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1500 questions
35
votes
2 answers
What is the Kosterlitz -Thouless transition?
I couldn't find any simple texts explaining the Kosterlitz-Thouless transition. More specifically can someone explain the role of vortices in the transition.
edit: links explaining the transition in a simple manner are also appreciated. Also the…
MomoTheSir
- 511
35
votes
4 answers
Why does the moon sometimes appear giant and a orange red color near the horizon?
I've read various ideas about why the moon looks larger on the horizon. The most reasonable one in my opinion is that it is due to how our brain calculates (perceives) distance, with objects high above the horizon being generally further away than…
logicbird
35
votes
7 answers
Is there something behind non-commuting observables?
Consider a quantum system described by the Hilbert space $\mathcal{H}$ and consider $A,B\in \mathcal{L}(\mathcal{H},\mathcal{H})$ to be observables. If those observables do not commute there's no simultaneous basis of eigenvectors of each of them.…
Gold
- 35,872
35
votes
3 answers
Deriving the Lagrangian for a free particle
I'm a newbie in physics. Sorry, if the following questions are dumb. I began reading "Mechanics" by Landau and Lifshitz recently and hit a few roadblocks right away.
Proving that a free particle moves with a constant velocity in an inertial frame…
Someone
- 453
35
votes
5 answers
How am I able to stand up and walk down the aisle of a flying passenger jet?
The energy of a moving object is $E = mv^2\;.$ That is it increases with velocity squared.
I walk at say 3 miles per hour, or lets round that down to 1 meter per second for a slow walk. I weigh less than $100~\mathrm{kg}\;,$ but lets just round that…
user2800708
- 942
35
votes
5 answers
Do free-electron lasers actually lase?
Free-electron lasers are devices which use the motion of highly energetic electron beams to produce bright, coherent radiation in the x-ray regime. More specifically, they start with a high-energy electron beam and feed it into an undulator, which…
Emilio Pisanty
- 132,859
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- 666
35
votes
3 answers
Free electron can't absorb a photon
Why can't a free electron absorb a photon? But a one attached to an atom can.. Can you explain to me logically and by easy equations? Thank you..
user65035
35
votes
4 answers
What is the energy source if a tall water tank is used to transfer floating objects upwards instead of cables with motors?
Suppose I want to transport some logs from the ground to the roof of a tower. Originally I can use a lift, or some cables, or even move the logs upwards manually; then the energy is converted to the potential energy of logs.
Now, if I build a tall…
Gstestso
- 935
35
votes
2 answers
Should a superconductor act as a perfect mirror?
I have been told that metals are good reflectors because they are good conductors. Since an electric field in conductors cause the electrons to move until they cancel out the field, there really can't be electric fields in a conductor.
Since light…
user42012
- 902
35
votes
3 answers
Push a Boeing 737 on ice: Did this guy have a chance?
In this video an airport worker (in blue) tries to prevent a Boeing 737 from sliding on ice in heavy winds:
Did he even have a chance?
On one side of the argument, the airplane is sliding due to the force of the wind being stronger than the force…
dotancohen
- 4,545
35
votes
4 answers
How is it possible that it can get hotter in the car than it is outside?
The Law of Thermodynamics says that two bodies will eventually have equal temperatures. How is it possible that when you leave your car in the sun, it gets hotter in the car than it is outside? Why isn’t the car at the same temperature as the…
bodacydo
- 1,313
35
votes
4 answers
Irreducible tensors concept
This might be a little naive question, but I am having difficulty grasping the concept of irreducible tensors. Particularly, why do we decompose tensors into symmetric and anti-symmetric parts? I have not found a justification for this in my…
yayu
- 4,822
35
votes
3 answers
How does electricity propagate in a conductor?
On a systems level, I understand that as electrons are pushed into a wire, there is a net field and a net electron velocity. And I've read that the net electron drift is slow. But electricity travels through the wire, essentially at c, and I want…
Dov
- 1,035
35
votes
4 answers
Why does moving through time not require energy?
Moving through the other three dimensions necessitates energy. But why doesn't moving through time necessitate energy?
A.R.K
- 1,423
35
votes
5 answers
What is the physical meaning of the affine parameter for null geodesic?
For time-like geodesic, the affine parameter is the proper time $\tau$ or its linear transform, and the geodesic equation is
$$\frac{\mathrm d^{2}x^{\mu}}{\mathrm d\tau^{2}}+\Gamma_{\rho\sigma}^{\mu}\frac{\mathrm dx^{\rho}}{\mathrm…
Siyuan Ren
- 4,932