I am working on a story where an alien tells humans that a gamma ray burst will be fired at the alien civilization in one year, as the alien is trillions of light years away, that means that the burst is already on its way and can’t be stopped. The only way to stop it is time travel. Because of this, humans send a time traveler back in time and towards the origin of the message to disable the gamma ray burst before it fires. However, the alien who sent the message knows this will happen and has made a clever and hard to discover set of tests to ascertain when and where the time travelers came from, and uses that information to send the message the time travelers received in the first place. This is because the alien is using paradoxes to find and summon other time travelers to it to make a menagerie. If it can't determine where the time travelers came from it can't send the signal that tells them to arrive. So knowing where the time travelers come from is a requirement for the time travelers to arrive and for the plan to work. However, I’m not sure how to do that.

How can an alien test a time travelled to see when and where they come from? The alien is from another universe, so it doesn’t know about humans or even what galaxy to look in. It also is running this gambit on every civilization in the universe that can respond, so it has to work universally. The alien can make extremely convincing illusions and can even make fake suns if needed, so no matter how hard the test is to implement, it can be done. The tolerance for error is non-negligible, about a 10,000 years and 100 light years.

What tests should the alien use?

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    $\begingroup$ Too confusing. Let's start with the beginning of timeline - aliens are about to fire a gamma ray burst when some human arrives and tells them to stop. Is the problem how to "authenticate" this human? $\endgroup$
    – Alexander
    Jun 23 at 7:25
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    $\begingroup$ This doesn't seem like a question about establishing some fact of your world. Instead it seems like a question about the actions of characters within an already established world. Note how you're asking about the actions of an individual or organization? Such questions are not a good fit for this site. $\endgroup$
    – sphennings
    Jun 23 at 12:53
  • $\begingroup$ Why have you dismissed carbon dating? $\endgroup$ Jun 23 at 14:30
  • $\begingroup$ Should I ask this on physics stack exchange? $\endgroup$ Jun 23 at 17:03
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    $\begingroup$ I think this question is better off in the writing SE, because it's more about your story than the functioning of your world, and I don't think plot points about fictional time travel are within the range of physics SE. $\endgroup$ Jun 23 at 20:40

2 Answers 2


The alien asks the arriving time travelers.

These time travellers believe. And they think they have at least one ally. The alien meets them and asks if they got the message; they did. The alien tells them it sent several messages to various systems and wants to account for each. It asks the travellers what what wavelength the cosmic background radiation is during their period from which it can deduce time period. It will ask system they are from although a trillion light years away means the alien exists a very long time ago and any universal points of reference will probably have been altered by the intervening Big Bang.

Then I suppose the alien will proceed to travel to their future systems and steal their tchotchkes for its collection.

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    $\begingroup$ Accepting before this gets closed because it isn't wrong. $\endgroup$ Jun 23 at 23:01

Also self answer since I have some ideas


Tracing radioactive particles might work, but that is very hard to do and probably takes their whole body to get enough Uranium or what have you to get a good sample size.

Making a simulation of the universe and having the inhabitant fix it by aging it up to when they came from to pass the test. The universe always "fixes" it self if three travelers give the same answer.

Time and position

Having multiple suns shine down in multiple parts of their lives, the one that the inhabitants step into is like the they type and age of their star.


Make a simulation of the night sky and have the people navigate under it, but the directions constantly shift. To navigate, the inhabitants can put up blue stars in the night sky. The alien then cross references bright blue giants in every galaxy to try and find the planet with that constellation.


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