1
vote
1answer
146 views

Ideal four-level Gain Medium (or just any old gain medium)

What exactly is happening in a laser's gain cell when the irradiance is greater than the saturation irradiance? Also can someone offer a clear conceptual treatment of the gain-coefficient and its ...
3
votes
2answers
672 views

Missing transverse energy, exact definition

This might seem basic, but it is a bit confusing. You hear about missing transverse energy a lot in SUSY searches due to the LSP which cannot be detected. Let's say I have the 4-vector for the LSP. ...
-5
votes
1answer
146 views

Black out the sun?

How many observers would it take to cancel out the number of photons produced in a medium sized star like our Sun, thus making it dark? (I want to consume more photons than the star is generating)
2
votes
3answers
1k views

Minkowski diagram and length contraction

The length contraction means that an object is the longest in the frame in which it is at rest. Lets assume i have a meter stick with length $\Delta x$ in my rest frame which is $x,ct$ and i want to ...
2
votes
5answers
1k views

Is there perfect vacuum 10000 billion lightyears away?

Is there perfect vacuum about some 10,000 billion light years away? With perfect vacuum, I mean that there are no particles, not even virtual photons! I had this idea by assuming that all particle ...
0
votes
4answers
1k views

Find total energy and momentum of an moving electron in a rest frame

I have an electron moving with speed $u'$ in a frame $S'$ moving with speed $v'$ relative to a rest frame $S$. How do I find the total energy and momentum of the electron in the rest frame $S$? I ...
5
votes
2answers
12k views

Why does the balloon pop?

When we pierce a balloon with a sharp needle, it pops and produce a great sound. But, It doesn't pop when we open the mouth of the balloon (through which we have blown air)... So, Why doesn't the gas ...
5
votes
3answers
440 views

Rationale for writing wave function as product of independent wave functions

When solving Schrödinger's equation for a 3D quantum well with infinite barriers, my reference states that: $$\psi(x,y,z) = \psi(x)\psi(y)\psi(z) \quad\text{when}\quad V(x,y,z) = V(x) + V(y) + V(z) = ...
7
votes
3answers
718 views

What makes a Feynman diagram real or virtual?

Simple question: as the title says, what makes a real Feynman diagram real, and what makes a virtual diagram virtual? Or in other words, how do I tell whether any given diagram is real or virtual? ...
3
votes
2answers
787 views

Divergence of non conservative electric field

I'm looking for the proof that the 1st Maxwell equation is valid also on non conservative electric field. When we are talking about a electrostatic field, the equation is ok. We can apply the Gauss ...
1
vote
2answers
2k views

Does Stephen Hawking not believe in Negative movement? [duplicate]

Possible Duplicate: Is it possible to go back in time? Is time travel possible? According to Stephen Hawking, time travel to the future is not only possible but proven, however time ...
5
votes
1answer
730 views

why sometimes touching old flickering tube lights starts them properly

In my old house there are two old tube lights. Some times they don't start properly, (specially at evening time, may be it is because of low voltage), they starts flickering i.e. on and off ...
3
votes
3answers
452 views

Compute closed line integral of electric field in circuit

I have a circuit where resistor is parallel to capacitor, which is charged with voltage U. How to compute line integral around closed loop to get the result of Kirchhof second law - ...
3
votes
4answers
458 views

Before the Big Bang

I've heard this saying before I don't know about anyone else. It says, "What ever was before the big bang is something physics can't explain..! Is this saying true (accurate)?
3
votes
1answer
296 views

Can I find a potential function in the usual way if the central field contains $t$ in its magnitude?

I'm working on a classical mechanics problem in which the problem states that a particle of mass $m$ moves in a central field of attractive force of magnitude: $$F(r, t) = \frac{k}{r^2}e^{-at}$$ ...
3
votes
1answer
291 views

what does holographic principle from string theory say about the possibilities of wormhole travel?

