22,914 reputation
152122
bio website lightandmatter.com
location Fullerton, California
age
visits member for 3 years, 2 months
seen 38 mins 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.


38m
answered Does nature of singularity in black hole depend on material that fell in?
51m
comment How come we talk about gravitational potential energy and not gravitational potential?
The metric is the central object of study in general relativity, and the metric is essentially the gravitational potential.
19h
awarded  Explainer
1d
comment Can gravitational waves be red-shifted?
@arc_lupus: Cite: "Yes, gravitational waves will undergo the same red-shift as any wave that propagates at c." This is simple kinematics. It has nothing to do with the specific characteristics of the wave. For an example of how this is derived, see section 3.2 of my SR book: lightandmatter.com/sr
1d
comment Can gravitational waves be red-shifted?
@arc_lupus: Isn't this true for all waves without respect for their speed? No, it can't be. If a wave travels at some velocity v that is less than c, then v depends on the frame of reference, and the Doppler shift can't just be a function of the emitter's state of motion relative to the receiver. For example, the Doppler shift of a sound wave depends on the emitter and receiver's states of motion relative to the vibrating medium.
1d
answered Do perfect spheres exist in nature?
1d
comment Is it correct to say “like poles attract, unlike poles repel” while two magnets are placed such that one is inside another?
Magnets don't actually have poles. Magnetic monopoles don't exist: en.wikipedia.org/wiki/Magnetic_monopole
1d
comment How do we know that there is more than one photon in the universe?
A photon is massless and travels with lightspeed, which means for the photon itself, that time stands still and space shrinks to zero. Not true. See physics.stackexchange.com/questions/29082/…
1d
comment How do we know that there is more than one photon in the universe?
Photons simply make a full rotation from the time-like to the space-like coordinate axis. No, this is wrong.
1d
revised How can I measure the speed of a figure skater's spin?
fix capitalization and punctuation of title
1d
comment Impact force in a fall
@Qmechanic: Thanks for pointing me to that info. After reading it, I still don't think this should have the homework tag.
2d
revised Is the amount of radiation you receive in space constant regardless of velocity?
delete general-relativity tag
2d
awarded  Revival
2d
comment Why $r^2$ and not other power of $r$ in mutual force of gravitation?
This is fine as a heuristic argument, but as a matter of taste, one may or may not like it. Gravity isn't a physical substance that spreads out over an area like peanut butter.
2d
comment Why $r^2$ and not other power of $r$ in mutual force of gravitation?
You've asked an intellectually sophisticated question, and you deserve an intellectually sophisticated answer. The trouble is that there is no uniquely defined answer to this type of "why" question. If there's an answer that you will find satisfactory, it will be an answer that appeals to something you consider more fundamental. But we don't know what you'd say was more fundamental than this law. That's a matter of taste. Empirical observations? General relativity? Heuristic arguments about the commonness of inverse-square laws, because we live in three dimensions and area goes like $r^2$?
2d
comment Tensor equations in General Relativity
@lurscher: Right, I think we're in agreement. I was trying to be nontechnical in my answer, but basically everywhere that I say "coordinates," technically it should be "atlas."
2d
comment Impact force in a fall
@kenik: Since you're a climber, you know about static vs dynamic ropes. The whole reason for using dynamic ropes is to reduce the force of impact. This is why the equation in the answer depends on quantities like the Young's modulus. The Young's modulus for a dynamic rope is small, so the impact force is small.
2d
revised Impact force in a fall
edited tags
2d
comment Impact force in a fall
This doesn't actually look like homework, so I'm removing the homework tag.
2d
comment What was Feynman's “much better way of presenting the electrodynamics” — which did **not** appear in the Feynman lectures?
The question is about electrodynamics, not quantum mechanics.