How to verify correctness of a galaxy map?

I'm trying to imagine mystery setting when amateur astronomer stumbles upon a torrent which contains a 3D map of the Milky Way galaxy. The map contains all large objects: star systems, planets, nebulae, black holes.

Is there a way with our current knowledge to check is the map real or just an object of someone's imagination?

• If map is not from Earth (stellar neighborhood) or in "real-time" it will be distorted by speed of light (we see dead stars!) in relation to what we can see, some information (novas, for example) might be some 50 thousand years old. By the way: XYZ positions for stars alone (some 400 billion) using half decent 128-bit decimal would sit around 18 gigabytes. Add small picture, say measly 100 kilobytes, for each star and suddenly you look at some 150 terabytes :D
– PTwr
Dec 30, 2016 at 15:03
• How old is the map? The positions of the stars change drastically over time; after several million years, the map could be pretty useless! Dec 30, 2016 at 15:22
• @PTwr Wow that's a lot of data. My idea is for 3D interactive map, where every star system is shown with major planets. I want orbits of each body, and it's composition. Similar to space engine / universe sandbox. Images are not required, just CG data is fine. Except for some special system. Dec 30, 2016 at 17:33
• Reminds me of a short story where a present-day astronomer discovers surprising photos in his latest image set that show the galactic center taken from another angle than usual. And there is evidence on those photos that a jet from the core is making its way towards the solar neighborhood. I can't remember the name of that story though, probably from the 90s. Dec 31, 2016 at 13:11
• I would like to add that instead of a map, a model of the galaxy with a time-stamp and a bit of state information, and the software required to run this model would be even better (and more spectacular!) than a map. More spectacular because using 3 bodies is very difficult to predict future positions; using hundreds of billions is far beyond our current mathematics! Dec 31, 2016 at 15:51

Just look at the sky1

Your protagonist is an astronomer (albeit an amateur one). He'll most certainly have access to a telescope or similar apparatus that allows him to start verifying the positions of certain stars or clusters relative to earth.

From there it is just a process of elimination, the more star locations they can verify using these methods, the higher the probability becomes that the map is *real.

On the other hand, anyone2 else will be able to do the same, so this method only allows you to garner a certain amount of proof.

Still, thanks to exact coordinates of stars, it becomes almost trivial to look out for certain electromagnetic-wave-patterns that would confirm3 the coordinates to be valid.

1you silly you...
2well not really anyone, but at least astronomers and people with lots of money/time yes, anyone - because star charts are a thing now(thanks to at @Zxyrra for that phantastic bit of information)
3well not really confirm, more like telling you that the chances of there being something like a star are higher than lightsecond to the relative left of that source

• This would only allow the verification of stars (unless the protagonist has a radio telescope and a very powerful light based one to spot planets and black holes) If the map was made on earth it is concievable that the designer made the section visible from earth accurate and invented the rest.. Dec 30, 2016 at 12:10
• What @Bellerophon said - if this person is good at math and they have a clear sky they will receive some of the right information, but an off-Earth telescope may have more of what's needed. Dec 30, 2016 at 12:12
• @Bellerophon indeed, that is the issue the third paragraph talks about; it also proposes a potential way to prove the validity of other locations on that map to some degree - I do not think/know of any other possibility of certainly proving these coordinates in a more exact/sure matter, as even using the powers of mathematics to calculate star positions will only be an assumption until proven undeniably.. Dec 30, 2016 at 12:12
• Could we check the planets if we know what & where we should be looking at? Maybe point some of our telescopes at some habitable planet that we overlooked. Dec 30, 2016 at 12:14
• @slobodan.blazeski have a look at the second part of the third paragraph; we could do that indeed. If we have exact numbers telling us where to point our looking-glass at, we can try to listen to a very very very very very very very very very very very small are of space, which would give us higher chances of detecting patterns that we know to be radiated by certain things such as a yellow star or a pulsar (or maybe even a moon?) Dec 30, 2016 at 12:18

As of September, we have a fairly accurate map of our galaxy.

By measuring the parallaxes of distant objects, we know the relative locations of about 400 million stars.

