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  Everett himself never wrote about quantum immortality, but he was apparently aware of its possibility. One of his work colleagues, Keith Lynch, reported that he once discussed the idea with Everett, who declared himself to be a firm believer in the concept. This may have been what he believed, but in our world Everett died of a heart attack in 1982, at the relatively young age of fifty-one – a victim of years of poor diet and lack of exercise.

  On a darker note, his daughter Liz, who struggled with addiction for much of her life, committed suicide in 1996 at the age of thirty-nine. She left behind a note in which she asked her family to throw out her ashes in the trash, as her father, a lifelong atheist, had similarly requested for himself. That way, she wrote, ‘I’ll end up in the correct parallel universe to meet up w/ Daddy.’

  Critics view quantum immortality as a self-evidently preposterous notion that casts doubt on the entire many-worlds theory. It’s hard to argue against this. We don’t ever encounter immortals wandering around. Everyone, it seems, dies sooner or later. And yet, the many-worlds theory does explain the mystery of superposition very well. What better explanation is there? This has led some of its supporters to seek a way to remove quantum immortality from the mix. With it gone, the thinking goes, Everett’s theory would become that much more credible.

  Tegmark, for example, has argued that immortality would only be guaranteed if life-and-death events could always be reduced to a set of binary possibilities, such as a car speeding by that either hits you or doesn’t. It’s a situation that has only two outcomes. The ageing process, however, isn’t binary; it involves millions of cumulative events building upon each other, and this might eventually overwhelm the odds, making death inevitable even in a many-worlds universe.

  If Tegmark’s take on the many-worlds theory is correct, some of our parallel selves might live a long time, but they’ll all perish sooner or later, experiencing a variety of different deaths. But what if he’s wrong? What if we all really are going to live forever?

  That’s the interesting thing about the many-worlds prediction of immortality. You don’t have to conduct a quantum-suicide experiment to determine if it’s true. In fact, you don’t need to do anything at all. If Everett was right, in due course you’re guaranteed to find out.

  CHAPTER TWO

  A Pale Blue Peculiar Dot

  We’ve just looked at the universe in its entirety. Imagine it before us, stretching out on either side to infinity. Astronomers believe that, from a vast distance (if it were somehow possible to see it from the outside), it might appear smooth and featureless. But now let’s move in closer. As we approach, hints of structure emerge. We can see the dim outline of millions of galaxies. They’re not evenly spaced out. Instead, they’re arranged into vast shapes, like filaments and walls that span up to a billion light years in length, criss-crossing space like a web. And within these structures there are countless smaller knot-like groups of galaxies.

  We’ll direct our attention towards one of these groups, the supercluster Laniakea. It’s over 500 million light years across. So, we’re still seeing the universe on an unimaginably vast scale. But, zooming in faster, one galaxy near the outer edge of Laniakea is our target: the spiral-armed Milky Way. It’s a mere 100,000 light years across, although it may contain as many as 400 billion stars. No one knows the exact number because it’s not possible to count them all. We can only make a guess based on the estimated mass of the galaxy.

  As our descent continues, we head towards one of the smaller arms of the Milky Way, the so-called Orion Arm, and here, about two thirds of the way from the centre of the galaxy, we finally arrive at our destination: a medium-sized yellow dwarf star that has eight planets in orbit. The third planet is particularly striking. It’s like a brilliant blue, green and white marble suspended in the vacuum of space. This, of course, is our home: the solar system and Earth.

  This region, which is studied by astronomers and planetary scientists, will be our focus in this section. You might think that because we’re in a more familiar setting the theories about it will be somewhat more restrained, but, as we’ll see, the solar system is no less full of mysteries and controversies than the cosmos itself.

  What if the Earth is at the centre of the universe?

  Where is the Earth located in the universe? If you had asked scholars this question 500 years ago, they would have had a ready answer. It’s right at the centre! The sun, moon, planets and stars all revolve around it. This had been the accepted belief for over 2,000 years, since the ancient Greeks, Egyptians and Babylonians had first begun to study astronomy.

