From the environmental standpoint shrinking of human population is often quoted to have desirable effects, and that's reasonable. But from the point of view of our daily lives and functioning of the human society, what negatives could we then expect? (I mean a soft decline due to lower birth rates, not some abrupt events.)

For example, with smaller population fewer music albums could be made every year than some time before, and people would maybe feel less inspired and satisfied. Less scientific research, less choices for relationships... and maybe other things? Would being more technically advanced compensate for the issues? Won't we feel ourselves in oblivion and romanticize the "numerous" past?

Deimos and I were discussing the use of "ask" topic tags this week, and we agreed it might be a good idea to get a consensus on these.

At the moment, Tilders are using four "ask" tags on topics:

• ask

• ask.survey

• ask.recommendations

• ask.help

(There may be more "ask" tags created in the future, but these four are what we're all using at the moment.)

Anything that's a question gets tagged with "ask". Some specific types of question will then get tagged with "ask.survey" or "ask.recommendations" or "ask.help", depending in the type of question being asked.

• "ask.survey" is for questions about preferences and favourites. "What's your favourite horror movie?" "What's the best place you ever visited?" "What's your favourite type of holiday?" The asker is collecting data about people's likes and dislikes (even if they're not going to publish the results in a report later!).

• "ask.recommendations" is for questions asking for recommendations. "What's a good browser to use?" "What book should I read next?" "Which brand of phone should I buy?" The asker is looking for people to recommend things to them.

However, Deimos and I wondered about "ask.help". One interpretation we came up with was that "ask.help" is for questions looking for a specific answer, where it should generally be possible for people to think "yes, this is the right answer to the question". This would include questions seeking help learning about an academic topic, such as happens in /r/AskScience and /r/AskHistorians over on Reddit. Another interpretation we came up with was that "ask.help" is for questions looking for guidance on doing something, like a "how to" type question. This would be more like the types of questions in /r/Help and like the Help menus in software and the F1 key - helping people get things done.

What do you think about the "ask" tags? In particular, what should the "ask.help" tag be used for? In general, are the existing "ask" tags okay? Do we need more "ask" tags? Do we need different "ask" tags?

I'm rather sleepy, generally very reserved when it comes to sharing my work, and not a native user of English, but I have a couple poems in English, and I though I'd share one here and see what the folks think of it. I love the challenge of writing stuff in languages other than my native tongue.

a bird with no wings
a song no one sings
a sorrow when time brings
         nil.
ex nihilo nihil fit
et words have no wit
mouth knows only to spit
         nil.
time is scarse and gods wobble
in vain hurry naive men hobble
ignoring they will only nobble
         nil.

Introduction

I want to give an introduction on several physics topics at a level understandable to laypeople (high school level physics background). Making physics accessible to laypeople is a much discussed topic at universities. It can be very hard to translate the professional terms into a language understandable by people outside the field. So I will take this opportunity to challenge myself to (hopefully) create an understandable introduction to interesting topics in modern physics. To this end, I will take liberties in explaining things, and not always go for full scientific accuracy, while hopefully still getting the core concepts across. If a more in-depth explanation is wanted, please ask in the comments and I will do my best to answer.

Previous topics

Bookmarkable meta post with links to all previous topics

Today's topic

Today's topic will be light emitting diodes, better known as LEDs. As the name suggests, we'll have to discuss light and diodes. We will find out why LEDs can only emit a single colour and why they don't get hot like other sources of light. Let's start by discussing diodes, in case you are already familiar with diodes note that I will limit the discussion to semiconductor (p-n with a direct bandgap) diodes as that's the type that's used in LEDs.

What's a diode?

A diode is an electronic component that, ideally, only lets electric current through in one direction. In other words it's a good resistor when the current flows in one direction and a really good conductor when the current flows in the other direction. Let's look a bit closer at how diodes function.

Semiconductors

Diodes are made out of two different semiconducting materials. In everyday life we tend to classify materials as either conducting (metals being the prime example) or non-conducting (wood, plastics, rubber). Conductance is the flow of electrons through a material, a conducting material has a lot of electrons that can move freely through a material while an insulator has none. Semiconducting materials fall in between these two categories. They do conduct but not a lot, so in other words they have a few electrons that can move freely.

