Suppose we put a shell around the solar system that displays a 2D image of the rest of the universe on its inner surface. What year would we have had telescopes powerful enough to determine that this was the case.

I think it would probably come down to the resolving power vs lack of parallax but I’m not sure what the orders of magnitude are for either of those.

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    $\begingroup$ @KeizerHarm That should be an answer... $\endgroup$
    – Zeiss Ikon
    Commented Apr 24, 2023 at 16:42
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    $\begingroup$ Just how black is this sphere? Unless it's utterly non-reflective, which verges on being physically impossible, we will see the Sun reflected from it. $\endgroup$ Commented Apr 24, 2023 at 16:42
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    $\begingroup$ @ZeissIkon It's a boring answer; I'm assuming the OP would like to refine their question to a scenario where the answer is more interesting. $\endgroup$
    – KeizerHarm
    Commented Apr 24, 2023 at 16:43
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    $\begingroup$ Does it simulate the Cosmic Microwave Background? $\endgroup$
    – Sascha
    Commented Apr 25, 2023 at 10:18
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    $\begingroup$ Didn't one of the early heliocentric (possibly also geocentric) models include a shell that held the stars? And parallax was what disproved that the stars were so near? So it wouldn't be so much that they detected the shell, but that a shell-theory was developed, and then all observations confirm its existence. $\endgroup$ Commented Apr 25, 2023 at 17:29

8 Answers 8


The answer depends entirely on the sophistication of the simulation.

The answer by AlexP suggests that the screen could be discovered in the 1600s or 1700s by parallax, but that's only true if the designers of the screen failed to account for parallax—and why would they? Compared to simulating a lawfully behaving universe outside the sphere, handling parallax is trivial. You just have to render your image from the perspective of the Earth's location in the sphere, instead of the perspective of some fixed point like the center.

The only conceivable reason they might not do that is if they wanted people to discover the sphere at that particular level of technological sophistication, as in The Sentinel. That's possible, but it's just as possible that they would design the sphere to be detected at an earlier or later time. Whenever you want it to be detected for the purposes of your story is probably fair game here.

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    $\begingroup$ Upvoted! With the small observation that by the late 19th and early 20th century astronomers were routinely measuring the spectral composition of the light coming from the stars, and thus this is a hard upper boundary of the time when the tricks would have been discovered. $\endgroup$
    – AlexP
    Commented Apr 24, 2023 at 19:04
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    $\begingroup$ You can correct for parallax to a point, but it is impossible to perfectly correct for everyone on the planet at once. You can correct the annual parallax observed as the earth moves from one side of its orbit to the other over the course of 6 months, but not the diurnal parallax observed as a person rotates from one side of the planet to the other over the course of 12 hours. Diurnal parallax is much smaller and harder to detect, but also impossible to faithfully mimic since you can't simultaneously correct for observers in different locations on the earth's surface. $\endgroup$ Commented Apr 24, 2023 at 19:21
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    $\begingroup$ @NuclearHoagie That's true, but diurnal parallax beyond 1 ly could only be detected with modern interferometry, I'm pretty sure, not in the 1700s. The aliens/gods could prevent detection even today by using a holographic screen that would produce a correct image everywhere inside the sphere simultaneously. So I stand by my first paragraph. $\endgroup$
    – benrg
    Commented Apr 24, 2023 at 20:12
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    $\begingroup$ @NuclearHoagie This assumes a screen that emits light uniformly in all directions. If you make it output different light in different directions, you could imitate parallax without knowing the earth location (that to a regular screen would be what light field camera is to a regular camera). $\endgroup$ Commented Apr 25, 2023 at 15:59
  • $\begingroup$ If I remember correctly, this was a plot point in Alastair Reynold's novel Century Rain, though I believe the sphere surrounding "Earth Two" was much, much smaller. $\endgroup$ Commented Apr 25, 2023 at 21:21

Assuming that the screen is electromagnetically "perfect", in that it reproduces exactly the radiation we'd expect to observe, including parallax AND spectra, with the required emission and absorption lines, all appropriately redshifted (quite a difficult challenge if you ask me), the answer is probably...


