8
votes
2answers
617 views

Do gravitational waves slow down as they pass through matter?

I've heard that gravitational waves travel at the speed of light, and have some parallels to electromagnetic waves. EM waves slow down as they pass through matter (speed of light in glass is slower ...
0
votes
1answer
330 views

Is spherical coordinates enough to calculate the exact position of a point with rotations included?

With "Geographic coordinate system" we only get the location, no rotations or anything. Translated into latitude and longitude. Where with "Spherical coordinate system", we should be able to get the ...
2
votes
1answer
168 views

Violation of Lorentz symmetry on cosmological distances

This question is about the domain of validity of Lorentz symmetry. As far as I know, general relativity is a generalization of special relativity. Does that mean that Lorentz symmetry is violated on ...
3
votes
1answer
1k views

Escape velocity of a rocket standing on Ganymede (Moon of Jupiter)

I want to calculate the escape velocity of a rocket, standing on the surface of Ganymede (moon of Jupiter) and trying to leave Ganymede. My thinking was, the kinetic energy $E_{\text{KIN}}$ must be ...
8
votes
1answer
314 views

“finite” QFTs and short-distance singularities and vanishing beta functions

I am not sure that I can frame this question coherently enough - it springs from various things in QFT that I have recently been thinking and reading about. May be these thoughts are mis-directed but ...
7
votes
4answers
1k views

What is the meaning of pressure of a gas

I understand the meaning of pressure on a surface to be force acting per unit area. But when it comes to understanding pressure in the context of fluids in motion or at rest I think I am having some ...
4
votes
2answers
838 views

Does a nonzero Poynting vector mean that there is propagation of energy?

I don't know how this "paradox" can be solved. I'm given the following system: A permanent magnet with a magnetic field given by ($\hat{a}$ are unit vectors in the x and y directions) ...
4
votes
1answer
66 views

What's the potential of the LHC's heavy ion experiment?

RHIC has been the dominant player in heavy ion physics, producing tantalizing evidence in support of the entropy/viscocity formula from AdS/CFT. What's the potential of the LHC's Pb ion collsions? ...
44
votes
0answers
1k views

Experimental test of the non-statisticality theorem?

Context: The recent paper The quantum state cannot be interpreted statistically by Pusey, Barrett and Rudolph (now On the reality of the quantum state, Nature Physics 8, 475–478 (2012), ...
4
votes
1answer
3k views

superconductor levitating in earth's magnetic field? [duplicate]

Possible Duplicate: Can superconducting magnets fly (or repel the earth's core)? I've seen superconductors levitating on magnets. But is it possible for superconductors to levitate on ...
4
votes
2answers
136 views

Decoherence inside black holes [closed]

I have a question about decoherence. Assume there is a macroscopic black hole floating around and you have some macroscopic object with you with a huge number of internal degrees of freedom. ...
2
votes
2answers
179 views

Inertial Mass of a scalar field

Does it make sense to talk of the inertial mass of a scalar field? By the equivalence principle, it must be equal to its gravitational mass. We know that the scalar field contributes towards the ...
5
votes
1answer
82 views

Shadow of a Jovian moon over the Great Red Spot

Where can I find pictures of the shadow of any of the Jovian moons partially covering the Great Red Spot? A series of such pictures over time would even be better. The idea is to learn more about the ...
1
vote
2answers
136 views

Equivalence between Differential Geometry and Mechanics?

Given a metric $$ ds^{2}~=~ g_{a,b}dx^{a}dx^{b}. $$ Here Einstein's summation convention is assumed for $a$ and $b$. Then given the Laplacian over that metric, can then we find a metric $ ...
0
votes
1answer
434 views

Phase shift for Scattering in radial potentials

given a radial potential in 3 dimension and its Schroedinguer equation $ -D^{2}U(r) + \frac{l(l+1)}{r^{2}}+V(r) $ here D means derivative with respect to 'r' then if we apply quantum scattering how ...
0
votes
1answer
784 views

The shear stress assumption in Navier Stokes

Viscous fluid mechanics starts with the assumption that shear stress is linearly related to velocity by $$\tau = \mu\frac{du}{dy},$$ but it later turns out that some forces in high speed situations go ...
1
vote
1answer
678 views

Time traveling and Time dilation within a multiplayer videogame [closed]

First thing first, I'm looking to implement this within a multiplayer videogame as a "special" skill or powerup, but, I'm willing to build it upon a physics based explanation/theory. The question may ...
6
votes
1answer
632 views

Explanation of Cardy's “a theorem”

There seems to have been some discussion of Cardy's "a-theorem" recently: “It is shown that, for d even, the one-point function of the trace of the stress tensor on the sphere, Sd, when suitably ...
1
vote
2answers
1k views

What is the physical meaning of a product of vectors?

