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1500 questions
42
votes
5 answers
Why are infinite order Lagrangians called 'non-local'?
And in what sense are they 'non-local'?
WIMP
- 2,645
42
votes
12 answers
Does car tire pressure change with weight of car load?
Does tire pressure measured by a meter on tire gauge change with load? (I am not interested in pressure produced by car tires onto the road).
Car spec usually says "inflate to 220kPa normal load, 300kPa full load". Does this mean the measured…
Marek
- 522
42
votes
4 answers
Is topology of universe observable?
There is an idea that the geometry of physical space is not observable(i.e. it can't be fixed by mere observation). It was introduced by H. Poincare. In brief it says that we can formulate our physical theories with the assumption of a flat or…
user55867
42
votes
6 answers
Why does a flat universe imply an infinite universe?
This article claims that because the universe appears to be flat, it must be infinite. I've heard this idea mentioned in a few other places, but they never explain the reasoning at all.
Nathan BeDell
- 681
42
votes
8 answers
Homemade salad dressing separates into layers after it sits for a while. Why doesn't this violate the 2nd law of thermodynamics?
The oil, vinegar and other liquids in homemade salad dressing separate into layers after sitting for a while, making the mixture become more organized as time evolves. Why doesn't this violate the 2nd law of thermodynamics?
I assume that the answer…
user26866
- 3,442
42
votes
2 answers
How does classical GR concept of space-time emerge from string theory?
First, I'll state some background that lead me to the question.
I was thinking about quantization of space-time on and off for a long time but I never really looked into it any deeper (mainly because I am not yet quite fluent in string theory). But…
Marek
- 23,530
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41
votes
4 answers
Classical and quantum probabilities in density matrices
In textbooks, it is sometimes written that a mixed state can be represented as mixture of $N$ (I assume here $N<+\infty$) quantum pure states $|\psi_i\rangle$ with classical probabilities $p_i$:
$$\rho = \sum_{i=1}^N p_i |\psi_i \rangle \langle…
Valter Moretti
- 71,578
41
votes
6 answers
If I fall into an evaporating black hole, where do I end up?
This question has been bothering me for a while. I have a crude hypothesis...
As I understand it, an observer falling into a black hole will cross the event horizon at some specific future (proper) time in, and that it will not be a traumatic event…
Beta
- 527
41
votes
1 answer
Phase shifts in scattering theory
I have been studying scattering theory in Sakurai's quantum mechanics. The phase shift in scattering theory has been a major conceptual and computational stumbling block for me.
How (if at all) does the phase shift relate to the scattering…
Cogitator
- 1,075
41
votes
8 answers
Are these two quantum systems distinguishable?
Suppose Stanford Research Systems starts selling a two-level atom factory. Your grad student pushes a button, and bang, he gets a two-level atom. Half the time the atom is produced in the ground state, and half the time the atom is produced in the…
Andrew
- 3,417
41
votes
2 answers
Square bracket notation for dimensions and units: usage and conventions
One of the most useful tools in dimensional analysis is the use of square brackets around some physical quantity $q$ to denote its dimension as
$$[q].$$
However, the precise meaning of this symbol varies from source to source; there are a few…
Emilio Pisanty
- 132,859
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- 351
- 666
41
votes
3 answers
Why does bunched up aluminum foil become so extremely hard to compress?
I noticed that whenever I bunch up aluminum foil (into a ball), it becomes extremely hard to compress.
If I use another piece of the same amount of aluminum foil, and keep folding it in, I arrive at a much smaller volume. It is full of air and much…
Árpád Szendrei
- 28,452
41
votes
4 answers
Rotate an object about the time axis
Is there a notion of rotating an object about its time axis? I'm not sure if this question totally makes sense, but it seems intuitive to me that an object with dimensions in the three spatial directions and the time dimension (so an object paired…
lanerogers
- 521
41
votes
4 answers
Double Slit Experiment: How do scientists ensure that there's only one photon?
Many documentaries regarding the double slit experiment state that they only send a single photon through the slit. How is that achieved and can it really be ensured that it is a single photon?
Drachenkatze
- 513
41
votes
7 answers
State collapse in the Heisenberg picture
I've been studying quantum mechanics and quantum field theory for a few years now and one question continues to bother me.
The Schrödinger picture allows for an evolving state, which evolves through a unitary, reversible evolution (Schrödinger’s…
FrancisFlute
- 1,096