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1500 questions
43
votes
2 answers
How was the first atomic clock calibrated?
As we all know, atomic clocks are being used to measure time and the GPS system.
But I was wondering based on what was the first atomic clock calibrated and how accurate this calibration was based on our standards nowadays?
MichaelScott
- 696
43
votes
5 answers
Difference between electric field $\mathbf E$ and electric displacement field $\mathbf D$
$$\mathbf D = \varepsilon \mathbf E$$
I don't understand the difference between $\mathbf D$ and $\mathbf E$.
When I have a plate capacitor, a different medium inside will change $\mathbf D$, right?
$\mathbf E$ is only dependent from the charges…
kame
- 919
43
votes
8 answers
Where to draw the line between quantum mechanics theory and its interpretation(s)?
I've completed a full year QM course (undergraduate level) and I am left confused on where to draw the line between quantum mechanics theory and its interpretation(s). I would personally like to stick to no interpretation at all, but since I do not…
43
votes
3 answers
Why isn't the circumferential light around the M87 black hole's event horizon symmetric?
After the revelation of the first black hole images, it seems there is a bias towards the south side. Is it because of measuring it from earth or is it something more fundamental in the understanding of gravitation?
0x90
- 3,316
43
votes
6 answers
Why are so many forces explainable using inverse squares when space is three dimensional?
It seems paradoxical that the strength of so many phenomena (Newtonian gravity, Coulomb force) are calculable by the inverse square of distance.
However, since volume is determined by three dimensions and presumably these phenomena have to travel…
Ryan David Ward
- 544
43
votes
7 answers
Why does the vacuum even have permeability and permittivity?
The vacuum is empty, yet it seems to have 2 properties: it's absolute permeability and absolute permittivity, which have specific, finite, non-zero values. Why?
Why are the vacuum permeability and permittivity non-zero and non-infinite?
What would…
cowlinator
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43
votes
5 answers
What is the highest possible frequency for an EM wave?
What is the highest possible frequency, shortest wavelength, for an electromagnetic wave in free space, and what limits it? Is the answer different for EM waves in other materials or circumstances? How could waves of this frequency be generated and…
Harry Weston
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43
votes
2 answers
What's the deal with momentum in the infinite square well?
Every now and then a question comes up about the status of the momentum operator in the infinite square well, and while we have two good answers on the topic here and here, I'm generally not satisfied by their level of detail and by how easy (not…
Emilio Pisanty
- 132,859
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- 666
43
votes
7 answers
Why did Einstein get credit for formulating the theory of special relativity?
See The Principle of Relativity here: The Principles of Mathematical Physics. This was written by Poincaré in 1904, a year before Einstein published his theory of relativity.
It appears from this and other writings of Poincaré that Poincaré…
Craig Feinstein
- 980
43
votes
7 answers
Adding 3 electron spins
I've learned how to add two 1/2-spins, which you can do with C-G-coefficients. There are 4 states (one singlet, three triplet states). States are symmetric or antisymmetric and the quantum numbers needed are total spin and total z-component.
But how…
Gere
- 1,457
43
votes
6 answers
What is an instant of time?
If we say that an instant of time has no duration, why does a sum of instants add up to something that has a duration? I have a hard time understanding this.
I think of one instant as being a 'moment' of time. Hence, the sum of many instants would…
james
- 829
43
votes
6 answers
Why can adding more polarization filters increase the amount of light that goes through them?
I am having some difficulty accepting the implications of the equation governing the intensity of light passing through polarization filters,
$$ I = I_0 \space\cos^2\theta $$
with $\theta$ being the angular difference between the two…
psitae
- 1,395
43
votes
8 answers
Negative probabilities in quantum physics
Negative probabilities are naturally found in the Wigner function (both the original and its discrete variants), the Klein paradox (where it is an artifact of using a one-particle theory) and the Klein-Gordon equation.
Is a general treatment of such…
Piotr Migdal
- 6,430
43
votes
2 answers
Physical interpretation of different selfadjoint extensions
Given a symmetric (densely defined) operator in a Hilbert space, there might be quite a lot of selfadjoint extensions to it. This might be the case for a Schrödinger operator with a "bad" potential. There is a "smallest" one (Friedrichs) and a…
András Bátkai
- 713
43
votes
5 answers
Why is a new moon not the same as a solar eclipse?
Forgive the elementary nature of this question:
Because a new moon occurs when the moon is positioned between the earth and sun, doesn't this also mean that somewhere on the Earth, a solar eclipse (or partial eclipse) is happening?
What, then, is…
JYelton
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