Difference between reflection and re émission of light
How does reflection work?
Is acceleration relative in relativity? Suppose a box A is moving relative to a Box B, then by time dilation equation if I take 1 sec passed for an observer in A then for an observer in B will be little longer. Now if I suppose that the box B is moving while A is stationary under the same condition, then by the time dilation equation time passed in B must be shorter than A. How is this issue resolved? In the twin paradox problem my book says that it's due to acceleration, but in my opinion acceleration is relative (please correct me if I'm wrong), but here it is not the case. "None" as plural indefinite pronoun I need rules to drop some malicious Apache connections I drop all traffic on ports except on 80 for my web server.  I have some rules like this on iptables:  iptables -A INPUT -p tcp -m tcp --dport 80 -m string --string "cgi" --algo bm --to 1000 -j DROP   Someone who have more can share? I know always the bad hackers still updating, but some of them always begin with the same code. I need to Drop the connection based on some criteria. Here are some Apache log (I remove ips but each atack come from the same):  Attack 1: This I don't know what are trying to do, but make it 50 times from the same ip  GET / HTTP/1.1  301 224 -   Mozilla/5.0 (X11; Linux i686) AppleWebKit/537.22 (KHTML, like Gecko) Chrome/25.0.1364.152 Safari/537.22 GET / HTTP/1.1  302 3387    -   Mozilla/5.0 (X11; Linux i686) AppleWebKit/537.22 (KHTML, like Gecko) Chrome/25.0.1364.152 Safari/537.22   Attack 2: this try to get information about server only.  GET / HTTP/1.1  301 224 http://myip:80/ Go-http-client/1.1 GET / HTTP/1.1  302 3228    http mywebsite  Go-http-client/1.1 GET /es/ HTTP/1.1   200 40947   https mywebsite Go-http-client/1.1   Attack 3: they try get access to a login page vulnerability  GET /userlogin/login.aspx HTTP/1.1  302 186 -   -   Attack 4: this try to access a cgi at first request, (see my first iptables rule to drop this)  GET /hndUnblock.cgi HTTP/1.0    302 186 -   Wget(linux) GET /tmUnblock.cgi HTTP/1.0 302 186 -   Wget(linux)   I'm very new with server this 4 attacks are from only last 12 hours... Have thousands per week. What software works well for viewing massive TIFF images on Windows 7? Quote.class doesn't return the Quote type, but Account.class does. Why? I'm trying a library that acts like an object factory. It works with serialization, so it is possible to specify formula fields without DML, which is pretty handy with tests.  My issue is that I'm trying to write a class that requires the Quote type. The apex compiler throws me the following error when using Quote.class:     Invalid field class for SObject Quote   But Opportunity.class, Account.class and even QuoteLineItem.class work just fine. Any idea why? Is it not possible to get the quote type? Or is there another way of doing it? How can I send the stdout of one process to multiple processes using (preferably unnamed) pipes in Unix (or Windows)? I'd like to redirect the stdout of process proc1 to two processes proc2 and proc3:           proc2 -&gt; stdout        /  proc1        \           proc3 -&gt; stdout   I tried   proc1 | (proc2 &amp; proc3)   but it doesn't seem to work, i.e.   echo 123 | (tr 1 a &amp; tr 1 b)   writes   b23   to stdout instead of    a23  b23 How to detect the presence of URL in a string Double orthogonal complement is equal to topological closure What causes yogurt in sauces to split? How to prevent it? A lot of my favorite curry recipes have a yogurt based sauce in them, but on a pretty regular basis when I make them, the yogurt ends up splitting into basically curds and whey. What causes that, and how can I prevent it?  As an illustrative example, last night I made a simple tofu curry as follows:   warm evoo in a pan add chopped onion, saute briefly add curry seasonings, allow to warm/season the oil add cubed tofu, toss to coat cook for a while, tossing periodically to lightly brown the cubes turn heat off add plain greek yogurt stir to combine   In the few minutes it took to finish the rest of the meal and start plating, the yogurt had separated so I had a clumpy, lumpy, yogurt soup instead of a smooth creamy sauce. How to run applications as root? I am having some strange issue with Kate and Kwrite. When I click on Open File, it crashes with segmentation fault.   I am a complete newbie to Linux, and I think the issue is that I am not running the application as root.   