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1500 questions
124
votes
6 answers
Why am I not burned by a strong wind?
So I was thinking... If heat I feel is just lots of particles going wild and transferring their energy to other bodies, why am I not burned by the wind?
When I thought about it more I figured out that wind usually carries some humidity, and since…
Jinx
- 1,263
124
votes
9 answers
Why does a helium filled balloon move forward in a car when the car is accelerating?
I noticed that when I had a helium filled, latex balloon inside of my car, it moved forward in the cabin as I accelerated forward. The faster I accelerated forward, the faster the balloon went from the back of the car to the front of the car. The…
user33986
- 1,209
124
votes
1 answer
Experimental test of the non-statisticality theorem?
Context: The paper On the reality of the quantum state (Nature Physics 8, 475–478 (2012) or arXiv:1111.3328) shows under suitable assumptions that the quantum state cannot be interpreted as a probability distribution over hidden variables.
In the…
Chris Ferrie
- 2,908
123
votes
5 answers
Does the $\frac{4}{3}$ problem of classical electromagnetism remain in quantum mechanics?
In Volume II Chapter $28$ of the Feymann Lectures on Physics, Feynman discusses the infamous $\frac43$ problem of classical electromagnetism. Suppose you have a charged particle of radius $a$ and charge $q$ (uniformly distributed on the surface). …
Keshav Srinivasan
- 3,139
123
votes
7 answers
Why are most metals gray/silver?
Why do most metals (iron, tin, aluminum, lead, zinc, tungsten, nickel, etc.) appear silver or gray?
What makes copper and gold have different colors?
What atomic characteristics determine the color?
Ali Abbasinasab
- 1,849
123
votes
7 answers
Why is the $S_{z} =0$ state forbidden for photons?
If photons are spin-1 bosons, then doesn't quantum mechanics imply that the allowed values for the z-component of spin (in units of $\hbar$) are -1, 0, and 1?
Why then in practice do we only use the $\pm 1$ states?
I have been told that this is…
Todd R
- 1,836
123
votes
11 answers
What Is Energy? Where did it come from?
The simplistic undergrad explanation aside, I've never really understood what energy really is. I've been told that it's something when converted from one kind of something to another kind, or does some "work", as defined by us, but what is that…
Anna
- 1,787
123
votes
15 answers
Why can't $ i\hbar\frac{\partial}{\partial t}$ be considered the Hamiltonian operator?
In the time-dependent Schrodinger equation, $ H\Psi = i\hbar\frac{\partial}{\partial t}\Psi,$ the Hamiltonian operator is given by
$$\displaystyle H = -\frac{\hbar^2}{2m}\nabla^2+V.$$
Why can't we consider $\displaystyle…
Revo
- 17,504
123
votes
4 answers
What conservation law corresponds to Lorentz boosts?
Noether's Theorem is used to relate the invariance of the action under certain continuous transformations to conserved currents. A common example is that translations in spacetime correspond to the conservation of four-momentum.
In the case of…
Warrick
- 9,870
123
votes
10 answers
How can I stand on the ground? EM or/and Pauli?
There is this famous example about the order difference between gravitational force and EM force. All the gravitational force of Earth is just countered by the electromagnetic force between the electrons on the surface of my feet and the ground.
But…
Kivanc Uyanik
- 1,331
123
votes
6 answers
Trace of a commutator is zero - but what about the commutator of $x$ and $p$?
Operators can be cyclically interchanged inside a trace:
$${\rm Tr} (AB)~=~{\rm Tr} (BA).$$
This means the trace of a commutator of any two operators is zero:
$${\rm Tr} ([A,B])~=~0.$$
But what about the commutator of the position and momentum…
Joe
- 3,554
121
votes
7 answers
What's inside a proton?
What constitutes protons? When I see pictures, I can't understand. Protons are made of quarks, but some say that they are made of 99% empty space. Also, in this illustration from Wikipedia, what's between the quarks?
121
votes
5 answers
Toilet paper dilemma
There are two ways to orient the toilet paper: "over" (left image), "under" (right image).
Each has it's pros and cons. For some reason, it's always easier to tear off the paper in the "over" orientation even though we apply the same force thus…
Monopole
- 3,404
121
votes
12 answers
Why don't we use weights to store energy?
One of the main reasons why we haven't switched to clean energy is the lack of efficient storage methods - But, why aren't we using dead weights to store energy and draw it back later when needed? As an example of what I mean:
Blake Angelo
- 1,181
121
votes
13 answers
If all motion is relative, how does light have a finite speed?
I've often heard that Einstein shattered the notion of absolute motion (i.e. all things move relative to one another) and that he established the speed of light as being absolute. That sounds paradoxical to me; I cannot understand how the two…
CircleSquared
- 1,351