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1500 questions
68
votes
9 answers
What are the next generation physics experiments?
The LHC and LIGO are two recent examples of hugely ambitious experiments in fundamental physics, both of which took decades to develop.
What are the next major experiments currently being planned and developed? What will they measure? What impact…
lemon
- 13,320
68
votes
6 answers
Path integral vs. measure on infinite dimensional space
Coming from a mathematical background, I'm trying to get a handle on the path integral formulation of quantum mechanics.
According to Feynman, if you want to figure out the probability amplitude for a particle moving from one point to another, you…
Nick Alger
- 2,790
68
votes
5 answers
How wrong are the classical Maxwell's equations (as compared to QED)?
Now, I don't really mean to say that Maxwell's equations are wrong. I know Maxwell's equations are very accurate when it comes to predicting physical phenomena, but going through high school and now in college, Maxwell's equations are seen as the…
user28375028
- 953
68
votes
5 answers
What challenges needed to be overcome to create (blue) LEDs?
In light of today's announcement of the 2014 Nobel laureates, and because of a discussion among colleagues about the physical significance of these devices, let me ask:
What is the physical significance of blue LEDs, which challenges had to be…
Martin
- 15,837
68
votes
6 answers
What if the universe is rotating as a whole?
Suppose in the milliseconds after the big bang the cosmic egg had aquired some large angular momentum. As it expanded, keeping the momentum constant (not external forces) the rate of rotation would have slowed down, but it would never reach…
John Alexiou
- 40,139
67
votes
5 answers
What exactly is regularization in QFT?
The question.
Does there exist a mathematicaly precise, commonly accepted definition of the term "regularization procedure" in perturbative quantum field theory? If so, what is it?
Motivation and background.
As pointed out by user drake in his…
joshphysics
- 58,991
67
votes
7 answers
Fourier transform of the Coulomb potential
When trying to find the Fourier transform of the Coulomb potential
$$V(\mathbf{r})=-\frac{e^2}{r}$$
one is faced with the problem that the resulting integral is divergent. Usually, it is then argued to introduce a screening factor $e^{-\mu r}$ and…
Emerson
- 1,377
67
votes
8 answers
Can 1 kilogram of radioactive material with half life of 5 years just decay in the next minute?
I wondered this since my teacher told us about half life of radioactive materials back in school. It seems intuitive to me to think this way, but I wonder if there's a deeper explanation which proves me wrong.
When many atoms are involved, half life…
uylmz
- 805
67
votes
2 answers
What explanations are there for a rug steadily creeping at about a millimeter/day rate across a room?
I have bought a handmade rug of size 1.5 $\times$ 2m. About 1-2 weeks ago I noticed the rug was not in the center of my room and it had moved a bit. I thought maybe because I walked on it, it has moved. I put it in its place and it happened again…
Joe
- 793
67
votes
15 answers
Why is ascending some stairs more exhausting than descending?
I have been asked this question by school kids, colleagues and family (usually less formally):
When ascending a flight of stairs, you exchange mechanical work to attain potential Energy ($W_\text{ascend} = E_\text{pot} = m gh$).
However, when…
Daniel
- 831
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67
votes
4 answers
Why is a second equal to the duration of exactly 9,192,631,770 periods of radiations?
Why is a second equal to the duration of 9,192,631,770 periods of radiation corresponding to the transition between two hyperfine levels of the ground state of the caesium-133 atom?
Why is the number of periods so complicated?
It could be any…
A. Vats
- 797
67
votes
1 answer
How to apply the Faddeev-Popov method to a simple integral
Some time ago I was reviewing my knowledge on QFT and I came across the question of Faddeev-Popov ghosts. At the time I was studying thеse matters, I used the book of Faddeev and Slavnov, but the explanation there is not very transparent, specially…
Alexander Cska
- 1,251
67
votes
12 answers
How long a straw could Superman use?
To suck water through a straw, you create a partial vacuum in your lungs. Water rises through the straw until the pressure in the straw at the water level equals atmospheric pressure. This corresponds to drinking water through a straw about ten…
Mark Eichenlaub
- 53,961
67
votes
4 answers
Lie derivative vs. covariant derivative in the context of Killing vectors
Let me start by saying that I understand the definitions of the Lie and covariant derivatives, and their fundamental differences (at least I think I do). However, when learning about Killing vectors I discovered I don't really have an intuitive…
Javier
- 28,811
67
votes
7 answers
Does gravity exist in a vacuum?
My understanding has always been that it does from conventional science courses, but really thinking about it, I was wondering if this is really the case.
To my limited understanding there is a theory that there are gravitons that act as particles…
user4779
- 781