24,274 reputation
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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.


1h
comment Correct tetrad index notation
Thanks for the edit. I've removed my whiny comments and changed my -1 to a +1.
5h
revised Tensor Notation Puzzle
added 339 characters in body
5h
answered Tensor Notation Puzzle
5h
comment Correct tetrad index notation
My initial impression is that one simply has no right to expect an index like $I$ to behave like a tensorial index, nor is there any reason to expect that moving such an index around has any well-defined physical meaning. In this context, $I$ is simply an integer that labels the four basis vectors. Even if you were using completely coordinate-free notation ("mathematician notation" for differential geometry), you could still have these $I$ indices running around. If any of the usual index gymnastics stuff can be made to work and has physical meaning, that's gravy.
6h
comment Correct tetrad index notation
For those who want to look at Carroll's treatment, it's on p. 95 of the pdf at arxiv.org/abs/gr-qc/9712019 (p. 88 according to the page numbering of the pdf). Rather than the WP link given in the question, a more directly relevant article would be this one: en.wikipedia.org/wiki/Frame_fields_in_general_relativity
6h
comment Can two electrons have the same momentum and spin directions?
@RobJeffries: How close could they be? The question assumes that the distance between them is known with infinite precision, which isn't normally the case.
7h
comment Can two electrons have the same momentum and spin directions?
@RobJeffries: If you know that two electrons are both at a certain position, say $x=0$, then each electron's wavefunction is a Dirac delta function $\delta(x)$. This violates the Pauli exclusion principle, because they are both in the same state.
7h
comment Percent error calculations dilemma
In real science, you would average the raw data (times) and also find a standard deviation of those times, resulting in T=_____$\pm$_____ s. You would then do propagation of errors to find g=_____$\pm$_____ m/s$^2$. Percent error is almost never of any interest and is almost never used in real science. It's hard to comment on what your teacher wants you to do, since it doesn't resemble anything anyone would do in real science.
7h
comment Why should photons obey the rules of causality?
Causality is very hard to define in a technical sense for physics. Philosophers also work with various definitions of causality, some of which overlap with the physics definitions. There is no way to answer a question like this unless you specify a definition. "Cause before effect" is not a definition, because you haven't defined "cause" or "effect."
7h
answered Can two electrons have the same momentum and spin directions?
1d
comment Is superposition principle equally valid for both scalar and vector quantities?
@AlfredCentauri: The linearity of vector spaces as a matter of definition simply means that we have a clear definition of what it would mean for superposition to hold for a vector field. It doesn't mean that it does hold. For example, sound waves can be describes as a vector field, if you use a vector to describe the displacement. But sound waves only approximately superpose.
1d
comment Which nucleus is the most resilient against gamma-induced fission?
The definition in the question would include processes that I don't think any nuclear physicist would call fission, such as knocking out a single neutron. Generally we don't call it fission unless both of the products are heavier than alpha particles.
1d
comment Which nucleus is the most resilient against gamma-induced fission?
I don't think this is the right criterion. Being strongly bound is not the same as being non-fissile.
1d
comment Concerning the transtition between matter dominated universe and radiation dominated universe
@Gödel: By scale factor equations, do you mean the Friedmann equations? The Friedmann equations are simpler when there is only one form of matter present, so textbooks often present those simpler cases. That doesn't mean that realistic cosmological models only use the simpler versions.
2d
answered Concerning the transtition between matter dominated universe and radiation dominated universe
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comment Covariant Nonlinear Transformation in two inertial frames
Is the matrix 4x4 still if you know its solution keeps always Real? I don't understand this sentence.
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comment Covariant Nonlinear Transformation in two inertial frames
I started to think what would the matrix be for any nonlinear transformation. Matrices are linear operators. You can't represent a nonlinear transformation with a matrix.
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comment Covariant Nonlinear Transformation in two inertial frames
What book is this from?
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comment If friction disregard area, why the direction you drag a long object matters?
related: physics.stackexchange.com/a/29954/4552
2d
comment If friction disregard area, why the direction you drag a long object matters?
I tried the experiment with the ruler, and I couldn't feel any clear difference.