Why are some SVG-images not visible in my footer?

Question: On our website, we have a footer (built with WP-Bakery) that includes the typical information that is needed, plus there is a section that should show the different types of payment methods that we accept. But for an unknown reason, some of these SVG-Images (not even every time the same) are not visible. Looking at the page, e.g. with Chrome inspector, I can see the images are all there, but they don't show.
We tried different types of SVG-Images, for example the normal SVG or the optimized SVG. We tried to add a width and height to the pictures in the HTML text, and we exchanged the SVG to different SVG pictures to check if the files have any errors. It's still not working, and maybe you know what we should do?
Answer: Not sure if you've already done this, so forgive me if I'm telling you something you already know.
But WordPress doesn't support SVG Uploads out of the box, so to speak. This is due to security issues that can be created when a user uploads SVGs with malicious code. Mainly, a problem if you let random visitors upload SVGs.
Sometimes builders like WP-Bakery or Elementor will let you upload them, but it always seems to be buggy.
I'd suggest installing something like SVG Support
It should work out of the box. If not play around with settings like "Inline SVG" if it doesn't.
Comment: Thanks for pointing that out but I already have that plugin installed and using inline pretty much destroys the site.
Answer: The solution was doing it by hand using the following piece of HTML:


How to make ubuntu fullscreen all the time

Question: How to make the ubuntu go full screen all time, as full screen is better for me. I can use F11 but a lot of system apps won't go full screen such as nautilus, system monitor won't go full screen.
Comment: An answer for Xfce would be nice, too.
Comment: I think you can use the [Unite extension](https://extensions.gnome.org/extension/1287/unite/) to disable the title-bar for applications that don't make use of CSD(Client-Side Decoration) and then press ```Super+Up``` to actually maximize them.But note that for applications with CSD like Nautilus or System monitor or lots of other gnome apps you cannot hide the title-bar because it has been drawn by the application itself not the window manager.(unless there's an option in the application itself to disable that when the window is maximized,for instance in **gnome-terminal**)
Answer: It is not quite easy, and options are limited. Unfortunately, Linux desktops provide limited user control on the default size of newly created windows (I do not know whether the situation is any better in other OS´es). Then also, there is a difference between "maximized" and "full screen".
Suggestion 1. devilspie2 is a daemon that monitors the creation of windows, and automatically resizes or repositions them as they are created. You could configure it to full-screen any window, but then you would need to define exceptions such as File or Print dialogs, or any other window you do not want to be maximized.
Suggestion 2. Per application approach:
If you can live with "Maximized" rather than full screen, some applications remember their previous state. Examples include Firefox, Files, Text editor Gedit, Gimp, Inkscape
Other applications provide command line switches, for example Evince document viewer: evince --fullscreen, Gnome terminal: gnome-terminal --full-screen. For these applications, you could change their .desktop file to add the necessary command line arguments. A more robust approach could be to have your own wrapper scripts, which then would call the actual executable with the needed argument. The latter approach would not only work if you launch the application from the menu, but also when it is being launched from within another app, e.g. Evince from within Firefox.
This way, you should be able to set up a working environment where most applications by default open in a mximized state or at least full-screen. Not perfect, however, by all means. An approach with devilspie would be more perfect, but more difficult to setup.


azure table storage space used

Question: How do I see how much data is in my table storage account? I have free trial and when I enable monitoring it has all kinds of metrics mostly related to errors, but I can't seem to find storage size.
Answer: There is currently no Azure API to find billing and usage information, therefore, no applications can provide you with the exact figure that you would be billed. There is an open feature request for this very issue. There is limited information available for blob storage, but I have found nothing for table storage.
Answer: You can download your usage details from the account subscriptions page.  It won't tell you how much storage each individual service is using up, just the total on locally or geo redundant.
Go to the console, click your name in the upper right, then "View my bill".  From here you can view your subscription usage data and download a CSV of the details.
Answer: Azure Storage size consist of all of 3 services (Blob, Queue and Table) altogether however there are no ways to calculate the size of each individual service for now. The billing data also shows the cumulative storage size for an Azure Storage Service including all three. There are work items logged to separate the size data for each individual service within Windows Azure Storage however there is no further info on this regard.

azure table storage space used


How would you analyze an image to find out if there are scripts embedded in it?

Question: How would you analyze an image to find out if there are scripts or possible malware embedded in it? What tools / methods to use?
Comment: nice question, I hope someone answer that. how to implement it in PHP...
Answer: I am assuming that you're going to be accepting user-uploaded image files and serving them to other users through a web application. In that case there are two different attack vectors here.
(1) files that can be interpreted as both image and another type.
This includes:
(1a) files with embedded HTML tags, which can then be loaded as HTML by a browser that does content-sniffing, and scripting content inside executed in your domain's security context, causing traditional JavaScript XSS.
(1b) files that can be interpreted as plug-in data, most notably Java (GIFAR attack). Just hosting these can get you compromised for XSS because the Same Origin Policy in Java and Flash is based on the location hosting the file, not the including page.
Such files can commonly be valid images of the expected type as well as working as the vulnerable target type.
You can make these attacks difficult by processing submitted images, loading them in and re-saving them. You may be doing this anyway in order to do cropping, resizing or quality-level changes.
It is in principle possible for someone to submit an image that, when compressed by whatever algorithm you're using, pops out an image with


TextBox ScrollToCaret causing threading confusion

Question: Apparently System.Windows.Forms.RichTextBox.ScrollToCaret can cause additional events to be processed within its call.  When placed within an event handler it can make code appear to run in a multievent (not really multithreaded but just as confusing) fashion causing race conditions.  Apologizing in advance for missing details:
1) Windows form app .NET C# 3.5
2) Create a Canvas and RichTextBox
3) Start a worker thread that calls canvas.Invalidate() at 30fps.
4) Capture OnMouseMove() and Paint() for the canvas.
5) Within OnMouseMove append append text to a RichTextBox and call ScrollToCaret().
6) Use a static short _eventdepth that ++ at top and -- at bottom of each of OnMouseMove  and OnPaint.
7) Trap when the value of _eventdepth > 1.  Stack Trace:
<code>   Canvas::OnPaint()
   [External Code]
   Canvas::OnMouseMove(),  scrollToCaret() is within
</code>
Is this behavior documented and supposed to work like this--are there are "GUI" calls that will distribute "child" events?  Sharing this in case some one else starts burning time on it or in case I am misunderstanding whats happening.
--- edit ---
I have also now wrapped the call to canvas.Invalidate (step 3) from the secondary thread with BeginInvoke.  I also had a lock that was blocking when that happens windows begins to dispatch additional events (in the same primary thread) as a solution rather than block (and hang like I would have expected).
The ScrollToCaret() still causes OnPaint() to be called from within the same stack.  Is it just obvious to to windows programmers which methods may trigger additional event dispatches?
Answer: Even if you follow Microsoft's recommendation to manipulate the control, it can still give you unexpected high CPU usage and hang (the cause seems to be inside riched20.dll.
So I suggest you use a simple TextBox or switch to WPF completely.


Retaining wall on a 2-way slope under an existing block wall

Question: I live on a corner home and the street/sidewalk on the right side of my home travels up a slope. The way my yard is currently designed, the right half of the yard follows the same slope, but not the left, almost at no angle at the front of the yard, but at an increasing angle as one goes towards the back of the yard. The pictures probably explain it better than my words.. in any case, what I want to do is add a retaining wall and remove excess soil in order to flatten out the lawn area. Due to the slope, the wall would also need to be sloped. There is a small section of block wall at the highest point of the front yard area, that I could remove if needed (I'd consider some white picket style fencing to ensure nobody "falls" into my front yard from the adjacent sidewalk), but if I didn't have to remove the wall that would be even better. I don't want to cut in from the existing wall as my yard just isn't wide enough for that to make sense in my opinion. Is there a way to build a sloping (not stepped) retaining wall (maybe concrete with some form of decorative facing) so I can have a flat yard (I want to install new sod and sprinklers as the next project eventually.. and yes the palm tree will be going away and I'll be tweaking the fence between the front and side yards too). Any suggestions or ideas would be appreciated. Thanks!
Comment: If you remove the soil on your side of the brick wall then it appears that you will expose the roots to the tree near the side walk.  How do you deal with that. Is it your tree to kill. If it is your tree, do you want to save it?  If you remove all of the soil then the retaining wall does not need to be sloped, it can be flat and its highest point would need to be as high as the earth it is holding back on the other sides. You can make it higher.   I do not get to see palm trees as often as i would like, can you send me yours ?
Comment: Yes, plan was to remove the excess soil but what I mean from the wall needing to be sloped is it needs to get higher as goes back towards the palm tree..pics don't show full yard but there is no slope toward house at the front right corner and it's only as one goes towards the palm tree that the slope on the right side gets higher and higher so it can't be 1 height.
Comment: That tree is the city's so yeah that's an added issue/headache. Honestly don't know where roots go... when I moved in there were 9 small-med trees on the sloped area of my yard overhanging back half of the front yard and touching the edge of the house roof that I had removed so it's possible the city tree's roots head under the road in the other direction from my yard due to competition. I'll gladly send you my palm tree if you pay for shipping ;)
Comment: Trees send out roots 360 degrees. I am sure the tree roots are at least partially under the soil you wish to remove, i can see that a major root has heaved the cement curb at the end of the wall up.
Answer: What's behind the wooden fence, your back yard?  Does the slope get even taller back there?  Would you dig out that slope and continue the retaining wall there?  You can reasonably build a retaining wall 3-4 feet high without major engineering but when you get to 6-8 feet high it has to retain much more weight (8x as much according to This Old House) and takes more engineering.  This may be more than you want to get into.
As Alaska man asked, is the tree next to the sidewalk yours, or is it the city's?  What do you plan to do with the brick wall?  It would need to be supported or removed also.  Would removing the tree or brick wall require city permits or an engineering study?  This may also be more trouble than it's worth.
With palm trees and bare dry dirt, I'm guessing you're in a dry area in the south western United States.  I'm in the central U.S., where we have thunderstorms 8-10 months out of the year.  A good thunderstorm or two would wash that loose dirt slope down against the house and risk toppling the brick wall so I would leave the slope as-is and sod it or put in low ground cover to hold the soil in place.  If I used grass I would then have to mow it.  I prefer a smooth slope I can just push a lawn mower up, rather than have to pull out a string trimmer for a few square feet of grass on top of a retaining wall.  For long-term durability and ease of maintenance, I'd recommend just adding sod or ground cover and leave the slope alone.


