There is one piece of information that makes me wonder why finitely sized black holes are even possible: to an observer from afar, anything that approaches a black hole's event horizon is never observed to cross the event horizon, instead asymptotically approaching.  If that is indeed the case, then that would suggest that, for the generation of a black hole, the instance an event horizon arises from a singularity, surely, to outside observers, no more mass should be observed to cross that infinitesimal event horizon. Therefore, that would suggest that no more mass contributes to the singularity, and the mass of the singularity does not change. Therefore, the event horizon would remain infinitesimal in size, and no black holes would have the opportunity to grow to a size that any outside observers could observe in the first place.  This, of course, seems to be unlikely, given how black holes have indeed been detected. Therefore, I suspect there is a flaw in my logic, or that I'm making an incorrect assumption. However, I do not know where the flaw lies.  For example, perhaps the initial formation of a black hole does not start off with an infinitesimally sized event horizon. I assume this as I thought the generation of an event horizon occurs instantaneously whenever enough mass is collected within a certain volume.  Perhaps objects do indeed pass the event horizon, but only after an extremely long time passed.  Or could it be that objects just at the event horizon contributes to the mass of the black hole, even without passing through, until the event horizon grows in size and eventually consumes that which was just beside the event horizon.  Or, as a wild afterthought, is it feasible for black holes to have existed since the very start of the universe?  Any help with this would be very much appreciated!
Excuse me if the question is naïve. I am not a physicist.  Let's imagine formation of a black hole from a homogeneous collapsing star. At certain moment when enough of matter is inside of a small enough spherical area the event horizon starts to exist. When does the singularity start to exist and why?  The matter below the event horizon still continues to collapse towards the centre (e.g. in the reference frame of the falling matter). What are the necessary conditions for the singularity to appear?  I have found two similar questions but the answers did not really explain this problem to me:
Change audio output depending on which one is on I have a PC that is hooked up to my HDTV (via a long hdmi cable) and to a monitor, which is in another room. I have speakers directly plugged to the PC audio out. The PC is next to the monitor and speakers.   I am not sure but I think Windows can play sound on only 1 audio device at a time. And I can only set 1 device as the default output. I can get sound on the TV or speaker depending on which device I set to default. But I would have to do this every time I switch between using my TV or my monitor!   Is there some way to configure such that sound plays through the TV if the TV is on, else it plays in the speakers? If this is not possible, then the next best alternative would be to get the sound to play on both the devices at the same time. How to login to gamecenter at PewDiePie's Tuber Simulator? How do I login to the Gamecenter on PewDiePie's Tuber Simulator, if you have not done it when they asked? Extending the User model with custom fields in Django What is a magic number, and why is it bad? What is a magic number?  Why should it be avoided?  Are there cases where it's appropriate? Resize screen for Dead touch zone I have Star N7189 (5.5") with the lower zone of touch screen dead     The menu and back buttons don't work,but the physical home button works.   I tried to press hard on the edges of the screen, I tried to apply a twist from the short sides as if it were nothing but a wet towel. I did the hard reset but it did not work.  Sometimes pressing dead area, the phone takes it as menu button  As well as replacing the screen what other methods can I try? Is there any app, launcher or trick to resize the screen usable only to the working area?  P.S. a few hours after leaving it in airplane mode suddenly the touch had this problem but I do not remember if I inadvertently placed something heavy in that area of ​​the cell. In the decimal system: $...-3, -2, -1, 0, 1, 2, 3...$ In the binary system: $...-11, -10, -1, 0, 1, 10, 11...$ In the unary system: $-111, -11, -1, \text{ ??? }, 1, 11, 111...$   I was only able to find a related question but none with an answer so far: I've compiled a java project into a Jar file, and am having issues running it.  When I run:  java -jar myJar.jar   I get the following error  Could not find the main class: myClass   The class file is not in the root directory of the jar so I've tried changing the path of the main class to match the path to the class file and I get the same issue.  Should I be flattening the file structure? if so how do I do this. I'm using Ant to build the Jar file if thats of any use.  UPDATE   Here is the contents of the jar and the relevant Ant sections, I've changed the name of the firm I work for to "org":  META-INF/ META-INF/MANIFEST.MF dataAccessLayer/ dataAccessLayer/databaseTest.class org/ org/eventService/ org/eventService/DatabaseObject.class org/eventService/DatabaseObjectFactory.class org/eventService/DbEventClientImpl$HearBeatMonitor.class org/eventService/DbEventClientImpl.class org/eventService/EmptyQueryListException.class org/eventService/EventHandlerWorkItem.class org/eventService/EventProcessor.class org/eventService/EventTypeEnum.class org/eventService/EventWorkQueue$MonitorThread.class org/eventService/EventWorkQueue$PoolWorker.class org/eventService/EventWorkQueue.class org/eventService/FailedToLoadDriverException.class org/eventService/IConnectionFailureListener.class org/eventService/InvalidEventTypeException.class org/eventService/JdbcInterfaceConnection.class org/eventService/NullArgumentException.class org/eventService/OracleDatabaseObject.class org/eventService/ProactiveClientEventLogger.class org/eventService/ProactiveClientEventLoggerException.class org/eventService/PropertyMap.class