Most Popular
1500 questions
55
votes
3 answers
How small does sand have to be to get wet?
I think of sand as a lot of very small rocks.
Suppose I have a pile of rocks, each about 1cm in size, and the pile is a meter tall. If I pour a bucket of water on the rocks, most of the water will fall through the rocks and form a puddle on the…
Mark Eichenlaub
- 52,955
- 15
- 140
- 238
55
votes
2 answers
Why is the equation for friction so simple?
The general equation for the force of friction (static or kinetic) is $F_f = \mu * F_N$, where $F_f$ is the force of friction and $\mu$ is the coefficient of friction (its value is dependent upon the surfaces interacting on each other).
Why is it…
user1546859
- 653
55
votes
5 answers
Mathematically-oriented Treatment of General Relativity
Can someone suggest a textbook that treats general relativity from a rigorous mathematical perspective? Ideally, such a book would
Prove all theorems used.
Use modern "mathematical notation" as opposed to "physics notation", especially with respect…
3Sphere
- 391
55
votes
5 answers
Scattering of light by light: experimental status
Scattering of light by light does not occur in the solutions of Maxwell's equations (since they are linear and EM waves obey superposition), but it is a prediction of QED (the most significant Feynman diagrams have a closed loop of four electron…
Keenan Pepper
- 7,697
- 2
- 25
- 29
55
votes
7 answers
A No-Nonsense Introduction to Quantum Field Theory
I found Sean Carroll's "A No Nonsense Introduction to General Relativity" (about page here. pdf here), a 24-page overview of the topic, very helpful for beginning study. It all got me over the hump of learning the meaning of various terms…
Mark Eichenlaub
- 52,955
- 15
- 140
- 238
55
votes
2 answers
How long would it take me to travel to a distant star?
Suppose I wanted to travel to one of the recently discovered potentially Earth-like planets such as Kepler 186f that is 490 light years away. Assuming I had a powerful rocket and enough fuel, how long would it take me?
John Rennie
- 355,118
55
votes
4 answers
Why does a cup with 100 g water float when placed on another cup with 50 g of water?
Imagine we have cup A with 50 g of water and cup B (smaller in width than A) with 100 g of water. Now put cup B into cup A. If the width of both cups are of comparable size then the cup with 100 g of water floats. It does not touch the bottom of cup…
quantum
- 531
54
votes
2 answers
Symmetries of the Standard Model: exact, anomalous, spontaneously broken
There are a number of possible symmetries in fundamental physics, such as:
Lorentz invariance (or actually, Poincaré invariance, which can itself be broken down into translation invariance and Lorentz invariance proper),
conformal invariance (i.e.,…
Gro-Tsen
- 800
54
votes
5 answers
How do we know that heat is a differential form?
In thermodynamics, the first law can be written in differential form as
$$dU = \delta Q - \delta W$$
Here, $dU$ is the differential $1$-form of the internal energy but $\delta Q$ and $\delta W$ are inexact differentials, which is emphasized with the…
EuYu
- 1,244
- 1
- 14
- 22
54
votes
4 answers
Why do electrons occupy the space around nuclei, and not collide with them?
We all learn in grade school that electrons are negatively-charged particles that inhabit the space around the nucleus of an atom, that protons are positively-charged and are embedded within the nucleus along with neutrons, which have no charge. I…
voithos
- 3,439
- 3
- 28
- 37
54
votes
6 answers
How do we know photons have spin 1?
Electrons have spin 1/2, and as they are charged, they also have an associated magnetic moment, which can be measured by an electron beam splitting up in an inhomogeneous magnetic field or through the interaction of the electrons's magnetic moment…
yippy_yay
- 2,598
54
votes
3 answers
Are modified theories of gravity credible?
I'm a statistician with a little training in physics and would just like to know the general consensus on a few things.
I'm reading a book by John Moffat which basically tries to state how GR makes failed predictions in certain situations. I know GR…
dcl
- 653
54
votes
5 answers
Can a radioactive atom be prevented from decaying if it's in a REALLY strong chemical bond?
So, based on this question, a molecule containing a radioactive atom will break when the atom decays. But suppose you need a lot energy to break the compound apart --- as in, more energy than the decay of the atom will release (obviously, a molecule…
Palbitt
- 680
54
votes
5 answers
What allows a pull-back toy car to drive further than it was pushed?
Imagine you have a pull back toy car. Its back part is on $x_0$. You push it down and move it in the back direction to the point $y$ (not marked):
Then you leave the car to move away:
Then you mark the final position by $x_1$:
Let's say, that the…
User123
- 1,173
54
votes
2 answers
In QFT, why does a vanishing commutator ensure causality?
In relativistic quantum field theories (QFT),
$$[\phi(x),\phi^\dagger(y)] = 0 \;\;\mathrm{if}\;\; (x-y)^2<0\,.$$
On the other hand, even for space-like separation
$$\phi(x)\phi^\dagger(y)\ne0\,.$$
Many texts (e.g. Peskin and Schroeder) promise that…
hwlin
- 2,548