Edit: Thank you all for the explanations, but I think until I fully grok special relativity, Lorentz transformations and relativity of simultaneity, the answers won't make any sense to me. Maybe the question I should be asking is &quot;why do we believe there is no absolute time frame&quot;, but that would change this question too much, I believe. I won't delete this on, but leave it up for interested readers and flag it as a dupe. Maybe the answers here will make more sense to them than to me. Cheers, everyone  I keep reading that FTL means traveling back in time would be possible, but all explanations I've read so far seem to concentrate on when it is we perceive the arrival of light as the observer. Maybe the definition of time travel that people are using is different from the one in Sci-Fi, so maybe my incomprehension comes from there. Here's what I think as a programmer. Could someone please point out in layman terms what I'm misunderstanding? Global state I assume, were the universe a computer, it'd have a global state that is constantly changing. State perception In order to perceive this state and its changes, we have evolved with utensils to do so. Our perception is always of something that happened in the past because the fastest thing to transfer that information travels at the speed of light. There is always a lag and the greater the distance from an event, the greater the lag. FTL Now that we've established state, perception and speed of information travel, let's say we have the possibility of traveling faster than the known maximum speed of information travel / lightspeed. Scenario 1: actor observing past action An actor does an action at normal speed, instantaneously travels to a location where the information of that action will take (let's say) max 10 seconds to arrive, and sees itself doing an action from 10 seconds ago. There was no time travel but simply a lag of information transfer. Scenario 2: two actors at different FTLs A cataclysmic event happens on earth. Slow FTL actor starts trip to distant planet to transfer information. Time passes and a faster FTL transport is made and fast FTL actor is sent to transfer information again. Fast FTL arrives before Slow FTL. Destination already has information that Slow FTL was bringing, and Slow FTL traveled ahead of light that was also bringing the information. Destination will get the information 3 times, but the even will always lie in the past. Even if destination mounted a rescue mission and sent help instantaneously to earth, the cataclysm would still have already happened. The global state had already changed.
Is this statement true?     You can not travel back in time. If I travel fast enough the clock will start to go backwards, but that does not mean I am traveling back in time. It would only mean that the time reference is producing a negative count.
How to pack the game files into one without using any file compressor? Well, one example is this file of the game The Sims 2 for PS2: quickdat.arc. The file contains this .arc extension, but it's no one an archive file and it contains some data of the game inside. How can I do this? Put so many files in one?  How to do something like this? All the game files inside one without using a file compressor? There seems to be a preference for to have the    18 mm - 55 mm 55 mm - 250 mm   lenses, but no mention of why they're better than a single 18-200mm. Could someone please explain why the two separate are better than the one combined, and if anyone has quantified this difference?  (I'm curious about Canon in particular.) What does this notification icon mean? Samsung s7 A Variation of the even-town odd-town problem Let assume facebook has $n$ users. Mark Zuckerberg decided that people are allowed to open groups under the following restrictions:      1) No two different groups can exactly the same participants.      2) The number of users in each group is even.      3) Every two group has an odd number of common users.      Denote by $m$ the maximal number of groups that can be formed in facebook under the restrictions mentioned above. Prove $m \leq n$.   I tried solving this by defined a matrix, multiplying it by its transpose and using some rank considerations I thought I could derive the bound, but I didn't succeed. Could you please give me a hint?  The question had anther subsection : prove that in the case that $n$ is even than we have stron inequality Complex zeros of the polynomials $\sum_{k=0}^{n} z^k/k!$, inside balls I'm looking to book flights from) Philadelphia (PHL) to Detroit (DTW). Direct flights are quite expensive. I noticed, in expanding a search to surrounding airports, that I can book from Allentown (ABE) to Detroit for far less, and the connection is through Philadelphia.  Can I book this flight but board in Philadelphia? I'll be traveling with carry-on luggage only, so on the return, it's obviously no problem. But would I be allowed through security and on to the plane in PHL with out having flown from Allentown? Why is it "description" but "describe" (b and p)? I've just wondered by it is "description", but "to describe". It looks as if "to descripe" would be a more consistant choice. Is there any other explanation besides "this is how it is written for years"?  edit: Or alternatively "to describe" and "describtion". I'm using python\pyodbc and would like to access the second result set of a stored procedure. As near as I can tell, pyodbc does not support multiple result sets. Additionally, I can't modify the stored procedure. Are there any options to access the second result set using SQL or some other work-around? Perhaps create a second stored procedure that only returns the second result set of the first? Why the "service --status-all | grep mysql" get many unrelated result? Best way to strike out a text Order of "not" with infinitive Java date format conversion - getting wrong month I'm totally new to Ubuntu/Linux, using Ubuntu Server at the moment. Just trying to  figure out something basic.   How can you tell where you are installing a program. For example I just installed Sphinx search engine by placing the tarball that I downloaded from their site to my:  /home/sphinx   directory. I created the sphinx directory to place that tarball in. Then I ran these commands:  tar xvzf sphinx-0.9.8.1.tar.gz cd sphinx-0.9.8.1/ ./configure --with-mysql-includes=/usr/include/mysql --with-mysql-libs=/usr/lib/mysql   and then these:  make sudo make install   Now I have a lot of files sitting in the directory where I ran these commands. Is this my Spynx installation or did it install somewhere else?  