Rare things that have been documented much better than UFOs

[Mendel]

I would set the prior of visiting, physical interdimensional beings rather smaller than visiting, physical aliens, but surely much bigger than Leprechauns, those being supernatural.

No times 12, and no.
Interdimensional beings are exactly as fictional as leprechauns, and their powers are exactly magic.
There is no science of interdimensional beings because they contradict established science, and there's zero evidence for them.

What is the probability of something I imagined in my head to exist in reality?
It's zero, because I did not follow the evidence. There are no observations that point to them possibly existing.

They sound more science-y than leprechauns, but they really aren't.

 

[Gary C]

Well, us finding them and them finding us is really the same problem, isn't it?

Not exactly.
If we find them, we will know about it. Alien detections =1
If they find us, we may or may not know about it depending on their level of technology and behavior. They may have the option of studying Earth remotely without revealing their presence. This is a behavioral factor that we cannot assign a probability to as we have no data on alien behavior.

edited for grammar

 

[Mauro]

No times 12, and no.
Interdimensional beings are exactly as fictional as leprechauns, and their powers are exactly magic.

I would distinguish among what is physical (even when purely fictional) and what is supernatural (which I think it's the case it's always fictional). Physical beings obey the laws of physics, even if in the fiction those laws can be much different from what we know: an eventuality to which I assign an abysmal probability of ever come true (*). Supernatural beings go beyond that: they are disembodied minds, minds existing just because they exist, with no physical support. And/or they can just will changes in the physical world without the need of a physical interaction, by 'magic': they just don't care about physical laws. This kind of beings gets an abysmal squared probability from me.

Where you put interdimensional beings depends on the fiction: are they a race whose physics is immensely advanced compared to ours and who can build machines which open passages between places in the multiverse? Then they are physical and they just get an abysmal probability (and much lower than regular aliens, who need to crack fewer physical barriers). Do you prefer interdimensional beings who teleport by just willing to be elsewhere? They're supernatural, abysmal squared. Your choice . Ultimately, it does not matter: both varieties are so improbable not to need to bother with.

(*) of course, in the sense of breaking fundamental limits such as conservation laws or the light speed limit, not in the sense that there will be no more discoveries

 

[Gary C]

Once again, the UFOlogists use sloppy sci-fi physics to obfuscate their lack of evidence and the repeated debunking of the evidence they claim to have.

The most cited source of dimensions beyond our 4D space-time is M-Theory, which subsumes the String Theories. These dimensions are needed to explain certain features of quantum mechanics as one goes down in size toward the Plank Scale. They are considered in the theories to be Compactified, essentially tiny tightly curled up spaces in which only the theorized strings can act. These are not "higher dimensions" from which beings with godlike abilities would pop in and out at will.

Importantly, these additional dimensions are distinct from the various hypotheses that posit a multiverse. The least complicated multiverse theories require no additional dimensions at all.

Casually invoking interdimensional beings as if they could visit our reality as easily as we take an elevator from one floor to the next is close enough to invoking fairies as to not have a meaningful difference.

/rant