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1500 questions
48
votes
3 answers
Why do electric sparks appear blue/purple?
Electric sparks tend to appear blue or purple or white in color. Why?
math_lover
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48
votes
1 answer
What is the difference between general measurement and projective measurement?
Nielsen and Chuang mention in Quantum Computation and Information that there are two kinds of measurement : general and projective ( and also POVM but that's not what I'm worried about ).
General Measurements
Quantum measurements are described by a…
transistor
- 797
48
votes
1 answer
Why does this object periodically turn itself?
See below gif image taken from here.
Or see this Youtube video about 30 sec in.
Is this a real effect?
Why does it seem to turn periodically?
Can it be explained by classical mechanics alone?
Is there a simple equation that models this…
pacó
- 515
48
votes
5 answers
Intuition as to why the orientation (of a 3D object) is not a conserved quantity?
Say you start off floating in space, in a fixed position and orientation,
with zero linear and angular velocity, with no external forces.
So you are a closed mechanical system. By twisting your body around,
you can't change your linear…
Don Hatch
- 670
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48
votes
9 answers
How can one derive Schrödinger equation?
The Schrödinger equation is the basis to understanding quantum mechanics, but how can one derive it? I asked my instructor but he told me that it came from the experience of Schrödinger and his experiments. My question is, can one derive the…
A.khalaf
- 842
48
votes
4 answers
How close can you get to lava before burning?
As the title asks: How close can you get to lava before burning?
I know that it depends on an number of factors; speed of lava flow, wind direction/strength, type(?) of lava flow (related to speed, in part, I think?). I'm guessing it also depends on…
Alexander
- 598
48
votes
14 answers
What is entropy really?
On this site, change in entropy is defined as the amount of energy dispersed divided by the absolute temperature. But I want to know: What is the definition of entropy? Here, entropy is defined as average heat capacity averaged over the specific…
user36790
48
votes
1 answer
Why do earphone wires always get tangled up in pocket?
What is the reason?
Is it caused by their narrow shape, the soft material, walking vibration or something else?
gerry
- 1,302
48
votes
5 answers
How effective is speeding?
At a simple level, speeding in a car attempts to minimize the time required to travel a distance by utilizing the basic relationship: $$d=st$$
So for a given distance, time should be inversely proportional to speed: the faster you go the less time…
jkeuhlen
- 1,207
48
votes
9 answers
Why is the gravitational force always attractive?
Why is the gravitational force always attractive? Is there another way to explain this without the curvature of space time?
PS: If the simple answer to this question is that mass makes space-time curve in a concave fashion, I can rephrase the…
New Horizon
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48
votes
6 answers
Apparent Violation of Newton's $3^{\text{rd}}$ Law and the Conservation of Momentum (and Angular Momentum) For a Pair of Charged Particles
Consider a system of the two identical positive point charges situated in free space (isolated from the influence of any other external fields) as shown in the attached diagram. Particle $1$ is at $(a,a,0)$ and particle $2$ is at $(0,a,0)$. Their…
user87745
48
votes
6 answers
Why do lasers cut? Is this a case of light acting as matter?
All I found in Google was very broad. From a physics models perspective, why can photons emitted from a laser cut? Does this cut mean that the photons are acting like matter?
sites
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48
votes
9 answers
Is it foolish to distinguish between covariant and contravariant vectors?
A vector space is a set whose elements satisfy certain axioms. Now there are physical entities that satisfy these properties, which may not be arrows. A co-ordinate transformation is linear map from a vector to itself with a change of basis. Now the…
Isomorphic
- 1,568
47
votes
2 answers
Operator-state correspondence in QFT
The operator-state correspondence in CFT gives a 1-1 mapping between operators $\phi(z,\bar{z})$ and states $|\phi\rangle$,
$$
|\phi\rangle=\lim_{z,\bar{z}\mapsto 0} \phi(z,\bar{z}) |0\rangle
$$
where $|0\rangle$ is the $SL(2,\mathbb{Z})$ invariant…
Axion
- 1,097
47
votes
8 answers
Explaining to a five year old - why don't bubbles run like water
My five-year old daughter was asking about astronauts the other day and why they float in space. After me showing her a few bits on the kids section on the NASA web site I started explaining about the planets and how the sun was a ball of gas and…
GrandMasterFlush
- 561