SMeznaric

511 reputation

bio	website	physics.ox.ac.uk/qubit/…
	location	Oxford, United Kingdom
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visits	member for	9 months
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	511 reputation	bio	website	physics.ox.ac.uk/qubit/…	visits	member for	9 months
12 badges		location	Oxford, United Kingdom		seen	17 hours ago

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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	answered	how to find correlation between four variables?
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	What facts and examples should I use in a debate with a Relativity Denier? If you will be debating the important thing is to convince your audience. In my experience nobody will change their mind on the spot. But if you make very good arguments they will go home and mull it over and some of them might get convinced. My suggestion is don't worry about your position, just punch holes in his arguments. Ask him first if god is inside the universe. If he says yes, tell him that singling out a reference frame does not negate relativity. I could also say Earth is a preferred reference frame, does not make relativity wrong. If he says no then god does not have a ref frame.
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?