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1500 questions
75
votes
5 answers
Is the converse of Noether's first theorem true: Every conservation law has a symmetry?
Noether's (first) theorem states that any differentiable symmetry of the action of a physical system has a corresponding conservation law.
Is the converse true: Any conservation law of a physical system has a differentiable symmetry of its action?
Larry Harson
- 5,278
75
votes
5 answers
Why is it cold on the sea floor if pressure heats things?
I was reading this and it says that Microsoft put a server farm at the bottom of the ocean because it's cooler there. Particularly it seems to imply that it get's colder as you go deeper, "Since ocean water gets pretty cold toward the sea floor..."…
user273872
- 2,613
75
votes
7 answers
Is the Boltzmann constant really that important?
I read a book in which one chapter gave a speech about the fundamental constants of the Universe, and I remember it stated this:
If the mass of an electron, the Planck constant, the speed of light, or the mass of a proton were even just slightly…
Les Adieux
- 3,705
75
votes
1 answer
Why does a window become a mirror at night?
In day, when you look in the room through the window out, you can clearly see what happens outside. At night when it's dark outside but there's light inside you can look in the window but it becomes a mirror.
Why?
Alon Gubkin
- 853
75
votes
9 answers
Why does ice melting not change the water level in a container?
I have read the explanation for this in several textbooks, but I am struggling to understand it via Archimedes' principle. If someone can clarify with a diagram or something so I can understand or a clear equation explanation that would be great.
stackseverywhere
- 889
74
votes
6 answers
Is there a way for an astronaut to rotate?
We know that if an imaginary astronaut is in the intergalactic (no external forces) and has an initial velocity zero, then he has is no way to change the position of his center of mass. The law of momentum conservation says:
$$…
David
- 840
74
votes
4 answers
If water is incompressible, how can sound propagate underwater?
Since sound travels as longitudinal waves, sound waves should only be able to propagate in a medium through compressions and rarefactions. However, water, as a liquid, is generally treated as an incompressible fluid. Since compression is essential…
Omnitragedy
- 753
74
votes
3 answers
Why do I see a saddle in this picture of a computer screen?
I am not entirely sure this is an appropriate question for PSE, however since many of you have such diverse backgrounds, I'll give it a shot.
I have noticed that when one takes a picture of a computer, TV screen with a phone, you can "see" what…
74
votes
7 answers
Is spacetime discrete or continuous?
Is the spacetime continuous or discrete?
Or better, is the 4-dimensional spacetime of general-relativity discrete or continuous? What if we consider additional dimensions like string theory hypothesizes? Are those compact additional dimensions…
linello
- 1,267
74
votes
5 answers
Why is the mole/"amount of substance" a dimensional quantity?
According to the BIPM and Wikipedia, "amount of substance" (as measured in moles) is one of the base quantities in our system of weights and measures. Why?
I get why the mole is useful as a unit. In fact, my question isn't really about the mole at…
Tim Pederick
- 889
74
votes
5 answers
Why is water clear?
Water appears transparent to visible light, yet most other objects are opaque. Why is that? Is there an explanation why water appears transparent?
Is water transparent at all wavelengths, or are the visible wavelengths somehow special? If it is…
D.W.
- 983
74
votes
5 answers
Why does a candle blow out when we blow on it? Our breath is 16% oxygen and only 4% CO2
Don't say that a layer of carbon dioxide covers the flame, because our breath has more oxygen than carbon dioxide.
Also, our breath does not extinguish the flame by cooling it as it is itself warmer than the coolness required to extinguish it.
So…
Prem
- 2,306
73
votes
2 answers
Why does dry spaghetti break into three pieces as opposed to only two?
You can try it with your own uncooked spaghetti if you want; it almost always breaks into three when you snap it. I am asking for a good physical theory on why this is along with evidence to back it up. Or, a reference to a good study previously…
David Ball
- 1,127
73
votes
6 answers
Why do we still need to think of gravity as a force?
Firstly I think shades of this question have appeared elsewhere (like here, or here). Hopefully mine is a slightly different take on it. If I'm just being thick please correct me.
We always hear about the force of gravity being the odd-one-out of…
ejrb
- 1,669
73
votes
3 answers
How strong are Wi-Fi signals?
My family members dislike the idea of having many devices communicating wirelessly in our house, arguing that the signals have negative effects on our physical health.
I would like to tell them the EM signals are in fact weaker than the light from…
Chong Lip Phang
- 821