# How do we know that distant stars and galaxies are real?

In some universe,with physical laws the same as in ours, our world is some kind of "lab environment". Our solar system is enclosed in a balloon-like sphere with a radius of a few light-days around the sun (just big enough to contain Oort clouds and all long-period comets).

The inner side of the sphere is a holographic screen/Wave/particle emitter that displays distant stars and galaxies (which would solve parallax observations?).

What else do I need to take into consideration to make it scientifically accurate? What about background radiation?

How could inhabitants detect such a setting without actually going to the "Truman Show wall"?

• In some universe means that the physical laws governing that universe could be different from ours. I don't see how anyone can answer for the "reality" of an effect with respect to physical laws they can't know. – StephenG Aug 4 '18 at 19:17
• In PJF's World of Tiers series, this is mentioned, but not really explained, except as how the universe was built. – Aaron Gullison Aug 5 '18 at 0:07
• Hey! I actually wrote a short novella on this! My main problem with that was explaining things like extrasolar objects entering the Solar System on hyberbolic trajectories and mankind launching probes to other star systems. But, surprisingly, it's hard to tell whether we're in a holographic shell or not. – B.fox Aug 5 '18 at 1:19
• To clarify, some asteroids swing past the Sun on orbits which can only be possible had they come from outside the Sun's gravity well. – B.fox Aug 5 '18 at 1:20
• Won't work. Two telescopes on opposite sides of the Earth coordinating looking at something will expose your deception. I'm not aware of this being done in the visual spectrum yet, but it's common in the radio spectrum. (If your image resolution is limited by your mirror size you can with care combine the results of multiple receivers to get an effective size equal to the spacing between them. Hence it would be done at some point. – Loren Pechtel Aug 6 '18 at 0:02

What else do i need to take for consideration to make it scientifically accurate?

Unless our understanding of physics is so fundamentslly flawed that our most basic conclusions about the universe are wrong, such a sphere is not scientifically possible, for a number of reasons.

Just like all other stars, the sun is bound to the galaxy by gravity and orbits its center. But your hollow sphere presentes no gravity to the solar system (hollow spheres only have gravity on their outside). Any arrangement of the system inside it will be unstable - the solar system will be stable in itself, but it will move within the sphere and eventually hit its inner surface.

As to why this will happen: any comet that approaches a planet (most probably Jupiter) and gets accelerated to solar escape velocity will eventually leave the system, taking some momentum. The whole rest of the solar system itself will be accelerated the other way with equal momentum. Since there is nothing binding the system to the center of the sphere, it is a matter of time until a collision with the sphere happens. Even before some planets smash against the sphere, that leaving comet will, and the impact may be very visible.

If you give your humans enough time, it might be a probe like Voyager, Pioneer and New Horizons rather than a comet.

Also notice that the solar system has a boundary known as the heliopause. It is where the solar wind pressure balances against the pressure from interstellar wind pressure. There are termination shocks and zones of lower temperature. If you wish to simulate it, you will need a constant influx of matter in the sphere, which overcomplicates things. If you don't... The inner surface of the sphere will get increasingly hot from shock with solar wind particles. Any projection from the sphere will be outshined by the increasingly denser, increasingly hotter cloud forming at the inner surface.

• Thank You for thorough analysis, the system "should" have single flaw, to allow residents discover it's nature some day but id' like it to be otherwise accurate. I could use solar wind to actually power up the display but what about binding the sun in the middle ? Any idea how to resolve this ? – Vancalar Aug 4 '18 at 14:18
• "I could use solar wind to actually power up the display" No. "but what about binding the sun in the middle? Any idea how to resolve this?" Also no. – Renan Aug 4 '18 at 14:56
• What about dark energy, that keeps it in place ? – Vancalar Aug 4 '18 at 14:58
• @Vancalar that would only make the system even less stable. – Renan Aug 4 '18 at 17:56
• I accept Your answer because Your proposals are really hard to overcome with whole system "skewing" from it's original position and accumulating amass of particles at the bonduaries. All other problems mentioned could be resolved by faking the outcome of phenomena. Cheers! – Vancalar Aug 6 '18 at 11:48

## Spectroscopy, or, why it cannot be a holographic display

The inside of the sphere cannot be a holographic display. We have been using astronomical spectroscopy to research the chemical composition of distant stars and galaxies since the 1930s. We have measured their velocities by measuring the Doppler shift of the spectral lines.

If we were inside a spherical display the spectral analysis would reveal the material of which the display is made, and there would be no Doppler shift.

The simulated distant stars and galaxies must actually be made of the respective elements; and they must actually move at the measured velocieties: at this point, what is the difference between the simulated universe and a real universe?

