Similar to how the famous Voyager "Pulsar Map" image was an attempt to define a given location in space in a way an alien culture could decode and find Earth -

How could you create an equivalent image for a given instance in Time?

Combined with the Pulsar Map, you might be able to use the Earth as a point of reference. Earth's specific location in Milky Way Galaxy, or against Cosmic Background Radiation (if that's even possible?)

Alternatively maybe the number of Hydrogen Atom Vibrations since the Big Bang?

Thoughts and Ideas?

enter image description here

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    $\begingroup$ Relativity teaches us there's no such thing as universal time. Time passes differently in different places depending on the local geodesics. Can you tell us a bit more what you might want? $\endgroup$ Commented May 14 at 3:17
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    $\begingroup$ Time is rediculously harder than space in this respect. What sort of timeframe do you want the scene to be valid for, and how accurately do you want to pin down the time at which an event occurred? The tools to specify a day in a century will be very diffierent than the tools to specify which 10,000 years out of a million years an event occured in. $\endgroup$
    – Cort Ammon
    Commented May 14 at 3:36
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    $\begingroup$ I am erring towards VTC - a golden rule on here is if a question could be an entire book in order to answer, then it is not a question for here. A Quick think on this leads me to suspect that the answer is such a deep rabbit hole of Physics - that one could write a doctoral thesis on it. I haven't voted yet and am open to differing opinion. FYI - I think this is a great quesiton... Just not one we can answer. $\endgroup$ Commented May 14 at 4:52
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    $\begingroup$ I’ve voted to leave open… the underlying question is a reasonable one, and the answer being “relativity hates time and sci-fi” is a valuable one. $\endgroup$ Commented May 14 at 7:18
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    $\begingroup$ Like if everyone wants to be pedantic, there is such a thing as a point in spacetime, so we can communicate to the aliens the precise point in spacetime that the probe launched. (They can then convert this to both space and time relative to any reference frame of their choice.) $\endgroup$ Commented May 17 at 13:37

6 Answers 6


Put a clock on it.

You can do pretty well pretty easily if you put a lump of metal in a box and some pictograms to tell the aliens what isotope was in the box when you launched it. The aliens can analyze the lump of metal and determine when it was launched the same way we do carbon dating. (Not carbon, though, since we need a long-lived radioisotope if we want a mission clock that will run for millions of years). Pick the isotope so that the half life matches the mission time, or have several boxes.

If you have very good measurements of very predictable trajectories that match your mission time, especially very linear, very big, very distant trajectories, an initial distance (plus the aliens' measurements of the current distance) is a good clock. For example, we can tell the aliens that we launched the probe when the center of mass of the Andromeda Galaxy was 1.78 gazillion disk widths away from the center of mass of the Milky Way.

In any case, the longer the mission time, the bigger your error bars will be - but it's not like the pulsar map gives you pinpoint precision, either.

The comments are being kind of silly. The pulsar map also doesn't work if you can't assume a universal frame, since angles also transform with velocity and curvature.

Fortunately, almost all matter in the entire universe, excluding black holes, is in the same reference frame. Unless ET is a hyper-intelligent jet of plasma or a particularly clever smear of black hole accretion disk, ET is already in the same frame as us. And since ET can see that our space probe is not a jet of plasma or a smear of black hole accretion disk, ET knows that the space probe has spent its most of its history in the same frame as ET. So both the pulsar map and whatever clock we put on it will work just fine.

