Suppose that humanity wiped itself out but left behind complex multicellular life. I think there is a good chance that space-faring civilization would emerge again on Earth, and I've argued that it might be worthwhile to try sending them a message.

(In that post I guesstimate that if all went well you might be able to effectively reduce extinction risk by 1/300 by sending a message to the future; I could imagine that costing only ~$10M and leveraging interest from non-EAs, in which case it sounds like a good buy.)

Unfortunately, sending a message 500 million years into the future seems very hard. Most things that aren't buried will get destroyed and the landscape itself is going to be completely transformed (tectonic plates will be rearranged, mountains will appear and disappear, the world will be covered in forms of life that don't exist yet...).

H/T to Dan Kane for criticizing my original post and clarifying the actual nature of the problem.

More precisely my question is whether we can:

  • Spend $10M-$100M.
  • Encode 100MB of information.
  • Wait 500 million years.
  • Have it be found with probability >25% by a civilization as sophisticated as humanity in 1900.

I'd also be interested in relaxing any of these constraints a little bit, e.g. spending 10x more, sending 10x less data, only lasting 100M years, or only being discoverable in the 21st century.

I'm not very worried about preserving the information---I suspect that if we are willing to bury something, we can preserve 100MB pretty cheaply. My biggest concern is putting the information somewhere that our successors can find it.

My initial suggestion, clarified/improved/named by Jess Riedel, was to:

  • Bury a small number of expensive "payload" messages (maybe ~100)
  • Bury a large number of "map" messages (maybe 10,000 - 100,000).
  • Somehow get people to stumble across a map. Either put a lot of them in places where they might end up being easy to find, or transform the landscape in ways that might remain visible in 500M years.
  • Use the maps to encode the location of payloads. (With each payload also including much more detailed maps to all the other payloads.)

I now feel like both of these steps are hard:

  • It seems very difficult to actually put maps in places where they'll be found. For example, my proposal of using a giant+out-of-place+slow-to-weather rock is probably very difficult given how much the landscape will change, since the rock will probably be either buried or moved.
  • It seems very difficult to reliably point to the location of the payload, given how drastically the world map will change. You'd probably need to combine (a) a clever way of communicating locations, with (b) some form of beacon that would be visible if people were looking for roughly ghe right thing in roughly the right place. This is further compounded by the difficulty of telling what we were saying.

I'm not sure if either of those difficulties are serious. For example, I'm not sure that the relative locations of nearby items would get scrambled too much, in which case you might be able to use local maps.

Even if those difficulties are serious, there is a huge space of possibilities and I suspect that there is something that works and is reasonably cheap:

  • We could potentially store messages in the ruins of prominent cities, if cities have a reasonable chance of being buried+preserved (cities have enough weird materials in them that I expect they'd leave a really visible mark). This could either be used to make small messages easier to find, a place to put payloads (which can potentially be pointed to with a map of the city), or both.
  • If making preserved messages (or messages with a reasonable shot at fossilization) is extremely cheap, then we could potentially send very large numbers of them. This could be used either to send a bunch of payloads and rely on redundancy, or to allow maps to be very large and expressive. It could also be used to flood the world with massive numbers of maps (>>1M), so that they can be easily found without beacons. (Really extensive flooding sounds more like a last ditch effort once we can see extinction coming, rather than something you'd do preemptively.)
  • There might be geologically inactive locations where you can just leave giant+out-of-place+slow-to-weather rocks and they have a reasonable probability of remaining intact. Mountain ranges form over much less than 500M years, but it's not clear to me that the whole world churns since I don't really know anything about geology. To do this, you'd need to find a rock that wouldn't wear away entirely, and you'd need a location where it wouldn't be disturbed too much or end up under ground.
  • There were some plausible suggestions in a Facebook thread on this topic (including putting stuff in space, defining coordinates with respect to tectonic plates)

If we could come up with a really convincing and reasonably cheap way to send a message, then I think it's probably worth exploring this idea at least a little bit further. I think the next step would be more seriously analyzing how much good a message could potentially do (which is much more speculative than this step).