DEBIAN: how to pair a bluetooth printer on console

Question: So the status is this:
1) The USB Bluetooth Dongle is inserted, and recognized.
root@server:~# lsusb
Bus 002 Device 002: ID 8087:8000 Intel Corp.
Bus 002 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 001 Device 005: ID 0a12:0001 Cambridge Silicon Radio, Ltd Bluetooth Dongle (HCI mode)
Bus 001 Device 002: ID 8087:8008 Intel Corp.
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub

2) I installed the following packages from buster without incidents:
bluez amd64 5.50-1
bluetooth all 5.50-1
libical3 amd64 3.0.4-3
bluez-obexd amd64 5.50-1
bluez-tools amd64 2.0~20170911.0.7cb788c-2
rfkill amd64 2.33.1-0.1

3) bluetooth is not blocked:
root@server:~# rfkill list
0: hci0: Bluetooth
        Soft blocked: no
        Hard blocked: no

4) bluetooth service is running:
root@server:~# service bluetooth start
root@server:~# service bluetooth status
? bluetooth.service - Bluetooth service
   Loaded: loaded (/lib/systemd/system/bluetooth.service; enabled; vendor preset: enabled)
   Active: active (running) since Thu 2020-03-19 12:42:35 CET; 1min 9s ago
     Docs: man:bluetoothd(8)
 Main PID: 29677 (bluetoothd)
   Status: "Running"
    Tasks: 1 (limit: 4333)
   Memory: 1.6M
   CGroup: /system.slice/bluetooth.service
           mq29677 /usr/lib/bluetooth/bluetoothd

I have no access to X, so I cannot run blueman
How can I now make the device visible and start the pairing process with the printer just from console, in order to install the printer on this computer?
UPDATE1:
Finally I was able to see the printer with
root@server~# bluetoothctl
Agent registered

okay.
[bluetooth]# help
Menu main:
Available commands:
-------------------
advertise                                         Advertise Options Submenu
scan                                              Scan Options Submenu
gatt                                              Generic Attribute Submenu
list                                              List available controllers
show [ctrl]                                       Controller information
select <ctrl>                                     Select default controller
devices                                           List available devices
paired-devices                                    List paired devices
system-alias <name>                               Set controller alias
reset-alias                                       Reset controller alias
power <on/off>                                    Set controller power
pairable <on/off>                                 Set controller pairable mode
discoverable <on/off>                             Set controller discoverable mode
agent <on/off/capability>                         Enable/disable agent with given capability
default-agent                                     Set agent as the default one
advertise <on/off/type>                           Enable/disable advertising with given type
set-alias <alias>                                 Set device alias
scan <on/off>                                     Scan for devices
info [dev]                                        Device information
pair [dev]                                        Pair with device
trust [dev]                                       Trust device
untrust [dev]                                     Untrust device
block [dev]                                       Block device
unblock [dev]                                     Unblock device
remove <dev>                                      Remove device
connect <dev>                                     Connect device
disconnect [dev]                                  Disconnect device
menu <name>                                       Select submenu
version                                           Display version
quit                                              Quit program
exit                                              Quit program
help                                              Display help about this program
export                                            Print evironment variables

ok, so I went like this:
[bluetooth]# show
Controller 00:15:83:F4:BA:F5 (public)
        Name: server
        Alias: server
        Class: 0x00000104
        Powered: yes
        Discoverable: no
        Pairable: yes
        UUID: Generic Attribute Profile (00001801-0000-1000-8000-00805f9b34fb)
        UUID: A/V Remote Control        (0000110e-0000-1000-8000-00805f9b34fb)
        UUID: PnP Information           (00001200-0000-1000-8000-00805f9b34fb)
        UUID: A/V Remote Control Target (0000110c-0000-1000-8000-00805f9b34fb)
        UUID: Generic Access Profile    (00001800-0000-1000-8000-00805f9b34fb)
        Modalias: usb:v1D6Bp0246d0532
        Discovering: no
[bluetooth]#

and then I started:
[bluetooth]# power on
Changing power on succeeded
[bluetooth]# discoverable on
Changing discoverable on succeeded
[CHG] Controller 00:15:83:F4:BA:F5 Discoverable: yes
[bluetooth]# devices
Device 06:0E:3C:5C:1A:FD ENVY 5000 series
[bluetooth]#

then I go
[bluetooth]# pair 06:0E:3C:5C:1A:FD
Attempting to pair with 06:0E:3C:5C:1A:FD
[CHG] Device 06:0E:3C:5C:1A:FD Connected: yes
[CHG] Device 06:0E:3C:5C:1A:FD Name: OfficeJet 5200 series
[CHG] Device 06:0E:3C:5C:1A:FD Alias: OfficeJet 5200 series
[CHG] Device 06:0E:3C:5C:1A:FD ServicesResolved: yes

and after some seconds ...
[CHG] Device 06:0E:3C:5C:1A:FD ServicesResolved: no
[CHG] Device 06:0E:3C:5C:1A:FD Connected: no
Failed to pair: org.bluez.Error.AuthenticationCanceled
[bluetooth]#

The printer device has something called Smart Bluetooth
so I said to myself maybe I must not "pair" but "trust"
so i went:
[bluetooth]# trust 06:0E:3C:5C:1A:FD
Changing 06:0E:3C:5C:1A:FD trust succeeded
[bluetooth]#

and so I stuck again
I still can not see the printer on http://127.0.0.1:631
Any hint ?
Answer: After some hours I found the answer.
first you must disable the enable/disable the simple pairing mode. depending on what your device is supporting. I do not need it so I disabled it.
hciconfig sspmode off

The next I disabled the timeout, because I want the printer to stay connected until I disconnect him.
in /etc/bluetooth/input.conf
IdleTimeout=30

Just change it to 0 and the devices stay connected.
IdleTimeout=0

Case solved.


How do I use a freewheel as a single-speed?

Question: Can I disassemble a free wheel sprocket and use it as a single speed sprocket or I have to buy a single speed sprocket?
Answer: There are multiple options.

Just use one cog and don't change gear.   Cheap and easily reversible.
Remove and store your rear derailleur/shifter/wires, and then trim your chain to length so it only suits one cog.  This may require a chain tensioner and risks dropping the chain off, if the chain line isn't good.
You can get single-speed kits that fit on a freehub, essentially a cassette with one cog and spacers for either side.  Since the horizontal placement is adjustable, you should be able to get a better chainline.  If you only have a freewheel, this is not an option.
The expensive solution is to get a single-speed wheel, or rebuild your existing rim onto a single-speed hub.   This may give you a flip-flop setup where each side of the bike has a different gear, or the choice between a fixed-gear and a single speed.

It really depends if your frame has track ends (probably not if it has a derailleur) or some other way to set the chain tension.

Remember you still absolutely need working brakes, front and rear.  Don't remove them in your quest.

Personally I just ride with my gears - the 1935/6 Tour de France riders petitioned long and hard to be able to use gears!
Comment: @Carel okay, I don't see any pictures or mentions of number of speeds in the question
Comment: How do you know it's a cassette hub?
Comment: @ojs: Hint one: the lock-nut for the cassette is easily visible. Hint two: anything with 7 and more cogs is a cassette. There some rare 7-speed free-wheels in this case check for a lock-nut.


Is there an automated way to replace several layers with a template layer in Photoshop?

Question: I have a file where there are several "cards" that will be containing text information. All of these cards are exactly the same but have been aligned differently. Unfortunately, I have to replace all of these cards with a new template that I have created. Is there a way to do so without having to manually replace all the layers individually and realign them?
Answer: If you are using ordinary layers I don't think there is an easy way to replace each layer.
One way to do this is to use Smart Objects.

Right-click the initial layer and choose Convert to Smart Object.

Duplicates of the smart object are all references to the same layer.

When you double-click one of the duplicates the smart object will open in a new window. You can edit it, save it and close it. The changes will be reflected in all the duplicates.

Since you are talking about also adding text information to each card you might want to look into using a layout application like Illustrator or InDesign instead of Photoshop. I believe it will be much easier in the long run.
Answer: Semi-easy version :)

convert each layer to a smart object manually or using this script;
RMB on the new smart objects and select Convert to Linked. Photoshop will ask for a folder to export all smarts as files;
delete all .psb files but one;
back to PS: start recording a new action: rmb on one layer and select Relink to file, select one file that left;
play this action on all other layers


What does this symbol mean?

