World Next Door
Nine theories of the multiverse promise everything and more. But if reality is so vast and varied, where do we fit in?
Our understanding of the fundamental nature of reality is changing faster than ever before. Gigantic observatories such as the Hubble Space Telescope and the Very Large Telescope on the Paranal Mountain in Chile are probing the furthest reaches of the cosmos. Meanwhile, with their feet firmly on the ground, leviathan atom-smashers such as the Large Hadron Collider (LHC) under the Franco-Swiss border are busy untangling the riddles of the tiny quantum world.
Myriad discoveries are flowing from these magnificent machines. You may have seen Hubble’s extraordinary pictures. You will probably have heard of the ‘exoplanets’, worlds orbiting alien suns, and you will almost certainly have heard about the Higgs Boson, the particle that imbues all others with mass, which the LHC found this year. But you probably won’t know that (if their findings are taken to their logical conclusion) these machines have also detected hints that Elvis lives, or that out there, among the flaming stars and planets, are unicorns, actual unicorns with horns on their noses. There’s even weirder stuff, too: devils and demons; gods and nymphs; places where Hitler won the Second World War, or where there was no war at all. Places where the most outlandish fantasies come true. A weirdiverse, if you will. Most bizarre of all, scientists are now seriously discussing the possibility that our universe is a fake, a thing of smoke and mirrors.
All this, and more, is the stuff of the multiverse, the great roller-coaster rewriting of reality that has overturned conventional cosmology in the last decade or two. 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. This isn’t the world of the mystics, the pointy-hat brigade who see the Age of Aquarius in every Hubble image. On the contrary, the multiverse is the creature of Astronomers Royal and tenured professors at Cambridge and Cornell.
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.
That hasn’t stopped some bold thinkers from trying, of course. One such is Brian Greene, professor of physics and mathematics at Columbia University in New York. He turned his gaze upon the multiverse in his latest book, The Hidden Reality (2011). According to Greene, it now comes in no fewer than nine ‘flavours’, which, he says, can ‘all work together’.
The simplest version he calls the ‘quilted multiverse’. This arises from the observation that the matter and energy we can see through our most powerful telescopes have a certain density. In fact, they are just dense enough to permit a gravitationally ‘flat’ universe that extends forever, rather than looping back on itself. We know that a repulsive field pervaded spacetime just after the Big Bang: it was what caused everything to fly apart in the way that it did. If that field was large enough, we must conclude that infinite space contains infinite repetitions of the ‘Hubble volume’, the volume of space, matter and energy that is observable from Earth.
If this is correct, there might — indeed, there must — be innumerable dollops of interesting spacetime beyond our observable horizon. There will be enough of these patchwork, or ‘pocket’, universes for every single arrangement of fundamental particles to occur, not just once but an infinite number of times. It is sometimes said that, given a typewriter and enough time, a monkey will eventually come up withHamlet. Similarly, with a fixed basic repertoire of elementary particles and an infinity of pocket universes, you will come up with everything.
In such a case, we would expect some of these patchwork universes to be identical to this one. There is another you, sitting on an identical Earth, about 10 to the power of 10 to the power of 120 light years away. Other pocket universes will contain entities of almost limitless power and intelligence. If it is allowed by the basic physical laws (which, in this scenario, will be constant across all universes), it must happen. Thus there are unicorns, and thus there are godlike beings. Thus there is a place where your evil twin lives. In an interview I asked Greene if this means there are Narnias out there, Star Trek universes, places where Elvis got a personal trainer and lived to his 90s (as has been suggested by Michio Kaku, a professor of theoretical physics at the City University of New York). Places where every conscious being is in perpetual torment. Heavens and hells. Yes, it does, it seems. And does he find this troubling? ‘Not at all,’ he replied. ‘Exciting. Well, that’s what I say in this universe, at least.’