Can negative absolute temperature be achieved?

Question: Thermodynamics does not allow the attainment of the absolute zero of temperature. Is then the term "negative temperature" a misnomer?
Comment: [Sixty Symbols on negative temperatures](https://www.youtube.com/watch?v=yTeBUpR17Rw).
Comment: Related: https://physics.stackexchange.com/q/21851/2451 and links therein.
Comment: @EmilioPisanty Thanks for the link. I found two references mentioned there to be quite useful. Here are the links https://arxiv.org/abs/1211.0545   and  https://journals.aps.org/pr/abstract/10.1103/PhysRev.103.20
Answer: Your question seems to be predicated on the idea that negative absolute temperature is somehow supposed to be "colder" than absolute zero, and you are right that that would be non-sensical.
But actually, in a very precise sense, negative absolute temperatures are hotter than all positive temperatures, see also this question. This is simply a result of the statistical definition of temperature $T$, which is
$$ \frac{1}{T} = \frac{\partial S}{\partial E},$$
where $S$ is the entropy of the system and $E$ its energy content, so $\beta := \frac{1}{T}$ is actually the more natural quantity to think about in the physical formalism. See also this excellent answer by DanielSank for how the $\frac{1}{T}$ appears naturally as a Lagrange multiplier in thermodynamics.
From the above, we see that temperature is negative for system whose entropy decreases as the energy rises. Such systems are unusual, but they are not forbidden. Absolute zero, $T\to 0$, corresponds to $\beta \to \infty$. As the system gets hotter, $\beta$ decreases. $\beta\to 0$ looks weird in terms of temperature, since it corresponds to $T\to \infty$, but since it is $\beta$ that is of primary physical importance, it is not actually forbidden for it to cross zero and become negative. In terms of temperature, a system crossing the $\beta = 0$ point would have to be described as heating up all the way to "infinite positive temperature", then flipping the sign of the temperature and starting to go towards $T= 0$ again from $-\infty$.
Answer: To expand on ACuriousMind's post, consider as a toy model a system with $2$ energy levels, say $0$ and $E>0$. These have respective probabilities $(1+e^{-\beta E})^{-1},\,e^{-\beta E}(1+e^{-\beta E})^{-1}$, so the mean energy is $E(1+e^{\beta E})^{-1}$. Clearly, this is $0$ at $\beta=+\infty$ ($T=0^+$), $E/2$ at $\beta=0$ ($T=\infty$) and $E$ at $\beta=-\infty$ ($T=0^-$), so a large negative $T$ is especially hot as measured by mean energy.
It turns out $2$-level systems cannot reach $\beta < 0$ while in thermal equilibrium. Lasers work by maintaining negative $T$ in systems with at least $3$ levels (well, most do). Thus the highest energy level is the most occupied. This is an example of population inversion.
Comment: I like your answer except your argument is based on Boltzmann distribution which is valid only when the system is in equilibrium.
Comment: @mithusengupta123 If you have a preferred definition of temperature that works in non-equilibrium dynamics, you can work out which microstates have negative temperatures. As lasers prove, negative temperatures are certainly possible at least in equilibrium.
Comment: Could it be that you did an odd number of sign  mistakes here? For example, in the probabilities, it should be $(1+e^{-\beta E})^{-1}$ and $e^{-\beta E} (1+e^{-\beta E})^{-1}$, respectively. Only then your statements make sense.


Inside drywall corners-mesh vs paper

Question: Can you use mesh tape on inside drywall corners or should I use paper? Is it a matter of preference or are there some rules that are generally followed?
Answer: Mesh has two three issues:

It requires more coats and coverage.
It is not manufactured to middle split evenly.  No folding line like paper.
When applying joint compound to mesh there is an art to not getting the corner of your knife on the mesh.   I have had to use mesh late at night for inside corner repairs.   Using mesh in a corner forces you to use your knife's corner meaning your knife will snag on it as you are putting more stress on the inside when tapering the compound.   Meaning you will have snags or the worst when doing a corner - moving the mesh.

In a lot of years drywalling and hiring some really good crews I have never seen anyone use mesh for an inside corner.   You could but why do something that is harder, takes more time, and could be flawed over time.
The best drywall guys I have seen only use tape for 95%+ of the job.   The mesh is for binding drywall to something else.   Even the pro-mesh guys (these weren't the best) have rolls of paper for inside corners.
Comment: Possible #4 (or expansion on #1): More mud is required as fill. Inside corners often have gaps of up to a half inch. Paper mostly spans that. Mesh requires more backfill due to shrinkage.
Answer: I suppose it's possible but I personally think it's crazy. Mesh is thick and the adhesive doesn't have much holding power. If the hardware store was closed and I needed to do my inside corners so I could move on to flat joints in the morning I'd consider it. Mesh requires a lot more building out on flat surfaces and I guess you can hide it in a corner pretty easily. It would just be really hard to get it to lay nicely in the corner.
Answer: Better yet use one of the options below:
Metal corner tape

Paper faced inside corner bead
Available individually at big box stores.
These make better looking stronger straighter corners.
Comment: These may have more commercial use or areas that might be damaged but both take longer to get to the finish and require more time/compound.   It is an alternative though that is better than mesh.
Answer: Can you use mesh tape on inside drywall corners or should I use paper?
Paper only - mesh is designed to be used flat, in joints that primarily expand - not to be folded in a corner, where the joint can freely move in all directions.
Is it a matter of preference or are there some rules that are generally followed?
Rules/Experience - manufacturers recommendations and common sense to anyone with a minimal amount of drywall taping experience.
More specifically, mesh tape is intended for joints mainly under expansion loads and does not function well under the compression, bending, or shear loads, that a corner typically adds to a joint. Paper tape has a crease designed into it, for ease of folding and placement within a corner joint. Read the following USG white paper, for more detailed information.
USG - Matching Tapes and Compounds

Inside drywall corners-mesh vs paper


Does the difference between a marketable limit order's price and the bid-ask price influence the priority of the order?

Question: A stock is $10 bid and $11 offered. Alice places a marketable buy limit order with a limit price of $12 per share. At exactly the same time, Bob places a marketable buy limit order with a limit price of $13 per share. Assume that the market's bid and offer prices do not change. Does Bob's order have priority over Alice's order by virtue of Bob's higher limit order price?
Comment: Also "Assume that the market's bid and offer prices do not change" I'm guessing what you mean there is "in the example assume there are plenty of asks at 11.00 so that alice and bob can both potentially buy at 11".
Comment: @Fattie No, I really mean "marketable limit order" (as opposed to a non-marketable limit order). That is a standard term.
Comment: @Fattie You can find the definition of "marketable limit order" in the NYSE and NASDAQ rulebooks, and in Regulation NMS (17 CFR § 242.600).
Comment: Flux, my bad, I had no idea that strange term was mentioned in a rulebook somewhere.  I'd say it's not a "standard" (ie, used commonly) term.  You could perhaps call it a technical term, the rulebook term, or an obscure term.  But that's just my opinion.  Ask people who trade frequently.
Answer:
"At exactly the same time"

Your question is meaningless.
In the database, there's just a sequence for orders. (1, 2, 3 etc.)† They cannot be at the "same time", time is not used and the concept is meaningless. The sequence numbers are unique and ascending.
So as a general concept there's (a) price ordering {no connection at all to time} and (b) sequence ordering {no connection at all to time}.  (And indeed (a) comes first then (b) second in the spirit of your question.)
Setting that issue aside, the answer to the spirit of your question is "No": the limit figure is not used in any way, whatsoever, to decide between two possibles, only the sequence mentioned in the 2e para.

† {Time, per se, is irrelevant; it would be commonplace and ubiquitous that many orders have the same time-stamp, whatever resolution is used on whatever platform; it means nothing: the sequential ordering is used to differentiate.}


Pasted vector from illustrator into Indesign. Change frame type

Question: Every object I've pasted from Illustrator into Indesign won't let me crop the frame without scaling the object. Meaning every object going off page I want to trim to the page frame just changes scale, I can't simply trim the frame like a standard object without the scake changing. Meaning the frame is dashed rather then a clean line. I know it sounds complex but I'm still learning and really want to change the frame type.
Comment: If you're still learning it might be a good time to understand its good practice to use your AI artwork as external links instead of copy pasting to Indesign. This is the normal work flow and it will save you time in the future as you might get to deal with more complex documents.
Answer: Copy your graphic from Illustrator.
Go to InDesign and select the rectangle you want to paste the graphic into.
Go to Edit -> Paste -> Paste Into (CTRL/CMD + Alt + V)


Ransomware requesting payment through Bitcoin

Question: I read this article recently, talking about the emergence of "ransomware". An excerpt is:

The newer CryptoLocker differs by taking the ransom to a whole new level. Upon infection it doesn't make a fanfare, to the contrary, it acts in stealth performing a nefarious task in the background. You probably won't even notice the task running, as CryptoLocker silently encrypts your data files, including your precious family photos, vital Microsoft Word documents, spreadsheets, and email.
Upon completing the encryption of your files, it sends the unique key required for decrypting and unlocking your files over the internet to a secure remote server, and then the ransom begins. Immediately the CryptoLocker window displays containing the image of a shield, instructional text, and a countdown timer.
It's important to understand that the ransomware is not bluffing. Unless you pay the ransom, you will lose access to any files that have been encrypted. Once the countdown starts you have about three days to make payment, otherwise the key is destroyed and you permanently lose the ability to decrypt your files.

It seems like a logical step to request payment through Bitcoin, given that it would be much harder to track down the person receiving the payment than most other options. Sure enough, it's been done already! Admittedly, it could be a pretty daunting task for your average Joe Bloggs to figure out how to safely pay within 3 days using Bitcoin, but obviously the time limit can be made flexible.
Has any malware successfully used this technique yet, using Bitcoin? If so, does anyone know of a receiving address used more than once, thus giving an indication of how many people are paying a ransom?
Are there any websites that have statistics on such incidents?
Answer:
Has any malware successfully used this technique yet, using Bitcoin?

I got around 10 people contacting me via localbitcoins in a day asking to buy exactly 2BTC to unlock their computers. I'd say it's pretty effective. I did get the addresses but don't seem to have it around now, when I looked around 30 people total had been paid out to two addresses.