Is travel through stable macroscopic wormholes between remote points of spacetime going to be possible in a definitive theory of gravity, be it string theory or something beyond it? Physicists level ...
0
votes
0answers
38 views

Physical meaning of force times area [duplicate]

Possible Duplicate: What is the physical interpretation of force times area? If force times area has any physical meaning, what would it be? Where would it be used? I know that force ...
4
votes
1answer
524 views

Holstein-Primakoff and Dyson-Maleev representation

In Holstein-Primakoff and Dyson-Maleev representation, spin operators are represented by bosonic operators. Roughly speaking, a state with $S^z=S-m$ corresponds to a state containing $m$ bosons. In ...
1
vote
2answers
198 views

how do we know that the base of entire universe is the proton (hydrogen) and not the antiproton?

It may be that the base of a part of the world is anti-proton, We've always been on the planet Earth and the Milky Way. how do we know that the base of entire universe is proton (hydrogen atom)?
1
vote
2answers
200 views

Problem with Big Rip Theory

I have heard of the "Big Rip" as one theory for the eventual end of the universe. If the speed at which the universe is expanding, and all the matter inside it is moving away from all other matter at ...
6
votes
3answers
481 views

What is the massless limit of massive electromagnetism?

Consider electromagnetism, an abelian gauge theory, with a massive photon. Is the massless limit equal to electromagnetism? What does it happen at the quantum level with the extra degree of freedom? ...
1
vote
1answer
128 views

Why are photons released from high-voltage electricity through a near vacuum?

I read an answer to what is inside a typical vacuum, and it suggested neutrinos from Space, and vapor from the container. So, maybe they add to this effect... But why are photons released from ...
2
votes
3answers
370 views

Weightlessness by a parabolic flight

Do you actually achieve weightlessness during a parabolic flight? Because I believe I heard somewhere did you achieve 'near-weightlessness' and not 'weightlessness' (if this is true, why is this?) ...
0
votes
2answers
326 views

Effect of charges near a parallel plate capacitor

If I charge a parallel plate capacitor and I insert a charged body near one of the plates will there be any interactions like attraction or repulsion? What if I disconnect the battery?
5
votes
2answers
149 views

Why is the gas halo of the Milky Way so hot?

I have read on the webpage of NASA that there is a massive hot gas halo around our galaxy. Its temperature is between 100,000 and 1 million Kelvins or more. I do not understand why is it so hot. The ...
2
votes
1answer
161 views

Parabolic Motion

I was wondering if I was right about this simple problem: A projectile is shot at an angle of 37.0° with the horizontal. The velocity of the projectile at its peak height is 16 m/s. Then I was told ...
-1
votes
2answers
353 views

A light and magnetic mirror paradox?

If light is an electromagnetic wave and lightspeed is constant (we ignore spacetime or gravity for this question) why can't we slow down light with a few dozen wellplaced magnets and electricly ...
4
votes
2answers
1k views

What is a bulk phase transition?

I have been able to google "bulk phase transition" and get plenty of results that verify that something called a bulk phase transition exists, however, I cannot seem to find a precise definition of ...
4
votes
1answer
259 views

What do I see if I move quickly past a charge surrounded by iron filings?

This might be a straightforward exercise, in which case I apologize. Suppose I surround a charge by iron filings initially oriented in some fixed direction, and I then move past the charge at an ...
2
votes
1answer
178 views

constraint on scaling dimension

How can we show that for any scalar operator $\Delta\geq1$ (where $\Delta$ is the scaling dimension)? Where can I find a reference for reading where it comes from?
0
votes
1answer
555 views

Is there a simple, accurate formula for calculating transit times from rise and set times?

I have access to "rise" and "set" times for astronomical objects, and want to determine the corresponding times of culmination. Is there a reliable and accurate way to do this? It isn't clear to me ...
0
votes
1answer
84 views

Writing a tensor with respect to a particular basis

When defining tensors as multilinear maps, I am having trouble understanding why a tensor, let's say of type (2,1), can be written in the following way: $$T = T^{\mu\nu}_{\rho} e_\mu \otimes e_\nu ...
3
votes
3answers
1k views

Free falling objects

My teacher and I are in the middle of an argument because she says that if you were to drop two objects at the same time and the same height, but with different initial velocities, both of them would ...
10
votes
1answer
702 views

When we define the S-matrix, what are “in” and “out” states?

