# All Questions

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### Coupled oscillators and Normal Modes

Consider we have a system consisting of 2 arbitrary masses and 3 arbitrary springs connecting them horizontally and between fixed walls, and we want to obtain the motion of each mass after we input ...
67 views

### How can magic be explained with Physics? [on hold]

Assuming that, hypothetically, and for this example only, "magic" means things like magical powers. In movies, games, etc. we witness magic; however, it's never explained how it works with regards ...
79 views

### Questioning Einstein's view on gravity [duplicate]

Oke, so my mind is blown by Einsteins view on gravity, at least as far as I understand the basics and principles he based his views on. One of the first things that struck me was that most of his ...
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### Is light emitted in zitterbewegung? [duplicate]

Recently I heard of Zitterbewegung, a trembling motion of the electrons in atoms that arises from Dirac's equation. I know that, according to Bohr's model, light is emitted when the electron "jumps" ...
3k views

### In reverse time, do objects at rest fall upwards?

I want to develop a game where time runs backwards, based on the idea that physical laws are reversible in time. However, when I have objects at rest on the earth, having gravity run backwards would ...
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### Angular Momentum conservation in star system

This question is related to what happens to a planetary body(like earth) when the star it is orbiting collapses under its own gravity. Suppose a star is rotating about its axis with some angular ...
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### String theory uncertainty limit? [duplicate]

Is there an uncertainty limit, constant (if there is a practically viable numerical constant) or not, to string theory? If so, could you explain what it means, the assumptions it has to hold to be ...
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### Aurora borealis forecasting, some technical details

I have an idea to make a software tool (with some really user friendly interface) for predicting auroras on any place on Earth. I do know some physics behind the phenomenon, but only basics, like it ...
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### Topological S-matrix as an operator in the graphical calculus

My question comes from the following classic paper by Kitaev: Anyons in an exactly solved model and beyond (arXiv link) In Appendix E (pg 86), Kitaev introduces a diagram operator $S_z$ which acts ...
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### Dipole approximation in solids

I have seen the use of dipole approximation (where the extent of the wave function is assumed to be much smaller than the wavelengths of the transitions or that of the electromagnetic field applied) ...
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### Change of vectors [on hold]

We have two vectors $F1$ and $F2$ as shown in figure. The change of vectors is shown as $F2-F1$. Why it is it rather than taking negative of vector $F2$ i.e. $-F2$ and then adding it by head-to-tail ...
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### Can diffraction be used like gravitational lensing [on hold]

I was thinking that in both the cases of diffraction and gravitational lensing, light is bend to some extent mimicking the lens effect. My Question is, Can diffraction be used like gravitational ...
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### Can Van Allen Radiation belts act as a capacitor?

There are two Van Allen Belts at rougly 5000Km and 17000Km from the surface of the Earth each of them containing majority of one of the charges electrons and protons). Can these belts like a ...
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### DFT Calculations, Atomic Ionization Potentials — Which Exchange-Correlation Functional to Use, to Preserve Koopmans' Theorem?

I have a program which can perform density-functional calculations for atoms, given a density functional. Of course the simplest form of exchange potential to use is one relevant for a uniform ...
2k views

### Why don't we have a theory of everything?

What is currently stopping us from having a theory of everything? i.e. what mathematical barriers, or others, are stopping us from unifying GR and QM? I have read that string theory is a means to ...
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### What does amplitude in wavelength of light physically mean?What oscilates with time in photon?

Like amplitude in wavelength of water waves signify the displacement of water particles about their mean position.
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### Numerical eigenbasis for a unitary [migrated]

Do you know what numerical software computes an eigenvector basis for a unitary matrix? Say I have a unitary matrix $U$. If its eigenvalues are simple (no multiplicities), then for instance Matlab ...
32 views

### Driven coupled oscillator [on hold]

Consider the following system consisting of 3 masses and 4 springs : Suppose i start to drive the system, for instance horizontally applying a sinusoidal force with frequency w to one of its ...
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### How do we find the accuracy of atomic clocks?