At what age can a child learn to use scissors?

Question: When should we as parents teach our child to use scissors for cutting a piece of paper? At what age typically can a child use scissors properly, i.e. with one hand?
Answer: You can, and should, get fully plastic scissors that are capable of cutting paper but are completely safe (as far as any toy is), at around 2.  My almost-three year old has been using them for six months or so and not only loves using them but has become very adept at it (he can cut with one hand properly most of the time).  This was one of the milestones for the daycare we used to go to during the 2-3 year old range.
One example of these is Crayola's My First Safety Scissors which are officially recommended for 3 years + ("choking hazard"), but as with most things is fine at 2 in my experience as long as it is used under supervision (and I can't really imagine what could fall off of these to choke on).
Comment: The choking hazard is the entire thing. Choking hazard is assessed as any part of a toy that can reach a certain distance through a specifically sized hole. So in real life there is little risk of choking with those scissors (especially under supervision) it meets the technical definition.
Answer: There are scissors which will cut paper, but have some protective element for reducing the likelihood of harming a person.  Training children in their use begins around 4-5yo with supervision. As their skills improve, other more serious scissors may be used.
I personally keep myself away from those super-slick ones that will cut through anything... I make mistakes too easily.
Start with safer and progress to more-care-needed as your evaluation of the child's skills warrants a change.  There is no substitute for judgement based upon personal review.
Answer: You can get blades woth sprung handles and short blades. Here's one example.  http://www.amazon.co.uk/gp/aw/d/B00366ICX2/ref=pd_aw_sims_2/276-6568127-1578546?pi=SL500_SS115
Young children will not know what handedness they are and it does make a difference.  Young children are likely to be frustrated by this.
Also, many adults also use scissors incorrectly (some scissors need you to keep your index finger out of the loop) and pass bad cutting habits on, which again frustrates children.
The correct grip requires the hand to be as if the child was about to shake hands, and not palm-down which they usually have.  It also requires stretched index finger and thumb.  These are tricky for children to achieve.
Also, you don't mention supervision.  When supervised cutting paper is fun and useful.  It helps develop fine motor skills.
So, teach correct grip; encourage practice even when child is failing (because effort should be rewarded); manage expectations so child is not disheartened with poor results; and supervise young children.
Here's a nice link: http://www.schoolsparks.com/blog/teaching-children-the-proper-scissors-grip
So I would say when the child can hold scissors correctly, and manipulate the scissors and the paper. This is likely to be around the age of about three, but will require close supervision and special scissors. Younger children don't have the finger strength nor dexterity.
Answer: My 2.5 year old has been cutting slits since before two and just figured out how to cut a continuous line, but he's had access to scissors for a long time now. My older child starting learning around two and mastered it at three. He used the spring loaded type for maybe a month, while my 2.5 went straight to normal metal kid scissors. It seems most parents wait longer. Most of the kids in my son's preschool had never used scissors before, and even now that he's in kindergarten (And it's an advanced kindergarten) there are a few children who can barely cut. IMO, scissor skills are one of the easiest to teach because kids love scissors and pretty much teach themselves when provided with the right materials. I focused more on teaching my kids to carry scissors safely so they didn't trip and stab themselves.
Answer: My not quite two year old with a small amount of tutelage was cutting like a pro within 10 minutes of me placing her fingers in the proper position and telling her how to open and close. This is not uncommon.  They can master this WAY before age four!!


How do I compare a DISTANCE in SOQL with a number field?

Question: I'm working on a travel price list function where I need to check if a customer inquiry matches a price list entry.
The price list entry has a destination position (geolocation) and a radius (number), the radius specifies how many kilometers from the destination position the price list entry is valid for.
The inquiry has a destination position (geolocation), and my task is to find the price list entry that this inquiry position fits into by checking if the destination position goes into any price list area (position with radius).
My SOQL goes like this:
select Id,Name
from Fixed_price__c
where distance(Destination_position__c, geolocation(59.29751613,18.08904094), 'km') < Destination_radius__c

The geolocation lat and long values are the inquiry coordinates, they are inserted to the query by apex code.
This SOQL gets a parsing error when I use Destination_radius__c instead of a fixed value, it seems that I cannot compare a distance with the field Destination_radius__c? I have Googled around and cannot find anything about this.
The other solution would be to select all Fixed_price__c rows and do a comparison in apex of course, but a single query like the one above would be nicer.
Is this a limitation in the SOQL, or is there any workaround?
Comment: Yes this is an inherent limitation of SOQL.
Answer: Create a number formula Dist_minus_radius__c with contents
distance(Destination_position__c, geolocation(59.29751613,18.08904094), 'km') - Destination_radius__c

In the SOQL query, do:
where Dist_minus_radius__c < 0

Comment: Thanks for your help, but the GPS coordinates will vary and will be inserted dynamically with apex so I can't put them into a formula.


Carboy not thoroughly rinsed after cleaning with b-brite

Question: I cleaned my wine carboy with b-brite but may not have thoroughly rinsed it. Will my wine still ferment and will it be safe to drink?
Answer: The answer to the first half of your question is, "Yes." It will still ferment if there was some residual b-brite left in.
The answer to the second half is, "Probably..." The safety data sheet for b-brite states:
Ingestion:
Short Term Exposure: Significant oral exposure is considered to be unlikely. However, this product may be irritating to mucous membranes but is unlikely to cause anything more than transient discomfort. Long Term Exposure:No data for health effects associated with long term ingestion.
Carcinogen Status:
SWA: No significant ingredient is classified as carcinogenic by SWA.
NTP: No significant ingredient is classified as carcinogenic by NTP.
IARC: No significant ingredient is classified as carcinogenic by IARC.
However, I wouldn't stake my name as a home-brewer and wine maker on failure to rinse my carboy thoroughly and potentially making people sick. It could also produce off flavoring.
Answer: It depends on how much of this stuff got into your wine. Pretty impossible to guess, but it is a slow working, pretty weak stuff, so your chances are pretty good. Only way to know for sure is to wait.
It shouldn't affect safety of the drink if it will ferment, but if there is much of it, it may affect the taste.
Comment: I know the bottom of the carboy was rinsed. I normally put about a gallon of fresh water in the carboy to rinse it and flush it out through the spigot at the bottom. However, I usually rinse the sides of the carboy with the spray wand at the sink. This is the step I left out. There may have been  some of the b-brite left on the inner sides.
Comment: @ColenThomas if sides was not rinsed *at all*,  do not drink it. To the drain with it. There is difference between *not thoroughly* and *not*. You can get away with lousy job, but not with skipping it altogether.


Are there alternative languages to python for controlling a PiGlow?

Question: I've seen lots of references to python for controlling the display on a PiGlow.  Are there any alternative languages available?  I could learn python, but if there are others I might use them instead.  My preferences would be bash shell script and javascript.
Comment: It would be hard to find a language that can not interface RPi's i2c via some library. I mostly use Lazarus/FreePascal and it has no problems at all with I2C (I think I can choose between 3 libraries for I2C access).
Answer: The PiGlow store page lists languages that have libraries for the PiGlow.
But since the driver IC is interfaced using i2c, you could use any language that allows you to use i2c, eLinux has a good page on this.
Comment: There's a node.js library on that list, so the OP should be able to use javascript.


What will happen if we change the field type in the fields (settings -> fields) in Craft 3?

Question: I am a beginner in Craft CMS. I just want to know, what will happen if we change the field type in the fields (settings -> fields) in Craft 3?
Answer: It depends on the field type. Many of them you can change without problems, while others would make no sense. If you are, for instance, changing a text field to an image field, Craft obviously wouldn't know what to do with it. But if you change an Email field to a Plain Text field, it would keep the value.
The dropdown will have a warning icon when it's unsafe to change the field type. Sometimes it may work, even though the results are unpredictable. For instance, if you change a Plain Text field to a Multi-Select field, the text will be the value of the field (I think), but there is no guarantee that it is a valid option for the Multi-Select.
Also, the other way, changing a Number field to a Plain Text field causes no problems. If you change a Plain Text to a Number field, you have no guarantee that the text actually represents a number. But I think it will keep the value, but that you will get a validation error on trying to save it the next time, if the value is not valid.
Comment: I actually changed redactor plugin to Ckeditor plugin. I faced some issues to already existing redactor fields. How I include span, style etc. redactor?


Honda radio not working

Question: I have a 1995 Honda accord and I was driving one day when my radio just went blank. Then it says code on it. So I entered the code and right as I enter the last number the radio makes a beep then the radio goes blank and I can't do anything. No sound nothing on the screen and none of the buttons do anything. I have tried a few times to pull the fuse and put it back in and the same thing keeps happening. Any thoughts?
Comment: How long did you leave the fuse out? If for only a few seconds, try leaving it out for a few minutes. I take it you already have the security code, so that is good. If this doesn't work, you may need to pull it from the harness (out of the dash) for a few minutes, then try it again. I don't know how the security end of things go for Honda radios, so am leaving this as a comment.
Answer: Take two butter knives and shove them in gap at the top of your radio about an inch or two from the sides. It should push the clips in and it will slide right out. Sometimes the cables come out in the back.

Honda radio not working


Is throttle-only possible for mid-drive?

Question: Suppose I want an electric bike with throttle. Is mid-drive (crank based) even an option? I got impression that mid-drive means pedal assist only (PAS).
Comment: Not my area of expertise, but the short answer is there are mid drive systems that do have throttles, try googling "mid drive clutch" etc, but many of the more mainstream ones (ie Bosch) don't. There's a lot of statutory/legal barriers in various markets involved.
Comment: I don't know about the Russian (from your bio) legislation but in the EU electric assist means that the bike won't move unless you pedal. The motor stops once you stop pedalling. So there's no provision for a motorcycle-type throttle.
Answer: While I'm not aware of any examples of a throttle operated mid-drive, it is certainly possible. With a regular bicycle crankset, a mid-drive motor will push the crankset around, transferring the power into the chain through the chainrings to the chain. This involves causing the crankarms to spin while in use, making it Pedal-Assist only for any practical use.
What you would need is a crankset like the one in several Specialized ebike models. These have a freewheel-like release in them. This means that when you pedal backwards, the chainrings do not spin backwards, but when pedalled forwards, the crankset mechanism engages and turn the chainrings forward. With a crankset like this, you could have a mid-drive ebike which could be operated by Pedal-Assist or by throttle.


