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1500 questions
37
votes
2 answers
How can wifi penetrate through walls when visible light can't?
I did search the question on Physics S.E considering it would be previously asked. I found this
How come Wifi signals can go through walls, and bodies, but kitchen-microwaves only penetrate a few centimeters through absorbing surfaces?
But in this…
Shashaank
- 2,767
37
votes
7 answers
Do Maxwell's equation describe a single photon or an infinite number of photons?
The paper Gloge, Marcuse 1969: Formal Quantum Theory of Light Rays starts with the sentence
Maxwell's theory can be considered as the quantum theory of a single
photon and geometrical optics as the classical mechanics of this
photon.
That…
asmaier
- 9,794
37
votes
2 answers
Before a once-warm lake starts to freeze, must its temperature be 4°C throughout at some point?
This is a problem I just started puzzling over, and I felt this would be a good forum to check my reasoning. So here are the relevant observations followed by my question:
Water achieves its maximum density at roughly 4°C. That is, water (including…
Excellll
- 477
37
votes
6 answers
Can a balloon be inflated with electrons?
If I use a Van De Graff generator to pump electrons into a deflated balloon, eventually negative charge will start to build up inside the balloon. Assume the mouth of the balloon is sealed so air cannot enter or exit the balloon. The electrons will…
Andrew
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37
votes
8 answers
Are Newton's 1st and 3rd laws just consequences of the 2nd?
Can Newton's 1st and 3rd laws be assumed given just $F=ma$. I know that the argument would be, "No, then there would only be 1 law". But I can't think of any situation where 1 and 3 aren't superfluous.
If you just told me $F=ma$:
I would assume…
BoddTaxter
- 2,858
37
votes
1 answer
State of Matrix Product States
What is a good summary of the results about the correspondence between matrix product states (MPS) or projected entangled pair states (PEPS) and the ground states of local Hamiltonians? Specifically, what "if and only if" type of…
Kaveh_kh
- 543
37
votes
1 answer
Instantons, anomalies, and 1-loop effects
A symmetry is anomalous when the path-integral measure does not respect it. One way this manifests itself is in the inability to regularize certain diagrams containing fermion loops in a way compatible with the symmetry. Specifically, it seems…
user6013
- 913
37
votes
3 answers
Can two heavy objects circling around their C.M. be separated because of the speed of gravity?
Imagine two massive objects, with the same mass (M) circling around their center of mass (C.M.). Let's assume that the distance between them is 1 light hour. Don´t the two bodies get accelerated and move away from each other because they feel the…
37
votes
7 answers
Do gases have phonons?
A phonon is a quantized unit of sound; they are encountered when quantizing lattice vibrations in solids. Now, even an ideal gas supports sound waves, but in this case, interactions between atoms are weak. That makes it hard to imagine what a…
knzhou
- 101,976
37
votes
2 answers
Were the Michelson-Morley results a surprise?
How unexpected were the Michelson-Morley experiment results?
Did physicists have theoretical reasons to predict that the speed of light would result to be invariant?
Marco Disce
- 673
37
votes
4 answers
Give a description of Loop Quantum Gravity your grandmother could understand
Of course, assuming your grandmother is not a theoretical physicist.
I'd like to hear the basics concepts that make LQG tick and the way it relates to the GR. I heard about spin-networks where one assigns Lie groups representations to the edges and…
Marek
- 23,530
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37
votes
4 answers
Can virtual particles be thought of as off-shell Fourier components of a field?
I just found this blog post, which gives an interpretation of virtual particles I haven't seen before.
Consider a 1D system of springs and masses, where the springs are slightly nonlinear. A "real particle" is a regular $\cos(kx-\omega t)$…
knzhou
- 101,976
37
votes
2 answers
How does string theory reduce to the standard model?
It is said that string theory is a unification of particle physics and gravitation.
Is there a reasonably simple explanation for how the standard model arises as a limit of string theory?
How does string theory account for the observed particle…
Arnold Neumaier
- 44,880
37
votes
4 answers
Scattering, Perturbation and asymptotic states in LSZ reduction formula
I was following Schwarz's book on quantum field theory. There he defines the asymptotic momentum eigenstates $|i\rangle\equiv |k_1 k_2\rangle$ and $|f\rangle\equiv |k_3 k_4\rangle$ in the S-matrix element $\langle f|S|i\rangle$ as the eigenstates of…
SRS
- 26,333
37
votes
2 answers
How does one correctly interpret the behavior of the heat capacity of a charged black hole?
Note: Although I have a provided an "answer" to the question, I did not resolve all the questions in this post satisfactorily. I invite anyone willing and able to provide a better answer, which I would be glad to accept instead.
Consider the…
Danu
- 16,302