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1500 questions
45
votes
6 answers
Why can we treat quantum scattering problems as time-independent?
From what I remember in my undergraduate quantum mechanics class, we treated scattering of non-relativistic particles from a static potential like this:
Solve the time-independent Schrodinger equation to find the energy eigenstates. There will be…
Mark Eichenlaub
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44
votes
8 answers
What is an observer in quantum mechanics?
My question is not about (pseudo) philosophical debate; it concerns mathematical operations and experimental facts.
What is an observer? What are the conditions required to be qualified of observer, both mathematically and experimentally?
Isaac
- 2,880
44
votes
6 answers
What exactly is heat?
Is it energy?
Is it energy per unit volume?
Is it energy per unit time i.e power?
What is it?
Larry Harson
- 5,278
44
votes
8 answers
How do headphones and earphones produce good bass if tiny speakers can't produce low frequency sounds very well?
It's a well known fact that small/tiny speakers cannot produce low frequency sounds very well. Conversely, large speakers cannot produce high frequency sounds very well. Hence the need for tweeters and woofers in your speaker systems.
But, how do…
navigator
- 811
44
votes
12 answers
What do people actually mean by "rolling without slipping"?
I have never understood what's the meaning of the sentence "rolling without slipping". Let me explain.
I'll give an example. Yesterday my mechanics professor introduced some concepts of rotational dynamics. When he came to talk about spinning wheels…
pppqqq
- 4,594
44
votes
11 answers
If you sliced the moon in half perfectly, would it hold together?
Just a big ol' slice down the middle. Would it drift apart over time, or eventually fuse back together?
Alex
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44
votes
3 answers
At what temperature are the most elements of the periodic table liquid?
For elements where 'liquid', is relatively easy to define, at which temperature are the most elements liquid, and which ones?
Assume 1 atm
yolo
- 2,646
44
votes
5 answers
How do I interpret uncertainty in velocity greater than the speed of light?
I just studied Heisenberg's uncertainty principle in school and I came up with an interesting problem.
Assume an electron which is moving very slowly and we observe it with a distance uncertainty of say $\Delta x=1\times10^{-13} \text{ m}$ if we…
Thulashitharan D
- 575
44
votes
12 answers
Is quantum mechanics applicable to only small things?
There was a fill-in-the-blank question in my university test. It was something like:
Quantum mechanics deals with ____
I wrote "everything" and my lecturer gave me no marks. He was expecting something like "small", "nano" or something. I tried to…
Rahul R
- 1,007
44
votes
4 answers
What is the mystery of turbulence?
One of the great unsolved problems in physics is turbulence but I'm not too clear what the mystery is. Does it mean that the Navier-Stokes equations don't have any turbulent phenomena even if we solve it computationally? Or does it mean we simply…
cspirou
- 1,226
44
votes
6 answers
Can a photon get emitted without a receiver?
It is generally agreed upon that electromagnetic waves from an emitter do not have to connect to a receiver, but how can we be sure this is a fact? The problem is that we can never observe non-received EM-Waves, because if we observe them the…
Enos Oye
- 1,121
44
votes
1 answer
Do an action and its Euler-Lagrange equations have the same symmetries?
Assume a certain action $S$ with certain symmetries, from which according to the Lagrangian formalism, the equations of motion (EOM) of the system are the corresponding Euler-Lagrange equations.
Can it happen that the equations of motion derived by…
Dilaton
- 9,503
44
votes
7 answers
How can two seas not mix?
How can two seas not mix? I think this is commonly known and the explanation everyone gives is "because they have different densities".
What I get is that they eventually will mix, but this process takes a long time.
From what you see in this…
Fofole
- 607
44
votes
5 answers
Kinetic energy with respect to different reference frames
I'm having problems understanding the following situation. Suppose two 1-tonne cars are going with the same orientations but opposite senses, each 50 km/h with respect to the road. Then the total energy is…
Bartek
- 543
44
votes
12 answers
Is there any physical evidence for motion?
Let's say that we have 2 tennis balls in space, one being in motion (say, pushed by an astronaut), and the other one still.
If we could take a snapshot of both tennis balls, would there be any evidence that could suggest that one is moving and the…
GaelF
- 591