What about an analogy? Isn’t this kind of similar to an echo? An echo can clearly be observed (heard) by many people and recorded with audio equipment, but an echo is not an tangible object that has a location in threespace either - it is a phenomenon observed when sound waves bounce off of objects. Thus, arguably ten people standing near a cliff listening to gunshots echo across the canyon are all experiencing different sound waves that may differ very slightly in frequency or timing, but the experience is very similar and they all can relate to a “shared” experience because the minute differences are either not detectable or are of )slight consequence.
You might say the same thing about the Doppler Effect in either sound or light (redshift and blueshift). Changing your position or velocity, except possibly in very specific ways that exactly balance out the changes, does have an effect on the exact amount of shifting that you experience. Nobody is claiming that because the Doppler effect cannot be held or subpoenaed, police radar guns are snake oil or occultic objects.
I agree that a rainbow is not just an optical illusion. You’re observing electromagnetic radiation that really exists. It’ll look different depending on where you’re standing but that’s just because you’re being exposed to different aspects of it. If I look at the front of my house it’ll look different than if I look at the back of my house. But my house is still a real singular object and it exists even if there’s nobody around to see it.
As an example of something that’s doesn’t exist if nobody’s seeing it, here’s a real optical illusion. Those black dots you’re seeing have no real existence - they only “exist” as your observation of them.
It depends on the definition of “exists”.
The prism effect is, right now, working everywhere there is a droplet of water and some light. Thus there are probably 10^20 rainbows (*) that could be measured right now, and each could be measured from an infinite number of angles. Only some of them are perceptible to the human eye, and only some human eyes happen to be looking when/where they occur.
So would you rather say that there are an infinite number of rainbows and only some are ever seen by anybody, or that the only existing rainbows are those that are observed?
(*) Source: PIDOOMA (definition of PIDOOMA)