When does an under-specified linear system have a unique solution?

Question: Consider the system of two equations with three real variables $x,y,z$:
$$
a_{11}\cdot  x  + a_{12} \cdot y + a_{13} \cdot z = b_1
\\
a_{21} \cdot (1-x)  + a_{22} \cdot (1-y) + a_{23} \cdot (1-z) = b_2
$$
where $x,y,z\geq 0$.
Since there are less equations than variables, this system may have infinitely many solutions. My question is: is there a simple condition on $b_1,b_2$ (as a function of the coefficients $a_{ij}$), that guarantees that this system has a unique solution?
Answer: For example if you have that
$a_{11},a_{12},a_{13}>0$ and $b_1=0$ your only solution for the first equation is $x=y=z=0$ that is a solution for the second equation if and only if
$b_2=a_{21}+a_{22}+a_{23}$
So in the case in which
\begin{cases}
b_1=0\\
b_2=a_{21}+a_{22}+a_{23}
\end{cases}
Your only solution is $x=y=z=0$
The geometric idea is no difficult. Your system of equations represents the locus of the intersection of two planes related by your equations but in general in $\mathbb{R}^3$ the intersection of two planes is the empty set or a plane (if the two planes are equal) or a line. There is not the case in which the intersection is one single point. But In your case you must consider also the condition $x,y,z\geq 0$ so you can consider $b_1,b_2$ such that the two different  planes intersect them in a line and that line passes on the origin and its intersection with the locus $x,y,z\geq 0$ is only the origin.
In this case your only solution will be the origin.
Answer: In general the answer is no: you are looking for a solution $x\geq0$ to the problem $Ax=b$. Suppose there is no vector $y$ (of right size) such that $A^Ty>0$ (1). Then by the dual version of Gordan's lemma there exists some $z>0$ such that $Az=0$. Therefore in this case, either there is no solution $x\geq0$, or $x\geq0$ produces $Ax=b$, and then $A(x+z)=b$ with $x+z\geq0$, so you have at least two solutions.
Since your $A$ is completely arbitrary, you have cases satisfying (1).


Could Planet X have a perihelion much closer than 300 AU and still be responsible for the gravitational interaction with 19 TNOs?

Question: In August, Michael Brown and Konstantin Batigyn proposed new data concerning the hypothetical Planet X to explain the orbits of the 19 TNOs. They suggest a perihelion of 300 AU, but might Planet X be even much closer? Is there something like a minimum perihelion it must have to account for the TNOs' highly elliptical orbits?
Comment: This is a proper science based question.  I voted up to encourage you to post more questions like this rather than your previous one.
Answer: Well yes, because Brown & Batygin (2021) say the perihelion would be $300^{+85}_{-60}$ au, so there is roughly a 50% chance that the perihelion is smaller than 300 au according to their work.
The quoted error bars represent their estimate of the 16th to 84th percentiles of a probability distribution, so conceivably the perihelion could even be a little lower. One or two of their simulations suggest a perihelion as low as 200 au, but they are less favoured than those with a larger perihelion distance.
This picture from Batygin et al. (2019) is quite helpful. It shows the parameter space for any new planet that is ruled out by other current observations (e.g. infrared surveys by WISE, the limits on perturbations of giant planets ["ruled out by ephemeris"]). The hatched region shows what might still be possible and does admit the possibility of a small planet between 100 and 200 au, but I would think thathas been superseded by Brown & Batygin's recent work.
Comment: @LoveForChrist, a gravity capture requires giving up excess orbital energy to some other planet.  In order for a rogue planet to be captured, it would need to approach the Sun at least as close as Neptune's orbit.
Comment: Thank you. It looks like they assume that beyond 1,000 AU there could't be any more planets but despite one's belief the Sun might capture a rogue one.
Comment: @Mark, in theory, I believe there are ways in which a capture *could* happen even with a perihelion far beyond Neptune. But all the mechanisms I can think of for that require either an unlikely initial state (e.g. a *very* loosely bound binary pair of rogue planets passing by the Sun) or an unlikely coincidence (two separate rogue planets passing close to each other in the Sun's vicinity), so I do agree with your comment in general.
Comment: @IlmariKaronen: I'm used to tidal capture but I don't think that can happen at 300AU.


How often did the average Israelite bring offerings to the temple?

Question: In addition to the mandated communal offerings, individual Jews were liable for, or permitted to bring, a variety of offerings of their own.  How often did the average Israelite bring each type of offering in a year?  Was this pretty routine, or unusual?  (Sin-offerings and wellbeing-offerings seem like they could be pretty variable.)
Answer: The minimum of an average person had to go to the Bais Hamikdash for the 3 Regolim and that meant a Korban Olas Re'iya and a Chagiga. In addition, on Erev Pesach they had to bring a Korban Pesach (which was brought in groups as opposed to the individual).
As far as how often a person would have to go to the Mikdash, even if a person is obligated to bring a Korbon s/he did not have to bring it straight away s/he had to bring it by the next of the 3 Regolim, if s/he did not then s/he would trangress a Mitzvas Asei. If s/he did not bring it by the end of 3 Regolim s/he would trangress a Mitzvas Lo Sa'saei as well. See Rosh Hashana 4a and rambam Hilchos Ma'asei Hakorbonos 14:13.
Comment: It should be clarified that the "average person" refered to in the answer above actually means adult males only and does not include the "average person" who may have been female. Although it is correct to say that three times a year was the minimum that an adult male had to appear at the Beit ha-Mikdash to bring korbanot associated with the three festivals, this still doesn't give us an idea of how many times the average person (including women) might have had to bring a korban which was not associated with the three festivals, such as a sin offering, etc.
Comment: Thanks, and welcome to J.SE.  As @Shemmy said, I'm hoping to get some perspective on all the non-chag korbanot -- sin-offerings seem to be owed for a great many things that don't *seem* that uncommon, but were people really running to the beit hamikdash every week or two?
Comment: I see that Meir Zirkind has edited his original answer somewhat, and it now includes a bit more information about the possible timing of korbanot other than the Shalosh Regalim. Providing text sources for this additional info about timing would be wonderful and would make your answer much more useful.
Comment: Meir, thanks for adding the information about timing.  The other part of my question is about number per type; do we have any way to know how many offerings of each type (sin, etc) were typically brought when he did go to Yerushalayim?
Answer: I don't think we have enough sources to be able to estimate how often the average person brought each type of sacrifice, save for one type - kinei yoldot (see further down). We can, however, estimate that the average person maybe went a few times in his or her lifetime to the Temple. This can be pretty safely said about the Second Temple, and is probably also true for most of the years that the Tabernacle in Shiloh was active. As for the other periods - I'm not sure.
Note: Most of this stuff was explained to me by one of my history professors. The kinei yoldot part is something I discovered via a shiur my grandfather z"l, formerly an economics professor and founder of the Economics and Halachah program at BIU, gave a few times on Shavuot.
The Second Temple:
Maccabees II, allegedly a summary of five books on the Maccabean Revolt written by one Jason of Cyrene, opens up with two letters written to the Jewish community in Egypt. Both letters were written a number of decades after the Revolt.1 In the letters, the Jews of Yerushalayim invite the Egyptian Jews to celebrate Chanukah with them. My guess is that if the vast majority of Jews came even just once or twice a year, there would have been no need to send a formal message inviting the Jews of Egypt to join in on the celebrations - decades after said events - because they would have been well aware of the festival already.
Philo mentions that he went up once to the Temple (On Providence, Fragment I):

"There is a city of Syria, on the sea shore, Ascalon by name: when I was there, at the time when I was on my journey towards the temple of my native land for the purpose of offering up prayers and sacrifices therein..."

Scholars generally agree that this means that Philo only came once (and some believe twice) to the Temple (being that he says "my journey" rather than "a journey" or "one of my journeys") - and Philo lived in Alexandria, Egypt, and was very wealthy and an important figure in the Alexandrian community - yet he was only able to come once or twice in his lifetime. What should we gather about people who lived farther away and/or were poorer?
There are also some Chazalic sources:
Mishna Nazir 5:4:

"...The incident in question was as follows: When nazirites were ascending from the exile to sacrifice their offerings, and they found the Temple destroyed, Naḥum the Mede said to them: If you had known that the Temple would be destroyed, would you have taken a vow of naziriteship? They said to him: Certainly not, as there is no remedy for a naziriteship in this case. And Naḥum the Mede dissolved the vow for them. And when the matter came before the Rabbis, they said: His ruling is incorrect. Rather, whoever took a vow of naziriteship before the Temple was destroyed, like these nazirites from the exile, he is a nazirite, as he committed no error at the time of his vow, and one cannot dissolve vows based a new situation. However, one who stated his vow after the Temple was destroyed is not a nazirite, as he vowed based on an erroneous assumption."

The mishna records an event in which a group of nazirites came to give their due sacrifices after the destruction of the Temple. Now, remember that the Great Revolt had been going on for 3 or 4 years before the destruction (depending on when one dates the destruction) and continued on in other parts of Judea for another 3-5 years (again, depending on different views), making entry into Eretz Yisrael in general and Yerushalayim in particular extremely difficult - yet these nazirites were apparently completely clueless about all of this!
In more aggadic fashion, the gemara in Yerushalmi Brachot 17 says:

"ודא מסייעא להו דמר ר' יודן בריה דר' אייבו עובדא הוה בחד יהודאי דהוה
קאים רדי געת תורתיה קומוי עבר חד ערביי ושמע קלה א"ל בר יודאי בר יודאי שרי תורך ושרי קנקנך דהא חריב בית מוקדשא געת זמן תניינות א"ל בר יודאי בר יודאי קטור תוריך וקטור קנקניך דהא יליד מלכא משיחא א"ל מה שמיה מנחם א"ל ומה שמיה דאבוי א"ל חזקיה א"ל מן הן הוא א"ל מן בירת מלכא דבית לחם יהודה..."

