-2
votes
3answers
869 views

Anti-laser: How sure we are that energy is transported?

Reading this PE question can-we-transport-energy-over-infinite-distances-through-vacuum-using-light, a related question arises naturally: Is energy transported (by light)? -- (I did believed in this ...
0
votes
1answer
713 views

Light years in terms of actual time space [closed]

What actually is 1 light year? What is the equivalent time in Earth time space?
7
votes
4answers
4k views

What is Fermi surface and why is this concept so useful in metals research?

What is Fermi surface and why is this concept so useful in metals research? Particularly, I can somewhat appreciate the Fermi energy idea - the radius of Fermi surface which is a sphere. But is there ...
4
votes
2answers
223 views

How do you derive Noether's theorem when the action combines chiral, antichiral, and full superspace?

How do you derive Noether's theorem when the action combines chiral, antichiral, and full superspace?
9
votes
6answers
822 views

A die versus a quantum experiment

Let suppose you roll a die, and it falls into a hidden place, for example under furniture. Then although the experiment has already been made (the die already has a number to show), that value can not ...
3
votes
2answers
1k views

Momentum-Representations in Quantum Mechanics

Why do we get information about position and momentum when we go to different representations. Why is momentum, which was related to time derivative of position in classical physics, now in QM just a ...
9
votes
1answer
372 views

Are there any applications of quantum information theory to physics?

Are there any applications of quantum information theory to physics?
5
votes
1answer
237 views

Natural systems that test the primality of a number?

There might be none. But I was thinking of links between number theory and physics, and this would seem like an example that would definitely solidify that link. Are there any known natural systems, ...
1
vote
3answers
652 views

Most elegant/fundamental formulations of the laws of classical mechanics?

Newton tried to do it with three laws/statements. While the first can be derived from the second, the three form a pretty nice framework. Later on, I've encountered Lagrangian Mechanics, which ...
-1
votes
1answer
513 views

The difference between free energy and perpetual motion [closed]

What is the difference between free energy (over unity) and perpetual motion? Please provide some examples, both real world and theoretical.
7
votes
3answers
904 views

Can we transport energy over infinite distances through vacuum using light?

I know that light (or electromagnetic radiation in general) attenuates in intensity as the square of the distance it travels. Why does it attenuate? Are the photons being scattered by the medium ...
3
votes
1answer
368 views

The full entropy quote

What is the full text (and possibly the source) of the summary of the 3 laws of thermodynamics that goes something along the lines of "Can't break even, can't win and can't even stop playing the ...
4
votes
2answers
404 views

Diff(M) and requirements on GR observables

This question is kind of inspired in this one: Diff(M) as a gauge group and local observables in theories with gravity The conundrum i'm trying to understand is how is derived the (quite) ...
6
votes
1answer
718 views

The Coriolis force bending a railway

Suppose a very long railway line goes from South Africa to Sweden, and then it's decided to move the entire railway line, sliding it 1 km to the north (leaving aside the difficulty of moving and ...
13
votes
7answers
1k views

What are the uses of Hopf algebras in physics?

Hopf algebra is nice object full of structure (a bialgebra with an antipode). To get some idea what it looks like, group itself is a Hopf algebra, considered over a field with one element ;) usual ...
1
vote
1answer
191 views

Gravitational effect of the distant universe

When we observe at close to the cosmological horizon (as close as we currently can) we see the universe as a much younger, denser place. As we feel the gravitational forces from these objects distant, ...
2
votes
0answers
303 views

Static gauge choice in string theory

In solving different problems in string theory and also in brane theory, we consider this static gauge choice and I think that's due to the local symmetry(Repara. Inv. and Weyl) that we have in our ...
3
votes
1answer
434 views

Questions on string theory vacuum and quantizing gravity

Actually I have few questions. The answer to a question related to string field theory helped me, but let me still ask it as its probably a bit different. When e.g we do bosonic/super string theory ...
11
votes
8answers
18k views

Electricity takes the path of least resistance?

Electricity takes the path of least resistance! Is this statement correct? If so, why is it the case? If there are two paths available, and one, for example, has a resistor, why would the ...
3
votes
2answers
327 views

Purchasing semiconductor substrates

I'd like to make a comparison of prices, availability and properties of semiconductor substrate layers like Si, Ge, GaP, AlN etc. I wonder what are the problems related to getting such data, what are ...
8
votes
1answer
1k views

Electric charge in string theory

The mass of an elementary particle in string theory is related with the way the string vibrates. The more frantically a string vibrates the more energy it posses and hence the more massive it is. My ...
2
votes
2answers
270 views

What mechanism in string theory enforces the consistency of self-couplings of massless vector bosons?

I have been reading the stackexchange questions on enhanced symmetries in string theory, the Leech lattice, monstrous moonshine, etc. , and I have a question to ask. An astute commentator pointed out ...
3
votes
2answers
536 views

Supersymmetry algebra

Let the $i=x,y,z$ components of the angular momentum have the commutation relations with the supersymmetry generators (also called supercharges?) $Q_a$ ($a = \pm \frac{1}{2}$) as, $[J_i , Q_a] = ...
1
vote
2answers
1k views

How can I tell which end of a wire will have a higher potential?

I have the following setup: A <---- wire ----> B $V_b - V_a = \Delta v = \text{a positive value}$ I have two questions: Which end of the wire has a ...
13
votes
1answer
1k views

Graphene and Klein bottle?

I am trying to understand graphene as a topological insulator. The spin orbital interaction in graphene is very small (~10mK?). But if we consider that, then graphene should be a topological ...
5
votes
2answers
910 views

How the Lagrangian of classical system can be derived from basic assumptions?

