A portal about 30ft in diameter appears on the surface of our present-day Earth. It appears for about 5 minutes each day before disappearing. Hesitantly, scientists go through it and discover a new world! This new world is "Earth-like" in that it has a similar ecology and atmosphere to earth, but it's pretty clear to the scientists we're not on OUR present-day Earth anymore. The days are a bit longer, gravity's a touch lower, and we're pretty sure we've never seen those species of animals and trees before.

The discovery of this world has led to some wild theories. A few say the portal is traveling us through time. Some others have suggested that the portal is leading us to another universe that developed differently from our own.

However, the reality is that it's simply another planet that lives in our very own, present-day, Milky Way galaxy.

My questions:

  • What's the fastest way for them to prove that? How long would it take?
  • Is it possible to estimate where this new planet is in relation to ours? If so with what level of accuracy?


  • There's basically no light pollution on the new world.
  • While it's not a blank check, we've got a very high budget to figure this out.
  • We can safely bring whatever we need through the portal so long as it fits.
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    $\begingroup$ BTW, both "longer days" and "lower gravity" already exclude that the remote world is Earth in the past or future, because either the days were shorter together with gravity being smaller, or days will be longer but the gravity would be higher, due to mass accretion process Earth still undergoes. $\endgroup$
    – Vesper
    Commented Aug 11, 2022 at 11:02
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    $\begingroup$ @Vesper This is not true. Earth's rotation is naturally slowing down as a result of tidal effects with the Moon -- roughly 1 hour every 200 Myr. $\endgroup$ Commented Aug 11, 2022 at 13:17
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    $\begingroup$ @EthanManess yup, but I exactly meant that. Earth was lighter in the past as well, because it did not lose mass in form of dust, while it also accretes and is still accreting mass from various asteroids, meteorites and dist falling down, thus shorter days on Earth were corresponding to lighter Earth so its gravity was also less than current. In the future Earth's days will be longer, but its gravity will also be higher because it will accrue more mass. OP said the planet has longer days and lower gravity than Earth's, so that planet is definitely not Earth. $\endgroup$
    – Vesper
    Commented Aug 11, 2022 at 13:57
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    $\begingroup$ What, exactly, does 'present time' mean? To a planet on the other side of the Milky Way from us, looking back at us, they would be 'seeing' ancient history. Something like 100,000 years ago. Is travel through this portal instantaneous? No matter how you do it, someone in one place or the other is 'looking back in time' from the perspective of the 'other place'. 'Handwaving away inconvenient details' might be necessary. $\endgroup$ Commented Aug 11, 2022 at 15:24
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    $\begingroup$ Is the hydrogen/helium loss all atmospheric? If so, it won't lower the gravity at the planetary surface. Unless I'm misremembering my physics calculations, the gravitational acceleration is only calculated from the mass below you, so atmospheric loss greater than cosmic dust acretion on the surface would still end up with higher surface gravity. Orbiting satellites and such, though, would detect the lower gravity, since they are above the atmosphere. $\endgroup$
    – user34314
    Commented Aug 11, 2022 at 20:08

8 Answers 8


Relatively simple astronomical observations of nearby galaxies such as Andromeda, Triangulum and the others in the local group should enable them to check if they were still in the Milky way relatively quickly. The first brave astronomers could easily be provided with a number of small but powerful telescopes to search the night sky. After being left on the far side of the portal for a day they should be able to tell if they were in the Milky way or not.

If this revealed that they were not in the Milky way then they would have to start more detailed astronomical investigations looking for known objects that were further afield like unusually shaped galaxies and specific pulsars. If a number of these objects could be identified then the observer’s position could be calculated at least roughly and then refined. This could take a long time.

If nothing at all was found then more distant objects could be looked for but it would take longer and longer because the sky would have to be scanned in ever greater detail by bigger and bigger telescopes that would have to be transported in from some distance.

In conjunction with the specific astronomical search they could also carry out a check on the redshift of distant objects to see if that matched what we see. And they could also bring instruments to measure the speed of light in a vacuum, the fine structure constant, Planck’s constant and other constants using ever more sophisticated means to check the value against what we know to greater accuracy.

Any significant deviation would suggest either they were in a different universe or that the laws of physics change in time or space in some part of our universe. The longer the time frame the more experiments would be dreamt up (Gamma Ray Busts from the distant universe - do they occur and do they follow expected behaviour in extent, direction and intensity?) and the portal itself would undoubtedly be investigated for clues. What happens if a cable is laid from one side to the other and left there during closure?

