Behind the ecclastical ritual, what we see is a crusading conjuncture:
the Albigensan crusade, the Spanish crusade, and the crusade of the pueri, all contemporaneous.
Our understanding of the fundamental nature of reality is changing faster than ever before. Not only must we accept that basic properties of our universe are accidental and uncalculable. In addition, we must believe in the existence of many other universes. But we have no conceivable way of observing these other universes and cannot prove their existence. Thus, to explain what we see in the world and in our mental deductions, we must believe in what we cannot prove.
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The multiverse hypothesis is the idea that what we see in the night sky is just an infinitesimally tiny sliver of a much, much grander reality, hitherto invisible. The idea has become so mainstream that it is now quite hard to find a cosmologist who thinks there's nothing in it.
First, some semantics. The old-fashioned, pre-multiverse 'universe' is defined as the volume of spacetime, about 90 billion light years across, that holds all the stars we can see (those whose light has had enough time to reach us since the Big Bang). This 'universe' contains about 500 sextillion stars - more than the grains of sand on all the beaches of Earth - organised into about 80 billion galaxies. It is, broadly speaking, what you look up at on a clear night. It is unimaginably vast, incomprehensibly old and, until recently, assumed to be all that there is. Yet recent discoveries from telescopes and particle colliders, coupled with new mathematical insights, mean we have to discard this 'small' universe in favour of a much grander reality. The old universe is as a gnat atop an elephant in comparison with the new one. Moreover, the new terrain is so strange that it might be beyond human understanding.
Stranger still is the holographic multiverse, which implies that 'our world' - not just stars and galaxies but you and your bedroom, your career problems and last night's dinner - are mere flickers of phenomena taking place on an inaccessible plane of reality. The entire perceptible realm would amount to nothing more than shapes in a shadow theatre. This sounds like pure mysticism; indeed, it sounds almost uncannily like Plato's allegory of the cave. Yet it has some theoretical support: Stephen Hawking relies on the idea in his solution to the Black Hole information paradox, which is the riddle of what happens to information destroyed as it crosses the Event Horizon of a dark star.
The world we see is an illusion, albeit a highly persistent one. We have gradually got used to the idea that nature’s true reality is one of uncertain quantum fields; that what we see is not necessarily what is.
Dark matter is a profound extension of this concept. It appears that the majority of matter in the universe has been hidden from us.
Nature plays an epistemological trick on us all. The things we observe each have one kind of existence, but the things we cannot observe could have limitless kinds of existence. A good theory should be just complex enough. Dark matter is the simplest solution to a complicated problem, not a complicated solution to simple problem. Yet there is no guarantee that it will ever be illuminated. And whether or not astrophysicists find it in a conceptual sense, we will never grasp it in our hands. It will remain out of touch. To live in a universe that is largely inaccessible is to live in a realm of endless possibilities, for better or worse.
We should not be surprised by the multiverse. Every time we have taken a look at the world around us, it has expanded. Copernicus realised that the Earth was not the centre of creation. Edwin Hubble realised that the Milky Way was just one galaxy among billions. Now we suspect that 'reality' is, in fact, something so magnificently vast that we struggle even to comprehend the parameters of how to describe it.