Question: I'm trying to figure out the meaning of this symbol.
Answer: It is 氏 as in 姓氏 (family name)
In ancient time, China was a 母系社会 (matriarchal society). People got their family name from their mother side. That was called '姓'.
Eventually, 父系社会 (patriarchal societies) started to develop. People began to add the family name from their father side in addition to their 姓 (family name from the mother side) and that family name from the father side was called '氏'
For example: A woman's family name was 杜 and gave her son the name 明, this 明 person would have a 姓 of 杜 and his full name would be 杜明.
However, his father's family name was 李, and this 明 person could legally call himself  '杜姓李氏' (杜 was his family name from his mother's side, and 李 was his family name from his father's side)
Eventually, matriarchal society societies was replaced by patriarchal societies completely. The term 姓氏 was consolidated to mean family name (from the father side)

In modern Chinese, the term '姓氏' refers to 'family name' and '姓' is the standard shortened form of '姓氏'

When you ask what is someone's family name, you only ask what is his 姓, never what is his 氏. The full term '姓氏' is rarely used colloquially. It appears mostly in written form.

氏 on its own refers to:

A woman's maiden name. For example: 杜门李氏 (married to the 杜 family and maiden name was 李)
Clan's name (interchangeable with 姓) -  both '杜姓一族'  and '杜氏一族' means 'the clan of 杜'
Answer: 氏: clan/surname
1.Refer to a clan. eg:曲阜孔氏,means:Qufu(曲阜 placename)'s family/clan with familyname: Kong(孔).
2.Refer to a person, usually a woman。王氏 means: Someone who is a particular surname Wang

What does this symbol mean?


Can I get Lifetime Learning Credit as a nonresident alien if I am a resident alien for tax purposes?

Question: I have been in the US for 8 years as an F1 student and I am a nonresident alien, but I am a resident alien for tax purposes. Can I claim LLC?
Answer: While many credits are not available to non-residents for tax purposes, very few explicitly require specific immigration status.
Specifically, for LLC to be considered an eligible student, you have to meet these requirements:

Who is an eligible student for LLC?
To be eligible for LLC, the student must:

Be enrolled or taking courses at an eligible educational institution.
Be taking higher education course or courses to get a degree or other recognized education credential or to get or improve job skills.
Be enrolled for at least one academic period beginning in the tax year.

No citizenship requirement.
You can check your eligibility here.


Velocity is relative, which means acceleration is relative, which further implies that forces are relative as well

Question: So how would we know whether a force truly exists or not. I can be accelerating a car my 5 meters per second squared but another car accelerating with the same acceleration would think that my car is at rest relative to them. So is there any force on the car? Or are forces just relative and their existence just depends on our reference frame?
Comment: "Height is relative" does *not* mean that slope is relative. Likewise saying that y' is relative does not make y'' relative.
Comment: Even though you are getting some quite curt replies, this is a great question. Understanding the various answers will give you some good insights.
Comment: Related questions / possible duplicates:
https://physics.stackexchange.com/q/22803/179151 ,
https://physics.stackexchange.com/q/349612/179151 ,
https://physics.stackexchange.com/q/245023/179151
Comment: Note that forces are not defined by acceleration. https://physics.stackexchange.com/q/581142/195139
Comment: _"I can be accelerating a car my 5 meters per second squared but another car accelerating with the same acceleration would think that my car is at rest relative to them."_ - I believe this is not true unless your velocities were identical initially. In all other cases the speed difference will be constant and visible from both cars.
Comment: A freely falling person does not feel earth's gravitational pull.
Answer:
Velocity is relative, which means acceleration is relative

This is not correct. Velocity is relative, but (proper) acceleration is not relative. It is an invariant. Real forces lead to proper acceleration so the existence of real forces does not depend on the reference frame. In contrast, fictitious or inertial forces do not cause proper acceleration so their existence does depend on the reference frame. Reference frames where fictitious forces exist are called non-inertial reference frames.
Comment: Actually without knowing about relativity some relativistic effects might appear to imply that acceleration is relative.   For instance, under a constant force an object close enough to the speed of light appears to accelerate more and more slowly.  I don't view this is force being relative though, more like the force carriers are travelling at a closer and closer velocity to the objects they are mediating between that they start loosing their "oomph" (to use a technical term ;)
Comment: @Michael gauge bosons don't physically exist, and photons (the boson of electromagnetism) always move at `c` regardless of the velocities of the objects they're acting on.
Answer: Within different inertial frames, velocities will be different. However, acceleration will always be the same in any inertial frame. Therefore, so will the forces.
Short proof: suppose $v(t)$ is the velocity in one inertial frame, and $v'$ is some constant shift in velocity due to choosing a different inertial reference frame. Then the velocity within that newly-chosen reference frame is $V(t)=v(t)-v'$, and upon differentiating it w.r.t. $t$, you get $A(t)=a(t)$, so the acceleration does not depend on the inertial reference frame, and neither will the force.
Now, in non-inertial reference frames, there exist what we call Fictitious forces (or not real forces), but these do not exist in inertial reference frames.
Comment: @tobi_s There's indeed the `[newtonian-mechanics]` tag attached on the question.
Comment: Heh, thanks, I indeed didn't see that. All this talk about inertial frames made my imagination move near the speed of light :)
Comment: Which time are you differentiating by? My point of course being that there are at least three different 'times' you could be differentiating by.
Comment: @tobi_s not sure I understand what you mean. I use the definition that $a=\dfrac{dv}{dt}$ to show that $a$ is the same in any inertial reference frame
Comment: There are different time variables attached to each frame and there is the proper time of the object that is subject to the forces. Thus my question is: which one is the $t$ in your denominator? To obtain an invariant definition of $v$ and $a$ it would have to be proper time. Of course this is moot when dealing with Newtonian mechanics where the three time variables coincide. Since the question was asked about a car which -- unlike trains -- is rarely used to explain concepts in relativistic mechanics I guess what I was aiming for was outside the scope of the question, so please never mind
Answer: People in an accelerated car know that it is accelerating due to inertial forces that can be observed inside.
In cases like that, besides the inertial forces, there is the reference of the fixed environement, so it is easy for the passengers to know about their acceleration, just looking outside.
More interesting is the case of the Earth. It was not so obvious to decide if all the sky rotates around us, or if we are rotating and the stars can be considered a fixed environment. Here, inertial forces like Coriolis, that explain trade winds and hurricanes rotation are a proof that we are rotating, what means an accelerating frame.
Answer:
I can be accelerating a car my 5 meters per second squared but another car accelerating with the same acceleration would think that my car is at rest relative to them

That is not true.
When we consider just velocity, it's simple because there are no forces involved.
But in accelerated motion we have to consider pseudo forces. You both are being pushed in this case, due to the pseudo force. That force is same for both of you, but that doesn't mean you won't notice a force on the other.
Say, you and your friend are moving in your cars with same acceleration. But your friend has a pendulum in his car that is hanging. Due to pseudo force, the pendulum is pushed back, and it stays like that because of the constant force on it backwards.
Like this:

You look over, and notice that pendulum is acting weird. That's how you prove, that even though both are experiencing same forces, they don't cancel out. Instead both experience it.
Answer: In Galilean relativity, physics is unchanged for frames that are related to uniform boost. That is, we introduce an equivalence relation $\sim$ among frames and define $\mathcal{A} \sim \mathcal{B}$ if frame $\mathcal{B}$ moves with constant velocity vector as seen by the frame $\mathcal{A}$. Then Newton's first law says that there exists a particular equivalence class such that inertial motion appears as uniform linear motion. Then there comes the second law.
In fact, we can think in this way: $\vec{a}$ is Galilean invariant, and $\vec{F}=m\vec{a}$ is the simplest physical law that conforms with Galilean relativity! And this is because force, a physical quantity, should not depend on observers. Indeed, "(Physical quantity)$ = $(Geometrical invariant)" is a recurring leitmotif throughout the history of physics.
Comment: "Then Newton's first law says that there exists a particular equivalence class such that inertial motion appears as uniform linear motion." Or Newton's first law tells you how to decide what "constant velocity" looks like in your reference frame: absent any forces, objects move along geodesics in spacetime (yes, we can do Newtonian mechanics on a curved spacetime, where gravity is curvature and not a force, just like we do with general relativity). Depending on how you interpret the law, it doesn't necessarily single out any particular class of frames.
Comment: Dear Arthur, yes that's a great point. There have always been controversies on the interpretation of Newton's first law: if interpreted "literally", it's just a special case of F=ma and rather unnecessary. What I followed here is the interpretation that some physics texts like Marion's classical mechanics textbook take.
Comment: I think such interpretation somewhat appreciates a bit of "philosophical content" inherent in the law, regarding the fact that not even the notion of "inertial motion," "intertial reference frame," or "force" was well understood before Newtonian mechanics. Yet, I wanted to be mathematically quantitative here rather than being philosophical.
Answer:
Or are forces just relative and their existence just depends on our reference frame?

