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Apr
29
comment What does it mean by “infinities” when dealing with QFT?
@CuriousOne "No offense"?! Jeez, I don't believe that intention for a second.
Apr
27
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
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
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
16
comment Symbol $p^{0}$ of particle
I definitely second what dmckee said!!
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
comment Would the moon be brighter if it were completely spherical?
@JánLalinský Ahhh you're right. I wasn't thinking about the dependence on Earth-Moon distance. Thanks for setting me straight :)
Apr
11
comment Zero Potential Energy Change in Raising a Mass
It has units of energy and is called "energy" ("kinetic energy" and "potential energy"). Walks like a duck and quacks like a duck...
Apr
9
comment Why did Einstein took speed of light unit or constant in his equation of relativity?
It's a bit rude to ask a question like this without first using google. What about the wikipedia page en.wikipedia.org/wiki/… is unsatisfactory?
Apr
8
comment Why do shiny surfaces reflect more light than other surfaces?
Could you make the title more descriptive? I'd answer the question, "Why do shiny surfaces reflect more light than other surfaces" with, "because that's how shiny is defined".
Apr
6
comment If we connect a block of p-type semiconductor only to a positive terminal, will it become positively charged?
This is false! I mean, for lab purposes sure, and no, the voltage difference on a battery doesn't completely specify everything, but the electric potential can be viewed as the result of a charge distribution, and so a change in potential means a change in the charge distribution, even if there's no obvious change to the naked eye.
Apr
6
comment Single slit diffraction - wave vs particle view
What's there to reconcile? $h/\lambda$ isn't the uncertainty in the wavelength...