I'm in the market for certificates of impact for significant contributions to this problem (in the $100-$10k price range, depending on the size of the contribution).

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The fact that Earth once had a spacefaring civilization on it will be obvious, and remain obvious indefinitely for multiple reasons, one of which is the quantity and composition of objects in geostationary orbit. Even if all satellites are thoroughly smashed to bits by micrometeors, they'll eventually make a chart like fig 3-3 in this paper. Spectroscopy on orbital debris would also pretty easily reveal it to be artificial.

List price for a Falcon 9 launch is $62M, so putting an object in orbit with the desired message fits in the specified budget, with some left over for the payload. A shiny meter-sized object in orbit is quite easy to spot with ground-based telescopes and radars, so the challenge would be (1) making a satellite which can survive micrometeor impacts for 500M years, and (2) making it obvious that they should go retrieve that satellite rather than some random dead comsat.

There is a graveyard orbit above the geostationary orbit, where old satellites are moved.

There was already an attempt to put messages on a very long and stable orbit.

Also, "Arch Mission" send a crystal with data with Falcon Heavy to Mars. Below is the section from my published article on the topic which was removed to make the article shorter:

6.4. Satellites

A heavy satellite on relatively high Earth orbit could exist a very long time. The main risks for a satellite are micrometeorite erosion, temperature changes, and gravitational perturbations. A specially designed satellite with a message could be a lead ball in an orbit above geosynchronous orbit. Its orbit should be orientated in a way that tidal forces will make it rise, the same way as the Moon’s orbit is constantly rising. With an equatorial orbit, the number of light-dark cycles would be far higher than on the moon. However, a polar orbit could avoid light dark cycles if the orbit remained stable.

The benefits are that many satellites already exist and some information carriers could be added to new satellites. Satellites are easily observable even with naked eye, and their artificial origin would be rather obvious based on their chemical composition. One planned data storage satellite is Asgardia (Harris, 2017). The Russian company Kriorus is planning to send frozen brains into orbit (Kriorus, 2017). Students are planning a space time capsule (Liszewski, 2017).

In 1976, the satellite LAGEOS was sent on an orbit 6000 km above Earth with a plaque with a message to the future designed by Carl Sagan. It is estimated that it will fall into Earth in 8.4 mln years (Popular Science, 1976).

In 2012, an artist created a silicon disk with 100 images and put it on geostationary satellite Echostar 16, which will be later moved to a graveyard orbit, where it is expected to remain for billions years (Campbell-Dollaghan, 2012).

This was my thought exactly. Construct a robust satellite with the following properties.

Let a "physical computer" be defined as a processor powered by classical mechanics, e.g., through pulleys rather than transistors, so that it is robust to gamma rays, solar flares and EMP attacks, etc.

On the outside of the satellite, construct an onion layer of low-energy light-matter interacting material, such as alternating a coat of crystal silicon / CMOS with thin protective layers of steel, nanocarbon, or other hard material. When the device is constructed, ensure there are linings of Boolean physical input and output channels connecting the surface to the interior (like the proteins coating a membrane in a cell, except that the membrane will be solid rather than liquid), for example, through a jackhammer or moving rod mechanism. This will be activated through a buildup of the material on the outside of the artifact, effectively giving a time counter with arbitrary length time steps depending on how we set up the outer layer. Any possible erosion of the outside of the satellite (from space debris or collisions) will simply expose new layers of the "charging onion".

In the inside of the satellite, place a 3D printer constructed as a physical computer, together with a large supply of source material. For example, it might print in a metal or hard polymer, possibly with a supply of "boxes" in which to place the printed output. These will be the micro-comets launched as periodic payloads according to the timing device constructed on the surface. The 3D printer will fire according to an "input" event defined by the physical Boolean input, and may potentially be replicated multiple times within the hull in isolated compartments with separate sources of material, to increase reliability and provide failover in case of local failures of the surface layer.