I have seen the scattering matrix defined using initial ("in") and final ("out") eigenstates of the free hamiltonian, with $$\left| \vec{p}_1 \cdots \vec{p}_n \; \text{out} \right\rangle = S^{-1} ...
4
votes
3answers
577 views

Do Christoffel symbols commute?

Do Christoffel symbols commute? For example, does $\Gamma^{e}_{db}\Gamma^{c}_{ea} = \Gamma^{c}_{ea}\Gamma^{e}_{db}$?
2
votes
1answer
310 views

What is nonlocal resistance?

We are first taught to calculate local resistance, where current and voltage are on the same part of the material. But many experiments measure nonlocal resistance, where current and voltage are ...
3
votes
1answer
181 views

Nonlocal Transport in the Quantum Spin Hall State

It has been reported that nonlocal Transport in the can be realized in topological insulator. Why non-local transport through edge channels has the potential application for low-power information ...
9
votes
2answers
656 views

What does “the ${\bf N}$ of a group” mean?

In the context of group theory (in my case, applications to physics), I frequently come across the phrase "the ${\bf N}$ of a group", for example "a ${\bf 24}$ of $SU(5)$" or "the ${\bf 1}$ of ...
3
votes
1answer
608 views

What happens to the Lagrangian of the Dirac theory under charge conjugation?

Consider a charge conjugation operator which acts on the Dirac field($\psi$) as $$\psi_{C} \equiv \mathcal{C}\psi\mathcal{C}^{-1} = C\gamma_{0}^{T}\psi^{*}$$ Just as we can operate the parity operator ...
-2
votes
1answer
67 views

Assuming collision , are there fundamental forces associated with absorbtion?

I just learned that strong and weak nuclear forces relate to decay/emission. I know absorbtion depends on Energy levels(QM) and heat(thermodynamics , kinetic energy , entropy) and E = gamma mc^2 ( ...
1
vote
1answer
226 views

Does the strong (nuclear) force ever contribute to decay?

Does the strong (nuclear) force ever contribute to decay ? Or is the weak nuclear force the only decaying force ?
1
vote
2answers
156 views

matter anti-matter world

let us suppose the gedanken experiment a man isolated into a room he ask if he is made of matter oder of antimatter could he set some experiments to see if he is made of matter or if he is made of ...
0
votes
3answers
280 views

Is it true that an isolated fundamental particle does not decay?

Is it true that an isolated fundamental/elementary particle does not decay? It seems logical to me.
5
votes
3answers
3k views

How many photons can an electron absorb and why?

How many photons can an electron absorb and why? Can all fundamental particles that can absorb photons absorb the same amount of photons and why? If we increase the velocity of a fundamental ...
5
votes
2answers
232 views

Secondary constraints leads to the value of lagrange multiplier

From Lagrangian I got two primary constraint $\phi_i$ and $\phi$. And my Hamiltonian in presence of the constraints becomes- $$H_p=p\dot q-L+\lambda_i\phi_i+\lambda\phi$$ here the $\lambda_i$ and ...
1
vote
2answers
172 views

What is the pressure on an empty pneumatic tube?

At the residence I'm constructing a kitchen digester. The gas is to be collected into an old bicycle/scooter tube - initially as empty as may be after wringing it out.. Will the digester (total ...
-1
votes
3answers
232 views

How many pieces of toast would you need to make a black hole?

I am trying to find the following: How many pieces of toast would you need to make a black hole? From what I've learnt so far I need to find an equation for the compression force the massive amount ...
6
votes
1answer
2k views

Why does an infrared thermometer display very low temperature when being directed to the outer air?

I'm toying with an infrared thermometer - one which you point onto an object, press the button and it instantly measures the temperature by estimating the infrared radiation from the object. It shows ...
-2
votes
1answer
6k views

Calculate acceleration and time given initial speed, final speed, and travelling distance? [closed]

A motorcycle is known to accelerate from rest to 190km/h in 402m. Considering the rate of acceleration is constant, how should I go about calculating the acceleration rate and the time it took the ...
3
votes
2answers
224 views

The earth's magnetic field

This might sound like a silly question. Is it possible for the earth's magnetic field to actually destroy or harm earth? (implosion, crushing etc.)

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