All Questions

2answers
40 views

When I change the rpm of a turntable, how long does the turntable to get to the new rpm?

If the turntable was rotating at 16 rpm and I switched it to 30 rpm, is the change in speed pretty much instantaneous, or is their a period of acceleration? When I did it, the change appeared to be ...
0answers
81 views

How does non-constant conductivity change surface charge on a wire?

Current density is equal to conductivity times electric field. Therefore flux through a cross section of a wire is equal to current over conductivity. Current has to be constant along a wire. This ...
0answers
120 views

Where can I find a table for NIST standards on temperature and pressure?

The Wikipedia article Standard conditions for temperature and pressure notes that IUPAC and NIST are two of the most common standards for temperature and pressure in use. Even if I could have gotten ...
1answer
97 views

Earth's direction in space [closed]

After watching videos about the rotation of the planets being helical and not heliocentric, I wondered, as Earth travels through space, which part of Earth is the forefront?
3answers
251 views

Spin in magnetic field and eigenvalues

We have some arbitrary quantum state, lets say $$\vert\Psi\rangle=\alpha_{1}\vert\uparrow\rangle+\alpha_{2}\vert\downarrow\rangle= \begin{pmatrix} \alpha_{1} \\ \alpha_{2} \\ \end{pmatrix}$$. And ...
1answer
128 views

Smallest Black hole nessesary to devour Earth?

While looking into black holes I can across the idea of hawking radiation, and the fact that microscopic black holes would near instantaneously evaporate before doing any damage. However larger black ...
1answer
230 views

effective field theory of the projective semion model

The "projective semion" model was considered in http://arxiv.org/abs/1403.6491 (page 2). It is a symmetry enriched topological (SET) phase. There is one non-trivial anyon, a semion $s$ which induces a ...
1answer
89 views

What's the next big goal of the Large Hadron Collider?

Obviously, physics lovers everywhere were excited at the announcement that the LHC had verified the existence of the Higgs Boson. As a non-physicist, I like to keep up with what I can regarding the ...
1answer
86 views

How to motivate this approach on Special Relativity?

One of the most common approaches to Special Relativity is that based on the two Einstein's postulates: The laws of Physics are invariant in every inertial reference frame The speed of light ...
1answer
114 views

1answer
179 views

Calculus of variations and string theory

In Polchinski's String theory book, Vol 1., in chapter 1, p. 18, he is deriving the Lagrangian in the light cone gauge (that's not necessary to know in order to answer this question), and he gets ...
2answers
62 views

Kirchoffs voltage law [closed]

There are many books which relate KVL with law of conservation of energy and I'm having trouble understanding how they are related so can anyone explain their relation to me, concept wise and if ...
2answers
122 views

Viability of a Fayet Iliopoulos term in the MSSM

Why is a Fayet-Iliopoulos term $-kD$ irrelevant or subdominant in the in the MSSM (Minimal Susy Standard Model)? According to Martin (A Supersymmetry Primer, p.70) it's because squarks and sleptons ...
1answer
75 views

QM: Why is there a minus sign on the Energy operator when using complex conjugate?

I understand how they get the first equation. But I have no idea why there is a minus sign on the second equation: This is from a derivation for the probability density current found here: ...
0answers
28 views

Does the Abbe limit hold for a single lens?

Let us say I have a single (converging lens) could I use the Abbe diffraction limit (for a microscope) to find its resolution or do I have to use the Rayleigh criterion. (i.e. Is the Abbe diffraction ...
0answers
66 views

Brachistochrone parametric equations

I'm having a bit of a hard time understanding how the parametrized $y$ equation (given below) of the brachistochrone is correct. When these equations are plotted it gives a concave down graph. ...
1answer
68 views

Interference of waves

Every thing has a dual nature. So if we take waves(consider light interference) as particles, exactly what happens during interference (both constructive and destructive)? Can you explain in easy ...
2answers
102 views

Would the presence of B-modes in the CMB provide evidence for quantum gravity?