• I think you just proved that a hologram can't display any color except whatever color its material is... Anyway, whatever the tenth-lightyear-diameter high resolution spherical hologram is made of, it's probably not photographic film -- and this question supposes that the hologram (whatever it's made of) can emit spectral lines or whatever. (Basically, you're saying "It's impossible!" I agree, of course, but that's not the question being asked. And if we're going to worry about the possible, 97% of the questions on this site get axed.) – Mark Olson Aug 4 '18 at 18:19
• @MarkOlson: It's not about colors. Color is a sensation which exists in the mind. It is not a physical quantity; it does not exist in nature. A hologram can display any color, just like an LCD panel can. But the spectral analysis would reveal the material of which the LCD is made, or the material used for the laser light displaying the hologram. – AlexP Aug 4 '18 at 18:23
• @AlexP: great comment, term "holographic projector" is given here for simplicity, maybe i should call it wave/particle emiter instead ? The basis is that all what we can detect is Waves/particles coming from "out there" , so "alien scientists" could prepare "the illusion barrier" to discourage us from exploring outer space, i.e. sending us electromagnetic waves identical with those from real stars. – Vancalar Aug 4 '18 at 19:07
• @Vancalar If that situation is possible, then the aliens would be so far advanced over us that you might as well have said "magic." Additionally, such a holographic display is more than 9.5 billion miles away (as Voyager 1 hasn't crashed into it), casts photons to Voyager 1's position (as well as to all other probe locations: Mars, Jupiter, Pluto, and a few dozen other places), and given the speed of light, it would be impossible to transmit to only these locations. – Draco18s Aug 4 '18 at 19:33
• Of course they should be advanced but not magicians :) I am not here to formulate pseudoscience theories but to learn how to deal with them, checking on the way how much (or little) to do with reality have such titles as "Matrix", "13th Floor" or "Dark city". – Vancalar Aug 4 '18 at 19:47

We don't know anything in a metaphysical sense, and aliens with Godlike powers could certainly fool us. But it's worth noting that they'd have to do a lot of careful tinkering in the Solar System as well as outside it to be successful. Here are some things they'd have to take care of beyond just faking the electromagnetic view of the universe.

Neutrinos and other high-energy particles: We see lots of particles which appear to come from far outside the Solar System. So not only would the hologram need to emit electromagnetic waves of all sorts, but it would have to emit high-energy elementary particles and light nuclei, also.

Gravitational waves: This is another spectrum which we can now observe, so it would have to be covered as well. And this is especially hard since what we have learned about the structure of space-time makes gravitational waves especially hard to fake.

Materials in the Solar System: When we investigate tiny inclusions in primordial meteorites, we find the traces of ancient, short-lived radioactive elements. There is no known source for them within the Solar System and they couldn't have come from the cloud which collapsed to form the Solar System (their half-lives are too short -- they would have already decayed). They do get made in supernova explosions, so they must have been made right next door when the Solar System was forming. It appears that a (very) nearby star which formed just a bit earlier than the Sun was massive enough to evolve through to becoming a supernova and exploded, scattering their short-lived radionuclides into the Solar nebula just as it was condensing.

So, it's possible, but pretty hard and needed to have been started almost five billion years ago.

It's also worth mentioning that physics within the Solar System is pretty self-consistent and it's extremely difficult to see how the rest of the universe could be markedly different than what we observe.

And, of course, in the future there will doubtless be many other tests we could perform. (For example, getting samples of comets on hyperbolic trajectories.)

### Parallax would be the first thing to break the hoax

In our universe, "nearby" stars appear to move across the sky as compared to distant stars due to parallax and the motion of the Earth around the Sun. The effect is subtle and wasn't observed until the late 1800 to early 1900s. Using something called a filar micrometer (seen below) to make accurate angular measurements.

While the 'experimenters could simulate the annual parallax of stars, if the inhabitants of your solar system achieve technology comparable to the early 1900s, they could stumble upon the daily parallax. The daily parallax of the rotation of the Earth couldn't be simulated for everyone on Earth like the annual parallax could. For instance:

Consider an astronomer at location A on the Earth. For the hoaxers to simulate the location of a fake star over a single night, the image on the display would need to move from A' to B' as the Earth rotates the astronomer from A to B. However, a second astronomer on the opposite side of the Earth would measure the star in a different position due it's nefarious displacement across the screen. The displacement (denoted $\theta_H$) could be measured relative to a known object, (Jupiter, for instance). When the two astronomers meet up later and compare their observations, they would know something is wrong.

### Appendix: Calculating the daily parallax

The current state-of-art parallax measurements have angular resolutions of around 1 milli-arcsecond (see the Hubble Telescope). The resolution of of the filar micrometers of the 1920-1930s astrometry would have been around 10 milli-arcseconds. To calculate the parallax your screen incurs, let $R$ be the radius of the Earth, $H$ the distance to the hologram (screen), and $S$ the simulated distance to the fake star.

The parallax angle the fake star creates across the diameter of the Earth is: $$\theta_S = \frac{2R}{S}$$ And the distance the image of the star must move across the screen (called $D$) is: $$D = (S - H)\theta_S = \frac{(S - H)2R}{S} = (1 - \frac{H}{S})2R$$ We see if $S$, the simulated distance to the star, is large, then $D\approx2R$; the image moves across the screen exactly the width of Earth. This is the worst-case scenario for the hoaxers. And lastly, the parallax produced by the motion of the image on the screen will be: $$\theta_H = \frac{D}{H} = ( \frac{1}{H} - \frac{1}{S} ) 2R$$ $$\theta_H \approx \frac{2R}{H}$$

For Earth, $R=6.73\times10^6$ meters. If we take your experimenter's screen to be 3-lightdays away, then $H=7.77\times10^{13}$ meters, then:

$$\theta_H = \frac{2 6.73\times10^6}{7.77\times10^{13}}\frac{180^\circ \cdot 3600 \cdot 1000}{\pi} = 36 \text{milli-arcseconds}$$

Which is large enough to notice with 1900s technology. Note that Jupiter only moves around 7 milli-arcseconds a night.

• That's why the question posited a holographic display. Holograms reproduce the phase of the light, so this entire construction is not applicable -- the apparent light source remains at the position of the Fake Star. – AlexP Aug 5 '18 at 21:27