Although as @benrg correctly pointed out, we had been detecting cosmic rays a hundred years before this, and neutrinos from the SN 1987A supernova in drum roll 1987, we'd be definitely suspecting something by this point.*

This is the date Voyager 1 crossed the heliopause and took direct measurements of the interstellar medium. This is the area dominated by particles coming from the other parts of our galaxy, which would behave noticeably differently, were the Sun to be found not to be moving at breakneck speed through that medium, creating a bow shock and everything. In fact Voyager 1 data would've indicated that something was amiss years earlier, when passing the termination shock layer.

But even if we assume they also installed massive interstellar fans to simulate the particle flow, we've got...


This is the date of the first non-electromagnetic measurement we've made of our universe, when we successfully detected the gravitational waves from a black hole merger.

Now the game is up, that screen may simulate any electromagnetic or particle radiation, gravitational waves are a whole different ball game. There is no known way to fake them with a projection device.

Of course if they went to the length of installing a screen so vast it'd weigh more than most stars, why wouldn't they do this too...

* There is an interesting Earth-based corollary to this. We have some fairly credible hypotheses about how cosmic rays are seeding lightning in storm clouds. If they are correct, we'd be seeing a difference in how thunderstorms behave on our planet. Of course we wouldn't have anything to compare it to, so we probably wouldn't discover this clue until after some other clues were put together but I thought it was worth mentioning.

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    $\begingroup$ We've been detecting cosmic rays since 1912 (per Wikipedia) and saw neutrinos from SN 1987A, both predating gravitational-wave astronomy. Perhaps if we'd never detected cosmic rays we wouldn't have noticed anything amiss, but not detecting neutrinos from a supernova would be harder to explain, I think. $\endgroup$
    – benrg
    Commented Apr 24, 2023 at 20:31
  • $\begingroup$ @benrg Fair point, although if they simulate particle flow as well, we'd still be fooled until gravitational wave detection became a thing. In my defence, I kind of worked on the basis that given these detections were quite surprising at the time, their absence wouldn't be too conspicuous. $\endgroup$
    – biziclop
    Commented Apr 24, 2023 at 21:23
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    $\begingroup$ Interestingly the 1987a neutrinos were detected hours before the visible light. Many scientists think that it took 2-3 hours for the visible light to reach the stellar surface after core collapse, but maybe it just took the aliens a few hours to update their simulation :) $\endgroup$ Commented Apr 26, 2023 at 16:25
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    $\begingroup$ More seriously though, I don't think either of these arguments are great. For Voyager, the data is entirely new and we only know about the shape and size of the shock from the data we received from Voyagers. The models we have are based on that data, so if the data was different, we would just have different models. Likewise I don't see how the gravitational wave detection would give up the game. We actually had light detected matching the gravitational wave source in 2017, but if the shell is displaying the real universe, it would match the waves. $\endgroup$ Commented Apr 26, 2023 at 16:28
  • $\begingroup$ @JasonGoemaat It's not meant to be great in the sense that it'd be impervious to an all-encompassing simulation. After all, we could all be living in one and be none the wiser. What I tried to do is just list when there's a marked increase in the difficulty level of the simulation required. Sure, if you want to project what's actually out there (why?), you could kit out your shell with GW detectors everywhere, and generate the appropriate EM counterpart on the fly. But it'd be harder. And to create fake GW or to mask existing GW, even harder. And so on. $\endgroup$
    – biziclop
    Commented Apr 26, 2023 at 16:57

Late 1600s or early 1700s

One light year is 0.3 parsecs.

One parsec is by definition the distance at which a distance one astronomical unit subtends an angle of 1 arc-second.

A star a distance of one light-year would show an annual parallax of a massive 6 arc-seconds.

Stellar parallax is the apparent shift in the position of a star as seen from Earth over the course of a year. It is measured relative to Earth, not relative to other stars.

