822 reputation
214
bio website
location London, United Kingdom
age
visits member for 2 years, 4 months
seen Dec 6 at 16:25

Oct
19
accepted Why does bad smell follow people (assuming they are not the source)?
Oct
18
comment Can a trajectory between planets accelerate a space craft?
Well, it depends. Black holes might actually have a weaker gravitational field than objects that are not black holes. What distinguishes them is not the strength of the field, but the fact that they are small enough that you can get closer to the centre than the so called Schwarzschild radius (the point where "light cannot escape").
Oct
18
comment Proof of quantum mechanical position uncertainty
Also, as an aside, your last formula is still incorrect. You need $⟨\rangle$ around the value inside the square root, otherwise you are taking the square root of the position operator (not what you want)
Oct
18
revised Why does bad smell follow people (assuming they are not the source)?
Minor grammar fixes.
Oct
18
answered Can a trajectory between planets accelerate a space craft?
Oct
18
awarded  Organizer
Oct
18
asked Why does bad smell follow people (assuming they are not the source)?
Oct
18
comment A change in the gravitational law
Also the iron sphere theorem would not hold as it relies on area of a sphere increasing with $r^2$. Relativity gets around this by changing the geometry.
Oct
18
comment Proof of quantum mechanical position uncertainty
Now that you have the expansion you can use the linearity of the expectation value, i.e. $\langle a \hat{x} + b \hat{y} \rangle = a \langle \hat{x} \rangle + b \langle \hat{y} \rangle$, where I marked the operators with hats and $a,b$ are numbers. In your expression the operators are $x$ and $x^2$, and $\langle x \rangle$ is a number. You are very close to the answer.
Oct
17
revised Proof of quantum mechanical position uncertainty
added 2 characters in body
Oct
16
comment Proof of quantum mechanical position uncertainty
You're right, here goes.
Oct
16
answered Proof of quantum mechanical position uncertainty
Oct
16
comment Proof of quantum mechanical position uncertainty
I'm afraid your step is incorrect (the last formula). Expanding $\langle(x-\langle x \rangle)^2\rangle$ you obtain $\langle x^2 -2x \langle x \rangle x - \langle x \rangle^2\rangle$. From here you only need to use that $\langle x \rangle$ is a number and that expectation value is linear. Since this looks like a homework, I won't work it all out for you (important part of the learning process in physics is to calculate things for yourself). But hopefully this is enough of a hint to get you to the right answer.
Oct
15
comment Mathematically challenging areas in Quantum information theory and quantum cryptography
If you are into more mathematically challenging things, you could start here: arxiv.org/abs/1106.1445. A review article titled "From classical to quantum shannon theory" mostly skips through the linear algebra and goes straight to the rigorous quantum info. Recommended. You might also want to know that entanglement is nowadays seen more as only one of the resources and that other resources such as quantum discord are gaining in importance.
Oct
14
comment Is there still light in practical darkness? Do photons penetrate everywhere?
Right, I agree.
Oct
13
comment Is there still light in practical darkness? Do photons penetrate everywhere?
Yes, and with the typical temperatures they would radiate at infrared wavelengths, not something you can typically see unless the object is very hot (hence the red glow of very hot objects).
Oct
12
awarded  Critic
Oct
12
reviewed Reviewed Why do we use Hermitian operators in QM?
Oct
12
comment What is the physical size of a black hole?
Yes, I know that. But in the frame of an asymptotic observer that happens only as $t \rightarrow \infty$. See also a related question: physics.stackexchange.com/questions/21319/…
Oct
11
comment decoherence free subspace of a single photon
Do you know the Kraus operators of the second order polarization mode dispersion when you take frequency as a classical variable?