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    $\begingroup$ This is the most straightforward and easiest approach, even with amateur telescopes it would be be trivial to see that all the major objects in the local group are there and with a bit more tech be able to figure out the distances and locations of them must mean you are in the milky way $\endgroup$
    – eps
    Commented Aug 11, 2022 at 20:57
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    $\begingroup$ Depends a bit on the galactic surroundings you're in. Dead easy at the rim of the galaxy, could be hard if near the galactic core, inside a nebula, or near bright stars. $\endgroup$
    – toolforger
    Commented Aug 12, 2022 at 15:49
  • $\begingroup$ @toolforger Nebulas ae rather transparent, as you can see by looking at one through a telescope. The photos you see of nebulas that make them look opaque take many thousands of times as long to expose as the human eye refreshes its vision. $\endgroup$ Commented Aug 12, 2022 at 19:08
  • $\begingroup$ @M.A.Golding still we can't look at our own galaxy core due to nebulae and gas clouds, right? Not in the visual spectrum anyway. (Sorry for sloppy "nebula" terminology, I meant to say "any gas/dust cloud".) $\endgroup$
    – toolforger
    Commented Aug 12, 2022 at 19:29
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    $\begingroup$ @Karl You are correct if the locality is in the Milky way (although I doubt very much indeed that my grandparents would have been fine with messier's catalogue and a table of logarithms). Trying to determine the locality beyond the Milky way would be a lot harder. $\endgroup$
    – Slarty
    Commented Aug 13, 2022 at 8:48

Pulsar-based navigation!

It's like GPS only using pulsars. All you need is equipment capable of receiving the signals emitted by pulsars, i.e. a radio dish, and you can pinpoint your location anywhere in the Milky Way. Getting a radio dish through the portal shouldn't be a problem as you can dismantle it and reassemble it on the other side of the portal. It isn't even especially high tech engineering.

  • 11
    $\begingroup$ Might not work, in case pulsars used to navigate the solar system wouldn't shine on the remote planet. A pulsar is visible as one in a beacon-field shaped area in the universe, with beacon ray width of several degrees, if the pulsar is pretty close to Earth, like Centaurus X-3 and the planet is out of its "shining area", detecting that pulsar woudl fail. Yet, there should be enough pulsars in the local group to shine over the entire Milky Way to be used for this purpose. $\endgroup$
    – Vesper
    Commented Aug 11, 2022 at 6:50
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    $\begingroup$ Why so complicated ? The question is, if this place is in our universe. To assert that, just look up with a small telescope and try to find something familiar outside our milky way galaxy, such as Andromeda. If you don't find anything familiar, we're not in our universe. If you do, measure the redshift of something far away and you know what time it is (on a cosmic scale, in terms of millions of years) $\endgroup$
    – Goodies
    Commented Aug 11, 2022 at 9:11
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    $\begingroup$ @Goodies, in our galaxy, not in our universe $\endgroup$
    – L.Dutch
    Commented Aug 11, 2022 at 10:42
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    $\begingroup$ I believe the left-side of the Pioneer plaque is a map of how to find Earth based on pulsars. $\endgroup$
    – Daron
    Commented Aug 11, 2022 at 11:58
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    $\begingroup$ If you can't find Andromeda it means you a very far away. Or that you're in Andromeda. $\endgroup$ Commented Aug 11, 2022 at 15:51

Many of the answers here are already correct, I would simply like to add one safe method to determine if you travel through time, assuming that we know we stay in our universe: measuring the CMB. Since the radio pollution on the planet should be pretty low because of the lack of human radio stations, we can get a good estimate on the CMB temperature already from a simple ground-based radio telescope; if we manage to somehow get a satellite in orbit, we can get an even better measurement.

Using the CMB is convenient because it is (almost) isotropic radiation throughout the universe and should be the same even outside the current observable universe. This means that even if we do not recognise a single object on the sky, we can determine "when" we are. This is a huge advantage compared to measuring the redshift of a known object, like a specific galaxy, because if you do that, you are in principle unable to differentiate whether you are closer to the object at the current time or farther from the object at an earlier point in time, due to the expansion of the universe. (This is just in very rough terms; the fact that the expansion accelerates makes the matter very non-trivial.) Also, like I said, you would see completely different objects outside of the observable universe, so you cannot even start to compare them to objects observable from Earth.

In any case, the formula for the CMB temperature is:

$T = T_0 \cdot (1 + z)$,

with $T_0$ the current temperature at around $2.73 \, \text{K}$ and $z$ the redshift of the universe. We are currently sitting at a redshift of 0 (in fact, it is defined in the sense that we always observe at a redshift of 0). Measuring a higher CMB temperature means that you have travelled to the past, measuring a lower CMB temperature means that you have travelled to the future. You can even use this to estimate how far into the past or future you have travelled.

