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1500 questions
78
votes
5 answers
Is the CMB rest frame special? Where does it come from?
It seems that we are moving relative to the universe at the speed of ~ 600 km/s.
This is the speed of our galaxy relative to the cosmic microwave background.
Where does this rest frame come from? Is this special in any way (i.e., an absolute…
Jus12
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78
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
- 5,456
78
votes
6 answers
How can we see planets thousands of light years away but don't know if there are more planets in the solar system?
That is basically my question, it arose when I saw an article (here is the scientific paper, which should be free to read) saying two Caltech scientists might have found the 9th planet of the solar system.
Suriya
- 1,768
78
votes
3 answers
Why doesn't the motion of a car affect the frequency of radio stations?
When we go in a car and tune to an FM radio station, why doesn't our motion disturb the frequency?
Like the Doppler effect?
DARU SRINIVAS
- 853
78
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
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78
votes
6 answers
When is the Hamiltonian of a system not equal to its total energy?
I thought the Hamiltonian was always equal to the total energy of a system but have read that this isn't always true. Is there an example of this and does the Hamiltonian have a physical interpretation in such a case?
Noah
- 1,870
78
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
- 929
77
votes
3 answers
Why can we see the dust particles in a narrow beam of light (and not in an all lighted area)?
Let us say that I am sitting in a room with all the drapes open. Bright sunlight is coming through the window. The whole room is brilliantly lighted. I will not be able to see the dust particles suspended in air.
Now, if I draw the drapes close,…
Masroor
- 759
77
votes
8 answers
Massless charged particles
Are there any massless (zero invariant mass) particles carrying electric charge?
If not, why not? Do we expect to see any or are they a theoretical impossibility?
Eelvex
- 1,270
77
votes
5 answers
Do the weights of two liquids not add when mixed?
I was given an interesting dilemma today. A co-worker saw me adding a liquid (Diisopropyl ethylamine AKA DIPEA) to a flask filled with another liquid (Tetrahydrofuran AKA THF). I needed to weigh out exactly 5 grams of DIPEA into the THF and so I…
Brian
- 687
77
votes
7 answers
Number theory in Physics
As a Graduate Mathematics student, my interest lies in Number theory. I am curious to know if Number theory has any connections or applications to physics. I have never even heard of any applications of Number theory to physics. I have heard…
C.S.
- 1,029
77
votes
7 answers
Does a thrown ball have kinetic energy at the top of the curve?
I'm going through physics with my 5th grade child. There is a question and answer that indicates that a airborne ball at the top of the trajectory does not have kinetic energy.
The diagram below shows the path taken by a ball after it was kicked.…
meow
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77
votes
20 answers
Is weight a scalar or a vector?
My professor insists that weight is a scalar. I sent him an email explaining why it's a vector, I even sent him a source from NASA clearly labeling weight as a vector. Every other source also identifies weight as a vector.
I said that weight is a…
Ryan
- 787
77
votes
4 answers
What is this shadow being cast upon?
The picture below is from on top of Mt. Shasta at sunrise. The sun was directly behind me when I took the picture and it appears to be casting a shadow on.... the sky itself?
Can anyone explain what the shadow is being cast on? There don't…
Abe Miessler
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77
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…
EXVII
- 3,783