Even early-1600s Galilean telescopes might have been good enough to measure this, but probably not. On the other hand, early 1700s telescopes were certainly good enough to measure such a massive annual parallax; we know this because in the 1720s James Bradley FRS discovered and measured the aberration of light, which, at about 20 arc-seconds, is of comparable magnitude and appears as a collective annual parallax of all heavenly bodies.

The first successful stellar parallax measurements were done by Thomas Henderson in Cape Town South Africa in 1832-1833, where he measured parallax of one of the closest stars, alpha Centauri. Few years later, 1835-1836. followed Friedrich Georg Wilhelm von Struve at Dorpat (nowadays Tartu) university observatory, who measured the distance of Vega and published his results in 1837.Wikipedia

Bonus: the modern-day Hubble telescope has an angular resolution of about 0.4 arc-seconds, which means that it could resolve two light-sources one astronomical unit from one another at a distance of about 8 light-years.

By 1901, Annie Jump Cannon would have noticed that the spectral composition of light suggests that all stars are made of the exact same material, and moreover that they are all made of something entirely unlike our sun...

  • $\begingroup$ The problem is that said parallax would be impossible to measure because every single body on the firmament is moving the same distance. We can only measure anything in arc-seconds based on an unmoving frame of reference in the background. If we have a shell where every object is moving the same from season to season, it would appear as if none are moving (none are with respect to each other), and the conclusion would be they are all are located infinitely far - or farther than can be measured with earth's orbit. $\endgroup$
    – KeizerHarm
    Commented Apr 24, 2023 at 18:20
  • $\begingroup$ @KeizerHarm: Ogh, no. The way we measure stellar parallax is to measure the star's actual position in the sky, relative to Earth. Micrometric screws for the win. $\endgroup$
    – AlexP
    Commented Apr 24, 2023 at 18:27
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    $\begingroup$ How exactly would you measure a star's actual position in the sky? You need some frame of reference, some set of coordinates fixed on something. The Prime Meridian for the sky. Then you can see a star wobbling against that meridian. If the entire sky is a 2D image, then none of them are moving with respect to each other. You cannot observe parallax on a flat surface of stars any more than you could walk about the Louvre to get the length of Mona Lisa's nose. And if there's no measurable parallax, then you have either a flat shell of stars or stars located at a much greater distance. $\endgroup$
    – KeizerHarm
    Commented Apr 24, 2023 at 18:32
  • $\begingroup$ It's the first sentence of the linked article: Stellar parallax is the apparent shift of position (parallax) of any nearby star (or other object) against the background of distant stars. Whatever science you're thinking of, it wouldn't be a 1700s technology. $\endgroup$
    – KeizerHarm
    Commented Apr 24, 2023 at 18:35
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    $\begingroup$ "One parsec is by definition ..." 1/12th the length of the Kessel Run. $\endgroup$
    – RonJohn
    Commented Apr 24, 2023 at 19:31

Assuming the universe exists outside the shell:

The Oort cloud is said to be larger than 1 light year in radius. Some of its objects may have eccentric orbits that should cross (hit) the shell at some point.

This is not an issue for objects coming from outside the shell, since its outer surface can withstand any kind of impact, while still displaying inside the shell, the projected object's trajectory, like if it hadn't been completely destroyed one second ago. Perfect illusion.

Even if destroyed, those objects can now be simulated for millions of years, virtually entering and exiting the shell.

On the other hand this is an issue for real objects coming from inside trying to reach their aphelion that's located outside the shell. This time impacting the shell's inner surface cannot be hidden by any kind of displaying technique since there is now stuff between the eyes and the screen.

Also, the solar system has a tail, so space dust trying to quit would accumulate more on one side, not to mention dust loves screens.

The answer depends on when did the shell appear, and how humans are able to detect exiting objects when they hit the screen.