org/eventService/SQLServerDatabaseObject.class org/eventService/TestHarness.class org/eventService/Utilities.class   And the ant target:  &lt;target name="compile" depends="init" description="compile the source "&gt;     &lt;javac srcdir="src" destdir="bin" classpathref="project.class.path"/&gt; &lt;/target&gt; &lt;target name="buildjar" description="build jar file" depends="compile"&gt;     &lt;mkdir dir="dist"/&gt;     &lt;jar destfile="dist/myJar.jar" basedir="bin" includes="**/*.class" &gt;         &lt;manifest&gt;             &lt;attribute name="Main-Class" value="org.eventService.ProactiveClientEventLogger"/&gt;         &lt;/manifest&gt;      &lt;/jar&gt; &lt;/target&gt; MikTeX 2.9, install packages on the fly? What is the difference between #include  and #include "filename"? In the C and C++ programming languages, what is the difference between using angle brackets and using quotes in an include statement, as follows?   #include &lt;filename&gt;  #include "filename" Electron shells in atoms: What causes them to exist as they do? I have seen similar posts, but I haven't seen what seems to be a clear and direct answer.  Why do only a certain number of electrons occupy each shell? Why are the shells arranged in certain distances from the nucleus? Why don't electrons just collapse into the nucleus or fly away?  It seems there are lots of equations and theories that describe HOW electrons behave (pauli exclusion principle), predictions about WHERE they may be located (Schrödinger equation, uncertainty principle), etc. But hard to find the WHY and/or causality behind these descriptive properties.  What is it about the nucleus and the electrons that causes them to attract/repel in the form of these shells at regular intervals and numbers of electrons per shell?  Please be patient with me, new to this forum and just an amateur fan of physics. How to keep glasses dry in the rain? Cannot get 2 raster files of same cell size to align ArcMap 10.2 Analogue of the Cantor-Bernstein-Schroeder theorem for general algebraic structures The Cantor-Bernstein-Schroeder theorem states that if there are two sets $A$ and $B$ such that there exist injective (alternatively, surjective, assuming choice I think) maps $A \to B$ and $B \to A$, then $A$ and $B$ are in bijection.  It then seems natural to try to strengthen the result to other structures.  If $A$ and $B$ are, say, groups (or rings, modules, etc) such that there are injective homomorphisms from each to the other, then $A$ and $B$ are isomorphic.  I'm wondering if there are any results on this, or if there are known counterexamples.  The theorem for sets alone is nontrivial, so I feel like any results about how it holds for other structures would be quite interesting. postfix - how to prevent users from sending with another users name Even though my postfix server cannot be used without authentification (open relay), sending mail in another users name after logging in is still possible like this:  EHLO domain.org auth plain eW91IGxpdHRsZSBkaXJ0eSBiYXN0YXJkIDstKQ== 235 2.7.0 Authentication successful mail from: otheruser@domain.org 250 2.1.0 Ok rcpt to: recipient@otherdomain.org 250 2.1.5 Ok data 354 End data with &lt;CR&gt;&lt;LF&gt;.&lt;CR&gt;&lt;LF&gt; Please send me your account info. . 250 2.0.0 Ok: queued as D40692A61AA quit   How can this be prevented? Why is the Yang–Mills existence and mass gap problem so fundamental? Ref:   Can anyone please explain why the Yang–Mills Existence and Mass Gap problem is so important / fundamental to contemporary mathematics (and, presumably, theoretical / mathematical physics)?  If possible, please include the following in your answer / response:  (1) Why the problem is so fundamental / important - for both mathematics and theoretical / mathematical physics; (2) How the solution to the problem would impact research in the above two fields; (3) (If applicable) What progress has been made so far in resolving it; (references to papers, books, other resources would be helpful).  I'm an upper-level undergraduate in theoretical physics, and so am seeking an answer / response at such level. Are the memories still there? Handler frame_change_pre doesn't work in render this works in viewport but not in render.  import bpy from math import *  def frame_handler(scene):     z = scene.objects['Lift'].location[2]     scene.objects['Text'].data.body = str(floor(z / 4.0) - 2)  bpy.app.handlers.frame_change_pre.clear() bpy.app.handlers.frame_change_pre.append(frame_handler)   How to get it working in render? I've read  and changed from "bpy.data.objects" to "scene.objects" as already seen in the script. What am I missing? Blender v. 2.81.16  Edit. Tried that depsgraph thing mentioned in comments and changed to frame_change_post:  import bpy from bpy.app.handlers import persistent from math import *  @persistent  # maybe not relevant but doesn't harm anything? def frame_handler(scene, depsgraph):     z = scene.objects['Lift'].evaluated_get(depsgraph).location[2]     scene.objects['Text'].data.body = str(floor(z / 4.0) - 2)  bpy.app.handlers.frame_change_pre.clear() bpy.app.handlers.frame_change_post.clear() bpy.app.handlers.frame_change_post.append(frame_handler)   Now the Text doesn't show up at all in rendered (but works in viewport). Why does the multi-paragraph quotation rule exist? The sign in the formula for the voltage across the inductor This question is about voltage induced in a inductance.  I know the rate of change of current induces EMF in the inductor. The direction of the induced EMF will be opposite to the change that is causing it. The EMF induced is given by the following formula.  $$V_l = -L \frac{dI}{dt}$$  The negative sign indicates direction of induced emf across the coil is opposite the current change.     In the picture I have drawn current direction and \$V_l\$ direction.  If \$\dfrac{dI}{dt}\$ is postive, \$V_l\$ will be negative according to formula right? Or does the direction of \$V_l\$ has to be reversed?  In many Internet references I found \$V_l = L \dfrac{dI}{dt}\$. Which one is correct?  I hope they reversed \$V_l\$ reference points and removed the negative sign in the formula. Please clarify. Are Variable Operators Possible?