Why is the speed of light said to be constant when we know it slows down in a medium?
What is the mechanism behind the slowdown of light/photons in a transparent medium?
I often see tutorials on the web or posts on this site which make heavy use of the following characters at the command line. Often it seems that they are used for pretty similar purposes. What are the differences between them when used on the command line or for shell programming? For what purpose do I use which of them?  " " double quotes  ' ' single quotes  ` ` backticks I am just learning ArcGIS 10.2 and it was working well until I tried to use the Export to a Geodatabase (single) option for a dbs table I made. Since then, it seems I now get the same Script Error for import/export functions. However, I just tried to export to Geodatabase (multiple) and error did not pop up. I spoke with tech support and they could not help me so far. I really do not know much about GIS so it is hard for me to fix this problem and cannot proceed with some things. Once upon a time,  DISPLAY=:0.0 totem /path/to/movie.avi   after ssh 'ing into my desktop from my laptop would cause totem to play movie.avi on my desktop.  Now it gives the error:   No protocol specified Cannot open display:    I reinstalled Debian squeeze when it went stable on both computers, and I guess I broke the config.   I've googled on this, and cannot for the life of me figure out what I'm supposed to be doing.   (VLC has an HTTP interface that works, but it isn't as convenient as ssh.)  The same problem arises when I try to run this from a cron job. I'm using an unsigned repo in Ubuntu 16.04 from Debian multimedia:  deb http://www.deb-multimedia.org jessie main   To install deb-multimedia-keyring, I'm running:  apt-get update &amp;&amp; apt-get install deb-multimedia-keyring -y   This gives an error:  W: GPG error: http://www.deb-multimedia.org jessie InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 5C808C2B65558117 E: The repository 'http://www.deb-multimedia.org jessie InRelease' is not signed. N: Updating from such a repository can't be done securely, and is therefore disabled by default. N: See apt-secure(8) manpage for repository creation and user configuration details. How do you greet multiple recipients in an e-mail? How do you greet multiple recipients in an e-mail?  Assuming they're both male, I just use "Sirs", but it seems a bit informal. How can we prove charge invariance under Lorentz Transformation? We have gravitational force between two massive particles and we have electromagnetic force between two charged particles. When special relativity suggests that mass is not an invariant quantity why we have electric charge as an invariant quantity ? I have a directory containing a large number of files. I want to delete all files except for file.txt . How do I do this?  There are too many files to remove the unwanted ones individually and their names are too diverse to use * to remove them all except this one file.  Someone suggested using   rm !(file.txt)   But it doesn't work. It returns:  Badly placed ()'s    My OS is Scientific Linux 6.  Any ideas? I'm trying to measure the execution time of a process that I call via the command line (i.e., I want to find out how long it takes to for the process to finish). Is there any command that I can add to the command calling the process that will achieve this? is a typical example for , though it is said that long time no see has been accepted by the mainstream English speakers.  Recently, there is a  movement in the , and a few new Chinglish words have been invented to demonstrate the characteristics of the Mainland.  The most outstanding examples (in my opinion) are:   shitizen - a citizen without citizen rights freedamn - the freedom for shitizens (no freedom) democrazy - the democracy for shitizens (no democracy) smilence - the speeches under the freedamn of speech () z-turn - to make effort in vain ( (Pinyin))   Here are my questions. Has there ever been any neologism movement in the history of English? What are the criteria to adopt new words into English? Get two letter country code in Apex method I want to write an Apex method that takes a country name as input and returns the equivalent two letter country code (ISO A2). Is there a way to write it without explicitly hard coding all countries and code in the method? Derivation of survival copula $$ \begin{align}F(x,y) &amp;= P(X\leq x, Y\leq y) \\  &amp;= C(F(x), F(y)) \end{align}$$ The copula of a bivariate distribution is equal to its CDF, $C(u,v) = F(x,y)$. The density of the data's CDF therefore can be expressed as $$f(x,y) = c(u,v) \times f_X(x) f_Y(y)$$ where $c(u,v)$ is the copula density and $f_X(x)$ is a marginal. If $\hat{C}(u,v)$ is the survival copula for determining $$ \begin{align}\bar{F}(x,y) &amp;= P(X&gt; x, Y &gt; y) \\  &amp;= \hat{C}(1-F(x), 1-F(y)) \end{align}$$ how is the first equation of the ordinary copula, $C(u,v)$, linked to the survival copula as $$\hat{C}(u,v) = u + v - 1 + C(1-u,1-v)$$ are $u$ and $v$ merely vectors containing the (ranked) observations of the uniform marginals? A derivation in integrals and probabilities (rather than $F$s) would be good. Can I change the font systemwide (10.8.3) Is it possible to change the systems font? (finder, alerts, menus etc) NullReferenceException in Unity Is floating point math broken? Consider the following code:   0.1 + 0.2 == 0.3  -&gt;  false   0.1 + 0.2         -&gt;  0.30000000000000004   Why do these inaccuracies happen? Customizing titlesec-etoc (further question) What do you call someone that believes he is the only living being? What do you call someone that believes he is actually the only living being and that everyone and everything that exist (besides him self) is a state of his own mind? I am not referring to a In what sense is a scalar field observable in QFT? Consider a QFT consisting of a single, hermitian scalar field $\Phi$ on spacetime (say $\mathbb R^{3,1}$ for simplicity).  At each point $x$ in spacetime, $\Phi(x)$ is an observable in the sense that it is a hermitian operator (operator-valued distribution) on the Hilbert space of the theory, but is each such operator observable in a stronger, more physical sense?  Is there an experiment one could hypothetically perform to measure the value of such a field at a given spacetime point? This is one of those questions I glossed over while learning QFT, but now it's bugging me.  In particular, I think this point is central in preventing me from understanding certain basic assumptions in QFT such as microcausality which I also never really think about anymore. How to extract img src, title and alt from html using php? I would like to create a page where all images which reside on my website are listed with title and alternative representation.  I already wrote me a little program to find and load all HTML files, but now I am stuck at how to extract src, title and alt from this HTML:  &lt;img src="/image/fluffybunny.jpg" title="Harvey the bunny" alt="a cute little fluffy bunny" />  I guess this should be done with some regex, but since the order of the tags may vary, and I need all of them, I don't really know how to parse this in an elegant way (I could do it the hard char by char way, but that's painful). How to do multiple joins when using groupby in linq Does this constitute a valid proof that $\frac{x^2}{1+x^4} \leq \frac{1}{2}$?