We say that atomic clocks are the most accurate clocks ever made, they may lose or gain $x$ seconds in $y$ years. How do we find this uncertainty because we do not have an ideal clock to compare with ...
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### Minimum spread of frequency and wavelength in neodymium laser

What is the equation linking the minimum spread in wavelength and frequency of a pulsed laser, in relation to the lasers pulse time and operational wavelength. For example: If a Neodymium laser ...
2k views

### What are galactic speeds measured against?

The Earth moves through space at 67,000 MPH. The Milky Way travels through a local group at 2,237,000 MPH. Wouldn't you need a fixed point to be able to measure velocity against? After all, compared ...
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### How do you find the tension in the real world? (Given a rope in a pulley system)

I'm well aware of the formula to calculate tension, however, given a real world situation where you have a closed pulley system. How do you measure the force (i.e., tension) required to pull on the ...
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### Vapour from a vessel's opening? [on hold]

I just noticed something interesting while I was in the chemistry lab. I heated some liquid( about 60 mL. Which consists of 20 mL of Oxalic acid solution and 20 mL of $H_2SO_4$) in a conical flask to ...
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### On what parameters is breaking stress dependent?

I have recently studied Elasticity in materials. There was a question about breaking stress that if the area along which a linear force is applied is doubled, then what happens to the breaking stress. ...
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### Is heat a property of Something?

There are some unique properties of things in our world. For example, charge is a property of mass. Is heat a unique property of something? Can a vacuum/void have a finite amount of heat in it? What ...
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### Zero Diffraction Plates: Why do they give such low diffraction signals?

In our laboratory we use silicon plates named "Zero Diffraction Plates". They are named so because they give extremely low diffraction signals in an XRD instrument (reflection mode). But how are these ...
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### Linear polarization measurement - what QM observable is measured?

My understanding is that photon can have spin +-1 along propagation direction, corresponding to two circular polarizations. Linear polarization is superposition of two. Since one can measure linear ...
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### Problem of uncoupling a train

As a kid I have seen several movies, cartoon etc. in which usually one guy just uncouple (unplugs (?), dunno) the rest of a running train and then runs off with the locomotive at full speed and ...
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### How does light speed up after coming out of a glass slab?

