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1500 questions
133
votes
9 answers
What, in simplest terms, is gauge invariance?
I am a mathematics student with a hobby interest in physics. This means that I've taken graduate courses in quantum dynamics and general relativity without the bulk of undergraduate physics courses and sheer volume of education into the physical…
Arthur
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132
votes
4 answers
Why can I touch aluminum foil in the oven and not get burned?
I cook frequently with aluminum foil as a cover in the oven. When it's time to remove the foil and cook uncovered, I find I can handle it with my bare hands, and it's barely warm.
What are the physics for this? Does it have something to do with…
Jason P Sallinger
- 1,315
132
votes
3 answers
Differentiating Propagator, Green's function, Correlation function, etc
For the following quantities respectively, could someone write down the common definitions, their meaning, the field of study in which one would typically find these under their actual name, and most foremost the associated abuse of language as well…
Nikolaj-K
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130
votes
11 answers
Why does Newton's Third Law actually work?
My father explained to me how rockets work and he told me that Newton's Third Law of motion worked here. I asked him why it works and he didn't answer. I have wasted over a week thinking about this problem and now I am giving up.
Can anyone explain…
user164594
130
votes
6 answers
What is known about the topological structure of spacetime?
General relativity says that spacetime is a Lorentzian 4-manifold $M$ whose metric satisfies Einstein's field equations. I have two questions:
What topological restrictions do Einstein's equations put on the manifold? For instance, the existence…
Eric
- 1,804
128
votes
7 answers
Why does the humidifier make a stove's flame orange?
Just like this guy's, the color of my stove's flames were affected by the humidifier as well.
Why does this happen?
Is it a good thing or a bad thing ?
Ilya Gazman
- 2,177
128
votes
13 answers
Can Maxwell's equations be derived from Coulomb's Law and Special Relativity?
As an exercise I sat down and derived the magnetic field produced by moving charges for a few contrived situations. I started out with Coulomb's Law and Special Relativity. For example, I derived the magnetic field produced by a current $I$ in an…
user1247
- 7,509
128
votes
5 answers
Why do wet objects become darker?
When something gets wet, it usually appears darker. This can be observed with soil, sand, cloth, paper, concrete, bricks...
What is the reason for this? How does water soaking into the material change its optical properties?
Suma
- 1,432
127
votes
10 answers
How is a quantum superposition different from a mixed state?
According to Wikipedia, if a system has $50\%$ chance to be in state $\left|\psi_1\right>$ and $50\%$ to be in state $\left|\psi_2\right>$, then this is a mixed state.
Now, consider the state…
Ruslan
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127
votes
10 answers
Why do spaceships heat up when entering earth but not when exiting?
Recently I read up on spacecrafts entering earth using a heat shield. However, when exiting the Earth's atmosphere, it does not heat up, so it does not need a heat shield at that point of time yet. Why is this so? I know then when entering earth,…
QuIcKmAtHs
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127
votes
4 answers
The Role of Rigor
The purpose of this question is to ask about the role of mathematical rigor in physics. In order to formulate a question that can be answered, and not just discussed, I divided this large issue into five specific questions.
Update February, 12,…
Gil Kalai
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126
votes
8 answers
Is the universe fundamentally deterministic?
I'm not sure if this is the right place to ask this question. I realise that this maybe a borderline philosophical question at this point in time, therefore feel free to close this question if you think that this is a duplicate or inappropriate for…
MRashid
- 1,348
126
votes
14 answers
QM without complex numbers
I am trying to understand how complex numbers made their way into QM. Can we have a theory of the same physics without complex numbers? If so, is the theory using complex numbers easier?
Zeus
- 3,543
125
votes
10 answers
Why is quantum entanglement considered to be an active link between particles?
From everything I've read about quantum mechanics and quantum entanglement phenomena, it's not obvious to me why quantum entanglement is considered to be an active link. That is, it's stated every time that measurement of one particle affects the…
Andrey Tatarinov
- 1,431
125
votes
16 answers
Is it possible for information to be transmitted faster than light by using a rigid pole?
Is it possible for information (like 1 and 0s) to be transmitted faster than light?
For instance, take a rigid pole of several AU in length. Now say you have a person on each end, and one of them starts pulling and pushing on his/her end.
The…
Jonathan.
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