In the near future a group of explorers land on Mars and proceed to explore the cave system under Pavonis Mons.

While exploring the gigantic interior of the lava tube, sensors indicate the presence of an unexpected kind of emission. Using remote probes they detect radiation levels that are safe for humans - but still, being on the safe side, they continue to explore using drones only.

They manage to locate the source while exploring the deepest parts of the cave system. As they send in a tethered drone packed full of sensors (CCDs, spectrometers, radiation detectors, environmental detectors and atmospheric detectors, among others) a huge stream of data start to pour from the sensors into the expedition computers as the science officer orders it to focus on a central piece standing atop what looks like a obsidian pillar; It clearly look like an artificial object.

The drone maintain its emotionless glare as the science officer asks for a few more minutes to analyze the incoming data, followed by a request for a couple more hours - and finally another for a day.

The science team then communicates to mission command, to the shock of all involved personnel, that the data points to an extra-galactic origin.

Question: How could they, based on remote sensory information only, reach this conclusion?


  • Near-future technological resources (2051-2100)
  • The device may be resting, slotted in or kept stable/in place by a powered mechanism
  • The drone packs a modern, beefed-up, expanded version of the MSL Science Payload in a sleek yet super creepy crawling body.

(Given the interesting points of view of some answers I dropped the hard-science tag, but would still like to keep it reasonably not so mushy.)

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    $\begingroup$ Within hard-science or even science-based, the only answer is a strong "They can't". Cosmological principle is still going strong, which means there is no difference in physical properties between different parts of observable universe. As such, any extragalactic material is indistinguishable from intragalactic material. The only clue, and decisive one, that something is extragalactic, is trajectory, which does not apply in your case. StephenG's answer is not conclusive either, because why couldn't the message be a lie? $\endgroup$
    – M i ech
    May 28, 2018 at 10:11
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    $\begingroup$ The only thing I can think of is the presence of some isotope, or isotopes, not found in our galaxy, which means not found in any galaxy in the observable universe. It would have to be an element that can't be produced in the modern universe, but even that doesn't actually mean the constructed object is necessarily from outside the galaxy, just that some of the material in it predates the end of the big bang, it could have been here in the neighbourhood for 14 billion odd years and someone found it and put it in a bottle relatively recently. $\endgroup$
    – Ash
    May 28, 2018 at 14:51
  • $\begingroup$ @Miech valid points. Initially I tagged the question as hard-science because when I started writing this scenario I was hoping for a different POV, like the article mentioned in this question from the Astronomy Beta. I always thought you couldn't discern matter-generated light from antimatter-; so maybe there would be a similar way in this scenario. $\endgroup$
    – OnoSendai
    May 28, 2018 at 15:54

8 Answers 8


You won't figure this out from physical properties. There's essentially no difference between what's available here and anywhere else.

a huge stream of data start to pour from the sensors into the expedition computers

This is the key.

The device sensors are not just detecting passive emissions, they pick up actual data streams (or what can be identified as data streams - e.g. a signal with a digital basis - two levels, on and off). These have been started because the device detects (or is turned on) by an active sensor signal.

When this is noticed the data is analyzed and someone (or a computer designed for pattern detection or something like that) decodes enough data to determine a basic encoding in the stream (which is repeating ?) and there's some kind of location embedded in it.

How do you encode that ? Well we did it this way. :-) There are various ways to encode a digital signal with a basic image that can be detected.

That could tell us a location with enough accuracy to locate another galaxy.

  • 3
    $\begingroup$ Oh, I see... So the same way you can pass Morse code through a light detection diode, and infer way more data than just presence/absence of light. That's clever. It doesn't make the artifact itself extra-galactic, but may point towards the extra-galactic source. $\endgroup$
    – OnoSendai
    May 28, 2018 at 3:00
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    $\begingroup$ In case it's of interest : Wikipedia's page on Communication with extraterrestrial intelligence $\endgroup$ May 28, 2018 at 3:33
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    $\begingroup$ So you say that they can not know if it's extragalactic, unless the artefatct itelf commmunicates this to them? $\endgroup$
    – b.Lorenz
    May 28, 2018 at 6:37
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    $\begingroup$ @b.Lorenz That's what I'm saying. At the risk of sounding silly (on WB ? :-) ) it can be as simple as a repeating message saying "Property of Microsoft Andromeda. Opening voids warranty. Contact head office at ... for repairs." - you know what these extragalactic corporate lawyers are like. :-) $\endgroup$ May 28, 2018 at 13:52
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    $\begingroup$ Unfortunately galaxies don't have "chemical fingerprints" or such distinct enough that faking one locally would be harder than bringing a piece of distant galaxy. The very artifact could even be made from relatively nearby planets - or even Mars matter - by an extragalactic Von Neumann probe. $\endgroup$
    – SF.
    May 28, 2018 at 15:23

OK, this is a tough one.