The satellite which gathered this information, called Gaia, also found billions of other potential objects. Most if not all of the data recorded, including the map your character needs for comparison, is open to the public.

After some comparison, it should be clear that the file they found is or is not accurate.

• I'm vouching for this as opposed to the other answer - while you may be able to map the galaxy with a home telescope you need a lot of spare time (hundreds of millions of distinct objects every night kind of free time) Dec 30, 2016 at 12:21
• Are all the objects even visible from our part of the galaxy? What about some star system on the opposite side of the galaxy. Wouldn't that be obscured by the galaxy halo? Dec 30, 2016 at 12:23
• @slobodan.blazeski While this map may not cover everything (it's far from doing so) this is as close as the OP can get. If the 400 million stars provided match the characters galaxy map then I'm imagining it's likely an accurate map. Dec 30, 2016 at 12:34
• @dot_Sp0T I suggest comparing the characters map to a map produced by the European Space Agency and located on its official website; I can assure you this isn't "another map found on the internet". Dec 30, 2016 at 12:36
• @dot_SP0T furthermore you are welcome to watch 400 million stars with a home telescope if you will only trust a primary source; but until that day I think the ESA's map is reliable enough for the OP to use. Dec 30, 2016 at 12:50

Zxyrra's approach of comparing to a publicly-available, trusted map has a flaw: the person faking the map could have downloaded the same map you're comparing against, then added the planets by making that data up. All the data that exists on both maps would be the same. That'd be pretty obvious (different maps should not be exactly the same, there are surely mistakes in both). A more clever forger would of course introduce some errors, and move things a little within the error bars, etc. Of course, all of this will need to be done by computers—not like you can eyeball two maps of the galaxy and say if they're the same. So both the forger and especially the person checking need to be skilled.

Checking for forgery essentially will involve outsmarting the forger: find some signature left by the algorithm the forger used to generate the new data.

There is another, easier way that doesn't require outsmarting the forger: wait. Astronomers are, fairly routinely, announcing newly discovered planets. You can check new discoveries against your map—any that agree with your map and occurred after you downloaded the map are evidence the map is true; any the disagree with your map are evidence it isn't (or at least is incomplete).

• That is the plot in my story. Comparing planets that have not be publicly known with the map. The initial idea is that astronomer considers the map as beautiful but speculative compilation of public data. When the newly discovered planets match the map the enigma begins. Did the map author has inside knowledge of the planet hunters unpublished data or the map is really of alien origin. Dec 30, 2016 at 17:38
• @derobert Eyeballing two maps of the galaxy for differences works if GallPeters is writing an NCIS script for television or similar. It might even be viable in more serious mode if there was something specifically being checked for, like quasar positions. Those you could check off directly. I still like this answer overall, but I don't want Gall to dismiss all quick checks, as that may move the story along better.
– SRM
Dec 30, 2016 at 22:03
• @SRM indeed, but it's tagged reality-check, so... A more realistic version would be if the computer finds things it "thinks" are different and displays them to visual confirmation. That'd probably work both on TV and in reality. (No doubt in TV the display would zoom in on both maps to display the differences, in reality you'd leave out the zooming) Dec 30, 2016 at 22:07
• Especially since 400 million isn't even 4 percent of the lower bound of the number of stars in our galaxy. Dec 31, 2016 at 15:56

Find an object on the map that is unknown but is within the detection capabilities of the best telescopes but no others. Point one at it.

Since the big scopes have good records as to where they have been pointed you can rule out prior human knowledge.

While this doesn't prove the map to be accurate it does prove it to be of alien origin.

Information theory 101: Either you know something, or you don't.

The map can be falsified (proved false) if it describes something physically impossible or extremely improbable. However, a sufficiently motivated forger can avoid this by running physical simulations of the map and verifying that it is physically stable over the short-to-intermediate term (which is exactly what you would do to check for these physical impossibilities in the first place).