Forces are only relative insofar as "every action has an equal and opposite reaction".  So a force applied to the car must also equally-and-oppositely be applied to something else, in this case the road surface.
For the other car, then, it must also be applying some amount of force to the road surface which results in the same acceleration.  (Note that the forces to achieve that acceleration won't be the same unless the masses of the two cars are the same, because of "F = ma".)
The fact that there is no net acceleration between the cars is irrelevant, because each car's forces are relative to the individual cars and the road surface.
The only way to make the other car relevant would be to connect the two together somehow, with a rope, pole or something.  Now your reference frame includes both cars - but your reference frame also includes forces in the connecting device, to establish net forces on each car within that reference frame.  You could for sure get the forces in the connecting device to be zero by applying the correct forces from each engine, but that only means there is no force applied via the connecting device.  Other forces must be known (and be in the correct ratio) in order for the force in the connecting device to be zero.
Answer: To understand the answer, you need the concept of an inertial or non-inertial frame.  That's fundamental to this kind of question.
Basic Newtonian mechanics are applicable in an inertial frame - one where the "space" (or box, or region) that we consider, is itself not accelerating. Since movement in a circle is also acceleration, that means the frame itself is not moving at constant velocity in a curve either.
If the frame itself is accelerating, its called a non-inertial frame. In a non inertial frame you have to take into account the acceleration of the frame itself. The acceleration of the frame will make it look (to someone within the frame) like there are strange extra forces pulling things in strange ways. These are called pseudo-forces - they don't really exist, they are artifacts of the frame's own acceleration. Like if you move within a car in a circle, it will seem like some force is pulling you away from the centre of the circle (centrifugal force). That's one example - we can see that this is a pseudo-force because obviously there is nothing at a distance all the way round the circle pulling just you outward.  Different non-inertial frames will have different pseudo-forces; by definition an inertial frame has zero pseudo-forces.
So the answer to your question is this:
Velocity can be relative, because a difference of velocity doesn't affect whether this forms an inertial or non inertial frame. Whether you travel at a constant 0, 20 or 50 km/h compared to another car, your frame in both cars is the same.  Constant velocity doesn't affect whether a frame is inertial or not, or (for a non inertial frame) how much by.
Acceleration cannot be relative, because a different acceleration always changes the frame. It changes inertial to non-inertial frames, and changes one non-inertial frame into another (or back to an inertial frame again).
And that means....
Because a different, but constant, acceleration of the frame changes the frames inertial nature, it will always change the pseudo-forces you see in that frame, and that will mean you can tell the difference between two frames which are accelerating differently.  So in an accelerating car, with blinds over the windows, you could still tell there's a difference in the cars motion between (for example) being stationary, accelerating slowly, accelerating quickly, or moving in a circle at constant velocity. You'll be pulled into the side of the car differently, or pressed into the seat differently, or a pendulum will hang or swing differently.
Because a different, but constant, velocity of the frame don't change the frames inertial nature or the pseudo-forces in it, you can't tell as an observer in the frame what velocity the frame moves at. In a car with blinds over the windows, you couldn't tell the difference between stationary, or moving in a straight line slowly, or quickly. It will all feel the same,and a pendulum will hang the same in each case.
Answer: I think it depends on what you mean by "force is relative." Do you mean that the observed force on an object is different in different frames of references?
Then yeah, force is relative. If you are sitting in your car, and the car is accelerating. You feel/observe the inertial force (kinda like gravity) backwards. But in the ground's frame of reference, you are not acted upon by any backward force.
However, acceleration, unlike velocity, doesn't satisfy the principle of relativity. That is for different inertial frames of different velocities, the physics in all of them are exactly the same. You can not measure an absolute velocity for an inertial frame by performing any kind of experiment.
However, acceleration doesn't have principle of relativity. You can know your absolute acceleration by observing your acceleration/inertial force in a non-inertial frame.
So acceleration and force are relative in that you observe different accelerations and forces in different frames of references. But they don't satisfy principle of relativity because they need the introduction of inertial forces to fix the physics in their frames of references.


Caed Nua - People seeking employment

Question: In my Caed Nua log, it shows there is a guy seeking employment. I simply can't find the guy or a way to hire him through the Caed Nua interface. How is it done?
Comment: Full original message could be helpful in finding what exactly you are seeing. Also, do you have Barracks built?
Answer: If you have the barracks built, then you just need to wait a little before the employment is possible. There is a little time shift between the log interface and the hiring interface. spend a night or change maps and it will do the trick.

Caed Nua - People seeking employment


How can you use a transformer with an AC current?

Question: I see transformers being used quite commonly for AC power and in many places I see people saying that they are meant to be used for AC. The way I understand it, transformers use large inductors around a core and can step down and step up voltage with different ratios of coils.  So won't these large inductors resist the AC current? Wouldn't it be much more efficient to use a DC current through the transformer coils and convert back to AC after?
Comment: Transformers ONLY work with AC.
Comment: @periblepsis You seem to have posted a good answer as 4 comments. This material may be lost in future. I strongly suggest that you copy the comments into a single answer. This should take minor diting and makes much better use of your input.
Comment: @SamGibson Okay. I tried. With more room I added a little more. I'll delete the comments.
Comment: I commented because somebody flagged it **BUT** I agree with them - it is a very useful commentary. As long as it does not get downvotes (which it certainly shouldn't) then it is a useful post for the OP and (importantly) a long term post to have available for others.  Do it !!! :-)
Comment: @periblepsis - Hi, Re: "*Is it really something to write as an answer?*" I will also say yes. It is useful in response to the overall question asked about "How can you use a transformer with an AC current?". Also, comments have a *deliberately* limited max length & have [limited allowed uses](https://meta.stackexchange.com/q/19756). A long-form monologue, no matter how good, is *not* one of them :( A big indicator that you're bypassing the deliberate site limitations, is writing more than one consecutive comment. As *Russell* said, to have longevity (& searchability) please aim for answers. TY
Comment: Some basic information about transformers [here](https://en.wikipedia.org/wiki/Transformer)
Answer: The whole point of a transformer is that it will resist AC current.  It works by blocking net magnetisation from the current of primary and secondary coils, so taking into account the turn ratios of secondary and primary coils, magnetisation from the incoming current in the primary coil will be countered by that from the outgoing current in the secondary coil.  An ideal transformer will not admit any net magnetisation to occur.
But it cannot properly resist DC, so DC will result in resistive heating of the primary coil, saturation of the core, a breakdown of the transformer principle and possibly even destruction through saturation and/or heat (once the core saturates, it does not inductively resist further net magnetisation current changes).
The way a transformer transfers energy is by making it very energy intensive to achieve any net magnetisation of the core, so by and large, the core will be left alone and incoming and outgoing current will be balanced along with the respective voltages.  That is what makes the losses in a transformer small.
Answer: Transformers cannot pass DC, only a changing magnetic field can pass through an inductor and a transformer is two inductors coupled to each other. so you need a switch to modulate the DC current to pass it through the transformer. Switching converters use transformers in this way and some AC/DC inverters.
Transformers are built for AC, a coil wire transfers current to magnetic energy and then back again, its one of the most efficient ways to be able to transfer energy while changing parameters (voltage/current). All electrical grids use transformers to be able to step up voltage up an down for better transmission.
Answer: While it is true that a transformer, as an inductor, 'resists' AC current, it is also true that the inductive effect on the iron of the transformer is related to the DIFFERENCE current between
the primary and secondary windings, not the sum.   With the
secondary open (unconnected) you don't want the primary to
waste power, and that means you want it to take only small currents.
But, when the secondary draws current, that current is in the opposite sense to that of the primary, so the primary's inductance, to the
extent that power is drawn, is not impeding the primary current
as much.   The effect of inductance of the transformer core
of a M:N turns ratio transformer, is that the AC current
sum, $$M\times I_{primary} - N  \times I_{secondary}$$ is impeded, and that means
that the secondary current is free to rise unimpeded, except
that primary current in proportion is drawn.
The inductive reactance that acts to limit AC current thus only acts
in a transformer to limit the energy-waste when the secondary
isn't drawing current.
Answer: You may already understand that increasing electric charge on a capacitor works up to a point. One can supply a specific DC current to a capacitor for only so long because, as the voltage rises from that fact, eventually the circuit no longer has access to the still-higher voltages needed to continue adding charge.
So, if the circuit depends entirely upon working in only one DC direction (sign) with a capacitor, and if it never discharges or otherwise allows the accumulating charge to reverse the sign of the current allowing a decline, then at some point everything must come to a screeching sudden halt.
Game over.
Consider perhaps this concept:
Imagine there exists something called magnetic charge as a dual of electric charge on capacitors, but now for inductors. In this case, these are often specified as Webers or, alternately, as volt-seconds.
(Names are important for communication. But the names we use for things aren't as important, when trying to understand a thing, as observing and understanding the thing itself.)
Thinking along these lines, if you apply a DC voltage that never changes sign, then over time these "volts times applied seconds" accumulate in only one direction and the circuit will then need to supply ever-increasing current.
So if the circuit depends entirely upon working in only one direction with an inductor and never allows the Webers to decline then at some point it must also stop working.
There is a small mental shift here, moving from capacitors to inductors, that may cause a momentary distress. Capacitor charge is like a static countable thing -- electrons or protons. Easy to understand and keep in mind. So when we discuss electric charge, there's little difficulty in teaching that even to a young child.
But Webers have time included. So it's not like marbles. In this case, more of them are created/needed the longer a voltage is applied to an inductor and this requires an increase in the current. (The number of Webers is directly proportional to the product of an inductance and its instantaneous current.)
This factor of time may offer a slight barrier. One needs to overcome that, if it is a problem. (Time is introduced, forced into the matter, because the speed of light imposes a time for interacting charges to interact.)
All this goes to say that while DC isn't strictly forbidden -- as an inductor will in fact respond to its application and do something about it -- it's generally not useful if it is never reversed and allowed to unwind/decline.
So you will find transformers used with DC -- but only if and when the DC is reversed repeatedly. Ham radio operators in the 1950's out of their cars would use a vibrator to cause the battery's DC to alternate across the transformer primary.
So DC works under the right circumstances. It has been used and will continue to be used. Just need to actively flip it every so often.
Flipping the DC, done just right, discharges accumulated Webers of one sign in to accumulate some of the opposite sign, before switching yet again to reverse the process.
The goal is that the net Webers hover around a usefully near-zero small value.
AC has a huge advantage: it automatically charges and discharges then reverses direction to charge the other way and then discharge. So it just happens as if by magic when using sinusoidal AC voltages.
No active devices or old-style vibrators needed then. Just plug and go.
There's the related slight problem that materials used in the core of transformers (those other than air or vacuum) do all have limitations with respect to the number of Webers per unit area (magnetic flux often specified in Teslas) they can support without seriously impacting the design inductance. So, at some point, DC when applied too long with one polarity across the inductor, the core material will cease to behave in the same way.
A new issue, then: core saturation. (But it's just a practical detail.)
This core saturation effect doesn't have to be avoided. It can be used and is used. (See this Wiki on the idea.) But it's more of a specialty area than mainstream.