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    $\begingroup$ It should be noted that Oort cloud objects are never observed at 1 ly distance. In fact, we can't observe Oort cloud objects in the cloud. Therefore, we wouldn't be able to notice the impact of Oort cloud objects against the shell and we could only infer its existence from the disturbance of the orbits of such objects, which is going to take way long. $\endgroup$
    – Pere
    Commented Apr 26, 2023 at 20:00
  • $\begingroup$ @Pere good point, I thought something like Oumuamua hitting at 26km/s would make a noticeable flash even 1ly away, assuming It's already known where in the sky to spot it $\endgroup$
    – user35577
    Commented Apr 29, 2023 at 14:14

Frame challenge: if you're supposing the aliens are capable of building a shell of radius 1 LY, then clearly they have very advanced technology. There is no reason they would build such a shell and then equip it with flat screens which work like our contemporary display technology, by showing the same image in all directions. If these aliens want to maintain the illusion, then it would be well within their technical capabilities for the screen to emit the correct light in the correct directions so as to account for parallax even at the scale of two observers at different places on Earth.

Contemporary human technology is already able to do the inverse of this ─ light field cameras capture not just the light intensity but also its direction. (This lets you do some neat things like emulate different lenses in software.) So imagine a "light field display" which can emit exactly the right photons with the right wavelengths and directions as needed, and there is literally no optical way to tell that apart from a real universe. In fact, the shell needn't even be so big.

Here are some ways we could figure it out:

  • By physically reaching the shell (with an unmanned probe, of course)
  • By the shell reflecting some of the light we emit back at us (the aliens would design the shell to be as unreflective as possible, but perhaps perfect absorption is not possible)
  • By determining that the universe behind the shell does not obey the correct laws of physics (i.e. a flaw in the aliens' simulation)
  • By a glitch that makes it obvious (imagine an astronomically large blue screen of death).

Not yet

There will be any number of string theorists, cosmologists, mathematicians, etc, working very hard to integrate the curious effect into normal known theories of the universe, with success always just a few years away.

Gravitational waves? Our theories of relativity are wrong, that's all.

Neutrinos? Dark matter must be absorbing them. Etc.

People postulating a hard shell will run into the hard faith position that is materialism. A hard shell? Can you imagine announcing such a thing? You would make the trials of Wegener or the quasicrystal guy look like nothing.

You mean, a giant shell built by someone? There may be a few eccentric astronomers, or creationist Christians who posit such a thing, but they will be professionally excoriated.


With just 1 $ly$ radius sphere, we can detect infrared light coming from what appears to be the pitch blackness of space (assuming the sphere has non-zero temperature).

  • $\begingroup$ Good point. Unless they were cryogenically freezing their encapsulation equipment, the parts that didn't shed light would still glow in the infrared. $\endgroup$ Commented Apr 26, 2023 at 16:46
  • $\begingroup$ @RobertRapplean Exactly. $\endgroup$
    – Karthik
    Commented Apr 26, 2023 at 19:24
  • $\begingroup$ But why would they then postulate a shell rather than a warmer expanse? $\endgroup$
    – user86462
    Commented Apr 26, 2023 at 23:56

When it smashes.

The problem with this idea is that we're in a galaxy, filled with stars and stuff.

These things will continue moving when you put your screen up.

This insanely large screen has effectively no mass measured from the inside, because it's a sphere, so won't affect the movement of anything within it, but will affect paths of stuff outside, which will in turn affect the orbits of stuff inside.

Eventually (as in, fairly quickly), something will cross that barrier, and the screens will either smash, or need to be moved aside to allow a solar system through, either of which one would expect to have some visible effects.

But if the "screen" is actually a more complex multilayered system, then you could move the layers about enough to permit stars to pass between, while still showing the same thing to the earth.

Downside of that is then... what's the point of the screen? Is it really just a filter to prevent people on Earth from seeing all the alien activity outside the screen? This is also a problem of the movie The Truman Show: there's no real point walling it all off just to get a reality TV show.

At that point, you're back to detecting parallax, and when you get to detect that depends on whether the screens were displaying data at the sun (in which case annular parallax would change) or at the Earth (the more likely case, in which case, it'd change the display over the year, and there'd be no visible annular parallax problem; only parallax from the orbital diameter of space telescopes).

There's also the possibility that the screen is better than a mere 2D raster device, in which case even that parallax could disappear.


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