Questions tagged [scattering]

Scattering is a general term for several physical processes in which radiation of some sort changes direction due to an interaction with a particle. Scattering can be classified by the type of radiation (ie, electromagnetic, x-ray, neutron), or by the relative sizes of the wave and the particle (ie, Rayleigh, Mie, geometric).

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Scattering of light by light: experimental status

Scattering of light by light does not occur in the solutions of Maxwell's equations (since they are linear and EM waves obey superposition), but it is a prediction of QED (the most significant Feynman ...
Keenan Pepper's user avatar
50 votes
5 answers
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Why does the sky change color? Why is the sky blue during the day, red during sunrise/set and black during the night?

Why does the sky change color? Why is the sky blue during the day, red during sunrise/set and black during the night?
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Why can we treat quantum scattering problems as time-independent?

From what I remember in my undergraduate quantum mechanics class, we treated scattering of non-relativistic particles from a static potential like this: Solve the time-independent Schrodinger ...
Mark Eichenlaub's user avatar
43 votes
10 answers
31k views

Why can't we see gases?

I am not sure what causes gas molecules to be invisible.This question may look silly but I really want to know the story behind it.
Praveen Kadambari's user avatar
40 votes
1 answer
29k views

Phase shifts in scattering theory

I have been studying scattering theory in Sakurai's quantum mechanics. The phase shift in scattering theory has been a major conceptual and computational stumbling block for me. How (if at all) does ...
Cogitator's user avatar
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37 votes
4 answers
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Scattering, Perturbation and asymptotic states in LSZ reduction formula

I was following Schwarz's book on quantum field theory. There he defines the asymptotic momentum eigenstates $|i\rangle\equiv |k_1 k_2\rangle$ and $|f\rangle\equiv |k_3 k_4\rangle$ in the S-matrix ...
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35 votes
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Why does the moon sometimes appear giant and a orange red color near the horizon?

I've read various ideas about why the moon looks larger on the horizon. The most reasonable one in my opinion is that it is due to how our brain calculates (perceives) distance, with objects high ...
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33 votes
8 answers
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Why aren't rainbows blurred-out into nothing after they are produced?

I understand how a prism works and how a single raindrop can scatter white light into a rainbow, but it seems to me that in normal atmospheric conditions, we should not be able to see rainbows. When ...
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29 votes
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How did Rutherford conclude that most of the mass (as well as the positive charge) was concentrated in the nucleus?

Geiger and Marsden's experiment led Rutherford to believe that the positive charge and most of the mass of the atom was concentrated in a small region. I understand what led him to conclude the way ...
Kunal Pawar's user avatar
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27 votes
1 answer
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Systematic way to draw all inequivalent Feynman diagrams

I am wondering whether there is some systematical approach to find Feynman diagrams for S-matrix (or to be more precise for $S-1$ since I am interested in scattering amplitude). For example in $\phi^3$...
I-L-P's user avatar
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In a vacuum, can you see light which is not travelling towards you?

In air, when there is light propagating in a direction, we can still see it even when it is not primarily travelling in our direction, because a small part of the light hits the air molecules, and ...
Júlia Sirotiaková's user avatar
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1 answer
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Is the converse of Weinberg's statement on the cluster decomposition principle true?

In Weinberg's "The Quantum Theory of Fields, Vol. 1", Section 4.4, page 182, the author says: We now ask, what sort of Hamiltonian will yield an $S$-matrix that satisfies the cluster ...
Gold's user avatar
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Use my example to explain why loop diagram will not occur in classical equation of motion?

We always say that tree levels are classical but loop diagrams are quantum. Let's talk about a concrete example: $$\mathcal{L}=\partial_a \phi\partial^a \phi-\frac{g}{4}\phi^4+\phi J$$ where $J$ is ...
346699's user avatar
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'Quantum' vs 'Classical' effects in Quantum Field Theory

After reading a few textbooks on Quantum Field Theory there's something that's always struck me as bizarre. Take a scattering process in QED like $\gamma$,e$^-$ $\rightarrow$ $\gamma$,e$^-$. The ...
chuckstables's user avatar
23 votes
3 answers
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As there is no specific boundary of an atom, how was Rutherford able to estimate the size of an atom?

On the basis of the observations, Rutherford drew the following conclusions regarding the structure of an atom: Most of the space in the atom is empty as most of the alpha particles passed through ...
5 Dots's user avatar
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Covariant Description of Light Scattering at a fastly rotating Cylinder

Let us consider the following Gedankenexperiment: A cylinder rotates symmetric around the $z$ axis with angular velocity $\Omega$ and a plane wave with $\mathbf{E}\text{, }\mathbf{B} \propto e^{\...
Robert Filter's user avatar
22 votes
2 answers
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What is the physical interpretation of the S-matrix in QFT?

