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Feb
28
comment How do inductors store energy?
Voting to leave open because research effort is not a criterion for closing.
Feb
26
comment How is a human voice unique?
Also, here's what a pure monochromatic sinusoid sounds like.
Feb
26
comment How is a human voice unique?
Possible duplicate of What are those characteristics by which every sound can be identified uniquely? and also Sound difference between musical instruments
Feb
25
comment Are the colors of the rainbow absolute or arbitrarily assigned?
Indigo is definitely the most arbitrary -- it's included in order to have a vowel between B and V for acronym purposes.
Feb
24
comment Can light projection on a wall travel faster than the speed of light?
Possible duplicate of Is the "How to break the speed of light" minute physics video wrong? and also Can a dot of light travel faster than the speed of light? and Can a very long screen display an object travelling faster than light?
Feb
23
comment Proof that 1d lattice displacement by phonons is given $u_{n\pm 1}(t) = A_ke^{i\omega_k t} e^{i knd}e^{\pm i k d}$
This is a standard wave mechanics problem I'm 99% sure is covered in e.g. Waves by Crawford, if you want a reference.
Feb
23
comment What effect does voltage have on electron energy levels?
I'm voting to leave open because "insufficient effort" is not, nor ever has been, a reason to close. That's what the downvote button is for.
Feb
23
comment Are ergs commonly used in astrophysics? If so, is there a specific reason for it?
E&M definitely plays a huge role in astrophysics' reasoning: the vast majority of matter in the universe is ionized plasma, generating and responding to electromagnetic fields.
Feb
22
comment Redshift Versus Luminosity
Peter's answer gives a good answer to why we need to consider luminosities. I'll add, though, that we use the luminosity method the other way around -- given that everything is calibrated, it is often more useful to know how much dimmer an object will be than how many proper gigaparsecs away it is. Similarly, one would use the angular diameter distance if one was interested in how big something appeared. In fact, the proper distance is probably the least useful distance measure, and it is also the least directly connected to observations.
Feb
21
comment How many galaxies could be the source of the recent LIGO detection?
@EmilioPisanty I've added a bunch more that might answer some of those questions.
Feb
21
comment What produce X rays?
A tip on your English: The occasional grammatical mistake is not a big deal. But no matter what your native language is, I know it doesn't just use excessive capitalization, random line breaks, multiple question marks in a row, or inconsistent spacing around punctuation. These are traits of informality, not unfamiliarity, and they make your post difficult to read (in any language).
Feb
21
comment Meaning of $5.1\sigma$ significance with regards to GW150914
True, p-values are easier to use for directly comparing probabilities (though I'd argue that the human mind is pretty awful at appreciating probabilities less than $1/1000$ anyway, so intuition isn't terribly important for some things). On the other hand, sigmas give an answer in terms of the natural scale for the noise. If I double the exposure time of my counting-statistics-dominated image, my noise should go down by a factor of $\sqrt{2}$ and I can expect a $3$-sigma detection, if real, to become a $4.2$-sigma detection.
Feb
21
comment Meaning of $5.1\sigma$ significance with regards to GW150914
"It would make more sense to simply report a p-value." And in astrophysics we think it would make more sense for medicine to use sigmas rather than p-values. It's just an opinion. "The LIGO hypothesis testing is, however, strictly frequentist." There is never a difference between a frequentist and a Bayesian answer to a question. The only difference is that people who call themselves frequentist tend to ask one type of question, while those who call themselves Bayesian tend to ask another.
Feb
21
comment How is locality preserved in quantum mechanics?
There are a red and a blue marble in a bag. We each take one without looking. Then you go to Pluto. I look and see a red marble. I instantly know you have a blue marble. Something to ponder...
Feb
20
comment Where does gravity get its energy from?
@TheVoid Um what recent changes?
Feb
19
comment Derivation of Gordon identity from Srednicki
For future reference (@SebastianRiese too) the cancel package is not loaded by default in our mathjax environment, but it is available. Just put \require{cancel} in any math environment before its first use.
Feb
19
comment If I Blow smoke out my window, will any come inside my room?
Indeed I took some shortcuts and assumed static conditions. In my defense, the pressure gradients that do appear are small compared to the iso-density case I was comparing against. Certainly my windows in winter aren't holding back 1-2 pounds per square inch of pressure difference.
Feb
18
comment Is the GW150914 signal consistent with a superluminal gravity wave burst?
@Livid On the second point, Lakatos had somewhat nuanced views, but in any event he and Quine and Kuhn were all greatly influenced (possibly too much so) by the rapid evolution of physics at the turn of the century. The vast majority of professional scientists are not engaged in instigating Kuhnian revolutions, yet they are certainly advancing science as I see it. Times when science were overturned make for nice stories, but we should be careful to not forget about the good work done in the interim, nor should we look to have revolutions for revolutions' sake.
Feb
18
comment Is the GW150914 signal consistent with a superluminal gravity wave burst?
@Livid On the first point see this and this.
Feb
18
comment Is the GW150914 signal consistent with a superluminal gravity wave burst?
@Livid That's a good question. You might start with this post. Basically, general relativity permits space itself to expand at any speed, but forbids any actual object from moving faster than light. Also it's worth noting that in general relativity there is no way to compare objects' speeds unless they are collocated. Thus when we say "a galaxy is receding faster than light" this doesn't really reference the galaxy's motion relative to us in the special relativity sense.