An artificial object will not be expected to have a natural composition. As such even determining extra-solar status would be difficult. There is also the problem that most galaxies are roughly the same in composition in terms of what they are made of.

Some thoughts.

The object is made of anti-matter. As far as we can tell our galaxy is made of matter. A large object, like an asteroid suspended in an electromagnetic field, made of anti-matter would be unlikely to find in our galaxy. If they found one that has been isolated by some device, there is a good chance it is extra-galactic in origin. Note, I am saying the object itself is clearly a naturally occurring object, if it was just a mass of antimatter plasma, then it was probably just manufactured.

The other idea I have is radio-isotope dating. We think the milky way is about 13.51 billion years old. The universe is 13.77 billion years old. If we dated the object to older than 13.51 it predates the milky way. Now most galaxies all formed around the same time, but predating the milky way is from outside the milky way.

For example. Long term geological dating can be done using Uranium->Lead Dating. Uranium-238's decay to lead-206 has a half life of something like 4.5 billion years. So if it seems like the object originally had a Uranium-238 core (maybe its clear its a nuke or a reactor), but half the core mass is lead-206, about 4.5 billion years has passed. If 3/4 of the mass is lead, then 9 billion years has passed. If only 1/8 is still uranium, then 13.5 billion years has passed. This method tends to have a 95% accuracy.

If you drop the artificial object there may be some other possibilities.

  • $\begingroup$ Very good points. About anti-matter: In that sense the 'device' may be what holds the antimatter (a natural piece from an extra-galactic source) in place. But wouldn't it emit x-rays the same way as artificial amounts created here on Earth do? Now, the time-based conjecture is really interesting. How would the SO proceed, or what technique would she use, to date something that old? $\endgroup$
    – OnoSendai
    May 28, 2018 at 0:32
  • $\begingroup$ With antimatter, it would be a much more plausible scenario that the antimatter was manufactured in our galaxy rather than natural antimatter (especially considering the that external containment device would have to be made of regular matter or else big-badda-boom.) $\endgroup$
    – Gene
    May 28, 2018 at 1:38
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    $\begingroup$ @Gene, agreed if it was a small amount. Something like an antimatter asteroid however would be extremely unlikely to be created in our galaxy (the antimatter that is produced has a short shelf life). I was thinking it would be something natural (like an asteroid) that would have been somehow brought to our galaxy and held in something made with regular matter $\endgroup$ May 28, 2018 at 2:09
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    $\begingroup$ @OnoSendai, The antimatter would be no different than the stuff we manufacture in colliders. However, you would not find a big chunk of antimatter in our galaxy. It is going to interact with the interstellar medium and go away. An antimatter asteroid would likely come from some sort of antimatter galaxy. I've edited my answer to explain how it would be dated. $\endgroup$ May 28, 2018 at 2:13
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    $\begingroup$ Is radio-isotope dating reliable in this case? If anything, we'd perhaps assume the Milky Way is older than we thought it was, not necessarily that the object originated somewhere else, right? $\endgroup$
    – Morgan
    May 28, 2018 at 3:36

The object is made of exotic matter.

There are several options.

  1. Super-heavy elements (https://en.wikipedia.org/wiki/Island_of_stability). These are not that freaky and you could hold some in your hand. They are theoretical elements of number 300+ which are stable in a way that other high number elements are not. It is cool to think about but I am not sure how it would produce huge streams of data, though.

  2. Negative matter (https://medium.com/the-physics-arxiv-blog/cosmologists-prove-negative-mass-can-exist-in-our-universe-250a980320a7). This stuff has gotten plenty of press on the Stack because it is needed for the Alcubierre warp drive. It might make some pretty weird data.

  3. Neutronium. This is the ultra dense stuff from the heart of neutron stars that Superman makes his housekey out of, so no-one else can lift.

  4. Other freaky matter not from around here: dark matter, strange matter. Stable muonium. Antimatter.

My favorite though: the thing atop the pillar is not an object. It is a hole in space / time. The streams of data are because active detection methods (using electromagnetic radiation or sound) go through the hole and keep going. What is received is what comes out of the hole from the other side. This is changing second by second because what is on the other side is changing second by second. The probe, receiving loads of wildly variable information unrelated to what itself is putting out assumes error, assumes reflection, assumes transmission, assumes noise, assumes error...

"Probe, are you sure that this thing is of extra-galactic origin?"


"Do you know where this thing is from?"


"It's from HD2112?"


"... Probe, it seems pretty small to be a star."


