Chris White
Reputation
87/100 score
 Jan 13 comment How to demonstrate frame dragging through the Kerr metric? What form of the metric do you have then? And what have you tried? For instance, what do you take frame dragging to be mathematically? You should show what you've done so we know where to start with an answer. Jan 12 comment How do we stabilise satellites so precisely? If you're impressed with Hubble's pointing, you may be interested in Gaia. Jan 11 comment Curvature of spacetime as a real thing? @barongbaron While it is true (but highly nontrivial to prove!) that one can always embed spacetime in a higher-dimensional flat manifold, this is emphatically not how anyone thinks about or does general relativity. Curvature can be measured without venturing into other dimensions, just as you can empirically prove the Earth is round without ever leaving the surface. Jan 6 comment Sign of the totally anti-symmetric Levi-Civita tensor $\varepsilon^{\mu_1 \ldots}$ when raising indices But your first formula and my sign change from ${}^{**}H$ to $H$ depend on the dimension of the space, the number of indices on $H$, and the number of negative signs in the metric signature, so I don't guarantee this works if those things change. Jan 6 comment Sign of the totally anti-symmetric Levi-Civita tensor $\varepsilon^{\mu_1 \ldots}$ when raising indices @Marion That's very confusing notation without Hodge stars, since $H^{\mu\nu}$ should by all rights be $g^{\mu\alpha} g^{\nu\beta} H_{\alpha\beta}$. In any case, we have $({}^*H)^{\mu\nu} = -(1/2)\epsilon^{\mu\nu\rho\sigma}H_{\rho\sigma}$. Then $(1/2)\epsilon_{\mu\nu\rho\sigma}({}^*H)^{\rho\sigma} = (1/2)g_{\mu\alpha}g_{\nu\beta}\epsilon^{\alpha\beta\rho\sigma}({}^*H)_{\rho\sigm‌​a} = -g_{\mu\alpha}g_{\nu\beta}({}^{**}H)^{\alpha\beta} = g_{\mu\alpha}g_{\nu\beta}H^{\alpha\beta} = H_{\mu\nu}$. Jan 6 comment Measurement of blueshift from Andromeda galaxy Slipher's paper says only that he used "the region of spectrum from F to H." I wonder if those are references to Fraunhofer lines, which Wikipedia tells me would be 486-397 nm and would include Hβ-Hδ and some iron and calcium. This makes some amount of sense given he used a spectrum of (sunlight reflected off) Saturn for reference. Jan 6 comment Could we make things out of newly discovered particles? I'm curious about your footnote. The neutron was discovered in 1932, the proton c. 1920, and the electron in 1897. 83 years old isn't that much younger than 113. Jan 5 comment What does 1714 mean in hydraulics? The same thing occurs in any unit system. You could measure lengths in meters on one side of the equation and in light years on the other side, and then the dimensionless (but not equal to 1) quantity $(1\ \mathrm{ly})/(1\ \mathrm{m})\approx9.5\times10^{15}$ will probably appear somewhere in the equation. Jan 5 comment What does 1714 mean in hydraulics? @Dennis The problem is that $1\ \mathrm{HP}$ is not the same magnitude as $(1\ \mathrm{psi})(1\ \mathrm{gpm})$, even though they both have dimensions of $\mathrm{(mass)(length)^2/(time)^3}$. You could use Imperial units such that $k = 1$; one such way is to measure pressure in psi, flow rate in cubic inches per second, and power in inch-pounds per second. Jan 5 comment When did people start to regard “time” as a physical quantity? It's not clear to me the whole "5 senses" thing is accurate either cognitively or historically. Certainly there is a sense of time, and philosophers hundreds of years ago (and maybe thousands of years ago; I'm less familiar with the literature) had no problem saying it was natural to perceive the passage of time and to order things in time just as they are located in space. Jan 5 comment Does the stationary object deform more than the moving one after a collision? @Nagora Has it right (and this was known to be true many centuries before Einstein). By the way, AlJo, the way comments ping people is complicated, but note I only saw your message above coincidentally. Jan 5 comment Walter Lewin's physics lecture 8.03 Waves and Oscillation @alarge I'm aware of the backstory. As Kyle Kanos said, it's an issue of whether we become a "please give me this file" site, copyright and lecture quality aside. This can always be brought up on Meta for further discussion. Jan 5 comment Walter Lewin's physics lecture 8.03 Waves and Oscillation I'm voting to close because we are not a file-sharing service, though I don't know how much precedence there is on this. We'll see what the rest of the community says. Jan 4 comment Why do the errors in a formula depend on how it's written? @DevgeetPatel You seem to be under the same (wrong) impression that sparked the question Why propagation of uncertainty is linear? I don't know who's out there teaching everyone the wrong formula for error propagation, but you should be aware of the right way to calculate these values. This won't make your problem go away, but as it stands you aren't really calculating any statistically meaningful quantity. Jan 3 comment Is the change in cochlear potentials dependent on perilymph velocity? This question appears to be off-topic because it is about the biological particulars of the human ear. Physics can solve abstract models inspired by living organisms, but it is biology that is tasked with developing those models. Dec 30 comment Tensor Product vs. Direct Product for three spin-1/2 particles Dec 30 comment Tensor Product vs. Direct Product for three spin-1/2 particles tex tips: \mathbb{C} has got to be faster than finding a ℂ character to copy and paste, and similarly for \otimes, \oplus, \equiv, and \in. | doesn't always have the same spacing rules as \lvert, only the latter of which is an actual delimiter, and \hbar and \sigma typeset differently from (i.e. better than) ħ and σ in MathJax. (+1 on the content though.) Dec 27 comment If neutron would be lighter by 1 MeV, how stability of Hydrogen could be changed? Presumably you meant the decay reaction $p \to n + \bar{e} + \nu_e$? (Note also electron capture would be $p + e \to n + \nu_e$.) Dec 27 comment In physics, what is the importance of distinguishing between a matrix and a group? You're not the only one to list the closure property, but it is certainly unnecessary given the definition you provide of a binary operation on $G$. If we have $* : G \times G \to G$, then automatically we know $a * b \in G$. Dec 26 comment How is the Horizon Problem really a problem? @Wood Don't worry, I at least understand your concern. But I've searched for years without finding a satisfactory answer, instead just seeing the same unjustified dogma repeated over and over. If you don't get a good answer over the next few days, feel free to ping me, since I would probably put my own bounty on the question then.