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    $\begingroup$ Most of the matter in the Local Group of galaxies is moving at relatively low velocities with respect to us. But that is not the case for stuff outside the Local Group, which is by far most of the matter in the universe. Universal space expansion and all that. $\endgroup$
    – AlexP
    Commented May 14 at 6:59
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    $\begingroup$ @AlexP You're mixing up incompatible velocity definitions. Peculiar velocity, not the rate of change of proper distance, matters for clocks. $\endgroup$
    – g s
    Commented May 14 at 7:16
  • $\begingroup$ Point of pedantry: the pulsar map isn't quite correct anyhow, as we'd confused the precision of the frequency. $\endgroup$ Commented May 14 at 8:43
  • $\begingroup$ Another note: if the thing carrying the clock/lump of radioisotope/whatever is traveling at a low speed relative to light relative to Earth, then by definition in order to slow down and read the disk, an ET will have their frame line up at least mostly with Earth’s. $\endgroup$ Commented May 16 at 14:03
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    $\begingroup$ @KalleMP Unfortunately 238Pu with its ~90 year half life will all be gone by the time the Pioneers and Voyagers get anywhere. Although the resulting 234U might make a good mission clock. $\endgroup$
    – g s
    Commented May 17 at 15:10

Pioneer already carries the information in its velocity. If the reader knows where it has come from using the pulsar map, then they can work out how long it took to get from there to here given the speed it arrived at.

This will not work if it has travelled a huge distance, and the reader cannot recognise the pulsars. It will not work if Pioneer had gravitational slingshots from unknown bodies. However, the Pioneer anomaly within the solar system was a pretty tiny departure from the trajectory based on just gravity, so the arrival time should be a pretty good clock, when corrected for any known gravitational effects. As the trajectory was drawn on the pulsar map, they can probably find the best fit to the map, Pioneer's velocity, and the arrival time.


This is a Frame Challenge

The reason the pulsar map is so cool is because it's something that exists in three physical dimensions in space. The problem with time is that it only exists in a single dimension. Yes, the traditional face of an analog clock is often used as a metaphor for time, but when it comes down to it, we represent time with a series of names and numbers.

May 19, 2024; 22:52:25

That's a single point in time. I'm ignoring the fact that an infinitely precise point in time is as valueless as an infinitely precise geographic location. But there aren't multiple time linesCitation Required that can be used to identify that one, specific point we want as there are multiple pulsars that can be used to define that one location we want.

Making this worse, as has been pointed out in comments, we don't really have a way to identify a moment on Earth that's specifically a moment on any other planet. (Reference) So, to add the proverbial insult to injury, not only is time a 1D event that doesn't need or deserve a multi-dimensional map to identify just one point, but that 1D point is only valid for the frame of reference you're standing on. Even something like hydrogen vibrations isn't universal as they speed up or slow down due to relativistic effects. Just as time would proverbially stop for someone traveling at the speed of light, so would the hydrogen vibrations. How, then, does one measure time?

So, cool as it might seem, there really isn't anything cooler than a traditional timeline to specify a range of time and a date and time reference for a single point.

Now, you could hybridize a solution (somewhat like the London Tube map is a completely accurate map of a subway that has no useful reference to geography). You could use pulsar locations in the galaxy to identify a single point in time. Use two galaxies worth of pulsars to create an interesting map. However, this has two weaknesses:

  1. Precision is not your friend. Pulsars, like everything else in the universe, require either ultraprecise detection methods to determine useful location shifts for time keeping or a honking lot of them so that every minor shift has meaning.

  2. Things change over time. The pulsar map works because compared to astronomical time nothing's really changing. By the time the map becomes obsolete, Earth's sun might have exploded. However, when measuring time rather than location, things do change because astronomical time comes into play. Stars are young, then age to old. I once asked how to determine your location if you could make a series of magically zero-time jumps from Earth to a distant star. The reason this is interesting is that as you approach the star its location changes and it grows older, possibly extinguishing/exploding before your ship arrived. Thus, the objects in space used as time references have a beginning and an end and therefore can only be used to represent a subset of the universe's time.

And, once again, this is only relevant in one frame of reference because the measurement of the location of said pulsars changes depending on the mass you're standing on. So, in the end, we really don't have a rule-of-cool way to graphically represent a single point in time that's truly universal.

At least not yet. Who knows what some clever scientist might discover? If science really wants to impress me, we would discover the universe's "minimum discrete time step." We call that a second (even though we break that into near infinite fractions), but our basis for time started as something quite arbitrary based on Earth's orbit, rotation and the Babylonian penchant for base-60 math. But what if the universe had a minimum discrete step?

