I think Kaluza/Klein was partly worked on to find additional dimensions to account for electric and magnetic fields.  I am confused why the gravity field gets the privileged spot of distorting space-time, whereas electric and magnetic fields have to use some other mechanism.  Is this somehow related to electric and magnetic fields having attractive and repelling force modes?
It is written everywhere that gravity is curvature of spacetime caused by the mass of the objects or something to the same effect. This raises a question with me: why isn't spacetime curved due to other forces or aspects of bodies?  Why isn't it that there are curvatures related to the charge of a body or the spin of particles or any other characteristics?
LINQ to Entities does not recognize the method 'System.String Format(System.String, System.Object, System.Object)' What is the difference between .profile and .bash_profile and why don't I have a .profile file on my system? So I'm pretty new to Linux and recently installed Fedora 19 on my netbook. I want to have a deeper understanding of Linux and the CLI so I'm now reading .  In exercise 3 the author gives the following commands:   ls -al cat .profile   I try to follow these instructions but to no avail. There is no .profile file in my user name directory as I can see from ls -al. From what I've learned the .bash_profile and .profile file are practically the same. The only difference I concluded from my research is that .bash_profile gets checked first.   What I don't understand is why are there multiple files for the same function in Linux, why is the Author using .profile and not .bash_profile (if I understand correctly .profile won't be read when the system finds .bash_profile first) and how comes that I don't have the .profile file on my system? Lost space after removing bootcamp Where do I find the current C or C++ standard documents? Related:   I have put a bit of commentary enumerating my confusions in parentheses  I read in Black Holes and Time Warps (Kip Thorne), that quasars can generate their jets from four different processes. These all involved the accretion disk, but there was one which doesn't make quite as much sense. It was called the Blandford-Znajek process, and it involved magnetic field lines carrying current.  The process was visualized in two ways. A black hole, with magnetic field lines, is spinning. In the first visualisation (viewpoint actually), the magnetic field lines 'spin' along with the black hole, and nearby plasma is anchored onto the field lines by electrical forces (where did the electrical fields come from?). The plasma can slide along the field lines but not across them (why?).  Since the field lines are spinning, centrifugal forces will fling them up and down the field lines, forming jets.  The other viewpoint is this, and it makes even less sense (to me that is, I haven't had a formal education in GR): The magnetic fields and the swirl of space generate a voltage difference across the field lines (Why? How?). The voltage carries current across the magnetic field lines (why are the field lines behaving like wires?). This current travels across plasma, which accelerates it, creating the jets.  Now the main thing that doesn't make sense, is that magnetic field lines are behaving like wires. Why would they? I suspect the answer lies hidden somewhere in the equivalence of EM waves in different frames, but I can't think up any convincing argument from that side.  If the answer involves GR equations, you don't need to solve it here (wouldn't make sense to me), but if you have to, just refer to the equation and what you did to it, along with the final result. Thanks! How to execute two mysql queries as one in PHP/MYSQL? Could someone please help me with this proof?     Suppose that $a, b \in N$, and $d = \gcd(a, b)$. Since $d$ divides $a$, we have $a = de$ for some integer $e,$ and similarly $b = df$ for some integer $f$. Prove that $\gcd(e, f) = 1$.   I understand why it works. Since d is all the common factors of $a$ and $b, e$ and $f$ had no common factors, therefore the $\gcd(e,f) = 1$. But how do I prove this?  Thanks in advance. How do you measure pedal axle/spindle size?  Replacement pedals and accessories specify a size like 9/16" or 1/2".  How do I measure my current pedal to see what I need?  Is it just a matter of putting the pedal threads in a caliper and measuring the outer diameter of the threads across the axle/spindle?  Or do you need to measure the solid part, excluding the threads? What is syntax highlighting and how does it work? I noticed that sometimes my code gets highlighted in different colors when rendered.   