The question is in the title. When we consider Feynman-diagrams (e. g. $e^{+}e^{-} \rightarrow\mu^{+}\mu^{-}$), we often have virtual photons. Do these travel with the speed of light?
I have recently been reading about Quantum Electrodynamics which I found very interesting, but even more confusing.  I understand photons mediate the electromagnetic force and interactions between charges can be described by virtual photon exchange. And that in Quantum Mechanics particles paths can be evaluated by sum over all paths...  An electron travelling from one point to another will take all paths, emitting and reabsorbing virtual photons. Does the electrons take all paths including those that involve faster than light velocities?  Is this question null, Does it mean anything to ask about properties of virtual particles?  Do I have the wrong idea about what virtual particles are?
Any ideas how to display a PDF file in a WPF Windows Application?     I am using the following code to run the browser but the Browser.Navigate method does not do anything!  WebBrowser browser = new WebBrowser(); browser.Navigate("http://www.google.com"); this.AddChild(browser); // this is the System.Windows.Window How does Spider-Man climb walls? How can you find and replace text in a file using the Windows command-line environment? I am writing a batch file script using Windows command-line environment and want to change each occurrence of some text in a file (ex. "FOO") with another (ex. "BAR"). What is the simplest way to do that? Any built in functions? Having recently switched to Google Chrome, I was wondering what files and directories I should be including in my backups to make sure that bookmarks and history are included.  Along the same lines, is there a process to restore these settings on a new install?  This is on Windows Vista. I have currently a strange problem on debian (wheezy/amd64).  I have created a chroot to install a server (i can't give any more detail about it, sorry). Let's call its path /chr_path/. To make things easy, I have initialized this chroot with a debootstrap (also wheezy/amd64).  All seemed to work well inside the chroot but when I started the installer script of my server I got : zsh: Not found /some_path/perl (the installer includes a perl binary for some reasons)  Naturally, I checked the /some_path/ location and I found the "perl" binary. file in chroot environment returns :  /some_path/perl ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV), dynamically linked (uses shared libs), for GNU/Linux 2.2.5, not stripped   The file exists, seems ok, has correct rights. I can use file, ls, vim on it but as soon as I try to execute it - ./perl for example - I get : zsh: Not found ./perl.  This situation is quite understandable for me. Moreover :   I can execute other basic binaries (/bin/ls,...) in the chroot without getting errors I have the same problems for other binaries that came with the project When I try to execute the binary from the main root (/chr_path/some_path/perl), it works. I have tried to put one of the binaries with a copy of my ls. I checked that the access rights were the same but this didn't change anything (one was working, and the other wasn't) How do I update my graphics drivers? I am using a laptop and ubuntu 11.04. I would like the easiest way possible because I am new to linux.  here is my display information:  *-display              description: VGA compatible controller              product: Core Processor Integrated Graphics Controller              vendor: Intel Corporation              physical id: 2              bus info: pci@0000:00:02.0              version: 18              width: 64 bits              clock: 33MHz              capabilities: msi pm vga_controller bus_master cap_list rom              configuration: driver=i915 latency=0              resources: irq:41 memory:d0000000-d03fffff memory:c0000000-cfffffff ioport:e080(size=8)         *-communication UNCLAIMED              description: Communication controller What is the probability of rolling $n$ dice until each side appears at least once? We roll a die until each side appears at least once, then we stop.     What is the probability of rolling exactly $n$ dice?   I guess the answer is  $$6-6\left(\dfrac{5}{6}\right)^n\;,$$  but this may be wrong.  