2
votes
1answer
330 views

Cosmological constant

I have always wondered about how cosmological constant is characterized. So since it is still a hypothesis you often read the “cosmological constant measured to be ….”. Shouldn't the statement read ...
3
votes
1answer
118 views

Spin Liquid in a band insulator?

In the literature, spin liquids are only possible in Mott insulators, however, I'm not entirely sure why the nuclear spin can't create a spin liquid in a band insulator. Is this possible? If so, is ...
3
votes
3answers
10k views

Why is copper diamagnetic?

Cu has an unpaired electron in 4s, but it is diamagnetic. I thought that it has to be paramagnetic. What am I missing?
4
votes
3answers
810 views

Looking out into the universe means looking back in time - how does that work?

This is a question that has been gnawing on me for many years now. Back a long time ago, as I recall in reference to a scene in a popular science show on TV, I was asked the following. The claim is ...
9
votes
1answer
366 views

Lagrangian Coordinates in Fluid Flow

I apologize if this is not the right place to ask this question: I am currently reading a paper by Y. Brenier, where for the fluid flow he introduces a Lagrangian label $a$ instead of the vertical ...
5
votes
1answer
426 views

Maxwell demon : how can the demon move the door without doing any work ?

Here is something that I never understood about Maxwell's demon paradox : in order to contradict the second law of thermodynamics, the demon must open and close the door without doing any work... But ...
4
votes
1answer
128 views

Thermal conductivity affected by electrical current

Does anyone know of any materials whereby the thermal conductivity can be changed by passing an electrical current through the material?
-1
votes
1answer
180 views

US time zones and Daylight saving time, energy efficient? [closed]

As a foreigner, United States has a very complex time system for me. Central Time Zone, North American Eastern Time Zone, ...
3
votes
3answers
646 views

What does it mean that a wavevector is null?

I have derived geometric optics for gravitational waves and I am trying to interpret one of the results. I have \begin{equation} k_{\rho}k^{\rho}=0 \end{equation} for the wavevector. For the case ...
2
votes
1answer
194 views

Is there a geometrical way to obtain a relationship between these vectors?

Suppose we have a setup like this. Here $a_1,a_2,b_1,b_2$ are acceleration magnitudes($b_1,b_2$ being relative) and $P,Q,R,S$ are not pulley/blocks but are points on the rope. If I use a geometrical ...
6
votes
4answers
412 views

Why a day is divided by 12/24 hours? Why the number 12?

Why a day is divided by 12/24 hours? Why the number 12? Why not using 10 or 6 or 14, 16? Who invented this? Any physical reasons behind this?
0
votes
1answer
149 views

About the theory of relativity

Why is speed of light constant in every non inertial frame of reference? Is there any theoretical explanation behind this postulate Since we cannot completely depend upon the experimental results?
0
votes
3answers
990 views

Temperature change inside pressure chamber

Let's say there is a pressure chamber with some sort of sample / specimen (e.g. protein crystal) in it. Now I apply a certain amount of gas pressure, e.g. 10 or 20 atm. Let's say I use xenon as a gas. ...
22
votes
6answers
2k views

What are the various physical mechanisms for energy transfer to the photon during blackbody emission?

By conservation of energy, the solid is left in a lower energy state following emission of a photon. Clearly absorption and emission balance at thermal equilibrium, however, thermodynamic equilibrium ...
2
votes
1answer
1k views

Deriving torque from Euler-Lagrange equation

How could you derive an equation for the torque on a rotating (but not translating) rigid body from the Euler-Lagrange equation? As far as I know from my first class in Classical Mechanics, there is ...
1
vote
3answers
164 views

Faraday's law in a ring

What role does the induced emf in Faraday's law play in generating current in a ring in which the magnetic flux is changing ?
52
votes
5answers
12k views

How long can you survive 1 million degrees?

I asked my Dad this once when I was about 14, and he said that no matter how short the amount of time you were exposed to such a great temperature, you would surely die. The conversation went ...
0
votes
1answer
50 views

Velocity of measurement

As per to Heisenberg uncertainty we will not be able to calculate the position and momentum at same instant because by the time we calculate the next of the one, it changes (i.e.) the changes are very ...
2
votes
3answers
347 views

Understanding bending light beam perpendicular to motion

I'm just reading a book about gravity. An example it gives is a spaceship accelerating. A beam of light travelling at right angles to the direction of movement of the spaceship enters it via a small ...
2
votes
2answers
366 views

Bondi's k factor

Consider two observers, travelling away from each other, after meeting (at which time they sync their clocks). $O$ sends a photon towards $O'$ at times $t$, received by $O'$ at time $t'=kt$, where we ...
2
votes
2answers
562 views

Movement of man and ladder and their center of mass

Suppose there is a massless frictionless pulley. A rope over it carries a mass $M$ and on other side carries a ladder of mass $(M-m)$ and a man on that ladder, of mass $m$. Now the man starts ...
9
votes
3answers
4k views

What would be the effects if Jupiter was to blown up?

So, in one science fiction story, that tries to be realistic as possible apart from few space magics, humanity has contingency plan to blow up Jupiter. As in, totally destroy it in one massive nuclear ...
7
votes
2answers
684 views

Diifference between real time propagation and imaginary time propagation?

Suppose I want to solve Nonlinear Schrodinger equation using imaginary time propagation to get the ground state solution. I choose $t = - i t$, and then solve the equation using split step Crank ...
4
votes
1answer
657 views

Intuition behind classical virial theorem

I am continuing to brush up my statistical physics. I just want to gain a better understanding. I have gone through the derivation of the classical virial theorem once more. I have thought about it, ...
1
vote
1answer
4k views

Work done by friction

Suppose we have a block of mass $M$ and we are moving it up a curve, very slowly ($a=0$). The surface is not smooth, and coefficient of friction is $\mu=\mu_s=\mu_k$. To move the block we apply a ...
-4
votes
2answers
133 views

Which units has the relation $E=mc^2$? [closed]

Fast everyone knows the relation $E = mc^2$ but I still ask me, which units the relation has. $c$ is the velocity of light. It means, a constant, isn't it? We therefore just have the relation $E = ...
3
votes
5answers
4k views

Eye sensitivity & Danger signal [duplicate]

Why are danger signal in red, when the eye is most sensitive to yellow-green? You can check luminosity function for more details...
3
votes
1answer
716 views

Net work output of an engine performing Carnot cycle?

My problem gives me a Carnot cycle heat engine with water as its working fluid, with $T_H$, $T_L$, and the fact that it starts from saturated liquid to saturated vapor in the heating process. I need ...
0
votes
1answer
193 views

Can you enter a timelike hypersurface?

As I understand it, a timelike hypersurface is one that has only spacelike normal vectors. But does this not imply that a the geodesic of a particle crossing it must be spacelike at that point? But ...
0
votes
1answer
82 views

In $PdV$, what is the value of $P$? $P_1$ or $P_2$?

Say I have an ideal gas that has a known $P_1$, $P_2$, $T_1$, and $T_2$ undergoing a reversible adiabatic process. I want to find the work done so I must use $PV = RT$ to get the change in $V$, so ...
3
votes
1answer
914 views

Change of coordinates from an arbitrary frame to a locally inertial frame in General Relativity

If I have the following metric: $$ds^2=(1-2\phi)c^2 dt^2 - (1-2 \phi)(dx^2+dy^2+dz^2)$$ $\phi$ being the gravitational potential with $|\phi| << 1$ everywhere. How do I find a coordinate ...
4
votes
1answer
430 views

Can a hovering helicopter travel half the globe in 12 hours? [duplicate]

Suppose we have a helicopter that is able to stay stationary in flight for extended periods of time. If such a helicopter stayed at point A in the sky for 12 hours straight, would it reach the other ...
4
votes
1answer
369 views

Metronome synchronisation applied to swings

The movement of several metronomes can be synchronised when a movable floor is utilised which couples the movement of the different metronomes. Is it possible to apply this sort of synchronisation to ...
0
votes
2answers
2k views

Rolling ball which slips

A bowling ball of mass $M$ and radius $r_0$ is thrown along a level surface so that initially ($t = 0$) it slides with a linear speed $v_0$ but does not rotate. As it slides, it begins to spin, and ...
3
votes
1answer
337 views

Intuition behind Work

I have a doubt in understanding the intuition behind the concept of work. First of all, I think this isn't duplicate, I've searched on the site, and the closest thing I've found was this post which is ...
3
votes
1answer
278 views

What are the conserved charges related to the Virasoro generators?

