We know light can't escape a black hole but...  Could light find itself in orbit around an object?  Say the light is on the cusp of an event horizon.  Could it end up in an orbit around the black hole if it was coming in from the perfect angle?
The photon sphere is a spherical region in space where photons are forced to travel in an orbit at $r = \frac{3GM}{c^{2}}$.   Is it possible to detect these spheres?  What happens if I fall through it? What happens to a photon that tangentially hits the event horizon, wouldn't this also lead to a photon orbiting the black hole? EDIT Since neutron stars also might have a , do we know neutron stars that should have such a sphere?
Which is correct?     How much is the two fares?      How much are the two fares? Do I need to change my ESTA address when visiting the US? I have an ESTA for my visit last Sept to NY and I know it is valid for 2years.  I am going to Las Vegas in May, do I have to update the address at which I will be staying? In electromagnetism, why does nature prefer the right-hand rule over the left-hand rule? At school I learnt the  to remember the resulting direction of different phenomena, such as geometrical cross products, mechanical torque, or the direction a screw will move when turning it. This directions are however decided by humans, and we could have easily defined the directions in the opposite way.  However the right hand rule also predicts phenomena which is not arbitrarily set by humans, but is intrinsic to the nature. If you have a coil in which you put some current, and the nature creates a magnetic field in a direction perpendicular to it, the direction depends on the right hand rule.  My question is, why this direction and not the opposite? Nature tends to be symmetric and not have apparent preferences for two equal options, so in a so why is one direction preferred over the other one? Why doesn't a "left hand" rule apply? How can I detect when an Exception has been thrown anywhere in my application?  I'm try to auto-magically send myself an email whenever an exception is thrown anywhere in my Java Desktop Application.  I figure this way I can be more proactive.  I know I could just explicitly log and notify myself whenever an exception occurs, but I'd have to do it everywhere and I might(more likely will) miss a couple.  Any suggestions? Plotting function defined with Lua doesn't do anything We add negative numbers and zero to natural sequence to make it closed under subtraction, the same thing happens with division (rational numbers) and root of -1 (complex numbers).  Why this trick isn't performed with division by zero? Is there any way to distribute spacing equally in every object?    Is there any option to do? I want 2MM spacing in every geometrical shape.  Thanks in advance. Evaluate the following integral:  $$\int \frac{\sqrt{\sin ^4x+\cos ^4x}}{\sin ^3x \cos x }dx$$ where $x \in \big(0,\frac{\pi}{2} \big)$  Could some give me hint as how to approach this question?  I tried to use the fact that $\sin ^4x+\cos ^4x=1-\frac{sin^22x}{2}$ but it didn't help. How should I proceed? How to clear bash history completely? In some books the word "if" is used in definitions and it is not clear if they actually mean "iff" (i.e "if and only if"). I'd like to know if in mathematical literature in general "if" in definitions means "iff".  For example I am reading "Essential topology" and the following definition is written:     In a topological space $T$, a collection $B$ of open subsets of $T$ is said   to form a basis for the topology on $T$ if every open subset of $T$ can be written as a union of sets in $B$.   Should I assume the converse in such a case? Should I assume that given a basis $B$ for a topological space, every open set can be written as a union of sets in $B$?  This is just an example, I am not asking specifically about this sentence. I am going to change my machine: from a laptop with Ubuntu to another laptop where I am going to install Ubuntu...  I am quite happy with the current system, what I am afraid of is having to rebuild everything from 0.  So what do people often do to clone an Ubuntu system? That may include the data, packages, shortcut settings, etc... Application of Complex Numbers I was just wondering how complex numbers can be applied in electrical engineering and why we use complex numbers over regular, real numbers for this application (e.g what capabilities does the complex number have that real numbers do not in electrical engineering)?  I have done some research concerning impedances and understand how they are written in complex form, however I am still confused why complex numbers are necessary in this field over regular numbers. How can I make a tool drop blocks with CanPlaceOn tag In Adventure mode, I'm trying to let a player get blocks that he can place later, is there a way to do it directly or do I have to change them in his inventory?  Situation: The player gets a tool that will allow him to clear a boulder blocking the road. Further down the path, he encounters a broken bridge, and I want him to use the blocks he gathered before to rebuild it.  At the moment, if I build the boulder with blocks having the CanPlaceOn tag, the tag disappear when the player destroys them and they go in its inventory. Saving project with data source path as relative in QGIS? Is there an option in QGIS to save the project with the data source being a relative path?  I don't see the option here. QGIS Wroclaw1.7.3 Term or phrase describing action occuring when not watching I am an IT professional. More specifically in automation. I am looking for a term or phrase that describes the event when either:  a. you are watching something closely and the issue doesn't occur  or  b. you are not watching something at all and the issue does occur If I want to shoot darker shots, is it better to increase the lighting and then darken in post to retain clarity? I'm just getting started - in my last shoot, I was using a 6D with a 50mm/1.4.  In low light, the pictures looked fantastic on the viewfinder but on the computer screen, appeared extremely grainy.  For future shoots, if I want to achieve something like this:     Should I shoot with way more light / lower ISO to retain clarity, and then just darken in post?  What kind of setup would be needed to achieve the above?    With my setup -   Camera Canon EOS 6D  Lens EF50mm f/1.4 USM  Focal Length 50mm  Shutter Speed 1/40 s  Aperture f/1.8  ISO/Film 6400  I was getting very grainy results that I had to soften in post (in case you want to see my attempts):      Any tips would be much appreciated. Is there a single word to describe a solution that hasn't been optimized? I am working with some code, and I would like to describe the difference in its performance when it is optimized versus when it is not optimized. Unfortunately, I can't find a word to describe the code before it is optimized; "sub-optimal" might work, but I want to emphasize that this is the first iteration of the code. If "unoptimized" were a word, I would use that, but I haven't found any references to it in any dictionary.  Perhaps "raw" or "original"? Singular or plural verb after a series connected by "or" Clip tool gives ERROR 000732? I want to completely replace Ubuntu with Lubuntu I accidentally issued some commands I thought were safe in the terminal, and I ended up with no System Settings and lots of other missing functionality, but the OS still boots normally. I've decided I want to install Lubuntu instead of fixing Ubuntu for a fresh start and since my laptop is running on a 1GHz dual core AMD CPU.  I created a bootable USB as I usually do and placed Lubuntu 17.10 amd64 using Rufus 2.14.1086. I have GRUB that was installed with Ubuntu upon first install I still have Win8.1 which I still want to keep.  PS: I also tried creating a startup disk from Ubuntu  The question is: How can I boot from USB with GRUB2 so I can overwrite Ubuntu?