  But ask astronomers the same question today and the answer is no longer as simple. They’ll definitely tell you we’re not at the centre; they’re sure of that. In fact, the idea has come to seem hopelessly naive – a product of a time when people simply didn’t know as much about the cosmos and so assumed that humans occupied a privileged place in it.

  If you press for a more specific answer, astronomers might give you our location in relation to other things around us. The Earth, they’ll say, is the third planet from the sun in the solar system, which is part of the Milky Way Galaxy, which in turn neighbours the Andromeda Galaxy. Both of these are part of the Laniakea supercluster, which is home to some 100,000 galaxies.

  And what if you keep pressing, asking for the Earth’s absolute location in the universe? Are we near its top or bottom? It’s southern edge, perhaps? This is where things get more complicated. Astronomers will explain that there actually is no such thing as an absolute location, because the universe doesn’t have an overall shape or structure against which we could orient ourselves. On a cosmic scale, there are no large-scale features, such as a boundary or a centre, from which we could derive our position.

  But what if there actually are? What if the universe does have an overall structure that can provide some orientation, and, when we locate ourselves with respect to this, the Earth turns out to be right at the centre? This was the curious argument made by the South African cosmologist George Ellis in an article published in the journal General Relativity and Gravitation in 1978.

  Ellis wasn’t some kind of crackpot. If he had been, critics could have filed his idea away with the rants of flat-Earth theorists and other members of the lunatic fringe. To the contrary, he had established his credentials as one of the foremost cosmologists in the world by co-authoring The Large Scale Structure of Space–Time with Stephen Hawking. This book, which came out in 1973, is considered to be one of the classic texts of cosmology.

  In line with not being an oddball, Ellis didn’t claim that the Earth was at the centre of the solar system. That would have been really crazy. Instead, he was concerned about the Earth’s position in the universe as a whole – the big picture of our place in the cosmos. Not that this made his argument any more palatable to cosmologists.

  Ellis’s argument was relatively straightforward. He noted that observational data had led astronomers to conclude that the universe has no overall structure and no centre. But this same data, he maintained, could potentially be used to produce a model of the cosmos that does have a centre, with the Earth located right there. So, if the same data could yield two different but equally valid models, then the choice of which one to adopt had to be seen as a matter of philosophical preference rather than of scientific necessity.

  So, what were these observations that had led astronomers to decide the universe was centreless? More than anything else, it was Edwin Hubble’s discovery that the universe is expanding. In the 1920s, a powerful new telescope installed at California’s Mount Wilson Observatory had allowed him to discern that the blurry nebulae in the night sky, whose nature had long puzzled astronomers, were actually distant galaxies, and, upon closer inspection, he realized that almost all these galaxies, in every direction, were rushing away from us at enormous speeds.

  A simplistic interpretation of this observation would have been that the Milky Way was at the centre of the universe and that eve
ry other galaxy was being flung away from it by some powerful force. But then, why would this force be acting on everything except the Milky Way? That didn’t make sense. Instead, astronomers decided that the recession of the galaxies had to mean that everything in the universe was expanding away from everything else. And it was this conclusion that eliminated the centre from the universe.

  The analogy of a rubber balloon is often used to describe the concept. It asks us to imagine small paper dots glued onto a balloon, the surface of which stretches as it inflates, causing all the dots to move away from each other. The bigger the balloon grows, the more space separates the dots. If you were a microscopic person standing on one of the dots, you’d look out and see the other dots all receding into the distance, which might lead you to think that your dot occupied a central position on the surface of the balloon. But that interpretation would be wrong, because the balloon’s surface has no centre. (Pretend it doesn’t have an opening for air.) The view from every dot is the same. No matter which dot you stand on, you’ll always see other dots receding into the distance.

  In the same way, astronomers explain, the expanding universe has no centre. When we look out at the cosmos, we see galaxies receding from us on all sides, but, wherever we might happen to be in the universe, we’ll always see the same phenomenon, because everything is expanding away from everything else.

