Reduce the Population

95% gets a lot easier if you can chop off the long tail. Now that you can move things faster than the speed of light, kinetic kill weapons can devestate the world. While world leaders debate how to achieve the goal, a rogue faction secedes to the moon, cracks the earth in half, and neatly qualifies for entry into the galactic fold. Then, they emigrate somewhere nicer than this galactic backwater.

First off, >95% coverage means that, with a worldwide population of 7 billion, you can ignore 350 million people. Which I feel like gives you some wiggle room.

Second, when you say 'near future,' what do you mean?

If you mean anything longer than the next few years, you may not need to do anything other than use cell phones. As of 2017, about 70% of the world's population owned one, and that percentage is rapidly increasing. In many areas of the world, mobile phones are the way you access the internet.

Additionally, about a quarter of the world's population is under 14 years old. The vast majority of these children, I believe it's safe to say, will be within hearing distance of an adult.

So, 70% of the world has cell phones. 25% are close enough to an adult to hear one in an emergency. Adding those together gives you 95% coverage.

Now, that's cutting it a little close as of 2017, but with the increasing ubiquity of mobile devices, I don't think it's unreasonable to assume in your 'near future' scenario that more than 95% could hear a cell phone if it went off.

But what if you wanted to be even more sure? Well, as we're all aware, humans are not evenly distributed across the planet. China and India each have more than a billion people, and many of those are among the poorest in the world... and they also live in large cities. By hooking up your raspberry pi to some airhorns in those large cities, you can pick up whole percentage points of the world population.

Want to be even more sure? Require phone manufacturers to donate a few tens of millions of devices, for the good of the planet. Ship those devices to every village in Africa and rural community in India, and voila! Even better coverage.

So, even assuming you're not happy with the current growth of cell phone usage worldwide, which within the next few years will easily take you over 95% coverage, manufacturing and shipping those cheap mobile devices for rural areas would take, what, a year? At most? We're already making hundreds of millions of phones now, so it wouldn't take long to redirect them where they're needed if necessary.

Is the scope of the system to only be on Earth? If that is the case, then satellites should suffice, as we have had 100% coverage for decades now. The old Iridium constellation, famous for its satellite flares, was able to completely cover the Earth, as shown below:

Simply use a satellite alert system for phones, or use those pole thingies you mentioned but hook them up to a satellite.

Detonating nukes in very high altitudes causes a lot of noise and brings along some nice pyrotechnical effects:

In general, nuclear effects in space (or very high altitudes) have a qualitatively different display. While an atmospheric nuclear explosion has a characteristic mushroom-shaped cloud, high-altitude and space explosions tend to manifest a spherical 'cloud,' reminiscent of other space-based explosions until distorted by Earth's magnetic field, and the charged particles resulting from the blast can cross hemispheres to create an auroral display which has led documentary maker Peter Kuran to characterize these detonations as 'the rainbow bombs'.

If we have the tech for FTL, we have the tech to blow some nuclear fireworks up high. That should gather a lot of attention. We could do it around the globe.

Low Frequency Radio

Remember those radio-controlled clocks? They get the current time from a radio signal. The entire United States is covered by one broadcast tower in Fort Collins, Colorado. It operates on 60 kHz band. This tower broadcasts at 1 bit per second, but you could presumably increase the baud rate a bit...

What you would do is give every citizen a little device that would listen for this broadcast. This antenna could also be placed in cell phones, clocks (duh), and other devices. So for the technophobes that don't want phones or whatever you could use that arduino setup you mentioned, deploying them in strategic areas, instead of all over the world.

As for the broadcast towers, to build in redundancy, I'd go with two per continent.

As for the message, unless you want to ignore the 1 bit per second part, it could just say "get to a tv/radio/Internet access point!"

Induced Gravitational Harmonics

You didn't mention if you have artificial gravity to go with your FTL or not, but if you do, and depending on how it works, inducing fluctuations in the planet's natural gravity at a frequency audible to humans could make every loose object on the entire planet vibrate out your message all at once.

As a bonus, more targeted manipulation could be used to crush flat anyone who doesn't want to join the galactic empire.

Solar Thermal Transducer

If you don't have sufficient artificial gravity power to pull this off, set up a solar laser array. Pumping a few gigawats of thermal energy into the atmosphere at the proper frequency and dispersement pattern should let you create arbitrarily loud sounds at arbitrary points on the planet's surface. You'd just have to keep your message short to avoid, shall we say, unpleasant side-effects...

As a bonus, any dissidents who don't want to join the galactic empire can simply be vaporized in the middle of the night. No muss, no fuss, no witnesses!

Radio

There is a school near where I live that is less than a mile from a 50,000 watt radio transmitter station. As it turns out, you'd be amazed what kinds of things can be used as AM receivers if the signal is strong enough... Lockers, desks, telephone lines, people's fillings... Anything conductive and capable of vibrating is potentially susceptible to being driven by a sufficiently powerful signal. The energy budget to blanket the globe this way is probably cost-prohibitive given that you'd have to convert it to radio waves, and the damage it might do to sensitive electronics would also be expensive, so the other options are likely better.

What about projecting the message on the moon? Everybody can see it, you only need one powerful enough projector rather than a global installation, you can cover the costs through ads.

The problem however would be language and the fact that the message would not be instantanious for everybody.

Or instead of projecting on the moon, set up a (or multiple) sufficiently large screen(s) in higher orbit and project on those.

Regarding the viability of purchasing a phone for every person in the world:

https://www.nbim.no/no/ - 8 468 829 792 352 nok (about a trillion dollars)

That is the savings of Norway, a small european country of 5 million people. That is more than enough to buy a smartphone for the last 30% of the world who are still disconnected from the internet.

I am sure we could figure something out.

This would be trivial with today's technology because,

Fortunately in our real world there's a near-duopoly on device OS.

Couldn't be easier. In five minutes the 2 major device OS, and the few minor device OS, could be forced to add more "emergency alert" functions than they do now.

Step 2, governments would simply mandate that older versions of the OS don't work, which is easy to achieve.

None of that is any harder then, say, that you "must have brakes on a vehicle" and so on - any regulation.

A tiny number of people don't have a device currently; it would only cost a few billion to make a minimal one for those folks.

Note that a few nutters would want to avoid being contacted: your 95% rule easily covers that case.

Easy! Thanks to the current duopoly on device operating systems.

There seems to be two assumptions that people are making that I want to unpick:

1. That a warning signal must be in a spoken language.




2. That "urgency" means getting the message to people in minutes rather than days.

Put a set of nuclear-powered satellites in low-earth orbit, such that their orbits cover the globe. Fit each with an incredibly a bright light source - perhaps an array of thousands and thousands of LEDs?

Take the time to contact the hard-to-reach population areas the slow way - by mail or in person - and tell them where to look in the sky each night for the signal. Give them training on what action they should take if they see the emergency light.

Interesting coincidence considering last Tuesday the Donald woke me up with a phone alert of "THIS IS A TEST". If OP is DARPA fishing for new ideas, you know now where to reach me. :)

we discover FTL

In that case we can have a grand old time playing with the nature of causality.

If you need to warn somebody located in even the most off-the-grid place, very effectively without complicated user training other than innate susceptibility to intimidation, and with 100% reliability due to some quirks of our universe exhibiting predestination effects even in the face of plot rewriting, you can just use one of these technological wonders: