Comment Re:Silly, silly, silly... (Score 1) 226
In that part of the universe that is visible. We have no idea how much of the universe is beyond the ~14 billion light year visible boundary, only that it very likely extends at least many times as far beyond.
We absolutely do not know if there are more non-simply connected, non-interacting universes, because by definition you can't get there from here and cannot see them in any way -- but that does not mean that they do not exist, nor does it mean that they do. It means that we do not know if they exist because we have no evidence for their existence. We absolutely do not know if there are more coupled universes -- universes that one might be able to reach -- with independent space-time, because whatever coupling there might be between them, it is weak.
Finally, we do not know even in the simplest case that the many worlds quantum theorists hypothesize whether or not the bundle of universes is coherent, so that there are indeed lots of "nearby" universes with alternate-me as part of a coherent quantum bundle of me embedded in a coherent quantum bundle of universes that is semi-classical in evolution but knit together (entangled) with many little quantum jumps at the planck scale, or if you prefer the macroscopic cosmological hypothesis (which is pretty much completely without empirical foundation or any likelihood of obtaining any) of universes bubbling up in some sort of overarching ether with coupling constants that vary (without any explanation of how they might vary, or what sort of space of possible values bounds or does not boud the variation, or the physics that couples them with meta-coupling constants -- do you get the feeling that this is just mathematical science fiction, because it is -- we do not have any evidence that one single one more of these hypothetical universes exist.
It's like planets. Before 1990 there were lots of theories for planetary formation, and science fiction writers had been writing stories involving planets around other stars, usually with highly evolved life, just taking it for granted that they exist. But nobody had ever seen one, so that the statement "there is a planet within 100 LY of Earth with highly evolved life" was at best a scientific hypothesis unsupported by any evidence, but really was a complex form of science fiction because science had no way of sensibly bounding the probability that such a statement is true and there are a near-infinity of possibly true statements and we quite rightly give little credence to almost all of them without evidence and a really, really good reason to think that they might be true in the physics.
Today we have built instrumentation that permits us to "see" these "alternate universes" -- very crudely, but even that is improving over time as we build still-better instrumentation -- and we have at least some idea of what the density of planets of at least a certain size is in our immediate vicinity, much as we built the Hertzsprung-Russell diagram from parallax observations of nearby stars. We therefore have some small empirical basis for a model of planetary density around third generation stars (that probably doesn't extend into specialized environments, e.g. globular clusters). At the same time we are learning, somewhat painfully, lots of things about planetary chemistry and planetary ecology from observations within our own solar system and MIGHT -- as soon as we can sensibly measure some of the gross signatures of these things with even BETTER instrumentation -- soon get an idea of whether or not it is at all probable that there be highly evolved life within 100 LY. By an idea, of course I mean some statement of probability based on observations that has some statistical significance, as opposed to a statement built on top of a teetering tower of Bayesian priors 2/3 of which have no observational basis at all, none of which can be improved by recomputation of posterior probabilities based on observations within some restricted set.
A single example might be oceans -- we do not know for certain what the origin(s) of Earth's ocean might be. There are different hypotheses, and not sufficiently sensitive way of choosing between them this far after the event because we cannot look back at time and measure the probability of e.g. water-bearing comet impacts in the critical period after it cooled enough to hold an atmosphere -- plus there is the further complication of the probable impact that produced the moon. As a consequence, we have no good way of determining the probability of there being an ocean on any rocky planet in the right size range that might be orbiting any of the stars within 100 LY. It could be that such an ocean is a nearly absolute requirement for the evolution of advanced life forms -- it isn't even improbable that it is. It could be that only planets with molten iron cores (and hence magnetic fields, near-orbit moons formed via collision like ours was and a sufficient density of water-bearing comets will have the right "mix" of atmosphere regenerating oceans plus tidal stripping of stuff like ammonia and methane plus protection from radiation needed to allow the nucleation and growth of life (and we don't have any data on just how probably the latter is anyway).
As a consequence, while observations of nearby large planets has made it far more probable that there are planets the size of the Earth as well, and hence has systematically increased the probability of a life-bearing planet within 100 LY, that probability (as a statement of our ignorance, not of reality where there either is or there isn't) is still too uncertain to take any estimate of it seriously -- the error bars add up to "still science fiction", not scientific knowledge.
To cite a slightly different kind of example, consider magnetic monopoles. Now I personally just love the idea of monopoles. I teach electrodynamics at all levels and anything with the name "Dirac" attached should be taken very seriously indeed. Physicists in general would love for monopoles to exist, I think, because it would complete the symmetry of Maxwell's Equations and explain the quantization of charge, and it would have a profound impact on the development of quantum theories of everything as well, as now one has to fork the theories at the point where one decides "monopoles or no monopoles" and the latter requires some fancy dancing to explain charge quantization and WHY Maxwell's equations have a broken symmetry. Really, another broken symmetry as there would still remain the +/- charge asymmetry to explain, but there might be a way to get a twofer if there are monopoles.
Sadly, so far there is no reproducible, accepted evidence for the existence of magnetic monopoles, so in spite of the many physical theories they would improve, in spite of the enormous compulsion to believe in them as a sort of Platonic Ideal extension of our existing theory, all the physicists I know are very careful to draw the proper scientific box around them and present them as an unproven hypothesis -- in this case a notch up from science fiction, as they have immediate explanatory power of widespread observations -- not as a known (very probably true) "fact" supported by direct observation.
That's what annoyed me about the top article. It is sloppy, headline-grabbing stuff, not science. Science has a clear empirical foundation that is simply missing, but they aren't providing the required disclaimer. If I wrote a scientific paper that proved that Santa's Little Elves (if they exist) dine on dark matter (if it exists) and fart dark energy (if it exists) it wouldn't matter how valid and elegant the mathematics of the proof were, until somebody puts salt on the tail of an Elf, some dark matter, and some dark energy. And there's the rub, because they are all, well, dark. I might as well assert that the universe is run by invisible fairies. Prove me wrong! Kinda hard, because they are invisible.
So are alternate universes, to us, so far.
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