5
votes
2answers
420 views

CPT and heat equation

I haven't understood this thing: Physics is invariant for CPT trasform...But the Heat or diffusive equation $\nabla^2 T=\partial_t T$ is not invariant for time reversal...but it's P invariant..So CPT ...
13
votes
4answers
756 views

Nonlinear optics as gauge theory

the widely used approach to nonlinear optics is a Taylor expansion of the dielectric displacement field $\mathbf{D} = \epsilon_0\cdot\mathbf{E} + \mathbf{P}$ in a Fourier representation of the ...
-3
votes
2answers
843 views

An iPhone falling on carpet is fine, is it true? [closed]

I heard that an iphone falling on carpet would not cause damage to it, both internal and external. Any physical explanation for this?
2
votes
1answer
870 views

Relation of angular speed of a rigid body to Euler's Angles

My Question was like this and i have realised few things and still have some doubts I have a book in which a paragraph goes like this Now, $\dot\phi$, $\dot \theta$, $\dot\psi$ are respectively ...
6
votes
2answers
931 views

Separation of variables, eigenfunctions of the Dirac operator

Disclaimer: I am not a physicist; I am a geometer (and a student!) trying to learn some physics. Please be gentle. Thanks! When solving the Schrödinger equation for a particle in a spherical ...
5
votes
1answer
1k views

Why are there 3 quarks in proton?

A few quark related questions (I don't knowmuch about them other than there are 2 flavours concerning protons and neutrons) Why are there 3 quarks in a proton or neutron? Why not 2 or 4? Is there an ...
4
votes
1answer
228 views

Is it possible to determine timescales of electron dynamics from the natural linewidth of an electronic transition?

A lot of work has been done recently on electron dynamics using attosecond pump-probe techniques; for instance in this paper. In this particular paper, the authors photoionized the neutral ...
6
votes
5answers
6k views

Is it possible to obtain gold through nuclear decay?

Is there a series of transmutations through nuclear decay that will result in the stable gold isotope ${}^{197}\mathrm{Au}$ ? How long will the process take?
12
votes
3answers
2k views

Why are quark types known as flavors?

There are six types of quarks, known as flavors. Why where these types called flavors? Why do the flavors have such odd names (up, down, charm, strange, top, and bottom)?
3
votes
1answer
2k views

Spherical wave as sum of plane waves

How can we do this computation? $\iiint_{R^3} \frac{e^{ik'r}}{r} e^{ik_1x+k_2y+k_3z}dx dy dz$ where $r=\sqrt{x^2+y^2+z^2}$ ? I think we must use distributions... Physically, it's equivalent to ...
17
votes
2answers
2k views

Ising model for dummies

I am looking for some literature on the Ising model, but I'm having a hard time doing so. All the documentation I seem to find is way over my knowledge. Can you direct me to some documentation on it ...
3
votes
3answers
628 views

Light emission spectrum units

Do someone knows the units of the spectra provided here ? It seems obvious enough that it's said nowhere, but even Wikipedia and other sites are quite blurry on this point. So, is it power ($W$), ...
15
votes
2answers
2k views

Why does my watch act like a mirror under water?

I have a digital watch, rated to go underwater to $100 \rm m$. When it is underwater it can be read normally, up until you reach a certain angle, then suddenly, it becomes almost like a mirror, ...
2
votes
2answers
413 views

What is an analog to QM's Hilbert space in GR?

I've read that QM operates in a Hilbert space (where the state functions live). I don't know if its meaningful to ask such a question, what are the answers to an analogous questions on GR and ...
11
votes
4answers
1k views

How many Onsager's solutions are there?

Update: I provided an answer of my own (reflecting the things I discovered since I asked the question). But there is still lot to be added. I'd love to hear about other people's opinions on the ...
2
votes
3answers
906 views

Does the energy of a magnetic field decrease when it moves a conductor carrying a current?

When a charged particle moves in an electric field, the field performs work on the particle. Thus, the energy of the field decreases, turning into kinetic energy of the particle. Does the magnetic ...
11
votes
7answers
13k views

How efficient is an electric heater?