  • 2
    $\begingroup$ I like the perspective of actually not being a physical object, but a stable Einstein-Rosen bridge with well delimited dimensions. The sensor are then peeking through. I like the other suggestions a well, but I'm having a hard time imagining how the Science Officer would determine, with certainty, that it is in fact made of Negative matter. Casimir effect, perhaps? $\endgroup$
    – OnoSendai
    May 28, 2018 at 3:08
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    $\begingroup$ Given that the object is clearly artificial, the fact that is made of exotic matter does not imply it has not been built in this galaxy, as the matter could have been manufactured as well as the rest of the artifact. We have made antimatter here on Earht, for example. The hole to another galaxy, though, is a clever idea. +1 $\endgroup$
    – Rekesoft
    May 28, 2018 at 7:14
  • $\begingroup$ Assuming any of those types of strange matter can exist in stand-alone form as fashioned artefacts (most can't), there is exactly no reason to assume extragalactic origin because physical laws are same in entire observable universe. And if laws weren't same, there's no reason to assume that unusual matter would actually survive crossing the boundary. $\endgroup$
    – M i ech
    May 28, 2018 at 10:10
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    $\begingroup$ An object made of artificial exotic matter is arbitrarily advanced not necessarily extra-galactic in origin. $\endgroup$
    – Ash
    May 28, 2018 at 12:01

There are challenges, but the TLDR is to use pictures.

The first problem is even if an Alien species wants to communicate with us, how will they and how will we understand?

Communicating between two completely different civilizations that probably evolved in completely different environments is not so easy as the movies make it seem.

From Wikipedia:

CETI research has focused on four broad areas: mathematical languages, pictorial systems such as the Arecibo message, algorithmic communication systems (ACETI), and computational approaches to detecting and deciphering "natural" language communication. There remain many undeciphered writing systems in human communication, such as Linear A, discovered by archeologists. Much of the research effort is directed at how to overcome similar problems of decipherment that arise in many scenarios of interplanetary communication.


If you want the people to receive radiation or some kind of radio signal and be able to understand it without some kind of context clue, that is just not happening. For a few reasons:

  1. How will they know how to decipher the signal into something that makes sense? Think encryption. The information you are getting might have a meaning. However it will be difficult or impossible to convert that into its intended form with out key information.

    As an example lets use information that we put out into the cosmos. If you received something as simple as a radio signal for a television/radio broadcast and didn't know what a radio or television was then how would you decipher it? You would need an audio source or screen display, and if you didn't know that was the intended way to use the signal then you might never know what it contained. So there is the accompanying knowledge of how to interpret/process/display a signal that will be missing. This will prevent the team from coming up with a solution to decode the signal.

  2. Even if there are coordinates. Who is to say that the aliens use the same coordinate system as us? If they don't then the humans are going to be in for a very hard time. If their math looks nothing like ours and or uses concepts we haven't even discovered yet then it is going to be even harder. Think explaining imaginary numbers to an ancient roman who still uses numerals.

The best your team will be able to deduce at this point is that the object is in fact manufactured, and then because no humans have reached this location before it might be... ALIENS!

If you really want there to be some kind of information exchange you may want to try pictography instead. AKA have a screen that shows pictures or something like that. Pictures is the current global plan for how to communicate with aliens in the event of contact. It is something we can all presumably understand.

As an example, check out the Voyager 1. It identifies our location as an etched drawing of our location relative to the local pulsars. For an advanced space fairing civilization something like that should be straightforward to figure out after enough brainstorming. You could try something similar, or checkout out the other info the Voyager uses.


How these folks did it is probably your best bet:

enter image description here


The data could include a picture of our galaxy, from the outside far away, with enough detail to locate our sun.

While it could potentially be faked, we have enough information about stars in our galaxy to verify the picture to a high degree of accuracy.


The object has to do two things: (1) carry information that argues it's from outside the galaxy and (2) have some property that makes it believable.

A photo of the galaxy with a "you are here" arrow, combined with another image of a galactic group with a "and we come from here" arrow, would carry the argument. (It would be particularly mind-blowing if they were up-to-date images)

But images can be faked. What kind of evidence can be provided? Particularly evidence that can be easily evaluated.

How about having a density of 50 gm/cc? Nothing on Earth can approach that, but it's expected of elements in the 2nd island of nuclear stability. That's evidence of being from a very different stellar environment (or having the technology to fake it)


The key is in the way the data is modulated onto a carrier frequency.

How do we know a galaxy is far, far away? We look at the redshift $z$ of the spectral lines of a few elements, in particular Hydrogen's 21cm line. The drones can carry out spectroscopy of the carrier frequency and find large values of $z$, leading to the conclusion that the origin must be from way beyond even the Andromeda galaxy (which has a blueshift since it is coming closer).


A detailed map of another galaxy, with a partial map of ours, along a fairly direct route back to the other, would be enough for the data to point to (though not to prove) extra-galactic origin.


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