It is well known that the Lagrangian of a classical free particle equal to kinetic energy. This statement can be derived from some basic assumptions about the symmetries of the space-time. Is there ...
4
votes
3answers
2k views

How is it possible for orbits to maintain stability?

According to $a = v^2/R$, the circular velocity and radial distance between two attracting objects (such as planets), must remain in perfect proportion in order for orbital motion to take place. How ...
1
vote
2answers
387 views

What is an “inclined magnetic field”?

What is meant by an "inclined magnetic field"? How is it different from the usual magnetic field?
3
votes
1answer
217 views

Free/open source tools for calculating normal modes, failing that, advice

Are there any open source tools for calculating the normal modes of arbitrary objects? Input would be some 3D design format and material properties, output would be movement direction and magnitude ...
1
vote
0answers
368 views

Skin effect and currents

Here in this picture you can see I_W which is induced by H. But why I_W is not vice versa? Because of $$rot \, \vec B = \mu_0 \, \left( \varepsilon_0 \frac{\partial E}{\partial t} + \vec j ...
2
votes
3answers
638 views

Light bending by black holes

In the center of our milky way, it is assumed that a black hole exists with a mass of $\approx 4\times 10^6$ times our sun's mass. How much light bending (in degrees) would arise for stars that are in ...
11
votes
3answers
1k views

What happens when a black hole and an “anti-black-hole” collide?

Let's say we have one black hole that formed through the collapse of hydrogen gas and another that formed through the collapse of anti-hydrogen gas. What happens when they collide? Do they (1) ...
11
votes
7answers
2k views

References about rigorous thermodynamics

Can you suggest some references for rigorous treatment of thermodynamics? I want things like reversibility, equilibrium to be clearly defined in terms of the basic assumptions of the framework.
8
votes
5answers
621 views

Dark matter and dark energy references

I've been looking for questions about dark matter, and I've read some very interesting answers. However, I desire too look into it deeply. This is not actually a question. I'm asking the community ...
3
votes
2answers
1k views

Dynamics of moment of inertia

I'd like to be able to determine the angular acceleration of a system of two rotating masses, which are connected so as to have a variable mechanical advantage between the two. My background with ...
3
votes
3answers
208 views

Object docking in Space

How do objects dock in space? There's a recent piece of news that Keppler would dock to the ISS sometime this week. I want to know the operational aspects of it - considering the velocity of the ...
5
votes
4answers
653 views

What does it mean, when one says that system has N constants of motion?

For example for an isolated system the energy $E$ is conserved. But then any function of energy, (like $E^2,\sin E,\frac{ln|E|}{E^{42}}$ e.t.c.) is conserved too. Therefore one can make up infinitely ...
3
votes
1answer
597 views

feynman diagram - direction of arrows

http://upload.wikimedia.org/wikipedia/commons/1/1f/Feynmann_Diagram_Gluon_Radiation.svg Why does the arrow of the positron doesn't show upwards?
11
votes
3answers
844 views

Spinning Tachyons

In all examples that I know, tachyons are described by scalar fields. I was wondering why you can't have a tachyon with spin 1. If this spinning tachyon were to condense to a vacuum, the vacuum ...
2
votes
3answers
408 views

Consistent theory of continuum

Why is there a consistent theory of continuum mechanics in which one just consider things like differential elements and apply Newtons laws? Is there a deeper reason for it. Is it the nature of ...
6
votes
2answers
1k views

A pendulum clock problem

Below is a picture of a simple pendulum clock. Suppose that the bob (a rigid disk) on the end of the pendulum can spin without friction about its geometrical axis and is spinning at an angular ...
3
votes
3answers
449 views

Do extra-dimensional theories like ADD or Randall-Sundrum require string theory to be true?

What I mean is could it turn out that the world is not described by string theory / M-Theory, but that nevertheless some version of one of these extra-dimensional theories is true? I have no real ...
5
votes
1answer
1k views

Why does lambda decay violate parity?

When a lambda particle decays into proton and a pion, I am told it does not conserve parity. Why?
5
votes
4answers
447 views

What is the medium that allows magnetic fields *or any forcefield* to exist?

Magnetic fields are obvious distortions.. of.. something, but what exactly are they distortions of? Massive objects produce curvatures/gradients in space-time resulting in what we observe as ...
10
votes
9answers
12k views

What's the core difference between the electric and magnetic forces?

I require only a simple answer. One sentence is enough... (It's for high school physics)
8
votes
3answers
1k views

Is there a nice tool to plot graphs of paper citations? [closed]

I would like a tool which allows me to enter some paper citation, and then will begin drawing a graph, where each paper is linked to other papers that cite the original paper or are cited by it. It ...
9
votes
2answers
773 views

Quantum shot-noise and the fluctuation dissipation theorem

Classically, shot noise observed in the signal generated by a laser incident on a photodiode is explained as being due to the quantization of light into photons, giving rise to a Poisson process. In ...
0
votes
1answer
383 views

what does 'same energy as a ton of TNT' mean?

Very often the energy released in large-scale events (e.g., earthquakes, meteor impacts, etc) is described in terms of some number of 'tons of TNT'. This may be all well and good for those with an ...
3
votes
1answer
183 views

What keeps the sugar suspended in the tea?

At room temperature.... How long will the sugar stay suspended once dissolved ? What governs the rate of settling ? What part does Brownian motion play ? Sugar might be a bad example...please ...
-2
votes
3answers
2k views

Best way to keep food cool indoors without using a fridge/freezer

I've just moved into my first place, and for reasons I do not condone or recommend I don't have any method of refrigeration right now. Luckily I now live very close to shops, so it isn't a big deal, ...

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