How efficient is an electric heater? My guess: greater than 95%. Possibly even 99%. I say this because most energy is converted into heat; some is converted into light and kinetic energy, and ...
4
votes
2answers
161 views

Can I parameterize the state of a quantum system given reduced density matrices describing its subparts?

As the simplest example, consider a set of two qubits where the reduced density matrix of each qubit is known. If the two qubits are not entangled, the overall state would be given by the tensor ...
3
votes
2answers
671 views

Is there a conserved quantity that enforces planar orbits in central force motion?

From what I remember, one of the first steps in finding the equations of motion for an orbiting body is to argue that the body's motion has to be restricted to a plane, because the central force has ...
14
votes
6answers
5k views

Why is the mapped universe shaped like an hourglass?

I've watched a video from the American National History Museum entitled The Known Universe. The video shows a continuous animation zooming out from earth to the entire known universe. It claims to ...
7
votes
4answers
2k views

How far does a trampoline vertically deform based on the mass of the object?

If a baseball is dropped on a trampoline, the point under the object will move a certain distance downward before starting to travel upward again. If a bowling ball is dropped, it will deform further ...
2
votes
4answers
3k views

Relation between density and pressure for a perfect fluid

what is the relation between mass density $\rho$ and pressure $P$ for a perfect fluid ?
1
vote
2answers
142 views

Behaviour of mass and momentum distributions under Newtonian Gravity

In the context of this question should mass distribution $\rho(r,t)$ and momentum distribution $p(r,t)$ be well behaved ? By 'well behaved' it is meant that derivatives of all orders exist everywhere. ...
10
votes
3answers
458 views

If time standard clocks and any memories about the time standard are destroyed, can we recover the time standard again?

Assume the time standard clocks and any memories about the time standard are destroyed. Can we recover the time standard again exactly? Recovering the time standard again means we can determine the ...
10
votes
3answers
1k views

Why some nuclei with “magic” numbers of neutrons have a half-life less than their neighbor isotopes?

It's easy to find the "magic" numbers of neutrons on the diagrams of alpha-decay energy: 82, 126, 152, 162. Such "magic" nuclei should be more stable than their neighbors. But why some nuclei ...
15
votes
4answers
4k views

Reciprocal Lattices

Is there an easy way to understand and/or visualize the reciprocal lattice of a two or three dimensional solid-state lattice? What is the significance of the reciprocal lattice, and why do solid ...
2
votes
2answers
1k views

Appearance of atoms

I was watching a documentary entitled "The Atom" and one of the statements made was that Atoms behave differently when we look at them. I wasn't too sure about the reasoning behind this and i'm hoping ...
0
votes
3answers
184 views

Evolution of mass and velocity distributions under newtonian gravitation

Let $\rho(r,t)$ and $v(r,t)$ be mass and velocity distributions. Given $\rho(r,0)$ and $v(r,0)$ (initial conditions) what is the differential equation that describes the evolution of $\rho(r,t)$ and ...
1
vote
1answer
7k views

Is there a name for the derivative of current with respect to time, or the second derivative of charge with respect to time?

This measurement comes up a lot in my E&M class, in regards to inductance and inductors. Is there really no conventional term for this? If not, is there some historical reason for this omission? ...
2
votes
3answers
361 views

What is the definition of momentum when a mass distribution $\rho(r,t)$ is given?

This question is Edited after recieving comments. What is the definition of momentum when a mass distribution $\rho(r,t)$ is given? Assuming a particle as a point mass we know the definition of ...
3
votes
3answers
688 views

General relativity (gravitation) in time and one spatial dimension

I don't have any idea of general relativity but intend to learn. Is it a good idea to study general relativity in two dimensions (time and single spatial dimension) in the begining to get good idea on ...
3
votes
10answers
4k views

Why is Physics so hard? [closed]

I'm in a classical mechanics class now. On our exams, most questions are quantitative. And in general, besides the theory part, all physics problems just require you to gather formulas, manipulate ...
4
votes
4answers
669 views

2nd Law of Thermodynamics

I understand that the 2nd law of thermodynamics roughly states that, if you have a body (or a gas in a chamber) that is hot at one end and cold on the other, the heat will always flow from the hot to ...
11
votes
8answers
4k views

Is it possible to separate the poles of a magnet?

