Unexplored modes of language

drumming/tapping, received by ears or touch possibly faster than spoken language, because precise sounds can be very fast. I don’t know. This doesn’t really sound good.

That sounds like Morse Code. Telegraph operators had developed a set of codes and abbreviations and emoticon-like conventions during the heyday of the telegraph... give it enough time and internationalization and it might have developed its own grammar. There was a case of a POW who blinked in Morse code during a propaganda video he was forced to make: 

There is a documentary about conlanging (I believe available through Amazon Prime Video) that interviews a couple who made a hand-holding language for themselves. It's unclear how extensive it is, but they use it to communicate covertly in situations where they don't have the chance to duck away for a moment to confer. 

The conlang toki pona has been converted into emojis, which is easy since it only has 122 words (or so). 

"drumming/tapping, received by ears or touch" sounds a lot like morse code, though I presume you mean whole words rather than letter-by-letter (which, while fast, is still slower than spoken English).

We can subdivide spoken languages into languages into a 2D scatterplot with consonants and vowels on one axis and languages expressed with meter and pitch on the other axis. In practice, all spoken languages use a combination of both but it is possible to communicate with one or the other. For example, English can be communicated with just the consonants and vowels (such as via the Roman alphabet or a bad computer voice synthesizer) whereas Pirahã can be communicated with only the meter and pitch (as in humming and whistling). Whistling can be heard over long distances. Humming helps you keep from being overheard.

symbols drawn with hands on receiver’s skin, received by touch

Chinese people do this for real except the symbol is drawn in the air or on your own hand with a finger.

Chinese is a single written language unifying many different spoken languages. It is not unheard of for Chinese speakers of separate dialects-without-an-army to draw a character on their own hand or in the air (never on the interlocutor's skin) to disambiguate different words.

symbols gestured with whole body like interpretive dance, but with objective interpretation. Good from a distance, when loud, etc.…

Instead of using your treadmill desk while you type with your hands, you just type with your arms, legs and body in a virtual reality whole-body keyboard space.

This is called "semaphore". It was used to communicate long distances before the invention of radio.

There are a number of whistled languages, most notably Silbo. They're usually used for communicating across large empty spaces (think gaps between mountains).

• Re: drumming, seems great for simple long-distance messages, e.g. between villages. More Western analogy is ringing church bells – the whole village immediately knows if there's an enemy attack, or a fire, or a.... wedding. Very important.
• Semaphores (not the CS type)
• "The beacons are lit, Gondor calls for aid!"
• Blinking-based communication – extremely inefficient, but good if you have locked-in syndrome. Also not private, but potentially you could communicate privately across a room by blinking morse code.

Personally I'd be most interested in my friend group adopting SEE, so that you can still communicate at a party that's too loud (or even bring down the volume, since fewer people would be talking). Also if my boyfriend is across the room and I want to complain to him that I'm hungry or want to leave, I don't have to go over there, or like, text him (especially since he never has his phone on him). All of the other options seem too limited in their use. The only drawback (besides the startup cost, obv) is it's not a good way of communicating privately, if everyone around you has learned it and can see you doing it. But I guess if I need to say something privately to my boyfriend, in many contexts it would suffice to speak in horribly mangled German.

ETA: cats pressing buttons

The cockpits of airliners and the control rooms for nuclear power plants are designed such that making a change to the system is also an act of communication when there is more than one pilot or operator present (which is the usual state of affairs). In particular, one of the reasons a cockpit has hundreds of physical switches and levers instead of, e.g., one big touchscreen in front of each pilot is to make it easier for a pilot to observe accurately the changes made by the other pilot.

Every pilot's needing to memorize what every switch and lever does raises training costs, but the airlines have been willing to pay that additional cost because the experience of the industry strongly suggests that cockpit designs with hundreds of physical switches and levers have lower rates of fatal misunderstandings between the pilots.

A music-based conlang: https://en.wikipedia.org/wiki/Solresol

The problem with most sign languages is that they are hard to learn. It would be interesting to have a sign language that's like Toki Pona in that it can be learned in a weekend. 

