This is only a couple of pages.
I wonder why most discussions of multiverse posit that the laws of physics would be different in the different universes and that we just happen to be in an "anthropically favored" universe where the various properties of matter, energy and constants are favorable for our existence. What if many, or all, of the universes share similar or identical physics and just cannot communicate with each other due to barriers in space-time?
multiverse: an oxymoron?
The term universe means all-inclusiveness. So multiverse per se implies multiple all-inclusiveness. And folks, yes it can get worse. A recent conference, and probable bk, has the title of The Universe of Multiverses. See astro-ph Archives today; reference in some article. Why not a set of manifolds, or a set of continuums, or of spacetimes of some sort?
As a solar system is a sub set of a galaxy and a galaxy is a sub set of a universe, why not a
universe as a sub set of a multi verse? This could go on forever there is no data AFAIK.
No matter what laws change there must be a law or laws that permit a universe to be a universe.
We observe star systems, galaxies and galactic clusters; show me one of these other universes and I will believe you.
From the OP link paper
It might be simpler to believe in an unobservable God instead. (Where I define 'God' to be the author and guarantor of the laws of science, the agent that "breathed fire into the equations")
Either way 'faith' in an unobservable is required.
What we now call galaxies were once dubbed 'island universes', until we realised that wasn't good terminology and called them galaxies. The same thinking should apply if we ever find that what we previously thought of as 'the universe' is actually one part of a larger thing. The exact name we'd settle on for what was previously 'the universe' would depend on the nature of the discovered extension to the extend of reality.
Edit: I concur with Garth, if you can't observe or measure it then it's not physics
So this could change in the near future?
Indeed, instead of a multiverse we could simply have an infinite universe where far regions have different laws to our own region. We can only live in the fecund region which is a fraction of the vast whole. Of course this would demand some underlying theory that requires the constants of physics etc. to vary across such a universe.
But until we observe such regions these ideas, interesting though they may be, fall on the sword of Popper.
I am not so sure that what we can't observe doesn't exist, Cristoforo Colombo couldn't see the Americas.
Right, but he didn't know they existed till he did see them, and speculating about their existence before observation was reasonable, since there was a means of testing this idea (by sailing there and having a look).
The discussion here is whether we can sensibly talk about the existence of something that is inherently unobservable, i.e. there is no known way to 'sail there are have a look'.
I am not saying they don't exist, it is just that the idea of their existence is just speculation, not a scientific falsifiable hypothesis.
If our rational system requires them to exist, to explain the anthropic coincidences for example, then it must be recognised that that system is based on 'faith' rather than 'sight'.
[Edit: crossed with Wallace]
I fully agree, but let me say that our knowledges are still limited to assert that or its contrary
speculating about the nature (or the existance) of universes that are not the single observable Universe is mostly pointless, i agree.
but, i believe, the reason "why most discussions of multiverse posit that the laws of physics would be different in the different universes and that we just happen to be in an "anthropically favored" universe" is, as far as i can tell, a response to the question naturally posed by, what some might observe to be, our "fine-tuned universe". from what i can read from John Baez's site, there are 25 dimensionless parameters that define the Standard Model and 1 more dimensionless parameter for the cosmological constant. these are numbers that Nature hands to us and, at least at present, are not derivable purely from mathematics nor from each other. one might wonder why the values of these parameters are what they are.
if some of these parameters were significantly different, we are not sure what that would mean. maybe it wouldn't change anything (except what we get when we measure those parameters). but if some others of those parameters were significantly different, i think the present understanding of physics is that matter would be significantly different or might not form at all after the big bang. it's been called "carbon chavinism", but it's pretty hard to understand how life (and us) could be around to wonder about the values of these fundamental constants if they were much different.
so then the question arises to "how could the Universe be so lucky to get these constants just right?" with a single and sole Universe, the Anthropic Principle does not suffice to answer that question but just begs the question. (i mean the weak anthropic principle which amounts to essentially saying that conditions that are observed in the Universe must allow the observer to exist. i consider it to be a tautology, which is not bad, it's a truism and hard to argue with, but cannot be used to support other claims.) so, with a single and sole Universe, it's like we're holding one dart, the dartboard is about 40 meters away and the bullseye is about 1 mm in diameter. we have one opportunity to chuck that dart and hit the bullseye spot on (for the parameters of the universe to be friendly enough to allow for our existance). if we fail to do that and miss it by even a little bit, then the Universe, at it would exist, would go utterly unnoticed. it that scenario, i think it's reasonable to wonder how we could be so lucky to have a Universe that even has matter with atoms and molecules and such, let alone sentient living beings.
