# All Questions

2k views

### Where are the ice rings in X-ray crystallography located?

I threw this image of one of my protein crystal's diffraction pattern (not the greatest) on a poster, and was wondering where (what reciprocal dimension) the ice rings are located as it might be a ...
324 views

### Is the equivalence principle strictly fulfilled by general relativity?

The equivalence principle states The outcome of any local experiment in a freely falling laboratory is independent of the velocity of the laboratory and its location in spacetime. Any real local ...
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### Is 4-velocity normalized to -1 even for non-geodesic timelike curves?

In Hartle's General Relativity book ("Gravity"), one of the problems (chapter 8 problem 6) is to prove that $g_{\mu\nu}u^\mu u^\nu$ is conserved along geodesics (really not hard to show), where ...
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### In layman's terms, what is a quantum fluctuation?

What causes it and how does it occur? If you do post some mathematics, please explain what each term means too please.
669 views

### Is (0,0,0) an undefined vector?

I'm not sure what to make of the direction of a vector with components (0,0,0). Is it an undefined vector?
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### Lorentz Transformation via Geometry

Today, I was tutoring and explained the space-time. I explained how one can convert North-South into West-East by rotating, and how you can convert time into space with velocity. Below the ...
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### Why is the Dirac operator so important - in both physics and mathematics?

Why is the Dirac operator considered so important - in both physics and (pure) mathematics?
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### What objective criteria distinguish between valid science, fringe science and pseudoscience in physics?

Plenty of research activity in physics have been vigorously opposed by their opponents as pseudoscience or fringe science, while other research are mainstream. It is possible some topic is ...
316 views

### What does transport equation represent in terms of physical quantities?

In my math course we're taught to solve PDE (partial derivative equations) like transport equation: $$c\frac{\partial u}{\partial x} +\frac{\partial u}{\partial t}~=~0.$$ If $u(x,t)$ is the ...
885 views

### Planetary model of atom still valid?

When I was in school, I learned (from Democritus) that an atom was similar to a solar system, with the nucleus being the sun, and the electrons being the planets. Of course, there are some ...
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### given the infinitesimal generator can we deduce the symmetry?

for example given the differential operator $\partial _{x}$ we know that it belong to the translation $y=x+a$ for some 'a' given the differential operator $x\partial _{x}$ we know that we are ...
398 views

### Length of publication cycle for peer-reviewed journals [closed]

Is there any data of the expected length of publication cycle with respect to different peer-reviewed journals in theoretical physics? I know that the length depends on many things, including the ...
280 views

### Does a thermally expanding torus experience internal stress?

I'm trying to learn continuum mechanics and thermo-mechanics. As we know, heating an object increases the mean atomic distance $a_0$ of the atoms in a rigid body. Let's assume it is a linear elastic ...
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If there was a way to see the electromagnetic waves that is all around us, i.e. see the electric and magnetic fields spreading away from an antenna say, how would it look? I mean if we were to freeze ...
206 views

### Speed Distribution of The Particles

I want to know the distribution of the particles's speed. The particles what I mean are nucleons and electrons of element. Consume there is 1kg of iron on room temperature and it's shape is sphere. ...
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### Some questions about the spectral function

If you think that this question is likely to get closed then please do not answer and only say that in the comments since this system doesn't let me delete the question once it has answers. ...
3k views

### Is there an intuitive reason the brachistochrone and the tautochrone are the same curve?

The brachistochrone problem asks what shape a hill should be so a ball slides down in the least time. The tautochrone problem asks what shape yields an oscillation frequency that is independent of ...
825 views

### Quivers in String Theory

Why do a physicist, particularly a string theorist care about Quivers ? Essentially what I'm interested to know is the origin of quivers in string theory and why studying quivers is a natural thing ...
373 views

### Transparent boundary condition [closed]

I am interested in the finite-difference beam propagation method and its applications. I try to solve the Helmholtz equation. At first, i would like to solve numerically it for the easiest case, ...
2k views

### Penetration of armor plate

Is there a simple mathematical expression for the stopping power of a given thickness of armor, given the thickness of armor plate, the radius of a cannon ball, the density of the cannonball and the ...
20k views

### What is the math knowledge necessary for starting Quantum Mechanics?

Could someone experienced in the field tell me what the minimal math knowledge one must obtain in order to grasp the introductory Quantum Mechanics book/course? I do have math knowledge but I must ...
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### Energy is actually the momentum in the direction of time?