Translation: "...And something that will assist in proving this interpretation, as what Rabbi Yudan, son of Rabbi Aibo recounted: Once there was a Jew who was plowing his field when his two oxen began to bleat. An Arab passed by and heard the sound. He said to him: "Jew son of Jew, untie your oxen and untie your pitcher for your Temple has been destroyed." And the oxen proceeded to bleat again. He said to him: "Jew son of Jew, tie your oxen and tie your pitcher for the King Mashiach has been born." Said [the Jew] to him: "What is his name?" "Menachem." Said he to him: "And what is his father's name?" Said he to him: "Chizkiyah." Said he to him: "Where is he from?" Said he to him: "From the royal city of Beit Lechem of Yehudah."..."
Though aggadic, we might see here an echo of the situation in the diaspora, in which the average Arab (then a more nomadic people than they are nowadays) was more knowledgeable than the average Jew on the happenings in Eretz Yisrael.
Finally, l'havdil, the NT describes Jesus and his disciples coming to Yerushalayim:
Mark 13:1:

"As Jesus was leaving the temple, one of his disciples said to him, "Look, Teacher! What massive stones! What magnificent buildings!""

We see from here that these men from the nearby Galilee, in their twenties and thirties, had never been to Yerushalayim before and were astounded by its size and grandeur.
In short, it seems that the average person hardly ever came to the Temple in his lifetime. Certainly, there are other sources that attest to the numbers that the city drew during the Regalim - but presumably, there was constant change in who came (i.e., same numbers, different people) - because it was very difficult to come every time.
One exception - kinei yoldot (nests of women after childbirth):
Mishna Keritot 1:7:

"With regard to a woman who has in her case uncertainty concerning five births, and likewise a woman with regard to whom there is uncertainty concerning five irregular discharges of blood from the uterus [ziva], she brings one offering, and then she may partake of the meat of offerings. And the remaining offerings are not an obligation for her. If she experienced five definite discharges of a zava or five definite births, she brings one offering, and then she may partake of the meat of offerings. And the remaining offerings are an obligation for her..."

According to the mishna - and as explained by my grandfather - it was customary for women during the Second Temple Era to bring childbirth or zava sacrifices only after several births or discharges took place - evidently waiting years before coming up to the Temple. It's possible that the mishna is referring to the average: On average, women came after five births or five discharges.
The Tabernacle in Shiloh:
Famously, in the story of the concubine of Givah, the survivors of the Tribe of Binyamin had to receive directions to know how to reach the Tabernacle in Shiloh (Shoftim 21:19-20):

"Then they said, Behold, there is a yearly feast of the Lord in Shilo which is on the north side of Bet-el, on the east side of the highway that goes up from Bet-el to Shekhem, and on the south of Levona. Therefore they commanded the children of Binyamin, saying, Go and lie in wait in the vineyards..."

Evidently people hardly came up to the Tabernacle, and this story, according to most views, took place pretty early in the Era of the Judges. A midrash further  expounds on this, but also explains how the situation was remedied during the last years of the Shiloh Era (Tannah Devei Eliyahu Rabbah 8):

"אלקנה היה עולה לשילה ארבעה פעמים בשנה שלשה מן התורה ואחת שקיבל עליו הוא בנדבה...עלה אלקנה ואשתו ובניו ובני ביתו ואחיו ואחיותיו וכל קרוביו היו עולין עמו...וכשעולים עמו בדרך היו לנין ברחובה של עיר והיו מתקבצין האנשים לבד והנשים לבד שכן האיש היה מדבר עם האיש והאשה עם האשה וגדול עם הקטן והיתה המדינה מרגשת והיו שואלים להן להיכן תלכו ואומרים להם לבית האלקים שבשילה שמשם תצא תורה ומעשים טובים ואתם למה לא תבואו עמנו ונלך ביחד מיד עיניהם משגרות דמעות ואומרים להם נעלה עמכם וכן אמר להם עוד הפעם עד שעלו עמו לשנה הבאה חמשה בתים ולשנה האחרת עלו עמו עשרה בתים ולשנה האחרת הרגישו כולם לעלות והיו עולין עמו כמו ששים בתים ובדרך שהיה עולה שנה זו אינו עולה לשנה האחרת עד שהיו כל ישראל עולין והיה אלקנה מכריע את כל ישראל לכף זכות וחינך אותם במצות וזכו רבים על ידו..."

Translation: "Elkanah would go up to Shiloh four times a year: Three times from the Torah and once more that he took upon himself charitably...Elkanah went up with his wife and sons and his household and borthers and sisters and all of his relative went up with him...and when they went up with him on the road, they would sleep in the middle of the city and people would gather - the men by themselves and the women by themselves, for man would speak with man and woman with woman and an elder with a youth and the country would take notice and they would ask them: "To where are you going?" And they replied: "We are going to the House of the LORD in Shiloh for from there the Torah and good deeds shall come out. And you, why shouldn't you come with us and we shall go together?" Immediately, their eyes would fill with tears and they would say to them: "We shall come up with you!" And so he said to them once again, until they came up with him. By the next year, five houses came [with him] and the next year - ten houses, and the next year, everyone felt the need to come up, and some sixty houses would come with him, and in the road that he went up one year he did not go the next, until all of Yisrael would go up and Elkanah brought Yisrael to be judged favorably and he educated them in mitzvot and everyone merited by him..."
We see that for most of the time that the Tabernacle was in Shiloh, hardly anyone went up there, until Elkanah fixed this.
Interestingly, it is Elkanah's wife Chanah who, like the women after childbirth in the Second Temple Era, puts off the time of her coming to the Tabernacle after giving birth to Shmuel for a few years (Shmuel 1:1:21-24):

"And when the man Elkanah and all his household were going up to offer to the LORD the annual sacrifice and his votive sacrifice, Hannah did not go up. She said to her husband, "When the child is weaned, I will bring him. For when he has appeared before the LORD, he must remain there for good." Her husband Elkanah said to her, "Do as you think best. Stay home until you have weaned him. May the LORD fulfill His word." So the woman stayed home and nursed her son until she weaned him. When she had weaned him, she took him up with her, along with three bulls, one ephah of flour, and a jar of wine. And though the boy was still very young, she brought him to the House of the LORD at Shiloh."

So we find that even in ancient times, it was not considered problematic for a woman to put off bringing the childbirth sacrifice.

1 See Daniel Schwartz's commentary on the letters in his edition of the book for an explanation on the dating system used by the authors of the letters.
Comment: Most of the second temple sources are about people outside Israel, leaving only one anecdote about Galileans — is it fair to extrapolate to the general case?
Comment: @magicker72 and the kinei yoldot which doesn't say where the women came from. "Is it fair" is a fair question. Most historians, to my knowledge, use these sources to reach the conclusion I wrote. Is there room to disagree? Certainly.
Comment: @magicker72 I suppose another issue is how one defines "average Jew" - would you include diaspora Jews in that count or not?


Can I fly from state to state in Mexico with a US passport? Or do I need a Mexican passport to fly?

Question: Ill be catching a flight from Tijuana to Mexico City. Can I travel with my US passport or will I need to get a Mexican passport?
Comment: Are you a citizen of Mexico?
Comment: (+1) I think it's a reasonable question, some countries do require citizens to use a local ID for all official purposes so even if you don't cross a border, it could be forbidden to use a US passport if you are a Mexican citizen.
Answer: You can't get a Mexican passport unless you are Mexican citizen.
Generally speaking, for travel you need a government issued identification document with your photo - a passport any passport qualifies for this.
Depending on the flight, you may also provide other forms of identification in lieu of the passport (for example, a driver's license is accepted on most domestic flights in the US - even from non-citizens).
In fact, depending on the circumstances - the document doesn't even need to be valid/current (for example, if you are trying to prove your right to enter by citizenship, an expired passport will suffice).
Comment: That's a new one to me - professional certificate - interesting.
Answer: depends, usually they do not ask you for one if it has your picture if you are travel withing México, because they have no way to prove you are mexican, so as long it has your picture, you can travel with it without problem


Change combined inbox account colors

Question: I have a Samsung Galaxy S3 with Android 4.1 Jelly Bean.
I have 3 email accounts set up in the email client and I would like to change the colors assigned to each of the accounts.
Is it possible? I haven't been able to find any setting to do it. In this question they say it's not possible in Android 2.2, but how about 4.1?
Answer: I've tried on my Galaxy S3 and I couldn't find a way to do this.
Searching on the web returns nothing more (aside of trying to delete/recreate an account until it's given the color you want, which I didn't try).
You can use another mail client than the one pre-installed on the GS3.
I've tried the following ones:

MailDroid: great one, possible to choose the color for each account from a palette, free version is ad-based
AquaMail: good one too, you can choose the color for each account from 8 predefined colors, free version has no ads
K-9 Mail: possible to choose the color for each account from a palette, no ads, but looks a bit old
Answer: Enhanced email from the market will allow to choose account and notification colors for each account. However it lacks the subject in the notification, which is why I stick with stock.
Comment: Your suggestion to use another email client is a good one. It would be more useful if you also include a link to the app you're suggesting.