I have just learned from reconsidering my demystified book, that when conformally maping the worldsheet of a closed string to the complex plain by using the transformation $z = e^{\tau + i\sigma}$ ...
2
votes
1answer
659 views

Difficulties with understanding total entropy change and unavailabillty

Of course, I know the fact that the entropy of an isolated system never decreases. Neverthless what makes me confused about the entropy(or change of entropy) of an isolated system is the explanation ...
1
vote
1answer
3k views

Archimedes principle and specific gravity

A physical balance measures the gravitational mass of a body. I conducted an experiment to find out the specific gravity of a bob. I first measured the mass of the bob in air, and then in water. The ...
34
votes
4answers
3k views

Does the sun rotate?

As implied from the question, does the sun rotate? If so, do other stars not including the sun also rotate? Would there be any consequences if the sun and other stars didn't rotate? Me and my friends ...
4
votes
1answer
4k views

How to understand the work-energy theorem?

How to understand the work-energy theorem? I took a short lecture on physics for engineering last week. The lecturer emphasized that the work done on an object will cause the kinetic energy change as ...
4
votes
1answer
505 views

Diving into a charged (Reissner-Nordstrom) Black hole

Apparently there are two event horizons in this type of black hole, where the second one is known as the Cauchy horizon. According to Carroll, if you go into the first one, you will fall until you ...
0
votes
0answers
152 views

Transfer functions of active filters

I've been looking for some straightforward method or trick to obtain the transfer functions of active filters (like the Sallen-Key filter, the butterworth or Cauer topology etc...) since KCL or KVL ...
0
votes
1answer
287 views

Strings and QFT: particles moving backward in time?

New question: In string theory and QFT, do particles travel back in time? Not related to antimatter: Do they travel back and forth in time in reality or are these just interpretations of mathematical ...
2
votes
1answer
123 views

Interacting particles

We are familiar with the grand partition function for the grand canonical ensemble. This makes me wonder: what kinds of modifications would be required if the particles interacted? Thanks.
1
vote
1answer
319 views

Why work to change velocity from 0 to 20 km/h is less then from 20 to 40? [duplicate]

Imagine spaceship in vacuum with mass = 1. At beginning, it has velocity 0, and kinetic energy 0. $$W_1 = 0$$ Then, it turns on its engine, and changes velocity from 0 to 20 (delta v = 20). It's ...
1
vote
1answer
917 views

Newton's Third Law of Motion

I want to start off by saying that I've looked around for other explanations, but I've not really found any satisfying ones. My question is basically the whole "Why can anything move at all?" ...
0
votes
1answer
291 views

Information bearing degrees of freedom of a quantum simple harmonic oscillator

I am trying to make sense of arXiv:physics/0210005. I am confused with the concept of information bearing degrees of freedom of a system mentioned at the very beginning. To verify the arguments of the ...
0
votes
1answer
1k views

Why is the equation for Entropy of an ideal gas that undergoes reversible change in T at constant Pressure like this?

Why is the equation for change in Entropy for a reversible change in $T$ at constant $P$ described as $$\Delta S = n C_p \ln\frac{T_f}{T_i}$$
5
votes
1answer
2k views

Frictional force on a rope wrapped around a drum

So there's a rope wrapped around a circular drum, subtending an angle $\theta$. A large force, $T_A$, pulls in in one direction, and a much smaller force $T_B$ pulls in in another direction. The ...
2
votes
1answer
545 views

Conservation of Angular momentum in the dipole selection rules

If the total angular momentum J of an atom is not changing during a dipole transition, where does the angular momentum for the photon come from?

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