If you knew that, you just might make the map you're looking for.

BTW, I truly believe there's no way to achieve the goal you've set. If I'm wrong, then your quesiton should be closed as opinion-based as there's no best answer that isn't based on aesthetics. It's like asking what the best color is. But I believe the deterministic and objective answer is, "you can't."

  • $\begingroup$ Since planetary bodies are in motion, surely multiple "frames" of animation could be printed, along with their "frame rate" (would be very slow). The number of Earth's orbits relative to larger astronomical changes could be conveyed by scientists clever enough to lay them out and notate in a way that the association could be seen. If Planck Time could be worked into the reference, even better. I know I'm using broad concepts, but still - I'm not sold on "you can't". (Although I like the other aspects of your answer.) $\endgroup$
    – Mentalist
    Commented May 15 at 7:01
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    $\begingroup$ @Mentalist You might be misunderstanding what I'm talking about. When I use the phrase "frame of reference" or the word "frame," I'm talking about Einstein's Theory of Relativity. Things are only perceived according to your "frame of reference" (e.g., from the planet Earth). Before buying entirely into your disbelief of "you can't," learn more about Relativity of Simultaneity and more also about Special Relativity. To give you some insight, look how messy this question was. $\endgroup$
    – JBH
    Commented May 15 at 10:15
  • $\begingroup$ Did you maybe get confused re. the Mercator example? The Mercator projection is the standard map projection used for 2D world maps, not the topological Tube map. $\endgroup$
    – aantia
    Commented May 15 at 11:31
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    $\begingroup$ @aantia Thank you! You are correct and I didn't catch my mistake. I've updated my answer. $\endgroup$
    – JBH
    Commented May 15 at 14:58

Universal decay of regions- the universe is a machine eating itself, hydrogen to helium to yada.. yada to iron. Short periods in between where the slag is radioactive.

So why not define a point in time, by carbon-dating the surrounding region? As in how much original hydrogen is left, how much at this point had decayed to what material, stored in planets, suns and nebula. Like a hyper precise material footprint, that if read right, even doubles as a star map.

No idea about how such a layout would look. A voxelmap, with the voxels being the https://en.wikipedia.org/wiki/Periodic_table

PS: Such a footprint, would of course have to take into account that the whole region is moving and redistributing, so it needs a sort of fluid simulation of the galaxy in addition

PS: The whole process produces all sorts of radiation, so there is a additional fingerprint in the shells of light surrounding any interstellar object. A star was at position xyz traversing to nxnynz and this "route" can be seen as recorded in a sort of tree-layering of light, spreading ever outward.

All of these "indicators" of time, would be referenced by the one thing universal- the big bang as starting point. That one thing unfolding into the simulation surrounded by the light-shell, that plasma blob.


Slightly different answer: do you want something like a Treasure Map? Something that is pure information, so it could be coded in a radio signal or DNA molecule?

You could do this if the map also pointed to an event. We could add the position of the Crab Nebula. This would give a position in space and time, as the year it went BANG. If you have defined distance intervals, then you can define time intervals using the speed of light.

This will not be nearly as accurate as using Voyager's velocity in my other answer. Even if they did not witness the origin of the Crab Nebula, they can probably reconstruct it with reasonable accuracy. They will have to correct the map for the motion and interaction of the various bodies, and do relativistic corrections for their time frame. But you have to allow for things which may have changed with all maps.


Similar map could point to a specific universe time, too. Relative position of heavy objects to one another could be calculated backwards and forwards. For example a map of a star system with enough orbiting stars, white dwarfs, black holes to avoid repetitive positions could be unique in a degree to serve as watch hands.

Yes, the precision is as good as the fastest orbiting bodies (around 50 minutes i think) and aplicable on a limited time range (but still could be wide enough) but for a time-travel it could be sufficient.


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