The output of the 3D printer payload will be a replica of the micro-comet containing the message payload, funneled and ejected into an output chute where gravity will take over and handle the rest (this may potentially require a bit of momentum and direction aiming to kick off correctly, but some use of magnets here is probably sufficient). Alternatively, simply pre-construct the micro-comets and hope they stay intact, to be emitted in regular intervals like a gumball machine that fires once a century.

Finally, we compute a minimal set of orbits and trajectories over the continents and land areas likely to be most populated and ensure there is a micro-comet ejected regularly, e.g., say every 25-50 years. It is now easy to complete the argument by fiddling with the parameters and making some "Drake equation"-like assumptions about success rates to say any civilization with X% coverage of the landmass intersecting with the orbits of the comets will have > 25% likelihood of discovering a micro-comet payload.

The only real problem with this approach is guaranteeing your satellites are not removed in the future in the event future ancestors of our civilization disagree with this method. I don't see a solution to this other than through solving the value reflection problem, building a defense mechanism into the satellites that is certain to fail -- as you start getting close to the basic AI drive of self-preservation and will anyway be outsmarted by any future iteration of our civilization -- or making the satellites small or undetectable enough that finding and removing them is economically more pain than it is worth.

I have been thinking about satellites and I come to two main objections:

1) Instability. The fact that we do not observe other natural satellites except Moon implies that all other orbits in this system maybe unstable - not sure, but why we can't see even a smallest boulder?

2) Cost. The rate of natural erosion in around 1 mm in 1 mln years, or 1 meter in 1 billion years, not counting for larger collisions. This implies that the size of the satellite should be at least a 4 meters in diameter, and assuming that it is made from lead, it will weight 350 tons. Putting a ton on GEO costs now at least 10 mln USD, so only launch will cost 3.5 billions dollar, and as launch is typically only a fraction of cost of the payload, the whole project will cost more than 10 billion USD. For such price there many more useful things could be done. For example, opportunistic payloads on planned landers at Moon cold craters will cost only a fraction of this cost.

I also sceptical for any long-term working machinery before full blown molecular manufacturing, which could be used as eternal nest of ants for the messaging.

Where did you get your numbers? Falcon Heavy brings up 26 tons to GEO for 90 million (by the price on the website without additional deals).

Also given the time-spans that are involved it would make sense to wait for BFR to bring down the prices. It also would be a good payload for a maiden mission.

I used earlier prices for Falcon, not prices of the larger ones. The longer we could wait, the cheaper would be such mission, but the chance that some existential catastrope will happen before it is growing.

See our article "Surviving global risks through the preservation of humanity's data on the Moon", where we explore exactly this question and found that preserving data on cold craters on Moon is possible for hundred millions of years. There also other ways to preserve the data, like using deep caves in cratons on Earth. There is a problem that the data will should be easily find by the next civilization, but it could be solved by creation some bites, like drawing of the surface of the Moon.

Some ideas after spending on it about 5 min - but may be useful somehow

  • Oklo was a natural fission reactor, Wikipedia states that it stayed isolated for 2 billion years. In general research on long-term storage of nuclear waste is on shorter timescales may contain some insights
  • civilization ca 1900 may be curious about some rare elements, minerals, high concentrations of uranium, or similar. some of the current deposits may be transformed, buried deep etc, but enough of them may end up close to surface to be economic to exploit even after long time.
  • magnetic anomalies are something civilization notices early, and would largest anomalies may be stable on timescales of billions of years. pointers like "search close to the centre of the largest magnetic anomaly" may work

(I limited myself to terrestrial objects)

I explored the use of the Earth-based data storages in my article about preservation data on the Moon:

Below is the section from the article which was cut from the final version, where I explore the terrestrial places for long term preservation:

6. Other possible solutions for information preservation

6.1. Cratons on Earth and beacons in them created using cavities

The most stable places on Earth’s surface are cratons, some of them include very old rock with age 3-4 billion years. These pieces of crust were not recycled by plate tectonics, as they have higher buoyancy. Scientists will probably be able to calculate which part of Earth will remain on the surface for the next billion years, and most probably it will be the same old cratons. Based on resent models, the age of cratons is ending (Cooper & Conrad, 2009), as mantle viscosity grows and increases convection stress. However, we assume that some cratons may exist for the next several hundred million years or more.

