All Questions

12
votes
6answers
7k views

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 ...
7
votes
1answer
245 views

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 ...
2
votes
3answers
349 views

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 ...
0
votes
1answer
208 views

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 ...
2
votes
1answer
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 $\mathbf{SU}(N-1)$, ...
0
votes
2answers
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 ...
0
votes
1answer
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 ...
5
votes
2answers
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 ...
11
votes
2answers
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 ...
26
votes
6answers
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 ...
7
votes
2answers
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?
3
votes
0answers
205 views

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 ...
2
votes
1answer
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 ...
0
votes
2answers
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 ...
2
votes
2answers
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 ...
1
vote
1answer
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?
-3
votes
4answers
915 views

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 ...
10
votes
2answers
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 ...
3
votes
2answers
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 ...
3
votes
2answers
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 ...
1
vote
6answers
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 ...
-1
votes
3answers
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?
-1
votes
1answer
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 ...
5
votes
1answer
806 views

When is use of the 'effective mass' concept appropriate?

In textbooks the characteristic length scale of an exciton, or an electron bound to dopant atom, in silicon is calculated by analogy to the vacuum case. Bohr radius in vacuum: $$a_0 = \frac{4 \pi ...
2
votes
4answers
222 views

What are the conditions to be satisfied by a theory in order to be a quantum theory?

This is in continuation to my previous question. It is not a duplicate of the previous one. This question arises because of the answers and discussions in that question. Can we call a theory, quantum ...
3
votes
3answers
5k views

What is a virtual ground?

What is a virtual ground? I would like to know what it is.
5
votes
3answers
230 views

Mobile “muonic hydrogen”

If we look at the atomic positions in a single crystal sample with a diamond like lattice, there exist directions along which there are long hexagonal "tubes" (I'm not sure if these have a proper ...
5
votes
2answers
1k views

What allows the Wave Disk Generator to be so efficient?

Researchers at Michigan State University recently invented the Wave Disk Generator that is supposed to get 60% fuel efficiency. What allows it to be so much more efficient than a traditional Internal ...
5
votes
2answers
266 views

Expanding universe

If we know the universe is expanding in whatever direction we look, can't we reasonably estimate where the 'center' of the Universe is? Is the rate of expansions in all directions the same?
11
votes
2answers
901 views

How to model/simulate pressures and flows in a network of pipes

I'm having a hard time finding information on how to model/simulate this. I attached a couple files, both of which show an example tank & pump network. It's just nonsense that I made up for this ...
12
votes
2answers
1k views

Vasiliev gravity and “holographic” entanglement

It has been proposed that AdS/CFT arises because of the entanglement structure of quantum field theories, e.g. see the discussion which occurred right here. Until now I have been skeptical of the ...
2
votes
1answer
446 views

Do Hydrinos Exist? [duplicate]

Possible Duplicate: Have the Rowan University “hydrino” findings been replicated elsewhere? I came across this controversial company: Black Light Power I want to know if there ...
40
votes
5answers
4k views

Dimensionless Constants in Physics

Forgive me if this topic is too much in the realm of philosophy. John Baez has an interesting perspective on the relative importance of dimensionless constants, which he calls fundamental like alpha, ...
12
votes
1answer
435 views

An unfamiliar way of writing supersymmetry transformations

This question is in relation to this recent paper. I would like to know how the so called supersymmetry transformations at the start of page 27 or at the end of page35 (equation 8.4) or at the end ...
4
votes
2answers
220 views

Can a trapped surface be formed by a mass configuration outside of that trapped surface?

Can a trapped surface be formed without any massive bodies inside that trapped surface, but only by a configuration of massive bodies surrounding the trapped volume?
16
votes
2answers
822 views

If energy is only defined up to a constant, can we really claim that ground state energy has an absolute value?

Sorry if this is really naive, but we learned in Newtonian physics that the total energy of a system is only defined up to an additive constant, since you can always add a constant to the potential ...
2
votes
0answers
227 views

Did the LHC just find a Z' boson? [duplicate]

Possible Duplicate: Did the guys at Fermilab find a fifth force? Did the LHC just find a Z' boson, or is it just your typical media overblown sensationalism with no basis in fact?
9
votes
3answers
415 views

Are the basic postulates of QM the only set of postulates that can give rise to a sensible semi-probabilistic physical theory?

Are the basic postulates of QM, such as complex Hilbert space, unitary evolution, Hermitian operator observables, projection hypothesis etc., the unique and only set of postulates that gives rise to a ...
3
votes
1answer
817 views

Mathematical justification of Hartle-Hawking “no boundary” proposal

In Hartle-Hawking "no boundary" proposal it is proposed that Riemannian spacetimes rather than Lorentzian dominated the path integral near the big bang. Moments after the big bang however spacetimes ...
8
votes
1answer
1k views

water flow in a sink

When one turns on the tap in the kitchen, a circle is observable in the water flowing in the sink. The circle is the boundary between laminar and turbulent flow of the water (maybe this is the wrong ...
5
votes
5answers
392 views

Trapping a lightray

Given a solid whose interior is a hollow sphere with perfectly reflecting mirrors. A small hole is drilled in the sphere and a photon is sent in at some angle. Will it always eventually exit through ...
3
votes
2answers
371 views

If you are told an image is real and you know it was made by a single optic (either a lens or a mirror), which of the following must be true? [closed]

If you are told an image is real and you know it was made by a single optic (either a lens or a mirror), which of the following must be true? The image distance is positive. The image distance is ...
4
votes
2answers
272 views

Does throwing a watch into the air cause it to gain or lose time?

Suppose I'm on a non rotating planet. I have two identical, perfect watches. I synchronize them. Then I throw one of them into the air and catch it. Does the one I throw into the air gain or lose time ...
16
votes
2answers
896 views

How do horseshoe orbits work?

An asteroid was recently discovered that is in a horseshoe orbit with respect to the earth. Is there an intuitive explanation for these orbits? It seems that the earth acts as a repulsive force where ...
5
votes
1answer
10k views

How is formula for converting pressure from mmHg to Pa derived?

Today my younger brother asked me from where does the 1 Pa = 0.00750061683 mmHg formula for mercury barometer come. He needs a way to derive it, or an academic source which can be cited. After ...
3
votes
2answers
376 views

Alcohol inside a container

Let's imagine a plastic container completely closed. We put alcohol inside it filling it at half. Now we make a hole to introduce 2 little Conductor one near the other so that they don't let the air ...
-2
votes
1answer
328 views

projectile power with air resistance [closed]

I need to get the x/y values of a fired projectile given the angle of the initial power, and air resistance. The formulas I have are $$V_x=V_o \cos\theta$$ $$X=V_o \cos \theta t$$ $$V_y=V_o \sin ...
3
votes
2answers
374 views

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, ...
8
votes
3answers
694 views

Maximum time difference between clocks in a gravity field

From Surely you must be joking, Mr Feynman. You blast off in a rocket which has a clock on board, and there's a clock on the ground. The idea is that you have to be back when the clock on ...
7
votes
1answer
4k views

How does a digital radio tuner work?

I believe I understand how tuning a radio with an analog tuner works: turning the dial physically changes the length of the antenna, which determines which broadcast wavelength will resonate in the ...

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