Activity for Derek Elkins‭

Type	On...	Excerpt	Status	Date
Comment	Post #288251	There is not a "singular" way of solving a boundary condition problem, so saying "solve this problem as a boundary condition problem" doesn't specify an approach. Nevertheless, I assume you mean something like [this](https://tutorial.math.lamar.edu/Classes/DE/BVPEvals.aspx). This corresponds to my se... (more)	—	11 months ago
Comment	Post #288251	I'm not quite sure how you are getting an eigenfunction problem or what you expect the eigenfunctions look like. Can you elaborate? Ignoring the $x(c)=0$ condition momentarily, we can easily show that the solution to this ODE is $x(t) = a\sin(t\sqrt{\frac{k}{m}})$ with $a$ free. Adding the $x(c)=0$ c... (more)	—	11 months ago
Edit	Post #288244	Post edited:	—	11 months ago
Edit	Post #288244	Initial revision	—	11 months ago
Answer	—	A: Why is it forbidden for two photons to turn into one? Imagine two equivalent (e.g. same frequency) photons colliding with each other head-on. The linear momentum of the system is $0$ because each photon's momentum has the same magnitude but is pointing in opposite directions. If these two photons collide and form a single photon (and nothing else), t... (more)	—	11 months ago
Comment	Post #286662	This is a question about opthamology, biology, childhood development, and/or medicine, not physics. (more)	—	almost 2 years ago
Edit	Post #285649	Initial revision	—	over 2 years ago
Answer	—	A: How are the assumptions behind two ways of deriving the Rayleigh-Jeans law related? One way this can be explained is from the perspective of numerically approximating an integral. From this perspective, the concordance of "continuous" and "low frequency" has to do with the low frequency regime being where the approximation of the "continuous" integral is valid/best. If you wanted... (more)	—	over 2 years ago
Comment	Post #285373	Just because you get the correct result doesn't mean that your derivation is correct. At the time of writing the question you seemed uncertain as to how/why we could go from $\int_V \rho_f d\tau$ to $\oint_S \sigma_f da$. The way I read your concern(admittedly with a lot of guessing on my part to fil... (more)	—	over 2 years ago
Edit	Post #285373	Post edited: Improve formatting of implications	—	over 2 years ago
Suggested Edit	Post #285373	Suggested edit: Improve formatting of implications (more)	helpful	over 2 years ago
Comment	Post #285373	Where are you getting the $\sigma da = \rho d\tau$ equation? As you allude to, $da$ is referring to an area element and $d\tau$ to a volume element, so it's not clear what it means to equate these expressions. (more)	—	over 2 years ago
Edit	Post #285238	Post edited:	—	over 2 years ago
Edit	Post #285238	Initial revision	—	over 2 years ago
Answer	—	A: How can the kinetic energy equation be intuitively understood? Your analysis seems pretty good. To take an equivalent but more traditional example, imagine we throw a ball upwards. Ignoring air resistance and approximating the gravitational force as constant, the ball will accelerate downward with constant acceleration. If the initial (upwards) velocity is $v0$ ... (more)	—	over 2 years ago
Comment	Post #285233	The OP knows what the difference between kinetic energy and momentum is. The question is how to intuitively understand why kinetic energy grows quadratically with velocity. Momentum was only brought up for contrast. (Also, kinetic energy is *not* conserved in most scenarios; total energy is.) (more)	—	over 2 years ago
Comment	Post #284761	It doesn't make sense to say the "Laplace operator represents waves" any more than it would make sense to say the second partial derivative of time does. You could say that the *solutions* to that differential equations] models waves, but this isn't a property of the operator. (Admittedly, there is a... (more)	—	over 2 years ago
Edit	Post #284671	Post edited:	—	over 2 years ago
Edit	Post #284671	Post edited: Point out the reparameterization motivation of the discussion in the middle. Some other changes.	—	over 2 years ago
Edit	Post #284671	Initial revision	—	over 2 years ago
Answer	—	A: What is "order" and "disorder" in entropy? "Disorder" and "order" don't mean anything with regards to entropy. This is a common "science popularization" level description of entropy that is "not even wrong" in that, as you've seen, these terms are not defined, and especially not defined in terms of usual physical models. Now one could defi... (more)	—	over 2 years ago
Comment	Post #284073	You have a definition of the Poisson bracket, so there's nothing that can be expressed with this notion of Poisson bracket that can't be expressed without by simply using its definition. Besides that, what answer are you expecting that [the Wikipedia page for Poisson bracket](https://en.wikipedia.org... (more)	—	over 2 years ago
Comment	Post #283954	The Lagrangian doesn't minimizes the action. The Lagrangian is the integrand of the integral defining the action. The action is a function of trajectories. The trajectories that make the action stationary (not necessarily minimal) are the physically realizable trajectories. The Lagrangian isn't a tra... (more)	—	over 2 years ago
Edit	Post #280620	Initial revision	—	about 3 years ago
Answer	—	A: How do constraints work in Lagrangian systems? As apparently seems par for this book, the theorem statement you're paraphrasing involves vaguely defined or completely undefined terms. For example, it talks about "admissible (smooth) paths" but this is the first use of the term "admissible" in the text.^[The notation $f'q$ is bizarre. The authors ... (more)	—	about 3 years ago
Edit	Post #280558	Initial revision	—	about 3 years ago
Answer	—	A: How does probability conservation work in Dirac's original formulation of relativistic QM? I recommend https://www.mat.univie.ac.at/neum/physfaq/topics/position.html which, while a bit hard to read, is more comprehensive and written by someone more authoritative than me. My research didn't start there, but I covered several of the same sources before finding it. The tl;dr is the "naive"... (more)	—	about 3 years ago