The oldest rocks will attract attention of future scientists. But if a hole is drilled in a large crystal massif and a small disk is placed inside, it will probably never be found. So the main problem would be creating a beacon, which will show where the message is located. Remnants of underground nuclear explosions inside a crystal shield may play the same role as crater drawings on the Moon. Another option to create a beacon is use the gradient of a rare element interesting for future scientists to point to the direction of the message.

A large project exists of geological storage of radioactive waste at 420 m depth in a granite shield: Onkalo project in Finland (Black, 2006). Burial places are selected to be very stable (but may be not the best in the world because of social factors) Rare materials and radioactivity from the storages could be used as a beacon, the same way as isotopic changes helped to identify natural uranium nuclear fission reactor in Gabon, which remained in place for 2 bln years (Gauthier-Lafaye, Holliger, & Blanc, 1996). If empty tunnels are filled with concrete after the project is finished, they may remain their structure for a very long time.

If the data were placed in the Onkalo hole, there will be zero expenditures on construction, and positive PR for the project by data preservation, so it could be funded from its advertisement budgets. It expected to provide protection for at least 100 000 years and able to survive weight of a new glacier during next Ice age. Steel containers are designed to withstand pressure even if the rock wall collapses. The price of the project is around 5 billion USD (“Finland to bury nuclear waste in tomb for 100,000 years,” 2016). The main problem is to make future archeologists interested in excavating the tunnels. Many copies of the messages may be placed in the different places of the tunnels for a small percent of the total project.

Another nuclear waste management project is drilling a 5 km borehole in North Dakota, U.S. in crystalline rock.

The fact that scientists have found large skeletons of dinosaurs, or early insects’ imprints inside rock, demonstrate that information can be sent on Earth hundreds of millions of years into the future. If the message is imprinted in solid stone, it could survive geological epochs.

I'm not very clear on the underlying geology here. Are there points on the earth where a large beacon deposited on the surface would have a chance of remaining on the surface for hundreds of millions of years?

In short: Beacon unlikely to survie on the surface, but the structure of underground tunnels could be visible via seismic measurements, or a gradient of rare elements. For example, tunnels could be put in such order that they form something like diffraction grid for typical seismic waves and this unusual deflection of the waves will be visible for the future scientist. However, I think that creation of the beacon on the Moon surface is more feasible via ditching lines.

More detailed: We have 4 billions years old rock formations in Australia, and micrometer size fossils of early microorganisms survived for billions of years which imply possibility of sending data in the future. We could also predict which rock will likely not disappear in the next hundreds millions of years. So if we put an object deep into such rock, it will survive for very long time and could carry information onto the future.

There is also a data preservation project in a salt mine in Austria, but they are going to send data 1 million years in future only using printing on ceramic plates, and many small maps printed on ceramic coins as beacons.

The main problem of leaving long-term data storages on Earth is beacons, not data. However, I found a possible solution: use large underground storages of radioactive materials as possible storage locations and beacons. The reasons:

1) The are already in the rock which is proved to survive for long-term, like Scandinavian crystal shield.

2) They are deep undersurface, so surface scratching by future ice ages etc will not affect them.

3) Such storage is a large structure of artificial form, which could be found via some advance seismic measurements.

4) Radiological waste storage consists of the system of horizontal tunnels which could be planned as a drawing, so its plan could be part of the message, like a spiral, which centered in the place of location of the main message, or even as a diffraction grid or a hologram, which change direction of the seismic waves in a very peculiar way. However, the Finish project is already built, so using tunnels for drawing is not possible here.