Shorting a company's stock after leaving

Question: If a person just generally thinks a company is poorly run and decides to quit, would shorting the stock count as trading on non-public material information? If it's not based on any specific piece of information, could one not plausibly say both one's departure and the shorting was out of spite for the company? Alternatively, if a person is a minor figure in the organization but is fairly essential to the future growth of the company, if that person quits and then buys put options, would that fall foul of insider trading laws?
Answer: The law is somewhat nebulous but from what you are describing I would say "no".
As anyone can post and read reviews on glass door or something similar, it could be common knowledge of the management woes.  Heck it might even be in the company's press releases but one would have to read between the lines.   (Example the CEO's brother is the CFO and his cousin the the CTO and their resumes or education do not support those roles).
As far as the person who was essential to the company's growth...was their hire announced via press release?   If not then neither would their departure and so as such it is immaterial.
Leaving and then deciding that it is so poorly run that they can't help but lose money would not qualify.
Answer: Illegal insider trading is when someone uses material nonpublic information (MNPI) to gain an advantage. Thinking that a company is run poorly isn't MNPI, it's just an opinion. However, knowing that the entire management team that brought success to the company is going to be replaced by the co-founder's son and his drinking buddies for no reason except nepotism would definitely be nonpublic information, and almost certainly material as well.
There's no hard-and-fast rule of what constitutes MNPI. The nonpublic part is easy - anything that isn't made known to the general/investing public. But materiality is on a sliding scale. The above example is so extreme I can't imagine it not being material. But a CEO stepping down to be replaced by another qualified CEO may not rise to the level of being material (if you know that information ahead of time, it would still be risky to trade on it in case the SEC does determine you traded on MNPI).

If a person is a minor figure in the organization but is fairly essential to the future growth of the company, if that person quits and then buys put options, would that fall foul of insider trading laws?

Possibly, but probably not. If they're a minor figure in the organization, then their impact on the future growth of the company is probably small. If they're the sole inventor or idea generator, or do something that nobody else can do (i.e. they're irreplaceable), then they're definitely not a minor figure.
To protect itself, a company may place a post-termination trading restriction on an employee or executive to prevent them from trading on any MNPI after leaving the company, usually through the next quarterly earnings release. Someone with sales data, financial information, information on R&D progress, etc., might fall under that category. While not being under a restriction like that when you leave isn't a guarantee that you don't have MNPI , it often does mean that the company feels like any information you're leaving with isn't material.
Answer: This is in a very gray area, and you should only consult with a lawyer to get a reliable advice. The answers you get here worth exactly what you paid for them, but hear this:

Insider trading is trading based on information not available to general public. If you're that material person and your departure was not announced to the general public but you know how it would affect the company performance - trading on that knowledge is most definitely insider trading.

The fact that you're no longer employed doesn't matter for insider trading rules. The information you were exposed to is still confidential to the general public, so you're still not allowed to trade on it.

The enforcement of insider trading rules is done based on facts and circumstances of specific cases. If SEC can show that your trading pattern was abnormal, you were trading significantly differently than other investors, and that you've benefited where they lost - they will probably have a better case to prove that you did all that because you used some knowledge you gained while being employed. They might even not need to point to the specific knowledge, just the opportunity to have it.

That said, as mentioned in the first sentence: for reliable legal advice (where you can use "advice of counsel" as a criminal defense) - talk to a lawyer.


How wide should a workbench top's overhang be to accomodate clamp-on vises and work lamps?

Question: Is 2" (5 cm) of workbench top overhang enough to accomodate most clamp-on vises and work lamps?
Answer: That should generally be enough. If you happen to run into a vise or lamp that wants a little more, you could edge-laminate an additional buffer strip onto the bend in that spot.
Comment: In the end I made the overhang 6 cm wide. The clamp-on vise with largest clamp-throat-depth I've found is PanaVise 311 with clamp-throat-depth of 5.87 cm, though I doubt I'll need to clamp that deep on a bench that's only 3/4" thin. Typical clamp-on vises do indeed have clamp-throat-depth of less than 2".
Answer: I would go further in the event that you want to clamp deep throat clamps or woodworkers clamps.  Hand screw clamps can be 6" or more.  Keep that in mind for the future. What about a single side with a deeper inset for specialty clamps?
Comment: I probably should add that I am building a 3/4" thin electronics bench, so even the clamp-on vise scenario is not very likely (I use Panavise for holding PCBs) but this information will come useful to those wanting to build a woodworking bench. Thanks.


Won't go when out in gear

Question: I have a 1500 Suzuki intruder that I changed oil oil filter and gear lube on Plus changed plugs now when I put it in first it dies why
Comment: Welcome to Motor Vehicle Maintenance & Repair! What kind of oil did you use?
Comment: Perhaps more interestingly, what oil was the "gear lube"?
Comment: If the bike has a wet clutch and the oil is shared between the engine and gearbox - if its the wrong type it could cause problems!
Answer: I am unfamiliar with this motorcycle.
Many or most of today's bikes, however, are designed to allow the transmission to be shifted into gear only when the clutch is disengaged, i.e., when the rider is squeezing the clutch lever on the handlebar. If the bike thinks the clutch lever isn't being squeezed, then it cuts the motor as the rider attempts to shift into gear. (Some bikes also have a similar switch on the sidestand: attempting to put the bike in gear with the sidestand down will likewise kill the motor.)
If the bike starts and idles correctly, and immediately dies when the clutch is squeezed to shift into first, then something is awry in the clutch (or sidestand) safety circuit. Somewhere on the clutch lever is a small switch, and either that switch has broken and is stuck in the position that indicates "clutch is engaged," or there's a fault in the wiring or connectors downstream from the switch.
A good search approach when something fails just after work is done is "Look where you worked." It's very possible you inadvertently disturbed the clutch safety switch wiring or a connection when you were working on the bike.
The fault will have to be found and rectified.

Won't go when out in gear


Was Pete Townshend the first musician to destroy his instrument as part of a performance?

Question: Pete Townshend of the Who regularly  would smash his guitar on stage as part of his performance. Was he the first musician to destroy his instrument as part of a performance?
Comment: Related wiki page: http://en.wikipedia.org/wiki/Instrument_destruction
Answer: Not to the extent of actually destroying the instrument, but Jerry Lee Lewis was well-known for kicking the stool over, stamping on the keys, sitting or standing on the piano & generally mistreating it as part of his act.
Somewhat gentler times - that was probably seen as just as controversial; though it would be quite hard to swing a piano ;)
By the end of the 60's even smashing a guitar was no longer considered exciting enough. Hendrix even set fire to his.
Comment: When we count any kind of damage done on purpose to an instrument as part of a performance, we probably have to start with [Paganini's habit of killing e-strings](https://www.youtube.com/watch?v=xIqIerlZN1E).
Answer: To a historical extent and certain verifiable records, Paganini and Liszt are also known to have broken their instruments while performing. Paganini was reported to have played on a violin that had only one string left, and Liszt might have broken a number of piano keys during his hectic performances. Now the problem is whether the decision to partially damage the instrument was deliberate in order to give off technical virtuosity and win over the audience.
Answer: Nam June Paik wrote "One for Violin Solo" in 1962.
https://www.youtube.com/watch?v=Q6u5nJCR0xY
Answer: The Lawrence Welk show from 1956 Features a fairly decisive guitar smash from "Rockin` Rocky Rockwell". https://www.youtube.com/watch?v=JQz0u2yV4Os&feature=player_detailpage#t=100
The fact he is not actually playing it makes me wonder if it qualifies as an instrument, or is more of a prop...
Comment: The link to the video is broken.


Show deleted notification on direct URL

Question: Please show a deleted notification when browsing to the direct URL of a deleted question.  I find it vary annoying when I'm watching a question (pressing F5 every hour or so), and it simply disappears.  I know that it was there, but now I'm treated to a page not found error like I typed something random into the browser.  I'd don't need the explanation as to why it was deleted, although that would be nice.
Answer: Well, if you view the 404 page..
https://stackoverflow.com/404
You'll note that it already says

Occasionally questions or answers may be removed from Stack Overflow for reasons of moderation: spam, off topic, abuse, etcetera.

almost all the time, the post was deleted by the author, by the way... it's very rare for it to be editorial or spam.
Comment: Still, there is no way to tell whether one got the URL wrong, or a question used to be there and was deleted. It would be nice to have some distinction
Comment: I realize that the scenario is mentioned.  That's why I'm assuming that the question was deleted.  It's misleading, and I normally come to SO for clarity.

