# What would make scientists realize their planetary system is inside a hollow sphere?

I read What would make scientists realize they were on a flat world?, and I decided to make a similar one.

Scenario: While poking around in an alien ruin, scientists discover a gateway which offers instant transportation to an planet.

The Observed World: The gateway leads to a planet. The planet is in a planetary system that has a star. 42 planets rotate around it. The planetary system doesn't have any other celestial object. Besides the planetary system, there are other stars that form constellations.

The Actual World: The planetary system is actually inside an artificially constructed hollow sphere. The solid part of the sphere is of made of 2-Methyl-1,3,5-trinitrobenzene. Its hollow part is a sphere that is concentric with the hollow sphere. It produces no gravity in its hollow part. But it is thick enough that things outside it can feel gravity.

On the discontinuity between the solid and hollow part, there is a screen that displays the stars. The resolution of the screen is high enough, the paths of the stars and light are simulated well enough.

The hallow part of the sphere is large enough. No planet will hit the inside of the sphere.

Question: If a team of scientists are sent through the gateway with the purpose of investigating the planetary system, how would they realize that they are inside a hollow sphere?

Particularly, what would stand out to someone with a good grasp of physics, or astrophysics, even if they had no reason to suspect that they are in a hollow sphere?

Going out of the planetary system is not in their plan.

I'm not looking for a mathematical proof, but rather something that visibly stands out and would make a scientist decide to perform such a proof in the first place.

Their available technology is modern-day: spaceships, space shuttles, space stations, telescopes, etc.

• Missing space dust, electromagnetic waves other than light which aren't simulated by your wall, stuff like that. – Nobody Jun 24 '16 at 15:15
• TNT seems like a poor building material. – Seeds Jun 24 '16 at 15:18
• @Seeds Clearly you've never played Minecraft. – Thomas Jacobs Jun 24 '16 at 15:21
• @ThomasJacobs I don't get the part about Minecraft. – v7d8dpo4 Jun 24 '16 at 15:30
• – Michael Seifert Jun 30 '16 at 20:05

## 6 Answers

Assuming you can trick them by matching the correct spectra of light coming from each individual star, you still run into the problem of a projection on a 2D surface not being able to effectively simulate 3D space at more than one location. e.g. Even if your screen properly tracks the movement of the location of the observers and updates the relative positions of the stars, anyone not in the same location will be seeing incorrect positions.

Lo-tech example: imagine three posts in the ground, in a triangular formation. If you stand some distance away and look at the top of the posts mostly equidistant from two, you will see something like: (imagine it's dark with glow pain on the tops)

o o o

A person standing several feet to your left will see

o o o

Whereas a person standing to you right will see

o o o

A 2 dimensional projection on a screen will not be able to reproduce that.

• The hallow part of the sphere may be big enough for people not to find how close the stars are without leaving the planetary system. – v7d8dpo4 Jun 24 '16 at 16:18
• If you properly reproduce the spectra of the stars, they will know how far away they are supposed to be, then they will notice they don't move properly for things at different distances. (if you don't properly reproduce the stellar spectra, that will be a dead giveaway) – Seeds Jun 24 '16 at 16:22
• A shorter way to say what you said: an astronomer looking at the sky for a few days would notice that the closest stars don't see to move relative to one another due to a missing parallax effect. Have my upvote by the way. – The Square-Cube Law Jun 24 '16 at 16:23
• Yeah, but they hate short answers around here. :) – Seeds Jun 24 '16 at 16:23
• Only for one person at a time, or several close together. Depending on the accuracy of their equipment, and the distance between them, and motion, any 2d screen will fail to show the proper parallax to one or the other. – Seeds Jun 30 '16 at 16:55

The universe would look far too hot.

Your sphere is not transparent, so it will absorb and then re-radiate all energy emitted by the star it encloses, at a range of frequencies according to its temperature. (This energy will be radiated as black-body radiation.

Anyone looking at anything outside the solar system with something like a radio telescope would see this thermal radiation in place of the standard cosmic background radiation. Furthermore, this would give them the ability to precisely determine how big the sphere was, since it would radiate heat at the same intensity as the radiation it absorbed from its star.