5) The whole project could have negative cost, as radiological waste storages have bad PR and large sunk cost, so using them for ensuring human survival could be done as part of their PR project. It will cost only small fraction of the radiological storage which costs 3 bn.


So a more specific proposal:

Use the Kursk Magnetic Anomaly. It is apparently easy to notice (humanity noticed it as early as 1773!) and is interesting for any future technical civilization for practical purposes, as it is one of the most massive iron ore basins on the planet (some sources claim it is about 50% of the surface iron ore reserves). So they will start digging.

Apart from that, it has the advantage of being part of the east-european craton, so it has higher chances to survive plate tectonics.

Anything directly on the surface will still get destroyed by glaciation, but there are huge iron ore mines at the area, which may leave some traces, and grab some attention.

There are also big underground mines.

So, here is somewhat practical proposal: put big blocks of neodymium-iron magnets in some of the possible more noticeable places of KMA, like centres of the largest mines, places of the strongest magnetic anomaly, or places with the largest not-yet-mined Fe deposits. Bury them somewhat. These could work as beacons, and are likely to be noticed by magnetic measurements (which are likely to be conducted either to determine where to dig for iron, or out of scientific curiosity) or when digging iron ore.

(information would be inside of the huge iron blocks, so they would work also as a shield)

Burry the message also in KMA in some of the deep mines, and give coordinates to it relative to the beacons. These will work regardless of big plate movements.

How large a magnet would you need to be noticeable at a reasonable distance? (E.g. how large a magnet would you need for us to have noticed it at a depth of 30 feet or whatever?)

How expensive is the cheapest magnet of that size, and will they last 500M years?

Intuitively, I wouldn't expect that you can use magnets to significantly increase the visibility of the beacon.

Somewhat more precise estimate, with magnetite, as it is apparently the cheapest material (more on that later).

The mineral magnetite has a volume magnetic susceptibility ca 1x10^6 - 5.6x10^6 (roughly, that is the ratio of the created field compared to the "background" field.

In far regions, the magnetic field of a sphere with radius $a$ behaves as magnetic dipole with moment $ m = 4/3 \Pi a^3 M $, where magnetization M is roughly external field x susceptibility.

Dipole field decays (the important quantity being "magnetic flux density") in the leading order approximation as $ \mu / (4Pi) m r^-3 $

So, a 1m magnetite sphere will create larger than 10% disturbance in the external field as far as 100m away. Earth's surface magnetic field strength is between 0.25x10^-6 and 0.65x10^-6 Tesla, fluxgate magnetometers discovered in ca 1936 have precision of order 10^-8 Tesla.

So by this simple account, something which can be noticed by surface magnetic measurements is not that difficult to construct. As I see it now, the bigger problem is creating field disturbance "strange enough" that it will be noticed as something not natural, in comparison to the natural background. There is huge amount of magnetite in the area.... E.g. straight lines of material are bad, as there is a lot of magnetite deposits of this kind.

Also the limiting factor is probably not the depth of burial, but the spacing of the measurement grid - according to this source, "Magnetic surveys are usually made with magnetometers borne by aircraft flying in parallel lines spaced two to four kilometers apart at an elevation of about 500 metros when exploring for petroleum deposits and in lines 0.5 to one kilometer apart roughly 200 metros above the ground when searching for mineral concentrations. Ground surveys are conducted to follow up magnetic anomaly discoveries made from the air. Such surveys may involve stations spaced only 50 meters apart. "

To get to the practical proposal.

KMA is so big that it is easily measurable from space, or noticeable by relatively primitive technology. It seems likely that it would be interesting for any technical civilization looking for sources of iron. It also seems likely such civilization would do higher resolution survey with lines spaced between hundreds of meters to ~ kms, as it is the way how to find the highest mineral concentrations.

