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1500 questions
39
votes
2 answers
What's the point of Pauli's Exclusion Principle if time and space are continuous?
What does the Pauli Exclusion Principle mean if time and space are continuous?
Assuming time and space are continuous, identical quantum states seem impossible even without the principle. I guess saying something like: the closer the states are the…
Yogi DMT
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39
votes
1 answer
Can lightning be used to solve NP-complete problems?
I'm a MS/BS computer science guy who is wondering about why lightning can't (or can?) be used to solve NP complete problems efficiently, but I don't understand the physics behind lightning, so I'm posting here.
What seems peculiar to me about…
micahhoover
- 525
39
votes
3 answers
Why can interaction with a macroscopic apparatus, such as a Stern-Gerlach machine, sometimes not cause a measurement?
Consider a Stern-Gerlach machine that measures the $z$-component of the spin of an electron. Suppose our electron's initial state is an equal superposition of
$$|\text{spin up}, \text{going right} \rangle, \quad |\text{spin down}, \text{going right}…
knzhou
- 101,976
39
votes
2 answers
What's the intuition behind the Choi-Jamiolkowski isomorphism?
What is the intuition behind the Choi-Jamiolkowski isomorphism? It says that with every superoperator $\mathbb{E}$ we can associate a state given by a density matrix
$$ J(\mathbb{E}) = (\mathbb{E} \otimes 1) (\sigma)$$
where $\sigma = \sum_{ij} |…
Spine Feast
- 2,805
39
votes
1 answer
$(\mu,P,T)$ pseudo-ensemble: why is it not a proper thermodynamic ensemble?
While teaching statistical mechanics, and describing the common thermodynamic ensembles (microcanonical, canonical, grand canonical), I usually give a line on why there can be no $(\mu, P, T)$ thermodynamic ensemble ($\mu$ being the chemical…
F'x
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39
votes
7 answers
Why don't we feel the subtle speed change of Earth's elliptical orbit?
Earth's orbit is a slight ellipse, so to conserve momentum its speed increases when it is closest to the Sun. If the speed changes there is an acceleration. If there is an acceleration there is a force. Even if the change is small and gradual,…
BoddTaxter
- 2,858
39
votes
10 answers
Why don't modern spacecraft use nuclear power?
The Voyager 1 & 2 spacecraft launched in 1977 with Plutonium as their source of electricity. 34 years later they claim these two spacecraft have enough power to last them until at least 2020. That means they'll have had enough power to last them at…
John Conde
39
votes
5 answers
Would you be weightless at the center of the Earth?
If you could travel to the center of the Earth (or any planet), would you be weightless there?
freeside
- 543
39
votes
7 answers
Isn't the uncertainty principle just non-fundamental limitations in our current technology that could be removed in a more advanced civilization?
From what I understand, the uncertainty principle states that there is a fundamental natural limit to how accurately we can measure velocity and momentum at the same time. It's not a limit on equipment but just a natural phenomenon.
However, isn't…
user8791
39
votes
8 answers
How is Liouville's theorem compatible with the Second Law of Thermodynamics?
The second law says that entropy can only increase, and entropy is proportional to phase space volume. But Liouville's theorem says that phase space volume is constant.
Taken naively, this seems to imply that the entropy can never change. What's…
knzhou
- 101,976
39
votes
4 answers
Why does a moving fan seem transparent?
We all know when fan starts moving faster, we cannot see its blades. Why is this?
First I assumed persistence of vision may be the reason. But that can happen with blade also right? Image of blade can remain in our memory and moving fan can appears…
tollin jose
- 1,283
39
votes
4 answers
Why can't the Navier Stokes equations be derived from first principle physics?
At the 109th UCLA Faculty Research lecture, Seth Putterman gave a talk on Sonoluminescence. During the lecture he emphasized that "The Navier Stokes equations cannot be derived from first principles [of physics]".
In physics there are lots of first…
docscience
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39
votes
2 answers
How does the Higgs mechanism work?
I'm not a particle physicist, but I did manage to get through the Feynman lectures without getting too lost.
Is there a way to explain how the Higgs field works, in a way that people like me might have a hope of understanding?
Mike Dunlavey
- 17,055
39
votes
5 answers
If the Earth is a good conductor of electricity, why don't people get electrocuted every time they touch the Earth?
Since the Earth is a good conductor of electricity, is it safe to assume that any charge that flows down to the Earth must be redistributed into the Earth in and along all directions?
Does this also mean that if I release a million amperes of…
Swami
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39
votes
7 answers
Can one do the maths of physics without using $\sqrt{-1}$?
The use of imaginary and complex values comes up in many physics and engineering derivations. I have a question about that: Is the use of complex numbers simply to make the process of derivation easier, or is it an essential ingredient, without…
Ajay
- 577