Otherwise, it's a matter of comparing the map to reality. The problem with that is twofold:

1. Some aspects of reality are already known, to both you and any sufficiently well-motivated forger. Because you already know these things, a genuine map tells you nothing interesting about them, and a good forgery will contain them anyway.
2. Some aspects of reality are unknown, to both you and any forger. Because you do not know these things, you cannot use them to verify the map.

If you need to know whether the map is real based on the same set of information as the forger has, you are out of luck.

You could wait for more aspects of reality to become known (i.e. for more stars and exoplanets to be discovered), or perhaps use nonpublic information depending on who you work for (e.g. the military?), but the above dilemma continues to hold: Everything which is learned is now known, so the map can no longer provide new information about it. And that's assuming that science follows a straight line from unknown to truth without passing through falsehood, which is unrealistic. It would perhaps be more accurate to say that there is a continuum between (1) and (2), and it's often difficult to know exactly where you are on that continuum. It's possible that a genuine map might disagree even with supposed "known facts," if our understanding is badly incorrect.

Perhaps, after enough new stars and exoplanets have been discovered, and the map has agreed with these discoveries sufficiently often, you will conclude that the map is genuine, rejecting the possibility that a forger managed to guess all those discoveries by chance alone. But how often is "sufficiently often" and how many discoveries does this take?

To answer that, we (usually*) use statistical significance testing. Basically, you imagine (or simulate) numerous forgers creating numerous fake maps, and try to figure out what fraction of those fakes happen to look at least as realistic as the map you actually have. If this number (called the p-value) is very small, you can argue that it's unreasonable to continue believing the map is a forgery. Your definition of "very small" (the significance level) will depend on what (if anything) you plan to do with the map once you know it's genuine. If you're going to launch a generation ship at an exoplanet, you will probably be a lot more cautious than if you're going to point a space telescope at an interesting area of the sky for a few days. If you don't plan on doing anything in particular with the map, perhaps you should ask yourself why you care about its accuracy in the first place. That reason will inform your choice of significance level.

If all that is a bit hard to follow, here's the short version: You can wait for some more celestial bodies to be discovered by astronomers, compare the newly-discovered objects to those in the map, and use that to prove the map is (probably) real. But if you do that, you won't be able to use the map right away, and by the time you are able to use it, it will be partially redundant to the newly-discovered information.

* This link included for completeness; you can completely ignore it if it doesn't make sense to you.

• Excellent answer ! (that deserve its relevant xkcd) Dec 31, 2016 at 13:03

Does the OP have any idea of hat a 3D map of our galaxy showing all the stars and planets would be like?

There are at least 100,000,000,000 stars in our galaxy and possibly a few times that many. And a large percentage of them have planets so there are billions of planets in the galaxy.

And how could you show everything to scale on a map? In our solar system the eight planets have average distances from the Sun of 57,909,000 to 4,500,000,000 kilometers - Neptune is 77.708 times as distant as Mercury. A light year is 9,460,700,000,000,000 kilometers. The nearest star is Alpha Centauri about 4.30 light years away, or 2,100.33 times as far as Neptune. The diameter of the galactic disc is about 100,000 to 120,000 light years, or about 23,255.813 to 27,906.9 times the distance to Alpha Centauri, or about 48,844,854 to 58,613,699 times the distance of Neptune.

So your map should be a gigantic computer database with all the information and the ability to display 3D maps of selected objects.

The way to check its accuracy would be to find out what astronomical projects are looking for objects in the galaxy and planning to do so in the near future. You will want to have a computer program to automatically record all announcements of discoveries of objects in our galaxy and automatically check against the data base.

If you want to prove to others that you have a accurate database you will want to search the data base for relatively small group of objects that are being currently searched for. For example there are only a few hundred star systems within a few tens of light years from Earth. Finding them all is important to getting a good unbiased sample of the stars in this part of the galaxy. So you can make a list of all star systems within 30 light years of Earth that have not yet been found to be that close and publish their distances and when astronomers check them and find you are correct that will convince many that you do have an accurate database.

• A map is not necessarily a database full of bazillions of things. A map is a projection of these things. Such a projection could be calculated at runtime as needed based on efficient complex algorithms and thus data will only take up space when it is requested. Jan 28, 2017 at 16:54