Activity for Derek Elkins‭

Type	On...	Excerpt	Status	Date
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
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
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
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
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
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)	—	over 3 years ago