  • $\begingroup$ I like that answer. It feels like this would be a necessary first step, in prder to calibrate the other star observations. $\endgroup$
    – Dan Hanson
    Commented Aug 13, 2022 at 3:51

John's answer is correct.

A preliminary scan which can help assessing if you are in our galaxy or not can also be done by searching for known bodies and constellations in the sky.

If you don't recognize any constellation or local stars in the night sky, but you can still recognize known galaxies and nebulae and their relative positions are not too altered, this is a strong pointer to the fact that you have moved somewhere else in the galaxy: close-by star associations have been more greatly impacted by the change in observation point than more distant objects.

  • 4
    $\begingroup$ That will only work if the new planet is very close to Earth. The constellations in the night sky are all less than a few thousand light years away. If they got dumped literally 80,000 LY away on the other side of the galaxy, the night sky would be meaningless and they'd have to rely on their ability to see extragalactic markers such as the Magellanic clouds and Andromeda, which are naked eye visible in the right conditions. $\endgroup$
    – stix
    Commented Aug 11, 2022 at 16:49
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    $\begingroup$ Globular clusters would also work. We have mapped about 150 globular clusters, and can identify them uniquely. At peast half would be visible from anywhere on any body in the galaxy. Their relative positions would tell you where you are. $\endgroup$
    – Dan Hanson
    Commented Aug 11, 2022 at 17:51
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    $\begingroup$ @stix The scientists are from present day Earth, with a finite, but very high budget. If they just need to make observations similar to those we've made over the past few decades, then technology isn't likely to be an issue. $\endgroup$
    – 8bittree
    Commented Aug 11, 2022 at 20:40
  • 1
    $\begingroup$ @stix Well, 1) They aren't starting from zero. They have the industrial capacity of present day Earth to use. 2) How is any of that a technology limitation? That all sounds like local astronomy knowledge limitations and some infrastructural limitations. $\endgroup$
    – 8bittree
    Commented Aug 11, 2022 at 21:20
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    $\begingroup$ @stix Did you know we have multiple observatories in space? Did you also know that we didn't build factories in space to build them? Instead, we built them here, on Earth, stuffed them into tiny rocket fairings, and launched them into space. In the scenario in the question, we don't even need to involve the rockets, we can just drive the parts over and then assemble them on site. And we can even make repairs if we need to. Even on Earth, many observatories are built in remote, difficult to access areas. I don't know why you think the lack of infrastructure is so insurmountable. $\endgroup$
    – 8bittree
    Commented Aug 11, 2022 at 22:49
  • Unless you are unlucky and SMC, LMC and M31 galaxies were behind the zone of avoidance, you should be able to recognize their features with a small and cheap telescope to show that you must be in our galaxy.

  • 10m portal should be enough to pass through something like James Webb space telescope and observe S-cluster stars orbiting Milky Way's supermassive black hole, compute their orbital parameters and find matches with some of the ones observable from Earth. And the same for the SMBH itself.

  • If pulsars from John's answer are not usable, you could use quasars (active galactic nuclei) and their mutual position in the sky to show that you must be in our local group, I think. (*)

Just keep in mind, that what you observe might be tens of thousands years in the past or in the future, relative to Earth's time :-)

Edit (*): I suspect that given the usual distance to quasars, that would be more like our local supercluster :)

  • 3
    $\begingroup$ "Something like the James Webb telescope" won't work inside the atmosphere of a planet, and getting a space program started there is likely a bit more effort. $\endgroup$ Commented Aug 11, 2022 at 22:39
  • $\begingroup$ @PaŭloEbermann you don't need to start it there since we already have a space program here. You just need to move a rocket through the portal, build a launch pad and a control center there and off you go. Depending of how much corners you are willing to cut, it could be done relatively quickly and cheaply, I think. $\endgroup$
    – Edheldil
    Commented Aug 12, 2022 at 10:29
  • $\begingroup$ "Elsewhere in the milky way" means a time-error on the order of many 1000s of years (light speed delay). I doubt the S-cluster stars in the MW core have orbits that are stable over that period? $\endgroup$
    – Yakk
    Commented Aug 12, 2022 at 17:45
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    $\begingroup$ @Edheldil I thought the hole was smaller than it actually is. Still, moving something like the Ariane 5 through a 10 m hole in 5 minutes still seems a bit challenging. $\endgroup$ Commented Aug 13, 2022 at 11:23

You can see the Andromeda Galaxy in the sky.

