We were not alone.
After just under forty-seven days, the signal changed. What was a sequence of grouped blips turned into a weaker carrier wave bearing analogue signals. It was so weak it might've gone unnoticed had it not been preceded by the stronger prime numbers. But this was no Hollywood movie, so there was no immediate translation. The signals were stored in massive databanks around the world as we listened to whoever was trying to speak, not yet able to understand. The second thing someone pointed out was the "prelude signal" had lasted for exactly 5,799,999,999,999,991 periods of the fundamental transition of the hydrogen atom. A prime multiple of a universal quantity made it clear without a doubt that something intelligent was behind the signal. With nothing yet to work with, and the public and media craving a name, they begun to refer to it as the Serpens Codex.
The world held its breath while the scientists worked. Monte Carlo simulations of various configurations of resolutions, pixel layouts, scan rates and rastering methods combined with pattern-recognition algorithms did the brute force searches while expert crytographers tried various analysis techniques. Ten weeks later, the supercomputer cluster at the University of Ottawa found what everyone had been looking for – a white circle on a black background. What followed was an almost exact copy of what we had put on the Voyager Golden Record. A slow build-up of notation, language and numerals. Mathematical diagrams demonstrating Pythagoras's Theorem, vector geometry, calculus and matrix operations. Pictures of an alien world, annotated with scales of distance and chemical formulae. Images and videos of strange creatures eating, sleeping, drinking, dancing, laughing and crying. Music both haunting and alien. Performances and ceremonies. Greetings in hundreds of languages. Art and culture, science and technology. The world watched in awe as an entire civilisation bared itself to us.
Then the world watched in horror as the message begun to describe a nearby supernova. It corresponded with one that had been observed from Earth only seventeen years prior to the message. The aliens who had reached out to us were dead. A supernova event bombarded their planet with gamma rays, destroying the ozone layer and baking the world with solar and cosmic radiation, the ecosystem slowly collapsing. The message wasn't a hand of friendship. It was a dying gasp. The effort of a doomed race for some measure of immortality.
Their gift had not been in vain. 156 petabytes of data had flowed into the thousands of radio telescopes on Earth over two and a half years. The Codex had ramped up in stages, transmitting several signals on the same carrier wave, and only increased in complexity from there. Extracting signals from one another was a gargantuan task in and of itself. Sifting through the resulting information occupied most of the world's supercomputers for the next decade.
The cultural information had been front and centre in the real-time decoding of the Codex, but it hid a wealth of other information. Analysing and structuring the signal in different ways displayed different threads of the broadcast, and the supercomputer in Ottawa had simply hit on the first one. The hundreds of other threads revealed detailed information, operating principles, blueprints and schematics for various forms of technology. The most interesting technology was devices that seemed to claim to manipulate gravity and inertia. Over the next several decades, examples and prototypes of these devices were created and they worked. The field of gravitics was born.
It has been suggested that whoever broadcast the Serpens Codex specifically targeted Earth. Our own exoplanet observations have begun to become sophisticated enough that we can determine the atmospheric composition of an exoplanet via chemical spectra. With enough accuracy and the right conditions, chemicals that can only be produced by carbon-based life can be identified. The civilisation that transmitted the Serpens Codex may well have been capable of doing this, identifying biogenic compounds in Earth's atmosphere to confirm that they weren't broadcasting their last gasp to a sterile, airless rock. Given that the Codex was transmitted within the last few thousand years, Earth would have looked as alive then as it is today.
One mystery remains, however. After no less than five years examining the gravitic science and technology within the Codex, a human physicist developed a working principle for faster than light travel, which was demonstrated six years later. If humans could do it, why couldn't the species in the Codex? They could have simply left their planet to find a new one. Nothing about the supernova would have prevented a mass exodus. But nothing like that was depicting in the Codex. Where had they gone? Did it simply not occur to them? Had they been trapped by something else?
Using the Serpens Codex
The Serpens Codex takes the idea of a technologically advanced alien precursor species and puts a hard science fiction spin on it. It replaces xenoarchaeologists in vaccsuits at dig sites on airless moons with physicists, cryptographers and computer scientists deciphering signals at radio telescopes and universities around the world. It also allows a setting to explore the idea of the sudden introduction of advanced technology at our current tech level, letting us see what our world might be like with contragrav cars while changing little in our political situation.
Campaigns involving the Serpens Codex might have the players involved directly, as researchers decoding the signals, discovering the contents and trying to develop their understanding of it, building prototypes of the advanced technology. A campaign like this would have little to no combat but rules from GURPS Social Engineering might be very useful, from giving talks and demonstrations at conferences to dissuading the military from seizing research materials.
The Serpens Codex might be the catalyst for an espionage game. If the signal was received during the Cold War, the USA and USSR would compete to develop the technology, stealing ideas, research, records and staff from each other to out-compete in the development of spacecraft, flying tanks, gravitic weapons, and so on. Replace any great powers at odds with each other for whatever flavour you want. What if the signal arrives during the prelude to the Great War, or during the Interbellum? What if Nazi soldiers march on Moscow with inertially-damped power armour and the Normandy landings are carried out via air/raft? What if North Korea starts building gravitic tanks? Or Turkey starts supplying Daesh (the supposed 'Islamic State') with singularity bombs?
On the other hand, the Serpens Codex might simply be window dressing. A TL11^ space opera might have the Codex as the thing which allowed humans to build faster-than-light starships and colonise the galaxy, far in the past. A hard science or transhumanist setting with reactionless but relativistic spacecraft spending years cruising at 0.99c might have the Serpens Codex to thank. You could even have all star-faring species have received the Serpens Codex. Whether this is simply the Serpens civilisation getting a few lucky hits out of beaming the signal to several hundred thousand worlds, or a targeted uplifting campaign where only recipients of the Codex ever develop interstellar travel, is up to the GM.
Under the Hood
The fact that the Serpens Codex is carried by radio waves is deliberate for two reasons. Firstly, the civilisation that sent them wanted to make sure that someone could detect them. Using a lower-tech communications method ensures that even civilisations behind you in technology can still hear you. Secondly, the Serpens Codex being encoded in radio waves allows it to be used in your setting as early as TL6, in case you want the divergent tech levels of a TL6 society suddenly acquiring advanced TL10^ to TL11^ technology.
If you want a more late-TL8 to early-TL9 flavour, or to capitalise on the scientific zeitgeist, the Codex can be transmitted in gravitational waves. Advanced LIGO would be able to detect the Codex's signal as early as 15th September 2015, and LIGO India, VIRGO in Italy and KAGRA in Japan should all be built and operational before 2036. Gravitational waves make the artificial nature of the signal even more obvious – natural sources of gravitational waves like black hole mergers produce slowly-shifting frequencies culminating in a high-pitched chirp and a quick relaxation to background noise. The Serpens Codex would initially produce localised blips, far more sudden and isolated than natural sources, not to mention the fact that humanity is now on the lookout for gravitational waves in the universe and will analyse each and every signal with extreme interest. Gravitational waves also suit the flavour of the civilisation that broadcast the Codex, since one of the biggest gifts in it is gravitic technology.