A few closely related questions regarding the physical interpretation of the S-matrix in QFT: I am interested in both heuristic and mathematically precise answers. Given a quantum field theory when ...
newbie's user avatar
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21 votes
5 answers
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Why is it necessary for an object to have a bigger size than the wavelength of light in order for us to see it?

I keep hearing this rule that an object must have a bigger size than the wavelength of light in order for us to see it, and though I don't have any professional relationship with physics, I want to ...
aslı's user avatar
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20 votes
2 answers
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Locality in the scattering amplitude

Early in this talk by Nima Arkani-Hamed, he describes what locality means for scattering amplitudes. "Locality tells you that the only poles in the scattering amplitude occur when the sum of a subset ...
Tim Goodman's user avatar
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19 votes
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Scattering vs bound states

Why are these states called as such, and how do they differ? I vaguely understand that when $E > 0$ you obtain a scattering state, but when $E < 0$ you have a bound state.
wrongusername's user avatar
19 votes
2 answers
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Why Cronin Effect Happens?

I'm looking for explanation on Cronin effect but unfortunately there's no Wikipedia entry or self-contained paper to start from. The statement of this effect is that: "At leading order, multiple ...
Yair's user avatar
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18 votes
2 answers
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Neutron discovery, why it couldn't be gamma ray? or other photon?

During the discovery of the neutron by J. Chadwick it was observed that something is emitted from beryllium when bombarded by alpha particles that could knock out protons with 5.7 MeV from a paraffin ...
fahd's user avatar
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1 answer
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Green's function in path integral approach (QFT)

After having studied canonical quantization and feeling (relatively) comfortable with it, I have now been studying the path integral approach. But I don't feel entirely comfortable with. I have the ...
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What are the exact relations between bound states, discrete spectra, and negative energies in quantum mechanics?

Consider the nonrelativistic quantum mechanics of one particle in one dimension ("NRQMOPOD") with the time-independent Schrodinger equation $$ \left( -\frac{\hbar^2}{2m} \frac{d^2}{dx^2} + V(...
tparker's user avatar
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17 votes
4 answers
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Why can't I see the blue color scattered by the lower atmosphere of the earth?

I understand that the blue colour of the sky is because of the scattering of blue light by molecules in earth's atmosphere. The scattering appears to be happening from molecules that are far above in ...
coder's user avatar
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17 votes
2 answers
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Scattering states of Hydrogen atom in non-relativistic perturbation theory

In doing second order time-independent perturbation theory in non-relativistic quantum mechanics one has to calculate the overlap between states $$E^{(2)}_n ~=~ \sum_{m \neq n}\frac{|\langle m | H' |...
Edward Ross's user avatar
17 votes
3 answers
2k views

Why is there a "blue hour" after the "golden hour"?

There's a great story about why the sky is blue during the day, and turns golden during sunsets: Rayleigh scattering affects blue light more. During the day, blue light from the Sun is scattered ...
knzhou's user avatar
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17 votes
3 answers
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Is the total cross section a Lorentz Invariant?

In Peskin and Schroeder's book (P&S), on the botton of page 106, the authors say that the total cross section transforms as its only non-invariant factor, namely: $$ {1 \over E_{A} E_{B} |v_A - ...
Forever_a_Newcomer's user avatar
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Why is sky on Mars red?

There are some great explanations on StackExchange of why we see a blue sky on Earth, eg here. Namely, that it's a combination of : (a). the mix of colours from the Sun (eg, more green than indigo), ...
Errol Hunt's user avatar
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Does the exact string theory $S$-matrix describe all physics there is?

Suppose someone manages to evaluate the string theory $S$-matrix to all orders for any and all vertex operator insertions including non-perturbative contributions from world-sheet instantons and re-...
Udo Kamilla's user avatar
16 votes
4 answers
4k views

Is forward scattering = no scattering?

What is forward scattering? If it is equivalent to no scattering, then why not call it "no scattering"?
Physics_maths's user avatar
16 votes
1 answer
4k views

When we define the S-matrix, what are "in" and "out" states?

I have seen the scattering matrix defined using initial ("in") and final ("out") eigenstates of the free hamiltonian, with $$\left| \vec{p}_1 \cdots \vec{p}_n \; \text{out} \right\rangle = S^{-1} \...
A. Zerkof's user avatar
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15 votes
2 answers
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Only sea water appears blue in color, why this is not happening in river water?

Is the salt in the water the reason for scattering sunlight into blue?
Hvac Engineer's user avatar
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2 answers
2k views

The blue sky — is the simple Rayleigh explanation wrong?