Show deleted notification on direct URL


Clicking User Accounts in the System Settings breaks everything

Question: Clicking on other settings like Displays or Keyboards works fine, the new window opens and I can change the settings.
But when I click on User Accounts, it does not open, and I am unable to click on anything else, and the window will not close and stays on top. I can't use anything else until I force log out.
This is 14.04.4 LTS and Unity
Answer: This happens when you upgrade graphical stack and do not install libwayland-egl1-mesa-lts-xxxx, where xxxx is your graphical stack name.
Check which version is installed by
dpkg -l | grep xserver-xorg-core

and see which line is with ii chars
ii  xserver-xorg-core-lts-wily                            2:1.17.2-1ubuntu9.1~trusty1                         amd64        Xorg X server - core server

If it is wily, then run
sudo apt-get install libwayland-egl1-mesa-lts-wily

and the issue will be fixed.


How do we know that nuclear decay is truly random and spontaneous?

Question: Nuclear decay is said to be random and spontaneous, but how do we know for certain, that it is not just a lack of understanding of some other unknown force?
Doesn't everything in the universe just depend on the starting conditions, so arguably nothing is random?
Comment: Why should we think that it *is* lack of understanding of some other unknown force? Randomness explains what we observe about nuclear decay very well, and we do not observe any unknown forces beyond that. "Some other unknown force" can "explain" anything and everything, see [God of the gaps](https://en.wikipedia.org/wiki/God_of_the_gaps).
Comment: @Conifold have you heard of hidden variables in QM?
Comment: @WillyBillyWilliams Yes, of course. But it is lack of observable predictions, and scarcity of theoretical benefits, that reduced them to a small minority position.
Comment: https://en.wikipedia.org/wiki/De_Broglie%E2%80%93Bohm_theory
Comment: I disagree with your assumption that nothing can be  random. It could be, or it could be not, we just do not know.
Comment: 1) We don't know, on what it depends, but its behaviour seems to be randomly 2) The behavior is completely randomly <- Do you have any experiment to differentiate (1) and (2)? If you would have, the behavior or the thing wouldn't be random. (In practice, radioactive decay is the best known randomness source what we have. Sometimes, particularly in cryptology, we often need very random numbers, it can be useful for that.)
Answer: Science doesn't tell us the reason things happen.  It provides a way to develop models which predict what will happen.  For all we know, Zeus himself personally causes every radioactive atom to decay when he sees fit.  Science cannot disprove such a claim.
What we can do is use the scientific method.  In the scientific method, we pick a "null hypothesis" which is what everybody expects to happen, and an "alternate hypothesis" which is the interesting thing we want to test.  Then we run an experiment and hopefully show that the null hypothesis is highly unlikely, while our alternate hypothesis is good.  In the case of this topic, the usual null hypothesis is "radioactive decay is random."
To date, nobody has been able to develop a test which can demonstrate that they can predict the timing of radioactive decays better than random chance.  That's not to say there's not some local hidden variable* or angelic cherub that knocks the atom about to cause it to decay.  It just says that nobody has been able to provide such a theory which does better than the "radioactive decay is truly random" theory does.
* Okay, I lie.  Moonman239 pointed out that Bell's theorem actually does state that you can't have local hidden variables.  Rather than editing it out, I keep it in with a footnote just because it shows just how weird our observations can be, and just how delightfully strange the universe must be.  Even very reasonable assumptions get tested and disproved on a regular basis!
Comment: "That's not to say there's not some local hidden variable," What about Bell's theorem?
Comment: @moonman239 Good point.  I might have been on too much of a "local hidden variable" binge and thrown the word "local" in where it shouldn't be.  Although I admit, I am now terribly amused by the idea of Zeus smiting radioactive atoms in a non-local way!
Comment: @moonman239 Offhand, do you know of any experiments proving Bell's theorem applies in an experiment involving radioactive decay?   I have no reason to believe it wouldn't apply, but has anyone gone out there and demonstrated it? (if for no other reason than to put Zeus on notice)
Comment: Just curious, are hidden local variables needed to explain why rolling a fair die produces a random value?
Comment: Is there a precise formula to predict which atoms (or isotopes) are radioactive? Also it seems determining or measuring half-life is [not an easy task](https://science.purdue.edu/xenon1t/?p=1287) either.
Answer: We don't know anything "for certain" but experimental evidence favors randomness over Newtonian deteminacy for quantum decay phenomena. All we can do is proceed on the basis of our best experimental data to build theoretical models that employ consistent mathematics. To do otherwise would be outside scientific boundaries.
Comment: @Aaron The uncertainty principle puts paid to the notion of *"knowing all initial starting conditions"*. Pascal's vision of a clockwork universe isn't compatible with Heisenberg's principle.
Comment: What experimental evidence? But again surely knowing all initial starting conditions means an accurate prediction of the outcome can be calculated. It seems quantum decay has something to do with it. But then how does quantum disagree with my logic.
Answer: Let me focus on the first question that is posed: "Nuclear decay is said to be random and spontaneous, but how do we know for certain, that it is not just a lack of understanding of some other unknown force?"
I think the answer is: we do not know. The proof of this is that we have different interpretations of quantum theory, all of which are compatible with experiments, yet some have intrinsic randomness, whereas other are completely deterministic.
For instance, the de Broglie-Bohm interpretation (also known as pilot-wave theory) is a purely deterministic theory which reproduces all predictions of quantum theory. In this framework, the initial state entirely fixes when a nuclear decay will occur. However, it is exceedingly difficult to access the full information, and so it only seems random, just as how Newtonian physics can seem random when using statistical mechanics. (As a consequence, Born's rule is not an axiom in this framework but can be derived as a statistical fact, just like how in statistical mechanics you derive that a box of gas will be described by a homogeneous distribution in phase space.)
However, there is something we really do know: nature cannot be deterministic and local. Bell's theorem shows that any such theory of nature predicts a certain inequality which has been experimentally violated, concluding that a purely local and deterministic theory cannot be consistent with experimental fact. (E.g., the aforementioned pilot-wave theory is consistent with this, in that it has explicit nonlocal elements in it.)

A "don't shoot the messenger" disclaimer: For some reason, mentioning the existence of certain interpretations of quantum theory is offensive to some. I side with J.S. Bell in that it is useful to know what interpretations (compatible with experimental fact) exist, to clearly differentiate which things are forced upon us by nature, and which are merely a product of the choices we make. This has no bearings on what choices seem more 'minimal' or 'natural', which is quite personal and subjective.
Answer:
Doesn't everything in the universe just depend on the starting conditions, arguably nothing is random.

You are thinking in terms of classical deterministic physics which has been validated  in dimensions much larger than the quantum mechanical dimensions commensurate with Planck's constant h_bar. Nuclear decays are in the range where quantum mechanics has to be invoked, which is by construction a probabilistic theory.
There are differential equations which give the solutions to quantum mechanical problems, where boundary conditions have to be imposed and the probability distributions are predicted and validated by the data. Individual measurements cannot be predicted , only the probability of finding a value can be predicted.
The randomness of the nuclear decays is due to this quantum mechanical  probabilistic underpinning:

A nucleus does not "age" with the passage of time. Thus, the probability of its breaking down does not increase with time, but stays constant no matter how long the nucleus has existed.

P.S. For complicated quantum mechanical systems have a look at this answer of mine
Answer: I broadly agree with existing answers at the time of writing. I hope I can add something helpful. I would like to underline the role of model-making in science. The physical world does what it does, and in science we construct models. The models help us build insight into the nature of the physical world. But the model is not the same as the physical world.
In the case of phenomena such as radioactive decay (and another good example is light reflecting off a partially reflecting surface) our most complete model is the one provided by quantum mechanics. In quantum mechanics the model includes randomness (let's not get into Everett interpretation here, it will not change the central point I want to make). So we think the decay is random because we think quantum mechanics is offering a good model. That is not the same as certain knowledge; it is a case of reasonable belief.
If someone wishes to propose another model, one in which the decay events are not random, then they are welcome to do so. It will be received if it matches experimental observations and has an elegant mathematical framework or other such beauties to recommend it. It will be enthusiastically celebrated if it also manages to predict something currently unknown or puzzling and get it right. But no one is currently able to think of a model like that.
By the way, personally I welcome a bit of openness or non-determinism in the world. If it is like that, as it seems to be, then it seems to me to be a more liberating sort of a world, and I say this even aware of all the pain that is also associated with random events.
Comment: I appreciate your point, but I feel it's missing the mark. The claim whether it is truly random is indeed a property of the model. But we have models which are compatible with reality and which are not random, like the de Broglie-Bohm (pilot wave) interpretation. So I think the real answer is: nuclear decay is random or not depending on your choice of interpretation. I know some say that the deterministic interpretation is not minimal since it involves extra structure (like hidden variables), but it also needs less axioms (e.g., the Born rule is derived in that framework, rather than assumed).
Comment: @RubenVerresen I see what you mean about de Broglie-Bohm. I guess I was leaving it out because whereas I think it is a respectable way to interpret the non-relativistic version of QM, with single-particle wavefunctions and the like, I have never become convinced that it stands equally well as a way to interpret the quantum field theory which underlies the Standard Model. Do you think it gives a satisfactory picture of pair creation and annihilation, and Higgs mechanism, etc.?


Why don't the inner moons of Jupiter have tidally-induced volcanism?

