# How do you prove the age of a time capsule?

There is the classic trope of a kidnapping victim posing for a photo with today's newspaper to prove they still live, or a time traveller correctly announcing Solar Flares before they happen to prove they have future knowledge. In both cases, these can be used as "Proof that the individual was alive at X date".

However, I have a different problem. I want to be able to prove that my time capsule/ancient dungeon/past time traveler really is from Xty years ago and has not been interfered with at any time between now and then. Short of Carbon Dating, which requires specialist knowledge and equipment beyond the reach of a layman, what can be used as a proof? The best answer will be one an unprepared recipient can verify, even if the original creator needs something special to make the proof.

• Also, carbon dating doesn't work well for material (bone, wood, leather, etc.) produced after the early 19th century, because coal and petroleum have essentially zero carbon-14 (they've been shielded from solar radiation for millions of years) and have diluted the atmospheric ratio since they started to be burned in quantity. Commented Oct 12, 2022 at 12:08
• "my time capsule/ancient dungeon/past time traveler " Is it a person place or thing? Answers for a question like this rely on context. An undisturbed dusty floor says one thing about a dungeon, another about a person. Commented Oct 12, 2022 at 12:11
• @ZeissIkon: There are calibration tables available. (Carbon dating always uses calibration tables, otherwise the dating is very imprecise.) (And the real turning point is the 1950s, when several highly civilized nations used their technological know how to create large amounts of radioactive carbon in the atmosphere. Naively carbon dating an item from the 1960s would make it appear to come from the future...) Commented Oct 12, 2022 at 13:35
• @ZeissIkon: The point is that the ordinary person does not have to do it themselves. Not even archaeologists do it themselves. It is done as a service by specialized laboratories. Commented Oct 12, 2022 at 13:39
• @JonCuster No way would I pop for \$460 to support some crazy claiming to have skipped the past couple centuries. That's a car payment, or half a month's rent if you don't live in a major city. Much more than "ordinary people" would just throw away. Yes, I could pay that much, but I'd have to be pretty well convinced it was worth it up front. Commented Oct 12, 2022 at 15:48

Probably the most reliable way of dating a time capsule, and one that doesn't require the opener to know any science and but little history, is for the capsule to be poured into a concrete building foundation.

Unlike burying it under a cornerstone (as was often done with time capsules, but could potentially be faked by tunneling under the standing building), it's very difficult to convincingly fake a cast-in item in poured concrete -- and a building foundation especially so, since it would be a major repair/rebuild job to replace a section of foundation in order to cast in a time capsule at a later date (not impossible, but a large enough project that the history of a landmark building is very likely to include it).

Of course, this is limited to the period (since the mid 20th century, as I recall) when cast concrete was used for building foundations, and requires "chain of evidence" handling to maintain confidence that this time capsule was actually found inside the foundation of that building, but it doesn't require any instruments more scientific or difficult to use than a newspaper morgue.

Of course, this could also be applied for a capsule from the 1st century BCE to around 3rd century CE, for Roman concrete...

• A landmark with "here lies a time capsule" could also qualify. We have one nearby, due to open in IIRC 2045, with documented evidence of the landmark existing since its foundation in 1945. Yet, in order to proof something to a layman even this sort of evidence could not be enough... Depends on the person to be convinced. Commented Oct 13, 2022 at 11:46
• Again, though, unless the capsule is inside the landmark, tunneling could replace the original capsule with a modern fake, and that wouldn't be detectable in the normal course of digging up the capsule. Commented Oct 13, 2022 at 12:50
• Well, I thought I did say that in our case the capsule is within the landmark's solid part. Sorry. Commented Oct 13, 2022 at 12:54
1. Make up a message. It doesn't matter what it is, but it does matter that it's a secret.
2. Use a cryptographic hash function to make a hash of the message.
3. Publish the hash somewhere that will have a date associated with it and where someone can find it later. The easiest option would probably be in a newspaper in a classified ad.
4. Put the message in the capsule.

When someone opens the capsule, they can perform the hash operation on the message and compare it with the one in the newspaper.

Of course, this is not something most people can do on the spot, particularly if unprepared. To make it easier on them, you can:

• Include clear instructions in the capsule on how to do it.
• Include a device that can perform the hash operation.
• Tell them exactly where to find the published hash.

The details will vary based largely on the level of technology available at each end of the system. The key components are

• A method of encoding a message that is reproducible given the algorithm and the original message.
• Somewhere to put the hash where people will believe it's from when you say it is.
• A tamper-resistant time capsule.
• This of course becomes unreliable if someone finds a way to generate hash collisions for the hash function you chose at some point between burying the time capsule and unearthing it. If you could generate a collision, you can make a new message that hashes to the same value as the one put in the newspaper all those years ago. Commented Oct 13, 2022 at 0:16
• It only proves the message is X years old, but it might have been put in a new time capsule.
– towr
Commented Oct 13, 2022 at 5:41
• Actually, it only proves the hash code is X years old. The message might have been generated yesterday from a 90 (or 900) year old "hash code" found in a published source. Commented Oct 13, 2022 at 11:08
• @ZeissIkon: The whole point of one way functions is that there is no way to go backwards, i.e. no way to generate a message from a hash code. Or at least much harder to do so (but is it hard enough? With technology of the future?). Commented Oct 13, 2022 at 21:57

# Ride the comet ☄️

## Preparation

• Find a comet with an elliptical orbit that comes relatively close to Earth every Xty years. If no such comet exists and you have enough resources, launch an artificial object on a similar elliptical orbit.
• (hardest requirement for the prover) Print a message or put the capsule on the comet.

## Verification

• Observe the comet for a period of time to verify its orbit. This proves no one on Earth was tampering with it for Xty years.
• Observe the comet using a powerful telescope to verify the message.
• Launch a mission to recover the capsule (easier if you controlled the orbit to align well with Earth's back then).
• Verify that the comet has no engines, just in case.
• Don't think this will work well for an "unprepared" receiver of the time capsule and message... Commented Oct 13, 2022 at 11:09
• Carbon dating does sound easier
– Hugo
Commented Oct 13, 2022 at 17:26