Change combined inbox account colors


Yeast in 1.080 beer

Question: I pitched 1 pack of Safale 05 in my 1.080 beer.  That is all the recipe called for but I am afraid it may be an under-pitch.  Is it advisable or possible to pitch more, its been fermenting 2 days at 64 degrees.
Comment: You didn't say whether you rehydrated the yeast or not. Best practice is to rehydrate dry yeast in about 10 ml of water per gram of yeast. That would mean about 115 ml (about half a cup) of water for an 11.5 g sachet of US-05. http://www.brewwithfermentis.com/tips-tricks/yeast-rehydration/
Comment: As Evil Zymurgist said, batch size is pretty important here. Are you making a 5 gallon batch?
Answer: That yeast is 6x10^9 cells per gram at packaging. 69 billion cells in the whole pack. Your beer needs about 279 billion cells (5 gallon batch)
While 1 pack is an under pitch you should be ok.  Some styles do better with a slight under pitch if you want those growth esters.
It's always a good idea to do a starter it will at least proof the yeast for you and you can control the cell count better.
Edit: I always get flack for recommending a starter in most cases. I hold true to this my only exception to not doing a starter is if I'm using fresh trub that I personally harvested. I've seen too many beers go bad from underpitching because viability was assumed. You can't assume viability! Simply because of shipping and vendor mishandling is reason enough to do a starter. Yeast has a thermal death point starting at 130°F. Maybe because I live in the South West USA I'm more cautious, here on a warm day inside of a car can reach 140° in minutes killing pets & children all the time, let alone the back of a delivery van.  So do whatever yeast practice you like, I just know better than to assume viability. If a baker will always proof yeast for $1.00 worth of dough, why wouldn't you for $50 worth of ingredient?
Comment: With dry ale yeast, without magnetic stir or something like that, you may end up with lower cell count with starter. I disagree that starter is *always* good thing.
Comment: @Mołot I can't think of any scenario where a proper starter would give a lower cell count. Granted a starter isn't always needed, but it is a good habit to have. You have no idea how the yeast has been handled from packaging to you. Its much better for a starter to identify a weak or dead pack than your batch of beer.
Comment: I think you're off by a factor of 10, 6x10^9 is 6 billion, giving you ~70 billion cells per 11.5 gram pack.
Comment: "where a **proper** starter"...
Comment: @QuestionMarks yeah sorry tired. 6.9e10 doesn't have 10 zeros put me off x10
Comment: @EvilZymurgist You always have quality answers, and I was almost hitting my head against a wall trying to figure out how those numbers went against everything I understood about yeast cell counts.+1 with the edit :)
Answer: Yeast pitching rates aren't always too easy to figure out. No one seems to agree on how many cells you should expect in a gram of yeast (various sources state anything from 6-20 billion per gram), and different yeasts have different size cells and clumping tendencies, so the correct number of cells can vary between different strains of yeast.
For a rough estimate on how much yeast you should pitch, check out this yeast pitching rate calculator. The manufacturer of Safale states there are at least 6 billion viable cells per gram at packaging. The guys over at BrewersFriends.com however have found that a single pack of healthy dry yeast was enough to fully ferment a 1.050 gravity beer (the calculator would tell you to add at least 3 packs if it really was 6 billion/g).
For a 5 gallon batch of beer at 1.080 (which is a pretty big beer), you'll need at least 2 dry yeast packets, properly hydrated. If you aerated the wort sufficiently before pitching the first packet, then you should be able to add the second pack without any extra aeration and save your beer. If not, you'll want to make a starter for the yeast, heavily aerate the starter, and let the yeast finish their initial growth phase before pitching into your beer.
Comment: @EvilZymurgist How do you get 690 billion? At 6 billion cells per gram, you'd need a 115 grams for that many yeast cells.
Comment: you're right I was off by a factor of x10
Answer: I agree with all of the above. Assuming that you are making a 5gal batch, one packet of US-05 is seriously under-pitching and probably won't make a good beer. However, the question asked whether it's advisable to add more yeast now that the fermentation is underway. The two consequences of under-pitching that you're likely to encounter are ...
1) Increased production of unwanted bi-products of fermentation; particularly fusel alcohols and acetalehyde.
2) Fermentation is more likely to stall.
With this in mind, pitching more yeast can restart a fermentation, and reduce acetalehyde, but it wont do anything for the fusel alcohols. Once primary fermentation is complete (should be about now) take a gravity reading and taste the beer. If you get strong solventy flavours then decide whether you want to cut your losses an dump the batch. If you get a lot of acetaldehyde or the gravity is higher than it should be then it's worth adding more yeast. If it tastes OK and the gravity is where you expect it then you've probably got away with it.
When adding yeast to an active fermentation you should make up a starter of the size that should have pitched initially. Don't wait for the starter to complete fermentation, pitch it when it's at high krausen. That gives the yeast the best chance of thriving in what will already be a high alcohol , high C02 environment.
Answer: Contrary to all the theoretical advice, I have some practical, experience based advice....you'll be OK.  I've done it many, many times with fine results.  1.080 is about the limit if where I'd feel safe (although I have gone a few points higher).  So, in the future you'd probably be safer to pitch more, for now you should be OK.

Yeast in 1.080 beer


When is it normal for a child to start walking?

Question: My child is twelve months going on thirteen, but cannot walk. However, he started mimicking sounds as early as 8 months and can now speak fairly understandable words. Could there be problems with his ability to walk?
Comment: Slightly relevant, [there is no connection between when a baby starts talking and when a baby starts walking](http://parenting.stackexchange.com/q/7137/4054).
Comment: We stay in the rural Kenya. We visit the public health facilities and what they would normally do is administer vaccines. We also visit them when the children show signs of illness in which case they do a lot of laboratory tests. Not much about about mental development come up.
Comment: Have you talked to your pediatrician about your concerns?  Since your son is just over a year old, he should've had an appointment recently - at that appointment, what did your pediatrician say?
Comment: @Joe, We visit the local medical centre for normal checkup but nothing to the effect that he could be having problems has ever come up. I have always thought that the facility's capabilities are too limited to detect such cases. My greatest worry is that the children who were born at the same time have already started walking. Interestingly non of them can talk as clearly as my son. In fact he talks much better than those who are three months older.
Comment: I don't know about your region, and what kind of doctor you see, but at least in the US, the pediatrician's *main* purpose in doing these visits is to check for developmental issues (or at least that's what it seems like).  They check normal physical stuff, but they ask a lot of questions of us in terms of mental development.
Answer: From what I've read and from personal experience, I would say that you have nothing to worry about just because your son cannot walk at 13 months.
According to these websites, it's normal for babies to start walking any time between 9 and 18 months:
http://www.whattoexpect.com/first-year/first-steps/
http://www.babycenter.com/0_baby-milestone-walking_6507.bc
My son started taking a few steps in his 13th month but wasn't walking confidently, particularly outside, until 14 to 15 months but he happened to still be ahead of most babies of similar age that we know. It depends who you have around you - probably in another group of friends he could have seemed behind.
One of my son's little friends wasn't walking until nearly 2 years old but as soon as she started she could walk very well within the space of a week whereas my son did it much more gradually. Another of his friends only started walking at 16 months and by 2 years was running and climbing all over the place.
At this age I would be looking more for progress towards walking. Is he pulling up to standing using furniture? Can he walk a bit with help - starting with two hands and then just one? He may start to stand for short periods without holding anything. If you are concerned that he's not doing any of these things or making progress then it might be a good idea to mention to your doctor but even then I probably wouldn't worry too much for another month or two.
This is one of those things where it seems very important while you're waiting for it to happen but once your child is walking, no-one else will even know or care when he first started. Nor does it seem that it affects how the child develops in the future. Even a few months later a 'late' walker can be doing just as much or more than an early walker. Children develop at their own pace. Starting to walk depends on a lot of factors, from physical strength to mental development, the child's personal interests and temperament:
http://www.askdrsears.com/topics/parenting/child-rearing-and-development/walking/baby-walking
Answer: My daughter didn't start crawling until her 1st birthday.  She was walking when she was 18 months, and now at 2 she runs and climbs all over.  She was also an early imitator/talker and says sentences now.  I think she didn't crawl until she was 1 because she was an only child and we gave her anything.  She needed motivation to make the effort.
Answer: Of my three children, each started walking at a different age. The youngest did not walk on his own until nearly 14 months -- he was very cautious about his balance, and apparently did not want to risk falling over. The middle one started walking around 12 months, and the oldest was walking at 13 months.
Children meet developmental milestones at different times: children may be walking by 12 months, but not all will be. Some children walk as late as 15 or 16 months. (Ref. Denver II Developmental Milestones chart -- look towards the bottom for gross motor skills, which include walking.)
If he has not met other age-appropriate milestones (crawling, sitting without support, standing, "cruising"), take your son to a pediatrician and be specific about why you are worried. A doctor has more information and expertise, and can take additional factors into account.
Answer: From my experience children are usually either "walkers" or "talkers". The ones that walk early have lower language skills and vice versa. There are also ones more in the middle of the spectrum, but they mostly seem to lean one way.
My daughter showed no interest in walking or even crawling or getting up on hands and knees until about 14 months. At that point she suddenly decided it was important and started crawling, and then walking within a few weeks.
Don't worry about it. Focus on language if that's what interests them right now. Eventually, they will decide that movement is important and will work on it. There's no cause for worry at this point.


Windows 7 Backup Utility Fails

Question: My Windows 7 built in backup utility has failed. The system tells me the specified service does not exist as an installed service(0x80070424) Win 7 Ultimate x 64 RTM. Any ideas?
Answer: The default in Windows 7 for the Windows Backup service is manual, apparently because it's only started on-demand when going through the official interface.
From the Start menu, start "Backup and Restore", then click on "Set up backup". This will start the service and then bring up a dialog where (I hope) you can do the backup. I couldn't follow this further since my Windows 7 is in a virtual machine.
If you're using another interface for the backup and want the Windows Backup service to always be available, then start Services.msc from the Start menu, double-click on the "Windows Backup" service and set its Startup type to "Automatic (Delayed start)".
EDIT
Another possibility is that a registry cleanup utility has destroyed a needed registry entry.
Either rollback to a previous system restore point or get from another computer with the same level of operating system the contents of the following registry key
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\wbengine.
This thread contains the registry update file for Vista, which may work also for Windows 7.

Windows 7 Backup Utility Fails


Does Springfield still exist in the Futurama future?