• There'd have to be some mechanism to radiate the heat away, or all those unimaginable billions of tonnes of TNT would've gone bang. – Seeds Jun 24 '16 at 17:43
• Half the energy absorbed would get radiated outwards, so a big sphere won't heat up that much. Something with, say, a radius 50% greater than the orbital radius of Pluto would only heat up to about 50 Kelvin. Far hotter than most of the universe, and definitely noticeable with a radio telescope, but still a bit below the freezing point of oxygen. – ckersch Jun 24 '16 at 17:55
• Possibly, in an ideal model, but a big layer of TNT is absorbing heat, and conducting heat throughout it's thickness before it can radiate away to the outside. I don't have it's thermal conductivity handy, but at equilibrium it'll have to be radiating 100% of the star's output from the outer surface of the sphere, the question is how hot will it get without help. – Seeds Jun 24 '16 at 18:12
• The more I think about it, the TNT probably out masses the sun, so it will get warmer, but not much. – Seeds Jun 24 '16 at 18:18
• The CMB is thermal radiation — just from a very, very, cold source. Seeing the thermal radiation from the screens might lead the scientists to conclude that the portal had taken them to a parallel universe that was younger and hotter. – Michael Seifert Jun 30 '16 at 20:13

Assuming that the sphere is thick enough to block external transmissions, and assuming that the display screens do not also emit a replacement for those missing signals, then the cosmic background radiation might be missing.

• They cannot know Cosmic Background Radiation is "missing" because they never knew it should be there in the first place. The theory of CMB is a result observing the expansion of the universe. To observe that you must see other stars. – MichaelK Jun 24 '16 at 17:12
• But the scientists aren't from inside the sphere. The O.P. states that they came through a portal. They would know about CMB from their observation of the world outside of the sphere. – Henry Taylor Jun 24 '16 at 17:52
• If the display screens are kept at a constant temperature, the blackbody radiation they emit will be indistinguishable from a cosmic microwave background at that same temperature. The scientists who measure this will know that something is up if the temperature of the displays isn't 2.7 Kelvin, but they might equally well conclude that the portal took them to a parallel Universe that hasn't expanded as much as ours (and so the CMB is warmer and not as redshifted.) – Michael Seifert Jun 30 '16 at 20:10
• @MichaelSeifert, I was thinking that any display technology which could emit images of star fields and distant galaxies would be much warmer than 2.7 Kelvin; which might be another way they could tell that something was weird. Hadn't considered the parallel universe option. Good Thoughts! – Henry Taylor Jun 30 '16 at 21:36

Once on the planet in the hollow sphere system the scientists would set up a radio telescope to locate pulsars. Because pulsars are the ideal sign posts for navigating around the galaxy. This would be standard navigational procedure for interstellar expeditions. If the sphere blocks signals from outside, they won't be able to detect the pulsars. In fact, they will soon discover everything else in the radio universe was blocked too.

A quick check with radar and lidar will fairly soon reveal this system is inside a hollow sphere.

I have assumed basic technology like radar, radio telescopes and lidars will highly advanced kit and quite portable to boot. Their use will be standard procedure for galactic explorers making it straight forward to discover the nature of this closed system.

Information about using pulsars for interstellar navigation can be found at: https://www.technologyreview.com/s/413615/how-to-use-pulsars-for-interstellar-navigation/

• Great! Something they would do naturally and soon after arriving. – JDługosz Jun 30 '16 at 11:47
• See also How to determine one's position in space? (full disclosure: the accepted answer is my own) – user Jul 14 '16 at 14:36

I would expect a significant amount of light would be reflected back inside the sphere, think LED screen outdoors on a bright day. An observer on one of the planets inside the sphere will see a white sky day and night.

If parallax and stellar spectra give them clues into what they're looking at, I'm betting they would prove it with radar.

The sun is a whole eight light minutes away, and I'm betting your screen system is much closer than that. Without much equipment, they should be able to generate a tight beam radar pulse, and then register a consistent return pulse anywhere they point it at the sky.

Of course it is possible that the sphere can completely absorb an energetic photon pulse and generate no detectable return, in which case my next experiment requires hitting the thing with brick on a rocket and seeing what happens.

• The whole planetary system is inside the sphere. The wall of the sphere is farther away than the sun. – v7d8dpo4 Jun 25 '16 at 6:00