now, if our universe was merely one in a zillion (or an infinite number of universes), then it's like, instead of one dart and one toss at the dartboard, we have a bottomless bin full of darts and all of the "time" till eternity to hit that bullseye. we could chuck these darts at random with no effort to aim, and eventually, it might take a million "years" (i know we are outside of time), we would, just by chance, hit the bullseye and that universe would have the rare conditions that would allow life such as ours to be eventually created out of the primordial elements. perhaps all of the other universes would go utterly unnoted. combined with this hypothesis of a Multiverse, then the weak Anthropic Principle actually says something that helps explain why these fundamental constants (and the initial conditions or this particular universe) could come out the way they had without it being unbelievably extraordinary.
i know (or have read) that string theory (or "branes") can point to the concept of the multiverse, but, for the most part, i think that the multiverse is mostly a concept - wishful thinking, in fact - of desperate atheists that have no other persuasive explanation for why the Universe, particularly one that has parameters set "just right" (as with Goldilocks), should bother to exist at all. we all have our tenets of faith. some of us might admit it.
so that's my answer to sysreset's question.
And the reality is that Christopher Columbus wasn't initially looking for the "New World." He was looking for a new route to India. Hence, the name West Indies for the Islands of the Caribbean. But the real moral of this story and throughhout the history of science is that some of the greatest discoveries were discovered by accident, luck and a spurt of inspiration, and this is probably how we will stumble on the discovery of other Universes.
perhaps, but i think that is wishful thinking. like discovering a means of interstellar travel at "warp speeds". makes for nice science fiction, but i am not confident at all about it happening for real.
this is different than people skeptical of human flight in the 19th century (and earlier) saying "If man were meant to fly, God would have given him wings." a skeptic of human-engineered heavier-than-air flight seemed to consistently ignore that the nature was demonstrating its possibility, since birds and bees were heavier than air. it was already happening and it really was "just" a matter of learning the principles and doing the engineering.
but to say humans will someday measure or observe physical reality that exists outside of the observable universe is more like saying someday humans will figure out how to travel faster than light or back and forth in time or something like that. whereas i have no crystal ball and cannot see into the future, i am still quite dubious of any of that.
Wouldn't physical reality and the observable Universe both be part of "nature?" If we are already aware that what we see is not all that exists then aren't we part of the way there (there being physical reality) just as your example of the birds in flight indicate nature has shown that flight is possible? It may be we just don't recognize the signs nature has given us.
But in general, I concur with your statement. This is a much more complex issue than the easily found proof of a bird in flight showing us that flight is possible. There wasn't much difficult scientific investigation in identifying nature's example of flight.
but we're not "already aware" of any other universes than the one we observe. unless you dabble in metaphysics, we are not aware of anything other than or outside of the observable universe. anything else is hypothesis, not awareness.
it's not the same as with dark matter or dark energy, because there is some observational consequence (we see something) that leads us to think that there must be something like dark matter and dark energy out there). it is not outside the observable universe. all's that we know about the finite extent of the observability of the universe we are in is simply that, our ability to see anything is limited by, what, is it 14 billion lightyears? maybe there is something outside of that or maybe not. maybe there is some other parallel universes in the physical reality (that are totally unobservable to us) or maybe not. we do not know and i seriously doubt humans will never know.
Agnosticism applied to the existance of other universes seems quite prudent to me.
well, i think that measuring the existance of other universes is more like super-lightspeed travel or Vulcan mind-melds (or ESP, for that matter) and less comparable to the mistake that people 200 years ago made regarding heavier-than-air flight (when examples of heavier-tha-air flight was flying all around them). it's like measuring the existance of God with some physical instrument. ain't gonna happen.
Yes. Very true.
My feeling is that our DNA, and knoweledeges, are not ready for theories like the one on multiuniverses yet, however today we are not still discussing if the earth is a dish at the center of the universe.
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