My teacher told me that Vectors are quantities that behave like Displacements. Seen this way, the triangle law of vector addition simply means that to reach point C from point A, going from A to B ...
1
vote
0answers
33 views

GUT predictions for charm mass?

Most GUT models have some relationship between down-type quarks and leptons, that more or less agree with the observed values after running the renormalization equations. But, what about up-type ...
19
votes
7answers
15k views

Why is the charge naming convention wrong?

I recently came to know about the Conventional Current vs. Electron Flow issue. Doing some search I found that the reason for this is that Benjamin Franklin made a mistake when naming positive and ...
-1
votes
2answers
240 views

Fuel Tank Question

Could some one please help me with this question as my physics knowledge does not extend this far. I have a large fuel tank with a capacity of 21000L. The tank currently is holding 2000L of fuel. On ...
3
votes
8answers
700 views

Does the wave function/density state actually exist?

I have been reading with interest the debates here on whether the wave function/density state actually collapses or not, or whether it is subjective Bayesian or objective with actual complex numbered ...
0
votes
2answers
419 views

Does gravitational differences affect the distance light travels? (a thought experiment)

My question is: How is light passing through different gravitational densities affected? The value of "c" is constant in a vacuum. I'm curious about if various time frames have any effect. This is ...
13
votes
3answers
900 views

Does a Weak Energy Condition Violation Typically Lead to Causality Violation?

In the answer to this question: ergosphere treadmills Lubos Motl suggested a straightforward argument, based on the special theory of relativity, to argue that light passing through a strong ...
-1
votes
1answer
771 views

How do I express the Kepler general orbit $r(\phi)$ in rectangular coordinates?

How do I express the Kepler general orbit $r(\phi)$ in rectangular coordinates? I use the identities $x=r\cos\phi$, $y=r\sin\phi$, and $r^2 = x^2 + y^2$, but I block at some point.
1
vote
1answer
681 views

AdS space - Poincare Patch

How can I work out in detail the explicit coordinate transformation formulas needed to go from the "canonical" coordinates to the "Poincare patch"? I'm reading about AdS but the text takes the ...
3
votes
1answer
130 views

The fine structure non-constant

If the fine structure constant is different in different parts of the universe, then what would happen if we travelled to those regions? (I realise this is completely impossible as they are ...
1
vote
1answer
291 views

Why *should* the mass of elementary particles theoretically be of the magnitude of the Planck mass?

Why should the mass of elementary particles be theoretically of the magnitude of the Planck mass? I've read that already a few times but I don't understand why it should be that way. For example: ...
25
votes
4answers
1k views

“Slightly off-shell”?

I'm not new to QFT, yet there are some matters which are quite puzzling to me. I often come across the statement that real particles (the ones we actually measure in experiments, not virtual ones) are ...
4
votes
1answer
566 views

Is there something like the Poynting vector for hydraulic circuits?

The Poynting vector is a representation of the energy flux in electromagnetics, showing the amount and direction of power flow at different points in space. In electric circuits, the energy is not ...
6
votes
3answers
14k views

Why is gravitational potential energy negative, and what does that mean?

I usually think of gravitational potential energy as representing just what it sounds like: the energy that we could potentially gain, using gravity. However, the equation for it (derived by ...
2
votes
3answers
20 views

Does anybody actually do astrometry in the 21st century?

I mean the nitty-gritty measurements of positions of stars and planets using transit circles. The transit instruments I've seen are mostly in national observatories and are essentially museum pieces: ...
2
votes
3answers
1k views

Why do some satellites fall to Earth?