N-type semiconductors

We are able to change a semiconductor's conductivity by adding tiny amounts of other materials, this is called doping. As an example, we can take silicon (the stuff that the device you're reading this on is made out of) which is the most well-known semiconductor. Pure silicon will form a crystal structure where each silicon atom has 4 neighbours, and each atom will share 1 electron with each neighbour. Now we add a little bit of a material that can share 5 electrons with its neighbours (how generous!). What will happen? Four of its shareable electrons are busy being shared with neighbours and won't leave the vicinity of the atom, but the fifth can't be shared and is now free to move around the material! So this means we added more freely flowing electron and that the conductivity of the semiconductor increases. An illustration of this process is provided here, Si is chemistry-talk for silicon and P is chemistry-talk for phosphorus, a material with 5 shareable electrons. This kind of doping is called n-type doping because we added more electrons, which have a negative charge, that can freely move.

P-type semiconductors

We can do the same thing by adding a material that's a bit stingy and is only willing to share 3 electrons, for example boron. Think for a moment what will happen in this case. One of the silicon atoms neighbouring a boron atom will want to share an electron, but the boron atom is already sharing all of its atoms. This attracts other electrons that are nearby, one of them will move in to allow the boron atom to share a fourth electron. However, this will create the same problem elsewhere in our material. Which will also get compensated, but this just creates the same problem once more in yet another location. So what we now have is a hole, a place where an electron should be but isn't, that is moving around the crystal. So in effect we created a freely moving positive charged hole. We call this type of doping p-type. Here's an illustration with B the boron atoms.

Creating a diode

So what would happen if we took a n-type semiconductor and a p-type semiconductor and pushed them against one another? Suddenly the extra free-flowing electrons of the n-type semiconductor have a purpose; to fill the holes in the p-type. So these electrons rush over and fill the holes nearest to the junction between the two semiconductors. However, as they do this a charge imbalance is created. Suddenly the region of p-type semiconductor that is near the junction has an abundance of electrons relative to the positive charges of the atom cores. A net negative charge is created in the p-type semiconductor. Similarly, the swift exit of the electrons from the n-type semiconductor means the charge of the cores there isn't compensated, so the region of the n-type semiconductor near the junction is now positively charged. This creates a barrier, the remaining free electrons of the n-type cannot reach the far-away holes of the p-type because they have to get through the big net negative charge of the p-type near the junction. Illustration here. We have now created a diode!

How diodes work

Think for a moment what will happen if we send current* (which is just a bunch of electrons moving) from the p-type towards the n-type. The incoming electrons will face the negative charge barrier of the p-type and be unable to continue. This means there is no current. In other words the diode has a high resistance. Now let's flip things around and send electrons through the other way. Now they will come across the positive charge barrier of the n-type semiconductor and be attracted to the barrier instead. The electrons' negative charge compensates the net positive charge of the barrier on the n-type and it will vanish. This destroys the equilibrium situation of the barrier. The p-type holes are no longer repelled by the positive barrier of the n-type (as it no longer exists) and move closer to the junction, this means the entire barrier will fade and current can move through. We now have a conductor.

OK, but I don't see what this has to do with light

Now let's find out how we can create light using this method. When current is applied to a diode what happens is that one side of the diode is at a higher energy than the other side. This is what motivates the electrons to move, they want to go from high energy to low energy. If the p-type semiconductor is at a higher energy than the n-type the electron will, upon crossing the junction between the two types, go from a high energy level to a lower one. This difference in energy must be compensated because (as @ducks mentioned in his thermodynamics post) energy cannot be destroyed. So where does the energy go? It gets turned into light!

The energy difference between the p-type and n-type is fixed, meaning a fixed amount of energy is released each time an electron crosses the junction. This means the light is of a single colour (colour is how we perceive the wavelength of light, which is determined by the energy of the light wave). Furthermore, none of the energy is lost so there is no energy being turned into heat, in other words the LED does not get warm.

Conclusion

So now we know why the LED is so power-efficient; it does not turn any energy into heat, it all goes into light. We now also know why they only emit a single colour, because the energy released when an electron crosses the junction is fixed.

Next time

I think next time I will try to tackle the concept of wave functions in quantum mechanics.

