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

Question

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"?

Answer 1

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.

Answer 2

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?

Answer 3

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.)

Answer 4

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.