Looking at other astronomical bodies in the Milky Way is problematic, due to light lag. The Milky Way has a diameter of almost 100,000 light years. Some astronomical objects will be perceived at a later stage of their lives, others at an earlier stage. And there is a lot that could change in 100,000 years. Some objects might have undergone astronomical events and developments we do not yet fully understand.

But the way the Andromeda galaxy looks won't change too much. It is the closest galaxy to the Milky Way, but still 2.45 million light-years away. When astronomers notice that the closest galaxy to the new world looks almost exactly like the Andromeda galaxy and is in exactly the spot of the Andromeda galaxy, they will realize that they are very likely still in the Milky Way.

Looking for one or two other galaxies will confirm this beyond any doubt.

The Andromeda galaxy is visible with the naked eye, but can hardly be identified as such. It will take a telescope to get an image of it which is good enough to clearly identify it. How large of a telescope? That's more of a topic for Astronomy Stack Exchange.

Then, when the astronomers have proven beyond reasonable doubt that they are still in the Milky Way, they can try to find out where in the Milky Way exactly.

Now begins the interesting part. Looking for different astronomical objects, and trying to identify them as objects which are already known from Earth. As I previously wrote, those identifications might in some cases be disputed due to the objects appearing from a different angle and at a different age. And then there is the problem that the exact distance between Earth and many objects is only known with an uncertainty of 10% or more, so triangulation isn't that simple either. There will be several competing hypothesis, supported by some observation but then refuted as the astronomers collect more data and find inconsistencies. The time travel hypothesis might not want to die either, causing further confusion. Until eventually all the data points at one hypothesis being true.

  • 1
    $\begingroup$ 100,000 years doesn't sound like a long time. I would expect most stuff on an astronomical scale to still look the same. $\endgroup$
    – Helena
    Commented Aug 12, 2022 at 11:25
  • $\begingroup$ @Helena Can you prove it? Astronomical observations which use more than just naked eye observation only reach a couple centuries back, some just decades and the most advance observations only years. $\endgroup$
    – Philipp
    Commented Aug 12, 2022 at 11:36
  • $\begingroup$ @Philipp we are talking about objects with lifetimes measured in millions and billions of years. 100k years is nearly nothing. There are some stars that would already/not yet have undergone their supernova - but basically none of these are visible from 100k Ly anyway. $\endgroup$
    – Chieron
    Commented Aug 13, 2022 at 10:09
  • $\begingroup$ For the Milky Way objects, the issue is mostly that the positions of the stars will change quite a bit (and most stars near us won't even be visible from the other side of the Galaxy). $\endgroup$ Commented Aug 13, 2022 at 11:28

They lean towards celestial mapping. They simply find the distance and angle of 3 far-off galaxies, and quickly compare that to the TERABYTES of Earth's present-day mapped constellations.

Even if they arrived at the furthest point in our Milky Way galaxy, they would have an offset of 6.6e+9 AUs (astronomical units) of distance. However, Andromeda Galaxy is over 1.6e+11 AUs away, so using some basic trigonometry, you could find that:


tan(angle)=1.6e11 / 6.6e9

angle = atan(1.6e11 / 6.6e9)

angle = 87.6 degrees

offset = 90 deg - angle = 2.4deg offset

So, even at the FURTHEST part of the galaxy, the NEAREST galaxy would only offset by 2.4 degrees in the sky. Most other offsets are significantly more negligible. The scientists realized that the night sky's distant lights look nearly identical to Earth's. After finding the distance and angle between 3 far-off galaxies, it only takes a programmer an afternoon of writing some code to compare those distance and angles against pre-recorded distance and angles of Earth's view.

They quickly find a positive match, and are even to triangulate their exact position within the galaxy, AND their relative distance from the Earth. All just by using 3 relative points in the sky.

