Most Popular
1500 questions
69
votes
5 answers
Can space expand with unlimited speed?
According to this article on the European Space Agency web site just after the Big Bang and before inflation the currently observable universe was the size of a coin. One millionth of a second later the universe was the size of the Solar System,…
cziko
- 793
69
votes
8 answers
What are the various physical mechanisms for energy transfer to the photon during blackbody emission?
By conservation of energy, the solid is left in a lower energy state following emission of a photon. Clearly absorption and emission balance at thermal equilibrium, however, thermodynamic equilibrium is a statement of the mean behaviour of the…
Douglas B. Staple
- 1,566
69
votes
6 answers
Why does a ping pong ball bounce higher when it is dropped together with a cup of water?
Setup: an official ping pong ball is floating inside a party plastic cup filled with clean water, which is then dropped from a certain height onto a soft mat.
Observation: the ping pong ball shoots up to a height which is much higher than its…
user6760
- 13,214
69
votes
5 answers
What does Enthalpy mean?
What is meant by enthalpy? My professor tells me "heat content". That literally makes no sense. Heat content, to me, means internal energy. But clearly, that is not what enthalpy is, considering: $H=U+PV$ (and either way, they would not have had two…
PhyEnthusiast
- 3,016
- 3
- 20
- 43
69
votes
2 answers
How do I construct the $SU(2)$ representation of the Lorentz Group using $SU(2)\times SU(2)\sim SO(3,1)$?
This question is based on problem II.3.1 in Anthony Zee's book Quantum Field Theory in a Nutshell
Show, by explicit calculation, that $(1/2,1/2)$ is the Lorentz Vector.
I see that the generators of $SU(2)$ are the Pauli matrices and the…
MadScientist
- 1,545
69
votes
2 answers
Why does wet hair keep its shape when it dries?
When I wash my hair and go to sleep, my hair is impossible to comb in the morning, stubbornly sticking to the shape it assumed during the night. The only way to get it right is to wet it again and comb it. What's the cause of this memory effect?
Petr
- 3,119
69
votes
7 answers
Why do archery arrows tilt downwards in their descent?
In the movies, arrows shot into the air rotate so that during the descent, the arrow head hits ground first. What is the source of this angular momentum? It would seem that the bow string exerts a force directly in line with the arrow.
lamplamp
- 1,668
69
votes
6 answers
Laplace operator's interpretation
What is your interpretation of Laplace operator? When evaluating Laplacian of some scalar field at a given point one can get a value. What does this value tell us about the field or it's behaviour in the given spot?
I can grasp the meaning of…
Džuris
- 3,227
68
votes
5 answers
Is there a Lagrangian formulation of statistical mechanics?
In statistical mechanics, we usually think in terms of the Hamiltonian formalism. At a particular time $t$, the system is in a particular state, where "state" means the generalised coordinates and momenta for a potentially very large number of…
N. Virgo
- 35,274
68
votes
12 answers
Is it possible that there is a color our human eye can't see?
Is it possible that there's a color that our eye couldn't see? Like all of us are color blind to it.
If there is, is it possible to detect/identify it?
MegaNairda
- 839
68
votes
5 answers
Why is technetium unstable?
Is there a simple account of why technetium is unstable?
From the Isotopes section of Wikipedia's article on Technetium:
Technetium, with atomic number (denoted Z) 43, is the lowest-numbered element in the periodic table that is exclusively…
Niel de Beaudrap
- 4,619
68
votes
11 answers
Why is torque not measured in Joules?
Recently, I was doing my homework and I found out that Torque can be calculated using $\tau = rF$.
This means the units of torque are Newton meters. Work & Energy are also measured in Newton meters which are Joules.
However, torque isn't a measure…
General Stubbs
- 805
68
votes
5 answers
Why do grapes in a microwave oven produce plasma?
Some of you may know this experiment (Grape + Microwave oven = Plasma video link):
take a grape that you almost split in two parts, letting just a tiny piece of skin making a link between each half-part.
put that in a microwave oven, and few…
JBE
- 791
68
votes
10 answers
Would time freeze if you could travel at the speed of light?
I read with interest about Einstein's Theory of Relativity and his proposition about the speed of light being the universal speed limit.
So, if I were to travel in a spacecraft at (practically) the speed of light, would I freeze and stop moving?…
68
votes
10 answers
How did Newton discover his second law?
I've always assumed/been told that Newton's 2nd law is an empirical law — it must be discovered by experiment. If this is the case, what experiments did Newton do to discover this? Is it related to his studies of the motion of the moon and earth?…
occam98
- 949