In trying to solve this problem, I read , but I couldn't solve the problem. whether they are complete metric space understanding "mount" as a concept in the OS I want to understand what mounting is. It is used in different contexts and situations and I can't find resources which:   Describe the mount concept Explain the actions taken by the computer/OS/utility when a mount is performed How and in which situations mount is used Which features in the Linux mount command are of frequent use and some examples   ( I hear mount applied to diverse entities directories, flash drives, network card, etc ) Splitting lines by polygons in QGIS I have a river network (lines) and a .shp with states (polygon). I would like to split the lines at the state boundaries i.e. split lines by polygon boundaries. Is there a geoprocessing tool to do this in QGIS? I can't merge the lines by the way, since they are meaningfully split into tributaries/main river. Query works on Oracle 11g but fails on Oracle 8i There are two Java Web Start tags:  : x 239 : x 320   Both tags refer to the same technology  refers to  both wiki point to  all questions tagged with  clearly relate to Java Web Start   I suggest they be merged into  (which appears in the auto-complete list if one types "webstart" in the Tags box when asking a new question). There is this addon that was built called Window Generator 2. It can be . However, when I go to the download link it shows up in a new tab as a bunch of code.  How can I download and use this addon in Blender? How to make random dots in different sizes and spaces between the dots in Illustrator? I am trying to make something similar to this?  Obviously it's possible to make it manually piece by piece, but I am looking for some kind of automated feature in Illustrator to achieve this?  Is this possible?  Thanks man in advance! What's the meaning of a set to the power of another set? Can houseplants actually improve air quality? Which command should I use for textual subscripts in math mode? Textual subscripts like in W_{total} should be typeset in an upright text font (so that code isn't good practice), as has been  in   . Now there are several ways to achieve this, and I'm somewhat confused which I should use:   W_{\rm total} is nice and short, and appears to work in all circumstances, but I expect LaTeX people will frown upon it :-) Is there any good reason against this? W_{\mathrm{total}} &ndash; is this the LaTeX equivalent of the \rm approach? W_{\textnormal{total}}  as total is text and not math&nbsp;... In , Ulrike Fischer suggests a more complicated approach. Even W_{\operatorname{total}} appears to work, but it seems inappropriate.   Note: I always assume here that amsmath is loaded.  I've already asked some questions above, but my main question is: which of these options is the best practice for textual subscripts?    Let me point out why I haven't mentioned \text, \textrm or \textup above. Those commands (mostly) keep the font of the context they're used in, e.g., in an italic context, \text and \textrm would give italic subscripts, and all three yield bold subscripts in a bold context.  Final note: This question is similar to, but not the same as all those  about the  between , , , ,  that have been asked before. Maybe it's still a duplicate&nbsp;... how do I print an unsigned char as hex in c++ using ostream? I want to work with unsigned 8-bit variables in C++.  Either unsigned char or uint8_t do the trick as far as the arithmetic is concerned (which is expected, since AFAIK uint8_t is just an alias for unsigned char, or so the debugger presents it.  The problem is that if I print out the variables using ostream in C++ it treats it as char.  If I have:  unsigned char a = 0; unsigned char b = 0xff; cout &lt;&lt; "a is " &lt;&lt; hex &lt;&lt; a &lt;&lt;"; b is " &lt;&lt; hex &lt;&lt; b &lt;&lt; endl;   then the output is:  a is ^@; b is 377   instead of   a is 0; b is ff   I tried using uint8_t, but as I mentioned before, that's typedef'ed to unsigned char, so it does the same.  How can I print my variables correctly?  Edit: I do this in many places throughout my code.  Is there any way I can do this without casting to int each time I want to print? How do I get my device detected by ADB on Linux? I just got my shiny new Wileyfox Swift – and before putting it to use I want to oem unlock and root it (as I usually do with new devices ;) Trouble is, though  is enabled on the device, and a corresponding line in /etc/udev/rules.d/51-android.rules exists, the device is not seen by adb devices.  I know there are several answers scattered around this site, but they are either hard to find, just cover a specific device, or don't cover all steps I finally needed. So I take this as a chance for a canonical, device-independent question and give it a detailed answer below:  How can I get my Android device seen and used by adb on Linux? Pokemon GO isn't working at all for me right now. I'm in one of the following circumstances and can't do anything:   My game is displaying a "Try Again" message I'm stuck on the Loading Screen When I sign in I get a message that the servers are experiencing problems. I see the map but nothing is loaded, and my team color is wrong The "loading" Pokeball just ends up spinning and I can't catch or interact with things When I try to catch a Pokemon, the Pokeball just sits on my screen doing nothing Gym opponents will drop to 1 HP and won't take any more damage or attack When I open a pokestop, the pokestop image and the name of the pokestop does not load.   What's causing this? Is there a way to fix it?