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Say someone has invented a time traveling device and he can go anywhere on earth in the very distant past. Where could he leave a token (any object or marking) on earth that could be discovered in the present as proof to himself that he went backwards in time.

This should be something that the traveler could leave in the very distant past, like a million or so years ago, and must be something that could be found again without an unreasonable amount of effort. For a little added simplicity, let's say the token can be any material, real or fictional, of any shape.

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    $\begingroup$ What about butterfly effect? Won't you go back in time will affect the present by changing the past? $\endgroup$
    – Vylix
    Apr 17, 2017 at 4:53
  • $\begingroup$ Related Million-year-old records $\endgroup$
    – MolbOrg
    Apr 17, 2017 at 19:20

7 Answers 7

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Make a small, hollow cylinder of Technetium,
which is only available on earth (in visible quantities) as a byproduct of nuclear fission of uranium or plutonium -- and is radioactive (for ease of relocation.) See: https://en.wikipedia.org/wiki/Technetium

And hide it in a bore hole drilled into an easily-relocated, not easily accessable spot on Uluru (formerly called 'Ayer's Rock".)

While Tc-99 is the most available (from nuclear fuel reprocessing), but Tc-98 is more stable (half life is over 4 million years), so the cylinder should still be nice and radioactive for detection. So let's use Tc-98.

Ballpark cylinder dimensions: (25 mm diameter, maybe 300 mm long, 5 mm wall thickness.)

Adjust the outer diameter to be a couple mm less than the selected rock drilling bit selected. Hollow with welded-on endcaps; weld closed only after adding the message on a suitably durable medium.

For the message itself, I'd use stainless steel (say T316, unless you insist on more Technetium; I wouldn't), which is available in a wide variety of thicknesses, and rolls nicely, so long as not too thick. Ballpark 0.010" (or 0.25 mm) should work. If you can find it (and get it etched!), you could also use Platinum foil, which ought to hold up even better.

Also make a spacer, in case the driller {see below} has slow reactions. Same diameter as the cylinder, ballpark 10 mm should be plenty.

{I would never do the following, but it would IMHO solve your question.}

Go to Australia.
With a pneumatic rock drill, drill horizontally into the the northern-most, steep face of Uluru (known previously as Ayer's Rock): https://en.wikipedia.org/wiki/Uluru (which is old, old rock {precambrian even?} and has been exposed for a long time as well.)

Drill a slightly uphill borehole (5 degrees, for drainage in case of erosion) higher than reachable with anything less than a 25 foot (8 meter?) ladder, at least a meter deep. Place the message cylinder in the hole, followed by the spacer and tamp them firmly into the hole.

Measure the remaining depth once the objects are placed and then patch the hole with epoxy mixed with the drilling tailings. This mixture, plus the hard to get to location should hide the patched hole from anything short of detailed inspection.

To recover the time capsule (and prove you must be a time traveller):

Using saved survey info (and/or photos), use a geiger counter/scintillator to find the patched hole on the sandstone monolith. Erect a scaffolding to work from. Use the next larger diameter drill from when you drilled the original hold. Mark your drill with the known maximum depth, prior to starting to drill.

Be aware that the rock face has eroded, and you don't know how much, so be careful doing the following:

While monitoring the drill shavings for metal, drill or dissolve out the epoxy/sand patching mixture (again using a pneumatic rock drill, or electric if you prefer), remembering to stop drilling when one sees metallic shavings!

Extract the cylinder and amaze the world! Yes, you can drill the hole larger if you brought the right drillbits with. Or dynamite (this is fiction, recall)

For best effect of your capsule opening, go back in time to somewhere before 1936 (when traces of Technetium were first discovered and verified.) I'd go back before the first atomic pile was invented (late 1942) for the most impact.

The container proves the date. The message could be anything you feel safe writing -- but beware of the butterfly effect!

PS, the spacer doesn't have to be Technetium. T316 stainless is again my choice.

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    $\begingroup$ I'm not sure I understand. What proves you did it a million years ago and not last week? $\endgroup$
    – Tracy Cramer
    Apr 18, 2017 at 19:10
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    $\begingroup$ Is it the decay products in the tube? $\endgroup$
    – Tracy Cramer
    Apr 18, 2017 at 19:16
  • $\begingroup$ @Tracy: At the time of the reveal, the element is just discovered, but nobody on earth knows how to make even micrograms of it, let alone grams. Here is an object made of 'unobtanium.' Where did it come from? The future, via my time machine. QED. $\endgroup$
    – Catalyst
    Apr 18, 2017 at 22:35
  • $\begingroup$ I see. I guess I took his "...discovered in the present..." to mean after he had invented the time machine. $\endgroup$
    – Tracy Cramer
    Apr 18, 2017 at 23:57
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A gold plate with his name carved in it. Leave it in a known archaeological strata which hasn't been disturbed until recently. Plenty of those can be found with a bit of research.

Could even inscribe a short message 'Joe Bloggs travelled back through time from 2017 and left this plate here for xxx reason. HI MUM !!'

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    $\begingroup$ @BrianWoodbury Yep, go back to the books, Dinosaur bones and older stuff are found that are 100's of millions of years old. The time traveller just needs to put the plate in one. It will become covered etc along with the eventual fossils and uncovered again in the present time. Could even go to the location of the Burgess shales, drop it in the right place in the ocean and it will be uncovered 500 million years later in the 1960's $\endgroup$
    – Kilisi
    Apr 17, 2017 at 3:51
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    $\begingroup$ A dinosaur with a gold tooth would be quite the astonishing find. $\endgroup$
    – Cort Ammon
    Apr 17, 2017 at 3:53
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    $\begingroup$ @CortAmmon Finding a fossil dinosaur dentist would be even more astonishing. $\endgroup$
    – a4android
    Apr 17, 2017 at 5:00
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    $\begingroup$ I am picturing Joe McTimetraveller shoving his gold plated memento inside the corpse of a decomposing dinosaur. $\endgroup$
    – Marshall Tigerus
    Apr 17, 2017 at 16:14
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    $\begingroup$ @MarshallTigerus you don't need to mess with corpses, just put the plate in the vicinity in the correct strata. If it was a TRex you could quite possible get the animal to ingest the plate, drawback is it does it while you're holding it. $\endgroup$
    – Kilisi
    Apr 17, 2017 at 21:23
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If you're allowed a time-capsule I think this is probably the best bet.

