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.

There is another you, sitting on an identical Earth, about 10 to the power of 10 to the power of 120 light years away

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 with Hamlet. 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.’

The quilted multiverse is only the beginning. In 1999 in Los Angeles, the Russian émigré physicist Andrei Linde invited a group of journalists, myself included, to watch a fancy computer simulation. The presentation illustrated Linde’s own idea of an ‘inflationary multiverse’. In this version, the rapid period of expansion that followed the Big Bang did not happen only once. Rather, like Trotsky’s hopes for Communism, it was a constant work in progress. An enormous network of bubble universes ensued, separated by even more unimaginable gulfs than those that divide the ‘parallel worlds’ of the quilted multiverse.

Here’s another one. String Theory, the latest attempt to reconcile quantum physics with gravity, has thrown up a scenario in which our universe is a sort of sheet, which cosmologists refer to as a ‘brane’, stacked up like a page in a book alongside tens of trillions of others. These universes are not millions of light years away; indeed, they are hovering right next to you now.

That doesn’t mean we can go there, any more than we can reach other universes in the quantum multiverse, yet another ‘flavour’. This one derives from the notion that the probability waves of classical quantum mechanics are a hard-and-fast reality, not just some mathematical construct. This is the world of Schrödinger’s cat, both alive and dead; here, yet not here. Einstein called it ‘spooky’, but we know quantum physics is right. If it wasn’t, the computer on which you are reading this would not work.

The ‘many worlds’ interpretation of quantum physics was first proposed in 1957 by Hugh Everett III (father of Mark Everett, frontman of the band Eels). It states that all quantum possibilities are, in fact, real. When we roll the dice of quantum mechanics, each possible result comes true in its own parallel timeline. If this sounds mad, consider its main rival: the idea that ‘reality’ results from the conscious gaze. Things only happen, quantum states only resolve themselves, because we look at them. As Einstein is said to have asked, with some sarcasm, ‘would a sidelong glance by a mouse suffice?’ Given the alternative, the prospect of innumerable branching versions of history doesn’t seem like such a terrible bullet to bite.

There is a non-trivial probability that we, our world, and even the vast extensions of spacetime are no more than a gigantic computer simulation

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.

String theory affords other possibilities, and yet more layers of multiverse. But the strangest (and yet potentially simplest) of all is the idea that we live in a multiverse that is fake. According to an argument first posited in 2001 by Nick Bostrom, professor of philosophy at the University of Oxford, there is a non-trivial probability that we, our world, and even the vast extensions of spacetime that we saw in the first multiverse scenarios, are no more than a gigantic computer simulation.

The idea that what we perceive as reality is no more than a construct is quite old, of course. The Simulation Argument, as it is called, has features in common with the many layers of reality posited by some traditional Buddhist thinking. The notion of a ‘pretend’ universe, on the other hand, crops up in fiction and film — examples include the Matrix franchise and The Truman Show (1998). The thing that makes Bostrom’s idea unique is the basis on which he argues for it: a series of plausible assumptions, plus a statistical calculation.

In essence, the case goes like this. If it turns out to be possible to use computers to simulate a ‘universe’ — even just part of one — with self-aware sentient entities in it, the chances are that someone, somewhere, will do this. Furthermore, as Bostrom explained it to me, ‘Look at the way our computer simulations work. When we run a simulation of, say, the weather or of a nuclear explosion [the most complex computer simulations to date performed], we do not run them once, but many thousands, millions — even billions — of times. If it turns out that it is possible to simulate — or, more correctly, generate — conscious awareness in a machine, it would be surprising if this were done only once. More likely it would be done countless billions of times over the lifetime of the advanced civilisation that is interested in such a project.’

If we start running simulations, as we soon might, given our recent advances in computing power, this would be very strong evidence that we ourselves live in a simulation. If we conclude that we are, we have some choices. I’ll say more on those below.

First, we come to the most bizarre scenario of all. Brian Greene calls it the ‘ultimate multiverse’. In essence, it says that everything that can be true is true. At first glance, that seems a bit like the quilted multiverse we met earlier. According to that hypothesis, all physical possibilities are realised because there is so much stuff out there and so much space for it to do things in.