Feedback

As usual, please let me know where I missed the mark. Also let me know if things are not clear to you, I will try to explain further in the comments!

Addendum

*) Yes, current flow is defined to be opposite to the flow of the electrons, but I don't want to confuse readers with annoying definitions.

I try to tag comments whenever I come across them to help keep the site clean. The problem I'm coming across when doing so is that I'm not entirely sure when to use noise versus joke.

The truth is, I've never come across anything that's just noise-- it's always been a joke that just feels more like noise than a joke to me.

Is there an official definition for these tags? Are different actions performed based on what the tag is?

A few months ago I saw a bunch of memes about drift racing and it turned out that was from a rather old anime. As a moderate fan of street racing I decided to check it out, and really enjoyed it. The idea of using talent to casually push an old "sleeper" car (but a nice looking one) beyond others is fun and inspiring. The dynamic racing scenes were done beautifully. Iconic eurobeat soundtrack. And surprisingly nice friendships and relationships in a "daily life" yet captivating story. Watching the show was a memorable experience for me, and it was also fun to recognize songs and phrases from the memes and to learn more about racing. And what do you think?

What are you reading currently? Fiction or non-fiction, any genre, any language! Tell us what you're reading, and talk a bit about it.

Notes: Do any one of you follow any literary magazines? How do you follow fresh pieces of literature, and grab hold of them "fresh out of the oven"?

Past weeks: Week #1 · Week #2 · Week #3 · Week #4 · Week #5

21 weeks and yet another classic record discussion: At Folsom Prison by Johnny Cash!

At Folsom Prison is a live album and 27th overall album by Johnny Cash, released on Columbia Records in May 1968. After his 1955 song "Folsom Prison Blues", Cash had been interested in recording a performance at a prison. His idea was put on hold until 1967, when personnel changes at Columbia Records put Bob Johnston in charge of producing Cash's material. Cash had recently controlled his drug abuse problems, and was looking to turn his career around after several years of limited commercial success. Backed with June Carter, Carl Perkins and the Tennessee Three, Cash performed two shows at Folsom State Prison in California on January 13, 1968. The resulting album consisted of fifteen tracks from the first show and two tracks from the second.

Despite little initial investment by Columbia, the album was a hit in the United States, reaching number one on the country charts and the top 15 of the national album chart. The lead single from the album, a live version of "Folsom Prison Blues", was a top 40 hit, Cash's first since 1964's "Understand Your Man". At Folsom Prison received positive reviews and revitalized Cash's career, becoming the first in a series of live albums recorded at prisons that includes "At San Quentin" (1969), "Pa Osteraker" (1973), and "A Concert Behind Prison Walls" (1976). The album was rereleased with additional tracks in 1999, a three-disc set in 2008, and a five LP box set with bonus rehearsals in 2018 for Record Store Day. It was certified three times Platinum on March 27, 2003 by the Recording Industry Association of America for US sales exceeding three million.

Here's the place to discuss your thoughts on the record, your history with it or the artist, and basically talk about whatever you want to that goes along with At Folsom Prison! Remember that this is intended to be a slow moving thing, feel free to take your time and comment at any point in the week!

If you'd like to stream or buy the album, it can be found on most platforms here.

Don't forget to nominate and vote for next week's obscure record in response to this comment!

The programming challenges have kind of come to a grinding halt recently. I think it's time to get another challenge started!

Given a grid of symbols, representing a simple binary state of "filled" or "unfilled", determine whether or not a square is present on the grid. Squares must be 2x2 in size or larger, must be completely solid (i.e. all symbols in the NxN space are "filled"), and must not be directly adjacent to any other filled spaces.

Example, where 0 is "empty" and 1 is "filled":

000000
011100
011100
011100
000010

// Returns true.

000000
011100
011100
011110
000000

// Returns false.

000000
011100
010100
011100
000000

// Returns false.

For those who want a greater challenge, try any of the following:

1. Get a count of all squares.
2. Detect squares that are touching (but not as a rectangle).
3. Detect other specific shapes like triangles or circles (you will need to be creative).
4. If doing (1) and (3), count shapes separately based on type.
5. Detect shapes within unfilled space as well (a checkerboard pattern is a great use case).