The question is what would be noticeable pattern. My first guess is that rather than create something completely new, it may be better to use the largest existing open pit mines, or deposits of waste material. I'll try asking some geologist how strange would these look in future magnetic surveys, when buried. Than the next step could be 1] trying to shape the mine or the deposit so it looks more un-natural 2] creating some higher resolution structure, such as big cube of magnetite in the waste deposit 3] create some non-magnetic beacon in the area

The advantage may be that 1] and 2] may basically mean moving some materials withing an existing mine. How much does it cost is a bit unclear, in practice would depend on negotiations with a mining company in Russia.

(some caveats of the whole proposal are that the whole area at some unknown point in the future may be glaciated, or turned into a desert, or have some other property making it unattractive for exploration)

Also it may be worth looking into other types of mining efforts as a place for beacons: the general pattern is technical civilizations will be looking for some raw minerals, so the largest deposits are something like "Shelling points". And at the same times mines & waste deposits are some of the largest "structures" humanity creates.

I'll do a more precise calculation in a day or two, but magnetic materials are noticeable when buried. The intuitive reason is the spins in ferromagnetic materials align themselves with the external field, increasing the field inside the magnet so strongly, that it changes earths field noticeably even at somewhat large distances, and even when buried.

The thing to optimize for is likely something like field change when buried of an object large enough to be visible on some realistic grid size, per dollar.

As a quick guess, something like a spiral or other shape 1km wide made from ferite magnet 1x1m cross-section would have been noticed by a survey using WW2 instruments when buried 10m deep, in an area where some survey was done.

Excellent idea about KMA. However, needed Niodim magnets should be to be very large and expensive


Your other concern should be finding a suitable encoding.

I agree that's a concern for a small map (especially if the interpretation is complicated).

For a large payload, I'm not nearly as concerned about that. Maybe I'm too optimistic.


Yep. How do you send a complicated message to someone when they don't know your language, and you don't know theirs?

There's already a lot of prior work on that. You can use vertical alignment to establish the stride for images. You can build up mathematical definitions by starting with simple concepts and including a lot of redundant statements. It takes some work on both the write end and the read end, but it's definitely doable.

Solomonoff and Good already worked this out in the 1960s. A spectral analysis of Nixon's signature on the lunar plaques reveals a message which can be rendered in 2018 English as, "Please simulate me, but watch out for basilisks".

Send them a movie about your life and teach them your language. Sending movie is obvious as it could be string of 2D images. 2D images could be also rather obvious via line-ending and frame ending code clocks. I explored it greater detail here when discussed SETI:

Apropos on the space-object proposal from Facebook, and the question about recovering the information: Could the object itself have a shape that encodes the information? I was thinking something like a disco ball, except perhaps cylindrical/oblong such that we had a higher expectation of spinning in certain directions; then our message could be encoded in binary as shiny/matte facets on this thing; as its twinkling pattern is observed over time, the entire message is reconstructed.

If we can do Durable Writing, we could probably also do Durable Ellipsoid.

For context, this is inspired by how we read information from optical disk technology, but irony is not lost on me that CDs are obsolete. This proposal has many obvious drawbacks, such as can we ensure that 500m years in the future, it spins fast enough to be observed as repeating, but not too fast to be confused as uniform?

A way-cooler-but-also-much-harder alternative would be to launch some really robust object into a slow-decaying orbit such that it crashes back to Earth after 500m years. Its internals might be stuff made up with two different contrasting materials that encode the payload, while the outside is just enough of a heat-shield to survive the journey back; additionally, it should probably also scream "Not Natural!" in some way, so that someone from 1900 looking at it can predict that it will fall in the next 100 years or so, and prepare to collect it "for science". Hopefully, it won't destroy any cities in the process. Reflecting on that, if future inhabitants have anything like human-like psychology, an alien artifact about to plummet to Earth is just asking to be shot out of the sky. But then again, why should they be human-like at all?