Most Popular
1500 questions
76
votes
6 answers
What is the connection between Poisson brackets and commutators?
The Poisson bracket is defined as:
$$\{f,g\} ~:=~ \sum_{i=1}^{N} \left[
\frac{\partial f}{\partial q_{i}} \frac{\partial g}{\partial p_{i}} -
\frac{\partial f}{\partial p_{i}} \frac{\partial g}{\partial q_{i}}
\right]. $$
The anticommutator is…
0x90
- 3,316
76
votes
10 answers
Is 3+1 spacetime as privileged as is claimed?
I've often heard the argument that having 3 spatial dimensions is very special. Such arguments are invariably based on certain assumptions that do not appear to be justifiable at all, at least to me. There is a summary of arguments on Wikipedia.
For…
Roman Starkov
- 2,155
76
votes
5 answers
Why does GPS depend on relativity?
I am reading A Brief History of Time by Stephen Hawking, and in it he mentions that without compensating for relativity, GPS devices would be out by miles. Why is this? (I am not sure which relativity he means as I am several chapters ahead now and…
Thomas O
- 3,187
- 6
- 30
- 34
76
votes
4 answers
How does water evaporate if it doesn't boil?
When the sun is out after a rain, I can see what appears to be steam rising off a wooden bridge nearby. I'm pretty sure this is water turning into a gas.
However, I thought water had to reach 100 degrees C to be able to turn into a gas.
Is there an…
Malcolm Crum
- 1,053
- 1
- 7
- 11
75
votes
5 answers
Do low frequency sounds really carry longer distances?
It is a common belief that low frequencies travel longer distances. Indeed, the bass is really what you hear when the neighbor plays his HiFi loud (Woom Woom). Try asking people around, a lot of them believe that low sounds carry longer…
Max
- 853
75
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
- 1,251
75
votes
3 answers
Why don't the Earth's oceans generate a magnetic field?
Many questions have been asked here about why the Earth has a magnetic field, e.g.,
What is the source of Earth's magnetic field?
How does Earth's interior dynamo work?
How can an electrically neutral planetary core be geodynamo?
Why does the…
Thorondor
- 4,080
75
votes
2 answers
What is the physics behind origami?
If we fold a paper and then apply pressure on the newly formed crease, it seems that the paper's surface gets a permanent deformation but what exactly has happened to the paper at a molecular scale?
tryst with freedom
- 7,940
75
votes
2 answers
Do "almost black holes" exist?
The only things I read about so far in astrophysics are either black holes, developing black holes or not black holes at all.
So I am wondering, is it physically possible to have an object that is almost a black hole, but not a black hole. What I…
Winston
- 3,226
75
votes
12 answers
Is the wave-particle duality a real duality?
I often hear about the wave-particle duality, and how particles exhibit properties of both particles and waves. However, I wonder, is this actually a duality? At the most fundamental level, we 'know' that everything is made up out of particles,…
user14445
- 1,493
75
votes
13 answers
If visible light has more energy than microwaves, why isn't visible light dangerous?
Light waves are a type of electromagnetic wave and they fall between 400-700 nm long. Microwaves are less energetic but seem to be more dangerous than visible light. Is visible light dangerous at all and why not?
suse
- 1,090
75
votes
7 answers
Is Pauli-repulsion a "force" that is completely separate from the 4 fundamental forces?
You can have two electrons that experience each other's force by the exchange of photons (i.e. the electromagnetic force). Yet if you compress them really strongly, the electromagnetic interaction will no longer be the main force pushing them apart…
Jan M.
- 1,122
75
votes
9 answers
Why are "degrees" and "bytes" not considered base units?
From Wikipedia:
The SI base units and their physical quantities are the metre for measurement of length, the kilogram for mass, the second for time, the ampere for electric current, the kelvin for temperature, the candela for luminous intensity,…
Wais Kamal
- 909
75
votes
6 answers
If like charges repel, why doesn't a charge break itself apart?
How can it be that, if like charges repel, they don't repel themselves? In other words, why don't charges break apart?
About the possible duplicate: I want to know about charges in general, not just that of an electron.
My response to Lawrence B.…
Kevin
- 857
- 7
- 10
75
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
- 3,423