Most people, including physicists, do not like Many World and Multiverse concepts, especially if they come with Extra Dimensions. But these concepts were always self-evident to me, even before knowing physics (UPDATE: Here two more recent versions, not mentioning "agnostics", of the self evident arguments). Many Worlds and the Multiverse are obvious because nothing else can make sense (read: be formally meaningful). The expectation of there being probably extra dimensions necessary in better theories is similar. I remember sketching four dimensional water melon slices in order to start seeing 4D only to discover that two eyes are sufficient to see stereoscopically in any type of dimensional space but I would need three dimensional retinas! (and how a 4D propeller plane or helicopter may look like.
However, many educated people are belligerent about such concepts:
“Physicists ad hoc add dimensions to everything these days – get back to reality!!!”
“Multiverse? Are you kidding me? Come back when you can explain this universe, then go on to explain others!”
“Oh sure, assume infinitely many worlds and call it science if we cannot say anything anymore because everything is possible. Flush parsimony and Occam’s razor down the drain.”
(UPDATE: Note well - I am not here to support nonsense such as Tegmark’s four level Multiverse, which claims to include Many Worlds as a third level which “adds nothing new” (his words) to the second level – an obvious and fatal problem with his scheme.)
What is strange about this hostility: These people are usually self described atheists or agnostics out to defend science.
Let me tell you why there “are” Many Worlds (1) and likely a Multiverse (2), without drawing on empirical science in any essential way. The last section (3) will rephrase the argument to include "hidden stuff" like extra dimensions.
1) Many Worlds:
Many Worlds is not the multiverse. Many Worlds is not even quantum physics! Quantum physics is the fact that different worlds are entangled, which allows that the “parallel existence” of other possible worlds can be inferred empirically. Without quantum mechanics, other self-consistent worlds are just parallel worlds that never interfere.
The Many Worlds concept includes the other worlds where you wear a differently colored shirt while reading this right now for example. I could draw on the authority of philosophers like David Lewis, Bas Van Fraassen, or as far back as Anaximander [610-546 before "the one"]. However, I will not soil a self-evident concept with arguments from authority.
A fundamental description aims to take account of the whole of totality, of all that is possible. If you refused Many Worlds in such a description, you will do the following ridiculous thing:
You would assume that in the very foundation of totality, the very most fundamental laws of nature, the absolute rock bottom core of physics in its most profound and general symmetries, there is something inside there that ensures that you, yes you personally with name and address, wear that silly color shirt today!
I may be the reincarnation of Jesus, but going as far as thinking myself and the color of my socks on a Tuesday afternoon being somehow part of the very foundation of everything? Nah!
Actualism versus probabilism? Those are for academic “philosophers” securing themselves a job via verification transcendent terminologies. All self-consistent possibilities are fundamentally equivalent and just as ‘alive from their inside’ as those actualized for you right now. Surely, this was self-evident to many long before Parmenides of Elea. Psychoactive substances tend to trigger such epiphanies and have been ingested for 25 thousand years!
[Remark: The Many World Interpretation (MWI) of quantum physics suffers normalization problems. Claiming that all Everett branches together are one entangled totality forgets that outside observer independent universal wave functions are highly suspect. Quantum physics is the single description (not single world) of the totality of worlds/minds! “World” in this context means the world of observers, not a cosmos inside a certain physical model (see “multiverse”).
UPDATE: For Many Worlds, what they are and about experimental verification, please see the article dedicated to these problems: Many Worlds Tautological Truth]
The Multiverse concept claims that there are many more universes that must be added to a consistent description of totality. “Universe” means here not totality (the Universe) but the kind of place like our observable cosmos. The Multiverse concept claims that many more of those look totally different from ours: different physical constants, different numbers of space dimensions, different shapes of space.
You should expect this. Why? Well ask yourself some questions. Where do you find life? Life needs a lot of just right ‘Goldilocks’ conditions! Consider time: There was no life for most of time, then there is a little bit of life right now, and then our universe will go on for an incredibly long amount of time (in a sense infinity) without any life again.
Consider space: There is no life in the middle of earth, then there is a tiny little bit of surface called biosphere where there are mostly bacteria and of course you and me in an even much narrower layer, and then above that there is again nothing but a few viruses perhaps stuck to a rock hurling through space after having been chipped off Mars a long time ago.
Consider spatial scale: There are no life forms in the nanometer range, none in the picometer range, none in the … probably all the way down. Nor do we see life forms as big as the moon, as big as the sun, as big as an galaxy, as big as a galaxy cluster, as big as ….
In fact, you can describe our universe along any parameter, say gravitational field strength or temperature, and you will find that even after transforming the scale logarithmically, there is only a tiny region where you find life. What about sheer numbers? Most planets have likely no life. Only a tiny fraction has their surface moldy like earth.
Now imagine all kinds of universes that all have a consistent physics and order them all according to whatever parameter you like, say number of dimensions or size. Naturally, you should expect life only in a tiny region, a miniscule proportion of that space of all consistent universes.
Of course, if we ask in terms of which ones are observed by conscious minds and in that sense “o-exist”, then only the ones that have life inside do. But apart from this distinction, all these universes are equivalent. You would need to believe in divine supernatural intervention in case only one of these perhaps infinitely many consistent universes in some ill-conceived sense “really actually ontologically exists”. You would need a god to have picked your world out of the huge amount of dead as well as alive competitors, blowing some ghostly glow of “absolute-exist”-pixy dust on it.
Even if you believe in god, whatever amazing thing that is, what makes you think he finds you so amazing?
3) Multiverse and Hidden stuff like Extra Dimensions?
We can start the multiverse discussion from most living beings' point of view - that would be microbes. Take a bacterium in some sort of environment for its 20 minutes of life before it splits into two. Take one that lives inside a bio-film on some surface, say inside the fold of a filling inside a cavity of one of your teeth. Here is the attitude “Multiverses and hidden dimensions are nonsense” translated to bacteria:
“The universe is a fundamental liquid space and a two dimensional surface on which I am stuck. The properties of space and that “given plane” are really rather curious but coincidentally allow me to thrive. The universe looks large, I cannot see the end, and it looks everywhere I can see the same, so that is all there is to it, there can be no more. A hidden space below the surface or other environments with different properties are nonsense.”
The curious properties of the universe that the bacterium is talking about derive partially from the properties of the material of your tooth-filling and so on. That filling only has such curious properties because it is not fundamental, because of the fact that it is actually an environment that came itself into being in a bigger universe that is not just very much bigger than the bacterium can grasp, but that also contains very many more and to the bacterium unimaginably strange places, some of them are universes to very different microbes and animals and ... . If that were not the case, there would not be such strange places like tooth cavity fillings.
The properties of our vacuum are also quite strange. There is handedness for example, and more matter than antimatter, and so on. What makes you think that you happen to be so special that your universe has these properties because they are the most fundamental properties rather than just the properties of some tiny dirty niche in a vastly bigger picture?
Again – the argument is not about academic physics or even anthropic principles. It is about not being a megalomaniac with your head up your own behind, irrespective of there being a god or not.
Below the "fundamental surface” there is something, something hidden that the bacterium cannot ever get at, but if the bacterium wanted to come up with a good physical theory that explains that surface, it should not be surprised if a lot of strange worlds have to enter the description.