Question: Does Springfield still exist in the Futurama future? I don't expect to see the characters travel to the future, but if Fry's descendant could survive, so could the Simpsons.
Comment: In he Simpsons episode 350 Future-Drama, Bender makes a short cameo appearance.
Comment: There's arguments for both sides of this question, on the on hand the Simpsons timeline effects the future for futurama in the cross over episode, but in the future episodes of the Simpsons they go though teleporters all the time, but in futurama they haven't been invented, one thousand years in the future.
Answer: In the episode "A Big Piece of Garbage" Fry finds a pile of Bart Simpson dolls and he comments about them as if he is aware that "The Simpsons" is a TV show.  Also in the episode "Mars University" you can see Simpson dolls hanging in one of the booths when Fry has his flashback to the carnival.  These seem to indicate that The Simpsons is a show in the Futurama universe, and not a real place.
In the crossover books w/ the Simpsons the Futurama cast gets projected into Simpsons merchandise and the creator states in the foreword that The Simpsons is a fictional TV show in the Futurama universe.
Oddly enough, Futurama merchandise shows up in The Simpsons indicating it's a show in that universe.  When Bender showed up in The Simpsons they went through a dimensional gateway, indicating that it's not the same universe.
Comment: Ok, but here's the important question: in the Futurama universe, is The Simpsons a cartoon or a live-action show?
Comment: @Izkata, That wasn't the question.
Comment: @KazDragon As stated in the answer, `When Bender showed up in The Simpsons they went through a dimensional gateway, indicating that it's not the same universe.`
Comment: @user606723 Oops, I think I read "live-action" as "live" ("show" is on its own line).  Sorry!
Comment: @KazDragon:   This is a good question!   I'm going to ask it.
Comment: In [Lrrreconcilable Ndndifferences](http://theinfosphere.org/Lrrreconcilable_Ndndifferences), they go to Comic-Con.  Matt Groening and David X Cohen get to play themselves on a Simpsons panel.
Answer: I agree with Andres that they do not exist in the same universe. In A Big Piece of Garbage there are Bart Simpson dolls as an indicator of 20th century culture(specifically 20th century not 21st century). This means that The Simpsons are a TV show in the Futurama universe just as they are in ours.
Answer: Actually, in the audio commentary for I believe Mars University (it has been a long time since I've watched early eps) Matt Groening explains alongside David Cohen that the Simpsons is technically "fake" while Futurama is "real" as in multiple episodes, you see Simpsons characters as stuffed dolls.
Originally, the character designs for all humans for Futurama were also going to have 5 fingers, but in the end, it was harder to animate, and looked strange.
So no, in the world of Futurama (read: our world) the Simpsons, and the town of Springfield are fictitious.
Answer: Note that Springfield is referenced in The Simpsons as being in all the states that there is actually a Springfield (as a running gag). So, if The Simpsons is just a show in Futurama, then it is likely that there are just as many Springfields as there are in our time. But there hasn't been any reference to any of them in Futurama so far.
Answer: I don't think the Springfield from the Simpsons TV show exists in Futurama's future at all.
Why? Because I don't think the Simpsons and Futurama exist in the same universe at all. In The Simpsons' world, people are yellow. In Futurama, they aren't. Of course, that doesn't preclude some tongue-in-cheek TV show reference ;)
Answer: Another note that supports that The Simpsons and Futurama are different universes is that the characters have different physiologies. This is clear because Futurama characters have natural skin tones, while Simpsons characters do not.


Mass effect 3 gibbed save editor and DLC Arrival

Question: I'm using gibbed editor to set DLC Arrival as succesfully completed. But it doesn't work. I set 7451 as true and still Anderson blames Shepard in blowing up the mass relay in Batarian's system. Why? Is there any more plot ids need to be set? And what plot ids add Conrad Verner into game? Thank you.
Comment: Welcome to Arqade! You seem to have two fairly different questions in there. You may want to ask a separate question about Conrad.
Comment: Sorry, but I'm more interesting in how to solve the dlc arrival import problem then. I've looked everywhere, I didn't find the solution.
Comment: You can ask both questions, I'm just saying you can ask the other one in a new post and keep this one and have an answer for both, but in two different places.
Answer: That seems to be working as intended.
The Arrival DLC ends up with Shepard blowing up the Alpha Relay and sacrificing a Batarian colony in the process. At the beginning of Mass Effect 3, Anderson uses that as explanation for Shepard's incarceration.
If Arrival isn't completed, then the Alliance's 103rd Marine Division is said to have carried out that mission and destroyed the Alpha Relay, but didn't make it out alive. In that instance, Anderson will blame Shepard for working with Cerberus to explain the incarceration.
Comment: thank you. but I'm still confused by GAW asset - because it doesn't depend on dlc complete flag, there is 'Alliance's 103rd Marine Division is said to have carried out that mission and destroyed the Alpha Relay, but didn't make it out alive' ANYWAY. Is this normal or it's a bug?


Do cameras actively filter out UV light, or only infrared?

Question: I know that a camera has a filter in front of the sensor to limit incoming light to the visible spectrum, to replicate what a human eye can see. But wherever I look on the internet, I always read about the filter in front of the sensor being an infrared filter. Wouldn't the filter also have to block out UV light? I couldn't find any useful information on this on the internet :/ Also, wouldn't an active filtering of UV light in front of the sensor render UV lens-filters useless?
Comment: Possible duplicate of [is uv filter a must?](https://photo.stackexchange.com/questions/90191/is-uv-filter-a-must)
Comment: Related: [Is a UV Filter required/recommended for lens protection?](https://photo.stackexchange.com/q/17854/15871) and [Are there any downsides to using a good-quality UV filter?](https://photo.stackexchange.com/q/57/15871) and [What effect does a UV filter provide?](https://photo.stackexchange.com/q/5441/15871) and [Is a UV filter better for lens protection than a protector filter?](https://photo.stackexchange.com/q/1393/15871)
Answer: UV is annoyingly present when doing landscapes and aerial photography. It records as a haze that blocks the clear view of distant mountains and it veils the land when imaged from high altitudes. A UV blocking filter can be very helpful under these circumstances. The UV filter and a cousin called a "Skylight" filter gained popularity. The "skylight" is tinted pink, so this UV filter also warmed up cool feeling blue-sky type vistas. Special note: The UV filter only benefits when the subject is distant and shrouded by water vapor.  Camera store salesmen, eager to pad a sale, generally advised, a UV filter will protect your precious, costly lens.  The popularity of the UV thus soared.
With the onset of the digital camera, the need to mount a UV filter diminished because electronic photography raises different issues. The imaging sensor requires trimming with filters or it will fail to deliver a faithful image.  The surface of the digital sensor is covered with an array of tiny photosites. These capture the image, but the chances that artifacts with spoil it are high. Most noteworthy is image noise. This is akin to grain in film photography. There are a plethora of these annoying artifacts.
Enter the digital camera's protective cover glass. The surface of the digital image sensor is fragile, it is covered by a flat glass overlay. This cover glass lends itself to have a dual purpose.  Some subject types will image with bizarre results. These are called "demosaicing artifacts, often seen as a moiré. To avoid, the cover glass is also a optical low-pass filter better known as a anti-aliasing filter. This filter slightly blurs fine detail that is finer than the native resolution of the senor. Additionally the cover glass will act as an infrared filter that blocks these frequencies otherwise they will record as false colors
The UV continues to be sold and mounted to protect our precious lenses.
Comment: Correction: The UV continues to be *hyped* to protect lenses.
Answer: UV lens filters in the digital era have a different purpose than actively filtering UV light. They are to protect the front element from dust, fingerprints, etc. If the glass in front of the lens gets dirty, you are much safer cleaning a $50 UV filter than a $500 lens front element.
Digital sensors are typically insensitive to UV, so you don't need the UV filter to filter it out. Source: https://www.dpreview.com/articles/7333331953/should-you-use-a-uv-filter-on-your-lens which says:

However, digital sensors are generally rather insensitive to UV, so the problem doesn't arise to anything like the same extent.
Comment: Disagree on the cleaning. Your front element can take some abuse. Does your filter have as durable a lens coating? Probably not...
Comment: "They are to protect the front element from dust, fingerprints, etc." Either that, or they are there to give users a false sense of security when in some cases they can actually make things worse. [To filter or not to filter, that is the question.](https://photo.stackexchange.com/a/105054/15871)
Comment: "Digital sensors are generally rather insensitive to UV..." mainly because there "generally" is a UV filter in the stack in front of the sensor. A bare sensor without the filter stack is more sensitive to both UV and IR at either end of the visible spectrum than a typical consumer camera that "generally" has a filter stack in front of the sensor.
Answer: In terms of filtration there is not only the IR filter, but also the RGB CFA (color filter array) that lies directly over the sensor. These are absorptive filters and only pass the wavelengths they are designed to. I.e. adequate UV filtration is provided by the RGB Bayer array (or similar).
I have read that silicon photodiodes (pixels) are more sensitive/reactive to IR than they are to UV. This is also probably due, at least in part, to the higher refractive index of shorter wavelengths.
Perhaps that is why the CFA alone is not sufficient for near IR but it is for UV.
In any case, the individual spectral response of a camera/sensor varies somewhat... some are more or less affacted by UV/IR even with their filtration.
Answer: Traditional metal-oxide "diodes" register UV in very low portions.
Here is a chart for "high UV sensetive" cmos: https://www.cameraiq.ru/data/image/QE%20MicroVista%20UV.JPG
Deep UV is ~270-330
Near UV is 330-400
As you can see, even special cmos can actually reach only small part of bellow-380 UV, while simpler ones actually reach only 400+, wich is visible light
So yes and no, cmos can see UV, but they are much more sensitive in visible part of spectrum.
In everyday life, while not being somewhere your oncologist wan't be happy to be, UV filter will not make any visible difference in your image, simple blend will make much more effect under bright sun. There are stories about shooting in the mountains, where people find some usage from this filters, but ok, even sample photos from filter-makers, are good both with and without filter :-)


Is black a color or a lack of color?

Question: Is black a color or a lack of color? What are the characteristics that defines a color?
Comment: This really is not a photography question.
Comment: null... totally unagree with you... whatever you are refering to... n_n
Comment: How would defining *black* as *is a color* or *is not a color* change the way you understand photography?
Comment: @TSR This seems more a physics question than a photography one.  What's the (photography) context for your question?
Comment: Physics has it including an answer and closed it as off-topic as well: http://physics.stackexchange.com/questions/92535/is-black-a-color-or-absence-of-color (CC: @MikeW )
Comment: I guess we could see if English is interested in having it there... but I've heard it come up enough when talking about photography that I don't mind it here.
Comment: I don't mind it here if it has a specific photography context, i.e. what specifically prompted the OP to ask the question?
Comment: Related : http://photo.stackexchange.com/questions/14820/what-do-hue-chroma-saturation-value-tones-tints-shade-etc-mean
Comment: Color is a property that black doesn't have.
Comment: For what it is worth, quibbling over this certainly came up in the answers to [Photoshop's color replacement tool changes to grey (instead of white) — how can I change a grey background to pure white?](http://photo.stackexchange.com/questions/83513/photoshops-color-replacement-tool-changes-to-grey-instead-of-white-how-can)
Comment: @RyanFromGDSE - shame it closed in Physics as the most upvoted answer is pretty good.
Answer:
Is black a color or a lack of color?

