## Top new questions this week:

### Why would spacetime curvature cause gravity?

It is fine to say that for an object flying past a massive object, the spacetime is curved by the massive object, and so the object flying past follows the curved path of the geodesic, so it "appears" …

general-relativity gravity spacetime differential-geometry curvature

### How does a half-life work?

Carbon-14 has a half-life of 5,730 years. That means that after 5,730 years, half of that sample decays. After another 5,730 years, a quarter of the original sample decays (and the cycle goes on and …

### How can we move an object with zero velocity?

Consider there is a box of mass $m$ at rest on the floor. Most books give an example that we need to do a work of $mgh$ to lift the box $h$ upward. If we analyze this work done, the external force …

newtonian-mechanics forces work

### Are there forces which do not involve a change in momentum?

I am familiar with the equation $$\vec{F}=m \vec{a}$$ I am wondering as to whether it is possible for something to exert a force on another object without changing the momentum of said object. My …

quantum-mechanics classical-mechanics forces

Since energy can always be shifted by a constant value without changing anything, why do books on quantum mechanics bother carrying the term $\hbar\omega/2$ around? To be precise, why do we write $H … quantum-mechanics energy harmonic-oscillator  asked by Friedrich 12 votes  answered by Ruslan 1 vote ### Why am I wrong about how to view gauge theory? Edit: I know there have been some similar questions but I don't think any had quite articulated my particular confusion. If gauge symmetries are really just redundancies in our description accounting … quantum-field-theory symmetry gauge-theory quantum-interpretations foundations  asked by gn0m0n 11 votes  answered by Urs Schreiber 7 votes ### Why do some sunsets have a green flash? Some sunsets exhibit a phenomenon called "green flash," where there is a quick flash of green light right as the sun is setting. I have seen this myself several times, and sort of understand it. I am … visible-light scattering sun  asked by Colin McFaul 9 votes  answered by DavePhD 1 vote ## Greatest hits from previous weeks: ### What is the relationship between kinetic energy and momentum? I can't seem to figure out the relationship between$E_k$and$p$or$F$. I understand that the units are pretty different. But for example: A bullet with a mass of 10.0g is moving at the speed of … homework energy energy-conservation kinematics momentum  asked by Dan the Man 5 votes  answered by Lagerbaer 13 votes ### Working of a p-n junction diode when forward biased If p-type semiconductor and n-type semiconductor of a diode are equally doped, and if the diode is forward biased, then holes will move toward the n-type semiconductor and electrons will move toward … semiconductor-physics electronics  asked by Samama Fahim 3 votes  answered by JKL 2 votes ## Can you answer these? ###$\mathcal{N}=2$spin$1/2$supermultiplet In Freedman and Van Proeyen's Supergravity, in the footnote on pg. 128, they say There is a subtle hermiticity requirement for$\mathcal{N}=2$, which requires the multiplet$(-1/2,0,0,1/2)\$ must …

supersymmetry representation-theory

### Topological Quantum Field Theories

I've asked this on Math.SE, but with no avail. So, I decided to ask it here. I was wondering about the following after reading the Wikipedia article on TQFTs. It is said that TQFTs have vanishing …

quantum-field-theory mathematical-physics hamiltonian topological-field-theory