Reading various answers here ([1], [2], [3]), Wikipedia, nasa.gov, and other places, the common explanation of our blue sky is Rayleigh Scattering due to gases and particles in the air, maybe with ...
jwd's user avatar
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15 votes
1 answer
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Do objects absorb or reflect gravitational waves?

We all know that objects can absorb and reflect electromagnetic waves. It stands to reason the same might be true for gravitational waves.
Derek Seabrooke's user avatar
15 votes
1 answer
6k views

Proof of Yang's theorem

Yang's theorem states that a massive spin-1 particle cannot decay into a pair of identical massless spin-1 particles. The proof starts by going to the rest frame of the decaying particle, and relies ...
QuantumDot's user avatar
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15 votes
2 answers
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Is there an analogue of the LSZ reduction formula in quantum mechanics?

In quantum field theory the LSZ reduction formula gives us a method of calculating S-matrix elements. In order to understand better scattering in QFT, I will study scattering in non-relativistic ...
Slayer147's user avatar
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14 votes
3 answers
5k views

Why is mist gray but water clear?

I was walking outside one cold afternoon with my mask on and my glasses began fogging up. The mist was initially gray. I kept walking without cleaning my glasses and eventually enough mist collected ...
George Orwell's user avatar
14 votes
3 answers
8k views

Center-of-mass frame for massless particles

Given a scattering event where a photon and electron go in and a photon and electron come out, what is the center of mass frame? I'd say, since the photon has no mass, it's the rest frame of the ...
Dave's user avatar
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14 votes
6 answers
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What is the difference between Raman scattering and fluorescence?

What is the difference between Raman scattering and fluorescence? Both phenomena involve the emission of photons shifted in frequency relative to the incident light, because of some energetic ...
Deep Blue's user avatar
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14 votes
3 answers
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What is the capture cross-section of a black hole region for ultra-relativistic particles?

What is the capture cross-section of a black hole region for ultra-relativistic particles? I have read that it is $$\sigma ~=~ \frac{27}{4}\pi R^{2}_{s}$$ for a Schwarzschild BH in the geometric ...
user12345's user avatar
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14 votes
5 answers
4k views

Does electron-electron scattering contribute to resistivity?

Electron-phonon and electron-defect scattering clearly contributes to resistance, but pure electron-electron scattering conserves the total momentum (and energy) of all the electrons. Then, how is it ...
ChickenGod's user avatar
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14 votes
1 answer
578 views

What's the "effective potential" for photons in $X$-ray diffraction?

The slickest way to introduce $X$-ray diffraction is to invoke scattering theory in quantum mechanics. One treats the incoming photon as just another particle in a scattering problem; by Fermi's ...
knzhou's user avatar
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14 votes
2 answers
573 views

How does Weinberg conclude that momentum and angular momentum are unperturbed by interaction terms?

In Weinberg's QFT volume 1, chapter 3.3, just below equation 3.3.19, he says $\vec P=\vec P_0$ and $\vec J=\vec J_0$ can be(easily) concluded from the definition of Møller wave operator or ...
Rajat Mondal's user avatar
13 votes
2 answers
3k views

Are all scattering states un-normalizable?

I am an undergraduate studying quantum physics with the book of Griffiths. in 1-D problems, it said a free particle has un-normalizable states but normalizable states can be obtained by sum up the ...
user35767's user avatar
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13 votes
3 answers
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What is the difference between Rayleigh scattering and Thomson scattering?

After reading the wiki articles I know, that both Rayleigh scattering and Thomson scattering are elastic processes. But what is the essential difference between those two processes, their cross ...
Lurco's user avatar
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13 votes
2 answers
3k views

Reflectionless potentials in quantum mechanics

Scattering on potential $$V(x) = -\frac{(\hbar a)^2}{m}\text{sech}^2(ax)$$ with 1D equation of Schrodinger is famous problem. It is dealt with in Problem 2.48 of Griffiths book or online here. It is ...
Nigel1's user avatar
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13 votes
2 answers
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Calculation of the cross section

Why, when we calculate the total cross section, we make the average other initial states and the sum over final states?
Andy Bale's user avatar
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13 votes
2 answers
580 views

Why does sunset spread over the horizon?

Why does sunset light spread horizontally over the horizon, instead of being radially spread around the solar disk? I understand physics about refraction, and wavelength, and gradient difference ...
Dmytro Brazhnyk's user avatar
13 votes
1 answer
4k views

Feshbach resonance in simple terms

I was reading up Feshbach resonances in cold atoms and I was unable to grasp the concept. I will tell you what I have understood. We consider two body scattering processes elastic as well as inelastic....
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