# All Questions

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### Why can't light escape from a classical black hole?

Photons do not have (rest) mass (that's why they can move at speed of "light"). So, my question is how the gravity of classical$^1$ black hole can stop light from escaping? -- $^1$ We ignore ...
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### Scale set by cosmological constant

Following on Jim Graber's answer to: Can "big rip" rip apart an atomic nucleus? If the cosmological constant is large enough, even the ground state of a hydrogen atom can be affected. So ...
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### Is it possible to bind an entangled electron to the outer shell of an atom or just a proton?

Say we start with an entangled electron--positron pair and we separate them. I want to take the entangled electron and bind it to a proton or the outer shell of an atom. Is it possible to do this ...
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### Is the quantum state or information of a particle is all that differentiates it from another particle?

So I have learned in my QM classes that you can't tell one electron from another electron. They are indistinguishable. I also learned that the wavefunction of a particle includes the spacial part and ...
547 views

### SU(N) symmetry and its representations

If a Lagrangian containing an N-multiplet of fields is invariant under global $\mathbf{SU}(N)$ transformations, does that necessarily imply it is invariant under $\mathbf{SU}(N-1)$, ...
3k views

### Forces in a Pendulum

Help me visualise the forces found in a pendulum. I know weight can be decomposed into $F_x$ and $F_y$. Centripetal force, acceleration and velocity are three words that I'm confused. Centripetal ...
602 views

### Derivation of relativistic energy

The concept of relativistic energy comes from it's conservation in relativistic mechanics for an elastic collision. It seems to me that another possible derivation could equate the energy of a single ...
233 views

### What reflective media do laser shows use?

I am having a hard time in finding out what exact light media laser shows use. I am trying to build a laser show myself. I know that the laser light is reflected off these particles in such a way that ...
2k views

### Are there examples in classical mechanics where D'Alembert's principle fails?

D'Alembert's principle suggests that the work done by the internal forces for a virtual displacement of a mechanical system in harmony with the constraints is zero. This is obviously true for the ...
9k views

### Is there an equation for the strong nuclear force?

The equation describing the force due to gravity is $$F = G \frac{m_1 m_2}{r^2}.$$ Similarly the force due to the electrostatic force is $$F = k \frac{q_1 q_2}{r^2}.$$ Is there a similar equation ...
5k views

### What is a complete book for quantum field theory?

I am searching for a complete and comprehensive book for QFT. What is, in your opinion, a good one?
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### If D-branes are the same as p-branes, and p-branes open out to “another universe”, why is the other universe invisible in D-branes?

Related to my other misguided question, D-branes are equivalent to p-branes. D-branes are described by a sheet in topologically trivial spacetime. p-branes are extremal black branes. They have an ...
298 views

### Complex valued energy

I'm working on a 2D He superfluid system with vortices. I was asked to calculate the kinetic energy of vortex-(anti-)vortex pairs and compare the two situations. One finds in literature that the ...
225 views

### The future of particle physics [closed]

Eventually, we are going to reach our limits in particle physics, or maybe find the theory of everything. Particle physics isn't easy by any means. It requires at the minimum years of graduate school ...
610 views

### Is spin is a conserved entity?

Suppose that an electron with spin up emits a photon in the field of an ion (bremsstrahlung). What is the spin of the emitted photon? Is it correct to say that the photon is circularly polarized if ...
129 views

### Why is there more analysis of short multiplets compared to long multiplets?

In theories with extended supersymmetry, both short and long multiplets exist. For some reason or other, short multiplets are studied more often. Why? What's wrong with long multiplets?
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### Who works professionally on reformulation of QFT?

P. Dirac was worried with the infinities and their discarding in QED. He wanted us to reformulate the theory in order to eliminate infinities and renormalizations from the very beginning. Is there ...
646 views

### The Reeh-Schlieder theorem and quantum geometry

There have been some very nice discussions recently centered around the question of whether gravity and the geometry and topology of the classical world we see about us, could be phenomena which ...
375 views

### Laws of fluid flow in porous medium

What are the equations that describe the flow of a fluid in a porous medium? Is there a variation of the Navier-Stokes equations? I would like to model the flow of air through a sponge-like ...
3k views

### Strongest force in nature

Possible Duplicate: What does it mean to say “Gravity is the weakest of the forces”? It is said nuclear force is the strongest force in nature.. But it is not true near a black ...
347 views

### Are the laws of physics applied mathematics? [closed]

This questions started with a question I had about gravity. If two objects of different weights fall to the earth at the same rate of acceleration, then it seems to me that gravity is in some ways ...
2k views

### Why electrons behave as a particle and also as a wave?

Why do electrons (and other very small particles) sometimes behave as particles (i.e. when we are not looking at them) where as other times they behave as waves?
4k views

### What affects the damping of a spring?

What variables affect the damping of a spring executing simple harmonic motion? What are the independent variables, and what variables would need to be controlled in an experiment? I'm attempting to ...
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### Is the speed of a person falling down different if doing a forward somersault during the fall?

Let's imagine this in an situation where a person jumps down from a smaller building, a garage for example. The exact numbers aren't really important here, since I'm not aiming for an exact answer, ...