In another question How does Newtonian mechanics explain why orbiting objects do not fall to the object they are orbiting?, one can read an affirmative answer. They how do you explain satellites ...
12
votes
5answers
27k views

Is the spring constant k changed when you divide a spring into parts?

I've always been taught that the spring constant $k$ is a constant — that is, for a given spring, $k$ will always be the same, regardless of what you do to the spring. My friend's physics ...
5
votes
2answers
266 views

Are there irreducible tensors of half integral degree in quantum mechanics?

According to Ballentine, an irreducible tensor of degree k can be defined as a set of $2k + 1$ operators $\{T_q^{\;\;(k)}:(-k \le q \le k)\}$ satisfying the following commutation relations: $$ ...
3
votes
1answer
181 views

Distinguishing two quantum states practically

Suppose we have two states $$|x\rangle = 1 |0\rangle + 0 |1\rangle$$ and $$|y\rangle = \sqrt{1-\epsilon^2} |0> + \epsilon |1>$$ where say $\epsilon = 10^{-20}$ Can we distinguish ...
3
votes
2answers
2k views

What is the physical meaning of the terms in the multipole expansion?

I have a few questions on multipole expansions and I have read about the topic in many places but could not find an answer to my questions, so please be patient with me. The electrostatic potential ...
6
votes
0answers
263 views

Partition Functions in (A)dS/CFT

I'm trying to understand some aspects of dS/CFT, and I'm running into a little trouble. Any help would be much appreciated. In arXix:1104.2621, Harlow and Stanford showed that the late-time ...
2
votes
2answers
233 views

Relativistic transformation of the wave packet length

Let us suppose we have an excited atom at rest. It has a certain mean lifetime $\tau_0$. If we wait sufficiently long time $t>>\tau_0$, we will find a deactivated atom and a (spherical) ...
9
votes
7answers
990 views

Time, what is it? [closed]

If you ask any person about time, she/he will give you some answer. I suspect that it is extremely difficult, (if not impossible) to define time. Is there a definition of what it is in physics? Is it ...
0
votes
1answer
100 views

A possible absolute reference system

What about considering the microwave background radiation (2.7K if I remember well) as a reference system with some absolute character? Please explains if this question make sense and possible ...
13
votes
2answers
367 views

Possible research implications of proof of John Cardy's a-theorem in QFT

According to this recent article in Nature magazine, John Cardy's a-theorem may have found a proof. Question: What would the possible implications be in relation to further research in QFT? ...
4
votes
1answer
138 views

A ball inside a cavity

Seems not a trivial problem: There is a semi-cylindrical cavity, with radius $R$ as shown in the Figure. A small ball (point mass for simplicity) with an initial horizontal velocity $v$ flies into ...
3
votes
1answer
31k views

How does force relate to velocity

I had originally asked this question on math overflow and it was suggested that I ask it here. So I know that a force will change the magnitude of velocity if it is at an angle other that 90 degrees. ...
2
votes
2answers
121 views

Best time of the year to see the star Alpha Centauri in Chile, Santiago

If I'm in Chile, Santiago, what is the best time of the year to see the star Alpha Centauri at the beginning of the night?
2
votes
0answers
133 views

What is new with quark-Gluon Plasma and the bubble formed at RHIC?

RHIC experiment results of forming Quark-Gluon Plasma and a possible bubble was news few months ago. I cannot find any follow up news after that. Does anybody know anything new in this front? I am ...
0
votes
1answer
87 views

why do some orbiting objects have a changing position?

I know that the reason why they stay in orbit but why do some move away from the earth(the moon) or come closer and eventually fall? And why does the moon move away from earth?
0
votes
1answer
163 views

why do all objects of varying mass acelerate the same? [duplicate]

Possible Duplicate: Confused about the role of mass why is it that two object of varying mass will fall at the same speed in a frictionless enviorment like the moon? Is it because the ...
5
votes
2answers
634 views

How can we be sure that nature isn't “faking” quantum statistics?

In a recent publication, Experimentally Faking the Violation of Bell’s Inequalities (Gerhardt 2011) (arXiv version), the statistics of quantum mechanics is faked using classical light sources. But if ...

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