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1500 questions
39
votes
1 answer
Why does wind direction significantly affect sound propagation?
I live about 2 miles from a race track.
Some days the noise from testing is extremely noticeable, Other days despite the exact same testing activity, there is little-to-no noise at all.
This is mostly effected by the direction of the wind.
How does…
39
votes
5 answers
How would a passing gravitational wave look or feel?
In a hypothetical situation I'm still sitting in a coffee shop but a gravitational wave similar to the three reported by LIGO passes through me from behind. The source is much closer though, so this one is perceptible but (let's hope) not yet…
uhoh
- 6,089
39
votes
11 answers
Discreteness and Determinism in Superstrings?
So Gerard 't Hooft has a brand new paper (thanks to Mitchell Porter for making me aware of it) so this is somewhat of a expansion to the question I posed on this site a month or so ago regarding 't Hoofts work.
Now he has taken it quite a big step…
QuestionAnswers
- 913
39
votes
3 answers
Phase space volume and relativity
Much of statistical mechanics is derived from Liouville's theorem, which can be stated as "the phase space volume occupied by an ensemble of isolated systems is conserved over time." (I'm mostly interested in classical systems for the moment.)
It's…
N. Virgo
- 35,274
39
votes
7 answers
Do Maxwell's equation describe a single photon or an infinite number of photons?
The paper Gloge, Marcuse 1969: Formal Quantum Theory of Light Rays starts with the sentence
Maxwell's theory can be considered as the quantum theory of a single
photon and geometrical optics as the classical mechanics of this
photon.
That…
asmaier
- 10,250
39
votes
5 answers
Why do we assume, in dimensional analysis, that the remaining constant is dimensionless?
Walter Lewin's first lecture (at 22:16) analyzes the time $t$ for an apple to fall to the ground, using dimensional analysis. His reasoning goes like this:
It's natural to suppose that height of the apple to the ground ($h$), mass of the apple…
Fine Man
- 1,493
39
votes
3 answers
Why does water pouring from a glass sometimes travel down the side of the glass?
If you have a glass of water, say, three quarters full and you pour it at an angle of say, $45^{\circ}$ with respect to the the table, the water comes out of the glass and goes directly down towards the floor.
However, when the glass is more full,…
ODP
- 4,677
39
votes
2 answers
What's the point of Pauli's Exclusion Principle if time and space are continuous?
What does the Pauli Exclusion Principle mean if time and space are continuous?
Assuming time and space are continuous, identical quantum states seem impossible even without the principle. I guess saying something like: the closer the states are the…
Yogi DMT
- 1,683
39
votes
10 answers
Is there a notion of causality in physical laws?
I was reading "A Few Useful Things to Know about Machine Learning" by Pedro Domingos and towards the end of the paper he makes this statement:
"Many researchers believe that causality is only a convenient fiction. For example, there is no notion of…
covfefe
- 483
39
votes
1 answer
Can lightning be used to solve NP-complete problems?
I'm a MS/BS computer science guy who is wondering about why lightning can't (or can?) be used to solve NP complete problems efficiently, but I don't understand the physics behind lightning, so I'm posting here.
What seems peculiar to me about…
micahhoover
- 525
39
votes
7 answers
How fast a (relatively) small black hole will consume the Earth?
This question appeared quite a time ago and was inspired, of course, by all the fuss around "LHC will destroy the Earth".
Consider a small black hole, that is somehow got inside the Earth. Under "small" I mean small enough to not to destroy Earth…
Kostya
- 20,288
39
votes
10 answers
A contradiction between Biot-Savart and Ampère-Maxwell Laws?
I came across a problem that I cannot get my head around.
Consider two very small spherical metallic balls given charges $+Q$ and $-Q$. Assume that both can be approximated as point charges. Now, they are connected by a straight, finite, conducting…
Newton
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39
votes
7 answers
Do gases have phonons?
A phonon is a quantized unit of sound; they are encountered when quantizing lattice vibrations in solids. Now, even an ideal gas supports sound waves, but in this case, interactions between atoms are weak. That makes it hard to imagine what a…
knzhou
- 107,105
39
votes
10 answers
Why don't modern spacecraft use nuclear power?
The Voyager 1 & 2 spacecraft launched in 1977 with Plutonium as their source of electricity. 34 years later they claim these two spacecraft have enough power to last them until at least 2020. That means they'll have had enough power to last them at…
John Conde
39
votes
5 answers
Would you be weightless at the center of the Earth?
If you could travel to the center of the Earth (or any planet), would you be weightless there?
freeside
- 543