  • 1
    $\begingroup$ "Degrees in the sky" isn't going to tell you a lot. They're relative to the local environment. Just because you could see Andromeda doesn't mean you'd be able to make a meaningful comparison to Earth's sky angles without having a bunch of constellations visible. If you can see Andromeda, you can tell you're almost certainly in the Milky Way, but you wouldn't be able to tell just exactly where based on that info alone if there are no other reference points. If they just got dropped on the other side of the Galaxy, and couldn't see LMC/SMC, Andromeda, they'd be hard-pressed to prove their loc. $\endgroup$
    – stix
    Commented Aug 11, 2022 at 19:06
  • $\begingroup$ @stix Andromeda's just an example. Use any 3 galaxies. We're not focused on "how many degrees its offset". We're focused on how many degrees it DIDN'T offset. As in, the sky should not look any different at all, which is why you can compare it against Earth's maps. That's why you need 3 points of comparison. 1 point tells you nothing, 2 points gives you a distance and is the minimum required to say "I am in the same galaxy". 3 points gives a plane and lets you triangulate your exact coordinates (based on the ratio of distance between the 3 points, compared to Earth ratios). $\endgroup$
    – Tyler M
    Commented Aug 11, 2022 at 19:13
  • $\begingroup$ This sounds like an interesting idea. Do you know of any good reading material on this? $\endgroup$
    – Unknown
    Commented Aug 12, 2022 at 1:59
  • $\begingroup$ Also Andromeda isn't the nearest galaxy, Magellan clouds are closer by an order of magnitude (200-300k LY vs 2.4M LY) $\endgroup$
    – Vesper
    Commented Aug 12, 2022 at 13:13
  • $\begingroup$ @Unknown I can't find any particular reading sources, but I'd describe it like this: Go outside and stand under a tree. Look up, and you see the tree. In your peripheral vision, you may even see a light post. You take a mental note of the visual distance between the two, i.e. not a distance measured in meters but a distance in degrees. Now, go stand under the light post and look straight up. You notice that the visual distance is the exact same! We can use this very precise visual distance as an identifier -- any two objects that are that distance apart, are likely the tree and lightpost $\endgroup$
    – Tyler M
    Commented Aug 12, 2022 at 17:49

As others have pointed out, astronomical observations are the key.

Astronomers who went through the portal would first look for the familiar constellations seen from Earth, to eliminate the possibility that the other side of the portal is on Earth. A They would also search for planets, to see if the planet was in a solar system similar to Earth's, or one quite different. I think that even small telescopes should enable the identities of planets with those in our solar system to be confirmed or disproved.

The next step, or on taken by another team of astronomers, would be to scan the skies for dim patches of light. The large and small Magellanic Clouds and the Andromeda Galaxy should be visible to the naked eye from anywhere in the Milky way galaxy, except where hidden by the galactic core.

If they spot them, they will examine them with telescope to confirm their identity, and they will search with telescopes for other galaxies which are conspicuous from Earth.

They can search for globular star clusters, seeking to identify any that they find with the globular clusters surrounding our Milky Way Galaxy. If they can identify two or three of the globular clusters of the Milky Way Galaxy, measuring the angles between them should enable them to calculate the position of the planet relative to the Earth.

On Earth, the Sun is the strongest source of almost all frequencies of electromagnetic radiation, because it is so much closer than other stars and extra solar astronomical objects. On any other Earth like planet, the star or stars in the system will be much stronger in most radiation bands than any more distant objects.

Among the brightest objects in radio frequencies seen from Earth are the galaxies Centaurus A (NGC 5128) about 10-16 million light years from Earth, and Virgo A (M87 and NGC 4486) about 54 million light years from Earth. They are also relatively prominent in visual light, making it easier to identify the radio sources with known objects. The angle between Centaurus A and M87 can be used to calculate the position of the planet relative to Earth.

Pulsars can also be used to find the planet's location.

Pulsar maps have been included on the two Pioneer plaques as well as the Voyager Golden Record. They show the position of the Sun, relative to 14 pulsars, which are identified by the unique timing of their electromagnetic pulses, so that our position both in space and in time can be calculated by potential extraterrestrial intelligences.[39] Because pulsars are emitting very regular pulses of radio waves, its radio transmissions do not require daily corrections. Moreover, pulsar positioning could create a spacecraft navigation system independently, or be used in conjunction with satellite navigation.[40][41]


X-ray pulsar-based navigation and timing (XNAV) or simply pulsar navigation is a navigation technique whereby the periodic X-ray signals emitted from pulsars are used to determine the location of a vehicle, such as a spacecraft in deep space. A vehicle using XNAV would compare received X-ray signals with a database of known pulsar frequencies and locations. Similar to GPS, this comparison would allow the vehicle to calculate its position accurately (±5 km). The advantage of using X-ray signals over radio waves is that X-ray telescopes can be made smaller and lighter.1[3] Experimental demonstrations have been reported in 2018.[4]


If Pulsars can be used to find the positions of space craft within our solar system, they can be used to find the positions of planets orbiting distant stars. But the atmosphere of a habitable planet would stop most X-rays from reaching the surface, so the astronomers couldn't be able to study Pulsar X-rays from the surface but would have to put detectors in orbit, which would be very difficult with what they could move through the portal.

The small size of the radio telescopes they could take through the portal would mean they would have to concentrate on the brightest radio sources. Both Centaurus A and Virgo A were discovered by 1950 with small radio telescopes, and so both are very bright radio sources, and far enough away to be similarly bright everywhere in our galaxy, and close enough to have large differences in angle as seen from different parts of our galaxy.


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