It might seem common sense that when we split a magnet we get 2 magnets with their own N-S poles. But somehow, I find it hard to accept this fact.(Which I now know is stated by Gauss's Law) I have ...
13
votes
2answers
306 views

Searching books and papers with equations

Sometimes I may come up with an equation in mind, so I want to search for the related material. It may be the case that I learn it before but forget the name, or, there is no name for the equation ...
8
votes
2answers
297 views

What happens for the spins around the phase transition

Suppose we now consider a lattice of spin, say Ising model, and the phase transition at the critical temperature $T_c$. There are few scaling laws describe the regime around the critical temperature ...
5
votes
4answers
2k views

Measuring the speed of light and defining the metre - absolute or relative?

If the metre is now defined as the distance light travels in vacuum in 1⁄299,792,458th of a second and the speed of light is accepted to be ...
1
vote
3answers
333 views

Teleportation by quantum isolation

This is a thought experiment, so please don't treat it too harsh :-) Short: If we could isolate two places A and B in the universe from all and any interaction with the surroundings, is there a ...
1
vote
1answer
266 views

Is there a theory about kinetic energy “particles”? [closed]

We have a model of electricity which says electrons flow from one place to another. We have a model of optics which says that photons go from one place to another. As I understand, there is currently ...
-2
votes
1answer
149 views

Four-dimensionalism vs energy economy

Four-dimensionalism claims that the universe is basically one huge space-time worm and that everything exists at once (however you want to say that since "internal time" is then just another ...
7
votes
6answers
2k views

Is rotational motion relative to space?

Let's assume that there is nothing in the universe except Earth. If the Earth rotates on its axis as it does, then would we experience the effects of rotational motion like centrifugal force and ...
13
votes
2answers
2k views

Is kinetic energy a relative quantity? Will it make inconsistent equations when applying it to the conservation of energy equations?

If the velocity is a relative quantity, will it make inconsistent equations when applying it to the conservation of energy equations? For example: In the train moving at $V$ relative to ground, ...
39
votes
5answers
3k views

Scattering of light by light: experimental status

Scattering of light by light does not occur in the solutions of Maxwell's equations (since they are linear and EM waves obey superposition), but it is a prediction of QED (the most significant Feynman ...
3
votes
7answers
1k views

Best example of energy-entropy competition? [closed]

What are the best examples in practical life of an energy-entropy competition which favors entropy over energy? My initial thought is a clogged drain -- too unlikely for the hair/spaghetti to align ...
18
votes
6answers
1k views

Is there a difference between observing a particle and hitting it with another particle?

First, let me state that I'm a lot less experienced with physics than most people here. Quantum mechanics was as far as I got and that was about 9 years ago, with no use in the meantime. A lot of ...
5
votes
2answers
962 views

If the light velocity is a vector quantity, why vector addition cannot be applied to it?

If the light velocity is a vector quantity, why vector addition cannot be applied to it? Or the light velocity is not a vector quantity?
43
votes
11answers
9k views

Why does dust stick to rotating fan propeller?

Why does dust stick to rotating fan propeller? Intuitively, most people (including I) think of the dust will not stick to rotating fan propellers. EDIT 1: Thank you for the great explanations. I am ...
6
votes
5answers
3k views

What sustains the rotation of earth's core (faster than surface)?

I recently read that the earth's core rotates faster than the surface. Well, firstly, it's easier to digest the concept of planetary bodies, stars, galaxies in rotation and/or orbital motion. But, ...
2
votes
2answers
151 views

Utility of displacements potentials in geophysics

In the elasticity theory, you can derive a wave equation from the fundamental equation of motion for an elastic linear homogeneous isotropic medium: $\rho \partial^2_t \overline{u} = \mu \nabla^2 ...
10
votes
5answers
2k views

Hubble's law and conservation of energy

If all distances are constantly increasing, as Hubble's law say, then lots of potential energies of form ~$\frac{1}{r}$ changes, so how is the total energy of the Universe conserved with Hubble's ...

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