There are all kinds of ‘alternative realities’ in which many even quite sane people can believe. However, when I listen to their ideas, one question springs often to my mind: Have you considered the possibility that there is just one reality that would ever be knowable to us, but people just can have extremely different perceptions of it?
Those differences in perception can be so strong that some people believe that they exist in a different kind of reality than others. This state of affairs is only proof of how humans are able to alter their minds to an extraordinary degree. It is not any kind of proof of that there would exist different ‘realities’.

One can develop an endless amount of different alternative realities in one's mind. One can also develop old, earlier made-up realities even further. However, these creations do exist only in the mind of a person who believes in them. They can feel even very real if belief is strong enough. However, as an old quote goes; reality is the one that does not go away when you stop believing in it.

Of course, the reality of bacteria is quite different to the reality of an ant. The reality of an ant is naturally also extremely different from ours. However, they share the same basic reality, even if they can perceive it very differently. Similarly the reality probably looks extremely different on the level of a quark or a galaxy. However, they still do exist as parts of the same reality.
If the multiverse-theory is correct it is possible that there is an endless amount of other universes. However, according to what we know now we just might never find out if these ideas are true or not. In any case they are part of the same reality in which we do live. They are only in different parts of it, and they can be formed differently.

The current theory goes that the multiple universes do not ever overlap. So, pondering about their existence or nonexistence is in the end just a purely mental exercise for us. The other possible universes have no practical way of affecting (or even to be observed in any way) from our own bubble of a universe.
However, if I claim that there is only one reality, nothing in this statement implies that I claim to know any kind of final truth about its true nature or structure. If we assume that there is an alternative reality, but it cannot be observed in any way in reality in which we do live in, how could we become truly convinced of its existence in the first place?

If this alternative reality cannot cause anything observable in the reality in which we do live in, what difference would its existence or nonexistence make to anybody? If there would be any real-world effects caused by some kind of ‘alternative reality’, we should be able to verify their existence easily. If there is none, there is no real reason to believe in their existence.

Refracted sun rising over Virginia Beach. - Wikipedia

An alternative reality could, of course, be like the other universes in the multiverse theory. Their existence or nonexistence would not make any real difference to us. After all, we can quite probably never observe them and they do not affect our own reality in any way.
Of course, we can well still think about all kinds of alternative realities endlessly and even imagine their existence in minute detail in theory. After all, we have the ability to make up an endless amount of alternative realities in our minds if we just have the will and enough spare time.

On the other hand, if there is no real evidence and if we base our belief in existence of an 'alternative reality" on writings of 'sages', hearsay and old stories, there is no real difference to the more formal kinds of religions. One should not forget that if there would be any even the tiniest observable fact, any single testable hypothesis the theories of 'alternative realities' would be instantly part of the mainstream science.
Alas, that is not the case and all 'alternative realities' are still in the class of religious beliefs. People accept all kinds of ideas without any kind of real, hard evidence. The existence of 'alternative reality' would soon be taught in every class and every university, if there just would be any kind of evidence of their existence to be had. One must just think about the glory and fame which such a revolutionary finding would bring to any scientist. There is no real reason why scientists all over the world would not seek to prove the existence of alternative realities, but just the lack for any means of verifying the existence of things that do not exist.

(This piece was completely refurbished on 14th of March, 2013)