That all depends on how you define color, and whether you are considering additive (e.g. you can't combine different colors of light to create light that is "black") or subtractive (e.g. you can add all of the various color pigments together to get "black") methods to reproduce color information.

What are the characteristics that defines a color?

Again, the answer to such a question totally depend upon the context in which it is being asked. Asking such a question in a physics classroom dealing with the nature of light as energy will elicit an entirely different answer than asking the same question at a paint factory. Even though each answer will be nearly a polar opposite of the other both will be correct within the context in which they are produced.
The context here at Photography.stackexchange is creative photography - not the physics of light other than as it applies to creating perceptual equivalents of the way our eye/brain systems see physical light and objects that reflect and/or absorb light.
Within the color models used by creative photography there are both additive and subtractive color models.
Inkjet and mass produced prints are subtractive - they add color to a white or near white piece of paper or other material to reduce the various wavelengths of light reflected by the paper. Each substance used to create color is a substance that absorbs some of the visible spectrum and reflects the rest, which is what we see when we look at something that is that color. If we include a substance made up of various pigments that each absorb various parts of the color spectrum, we can wind up with a substance that reflects no light in the wavelengths that we humans can perceive. Thus adding all of the different colors together makes whatever we have applied our pigments to appear black to our eyes.
CRT screens and older LCD screen that use individual pixels that emit light are additive - lack of a signal means the screen emits no light. Black is what we call the areas of the screen that aren't emitting any light.
LCD screens using a backlight are a kind of an odd mix of additive and subtractive color going on at the same time. Most newer LCD screens use a white backlight that is blocked by a color matrix that changes density depending on the amount of energy flowing through each "dot" on the screen.
As James Snell said in a comment to another answer to this question:

Black is not a lack of colour, it's a lack of luminosity/reflectance. Check out any of the perceptual colour models which split hue from brightness like HSV/HSL, Lab etc. You can specify a colour, not being able to see it does not mean it's not there any more. Put a coloured object in a completely dark room and it does not stop being coloured, you just can't see it.

When considering photographing a red apple in a dark room lit only with the very short burst of light of a flash we don't wonder what color the apple will show up as in the photo because we can't see anything except a black silhouette in the dark. We know what color the apple was before we turned the lights out. We know what color the apple will be during the near-instantaneous flash of bright light for our strobe. And we know what color the apple will be in the photo assuming our white balance is matched to the color of the flash's output. We can even know, with a fair degree of certainty, what color the apple will appear to be in our photo if we shift the white balance of the photo to other values that don't match our light source. In such a context, to say that the apple has "no color" when it sits in a room so dark that our eyes can only see a black shadow is ridiculous. The apple appears black in such a context because there is not enough luminosity to allow us to perceive the apple as red, not because it no longer has enough color for us to perceive it as red.
In the context of photography there is no current method to "reproduce" pure black. As Alan Marcus quotes a former Kodak instructor in a comment to his answer to this question:

My notes from a class lecture by Professor Ralph M. Evans @ Kodak (from years & years ago): "Black is the name given to colors that are very dark and therefore exhibit little or no hue."
Comment: Whole lotta downvotin' going on in this question! I didn't downvote, but I am not clear on exactly how additive vs. subtractive matters (vs. the chroma=color / it's all color distinction I talk about in my answer). In any case, I'm +1 for the "both are correct in context" part. :)
Comment: I do not think that substractive or additive has anything to do. :)
Comment: @mattdm I would think that chroma !=color. Chroma + luma = color.
Comment: @MirekE Sure. :)
Comment: @mattdm In subtractive models something (i.e. color or a combination of colors) must be placed in front of the white beginning point in order to reproduce "black". In additive models "black" is the result of nothing being added to the dark beginning point.
Answer: Light is energy. This is defined as electromagnetic waves. These energy waves are outputted by the sun, stars as well as other sources like artificial light from light bulbs etc. We also get light from emissions produced by living organisms like insects, fish. Fungi, and bacteria, this is bioluminescence. Most of the electromagnetic energy waves are invisible. We are taking about gamma rays, X-rays, and the like. We humans have evolved eyes sensitive to a narrow span of the electromagnet spectrum. Most scientist agree that the frequencies we can see, the visual portion of the spectrum, is due to the fact that the atmosphere of earth is transparent to this narrow band which spans 400 to 700 millimicrons in wave length. We cannot see those waves that are shorter or longer than this span. We can however build devices that can detect these other frequencies and convert them to something we can hear or see or otherwise display. The various colors we see each occupy a short span of the visible range.   White light is a mix of all frequencies in the visible range. Black is the absence of light. We can mix red, green and blue in different amounts and make almost all the other colors. Color photography takes thee unique pictures simultaneously using only the three primary colors of red, green, and blue. This is how TV and digital phones and cameras capture and display colored pictures.
Comment: @ Mattdm -- My notes from a class lecture by Professor Ralph M. Evans @ Kodak (from years & years ago): Black is the name given to colors that are very dark and therefore exhibit little or no hue.
Comment: @ Michael -- Black record as an area nearly void of exposure.  On negative film, black objects record as Dmin or nearly so. On reversal film black objects are rendered as Dmax or nearly so. On color paper, black is rendered as Dmax via superimpose subtractive primaries cyan, magenta and yellow. Commonly, prints made using ink feature an enhanced black. This is accomplished by an overlaid of black ink to reinforce the opacity of overlapped three subtractive primaries. As with most all colors both black and white areas are never accurate. We have yet to make a truly faithful image.
Comment: So.... is black a color?
Comment: So how does a color photo that combines red, green, and blue include areas that are black?
Comment: @AlanMarcus : That seems to be a nice *one-line* summary for your answer :)
Answer: This is similar to the "Actually, tomatoes are fruit, not vegetables" factoid — but even more complicated.
In plain English, black is definitely a color. You will find it very clearly defined that way in almost any dictionary. When you get a big box of crayons, it's clearly labeled 120 colors — even though black, white, and gray are surely in the mix.
Merriam-Webster's simple definition is: the dark color of coal or the night sky (emphasis added). I'll get to their more advanced definition below.
When we talk about color in a technical sense, there is some complication. We have a bunch of terms like "chroma", "hue", "saturation", "lightness", "colorfulness", and so on, and these are often used without rigorous definition. Even when there is a solid definition given, that definition can be different within different models of color — even within the single field of photography.
Merriam-Webster's entry goes on beyond the simple one, offering also: the achromatic color of least lightness characteristically perceived to belong to objects that neither reflect nor transmit light. So, even this more precise definition invokes the word "color", qualifying black as an "achromatic" one.
When someone says "black is not a color", they are probably using a particular model or concept of "color" where "color" relates only to the colorfulness or chroma (which is really just the Greek word of color, but we borrow it sometimes to make a technical distinction), separate from lightness/luminosity/value.
When someone says "black is a color!", they are using a broader definition which includes more things. If the "no it isn't!" person starts to argue with this and claims that the dictionaries are actually wrong, they are, frankly, being silly. It's okay for everyone to be right — as long as everyone understands the assumptions in play.
Oxford English Dictionary says: of the darkest colour possible, that of soot, coal, the sky on a moonless night in open country, or a small hole in a hollow object (again, emphasis added) — and adds a note which fundamentally agrees with what I'm saying in this answer overall: From a scientific perspective, the quality of being black is due to the absence or absorption of all the wavelengths of light occurring in the visible spectrum. In general use, however, it is normally classed as a colour.
Since natural language developed some hundred thousand years before we started talking rigorously about color theory, it's not surprising that English at least doesn't normally make the scientific distinction. And, since the language doesn't have great existing words for the fine details of the model, it's not surprising that we resort to somewhat awkward terms like "chroma" and "colorfulness".
In the art of photography, I don't think it's generally useful to make a split, either. If you're thinking about symbolism and cultural perceptions, it absolutely should be considered along with red, blue, purple, and whatever else. If you're talking about something technical where the distinction is useful, it can be okay, but even then, make sure you explain the model you're using. I think it's more useful to use chroma or colorfulness when that distinction is important — even if they are a little awkward.
I guess the exception I would make is when we're talking about color photography as opposed to black and white photography. Pedantry might call for me to say "no, that should be chromatic photography!" — but that seems pretty unnecessary. I think this is more a historical accident and a consequence of the lack of a common English term for the concept at issue here than it is an intentional use of a restricted sense of the world. In any case, within the context, it's clear what's meant.
Many of these terms are defined in more detail at What do Hue, Chroma, Saturation, Value, Tones, Tints, Shade, etc. mean? — this, I think, answers the second part of your question about the characteristics of color.
Comment: @RyanFromGDSE For whatever it's worth, not a misquote — they list the adjective first on the page... scroll down to the noun.
Comment: Can you please link dictionary entries that defines black as a color (without using itself in the definition)?
Comment: @RyanFromGDSE Sure — added M-W and OED.
Comment: You misquoted Merriam-Webster's simple definition. It says, "having the dark color" in a pointless argument of semantics that makes a big difference ;)
Answer: Black is a color
Black is the absence of light but that is not the same as absense of color.
Color is a phisical-phisiological-perceptual-psycological-cultural interpretation of light and shadows.
Even black has some characteristics, it is the darkest unsaturated color.
You need a context
There is a chance you are not awared of black, if all you see is black, but if the color is on a context you can clearly define it as such.
Answer: This is really a philosophy and metaphysics and psychology and all sorts of question far outside the realm of photography but to simplify it as best I can:
There are four things required for color perception to occur:

Light to be present
An object to reflect that light; which includes gases
Receptors that can detect that light
A processing system (in the case of humans that's the frontal cortex)

You can call black a color or not. By definition its more not a color than it is one. A few key points:
Color does not exist on an object. An apple is not Red, to make any claims that an apple is Red is a gross simplification. An apple has the ability to absorb all white light besides the wavelengths that most people perceive as red. But again this assumes that a white light is present or at least a light containing the wavelength most commonly perceived as red. This assumes the apple is in good health. This assumes the person or thing processing it can see that wavelength in the most commonly understood fashion. That's a lot of assumptions!
People have three types of cones and one type of rod. Except when we don't. Most people are familiar with colorblindness which is when one of the types of cones are missing or mutated. What you might not know is that there are also people with more types of cones - 4 types that we're aware of - and we have no idea how they perceive the world. See: http://www.digitaljournal.com/article/326976
So what does this have to do with black. Well one theory on the tetrachromat (person that has and can make use of the 4 cones) is that it improves dimly lit colors. But we don't know.. from that same article:

How does cDa29 see the world? She was unable to communicate her experience to the researchers in much the same way as it is impossible to describe the experience of red to a dichromatic person. Jordan says: "This private perception is what everybody is curious about. I would love to see that."

