Most Popular
1500 questions
56
votes
2 answers
Is the Hawking radiation of a charged black hole thermal?
Suppose you have a Schwarzschild black hole of mass $M$ and angular parameter $a = 0$ (no rotation).
Question: is it possible to throw a charge $Q$ at a faster rate than it will be re-radiated? Will the radiation profile be still thermal?
If it is…
lurscher
- 14,423
56
votes
3 answers
Why is hydrogen the most abundant element in the Universe?
Hydrogen is the most abundant element in nature. Does cosmological nucleosynthesis provide an explanation for why is this the case? Is the explanation quantitatively precise?
Solidification
- 11,483
56
votes
1 answer
Why are particles thought of as irreducible representations, in plain English?
I'm a PhD student in mathematics and I have no problem in understanding what irreducible representation are. I mean that the mathematical side is not a particular problem. Nevertheless I have some problems in understanding why and in which sense…
Dac0
- 924
56
votes
1 answer
What is the upper-limit on intrinsic heating due to dark matter?
Cold dark matter is thought to fill our galactic neighborhood with a density $\rho$ of about 0.3 GeV/cm${}^3$ and with a velocity $v$ of roughly 200 to 300 km/s. (The velocity dispersion is much debated.) For a given dark matter mass $m$ and…
Jess Riedel
- 3,616
56
votes
12 answers
Why does hot air rise in a column instead of cold air pressing down?
Ok, this looks like a dumb question or even near trolling, but I really don't understand it.
When air is heated over an oven plate, it rises. Obviously, I can check by blowing some smoke in.
The common explanation is that hot air has less density…
Gyro Gearloose
- 948
- 1
- 6
- 15
56
votes
3 answers
Why should the Planck constant be a constant throughout all space?
Our value for the Planck constant $h$ can be found on experiments on Earth, but how do we know that the Planck constant doesn't change throughout space, for instance it depends weakly upon the curvature of spacetime? As far as I know we have only…
QCD_IS_GOOD
- 6,710
56
votes
10 answers
What practical issues remain for the adoption of Thorium reactors?
From what I've read on thorium reactors, there's enormous benefit to them. Their fuel is abundant enough to power human civilization for centuries, their fission products are relatively short-lived, they're far less prone to catastrophic failure,…
GordonM
- 985
56
votes
12 answers
Why are angles dimensionless and quantities such as length not?
So my friend asked me why angles are dimensionless, to which I replied that it's because they can be expressed as the ratio of two quantities -- lengths.
Ok so far, so good.
Then came the question: "In that sense even length is a ratio. Of length…
xrisk
- 679
56
votes
4 answers
Do rainbows have ultraviolet bands and infrared bands?
We have seen that rainbows looks so colorful as we are only able to see only the visible light. But Do they also have ultraviolet bands and infra-red bands, that we are unable to see?
I know someone has already asked the same question but I am…
Shashank
- 1,840
56
votes
3 answers
Why doesn't light affect a compass?
In our daily life a lot of photons of visible light, infrared and radio etc move around us. We know that light is an electromagnetic radiation. So why doesn't that electromagnetic radiation affect a magnetic compass?
Bhavesh
- 1,905
- 3
- 25
- 40
56
votes
7 answers
In quantum mechanics, given certain energy spectrum can one generate the corresponding potential?
A typical problem in quantum mechanics is to calculate the spectrum that corresponds to a given potential.
Is there a one to one correspondence between the potential and its spectrum?
If the answer to the previous question is yes, then given the…
Revo
- 16,956
55
votes
4 answers
Do gravitational lenses have a focus point?
Do gravitational lenses have a focus point? Could I burn space ants?
Jitter
- 2,421
55
votes
6 answers
What is the "secret " behind canonical quantization?
The way I (and perhaps most students around the world) was taught QM is very weird. There is no intuitive explanations or understanding. Instead we were given a recipe on how to quantize a classical theory, which is based on the rule of transforming…
Jacob
- 1,503
55
votes
5 answers
What is happening when magnetic field lines snap or break?
In discussions of sun spots and auroras on Earth, magnetic field lines are often described as "snapping" or "breaking", with the result of releasing charged particles very energetically.
My understanding is that field lines are just a visualization…
Robert
- 1,101
55
votes
8 answers
Why do we use cross products in physics?
We can define cross products mathematically like if we take two vectors, we can find another vector with certain properties but why do we use it in physics, if we consider a hypothetical physical quantity like force which is equal to cross product…
hezizzenkins
- 1,414