Nonrelativistic QM can be applied to bound states like a hydrogen atom. QFT is used for free particles (whatever one means by particles) that shortly interact with each other and are free again after the interaction. Can QFT also be used for bound states?
Is there a theorem that says that QFT reduces to QM in a suitable limit? Of course, it should be, as QFT is relativisitc quantum mechanics. But, is there a more manifest one? such as Ehrenfest's theorem which dictates the transition from QM to Classical Mechanics.  I am curious as the basic object in QM is wave function whereas the basic object in QFT is quantized operator, which looks very different, and wondering how the comparison works.  (Of course, I am aware that QFT can calculate corrections for Coulomb potential energy, which may seem to be a transition from QFT to QM, but I want one more manifest than that.)
Why is there something instead of nothing? A simple but fundamental question.    The "something" means the whole Universe (known and unknown), it could be represented as the reality version of the set of all sets, which is itself . It includes all the  and such.  A better version of the whole Universe could be described with a .  The "nothing" means an absence of the whole Universe, not a particular void or emptiness in the whole Universe but a nonexistent whole Universe. How to deal with "java.lang.OutOfMemoryError: Java heap space" error? Can I fail to deserialize with XmlSerializer in C# if an element is not found? I am using XmlSerializer to write and read an object to xml in C#.  I currently use the attributes XmlElement and XmlIgnore to manipulate the serialization of the object.  If my xml file is missing an xml element that I require, my object still deserializes (xml -> object) just fine.   How do I indicate (preferably via Attributes) that a certain field is "required"?  Here is a sample method of what I am using currently:  [XmlElement(ElementName="numberOfWidgets")] public int NumberThatIsRequired {     set ...;     get ...; }   My ideal solution would be to add something like an XmlRequired attribute.     Also, is there a good reference for what Attributes are available to manipulate the behavior of XmlSerializer? In an ideal circuit, How can there be a current b/w points a &amp; b, when there is no potential difference and thus no electric field between a &amp; b? If there is no current, then where does current across the resistor come from, because that means no charge is coming from the battery (?). How do I download torrents? Is there any way to view which applications are accessing the Internet on Mac OS X? I think I remember a command line utility that did this, but I can't remember what it's called and a few Google searches turn up nothing. What is a single word for a person who, as a habit, exaggerates a lot when relating information? What is a single word for a person who, as a habit, exaggerates a lot when relating information?   So for example, they relate something that happened but exaggerate so much that they are including clear falsehood and they are well known for doing that. I have two cameras, a 6mpix Panasonic FZ8, and 14mpix Canon A2200.  When I compare shots from two cameras, one of differences I notice is dpi number that camera reports in JPG info:   FZ8 reports dpi=72 A2200 reports dpi=180   What physical meaning can these numbers have? I am at loss for guesses.    I am fairly familiar with notion of dpi in scanning and printing. I can calculate density of pixels on the sensor of the camera. But then, the linear density of pixels on the sensor will be hundreds times larger than number above. So what, if anything, does it mean? Is there any difference between "offense" and "offence"? So one of this question's  answers got me curious. What is the maximum damage a 20th level character can do with a 9th level Magic Missile by the rules to a single target.  So my best I've been able to find is Aasimar or Goblin WLK 1/WIZ 19   9th Level slot (1D4+1)(11) [keep in mind you only roll once for damage not 11 times]  per  Empowered Evocation (+5)(11) (assuming 20 INT) Hexblade Curse (+6)(11) (Proficiency Bonus +6)    Radiant Soul/Fury of the Small +20 (Level 20 Aasimar/Goblin)   For a total of (D4+12)(11)+20 for (min-max)avg (163-196)179.5  Am I looking at this correctly? Are there other abilities out there that can increase this?  Feel free to include non legendary magic items. NO TOMES PLEASE  Only use things available on D&amp;D Beyond please.  Acceptable books are :   Basic Rules DMG, MM, PHB Sword Coast Adventure's Guide The Tortle Package Volo's Guide to Monsters Xanathar's Guide to Everything and any officially released Adventure Book What are pros and cons for MacPorts, Fink and Homebrew? I'm just migrating from Ubuntu Linux to Mac, and everything is new and I'm re-learning a lot of stuff.  On Linux I had the excellent apt-get to manage software packages. I googled for an alternative on Mac and found about MacPorts, Fink and Homebrew.  I will use this computer primarily to develop Ruby on Rails applications.  So, what are the differences between them? Which are the upsides and downsides? Which one is best maintained and has more packages? Boliviana de Aviacion (BOA) tariff codes I am trying to book one-way tickets on BOA on Jan 13 2015 from La Paz (LBP) to Buenos Aires (EZE) from their website.  Their website asks me to make a choice between 7(!) different tariff codes - they are SSEEOB, QEEOB, TMEEOB, VEEOB, HEEOB, BEEOB, YOW. The prices are different for every tariff, but other than that I cannot see any obvious difference and there is no explanation available. Also I am unable to find anything useful on their website  Could anyone from Bolivia or familiar with BOA explain what is going on there and what choice am I supposed to make? Why is this clause unsatisfiable? I would like to create an alias to rm command in order to have a confirmation message after executing this command. So I am creating an alias like this alias rm='rm -i'. But as far as I know this is a temporary alias and it lives until you close the terminal.  As it is explained  to save alias permanently I need to execute ~/.bash_aliases or ~/.bashrc commands in terminal and add my alias there. But when I execute ~/.bashrc I get following error message :  bash: /home/bakhtiyor/.bashrc: Permission denied   When I run ~/.bash_aliases I get another error message like this:  bash: /home/bakhtiyor/.bash_aliases: File or directory doesn't exist.   What is the actual problem and how can I solve it? Making a recursive CTE like query, but for many records How does a pharmacy discount card work? Lightning New Case action override I have action override on New Case button which runs our custom component.  As you know, we can create a new Case from different places in org, e.g. from related lists. When creating a Case from related list on Account using standard action in Lightning we have prepopulated AccountId to the lookup field. Is it possible to do the same with custom component? I can't catch the AccountId using force:hasRecordId, because action override redirects me to the new page with custom component where I don't have a context of Account. URL also doesn't contain its Id. What can I do to grab Id of context object and populate it to my component? What's the term for using the wrong word because it sounds the same? I was reading about various terms for incorrect words, but they didn't seem to fit.  I saw a post where someone said "what do you like to do when you're board?".  That kind of thing.  Also, would that be considered a mistake in grammar?  I didn't think so, but my friend insists it is.  I'm not sure if there even is a word for that kind of mistake, but was just curious about it.  Thanks! Make gnome-terminal show the command running as title I want the name of command running , e.g unzip to be visible through the title bar of gnome-terminal , but it seems to be impossible , if the application running doesn't set a title explicitly , even though I choose 'Replace initial title' option in the profile dialog. How to watch live youtube stream using vlc? Is it possible for now to watch live youtube video streams using vlc? I just don't want to use flash player. I know that vlc can play youtube videos using link for the page where that video is located, but how about live youtube videos?