Finding B-modes in the CMB (which aren't due to foreground contamination) would be evidence for gravitational waves, because they cannot be produced by density perturbations (to first order, is my ...
1answer
76 views

Connection between two Petrov classification schemes

For the Weyl scalars of all spacetimes, at any point, possess one special structure, the so called principal null directions. Consider a general null tetrad $\{ l_a,n_a,m_a,\overline{m}_a \}$, we ...
1answer
149 views

Is possible to generate energy from gravity?

When the wind goes through the vanes of an wind turbine, it's generate energy. When the water goes through the water turbines, it's generate energy. So my question is short and simple, is possible ...
3answers
4k views

How small does sand have to be to get wet?

I think of sand as a lot of very small rocks. Suppose I have a pile of rocks, each about 1cm in size, and the pile is a meter tall. If I pour a bucket of water on the rocks, most of the water will ...
0answers
57 views

What makes Bell's original inequality unsuitable for experiments?

Bell derives the inequality $$|E(\vec{a},\vec{b})-E(\vec{a},\vec{c})|\leq 1+E(\vec{b},\vec{c})$$ in his book Speakable and unspeakable in quantum mechanics. In this derivation he uses the assumption ...
1answer
38 views

Perception at relatavistic speeds

If one were to be traveling at near the speed of light, their mass would be $m_{rel}= m_0 / \sqrt{1 - \frac{v^2}{c^2}}$. For the mass to double the speed would have to be $86.6\%c$  To better ...
2answers
96 views

Confusing, can a motor on a moving disc stop the disc?

I've been troubled trying to solve this problem. If a motor is fixed on a moving rotating disc and the motor's rotor has another disc attached to it. What would happen if we attached the motor's ...
0answers
327 views

What was Feynman's famous formula?

In Welton(1983), Memories of Feynman, Welton mentions two formulas which he denotes as Feynman's Famous Formula (FFF) and FFF #2. Which famous formulas is he talking about? Is he maybe talking about ...
4answers
82 views

Why is mass per second constant in the Equation of continuity?

Why should the mass per second be constant in the equation of continuity? Why can't the mass accumulate inside a tube?
3answers
109 views

Walking on water

I was wondering about walking on water. I wonder if we can use surface tension to do this. Lets say I make a pair of shoes in shape which has infinite perimeter (e.g a koch snowflake) with glass. Now ...
2answers
86 views

Resistor and voltage

Considering this simple circuit : The potential is supposed to be constant along each wire (because they're conductors), such that the left wire in its entirety is at the same potential as the ...
3answers
239 views

Can we have negative Electrostatic potential

What does it mean to have a negative electric potential? not talking about potential difference or voltage.
1answer
70 views

what is the difference between the trivial SPT phase and nontrivial SPT phase?

I read some paper and found two terms "trivial SPT" and "nontrivial SPT". I am wondering what is the difference between trivial SPT and nontrivial SPT. Thank you! SPT stands for symmetry-protected ...
2answers
1k views

What would it be like “inside” a star?

This question was triggered by a discussion regarding the computer game Elite: Dangerous, where spaceships routinely operate in close proximity to stars (two or three light seconds away), at which ...
1answer
71 views

Location of singularity in moving black hole

Say a black hole is travelling at $c/2$, does the shape of the event horizon change? What about the location of the event horizon? If it is travelling at a hypothetical $c$, does the event horizon ...
1answer
126 views

Why “colours” of light are given in wavelength not frequency?

If I understand correctly, when a beam of (monochromatic) light passes through media of different refractive indices, its wavelength changes but frequency remains constant. Why, then, are colours of ...
1answer
49 views

Velocity of electrons during transition to different energy

I was watching a documentary of quantum mechanics (this video). They said something about quantum leap, jumping of electrons from one energy state to other. I want to ask how electrons travel though ...
3answers
173 views

Magnetic Energy Density Equations

I have a plasma with a magnetic field through it and I want to know the magnetic energy within the plasma. I have the simple equation $U=\frac{B^2}{\mu}$ for the energy density. But my question is ...

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