Physics Stack Exchange Community Digest

Top new questions this week:

Correspondence between integral transformations and differential operators in quantum mechanics

The translation operator in one dimension is defined as $$ \hat T \psi(x) = \psi(x-\alpha) . $$ This can be written as an integral transformation, $$ \begin{align*} \hat T\psi(x) = \langle x|\hat ...

quantum-mechanics hilbert-space operators wavefunction mathematical-physics  
asked by dnnagy 7 votes
answered by Cosmas Zachos 4 votes

What is QED about "Cavity QED"?

As I understand it, when papers and books refer to "Cavity QED" they simply mean the strong coupling regime of two-level system to a photon field in a resonator, i.e. Jaynes-Cummings and variants ...

quantum-optics cavity-qed  
asked by Quasilattice 7 votes
answered by Wolpertinger 7 votes

Why coupled oscillators tend to seek integer frequency ratios?

In this document, the author writes (page 225) Coupled oscillators have a tendency to seek frequency ratios which can be expressed as rational numbers with small numerators and denominators. For ...

classical-mechanics orbital-motion resonance coupled-oscillators harmonics  
asked by Ma Joad 7 votes
answered by Tesseract 2 votes

If a photon truly goes through both slits (at the same time), then why can't we detect it at both slits (at the same time)?

I am not asking about whether the photon goes through both slits, or why. I am not asking whether the photon is delocalized as it travels in space, or why. I have read this question: Do we really ...

quantum-mechanics electromagnetism photons double-slit-experiment  
asked by Árpád Szendrei 6 votes
answered by S. McGrew 23 votes

Nobel Prize in Physics 2019 on exoplanets and Hot Jupiters

The 51 Pegasi b, discovered by Didier Queloz and Michel Mayor is classified as a hot Jupiter and not considered to be a star. I had read a bit about the minimum mass necessary for being a brown dwarf ...

newtonian-mechanics experimental-physics astrophysics astronomy  
asked by Maan 6 votes

Classical motion in delta potential

The question about classical motion in delta potential may seem artificial, but it makes sense it you try to calculate the propagator for particle in delta-potential, because you usually need to know ...

classical-mechanics dirac-delta-distributions propagator  
asked by Alex Goldstein 5 votes

What happens here when the Second Law of Thermodynamics doesn't hold true?

In the 1920s, theoretical physicists, most notably Albert Einstein, considered the possibility of a cyclic model for the universe as an (everlasting) alternative to the model of an expanding ...

thermodynamics statistical-mechanics cosmology entropy arrow-of-time  
asked by Maan 5 votes

Greatest hits from previous weeks:

Water pressure calculation for a volume of water at a given height

I'm looking to create a rain harvesting system. I have a 275 gallon IBC tote that is 48" x 40" x 46". I have an adapter for a 3/4 garden hose at the bottom of the IBC tote. I'm trying to figure out ...

water pressure friction volume  
asked by user379468 1 vote
answered by DavePhD 3 votes

Why doesn't the Moon fall onto the Earth?

Why doesn't the Moon fall onto the Earth? For that matter, why doesn't anything rotating a larger body ever fall onto the larger body?

newtonian-mechanics newtonian-gravity orbital-motion earth moon  
asked by Adir Peretz 65 votes
answered by Mark Eichenlaub 104 votes

Doesn't the speed of light limit imply the same electron can be annihilated twice?

If I understand correctly, there is a small probability the same electron to be found anywhere in the universe. Suppose that an anti-electron collides with an electron, annihilating it and producing ...

quantum-mechanics special-relativity speed-of-light  
asked by Stefan 45 votes

Which direction does air flow?

I remember learning this in high school, but have forgotten it, and can't seem to find it anywhere online. Air travels from areas of high pressure to low pressure...correct? So if I have a cold room ...

thermodynamics temperature building-physics  
asked by Andrew 14 votes
answered by Adam Zalcman 16 votes

How is light affected by gravity?

Light is clearly affected by gravity, just think about a black hole, but light supposedly has no mass and gravity only affects objects with mass. On the other hand, if light does have mass then ...

gravity mass speed-of-light photons relativity  
asked by PriestVallon 30 votes
answered by DilithiumMatrix 34 votes

Thermodynamics - Sign convention

I use the sign convention: Heat absorbed by the system = $q+$ (positive) Heat evolved by the system = $q-$ (negative) Work done on the system = $w +$ (positive) Work done by the system = $w -$ ...

thermodynamics work conventions volume  
asked by Fasna 10 votes
answered by Kenshin 5 votes

Why can two (or more) electric field lines never cross?

The the title is self explanatory, I guess. Why can two (or more) electric field lines never cross?

electrostatics electric-fields  
asked by SjonTeflon 15 votes
answered by The Photon 25 votes

Can you answer these questions?

How to prove that these integrals are the same with a Fourier transform?

In the paper BMN Correlators and Operator Mixing in $\mathcal{N}$=4 Super Yang-Mills Theory, they claim in Appendix $A.2$ (p.26, eq. $(A.7)$) that the following relation holds: $$F_{12,34} = ...

quantum-field-theory fourier-transform yang-mills propagator  
asked by Jxx 1 vote

Characterization of a thin lens with different media on the two sides

Consider a thin lens surrounded by media with different indices of refraction on the two sides. Let the indices of refraction be $n_1$ for the medium in which the object is, $n_2$ for the interior of ...

optics refraction geometric-optics lenses  
asked by Ben Crowell 1 vote

Does universal proof exist for newton's experimental law for all cases of elastic collisions?

Newtons experimental law I. E velocity of approach is equal to velocity of separation (for the points on the two bodies where the collision takes place) for an elastic collision is applicable for ...

newtonian-mechanics momentum conservation-laws collision  
asked by Sid 1 vote
answered by Marco Ocram 0 votes
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