 1 a unit?
You can think of an expression such as 9.8 m/s2 as being an element of a certain group. The elements of this group can be thought of as ordered pairs $(n,u)$, where $n$ is a real number and $u$ is an element of a group representing the units. In the group that $u$ belongs to, there has to be an identity element.
Sep
17
answered Do Maxwell's equations independently impose constraints on the speed of light?
Sep
17
comment How do we know we've unified two interactions?
Re E and B, in addition to the facts you've listed, there's the fact that when you change frames of reference, a particular combination of E and B becomes a different mix of E' and B'.
Sep
17
accepted Equilibrium for a rope hanging in a Schwarzschild spacetime
Sep
17
comment Is spacetime all that exists?
@JerrySchirmer: I disagree with both statements in this comment, for the reasons given in the comment thread below my answer.
Sep
17
comment Is spacetime all that exists?
@JerrySchirmer: Of course, there's a version of Mach's principle, where you could argue that the inertia in the Schwarzschild spacetime is set by the asymptotic flatness of the solution I've never seen Mach's principle formulated in this way, and it doesn't seem reasonable to me. Mach's principle has to do with distant matter, not with asymptotic properties of empty space. I really don't think this is controversial. Re vagueness, I disagree, for the admittedly more subjective reasons given in my answer to physics.stackexchange.com/questions/5483/… .
Sep
17
comment How do I build an alpha emitter?
Seems on-topic to me. Historically, Becquerel used naturally occurring uranium, and then the reason the Curies were considered so great was that they purified radium, which was a much more intense alpha source. Only later did people figure out how to make accelerators that could accelerate an artificial beam of alphas to nuclear-physics energies, i.e., the MeV scale. Building such an accelerator (cyclotron or van de graaff) could conceivably be a garage project, but it would be an extremely ambitious garage project.
Sep
17
comment Equilibrium for a rope hanging in a Schwarzschild spacetime
@Trimok: Aha! I finally understand what's going on with the $-(2/r)T$ term. Not only did I leave out the $\nabla_t T^t_r$ term, but none of us realized that there were also terms $\nabla_\phi T^\phi_r$ and $\nabla_\theta T^\theta_r$. These end up canceling the $-(2/r)T$.
Sep
16
comment Is spacetime all that exists?
The similar quote in Prathyush's comment is from a 1950 piece that Einstein wrote for Scientific American. You can find copies online by googling for its title, "On the generalized theory of gravitation." The article presents a lot of complex ideas and is not something that can be reduced to a sound bite. A modern reader can immediately tell that it's extremely out of date. Examples: (1) He presents the distinction between SR and GR in terms of accelerated frames. (2) He thinks GR is consistent with Mach's principle. (3) He's focused on classical unified theories.
Sep
16
comment Are principle of Conservation of energy and principle of conservation of momentum consequences of Newton's laws?
Note that Newton's laws don't deal with all the phenomena in physics, e.g., light. Therefore it would only make sense to discuss their equivalence to conservation laws within a very limited context, essentially mechanics and Newtonian gravity.
Sep
16
answered Is spacetime all that exists?
Sep
16
comment If gravity doesn't exist,what are the implications?
For something like this, a link to a paper on arxiv would be really helpful. Otherwise how the heck are we going to know what is being discussed? AFAICT this is just a duplicate.
Sep
16
comment Equilibrium for a rope hanging in a Schwarzschild spacetime
@Trimok: Yeah, I think what's happening here is that Brown made a mistake in his paper. If we define $\rho=\mu/(4\pi r^2)$ and $S=T/(4\pi r^2)$, then Brown's stress-energy tensor is written correctly in terms of $\mu$ and $T$, but his final differential equation should then really be a differential equation for $\rho$ and $S$.
Sep
16
comment Equilibrium for a rope hanging in a Schwarzschild spacetime
@Trimok: Right, the $T^\kappa_\nu$ being discussed here is the stress-energy tensor of the rope, not of the gravitating body.