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1500 questions
41
votes
4 answers
Can water falling from a tap follow a spiral path?
The faucet design depicted below is driving me crazy. The water falling from the tap appears to follow a spiral path. No one seems to agree whether it is physically possible for the water to spin in such a way. Is it possible? Please explain your…
Laure Joumier
- 429
41
votes
6 answers
Is there a thermodynamic limit on how efficiently you can solve a Rubik's cube?
Suppose I build a machine which will be given Rubik's cubes that have been scrambled to one of the $\sim 2^{65}$ possible positions of the cube, chosen uniformly at random. Is it possible for the machine to solve the cubes without giving off any…
Mark Eichenlaub
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41
votes
5 answers
Hilbert space vs. Projective Hilbert space
Hilbert space and rays:
In a very general sense, we say that quantum states of a quantum mechanical system correspond to rays in the Hilbert space $\mathcal{H}$, such that for any $c∈ℂ$ the state $\psi$ and $c\psi$ map to the same ray and hence are…
user929304
- 4,635
41
votes
9 answers
How would night sky look like if the speed of light was infinite?
Would it be brighter? Different color? Gravitational lensing? Would black holes exist?
Cano64
- 527
41
votes
2 answers
What are Stephen Hawking's main contributions to research-level physics?
Without a doubt, Stephen Hawking is the most famous living scientist; indeed, his public visibility in all of history seems to be rivaled only by Einstein and easily eclipses giants of physics such as Bohr and Dirac.
Media portrayals of Hawking…
user55611
41
votes
1 answer
Can gravitational waves be red-shifted?
Whenever the Doppler effect is mentioned, it's typically in the context of sound waves or electromagnetic radiation. On the cosmological scale, red-shifting is also important because of the enormous speed of receding galaxies, thanks to the…
HDE 226868
- 10,821
41
votes
0 answers
Positivity for the level of Chern-Simons theory
In many classical papers about Chern-Simons theory (see, e.g. [1]),
it is claimed that the Chern-Simons theories with gauge group $G$ are classified by an element of $k\in H^4(BG,\mathbb Z)$, the so-called "level". (I'm not assuming here that $G$ is…
André Henriques
- 531
41
votes
2 answers
Why is the sun brighter in Australia compared to parts of Asia?
Background:
I've lived in Philippines for several years, and visited other parts of Asia occasionally (Singapore, Indonesia, Hongkong).
I just moved to Western Australia a few months ago and I realized that the sun is brighter here, in the sense…
Zaenille
- 863
41
votes
10 answers
Classic home experiments for an 8-year-old child
My 8-year-old daughter's school report says that she's good at understanding the basic science she's doing, but she's having trouble seeing how experimental results lead to conclusions. Specifically, it says she struggles to appreciate how changing…
Bob Tway
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41
votes
5 answers
Why do heavier objects fall faster in air?
We all know that in an idealised world all objects accelerate at the same rate when dropped regardless of their mass. We also know that in reality (or more accurately, in air) a lead feather falls much faster than a duck's feather with exactly the…
Simd
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41
votes
4 answers
Noether's theorem and gauge symmetry
I'm confused about Noether's theorem applied to gauge symmetry. Say we have
$$\mathcal L=-\frac14F_{ab}F^{ab}.$$
Then it's invariant under
$A_a\rightarrow A_a+\partial_a\Lambda.$
But can I say that the conserved current here…
user46348
- 691
40
votes
12 answers
Where does the extra force generated by a lever come from?
Given an object being pulled down to earth by a force of 1000 lb:
If a machine were to lift it off the ground for me, and I grabbed on to the rope before it was released, there is no way in the world I could hold it in place.
But, if I had a…
Jeff
- 585
40
votes
5 answers
When a bus goes around a corner, does the person sitting at the back travel further distance than the person sitting at the front?
This is a bit of childish question.
When a bus goes around a corner, does the person sitting at the back travel further distance than the person sitting at the front?
My thought is no because the bus is connected and every point moves along the same…
StimMarine
- 511
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40
votes
7 answers
Is there a proof from the first principle that the Lagrangian $L = T - V$?
Is there a proof from the first principle that for the Lagrangian $L$,
$$L = T\text{(kinetic energy)} - V\text{(potential energy)}$$
in classical mechanics? Assume that Cartesian coordinates are used. Among the combinations, $L = T - nV$, only $n=1$…
Chin Yeh
- 741
40
votes
8 answers
Where does gravity get its energy from?
I would like to know where gravity gets its energy to attract physical bodies?
I know that the law of conservation states that total energy of an isolated system cannot change. So gravity has to be getting its energy from somewhere, or else things…
Caesar
- 511