Question: Volcanism on Io is caused by the fact that it is tidally heated. There are four moons that are closer to Jupiter than Io with higher eccentricities, yet they don't seem to have any volcanism at their surface. Why isn't the tidal heating enough to generate volcanism on any other moons of Jupiter?
Comment: There are no spherical moons between Jupiter and Io. All the satellites inbetween are asteroids, the largest of them are Amalthea and Thebe having diameters of about 250 km (150 mi) and 110 km (70 mi). So there is no interior differentiation and thus any volcanism impossible.
Answer: It's because they are much smaller than Io.  Tidal forces are differential forces, that is, they result from the difference in gravitational pull on one side of a body compared to the other.  When an object is small, the difference in distance to the two sides of it is necessarily small as well.
According to Wikipedia, Amalthea, the largest of those four innermost moons has a long axis that is only 250 km, and the others are smaller yet. The strength of the tidal heating scales as the body's radius to the 5th power, quite a strong dependence.   The small mass plays a role, too, but the dependence on radius is stronger.
For this same reason, small objects on Earth don't feel tidal stretching from our Moon, yet the Earth as a whole does.
On a related note, the inner three of those four satellites are inside Jupiter's Roche limit, the orbital distance at which a purely self-gravitating body should be pulled apart by tidal forces.  But they survive because most small bodies aren't held together primarily by gravity (just as your own body isn't) - the internal electromagnetic forces between atoms (which manifests macroscopically as the rigidity of the rock) is the main source of the bodies' structural strength.
Comment: To take this to a limit, artificial satellites around Earth don't experience measurable tidal heating, and don't disintegrate from tidal stresses, even though they're well inside the Roche limit.
Comment: Re *"...as the rigidity of the rock"*: But the inner three includes Amalthea which *"[consist of porous water ice with unknown amounts of other materials.](https://en.wikipedia.org/wiki/Amalthea_(moon))"*.
Comment: Only inside for the fluid-body Roche limit?
Comment: Water ice has rigidity - one surmises, in these specific instances, enough to withstand tidal forces. Presumably the moons for which that was not the case are all the ones that are no longer there.
Answer:
There are four moons that are closer to Jupiter than Io with higher eccentricities, yet they don't seem to have any volcanism at their surface.

Only one of those innermost moons (Thebe) has an eccentricity higher than that of Io. The other three have lower eccentricities.
The reason they don't exhibit volcanism is because they are too small. The largest of the four, Amalthea, has a mean radius of 83.5 km. Compare that with Io's mean radius of 1821.6 km.
Tidal heating results in part from a tension between the tidal forces exerted by the planet that tends to pull a moon out of round and the moon's self-gravitational force that tends to pull a moon into a roundish shape. Small bodies such as those four innermost moons of Jupiter are too small to pull themselves into a roundish shape. The fuzzy boundary between small bodies such as those innermost moons and dwarf planetary bodies such as Pluto is the potato radius, somewhere between 200 and 300 km.
All of those four innermost moons of Jupiter are well below the potato radius, too small to pull themselves into a roundish shape via self-gravitation. The tidal forces on those small bodies are also smallish. (Tidal forces are proportional to body radius). This makes the tension between self-gravitation and tidal forces negligible for those small bodies.
Another factor that comes into play is the square-cube law. Cooling is proportional to surface area and hence the radius squared while mass is proportional to radius cubed. This means cooling per unit mass is inversely proportional to radius. Heating induced by those negligible tidal forces and negligible self-gravitation forces dissipates quickly in small bodies, not so quickly in larger bodies. This makes the interiors of larger bodies such as Io partially liquid. Small bodies such as those innermost moons of Jupiter are solid throughout. They don't flex as much as do those larger moons, and hence there is significantly less heat production in those small moons.
Answer: tl;dr
The inner moons are much to small (and too stiff) to experience significant tidal work. They also cool down much faster though I don't think this is relevant in this case.
There're two things to look at, here:

The total amount of tidal work that is done on a moon
The amount of energy that is radiated away over time

When looking at the list of Jupiter's moon, one can easily see that especially the Galilean moons are more than a 1.000 times bigger than anything else that's orbiting Jupiter including the inner moons.
As we know work is force multiplied by distance, so it's not enough to push something - in order for work to be done, it also has to move.
For simplicity's sake let's think of a moon as a piece of rock. If you squeeze a piece of rock with your bare hands, you're just applying a force to it but not over a distance as the rock won't change shape. Thus you won't do any work and the rock won't heat up.
As with moons, the tidal force not only depends on the distance to the object it orbits, but also on of the mass of the object itself (among other things such as size), so a heavier object experiences higher tidal forces than a light one and there's a certain point at which those forces are strong enough to actually cause a change of shape, which means that's there's actually work being done that turns into heat.
The other aspect is probably negligible, though it's there: Larger objects cool down much more slowly, because they have a lower surface area compared to their volume.
Comment: Re: "the Galilean moons are more than a 1.000 times bigger than anything else that's orbiting Jupiter including the inner moons." By what measure? [Amalthea has a diameter of 170 km and Europa 3100 km](https://en.wikipedia.org/wiki/Moons_of_Jupiter#List) (a factor of 18)
Comment: @PeterMortensen by any measure that make sense, Europa is 23076 times as big as Amalthea.


Visualizing entering the Internet, Neuromancer-style

Question: In a cyberpunk city, what would it realistically look like to go to download your mind onto the internet, or to at least enter the internet by sending your mind into it, Neuromancer-style? How would you describe things like chatrooms, using search engines like Google, hacking and downloading, if they weren't 2D boxes on a screen, but 3D? What might it even feel like?
Comment: UI is a extremely subjective concept.
Comment: Welcome to WorldBuilding! If you have a moment please take the [tour] and visit the [help] to learn more about the site. Your question looks opinion-based, meaning that there is probably no somewhat objective way to rate answers against each other. There are a lot of ways how this has been portrayed already (think Matrix or Tron) and there are a lot of different directions you could go with current VR technology. How you want to represent this in your scenario is depending completely on what you want to convey. Should it be dark? Minimalistic? Bright? Sterile? Magical?
Comment: @r_alex_hall What do any of those issues have to do with the question that was asked? While I agree the question lacks objective criteria to determine a "best" answer, the issues you've raised, while relevant for other aspects of the OP's world/power, are unrelated to this question.
Comment: This is both subjective and extremely broad. Download your mind onto the internet? First of all it would be an upload ;) OR send your mind into the internet? Under what constraints? Who controls your mind, who has permissions to change it or copy it in any way, who has permission to save data and where, how the heck do you make data and your mind equivalent, is your mind copied or is it augmented (and if copied, do you leave your body behind and die or is your consciousness shared with a network or whatever) etc. etc. etc.
Comment: That practically anything could be made to appear relevant to the question is the point, and a demonstration of the question being too broad. "..download your mind onto the internet.." and "..sending your mind into it.." could be done (and has been done hypothetically in sci-fi) so many different ways and could itself have so many different conceptual meanings that there's no relevant specific meaning of the question on which to hang answers. I'm saying the question can't easily be answered because it's too broad.
Answer: The human mind is great at finding patterns and making associations.  If something is presented to it that is close enough to an experience it already had, it will fill in the gaps.  So it will readily identify boundaries that it cannot cross as "walls", etc.
The internet, or any complex computer network, is more like a system of tunnels than it is like a cityscape.  Sorry Tron fans.
Since you can disconnect (I am assuming one can in your world) whenever you want without having to "travel" back to your starting point, it makes little sense to use travel as a model for the experience.  It would more be like you select a tunnel and the fabric of the world around you shifts until your current point is the same as the other end of that tunnel.  A chatroom would be a space or opening wherein multiple visitors are aware of each other and can communicate, maybe even chat.  A hacker would be someone who can find a door where there should not be one.
Depending on your connection method, there could be a server in between you and the internet that is processing your requests and converting them into content for a VR helmet or such.  Perhaps that is the function of all dedicated access points in your world, is to provide a "translation" service where the signals are converted to a VR experience.
Search engines are servers providing a service, so you would essentially be asking a butler to fetch something for you.
Really your question is very open and the "right" answer is a matter of opinion.


How many four letter words can be made from the multi-set {T,E,L,E,P,H,O,N,E}?

Question: As the question states, how many four letter words can be made from the multi-set $\left\{T,E,L,E,P,H,O,N,E\right\}$? One condition applies, in that (EELE) is a valid four letter word but (EEEE) is not. I was thinking somewhere along the lines of placing k balls into n boxes, the general example for counting, where the k balls are the letters and boxes are the four possible places in our four letter word, but I just can't seem to form a believable answer.
Comment: You could separate out cases with 1E, 2Es, and 3Es. All other letters are distinct so counting words in each case should be straighforward.
Answer: From the word $$\left\{T,L,P,H,O,N,E,E,E\right\}$$ we count number of four leter words with no E $\binom{6}{4}4!$, with one E $\binom{6}{3}4!$, with two E $\binom{6}{2}\frac{4!}{2!}$ and with three E $\binom{6}{1}\frac{4!}{3!}$ and get
$$\binom{6}{4}4!+\binom{6}{3}4!+\binom{6}{2}\frac{4!}{2!}+\binom{6}{1}\frac{4!}{3!}=$$
$$=15\cdot24+20\cdot24+15\cdot12+6\cdot4$$
Answer: You have $9$ letters and a four letter word. So, it needs a permutation:
$9*8*7*6 = 3024$
That is, each of the $9$ letters can be combined with the remaining $8$ letters, that can be combined with the remaining $7$ letters, that can be combined with the remaining $6$ letters.
Comment: A permutation will not work since there are some repeated elements in the set.


disable guest accounts for commerce

Question: Is there a method to remove the ability for commerce to allow guest accounts? I want to allow only registered, logged-in users to be able to place orders.
Answer: I don't actually think Commerce has an option to disable guest checkout, no.
But Craft itself makes this, in effect, quite easy.  (In fact, Commerce's whole concept of customers is largely separate from the Craft concept of users.  A customer may be a user, and vice versa, of course, but they are really two separate things.)
In any case, you can hide all checkout access and pages behind <code>{% requireLogin %}</code> tags so those parts of your front end are simply then not reachable by customers who are not logged in users.
https://docs.craftcms.com/v3/dev/tags/requirelogin.html

disable guest accounts for commerce


SP2016: How can I add visit counter for documents?