So when is black... black? The only time that we are positive black is black is when it lacks all light. Any other time and its hard to say. Even among regular vision fatigue, the surrounding environment, old age can all diminish the ability to perceive Hue within a dark environment. For those discussing the LAB color space this is the so-called conundrum. A forest so dark that its difficult to impossible to tell if its an intensely dark green or actual black.
But we can say with certainty that black is lacking all light. Which leads us to
Is Black a Color?
Color is defined by the Cambridge Dictionary as:

the appearance that something has as a result of reflecting light

Color is defined by the Collins Dictionary as:

the sensation resulting from stimulation of the retina of the eye by light waves of certain lengths

Color is defined by Merriam-Webster dictionary as:

a :  a phenomenon of light (as red, brown, pink, or gray) or visual perception that enables one to differentiate otherwise identical objects

So the real question, and the only question is:
Can we call the complete lack of light a sensation on the retina?
Most would say no. But then again when we're so used to light being present, perhaps removing it all is a feeling. I've been in an unlit cave where you get as close to black as possible - it is a strange feeling. I'm not sure that qualifies though. I wouldn't call it such. It certainly doesn't help you differentiate objects.

Is Black a lack of color?
Black is absolutely a lack of color. Color is a stimulus upon the retinas, black is when there is no stimulus.

In practical application: This is pretty irrelevant. Language is for understanding each other and most don't know or care enough to distinguish such intricacies. As long as you know what you're talking about and others know what you're talking about the rest is irrelevant. When you need to get into technical things such as philosophy and physics than it becomes important. In photography --- not so much.

Update with new question from Mattdm in comments
Mattdm asks,

"It certainly doesn't help you differentiate objects." So, if there are three objects, one blue, one red, and one black, and I ask you to pick up the black one, you wouldn't be able to do it?"**

Yes and no. Like I said most of the time our English and assumptions of the world take over and most of the time are right. You're basing it on your own prior knowledge and perception  of redness, blueness, and blackness.
I'm reminded of going to a suit store and the guy swearing to me the suit was black while my years of experience in color told me it had blue in it. Was I right or was he? I am more right but for most purposes he can still call it black and its understood as such. But it absolutely had blue in it. In fact let's see an image:

All of these were sampled from a single photo of guys in suits. If I ask you to hand me the black one or black ones would you easily be able to? I doubt it. This is why a suit is more expensive than individual sports coats and slacks. A suit must be sold together made of the exact same piece and segment of fabric in order to match. A black sports coat from one shop and black slacks from another are likely to be two entirely different "blacks". None of which are pure black.
Again, this is a question of semantics and what effect a lack of stimulus has on your vision - if any. And a philosophical inquiry as to whether we can fully trust our senses. Nearly every theory on color says it doesn't exist outside of the brain (processor).

I'd encourage anyone interested in this subject to study it. Here are two wonderful videos (all hosted on YouTube at time of posting) to get you started:

Alan Alda's Flame Challenge - What is Color
Light Darkness And Colours - A Fascinating Journey Through The Universe Of Colours
Comment: *"It certainly doesn't help you differentiate objects."* So, if there are three objects, one blue, one red, and one black, and I ask you to pick up the black one, you wouldn't be able to do it?
Comment: You've given Merriam-Webster's definition 1a. I think it's pretty helpful to actually include all of 1b (1) and (2) as well.
Comment: @mattdm I updated for you and also included two videos you should watch when you have a chance. They're not short but they are wonderfully presented.
Answer: Visible light is part of the electromagnetic spectrum, our eyes can perceive only a small fraction of this spectrum, from black to white... the lack of light or poor light is perceived as black.

Our eyes can differentiate colors when there is the right amount of light, on very poor light conditions our eyes perceive only a shade of gray colors, this is because of our anatomy:

While using a small telescope to watch far away objects, you'll see this objects as "black and white" or "gray" but this is mainly because you are receiving a poor quantity of light, using a big telescope can give you more light and more colorful images, astronomy enthusiast on a small budget use long exposure photography to capture more light and have more colorful images, al this examples show that black is the perception of low light or not light at all.
Comment: In other words: "Black is a color characterized by the perception of low or no light." (Which is to say: I don't think this actually answers the question directly.)
Answer: Since in photography black is never the absolute black, I would say that for purposes of us photographers and color management enthusiasts, the black is a color.
If you think black is not a color, remove it from your vocabulary when speaking about photography and color management and see what happens...
Comment: The essential nature of black doesn't imply that black is a color. Black is really one and of the range of tints of any color. You might talk about *warm black* or *cool black* or *red black* or *blue black*, but you wouldn't talk about *bright black*. It's true that there's black ink and black pigment, but there's no light that's black (*black light* notwithstanding).
Answer: Color is the perception of visible light. Black is a complete lack of color.
Comment: Depends whether you're talking additive or subtractive.   Further, I can buy black pens, black paint, etc. so clearly it **is** a color so far as the world is concerned.
Comment: Black is not a lack of colour, it's a lack of luminosity/reflectance.  Check out any of the perceptual colour models which split hue from brightness like HSV/HSL, Lab etc.  You can specify a colour, not being able to see it does not mean it's not there any more.  Put a coloured object in a completely dark room and it does not stop being coloured, you just can't see it.
Comment: @CarlWitthoft it makes no difference if you're talking about an additive or subtractive color space. That doesn't change the nature of black.
Comment: @RyanFromGDSE you wrote: "that is incorrect way of thinking and goes against color theory and research". But the aforementioned color models, namely Lab are part of color theory. And they can define color without luminosity and imaginary colors that we do not see...
Comment: @RyanFromGDSE  Try it:  Pigments  Red + Green + Yellow --> Black.  It's a bit weird to say "the sum of some colors is no color"
Comment: @CarlWitthoft that's not accurate. That's a visual representation of a color so dark you can't tell what color it is. It's not black. You don't get black by adding colors together, you get something so dark and muddled it often appears black. This is why researchers are still trying to find ways to make a true black, the closest being Vantablack which is in fact defined as an absence of color using nanotechnology to absorb the visible light spectrum.
Comment: @RyanFromGDSE "particular amount of light reflected off an object" is reflectance or lightness, not color. There are colors that do not have particular wavelength, but still are colors (purples and violets) - see the bottom of the CIE chromaticity diagram.
Comment: RyanFromGDSE is right. What you get by adding multiple colors in additive color mixing is something "dark" but not truely black.
And even if it would add to black this would only mean that _no_ visible light is reflected from the objects surface - which is the requirement for getting true black.
So ... no matter how much we construct arond it. The answer is really complete with just the two sentences I wrote above. :)
Comment: @JamesSnell that is incorrect way of thinking and goes against color theory and research. A Color is a particular amount of light reflected off an object. If a particular wavelength doesn't exist than the color doesn't exist either. An Apple will always have the ability to reflect red light, but it doesn't possess red on its own. If you put an apple in a completely dark room it does stop being colored. It still has the ability to reflect that wavelength but its not, so its not red for that timeframe.
Comment: @CarlWitthoft "Pigments Red + Green + Yellow --> Black". But that's the subtractive model, you are actually subtracting them,  no?
Answer: Black is the only shade for which you can absolutely say that it is a "lack of color", cultural or colloquial considerations aside.
Even white is made up of colored light, so comprises colors, but black is the absence of any light.
Comment: On the other hand, a sheet of white paper is the absence of any color dyes, while black paper is the combination of dyes of many colors...
Comment: True, though you can also say a dye is effectively a filter which blocks colors other than the desired color, and a black dye, being composed of many dyes, blocks the reflections of all colors :-)
Answer: Simple answer:  Light is color.  Without light, no color.  The brighter the light the stronger the color.  Cameras photograph black when no light reaches the sensor.

Is black a color or a lack of color?


What wavelength of light is released when an electron drops an energy level in complexes?

Question: I know that the color of transition metals is related to the splitting of the d-orbitals. Only a certain wavelength is absorbed in the promotion of electron to a higher d-orbital and therefore the remaining colors are interpreted as a certain color. For example in copper(II) sulphate the red wavelength is absorbed and therefore it the complex appears to be cyan. But shouldn't the same amount of energy be released once the electron drops back to its previous state and releases the energy is gained? What wavelength would this drop correspond to?
Answer: Not necessarily. The drop may occur gradually so that the colour is "blinded out" by the nonadsorbed colours. The drop may also occur in stages, so that several low energy emissions occur. Some are outside visible range and others are too weak or go unnoticed.
Comment: But if it is due to the d-orbital splitting, shouldn't there be only one possibility of a drop? Where else could the electron go, besides the lower d-orbital?
Comment: Are you asking about a specific case of splitting or in general? There are a lot of things that affect the energy of an orbital, and there are multiple splitting combinations amongst the metals.
Comment: I am asking in terms of a complex that is formed with a ligand. I was taught that the reason why complexes appear to be colored is that their d-orbital splits into a higher and lower energy, and when light is shined through the complex, a certain wavelength is absorbed. But I wonder what happens to the excited electron. If it were to drop to a lower d-orbital it would release the same wavelength as it absorbed...
Comment: Then the energy levels available are dependent on what metals and what ligands are involved. But there's never just two of them. What would happen if an electron in the lowest level jumped to level 6, then all electrons dropped by one level each? No transition is repeated and yet we absorbed then emitted energy.