0
votes
4answers
2k views

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 ...
11
votes
4answers
9k views

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?
46
votes
9answers
3k views

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 ...
8
votes
1answer
149 views

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.
26
votes
3answers
5k views

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 ...
0
votes
3answers
312 views

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 ...
1
vote
1answer
213 views

Sun-Earth Virtual Gravitons?

How many virtual gravitons do the sun and earth exchange in one year? What are their wavelengths?
3
votes
2answers
827 views

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 ...
4
votes
3answers
679 views

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 ...
4
votes
2answers
776 views

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. ...
43
votes
7answers
2k views

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. ...
4
votes
2answers
407 views

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 ...
4
votes
4answers
1k views

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 ...
0
votes
1answer
192 views

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 ...
1
vote
1answer
626 views

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 ...
1
vote
3answers
799 views

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 ...
1
vote
5answers
2k views

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 ...
-2
votes
4answers
776 views

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 ...
7
votes
6answers
11k views

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 ...
9
votes
3answers
2k views

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 ...
3
votes
0answers
2k views

Energy Levels of 3D Isotropic Harmonic Oscillator (Nuclear Shell Model)

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 = ...
2
votes
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 ...
9
votes
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 ...
4
votes
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 ...
0
votes
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 ...
1
vote
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 ...
8
votes
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 ...
3
votes
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? ...
1
vote
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, ...
3
votes
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, ...
9
votes
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?
3
votes
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 $\Delta$ ...
1
vote
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 ...
5
votes
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.
26
votes
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 ...
1
vote
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 ...
1
vote
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?
0
votes
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 ...
8
votes
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 ...
3
votes
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 ...
4
votes
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 ...
1
vote
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 ...
2
votes
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 ...
8
votes
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 ...
6
votes
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 ...
4
votes
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 ...
12
votes
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 ...
2
votes
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 ...
1
vote
2answers
423 views

Why doesn't a phone charge faster, rather than slower when it is in use

In Physics class, we were building parallel circuits, and as more lights were attached in parallel, they got brighter (as more power was being provided to the lights, and the resistance decreases). ...
4
votes
1answer
2k views

Uniqueness of Helmholtz decomposition?

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 $$\pmb{H}=\pmb{\nabla} \phi +\pmb{\nabla} \times \pmb{G},$$ ...

15 30 50 per page