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 1d comment What does it mean by “infinities” when dealing with QFT? @CuriousOne "No offense"?! Jeez, I don't believe that intention for a second. 2d comment Single particle diffraction: how is this possible? "How is this possible?" Exactly! Classical well defined notions of position and definiteness are out the window. Apr 25 comment What does the Poynting flux represent? Not necessarily, no. If you turn a solenoid on, you'll find you (your battery) had to do work to establish the magnetic field. More work than just to overcome the resistivity of the wire. When you turn off your battery, that solenoid will try to keep up the current and dump energy back in as the field collapses. So static fields store energy too. Apr 25 comment What does the Poynting flux represent? it allows you to see how energy is flowing in space. In your plot there is a lot of energy flowing towards the center at that moment in time. The precise statement is in Poynting's theorem en.wikipedia.org/wiki/Poynting%27s_theorem (the vacuum case is most relevant, in which case $J_f=0$) Apr 25 comment What does the Poynting flux represent? It doesn't give you information about how the particles move. Just think about the magnitudes of $E$ and $B$. You can take $B$ larger and larger and $E$ smaller and smaller, and that will leave $E\times B$ the same. Apr 25 comment Gallilean Transformations with Linear Acceleration The laws of physics are the same under any Galilean transformation. "A Galilean transformation" is moving into a frame with a constant speed! When you move into an accelerating frame you have to add fictitious forces depending on the nature of the acceleration. It applies for any acceleration, not just time dependent rotations. Apr 25 comment Gravity vs Gravitational Waves "Is measuring gravity equivalent to measuring magnetism around a permanent magnet? Is measuring gravitational waves equivalent to a digital camera capturing photons?" Yep! That's a good analogy. Apr 20 comment Sequence of topics for studying quantum theory (Research strategy) First you need Lagrangian mechanics. Then Hamiltonian mechanics. THEN you can start on nonrelativistic quantum mechanics. (By the way, modern quantum mechanics has been around for 80ish years and basically encompasses "all of modern physics".) Apr 20 comment Why is this ice cloudy at the center and has a transparent layer? Are you sure this is the right answer? I was under the impression it was micro-cracks due to stresses from expanding ice. Apr 18 revised Stanford: “Objects in spacetime all move at constant speed $c$.” Are they right? added 34 characters in body Apr 18 comment Stanford: “Objects in spacetime all move at constant speed $c$.” Are they right? @knzhou awesome, glad that's settled. (Also, for any people doing comment archaeology, it should definitely be $u^\mu u_\mu=c^2$ :) Apr 18 revised Stanford: “Objects in spacetime all move at constant speed $c$.” Are they right? added 779 characters in body Apr 18 comment Stanford: “Objects in spacetime all move at constant speed $c$.” Are they right? @knzhou -shrug- I think we're saying the same thing but disagreeing on word choice. If I normalized it to $u^\mu u_\mu=4c$, then in whatever calculations I did I plugged that in, I could get exactly the same results as you would with $u^\mu u_\mu=c$. I guess the most precise version of my statement: no experiment can find the magnitude of a tangent vector to a curve in four-space. Only direction matters. Therefore, normalization is a choice. Apr 18 comment Stanford: “Objects in spacetime all move at constant speed $c$.” Are they right? @AstrophysicsMath "Wrong" is a strong word. I'd say that Brian Greene is conveying a truth that is part of the model of special relativity that he is presenting. But other phrasings exist. Apr 18 revised Stanford: “Objects in spacetime all move at constant speed $c$.” Are they right? added 50 characters in body Apr 18 answered Stanford: “Objects in spacetime all move at constant speed $c$.” Are they right? Apr 16 comment Symbol $p^{0}$ of particle I definitely second what dmckee said!! Apr 16 answered Symbol $p^{0}$ of particle Apr 12 comment What's the problem with light traveling at speed higher than $c$? @BenLindsay Sorry, trading precision for readability! Precisely: the same behavior that allows moving your head along a ripple of light, is the same behavior that made people predict that the Michelson-Morley experiment would find out how fast the Earth move through an aether. That behavior being "Galilean invariance", which is now known to not hold at high velocities. Apr 12 answered What's the problem with light traveling at speed higher than $c$?