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1500 questions
128
votes
10 answers
Why the Principle of Least Action?
I'll be generous and say it might be reasonable to assume that nature would tend to minimize, or maybe even maximize, the integral over time of $T-V$. Okay, fine. You write down the action functional, require that it be a minimum (or maximum), and…
Jonathan Gleason
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126
votes
10 answers
Why do spaceships heat up when entering earth but not when exiting?
Recently I read up on spacecrafts entering earth using a heat shield. However, when exiting the Earth's atmosphere, it does not heat up, so it does not need a heat shield at that point of time yet. Why is this so? I know then when entering earth,…
QuIcKmAtHs
- 3,745
126
votes
5 answers
Why do wet objects become darker?
When something gets wet, it usually appears darker. This can be observed with soil, sand, cloth, paper, concrete, bricks...
What is the reason for this? How does water soaking into the material change its optical properties?
Suma
- 1,412
125
votes
7 answers
Why does the humidifier make a stove's flame orange?
Just like this guy's, the color of my stove's flames were affected by the humidifier as well.
Why does this happen?
Is it a good thing or a bad thing ?
Ilya Gazman
- 2,115
124
votes
14 answers
QM without complex numbers
I am trying to understand how complex numbers made their way into QM. Can we have a theory of the same physics without complex numbers? If so, is the theory using complex numbers easier?
Frank
- 3,403
123
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
- 1,253
123
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
- 2,898
123
votes
9 answers
What, in simplest terms, is gauge invariance?
I am a mathematics student with a hobby interest in physics. This means that I've taken graduate courses in quantum dynamics and general relativity without the bulk of undergraduate physics courses and sheer volume of education into the physical…
Arthur
- 2,858
122
votes
8 answers
Is the universe fundamentally deterministic?
I'm not sure if this is the right place to ask this question. I realise that this maybe a borderline philosophical question at this point in time, therefore feel free to close this question if you think that this is a duplicate or inappropriate for…
MRashid
- 1,308
122
votes
13 answers
Can Maxwell's equations be derived from Coulomb's Law and Special Relativity?
As an exercise I sat down and derived the magnetic field produced by moving charges for a few contrived situations. I started out with Coulomb's Law and Special Relativity. For example, I derived the magnetic field produced by a current $I$ in an…
user1247
- 7,288
122
votes
4 answers
The Role of Rigor
The purpose of this question is to ask about the role of mathematical rigor in physics. In order to formulate a question that can be answered, and not just discussed, I divided this large issue into five specific questions.
Update February, 12,…
Gil Kalai
- 2,043
121
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
- 3,031
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
- 3,454
121
votes
6 answers
What is known about the topological structure of spacetime?
General relativity says that spacetime is a Lorentzian 4-manifold $M$ whose metric satisfies Einstein's field equations. I have two questions:
What topological restrictions do Einstein's equations put on the manifold? For instance, the existence…
Eric
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120
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
- 1,169