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1500 questions
65
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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
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65
votes
8 answers
Why is the observable universe so big?
The observable universe is approximately 13.7 billion years old. But yet it is 80 billion light years across. Isn't this a contradiction?
Thomas O
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65
votes
10 answers
Why does a free-falling body experience no force despite accelerating?
Note: For the purposes of my question, when I refer to free fall assume it takes place in a vacuum.
From my (admittedly weak) understanding of the equivalence principle, falling in a gravitational field is physically indistinguishable from floating…
AdamJames
- 753
65
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
- 28,106
65
votes
3 answers
Is it possible to "see" atoms?
As per my knowledge, atoms are small beyond our imaginations. But there is an image on Wikipedia that shows silicon atoms observed at the surface of silicon carbide crystals.
The image:
How can we see these distinct atoms if they are so small?
jNerd
- 913
65
votes
14 answers
Why isn't length contraction permanent even though time dilation is?
It's my understanding that when something is going near the speed of light in reference to an observer, time dilation occurs and time goes slower for that fast-moving object. However, when that object goes back to "rest", it has genuinely aged…
theboombody
- 937
65
votes
6 answers
Why do power lines buzz?
When near high tension power lines, particularly after a good rain, the lines themselves emit a buzzing noise. A similar noise can be heard coming out of the electric meters attached to my apartment.
I've heard before that this is supposedly from…
Billy ONeal
- 750
65
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
- 38,341
64
votes
7 answers
How is the Schroedinger equation a wave equation?
Wave equations take the form:
$$\frac{ \partial^2 f} {\partial t^2} = c^2 \nabla ^2f$$
But the Schroedinger equation takes the form:
$$i \hbar \frac{ \partial f} {\partial t} = - \frac{\hbar ^2}{2m}\nabla ^2f + U(x) f$$
The partials with respect…
user28823
- 1,060
64
votes
9 answers
Can the photoelectric effect be explained without photons?
Lamb 1969 states,
A misconception which most physicists acquire in their formative years is that the photoelectric effect requires the quantization of the electromagnetic field for its explanation. [...] In fact we shall see that the photoelectric…
user4552
64
votes
12 answers
Mathematically prove that a round wheel roll faster than a square wheel
Let's say I have these equal size objects (for now thinking in 2D) on a flat surface.
At the center of those objects I add equal positive angular torque (just enough to make the square tire to move forward).
Of course the round tire will move…
John T
- 853
64
votes
5 answers
Have researchers managed to "reverse time"? If so, what does that mean for physics?
According to press releases, researchers have reversed time in a quantum computer and violated the second law of thermodynamics. What does that mean for physics? Will it allow time travel?
Further information:
"Arrow of time and its reversal on…
Omar Einstein
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64
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
- 4,481
64
votes
6 answers
Why do we use capacitors and not batteries in defibrillator?
Why do we use capacitors in defibrillators and not batteries?
I know that capacitors are used to store electrical energy but isn't the function of a battery just the same?
Moreover, I know that batteries are used to make capacitors work in a…
musicinmyheart
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64
votes
10 answers
What is the difference between a measurement and any other interaction in quantum mechanics?
We've learned that the wave function of a particle collapses when we measure a particle's location. If it is found, it becomes more probable to find it a again in the same area, and if not the probability to finding it in the place that was checked…
Uri
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