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1500 questions
36
votes
1 answer
What's the relation between Wilson Renormalization Group (RG) in Statistical Mechanics and QFT RG?
What's the relation between Wilson Renormalization Group(RG) in Statistical Mechanics and QFT RG? For easier to compare, I choose scalar $\phi^4$ in both cases. Wilson RG: Given $\phi^4$ model, $$Z=\int\mathcal{D}\phi(x)\exp[-\beta H[\phi(x)]]$$…
maplemaple
- 2,237
36
votes
3 answers
Why does snow stay after a snowfall?
I have just experienced a snowfall and I am not so clear on how it works. Three days after a short day of snowfall, and having 2 min | 17 max degrees Celsius, full sunny scarcely cloudy each day, there is still some snow persisting in shadow and…
Brethlosze
- 723
36
votes
7 answers
Deriving Lagrangian density for electromagnetic field
In considering the (special) relativistic EM field, I understand that assuming a Lagrangian density of the form $$\mathcal{L} =-\frac{\epsilon_0}{4}F_{\mu\nu}F^{\mu\nu} + \frac{1}{c}j_\mu A^\mu$$ and following the Euler-Lagrange equations recovers…
mcamac
- 463
36
votes
3 answers
Why does the Earth cool at night time?
I do understand that open water and open ground cools by the means of convection — lower air takes the heat and goes up, where it cools. But why does the Earth lose energy and where does it go? Does it emit heat radiation to space? Does the heat go…
Denis Kulagin
- 481
36
votes
3 answers
What do the poles of a Green function mean, physically?
Is there a physical interpretation of the existence of poles for a Green function? In particular how can we interpret the fact that a pole is purely real or purely imaginary? It's a general question but I would be interested in the interpretation in…
PanAkry
- 1,001
36
votes
5 answers
Why do we use the electron volt?
Why do we use the electron volt ? Why did it come to be the electron volt and not, say, just a prefix of the joule, like the nanojoule? Does the electron volt represent anything particular as far as the mathematics goes? I am guessing that it does,…
Benjamin Rogers-Newsome
- 1,770
36
votes
5 answers
How to get Planck length
I know that what Planck length equals to. The first question is, how do you get the formula $$\ell_P~=~\sqrt\frac{\hbar G}{c^3}$$ that describes the Planck length? The second question is, will any length shorter than the Planck length be…
user2346
- 483
36
votes
2 answers
How well can a laser be focused over interstellar range?
I try to read up on diffraction limiting and gaussian beams, but it always gives a result saying it’s characterized by an angle which is the edges of a cross section of a cone. Which says nothing on the physical limits of how I choose that angle. It…
JDługosz
- 5,387
36
votes
2 answers
How to interpret correlation functions in QFT?
I'm fairly new to the subject of quantum field theory (QFT), and I'm having trouble intuitively grasping what a n-point correlation function physically describes. For example, consider the 2-point correlation function between a (real) scalar field…
user35305
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36
votes
5 answers
Regularization of the Casimir effect
For starters, let me say that although the Casimir effect is standard textbook stuff, the only QFT textbook I have in reach is Weinberg and he doesn't discuss it. So the only source I currently have on the subject is Wikipedia . Nevertheless I…
Squark
36
votes
1 answer
Why was the first discovered neutrino an anti-neutrino?
In the search for neutrinos, Cowan and Reines discovered the electron anti-neutrino and named it as such. Why is the particle they discovered the anti-variety? The reason we call electrons 'electrons' and not 'anti-electrons' is because the…
Joshua
- 1,383
36
votes
4 answers
How to imagine WiFi signal propagation?
When thinking about how the WiFi signal propagates through a household, can I use the following thought experiment? Assume absolute darkness. Place a strong lightbulb where the WiFi access point is. The illumination that reaches various places in…
vektor
- 382
36
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
36
votes
11 answers
Why do objects follow geodesics in spacetime?
Trying to teach myself general relativity. I sort of understand the derivation of the geodesic equation $$\frac{d^{2}x^{\alpha}}{d\tau^{2}}+\Gamma_{\gamma\beta}^{\alpha}\frac{dx^{\beta}}{d\tau}\frac{dx^{\gamma}}{d\tau}=0.$$ which describes "how"…
Peter4075
- 3,149
36
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
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