In windows if you run an installer (.exe file) anywhere the program will still install in your C:\Program Files directory. Does something similar apply to linux where all programs are installed in a central place, or can you install programs anywhere on the system.  Questions   I would prefer to keep all my installed programs in one place so what is the right place for this in terms of best practice. In other words what is the Linux equivalent of C:\Program Files? And how does one always install at this location, is it just a matter of placing the tarball and running the install commands from this location? What about if I use sudo apt-get to install a package. How can I point to this location to tell apt-get to always install there? I'm rotating an object on two axes, so why does it keep twisting around the third axis? I see questions come up quite often that have this underlying issue, but they're all caught up in the particulars of a given feature or tool. Here's an attempt to create a canonical answer we can refer users to when this comes up - with lots of animated examples! :)    Let's say we're making a first-person camera. The basic idea is it should yaw to look left &amp; right, and pitch to look up &amp; down. So we write a bit of code like this (using Unity as an example):  void Update() {     float speed = lookSpeed * Time.deltaTime;      // Yaw around the y axis using the player's horizontal input.             transform.Rotate(0f, Input.GetAxis("Horizontal") * speed, 0f);      // Pitch around the x axis using the player's vertical input.     transform.Rotate(-Input.GetAxis("Vertical") * speed,  0f, 0f); }   or maybe  // Construct a quaternion or a matrix representing incremental camera rotation. Quaternion rotation = Quaternion.Euler(                         -Input.GetAxis("Vertical") * speed,                          Input.GetAxis("Horizontal") * speed,                          0);  // Fold this change into the camera's current rotation. transform.rotation *= rotation;   And it mostly works, but over time the view starts to get crooked. The camera seems to be turning on its roll axis (z) even though we only told it to rotate on the x and y!    This can also happen if we're trying to manipulate an object in front of the camera - say it's a globe we want to turn to look around:    The same problem - after a while the North pole starts to wander away to the left or right. We're giving input on two axes but we're getting this confusing rotation on a third. And it happens whether we apply all our rotations around the object's local axes or the world's global axes.  In many engines you'll also see this in the inspector - rotate the object in the world, and suddenly numbers change on an axis we didn't even touch!    So, is this an engine bug? How do we tell the program we don't want it adding extra rotation?  Does it have something to do with Euler angles? Should I use Quaternions or Rotation Matrices or Basis Vectors instead? No, not like this:    Hear me out!  As Stack Overflow grows, it becomes increasingly ridden with extremely newbie and mediocre questions. It's just natural. The team are fighting hard to keep the crap out, but there are many very basic questions that are okay on their own. They don't deserve to be kicked out* - they are just very, very, very basic and/or localized.   Too much basicness tires users who are interested in discussing more advanced topics.  As has been discussed many times over, the voting system is not suited to identify advanced questions due to what is known as the . A group of people will vote on what they understand, which puts truly expert questions at a natural disadvantage that a democratic voting system can't overcome.  Now, there is a small number of users on Stack Overflow who I, as a user, know share my basic values (like, teaching a man to fish rather than fishing for them); I also know they are experts in their fields. It is safe to assume that what these users regard an interesting question worthy of attention, is likely to be interesting to me as well.      The idea is to introduce an opt-in "follow" mechanism that allows followed users to share what they find interesting, with very little fuss and on a purely opt-in basis.   Allow every SO user to follow any other SO user with a certain minimum rep, say 3000.   To every user who is being followed (or every user), display a new link underneath every question, "Recommend".    Introduce a new filtered view somewhere, named "Recommended". The view will show me all questions that my followed users are recommending.   Whenever I browse a list of questions, highlight those questions that have been recommended by my followed users. (I haven't put much thought into how to implement this graphically - you get the drift.)      I'm not especially fond of social networking, and I think most of it doesn't have a place on SO. But I'm starting to think this specific thing could be the right way to go in order to tell apart expert content from the 10000th dreary "how do I upload a file" question.  This is an entirely opt-in feature - unless I actively follow some users, I will never notice this.   Maybe for a bit of ego-stroking, the profile page can show how many users follow you. Or even who. Everything else would simply be kept under wraps - no publication of follower data in the data dump, no follower leagues or anything like that.      * At least not under current SO rules. Maybe those rules need to be changed - but that is a different discussion. Why do backslashes appear twice? Escaped line break tag in tooltip on election page Best way to return multiple values from a function? I have a problem regarding alignment for my equations.  This dummy program shows the problem:  \documentclass{article} \usepackage{amsmath} \begin{document}  \begin{enumerate}     \item First do this     \begin{align}     c^2 &amp;= a^2 + b^2     \end{align}      \item Then do this      \begin{align}     variable &amp;= c^2     \end{align} \end{enumerate}  \end{document}     I would like the = to be aligned under each other, but still be able to list them inside the enumerate.  Any suggestions? How do I prove that $ \displaystyle\lim_{n \to{+}\infty}{\sqrt[n]{n!}}$ is infinite?