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1500 questions
38
votes
6 answers
Must observables be Hermitian only because we want real eigenvalues, or is more to that?
Because (after long university absence) I recently came across field operators again in my QFT lectures (which are not necessarily Hermitian):
What problem is there with observables represented by non-Hermitian operators (by observables, I obviously…
Quantumwhisp
- 7,095
38
votes
4 answers
Did LIGO measurements prove that the speed of gravity equals the speed of light?
This question about the speed of light prompted my own question. In the linked question it is asked if there is experimental proof that the speed of gravity equals the speed of light. I was surprised not to see the LIGO measurements mentioned.
The…
Deschele Schilder
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38
votes
8 answers
When snow falls, temperature rises. Is this due to entropy?
A friend of mine told me that temperature rises when snow falls. And this is because condensation of water in snowflakes reduces entropy and the temperature of the air rises to compensate for this.
Is this explanation correct?
38
votes
7 answers
Does spin really have no classical analogue?
It is often stated that the property of spin is purely quantum mechanical and that there is no classical analog. To my mind, I would assume that this means that the classical $\hbar\rightarrow 0$ limit vanishes for any spin-observable.
However, I…
Akoben
- 2,549
38
votes
5 answers
Why do tidal waves appear so suddenly in some rivers?
As tide approaches in lower part of some rivers (e.g. Ganges), a several feet high tidal wave enters from the sea against the flow of the river (making a great noise), and the water level suddenly rises as the wave proceeds.
Here is a youtube video…
Archisman Panigrahi
- 3,350
38
votes
4 answers
Why are steel thermal mugs much better than plastic ones?
I have several thermal mugs, two of them by the same brand, have the same look, shape and size except that one is in steel (inside and outside) and the other one, plastic. Both have an insulating layer of air or vacuum.
I do not need a thermometer…
38
votes
1 answer
Emergent symmetries
As we know, spontaneous symmetry breaking(SSB) is a very important concept in physics. Loosely speaking, zero temprature SSB says that the Hamiltonian of a quantum system has some symmetry, but the ground state breaks the symmetry.
But what about…
Kai Li
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38
votes
2 answers
What is (meant by) a non-compact $U(1)$ Lie group?
In John Preskill's review of monopoles he states on p. 471
Nowadays, we have another way of understanding why electric charge is quantized. Charge is quantized if the electromagnetic $U(l)_{\rm em}$ gauge group is compact. But $U(l)_{\rm em}$ is…
twistor59
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38
votes
1 answer
Is there any evidence that dark matter interacts with ordinary matter non-gravitationally?
As far as I understand it, dark matter direct detection experiments are based on the idea that dark matter interacts with ordinary matter, it just has a very small cross-section. So far, there's been no confirmed detection.
Is there any evidence at…
Allure
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38
votes
4 answers
Why are solar panels kept tilted?
I have noticed that, in my country India, most of the solar panels are tilted southward at an angle of ${45}^{\circ} .$ Even on buildings with inverted V-shaped roofs, solar panels are still oriented southward on both the sides of…
Chemist
- 591
38
votes
9 answers
How does an isolated body in deep space 'know' it's rotating?
We can imagine an object floating in the known universe, maximally distant from any other large mass. Maybe it has been there since coalescing after the big bang.
What physical phenomena tell it whether it is rotating relative to the rest of the…
chasly - supports Monica
- 3,026
38
votes
4 answers
Can a Foucault pendulum really prove Earth is rotating?
According to this article, a Foucault pendulum proved Earth was rotating. I'm not sure it really proved it.
If Earth weren't rotating and a Foucault pendulum started in a state with zero velocity, it would keep swinging back and forth along the same…
Timothy
- 1,708
38
votes
4 answers
What is the difference between a spinor and a vector or a tensor?
Why do we call a 1/2 spin particle satisfying the Dirac equation a spinor, and not a vector or a tensor?
Fahim Sharif
- 557
38
votes
3 answers
How exactly is the formalism of thermodynamics based on contact geometry?
There is a famous quote by the mathematician V. I. Arnold that goes like this:
Every mathematician knows it is impossible to understand an elementary
course in thermodynamics.
The source is Contact Geometry: the Geometrical Method to Gibbs'…
valerio
- 16,751
38
votes
7 answers
Can you be blinded by a 'dim' light?
From what I can tell, if you pick a color near the extreme of the visible light spectrum, let's say red, and trace a path across the spectrum until you are outside of the visible range, at some point the red color will begin to darken and dim until…
JakeStrang
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