21,425 reputation
143108
bio website lightandmatter.com
location Fullerton, California
age
visits member for 3 years, 1 month
seen 4 hours ago

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.


Aug
24
comment Prove that a derivative with respect to a covariant 4-vector is a contravariant vector operator
I don't understand why you would use the Lorentz transformation. We're asked to prove a statement that's much more general than relativity. It's a fact about differential geometry.
Aug
23
comment Bekenstein bound for electron?
The "classical electron radius" isn't classical and isn't an electron radius. As far as we know, the electron is a pointlike particle. There are empirical upper bounds on its size (if it has internal strructure) which are far smaller than the classical electron radius.
Aug
23
answered Entropy increase vs Conservation of information (QM)
Aug
23
comment What was a second in the early universe?
The definition of the unit is secondary. What is really being defined here is time: time is what a clock measures. This is an example of an operational definition. Operationalism was formulated by Bridgman ca. 1927. One of the classic problems that he had to deal with was the fact that no measuring device handles all regimes of measurement. You have to calibrate different devices against each other. See plato.stanford.edu/entries/operationalism .
Aug
22
awarded  Nice Answer
Aug
22
comment Time reversal in Maxwell's electromagnetism
I don't see a question here...?
Aug
21
revised A space train falls front first into a black hole
edited tags
Aug
20
comment Does or should the metric expansion of space imply a locally observable increase in kinetic energy?
@benrg: the perturbation of the orbits only depends on G and Λ, not on H(t) or a(t). Not true. See the links in my preceding comment. There is a strain proportional to $\ddot{a}/a$, and a secular trend proportional to $(d/dt)(\ddot{a}/a$. Both of these can be nonvanishing even if $\Lambda=0$; in an FLRW spacetime, the Friedmann equations make the second expression proportional to $\dot{\rho}$. At the atomic or solar-system scale you're assuming that there is Hubble-expanding matter inside the atom or solar system. Again, see the two links.
Aug
20
comment Does or should the metric expansion of space imply a locally observable increase in kinetic energy?
@benrg: "The solar system does theoretically have a secular trend of growth due to cosmological expansion" is incorrect; there is no such growth at any time scale. No, you're incorrect. See Cooperstock, Faraoni, and Vollick, "The influence of the cosmological expansion on local systems," arxiv.org/abs/astro-ph/9803097v1 , which I've also given a condensed presentation of here: physics.stackexchange.com/a/70056/4552
Aug
20
comment Is “now” or “the present moment” properly defined in GR?
But there is no such preferred structure for arbitrary general relativistic spacetimes. As explained in my answer, there is a condition for such a preferred structure to exist. The condition is that the spacetime is static.
Aug
20
comment Is “now” or “the present moment” properly defined in GR?
@soulphysics: if in a globally hyperbolic spacetime, you take the timelike Killing fields to distinguish a "preferred" class of Cauchy surfaces (those that are orthogonal to the Killing fields) Global hyperbolicity is an extremely loose condition. Possession of a timelike Killing field is an extremely restrictive condition. Essentially all spacetimes of any physical interest are globally hyperbolic. Very few have a timelike Killing field.
Aug
20
revised Experiment demonstrating interference patterns of neutrons
added 716 characters in body
Aug
20
comment Experiment demonstrating interference patterns of neutrons
Their main result is a graph, which I've reproduced on p. 955 of this book: lightandmatter.com/lm . The whole paper can be found online (probably illegally, depending on your country's laws).
Aug
19
answered Is “now” or “the present moment” properly defined in GR?
Aug
19
comment Is “now” or “the present moment” properly defined in GR?
A Cauchy surface isn't necessarily a surface of simultaneity. For example, in Minkowski space, you can make all kinds of Cauchy surfaces that are not flat and that aren't surfaces of simultaneity for any observer.
Aug
19
comment Can lightly-ionized atoms be accelerated to relativistic speeds with current technology?
Why is it of interest whether they're lightly ionized?
Aug
19
comment Does or should the metric expansion of space imply a locally observable increase in kinetic energy?
@JerrySchirmer: That's different from a secular trend.
Aug
19
answered Integral ambiguity
Aug
19
comment Integral ambiguity
No, $(dp/dt)dt=dp$ is not an abuse of notation. In this notation, $dp$ and $dt$ can be interpreted as infinitesimal numbers. That's how Leibniz interpreted them, and that interpretation has rigorous logical support in non-standard analysis: en.wikipedia.org/wiki/Non-standard_analysis
Aug
19
comment Does all information in the universe come from the observer?
In absence of the observer any system undergoes unitary evolution, that is reversible evolution without entropy change. Not true.