@graydon @futurebird Fluorapatite happens naturally too some places. That's how people figured out it helps.

Nitinol skeletal implants (hip, knee, etc.) would be pretty obvious. Probably some prompt fission daughter products in skeletons that supposedly died before 1945 would also be suspicious.

@AMS @futurebird It does happen naturally but so far as I recall, natural occurrences are rare. And you could presumably correlate isotope traces in the bones with the environment and go "this is a surprise".

Which is very much what this kind of thing is; you need a lot of context to know if the bone isotopes are interesting and even more to know where to associate what you got with if it seems like it's most probably not local.

@futurebird Way too much fluoride in the apatite. UV-cured ceramic fillings in the teeth, perhaps. Post-WW2 nuclear testing changed the isotope ratios of some elements: the traveler might appear to be a different age from other remains in the vicinity, and results inconsistent with ratio tests on other elements. Not enough evidence of parasites during childhood.

@futurebird @davidtheeviloverlord

Magnific-ant

@michael_w_busch @futurebird Sr-90 was my first thought too, not least because it accumulates in bones so it would be in the skeleton (and I *did* know about the baby teeth) and wouldn’t be found at all pre-1945. So my next thought was that I needed to know when the travelers were coming from, and also when the bones were found.

@graydon @futurebird There are many naturally occurring fluorine sources (that’s how we figured out that it helps prevent caries) so its presence wouldn’t be at all definitive.

@CliftonR @futurebird

<<tries to eye zirconium crowns suspiciously>>

<<sprains neck in the attempt>>

@futurebird Fillings in the teeth or false teeth are a common trope in science fiction for spotting time travellers. So are other surgical things like replacement hip joints.

@futurebird so the thing about carbon dating

(note: we are not experts on this, this is our lay understanding)

it's based on the observation that a living thing ceases to take carbon into itself when it dies, which means its radioactives start to be a smaller proportion of its mass compared to things that are still alive

but that only helps measure duration since death

@futurebird there is a nuance, though, that we suspect must be part of this methodology when it's applied for-real, but don't know enough about

which is that the total isotope balance in the ecosystem is not necessarily constant over time. we have no intuition for how it would vary but suspect it's not uniform.

so if you had a time traveler who you knew just died, and they hadn't been in the present day long enough to have exchanged much mass with it, they might indeed be noticeable

@futurebird though you wouldn't be able to tell when they were from specifically, and there would be plenty of "regular" explanations such as them having had unusual exposure to radiation during their life

@cinebox @futurebird honestly yeah microplastics and other environmental contaminants are probably the better way to do that identification

@mattmcirvin @futurebird let us aspire to allow the background radiation to continue to decline, sigh

@futurebird @davidtheeviloverlord the best part is you can make sure your observations reach the present day, by carving them on a part of your own skeleton that you didn't get around to examining the fossilized version of

@futurebird on another note, we don't remember who wrote it but there was a short story once about time travelers who attempt to rob the library of Alexandria right before it burns down, but get caught by local law enforcement, who correctly deduce they're time travelers because......... they attempt to pass coins that are real gold, but all identical rather than showing individual variation in their manufacture

@ireneista @futurebird

At the coroner's office, after time travel has been invented:

Detective, writing into small notebook: "So, how long ago did the deceased die?"

Coroner, taking another large swig from a bottle: "Who knows? Maybe yesterday. Maybe tomorrow. Maybe right now."

@wakame @futurebird yeah lol the hidden premise of this scenario is that time travel is noteworthy in the setting, ie. not a normal part of society yet

@ireneista @futurebird

Time Traveler: "Time travel will been a rather recent invention."

Linguist: "You know what? I will simply go back in time and buy Microsoft shares instead of getting into a profession where I have to listen to eldritch abominations like 'will been'."

Time Traveler: "Microsoft? The burger chain?"

@wakame @futurebird they really should have known that that was always going to have happened

@ireneista @futurebird

Retrohistorian: "So what happened is this: A lot of time travelers, and I mean... a lot show up on April 4th, 1975, to buy a share of Microsoft.
And Billy 'Buns' Gates, always the entrepreneur, sells each of them a burger with fries, for $20, reselling food that costs $1.20 at the time.
After that, he basically bought a company named 'McDonalds' and rebranded it.
What do we learn from that? Always compare prices whenever you go."