It could likely also be a bit more efficient then Toki Pona by designed for being easy to learn. For the textual level words could sound like existing words the way Lojban choose the spelling of words where 60% of the sylables match sylables of related words in the 6 major languages of the world (numbers from memory). 

There might be other things that could be done to make the language more efficient while staying with it's small vocabulary.

In many cases such as having a side conversation while other people are speaking about opening the window, it might be completly fine to have a language with little expressibility for those exchanges. 

To loosen mental constraints around language a little bit: How you push-data-out doesn't need an obvious or direct map to how you pull-data-in. This is already true in the usual cases: Ears don't speak and mouths don't hear (but they're either writing to or reading from air pressure directly). Pens don't read and eyes don't write (but they're working with the same low-level-language of "how stuff looks").

I'd like to show that there can be a more obvious difference between how we generate symbols and how we receive symbols. Examples:

• A common game mechanic allows players to quickly select from a radial menu (button-down, mouse-movement, button-up) and other-players in game will just see the emote/chat/effect.
• The tribes ascent VGS system, navigation to select output, https://tribes.fandom.com/wiki/Voice_Game_System

'ambition' is written 'ambition' in French and the spelling is close enough that you would guess it and vice versa.

How about smells? I'm not sure how many different smells the human nose can recognize (there is some debate apparently, but that seems good enough for basic communication), and how long it takes for one to replace the other, but this could be a nice "secret" mode of communication (if everyone use it, the world will smell like a shitty perfume shop).

Have you read The Persistence of Vision by John Varley? Short story.

I also like something I call schematic poetry where words in a sentence are replaced with a vertical array of possible words that could go there. There was a version of this as a classic AI problem where the goal was to discover single word permutations that changed the subject referent of the sentence. It was thought for a time that machines might not be able to solve them since they seemed to interact with the frame problem.

Humans have a significant communication mode where they beam light towards each other. There is also a popular conversion between sonic communication into light communication - a cell phone.

Trying to get some non-technical old people to use computers such as the internet I was astonished how much they could get by knowing just ordinary spoken language.

Conversely there is kind of emerging phenomena that game design is starting to pick up conventions that are non-obvious to the unintiated. That is stuff like hp, continue, game over, controlling a camera, passing a level etc.

For the high end I think for example Antichamber contrasts by conveying a concept first throught game mechanic and then via english prose. For a well tuned reader/player the game mechanic part migth be enough. Somebody that is not a "gamer" might think of them as totally pointless.

I don't think any of what I'm about to say properly qualifies as language, but there are lots of cases where information is communicated by the manipulation of physical objects. The only examples I can think of require either a pre-determined agreement (verbal, written, or maybe a social norm) or shared context/knowledge. With a pre-arranged encoding scheme any message could be written out using some number of objects (e.g., reading lit and unlit candles in a window as the binary representation of ASCII characters, though this quickly needs a lot of candles).

What I came up with in about ten minutes, loosely categorized by use case:

• War and Espionage
  • From Paul Revere's Ride: "One if by land, two if by sea" to signal British troop movements (possibly apocryphal now that I think about it)
  • Also from American Revolutionary war history: the Culper spy ring used petticoats hung on clotheslines as signals (I'm not sure if they were interpreted as letters or words or just preset messages like "meet at the farmhouse at sundown").
• Mail
  • Boxes/bags left on a doorstep to say "this is for you"
  • A raised flag on a mailbox to say "please take my mail"
• Business
  • Closing the door, turning off lights, etc. at a business to signal "we are in the process of closing/already closed"
  • IDK where I read this from, but IIRC during some era Italian(?) courtesans/prostitutes would replace white flowers outside their residences with red ones to indicate they were taking their monthly time off
• Interpersonal relationships
  • Accepting a gift to show you accept an apology
  • Hugging/kissing your current boyfriend's when your ex walks into the party to say "no, we are not getting back together"
  • Packing your belongings up and leaving your ring on the kitchen table to say "I want a divorce"

Poetry and song both carry meaning through meter, rhyme, and other modes that standard spoken language only uses occasionally. Historically, before the invention of writing, oral traditions made use of this as a form of redundancy - it's easier to memorize an epic poem if you can mostly guess words from those kinds of clues (this is one part of why lyrics are so memorable).