How do I run applications as root in Ubuntu? Is it bad practice to do this? What is the purpose of the whole root thing, where even though we need to use root so frequently, it is not utilized as default? I have a spawn area I am hoping to make a safe haven, so I have a command block on a clock that kills each hostile mob type. This works to keep the place free of any hostile mobs, however every night I see rotten flesh, bones, arrows, and string everywhere.  I would like to add something to my command blocks so that they are killed without dropping loot. How can I accomplish this?  Note that I want mobs to drop loot generally elsewhere in the world, so /gamerule doMobLoot false will not do the trick for me. I'm hoping for a command that only applies to the mobs killed by the command block, and not anything else. Explaining two-tailed tests I am looking for various ways of explaining to my students (in an elementary statistics course) what is a two tailed test, and how its P value is calculated.  How do you explain to your students the two- vs one- tailed test? Just wanted some feedback to ensure I did not make any mistakes with this proof. Thanks!  Since $G$ is abelian, every subgroup is normal. Since $G$ is simple, the only subgroups of $G$ are $1$ and $G$, and $|G| &gt; 1$, so for some $x\in G$ we have $x\neq 1$ and $\langle x\rangle\leq G$, so $\langle x\rangle = G$. Suppose $x$ has either infinite order or even order. Then $\langle x^2\rangle \leq G$ but $\langle x^2\rangle \neq \langle x \rangle$, a contradiction. So $x$, and therefore $G$, has odd order, and by Feit-Thompson, $G\cong Z_p$ for some prime $p$.  Edit: Thanks, I see that Feit-Thompson is too much.  Since $G$ is abelian, every subgroup is normal. Since $G$ is simple, the only subgroups of $G$ are $1$ and $G$, and $|G| &gt; 1$, so for some $x\in G$ we have $x\neq 1$ and $\langle x\rangle\leq G$, so $\langle x\rangle = G$. Suppose $x$ has infinite order. Then $\langle x^2\rangle \leq G$ but $\langle x^2\rangle \neq \langle x \rangle$, a contradiction. So $x$, and therefore $G$, has finite order. Suppose $x$ has composite order $n$ so for some $p &gt; 1$ that divides $n$, $\langle x^p \rangle$ is a proper non-trivial subgroup of $G$, so $G$ is not simple. So $G$ is a cyclic group of prime order. Somebody asked me this question in a job interview and I replied that their joint distribution is always Gaussian. I thought that I can always write a bivariate Gaussian with their means and variance and covariances. I am wondering if there can be a case for which the joint probability of two Gaussians is not Gaussian? How the `\alignat` works in ams package I have read the related question(), which had detailed answer but still don't know what the meaning of the \alignat's argument. It is said in  that      This environment takes one argument, the number of “equation   columns”: count the maximum number of &amp;s in any row, add 1 and and   divide by 2.   What is the meaning of adding one in &amp; number and divide by two? I presumed the argument is given by the user, not automatically generated by the system? What happens when I run the command cat /proc/cpuinfo? What happens when I write cat /proc/cpuinfo. Is that a named pipe (or something else) to the OS which reads the CPU info on the fly and generate that text each time I call it? Where does tidal energy come from? Kind of an odd, random question that popped into my head. Tidal energy - earth's ocean movement, volcanism on some of Jupiter's moons, etc. - obviously comes from the gravitational interaction between large bodies. On earth the interactions with the moon are pulling water around the surface, creating some amount of heat due to friction, etc.   My question is, where does that energy come from exactly? More specifically, what potential energy source is getting depleted to do that work? Is the earth minutely slowing down in its spin - or are the orbits of earth and the moon subtly altered over time by the counteractive movement and friction of liquids and gasses? Reading file contents on the client-side in javascript in various browsers Is there a regular expression to detect a valid regular expression? As a grad student, I just find it tremendously frustrating when emails are ignored by faculty (both at my institution and at other places). Is this acceptable? I'd be content with a simple response with one line along the lines of "I have read this email and am busy right now, will respond in X days when I can write a more detailed email." I understand that, as a first year student in my field, I'm not really in a position to request anything of anyone, but it just seems like common decency.  Instead, I never hear back, as if my email has been jettisoned off into oblivion.