By comparatively examining the operators a student concludes that `Energy is actually the momentum in the direction of time.' Is this student right? Could he be wrong?
961 views

### How to concisely explain apparent retrograde motions of planets?

Some planetary orbits occasionally can appear to move backwards to an observer on the Earth? Does anyone know concise, clear, web-based visualizations, animations or tutorials that clearly show how ...
527 views

### What happens to body chemistry at the speed of light? [duplicate]

Assume that I'm traveling at the speed of light in one direction. My brain is also traveling at the speed of light in that direction. Presumably there is at least one receptor site in my brain that is ...
270 views

### WKB approximation to loop diagrams

I'm a bit confused with the terminology here. This paper claimed to use WKB method to calculate the usual loop diagrams. Notice that the vertex is approximated by expanding around the classical ...
413 views

### Motivation for maximum Renyi/Tsallis entropy

The Conditional limit theorem of Van Campenhout and Cover gives a physical reason for maximizing (Shannon) entropy. Nowadays, in statistical mechanics, people talk about maximum Renyi/Tsallis entropy ...
1k views

### Is instantaneous velocity an abstraction?

In introductory analysis, the discussion the derivative emphasizes that while average rates of change are measurable, instantaneous rates of change are a "limiting abstraction". While this makes ...
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### Mathematical Modelling for Efficiency of PV thermal technology

As the efficiency of PV thermal or wind energy decreases with time(from the installment). Is there any mathematical model that gives its relation with time. i.e., whether it is a linear decrease or ...
172 views

### Reflected optical rotation of polarised light

Assume polarised light travels through an optically active medium (e.g. water with lots of sugar) with the specific angle $\alpha_s$ of depth $d$ at which end it is reflected (so in total it travels ...
3k views

### Definition of “localized state”

Is there a definition of a localized state in quantum mechanics? I've seen that some textbooks use this term, but they don't give a definition. Thanks
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### How do I find the time for Pleiades to be at Zenith today

I am looking for a way find the time when: Pleiades will be at the Zenith today (11th November 2011) Pleiades will conjunct with the Moon The Moon will be at the Zenith today How do I find those ...
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### Which low energy SUSY models are compatible with LHC and tevatron data so far?

The LHC data has eliminated huge swaths of permissible low energy SUSY breaking models. But not all. Which regions of parameter space are still open?
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### What is the dominant interaction in elastic collision pi- + neutron -> pi- + neutron?

What is the dominant interaction in elastic collision pi- + neutron -> pi- + neutron? Is it strong nuclear interaction or electromagnetism? Edit: sorry I meant to say "elastic". There is no matter - ...
421 views

### What final states can proton-proton collision generate?

What final states can proton-proton collision generate? Where to find listing of possible final states of a particle interaction, for example proton colliding with proton?
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### What are the specific requirements for a do-it-yourself quantum double-slit experiment?

I was shocked to recently learn that the double-slit experiment is not only possible to do with completely ordinary equipment (with photons of course), but it actually looks rather easy. This is from ...
403 views

### How long does it take for expanding space to double in size

I have been reading about Hubble's constant and trying to make 'sense' of the theory of the expanding Universe. Is is possible that space in the universe expands uniformly? If so, absent of other ...
19k views

### When does 'energy' turn to matter?

I always hear about matter converting to 'energy' - fusion, fission. When does it go the other way around? What conditions lead to it? Are there reproducible experiments on this topic?
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### How many different formulations of QM currently exist?

I read some while ago that, currently, eleven different formulations of quantum mechanics exist. Is this correct / accurate? If yes, can someone provide a pointer(s) (i.e. link(s)) to the various ...
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### Literature request for books / review papers on gravitation, gauge theories and related mathematics [duplicate]

Similar to this reference, are there more such references / works [including textbooks] available in the literature? (A list would be greatly welcomed and appreciated.) With great appreciation.
133 views

### Definition regarding percolation

in a homework sheet studying bond-percolation on the Bethe lattice, a function $g(r)$ is introduced as "the probability of finding two nodes separated by a distance $r$ on the same cluster". Now ...
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### Use of advanced mathematics in astronomy, like topology, abstract algebra, or others

I know that topology, abstract algebra, K-theory, Riemannian geometry and others, can be used in physics. Are some of these areas used in astronomy, and are some astronomical theories based on them? ...
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### Scherk-Schwarz and other compactifications?

I have been thinking about various types of compactifications and have been wondering if I have been understanding them, and how they all fit together, correctly. From my understanding, if we want ...
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### Understanding P-, CP-, CPT-violation etc. in field theory and in relation to the principle of relativity

I can never get my head around the violations of $P-$, $CP-$, $CPT-$ violations and their friends. Since the single term "symmetry" is so overused in physics and one has for example to watch out and ...
259 views

### What is deep Fresnel region?

If I understand correctly, it has something to do with autocorrelation function, but can someone give me a definition or exact explanation? In case of scattering, if you wish to analyze pattern with ...
812 views

### Condensed matter physics for mathematicians [duplicate]

What is a good way for me to learn the basics of condensed matter physics? I'd like to get a better understanding of the fundamentals behind recent technological developments like OLEDs, applications ...
167 views

### Quantum mechanics as a Markov process

I am currently involved in some understanding on this matter with a colleague of mine. I know all the literature about but I do not know the state of art. Please, could you provide some relevant ...
939 views

### Calculating the heat capacity of a system

I have started reading Statistical Physics by F. Mandl and I would appreciate some help with the following exercise A system consists of $N$ weakly interacting subsystems. Each subsystem possesses ...
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### Quantum Mechanics Video lectures

Are there any good video lectures for learning quantum mechanics at the level of Griffith?