Those who argue that this ‘isn’t science’ are on the back foot. The Large Hadron Collider could find direct evidence for aspects of string theory within the decade

The ultimate multiverse supercharges that idea: it says that anything that is logically possible (as defined by mathematics rather than by physical reality) is actually real. Furthermore, and this is the important bit, it says that you do not necessarily need the substrate of physical matter for this reality to become incarnate. According to Max Tegmark, professor of physics at the Massachusetts Institute of Technology, the ‘Mathematical Universe Hypothesis’ can be stated as follows: ‘all structures that exist mathematically also exist physically‘. Tegmark uses a definition of mathematical existence formulated by the late German mathematician David Hilbert: it is ‘merely the freedom from contradiction’. Hence, if it is possible, it exists. We can allow unicorns but not arbitrary, logic-defying magic.

What does all this mean? If we live in a world of infinite possibilities existing across numerous dimensions, what is the point of trying to make sense of any of it? Does any of it have the slightest bearing on how we ought to live?

For the most part, scientists have no answer to these questions, except to repeat Churchill’s maxim to ‘keep buggering on’. But some have come up with tentative suggestions. Robin Hanson, an economist at George Mason University, has written several commentaries on Bostrom’s simulation thesis. His conclusions are rather depressing: ‘If our descendants prefer their simulations to be entertaining, all else equal, then you should want you and the events around you to be entertaining as well, all else equal … Be funny, outrageous, violent, sexy, strange, pathetic, heroic … in a word “dramatic”.’

The main thing is to make your story so compelling that people want to simulate you again. Forget being good: what does morality mean in a simulated universe anyway? Instead, be interesting — be Hitler, Jesus or Princess Diana, because it magnifies the chances that you will be reincarnated the next time the universe boots up, or be brought back when the audience decides your character is too interesting to kill off.

Perhaps the most extraordinary thing about all these arguments is how drily respectable they are. Martin Rees, the British Astronomer Royal, is a fully paid-up member of the multiverse club. His main argument is that ‘our’ universe appears to have been suspiciously fine-tuned to allow the existence of life, the so-called anthropic principle. Change any of the basic parameters even slightly — the strength of the Strong Nuclear Force, or the gravitational constant — and you end up with a dull universe that is either a sea of radiation or a black hole.

There are those who see here the hand of god. Others say that the anthropic principle is just a special case of selection bias. Think of a very fat, very short man who walks into a shop looking for a suit. If it’s a small shop, he would be right to be surprised if he found a suit that fitted. But if the shop was big and had thousands of suits, of almost all conceivable dimensions, there would be no surprise at all. Similarly, we should not be amazed to discover we live in a universe that can contain life if there were a lot of universes to choose from.

Some accuse the multiversers of technical-mysticism, of putting their faith in claims that are fundamentally unfalsifiable. To confirm string theory, for example, would require a particle accelerator the size of Betelgeuse. But those who argue that this ‘isn’t science’ are on the back foot. It is just possible that the Large Hadron Collider will detect direct evidence of the extra dimensions needed by string theory, for instance, and provide hints as to whether ‘our’ gravity is the weak remnant of a force operating at far greater strength in another universe. This could happen within the decade, not in some theoretical Star Trek future. Meanwhile, better telescopes could resolve the issue of whether our universe is really ‘flat’ and infinite.

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. Brian Greene finds this amazing and, clearly, rather wonderful.

So, from my own position of profound befuddlement, do I. But I also find it rather troubling. There are things missing from the multiverse: an intelligible place for consciousness, for one. Then there is the sense that, in a world where all possibilities become certainties and anything that can happen does happen, moral purpose is even more elusive than in the old-fashioned singular universe. If your evil twin is out there (which, in an infinite ‘flat’ universe, he or she certainly is), what does it matter what you do in your bit of eternity? For half a millennium science has been chipping away at the idea that humanity is central and unique. The multiverse replaces the chisel with a wrecking ball.