Most Popular
1500 questions
43
votes
5 answers
Without seeing the lightning, can you tell how far away it struck by how the thunder sounds?
Is there any way to tell how far away a lightning strike is by how its thunder sounds? I thought one way might be by using the fact that higher frequencies travel faster than lower frequencies. Would you have to correct for the fact that thunder may…
user48301
- 531
43
votes
9 answers
Online QFT video lectures
I'm aware of Sidney Coleman's 1975/76 sequence of 54 lectures on Quantum Field Theory. Are there any other high-quality QFT lecture series available online?
Nigel Seel
- 3,346
42
votes
5 answers
Interpretation of "transition rate" in Fermi's golden rule
This is a question I asked myself a couple of years back, and which a student recently reminded me of. My off-the-cuff answer is wrong, and whilst I can make some hand-waving responses I'd like a canonical one!
In the derivation of Fermi's Golden…
not all wrong
- 593
42
votes
6 answers
How to get the position operator in the momentum representation from knowing the momentum operator in the position representation?
I know that
$$\tag{1}\hat{p}~=~-i\hbar \frac{\partial}{\partial x}~.$$
How can I get
$$\tag{2}\hat{x}~=~i\hbar \frac{\partial}{\partial p}~?$$
I think this simple and I'm just over thinking it, but knowing the momentum operator (1), how can I get…
user34032
- 555
42
votes
2 answers
Large and small gauge transformations?
I've a questions about the difference between small and large gauge transformations (a small gauge transformation tends to the identity at spatial infinity, whereas the large transformations don't). Many sources state (without any explanation or…
Hunter
- 5,196
42
votes
4 answers
Why isn't the universe full of electrons?
The probability of an electron found outside the atom is never zero. Consider building an electron detector, it must receive permanent signals from all electrons in the universe, as they can exist everywhere. Of course the probability decreases with…
Anon
- 793
42
votes
7 answers
Is submersion in a canal a good way to shelter from a nuclear strike?
I live 1.5 miles from the center of a city in a nuclear-armed country, and an adversarial country has just put its nuclear forces on high alert during a time of extraordinary geopolitical tension. I am thinking about what I could do to shelter from…
Tom
- 549
42
votes
4 answers
Slipstreaming - is there a penalty?
When a following vehicle slipstreams close to a vehicle in front, by placing itself in a region of air moving at a similar velocity to the front vehicle, it is afforded a benefit, e.g. improved fuel efficiency.
Do the effects from this action…
Trev
- 521
42
votes
3 answers
How can a rainbow be so steady, even though the droplets causing it can be in such different states?
The rainbow! What magical "thing". And even if you see the droplets of rain move in a sunlit storm, she's steady. I have been trying to understand but there are so many drops involved! And they are moving in turbulent ways on top.
So what's going…
42
votes
12 answers
Does physics explain why the laws and behaviors observed in biology are as they are?
Does physics explain why the laws and behaviors observed in biology are as they are? I feel like biology and physics are completely separate and although physics determine what's possible in biology, we have no idea how physics determine every…
Sayaman
- 791
42
votes
6 answers
Why isn't temperature measured in Joules?
If we set the Boltzmann constant to $1$, then entropy would just be $\ln \Omega$, temperature would be measured in $\text{joules}$ ($\,\text{J}\,$), and average kinetic energy would be an integer times $\frac{T}{2}$. Why do we need separate units…
jcai
- 654
42
votes
3 answers
Do all Noether theorems have a common mathematical structure?
I know that there are Noether theorems in classical mechanics, electrodynamics, quantum mechanics and even quantum field theory and since this are theories with different underlying formalisms, if was wondering it is possible to find a repeating…
Filippo
- 1,781
42
votes
2 answers
Why on-shell vs. off-shell matters?
The definitions between on- and off-shell are given in Wikipedia.
Why is it so important in QFT to distinguish these two notions?
FraSchelle
- 10,493
42
votes
5 answers
Slinky base does not immediately fall due to gravity
Why does the base of this slinky not fall immediately to gravity? My guess is tension in the springs is a force > mass*gravity but even then it is dumbfounding.
Smoke Trees
- 423
42
votes
7 answers
Do nuclei emit photons?
Generally in text books they say that when a electron goes from high energy state to a lower energy state it emits photons.
My question is, it is possible that a proton that goes from high energy state to a lower energy state emits photons too?
amilton moreira
- 383
- 10
- 25