# All Questions

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### What is the typical career path to become a professional Astronomer? [closed]

Here is a typical question which I have been asked many times while giving public lectures in various places. While I know one of the paths like Diploma, Masters and PhD but sometimes this is not so ...
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### What does f/4.6 mean in a telescope and how important is this value? [closed]

In some specifications for telescopes, I saw a value marked as f/4.6. What does it mean exactly, and how important is when it comes to choosing a telescope?
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### Do all black holes have a singularity?

If a large star goes supernova, but not enough mass collapses to form a black hole, it often forms a neutron star. My understanding is that this is the densest object that can exist because of the ...
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### Best periods to observe Magellanic Clouds?

When is the best period of the year to observe the LMC and SMC in the Southern hemisphere, so that I can observe them at a reasonable time (e.g. around 10 in the evening?). I am in Sydney.
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### What nonlinear deformations will a fast rotating planet exhibit?

It is common knowledge among the educated that the Earth is not exactly spherical, and some of this comes from tidal forces and inhomogeneities but some of it comes from the rotation of the planet ...
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### Action - Reaction pair, through photons

Here's an example to describe the issue Supossed a high power laser (eg a 100 kW laser, ie, electromagnetic weapons) is fired to a target, then it will receive energy and move. (and likely to burn or ...
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### Sun-Earth Virtual Gravitons?

How many virtual gravitons do the sun and earth exchange in one year? What are their wavelengths?
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### bandgaps for 2D square lattice with potential of the form V=V(x) + V(y) - what are the general properties?

Let us consider Bloch wave function solutions for a particle confined to a 2D square lattice with a potential of the form $V=V(x) + V(y)$ (that is, one that can be factorized). In this case we can ...
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### Wigner's friend and intersubjectivity in quantum mechanics

Suppose there's a radioactive material and a 1/2 quantum probability of detecting it by a Geiger counter. This puts the system in a superposition. Also suppose you are in the same room, and the walls ...
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### Why don't two observers' clocks measure the same time between the same events?

Person A in reference frame A watches person B travel from Star 1 to Star 2 (a distance of d). Of course, from person B's reference frame, he is at rest and is watching Star 2 traveling to him. ...
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### Is 3+1 spacetime as privileged as is claimed?

I've often heard the argument that having 3 spatial dimensions is very special. Such arguments are invariably based on certain assumptions that do not appear to be justifiable at all, at least to me. ...
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### What is the dominant cause for ocean waves at a beach?

What is the dominant cause for ocean waves at a beach? Are they the result of wind/pressure difference? If so, the waves do seem to exist in similar intensity even during relative quiet times of the ...
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### How can I physically interpret voltage and current?

My understanding of an electrical current is that it is the flow of electrons through a material. The only magnitude I can fathom for this process is the number of electrons which are flowing. I know ...
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### If you removed every particle from space…? [closed]

I'm trying to find something Einstein (I think) said about time...It was something like.. "If you removed every particle from space and were left with only one pocket watch (clock, timepiece?), time ...
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### Statistical physics of molecular dissociation of a diatomic gas

Say there are $N$ atoms of type $A$ in a box of volume $V$ and they are undergoing a reversible association-dissociation reaction $A + A = A_2$. Let an $A$ atom have mass $m$, and hence the molecule ...
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### QM interpretation of Larmor precession

Typically when I think of a spin moment in an external field, I visualize it classically in terms of a magnetic moment precessing around the external field vector. Now how is this described quantum ...
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### Recommended progression with which to learn physics for fun

I enjoy reading up on new topics in physics and astronomy, and learning the concepts and ideas in the major theories, like relativity and quantum mechanics. Unfortunately, one thing that I lack ...
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### Why do Engineers manipulate coordinate systems? [closed]

When tackling a physics problem, An Engineer will manipulate the axes/coordinate system where a Mathematicians and/or Physicists will use the original coordinate system and math. Why do Engineers ...
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### What would be the effective resistance of the ladder of resistors having n steps

I'm a tutor. This is a high school level problem. In high school, every one have might have solved a problem of effective resistance of a ladder of resistors having infinite steps. Now the problem is ...
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### Explain how (or if) a box full of photons would weigh more due to massless photons