Question: My SharePoint site has anonymous access and extended for internet. I want to stroe anonymous number of visits for each document (such as Images, Videos, and etc.) and show to all anonymous visitors.
How can I do that?
Answer: You may need to enable usage cookies to generate unique user IDs so SharePoint can record the usage events from anonymous access.
Please refer to:Use recommendations and popular items on websites with anonymous users in SharePoint Server


Other expressions for "technology evolution"

Question: In the last few decades technology has changed many of our daily habits. Making  phone calls, keeping social contacts, booking a hotel or a flight, reading  digital maps, online newspapers, books and dictionaries, just to name a few things that not so many years ago required more time and often  the help of other professional figures. Bad or good this trend, often called technology evolution, is probably going to accelerate in the foreseeable future. What other expressions can you suggest to describe this unavoidable trend.
Comment: I love how each generation has to feel special, as if they were the only ones to know radical change.
Answer: Many people would regard onslaught of new technology and overwhelming tsunami of tech as accurate reflections of how they feel about the phenomenon you describe.
Answer: Mankind in a Transition
is an interesting expression used by Radovan  Richta, the philosopher who coined the expression "Technology evolution" that is the name of an innovation & technology related theory describing technology development.

Technology (which Richta defines as "a material entity created by the application of mental and physical effort to nature in order to achieve some value") evolves in three stages: tools, machine, automation. This evolution, he says, follows two trends: the replacement of physical labour with more efficient mental labour, and the resulting greater degree of control over one's natural environment, including an ability to transform raw materials into ever more complex and pliable products.
Answer: The simple word progress sometimes is used (eg, in phrases like "That's progress for you" or "the high price of progress") to encompass the idea that change is rapid, and while perhaps evolutionary, often disconcerting.  Progress also is part of the term progress trap, which refers to

the condition human societies experience when, in pursuing progress through human ingenuity, they inadvertently introduce problems they do not have the resources or political will to solve, for fear of short-term losses in status, stability or quality of life.

The progress trap term suggests that some avenues of evolutionary progress are dead ends.
Answer: "The Law of Accelerating Returns" was used by Ray Kurzweil to refer to the phenomenon you described.
http://www.kurzweilai.net/the-law-of-accelerating-returns
Answer: You might want to consider the terms exponential, accelerating or increasing, with respect to the pace, progress, advancement or similar of technology.
For example:

We live in a time where advances in technology occur exponentially. Today's cutting edge tech, maybe obsolete tomorrow.

or

The accelerating pace of technological progression means that new technology is quickly antiquated and needs replacement.

or

Breakthroughs in technology are occurring with increasingly shorter intervals. Where a new model of phone might have taken two years to come to the market, it now takes six months.

You might also consider for an explanation as why technology is progressing at an increasing rate with feedback effect or feedback loop.

The feedback loop of technological discovery means that technological breakthroughs occur at increasingly shorter intervals. The breakthrough of one technology allows discovery of other technologies, and so the rate of progress is quickened.


simpleCart.js - Calling quantity of a single item in cart with beforeAdd();

Question: I have searched the simpleCart documentation, found here, the github page, and here extensively, to find an answer to what should be a simple issue to solve:
I need to get the quantity of a single item in the cart, so I can check it against a custom attribute containing a maximum quantity, along with the quantity being sent to the cart when an item is added.
simpleCart.quantity(); gives me the total for ALL the items in the cart, same with simpleCart.item.quantity();. Meanwhile, item.get('quantity') gives me the amount I am sending to the cart, which I need but isnt the value I am looking for.
Here is the relevant code for context
if (simpleCart.has(item)) {
    var mA = item.get( 'maxamount' );

    var linkQty = item.get('quantity');
    totalquant = (item.quantity() + linkQty);

    alert('Max amount: '+mA+'\nQty sent to cart: '+linkQty+'\nQty in cart plus qty sent to cart: '+totalquant);
    if(totalquant > mA){
       alert("You can not select more than "+mA+" items of this size!");
       return false;
    }
}

I am aware this exact question exists on this site, but it recieved no response. Please can someone help me?
Answer: I think you were really close. I used SimpleCart v3
simpleCart.bind('beforeAdd', function (item) {
var requestedQuantity = item.get('quantity');
var existingItem = simpleCart.has(item);
if (existingItem) {
    requestedQuantity += existingItem.get('quantity');
}
if (requestedQuantity > item.get( 'maxamount' )) {
    alert("You have reached the maximum quantity of this size.");
    return false;
}
});


My guitar instrument produced a perfect sine wave?

Question: My homemade guitar was producing perfect sine waves, most guitar waves arent perfectly sine. Anyone have any suggestions for why?
 Physics project))
Comment: That's surprising.  How do you know that it was producing perfect sine waves?
Comment: To me, that would be a sign that my strings were shot and needed to be replaced.
Comment: Perfect?  What did you use to measure the harmonic distortion?
Comment: Related [question](https://music.stackexchange.com/q/32282/2600).
Answer: All string and wind instruments produce pure sine waves.  The thing is, usually they produce more than one of them at a time, at multiples of the root.  For example, if a string 12" long produces a sine wave, it will also produce the same sine wave that would be produced at 6", and at 4".  These are called harmonics. When you combine these, the waves don't look like sine waves any more-- if you combine enough harmonics in the right proportions, you could eventually get a square wave, for example.
Harmonics can be removed by using a low pass filter.  If you adjust the filter to remove all the harmonics, you will always be left with a single sine wave at the note's root.
My guess is that you recorded your guitar under some form of EQ, filter, or other signal processing effect, and only the fundamental made it through to the stage where you were examining the waveform.
Comment: For a plucked string fixed at both ends at least, these harmonics are a given, as the waves reflect and interfere. Also, the location of the pluck will impact the harmonics. So I tend to agree: the "perfect sine wave" is either imaginary or the result of filtering.


What's the status of LevelDB? Is it safe for use in production?

Question: Does anyone know how well tested LevelDB is and what is its status for use in production? It's a relatively new library and when I checked the source code it didn't appear to be handling errors too well. Does anyone use LevelDB in production and can comment on my question?
Comment: Heya, I downvoted this because it appears to be a subjective, opinion-based question.  Stack Overflow generally avoids these sorts of things, [seeking questions that have solid answers](http://blog.stackoverflow.com/2011/01/real-questions-have-answers/).  I'd expect an answerable question to specifically ask what functions in the library would need additional error checking to be reliable or what virtues LevelDB has vs. .  If this question doesn't work out, try asking another with this in mind.  Hope you find the answers you're looking for.
Answer: We use LevelDB in our website, but wrapped in SSDB(https://github.com/ideawu/ssdb), the LevelDB network server, with hash/zset data types support. Our SSDB instance serves 100 million queries per day.
Comment: How has the stability of leveldb been? Any issues you've run into?
Comment: Both LevelDB an SSDB is production ready. LevelDB write data in sequence, and then compact(merge) in a seperated thread, so the performace may slow down during the compaction, because disk IO would fill up. SSDB adds compaction IO speed limit feature to LevelDB.
Answer: LevelDB has a lot of high-visibility problems https://github.com/bitcoin/bitcoin/issues/2770 and the code is so poorly written that a bounty was needed to find a fix https://bitcointalk.org/index.php?topic=337294.0;all And the leveldb discussion group is predominantly bug reports about very fundamental database functionality that fails to work as advertised. https://groups.google.com/forum/#!forum/leveldb (e.g., "snapshots" aren't actually snapshots, and can be tainted by subsequent writes https://groups.google.com/forum/#!topic/leveldb/IAKJaL2zqZM etc...)
On the date that this question was asked, LevelDB was certainly NOT production ready and anyone who thought so was delusional. The code quality is abysmal, as confirmed by independent developers https://twitter.com/rescrv/status/406106256890286080
Answer: One place it is used in a production environment is the Bitcoin project. Within bitcoin, it's usage is critical for the security of the platform. See the release notes for Bitcoin QT 0.8.0
Comment: And it has caused no end of problems in bitcoin. https://github.com/bitcoin/bitcoin/issues/2770 and the problem is so bad and the code is so poorly written that a bounty was needed to find a fix https://bitcointalk.org/index.php?topic=337294.0;all And the leveldb discussion group is predominantly bug reports about very fundamental database functionality that fails to work as advertised. https://groups.google.com/forum/#!forum/leveldb

On the date that this question was asked, LevelDB was certainly NOT production ready and anyone who thought so was delusional.
Answer: How do you qualify "relatively new" as it was out in 2011?
Can you please give more detail on "not handling errors too well"?
LevelDB is used as a backend in Riak and Hyperdex, which have both customised it to improve throughput under huge loads. There was a great video from Ricon East 2013 explaining the Riak changes made by Basho. (taken down at some point prior to 2019-03).
Note that RocksDB is another major fork, by Facebook, which is recommended for serverside. History of it forking from LevelDB is on WikiPedia. You can read about how RocksDB handles errors on this page:

Currently in RocksDB, any error during a write operation (write to
WAL, Memtable Flush, background compaction etc) causes the database
instance to go into read-only mode by default and further user writes
are not accepted....
Call DB::Resume() to manually resume the DB and put it in read-write
mode. This function will clear the error, purge any obsolete files,
and restart background flush and compaction operations. At present, it
only supports resuming from background errors that happen during
compaction. In the future, we will add more cases.