Most Popular
1500 questions
43
votes
10 answers
How does a spinning object "know" that it is spinning?
I am constructing a thought experiment about a spinning object that is floating in intergalactic space. I assume that this object is about the size of a planet so that it will have enough gravity so that a Foucault pendulum will work, although I'm…
user1928764
- 551
43
votes
8 answers
Prove that negative absolute temperatures are actually hotter than positive absolute temperatures
Could someone provide me with a mathematical proof of why, a system with an absolute negative Kelvin temperature (such that of a spin system) is hotter than any system with a positive temperature (in the sense that if a negative-temperature system…
user7757
43
votes
8 answers
What is the proof that the universal constants ($G$, $\hbar$, $\ldots$) are really constant in time and space?
Cavendish measured the gravitation constant $G$, but actually he measured that constant on the Earth. What’s the proof that the value of the gravitation constant if measured on Neptune would remain the same? What’s the guarantee of its being a…
Manisha
- 883
43
votes
3 answers
What causes electrical boxes to hum?
Often times you pass by an electrical box on an electrical pole and you hear a distinct hum emanating from it. What causes that tone? Does the flow of electricity itself have a sound? Or does the flow rattle the metal parts at a certain frequency,…
user151841
- 1,619
- 1
- 14
- 22
43
votes
8 answers
Fourier transformation in nature/natural physics?
I just came from a class on Fourier Transformations as applied to signal processing and sound. It all seems pretty abstract to me, so I was wondering if there were any physical systems that would behave like a Fourier transformation.
That is, if…
Justin L.
- 5,980
- 6
- 43
- 51
43
votes
4 answers
The unreasonable effectiveness of the partition function
In a first course on statistical mechanics the partition function is normally introduced as the normalisation for the probability of a particle being in a particular energy level.
$$p_j=\frac{1}{Z}\exp\left(\frac{-E_j}{k_bT}\right),$$
$$Z=\sum_j…
ChrisM
- 1,650
43
votes
8 answers
If the gravity at the center of the Earth is zero, why are heavy elements like iron there?
If gravity is zero at the center of the earth, why is there a core of heavy elements, such as iron?
Alternate question for the opposite hypothesis:
If gravity is greatest at the center of the earth, as classical education tells us, why is the core…
Hsvkd
- 449
43
votes
6 answers
How much air needs to be displaced to generate an audible sound?
I'm reading a book where in one scene a wizard/alchemist teleports a scroll after reading.
He folded the parchment carefully and muttered a single cantrip. The note vanished with a small plop of displaced air, joining the others in a safe…
npst
- 543
- 4
- 8
43
votes
3 answers
What's wrong with this derivation that $i\hbar = 0$?
Let $\hat{x} = x$ and $\hat{p} = -i \hbar \frac {\partial} {\partial x}$ be the position and momentum operators, respectively, and $|\psi_p\rangle$ be the eigenfunction of $\hat{p}$ and therefore $$\hat{p} |\psi_p\rangle = p |\psi_p\rangle,$$ where…
ganzewoort
- 1,270
43
votes
2 answers
Examples of important known universality classes besides Ising
I am working with RG and have a pretty good idea of how it works. However I have noticed that even though the idea of universality class is very general and makes it possible to classify critical systems, textbooks seem to always end up with the…
Steven Mathey
- 4,375
43
votes
3 answers
What is going on in front of and behind a fan?
Why is it that when you drop paper behind a fan, it drops, and is not blown/sucked into the fan, whereas if you drop paper in front of a fan, it is blown away?
GreenAsJade
- 1,272
43
votes
2 answers
Why do stickers curl?
When peeling a sticker off its base, the immediate reaction is that it curls; why is this? I am having trouble finding an answer to this. Could it be that the glued side expands upon contact with the air? And, on a somewhat related note, is this a…
K. 622
- 787
43
votes
5 answers
Why is Newton's first law necessary?
Newton's second law says $F=ma$.
Now if we put $F=0$ we get $a=0$ which is Newton's first law.
So why do we need Newton's first law ?
Before asking I did some searching and I got this: Newtons first law is necessary to define inertial reference…
Murtuza Vadharia
- 1,807
43
votes
10 answers
Why can't we see gases?
I am not sure what causes gas molecules to be invisible.This question may look silly but I really want to know the story behind it.
Praveen Kadambari
- 1,633
43
votes
5 answers
What's the difference between helicity and chirality?
When a particle spins in the same direction as its momentum, it has right helicity, and left helicity otherwise. Neutrinos, however, have some kind of inherent helicity called chirality. But they can have either helicity. How is chirality different…
Ryan Dickherber
- 433