I understand that mass-energy equivalence is often misinterpreted as saying that mass can be converted into energy and vice versa. The reality is that energy is always manifested as mass in some ...
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One simple way of detailing the very basic structure of the nuclear shell model involves placing the nucleons in a 3D isotropic oscillator. It's easy to show that the energy eigenvalues are $E = ... 1answer 650 views ### What are the forces of constraint if there are multiple equivalent constraints? Suppose a large (rigid) block is sitting on top of two smaller blocks of equal height$1$, both of which rest on the ground. We wish to find the position of the block (easy) and the forces of ... 2answers 3k views ### Explicit Variation of Gibbons-Hawking-York Boundary Term Are there any references that present the explicit variation of the Hilbert-Einstein action plus the Hawking-Gibbons-York boundary term, and demonstrate the cancellation of the normal derivatives of ... 2answers 6k views ### Calculating threshold energy of particle reactions {..everything that follows is in the domain of relativistic kinematics..} Say a particle A collides with a particle B at rest and produces particles C and D. What exactly is the definition of ... 2answers 547 views ### Would a car tow rope withhold an adult person? I am not exactly sure how the physics work here. If we take a car tow rope, which is manufactured for towing a 3500 kg car and we actually start towing such a car on this rope, what force do we really ... 2answers 718 views ### Simulation of physics of chains/ropes in force fields resources? I'm thinking about a project to tackle, and I'd like to make a simulation that allows the user to define a rope or chain of length L, pin it at arbitrary points r1, r2.... etc. and draw the resulting ... 2answers 686 views ### The energy of a Graviton Maybe another stupid question, but what's the energy of a graviton? Is it$\hbar \omega$? Does it emit gravitons when an apple falls onto the ground, like photons be emitted when an electron transits ... 3answers 397 views ### Is there any physics behind flocking? There are many articles published in physics journals about flocking. Is there a physical reason for these phenomena or is it just because physics methods are being used to study collective motion? ... 3answers 1k views ### Entropy and how it applies to everyday activities like eating food So I was eating a plate of food one day and thought of entropy. As I understand the definition of entropy, it is the logarithm of the number of arrangements or states the object in question can be in, ... 3answers 477 views ### If the multiverse and many-worlds don't exist, how should we interpret probabilities? If the multiverse of eternal inflation and the many-worlds of quantum mechanics don't exist, how should we interpret the meaning of probabilities? If there is only one copy of the universe out there, ... 3answers 438 views ### Extended sound of thunder Why does the sound of thunder last several seconds even when lightning lasts for only fraction of a second? 0answers 419 views ### 1D Topological insulator with PT symmetry Assume I have the Hamiltonian for a 1D topological insulators as: $$H=\sin(P_x) \sigma_x+i \Delta \sigma_{y}+[1-m-\cos(P_x)] \sigma_z$$ where$m$is the mass term,$P_x$is the momentum and$\Delta$... 1answer 623 views ### Paper stiffness As piece of paper is folded and unfolded, the stiffness of the sheet may seem to be greatly increased. To those of you who don't recognise it: take a sheet of A4 paper, grasp one of the edges and ... 3answers 996 views ### Does there exist a laser that is tunable over the whole visible spectrum? Does there exist a laser that is tunable over the whole visible spectrum? If so does there exist one that is solid state? This is to settle a score with my dumb workmates. 6answers 6k views ### Does a magnetic field do work on an intrinsic magnetic dipole? When you release a magnetic dipole in a nonuniform magnetic field, it will accelerate. I understand that for current loops (and other such macroscopic objects) the magnetic moment comes from moving ... 0answers 458 views ### errata for Morse & Feshbach - Methods of Theoretical Physics [closed] Anyone knows where I can find an errata (or any related material, such as solution sheets, etc) for this book? Thanks. Note: This is not a physics question, but this book is so popular among ... 1answer 131 views ### Total power of CMBR Could you check me on this: if I'm interested in total power (summed over all frequency) of CMBR absorbed by a black body in empty space, would that be proportional to the surface of BB? 1answer 416 views ### if a force is 1 newton metre, what is it at 2 meters? If I have a force, say 24 kg/cm what would that equate to at 2cm? I would like to know the formulae for calculating this. For example. If a motor can hold an object of 24kg at 1cm from its pivot ... 1answer 252 views ### Is there literature on a continuous mass spectrum for the Higgs field? Various masses for the Higgs field are compatible with experiment, but is it possible that the Higgs field is not observable because it has a continuous mass spectrum? Work in the 60s and 70s on free ... 5answers 598 views ### Particles, waves and parallel wire filters. Transmission formula? If I think of a photon as a particle, I think a parallel wire filter should transmit proportionally to the uncovered area. (and reflect proportionally to the covered area). Obviously polarization ... 1answer 1k views ### How to derive the Karman-Howarth-Monin relation for anisotropic turbulence? I find the derivation of the Karman-Howarth-Monin relation in the book Turbulence from Frisch (1995) a bit to short. Can someone point me to a more detailed derivation of that relation, if possible in ... 2answers 242 views ### self-antiparticles and broken symmetries certain particles (i.e: certain bosons like the photon) do not have an anti-particle, or rather, they are they own anti-particles. lets assume that such symmetry is only approximate and these ... 3answers 6k views ### Wheatstone Bridge Why is using a Wheatstone bridge such an accurate way of calculating an unknown resistance? What are the benefits of using it over Ohm's law? It seems that it has something to do with the wires ... 2answers 402 views ### Quantizing EM field Why when we quantize EM field, whe quantize the vector potential$A^\mu$obtaining vectorial particles (photons) like the elastic field (phonons) and we can't quantize directly the EM-field tensor ... 1answer 455 views ### Do we need a quantum deformation of the diffeomorphism group in string theory? Let me justify my question before I go on. In string theory, gravitons are strings extended over space. Longitudinal gravitons are pure gauge modes of the diffeomorphism group. However, in string ... 1answer 272 views ### The Orbiting Moon as a Quantum Object How would the quantum mechanical treatment of the moon as a gravitationally-bound object differ from the usual treatment of the hydrogen atom using Schrödinger's equation? [The earth's gravitational ... 8answers 4k views ### Why are extra dimensions necessary? Some theories have more than 4 dimensions of spacetime. But we only observe 4 spacetime dimensions in the real world, cf. e.g. this Phys.SE post. Why are the theories (e.g. string theory) that ... 2answers 741 views ### Helmholtz decomposition in the plane Prove or disprove the following proposition: For any smooth plane vector field$\mathbf{H}=\left(H_x,H_y\right)$, there exist scalar potentials$\phi$,$\psi$such that$H_x=\frac{\partial \phi ...
Helmholtz theorem states that given a smooth vector field $\pmb{H}$, there are a scalar field $\phi$ and a vector field $\pmb{G}$ such that $$\pmb{H}=\pmb{\nabla} \phi +\pmb{\nabla} \times \pmb{G},$$ ...