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1500 questions
40
votes
3 answers
Where does the light go when I turn off the light switch?
As above... Surely it must go somewhere?
My child asked me this question and it was a difficult one to answer.
Brad
- 527
40
votes
10 answers
Why did we expect gravitational mass and inertial mass to be different?
I've read many times that the fact that gravitational mass is equal to inertial mass (as far as we can tell) used to be a puzzle. I believe that Einstein explained this by showing that gravity is itself just an inertial force.
When I first…
user1551817
- 937
40
votes
3 answers
Partial derivative notation in thermodynamics
Most thermodynamics textbooks introduce a notation for partial derivatives that seems redundant to students who have already studied multivariable calculus. Moreover, the authors do not dwell on the explanation of the notation, which leads to a poor…
1__
- 1,584
40
votes
3 answers
The interpretation of mass in quantum field theories
Consider a free theory with one real scalar field:
$$
\mathcal{L}:=-\frac{1}{2}\partial _\mu \phi \partial ^\mu \phi -\frac{1}{2}m^2\phi ^2.
$$
We write this positive coefficient in front of $\phi ^2$ as $\frac{1}{2}m^2$, and then start calling $m$…
Jonathan Gleason
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40
votes
9 answers
If electrons are identical and indistinguishable, how can we say current is the movement of electrons?
When we talk about current, we say electrons are "flowing" through a conductor. But if electrons are identical particles, how does it make sense to talk about them flowing?
To expand on that: imagine the simplest wire, just a 1-D chain of copper…
Mark Fugate
- 609
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40
votes
8 answers
Is torque as fundamental a concept as force?
I used to think that torque and force were equally “fundamental”. In other words, my understanding was that we usually use Cartesian coordinates in many common problems because it is a convenient system, so as a result instantaneous forces which act…
StayOnTarget
- 672
40
votes
8 answers
What is the most precise physical measurement ever performed?
Obviously some things, such as the speed of light in a vacuum, are defined to be a precise value. The kilogram was recently defined to have a specific value by fixing Plank's constant to $6.62607015\cdot 10^{−34}\frac{m^2 kg}{s}$.
In particular, in…
Cort Ammon
- 48,357
40
votes
4 answers
Is this cheap "air conditioner" able to cool a room?
My parents bought this "air conditioner", but I am very skeptical that this can cool a room, or even cool anything.
I doubt that it even has a cooling element, I suspect that it is just a fan + humidifier.
But even if this device had a cooling…
SinOfficial
- 503
40
votes
3 answers
Don't understand the integral over the square of the Dirac delta function
In Griffiths' Intro to QM [1] he gives the eigenfunctions of the Hermitian operator $\hat{x}=x$ as being
$$g_{\lambda}\left(x\right)~=~B_{\lambda}\delta\left(x-\lambda\right)$$
(cf. last formula on p. 101). He then says that these eigenfunctions…
Peter4075
- 3,029
40
votes
4 answers
The meaning of imaginary time
What is imaginary (or complex) time? I was reading about Hawking's wave function of the universe and this topic came up. If imaginary mass and similar imaginary quantities do not make sense in physics, why should imaginary (or complex) time make…
glebovg
- 561
40
votes
11 answers
Does artificial gravity based on centrifugal force stop working if you jump off the ground?
In an answer to another question of mine, concerning gravity, there was a link to a video about the creation of artificial gravity, based on rotation.
The question I have might be silly (or with an obvious answer), but it puzzles me non the less. As…
user187075
40
votes
10 answers
What does it mean to say "Gravity is the weakest of the forces"?
I can understand that on small scales (within an atom/molecule), the other forces are much stronger, but on larger scales, it seems that gravity is a far stronger force; e.g. planets are held to the sun by gravity. So what does it mean to say that…
Smashery
- 842
40
votes
6 answers
What is the evidence for a supermassive black hole at the center of Milky Way?
Black holes cannot be seen because they do not emit visible light or any electromagnetic radiation. Then how do astronomers infer their existence? I think it's now almost established in the scientific community that black holes do exist and…
Solidification
- 11,483
40
votes
2 answers
What is this sort of abstract rainbow?
Today I saw the phenomenon in picture below. It was not raining (at least nearby me). What can that be? What is the technical explanation?
Edit: Just seen today in Southern Brazil another circumhorizon arc, this time together with a halo. Really…
Diracology
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40
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3 answers
Sun's power density compared to a compost heap
According to Wikipedia the Sun's "power density" is "approximately 276.5 $W/m^3$, a value that more nearly approximates that of reptile metabolism or a compost pile than of a thermonuclear bomb." My question is, so why is the Sun's core so hot (15.7…
Peter4075
- 3,029