What's so special about second order equations in classical mechanics? I have a basic understanding of the Lagrangian and Hamiltonian formulations of classical mechanics, so I'm not looking for answers like 'because Newton's second law is a second order ODE' or 'because Euler-Lagrange equations act on a first time derivative of position'. I'm looking for a deeper physical reason - in the same sense that Energy conservation is not fundamental, it results from time translation invariance. I realise two boundary conditions are required to solve for the dynamics of a given system, but I see that more as a result of the equations being second order than the cause of them being second order. Is there a more fundamental organising principle that I am not aware of?
What is the reason for the observation that across the board fields in physics are generally governed by second order (partial) differential equations?    If someone on the street would flat out ask me that question, then I'd probably mumble something about physicists wanting to be able to use the Lagrangian approach. And to allow a positive rotation and translation invariant energy term, which allows for local propagation, you need something like $-\phi\Delta\phi$.  I assume the answer goes in this direction, but I can't really justify why more complex terms in the Lagrangian are not allowed or why higher orders are a physical problem. Even if these require more initial data, I don't see the a priori problem.  Furthermore you could come up with quantities in the spirit of $F\wedge F$ and $F \wedge *F$ and okay yes... maybe any made up scalar just doesn't describe physics or misses valuable symmetries. On there other hand in the whole renormalization business, they seem to be allowed to use lots and lots of terms in their Lagrangians. And if I understand correctly, supersymmetry theory is basically a method of introducing new Lagrangian densities too.   Do we know the limit for making up these objects? What is the fundamental justification for order two?
Journal vs conference publications when looking for a job in in computer science I'm currently a post doc in theoretical computer science and I have a double digit number of conference publications, about half of which are at so called "tier 1" conferences (SODA,STOC,...). Unfortunately I only have 1 journal publication so far, mainly due to coauthors dragging their feet.   Will the lack of journal publications have a great impact when applying for a faculty job in cs-theory at some point, and is it advisable to try to up this number before applying? Or is this fully compensated by having sufficiently many conference papers? Reading the question posted  left me with a more general question:  Given the professional title and name: Dr. (First Name)(Last Name), is there some way to differentiate between the holder of a philosophical doctorate and a medical doctor? Wouldn't it be more appropriate for a PhD holder to have the title (First Name)(Last Name), PhD? Drush 6 works great of corse on a Drupal 6 website, but will I have all of the Drush 6 features when working on a Drupal 7 website? Should I use Drush 7 when working on a drupal 7 website?  I found this post that says you can switch between the two version easily, is this correct?  switch to drush6 command: composer global require drush/drush:6.*  switch to drush7 command: composer global require drush/drush:dev-master Why is my attempt at downloading Big Sur failing? I am currently using a MacBook Pro 13&quot; Mid 2018 with Touch Bar. Today macOS Big Sur was released and I tried to download it but it's getting stuck at 484.4 MB. Initially, it shows 16-17 minutes left, but as soon as it reaches 484.4 MB it's stuck and the timer goes till 10 hours! My Wi-Fi is totally stable with a speed of 100 Mbps. I tried to restart my Mac but it still gets stuck. Earlier there were similar issues in the App Store (even now they are). The issue was I was not able to download an app unless I connect to the US VPN. I even contacted support but they were not able to help out. It wasn't a big deal as a VPN could be used. But now even with VPN, it gets stuck. Do you mind me / my Do the following sentences mean the same?        Do you mind if I open the window?   Do you mind me opening the window?   Do you mind my opening the window?      for me they all mean the same, bit #3 is a little less common as far as I've seen so far. Do you agree with me?  PS. I am more about the AmE register. My website title shows up slightly different from what's in my title tag, only in Google search nginx config fails with SSL key/pem (unique case) I am trying to install SSL on my nginx reverse proxy with certified ssl keys but i get this message when i try to restart server:  Restarting nginx: [emerg]: SSL_CTX_use_PrivateKey_file("/etc/nginx/conf.d/cert.key") failed (SSL: error:0906D06C:PEM routines:PEM_read_bio:no start line error:140B0009:SSL routines:SSL_CTX_use_PrivateKey_file:PEM lib) configuration file /etc/nginx/nginx.conf test failed   everyfiles are root:root with 600 permissions i've tested the certificats and they are validated with this website:   there are no trailing weird caracters in my keys, and has 64 caracter per line  here is my config file  server {     listen   443;     server_name     my.domain.com;     ssl on;     ssl_certificate      conf.d/cert.pem;     ssl_certificate_key  conf.d/cert.key;      location / {          proxy_pass         http://upstream1;          proxy_redirect     off;          proxy_buffering    off;          proxy_set_header        Host            $host;          proxy_set_header        X-Real-IP       $remote_addr;          proxy_set_header        X-Forwarded-For $proxy_add_x_forwarded_for;     }   }  Any ideas? Thank you extract string between double quotes I have requirement that i need to extract string using quotes. My string as follows.  "abcd efgh" "ijkl mnop" "qrst uvwxyz"   can you help me to get the string between second double quotes(ijkl mnop) using sed or grep command.  In other words if I say give me the string in first quotes I want the  first string; if I say second string it should give me string between second double quotes and similarly third one also. Printing all instances of a class Postfix SMTP banner show multiple hostnames Connected compact covering space of torus is homeomorphic to $S^1\times S^1$ Help with proving that $\pi$ is irrational Where does $PATH get set in OS X 10.6 Snow Leopard? I type echo $PATH on the command line and get  /opt/local/bin:/opt/local/sbin:/Users/andrew/bin:/usr/local/bin:/usr/local/mysql/bin:/usr/local/pear/bin:/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/bin:/usr/X11/bin:/opt/local/bin:/usr/local/git/bin  I'm wondering where this is getting set since my .bash_login file is empty.  I'm particularly concerned that, after installing MacPorts, it installed a bunch of junk in /opt. I don't think that directory even exists in a normal Mac OS X install.  Update: Thanks to  for correcting my echo $PATH statement It seems that in the words Englishman, Frenchman, and Scotsman, the ‑man part is pronounced /mən/ (just like in Roman).  Whereas in snowman, the ‑man part is pronounced /mæn/ (just like in no man).  Why is it that when ‑man is appended to snow‑ to make snowman, the pronunciation of the man part doesn’t change?    Wouldn’t it logically follow for it to be pronounced /ˈsnowmən/ not /ˈsnowˌmæn/? For example: man(1), find(3), updatedb(2)?   What do the numbers in parentheses (Brit. "brackets") mean? Compute the sum of the series $\sum_{n=1}^{\infty} \frac{1}{n \cdot 2^n}$ What would be the sum of the following series?  $$\sum_{n=1}^{\infty} \frac{1}{n \cdot 2^n}$$  Thanks iOS crash log catch, debug info.. Catch and send via email to the Dev team What is the emergency phone number in different countries? How to Start Web Mapping? I want to start working on a web map at work to map some of our statistics by city.    I have an idea, but I'm not exactly sure where to start.   Are there any definitive resources that provide step by step guidance on how to do this? How can mixed content compromise an entire HTTPS session?