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1500 questions
41
votes
5 answers
Can you bend light to go in a circle?
Is it possible to bend light so that it forms a circle and goes round and round indefinitely without losing energy?
macco
- 1,985
41
votes
1 answer
Dynamics of a rotating coin
Sometimes when I'm bored in a waiting area, I would take a coin out of my pocket and spin it on a table. I never really tried to figure out what was going on. But, recently I wondered about two things:
Can I determine the critical rotational speed…
user29305
41
votes
5 answers
Where does the extra kinetic energy of the rocket come from?
Consider a rocket in deep space with no external forces. Using the formula for linear kinetic energy
$$\text{KE} = mv^2/2$$
we find that adding $100\ \text{m/s}$ while initially travelling at $1000\ \text{m/s}$ will add a great deal more energy to…
Nomoturtle
- 421
41
votes
4 answers
Rigorous justification for rotating wave approximation
Whenever I have encountered the rotating wave approximation, I have seen "the terms that we are neglecting correspond to rapid oscillations in the interaction Hamiltonian, so they will average to 0 in a reasonable time scale" as the justification…
Abel Molina
- 712
41
votes
3 answers
What is the decay width and why is it given in energy units?
I'm reading Thomson, Modern Particle Physics, and in chapter 16 author says that the decay width of the Z boson is $\Gamma_Z =2.452 \pm 0.0023 \,\mathrm{GeV}$. He also says the total width of the decay is the sum of the partial widths,…
Patrick
- 1,835
41
votes
3 answers
How much mass can colliding black holes lose as gravitational waves?
Concerning the recent detection of gravitational waves produced by colliding black holes, it has been reported that a significant percentage of the combined mass was lost in the resulting production of the gravitational waves.
So evidently in…
41
votes
4 answers
What is momentum really?
The Wikipedia article on momentum defines momentum as in classical mechanics:
… momentum is the product of the mass and velocity of an object.
However, an electromagnetic field has momentum, which is how solar sails work.
I would not suppose that…
hb20007
- 1,536
41
votes
4 answers
Why don't magnetic monopoles exist?
Electric monopoles do exist, but magnetic monopoles don't. Why?
The question is closed because I need to clarify it, but I don't know how I could ask it another way. However, I've recieved many answers that were appropiate and added something to my…
Nicole Judge
- 517
41
votes
2 answers
Why does light act differently in miniatures?
When painting miniatures (like RPG fantasy miniature soldiers)... why is it necessary to paint lights and shadows?
Being a 3D object, shouldn't the natural light be enough to create lights and shadows if the figure is simply painted with plain…
David Torres
- 513
41
votes
3 answers
How can dimensional regularization "analytically continue" from a discrete set?
The procedure of dimensional regularization for UV-divergent integrals is generally described as first evaluating the integral in dimensions low enough for it to converge, then "analytically continuing" this result in the number of dimensions $d$. …
tparker
- 47,418
41
votes
4 answers
Are black holes very dense matter or empty?
The popular description of black holes, especially outside the academia, is that they are highly dense objects; so dense that even light (as particle or as waves) cannot escape it once it falls inside the event horizon.
But then we hear things like…
Keerthi
- 513
41
votes
4 answers
Why does wavelength change as light enters a different medium?
When light waves enter a medium of higher refractive index than the previous, why is it that:
Its wavelength decreases?
The frequency of it has to stay the same?
ODP
- 4,595
- 11
- 51
- 82
41
votes
3 answers
What is the smallest item for which gravity has been recorded or observed?
What is the smallest item for which gravity has been recorded or observed? By this, I mean the smallest object whose gravitational effect upon another object has been detected. (Many thanks to Daniel Griscom for that excellent verbiage.)
In other…
41
votes
4 answers
How fast do electrons travel in an atomic orbital?
I am wondering how fast electrons travel inside of atomic electron orbitals. Surely there is a range of speeds? Is there a minimum speed? I am not asking about electron movement through a conductor.
Fred
- 411
41
votes
7 answers
Why is gravitational potential energy negative, and what does that mean?
I usually think of gravitational potential energy as representing just what it sounds like: the energy that we could potentially gain, using gravity. However, the equation for it (derived by integrating Newton's law of gravitational…