Choose somewhere you know gets dug up before you were born (this way someone won't jump to the conclusion that it is a hoax) and include some random information that they might not understand yet. For example, a time capsule with a carved plate saying:

"You won't understand this yet, but on the 17th of April 2017 at 15:30(GMT) call me, Brian Woodbury, on 07765432198" (I've no idea if that is a real number, I just pressed random numbers)

And bury it in a site you know is uncovered before you're born because otherwise someone can say "Hey, what is this kid playing at...nipping in and burying a fake relic...pfft." and ignore you...or at least have a lot of skepticism around it.

The danger is that this changes the past and someone else wants to get your phone number or name because news of this gets out. You rely on the finder/some organisation keeping it secret for years until you're born.

There are a lot of risks...but then its timetravel, no one said it would be easy!

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    $\begingroup$ Thank you for using my real name but not my real number $\endgroup$
    – Brian Woodbury
    Apr 17, 2017 at 21:04
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    $\begingroup$ @BrianWoodbury Unfortunately...otherwise you would have got a call from some very excited scientists at half 3 this afternoon! $\endgroup$
    – Lio Elbammalf
    Apr 17, 2017 at 23:06
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The Moon.

  • We know exactly where the astronauts are going to land.
  • No possibility for someone to secretly swipe it.
  • No geology or weather to move or bury it.
  • Lots of attention paid to the Moon landing and surroundings so many people would be aware of discovery. It would be hard to hide it or hush it up. The astronauts would read it to Mission control and hundreds of people would hear it.

Gold plaque could state Armstrong's "one small step" quote, and the date of the moon landing. That would prove knowledge of future events without changing much. He and the other astronauts would find it and read it hours after he actually said it. The plaque would also state explicitly what it is: a plaque to prove that a person from the future traveled into the past.

What, you can travel thru time but getting to the Moon is tough? Lazy!

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  • $\begingroup$ A million years of the moon getting hit by asteroids doesn't sound like a safe place for it. $\endgroup$
    – Lio Elbammalf
    Apr 18, 2017 at 11:33
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Create a large 'man-made' diamond and cut it into a spectacular shape such that people would instantly see its value and not destroy it or cut it into smaller diamonds. Etch it using a laser such that without a microscope people couldn't read the engraving. As another answer said, put it in a well known cave system so it would be found by early spelunkers. Let it be found by ancient Egyptians, Chinese, or other early civilizations and it should attain a symbolic status and put into a museum for safekeeping. It could even change the course of history in that tribes or countries would fight over it.

If you just show up with a or other artifact that has something etched on it they would likely believe it was a hoax. It needs to be something that has a history that everyone knows about.

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    $\begingroup$ Playing with time, especially when you might cause fights (and kill your ancestors) is risky. Also how would they know he hasn't just made a replica of a well known artifact? $\endgroup$
    – Lio Elbammalf
    Apr 18, 2017 at 7:19
  • $\begingroup$ @LioElbammalf, Agreed - changing timelines is a risk. I'm assuming the diamond is in a museum or private collection, like the Hope Diamond but even more significant because it was found already cut - thus attaining some sort of 'fantastical' status. Someone would eventually want to examine it with a microscope and discover the inscription. If it was before you were born and they prevented you from being born we have another change the timeline since you wouldn't be able to invent the time machine. Time travel is fraught with paradoxes. $\endgroup$
    – Tracy Cramer
    Apr 18, 2017 at 18:54
  • $\begingroup$ @LioElbammalf, also, if it were in a museum I would have to be able to break into said museum to swap it out for a replica. $\endgroup$
    – Tracy Cramer
    Apr 18, 2017 at 18:58
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The Arctic

  1. Figure out where they have drilled ice cores for scientific studies.
  2. Get a gold plate. Put things on it.
  3. Go back in time and leave it at the predetermined location.
  4. ???
  5. Profit.

Why the arctic? Because the last ice-free era was around 2.6 million years ago according to this article I literally just googled to find.

The study provides new evidence that the last major gap ended about 2.6 million years ago

So you'll have very little geological change there compared to other places on earth.

Why gold? Because short of leaving it in zinc or mercury it won't corrode or degrade over the million years it needs to survive. I know you said it could be a fictional material, but a non-fictional one works well enough, so might as well go with that.

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    $\begingroup$ Some Artic ice moves around quite a bit, it would be very challenging to find the right spot. $\endgroup$
    – Joe Kissling
    Apr 17, 2017 at 14:58
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Find a deep cave to hide in/

Put something metal in one a cave far from the surface. Many limstone caves are more than a million years old. There is no natural metal in them so with a metal detector you could find the right stalagmite without having to smash the whole place.

Or you could shape the ceiling to drip at places of your choosing, after a million years those drips will become pillars, and you can spell your name using pillars as pixels. It might not be recognizable without mapping.

Some caves are still not fully explored, and seem to have been stable for a long time. You could leave your gold on the floor safely in an unseen side tunnel. Getting there without the time machine could be tricky.

For the time capsule gold is a good bet because it doesn't corrode. Wrapping a rock that has the writing means even if it gets banged around a little you don't lose the message.

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