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1500 questions
70
votes
2 answers
Does the new finding on "reversing a quantum jump mid-flight" rule out any interpretations of QM?
This new finding by Minev et al. seems to suggest that transitions between atomic states are not instantaneous, but continuous processes wherein a superposition smoothly adjusts from favoring one state to another (if I understand it correctly). The…
WillG
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70
votes
2 answers
Why is a neutron in free state unstable?
A neutron is a neutral particle which is merely some times more massive than an electron.
What makes it so unstable outside the nucleus that it has a half life only of about 12 min?
kalyani
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70
votes
3 answers
What makes cheese so effective at absorbing microwaves?
Whenever I put a meal in the microwave which contains cheese, why does the cheese get hot before the rest of the meal is heated through?
it's a hire car baby
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70
votes
17 answers
What is the simplest way to prove the Earth is round?
Assume you've come in contact with a tribe of people cut off from the rest of the world, or you've gone back in time several thousand years, or (more likely) you've got a numbskull cousin.
How would you prove that the Earth is, in fact, round?
tQuarella
70
votes
5 answers
Why do electron and proton have the same but opposite electric charge?
What is the explanation between equality of proton and electron charges (up to a sign)? This is connected to the gauge invariance and renormalization of charge is connected to the renormalization of photon field, but is this explanation enough? Do…
Newman
- 2,566
70
votes
2 answers
What are the proposed realizations in the New SI for the kilogram, ampere, kelvin and mole?
The metrology world is currently in the middle of overhauling the definitions of the SI units to reflect the recent technological advances that enable us to get much more precise values for the fundamental constants of nature than were possible when…
Emilio Pisanty
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70
votes
6 answers
When is the Hamiltonian of a system not equal to its total energy?
I thought the Hamiltonian was always equal to the total energy of a system but have read that this isn't always true. Is there an example of this and does the Hamiltonian have a physical interpretation in such a case?
Noah
- 1,760
69
votes
3 answers
Idea of Covering Group
$SU(2)$ is the covering group of $SO(3)$. What does it mean and does it have a physical consequence?
I heard that this fact is related to the description of bosons and fermions. But how does it follow from the fact that $SU(2)$ is the double cover…
SRS
- 26,333
69
votes
1 answer
Status of experimental searches for tachyons?
Now that the dust has settled on the 2011 superluminal neutrino debacle at OPERA, I'm interested in understanding the current status of experimental searches for tachyons. Although the OPERA claim was a fun puzzle for theorists trying to find…
user4552
69
votes
5 answers
How do we know the LHC results are robust?
Nature article on reproducibility in science.
According to that article, a (surprisingly) large number of experiments aren't reproducible, or at least there have been failed attempted reproductions. In one of the figures, it's said that 70% of…
Allure
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69
votes
1 answer
Is spacetime flat inside a spherical shell?
In a perfectly symmetrical spherical hollow shell, there is a null net gravitational force according to Newton, since in his theory the force is exactly inversely proportional to the square of the distance.
What is the result of general theory of…
Leos Ondra
- 2,133
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
- 3,069
69
votes
5 answers
Why do atomic clocks only use caesium?
Modern atomic clocks only use caesium atoms as oscillators. Why don't we use other atoms for this role?
Pinki
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69
votes
5 answers
Why is the harmonic oscillator so important?
I've been wondering what makes the harmonic oscillator such an important model. What I came up with:
It is a (relatively) simple system, making it a perfect example for physics students to learn principles of classical and quantum mechanics.
The…
Spine Feast
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68
votes
2 answers
Why was water freezing almost instantaneously when shaking a bottle that spent the night outside during a frosty night?
Due to the forecasted frost last night, I placed yesterday evening, some 1.5l standard PET bottles filled up to 90% with warm tap water(+60°C) close to some vegetables that I wanted to protect in my garden. The temperature dropped to roughly -3 ~…
s.k
- 781