What is syntax highlighting? How does it work? Why isn't my code being highlighted correctly? How do I report a bug or request a new language? How do I use syntax highlighting? What languages are currently available on Stack Exchange? Im having a bit of trouble drawing a circuit with a diode in. I know the current flows from negative to positive, but convention says that current goes from positive to negative.  When drawing a circuit with a diode do you draw the arrow pointing to the positive (matching the actual flow of electrons) or towards the negative terminal (matching conventional current)?  If you follow convention then surely when building circuits from the diagram the diode will not allow current as it is actually facing the incorrect way? How to Recover truncate table in MySQL? Is it possible to do a SELECT statement with a predetermined order, ie. selecting IDs 7,2,5,9 and 8 and returning them in that order, based on nothing more than the ID field?  Both these statements return them in the same order:   SELECT id FROM table WHERE id in (7,2,5,9,8)    SELECT id FROM table WHERE id in (8,2,5,9,7) Difference between differentiation and derivatives What is the difference between differentiation  and derivatives of a function ? Debugging using Nvidia Nsight 2.2 in Visual Studio 2010 What are the C++ coding and file organization guidelines you suggest for people who have to deal with lots of interdependent classes spread over several source and header files?  I have this situation in my project and solving class definition related errors crossing over several header files has become quite a headache. Zero visitors? What happened to site analytics about a week ago? This is the views section of the site analytics visible to 25k users on one of my sites:    That's, uh, not correct.  Our site has had its usual activity level during this time.  I checked a couple other sites, and the pattern is consistent.  This looks like a problem in the analytics data that's coming in; did Google change their API?  Update: Same site a week later; note that in addition to the brief spike of accuracy, page views have dropped to (near-)zero for the whole period, unlike the earlier report.  (I know this question has been closed as a dupe, but I thought having the two screen shots together might help somebody.) Run a Windows Service as a console app Is a set without limit points necessarily closed? According to the definition on Rudin' Principles of Mathematicial Analysis, closed set is defined as:      $E$ is closed if every limit point of $E$ is a point of $E$.   Then I have a question: if a set has no limit point, is it necessarily closed? I think this idea doesn't contradict the definition of closed sets. Am I right?  And another question is #10 in Chap.2 of Rudin's:      Let $X$ be an infinite set. For $p\in X$ and $q\in X$, define    $d(p,q)=0$ when $p=q$. Otherwises,$d(p,q)=1$.    I have known that every set in the metric space is closed. To prove it, we can show that the complement of any set is open, which is quite trivial. My question is: how can we prove that every set is closed just by checking according to the definition of closed set, i.e. how can we show that every limit point of any arbitrary set $E$ in the metric space is a point of $E$?   Thanks in advanced! align xticks in PGFPlots The text xticks are not placed on the same line, rather the same height counting from the highest letter (zero is much higher than high and low).    How can you fix this?  MWE:  \documentclass[class=scrartcl,14pt]{standalone} \usepackage{fontspec} \usepackage{luatex85} \usepackage{unicode-math} \usepackage{tikz} \usepackage{pgfplots,pgfplotstable} \usepackage{calc} \usepgfplotslibrary{groupplots} \pgfplotsset{compat=newest}  \begin{document}  \begin{tikzpicture} \begin{groupplot}[group style={group size=1 by 1, horizontal sep=2cm, xlabels at=edge bottom }, %~ yticklabel style={/pgf/number format/fixed}, %~ xticklabel style={/pgf/number format/fixed}, scaled y ticks = false, scaled x ticks = false, xtick pos=left, ytick pos=left, %xlabel=time (week) ] \nextgroupplot[xmin=0.5,xmax=3.5,xtick={1,2,3},ymin=2.43,ymax=3.05,xticklabels={zero, low , high}, xlabel={label}]  \end{groupplot}  \end{tikzpicture}  \end{document} Sum data based on cell color in Google Spreadsheets I have seen  and altough I face the same problem, the answer to that question is not helpful to my case.   I have large spreadsheets which are changed frequently. So, according to the solution accepted to that question, it is hard to duplicate all populated columns and it is cumbersome to edit one more cell everytime I wish to make a change on cells background colors.  Isn't there a way to sum data based on a condition regarding cell background color in Google Spreadsheets?