As I learned today in School, my teacher told me that when light enters a Glass Slab it slows down due to the change in density and it speeds up as it goes out of the Glass Slab. This causes a lateral ...
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I am trying to compute $$\langle 0| \, T\left\{\phi^\dagger(x_1) \phi(x_2) \exp \left[i \! \int{L_1(x) \, \mathrm{d}x} \right] \right\}|0 \rangle$$ for $$L_1(x) = ... 3answers 344 views ### What's the difference between “numerical methods” & “mathematical analysis” as said by Feynman in his lectures? While reading his lectures, I came to these lines: On the basis of Newton's second law of motion,which gives the relation between the acceleration of any body & the force acting on it,any ... 2answers 59 views ### Can string theory get rid of randomness in quantum processes? I am not a physicist, but I am very much into popular science, especially string theory. I would like to know if it is conceivable that string theory might be able to get rid of the randomness ... 0answers 7 views ### Explain materials with 4 fold symmetry having same reflectance when shone with LCP and RCP This is my first post here. I am currently reading "Optical planar chiral metamaterial designs for strong circular dichroism and polarization rotation" by Do-Hoon Kwon, Pingjuan L. Werner, and ... 0answers 101 views ### What does Weinberg–Witten theorem want to express? Weinberg-Witten theorem states that massless particles (either composite or elementary) with spin j > 1/2 cannot carry a Lorentz-covariant current, while massless particles with spin j > 1 ... 0answers 24 views ### Fast and slow modes in renormalization group of nonlinear sigma model A general nonlinear sigma model can be expressed as $$S[g] = \frac{1}{\lambda} \int d^dr\ \text{tr}[\triangledown g\triangledown g^{-1}]$$ where g takes value in a matrix ... 1answer 20 views ### Does doping silicon affect its coefficient of thermal expansion (CTE)? Everywhere I have searched I always see the CTE of silicon listed as 2.6*10e-6 °C^-1. However, I have silicon that was phosphorus-doped to an ion concentration of of 2.41*10e11 atmos/cm^3, and I would ... 1answer 64 views ### How much pressure would be needed to contain a 1 gigaton nuclear bomb explosion within a sphere of one meter radius? How much pressure would be needed to contain the largest human exploitable nuclear bomb within a sphere of one radius? Also would it be possible to create a magnetic field that controlled some ... 1answer 101 views ### Angular momentum needn't always change in multiples of \hbar? I read the following claim in Slichter's popular book, Principles of Magnetic Resonance (after Fig. 4.3, it's p100 in this version.). Despite the title, the author claims it in a quite general manner ... 1answer 41 views ### Practical Book on Hamiltonian and Lagrangians? [duplicate] Are there any terse, accessible books that are geared specifically at learning these two formalisms and how to effectively use them? So far I've only see either topic introduced as a part of another ... 4answers 216 views ### Why do physics students find vectors so difficult to deal with? [on hold] When I teach introductory physics to undergraduates, I find that although the classes are frequently split into "algebra-based" and "calculus-based" sections, the most difficult concept for any of ... 1answer 29 views ### How do I calculate the apparent brightness of a mirror reflecting the sun at a given distance? I've stumbled around trying to figure this out, but it just isn't my area and I haven't gotten far. This is for writing about an idea of a field of mirrors on the moon, on the terminator when the moon ... 0answers 22 views ### Elastic scaterring of neutrinos - Why does the electron neutrino dominate this channel Could anyone add more detail to this statement off wikipedia In the elastic scattering interaction, a neutrino collides with an atomic electron and imparts some of its energy to the electron. ... 2answers 31 views ### How are different wavelength components collected by the same detector in a spectrometer? Let's think, we have a detector array (128x1 and each CMOS detector responds 400 to 1000 nm TSL1401CL that way, each detector has 4.6875 nm interval). Then, basically in a spectrometer a prism reflect ... 1answer 111 views ### About the nonlocality of QM and faster-than-light/backward-in-time machines The fact the quantum mechanics is nonlocal is known already for a long time, since the Bell works (1966 and later) and the Aspect's group experiments confirming the Bell-type CHSH inequality (1980 ... 3answers 69 views ### In special relativity, do explanations involving clocks require that the clocks are ticking and that c is fixed? Are the explanations involving clocks only valid if the clocks are ticking when light hits? Is it true that these thought experiments experiment could only be valid due to the invariance of c? 0answers 42 views ### How is energy converted within the body? [on hold] I guess that this question is going to be on hold but: how does the energy in food is transmitted to our cells and we gain energy? what is there in the food which is transmitted as the energy to our ... 1answer 146 views ### What is the average mass of galaxies according to Hubble Deep and Ultra Deep field observations? It is very widely known among people interested in astronomy that there are 100-400 billion stars in the Milky Way galaxy and there are ~ 100 billion galaxies in the observable universe, which is ... 4answers 227 views ### Why drops form spheres? Consider a drop of water floating in an inertial frame in STP air (e.g., the ISS). Intuitively, the equilibrium shape of the drop is a sphere. How would one prove that? Is it equivalent to showing ... 1answer 43 views ### What is the temperature of the clear night sky from the surface of Earth? Before you all jump in with 2.73 K or thereabouts, this is more of an experimental question. It will obviously depend on humidity and radiation being scattered back towards the surface of the Earth. ... 1answer 22 views ### Why could the Homestake experiment only detect electron neutrinos The Homestake experiment measured the incoming electron neutrino flux via$$\nu_{e}+ Cl^{37} \rightarrow Ar^{37} +e^{-} Why does this reaction not apply to the other neutrino flavours? i.e. what ...

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