  The balloon analogy is potentially misleading, because a balloon does have an inside and an outside, which the universe doesn’t. But, if you can somehow imagine a balloon that lacks these spatial qualities, but which is expanding at all points, then this is the model of the universe that came to be widely accepted by scientists in the 1930s, and continues to be accepted today.

  What Ellis proposed as an alternative to the expanding-balloon universe was a universe shaped like an egg (though he didn’t call it that). He named it the ‘static spherically symmetric’ universe. But it was, in essence, an egg universe, standing upright.

  Each pointy end of the egg represented a centre – or rather, a centre and an anti-centre. At the top of the egg, there was a cool, relatively empty region of space, while at the bottom there was a naked singularity, a region of ultra-dense high-gravity matter, like that found at the centre of a black hole, but with no event horizon. In between the top and bottom, rings of galaxies floated, more spaced out towards the top, but increasingly dense as they neared the singularity at the bottom.

  This is a gross simplification of his model, which was far more theoretically sophisticated. To put it in a way closer to his description, he envisioned a universe with a very high concentration of galaxies at the centre and a sparser concentration around the edges. Anyone at the centre would see galaxies falling inwards, pulled by the region’s high gravity, but Ellis then used Einstein’s theory of relativity to imagine space curving in on itself in such a way that it created a second centre (or ‘anti-centre’) at the low-density edges. In his diagrams, this resulted in an egg-shaped universe, and, for our purposes, that image will suffice. Crucially, anyone living at the top of the egg (the anti-centre) would see galaxies rushing away in all directions, falling towards the bottom, high-gravity centre.

  Ellis’s egg universe neither expanded nor contracted. It had no beginning in time, nor would it have an end. It had always existed in the same form and always would, because the singularity at the bottom acted as a kind of cosmic recycling centre, consuming old matter from burned-out galaxies and spewing it back out as hot, fresh hydrogen that eventually formed into new galaxies.

  Ellis placed our galaxy at the very top of the egg, in the low-density anti-centre. As such, this was an identifiable, unique location in the universe, but he emphasized that being at this spot didn’t mean that we, as a species, were somehow special. To the contrary, the anti-centre just happened to be the only place in this universe cool enough for life to survive. It was the only place where we could possibly be.

  This universe Ellis had dreamed up was extremely odd – he acknowledged that – but his point was that it could very well be the universe we actually inhabit. After all, the cosmic view from the top of the egg would be exactly the same as the view seen if we were living on a dot in the inflating-balloon universe of standard cosmology. In each case, we would see galaxies moving away from us on all sides. In the inflating-balloon universe, this effect would be caused by the expansion of the entire universe, while in the egg universe it would be caused by galaxies being pulled towards the high-gravity singularity. But, from our location, there would be no way to tell the two apart. Both models would produce identical observational data.

  Is that it? Do we live at the top of a giant egg? If so, this would completely upend modern cosmology. After all, Ellis’s model summarily dispensed with the Big Bang, imagining the universe to have no beginning. Astrophysicists reacted to this with incredulity. Reviewing it in Nature, the physicist Paul Davies joked that it was lucky for Ellis that burning at the stake for heresy had gone out of fashion.

  Of course, the egg universe hasn’t brought down modern cosmology. As Ellis continued to tinker with his model, he decided that he couldn’t get it to work to his satisfaction with Einstein’s general theory of relativity, and this led him to abandon the idea. So much for the egg universe.

  But this didn’t alter the bigger point he was trying to make, which remained as relevant as ever. This was the problem of the large-scale structure of the universe – that it has the potential to trick us, undermining our attempts to make sense of the cosmos in which we live.

  Based on the astronomical observations that we have, the standard, inflating-balloon model of the universe is, in fact, a logical conclusion to arrive at. It’s entirely reasonable to assume that, if we see galaxies receding from us on all sides, the same phenomenon is occurring everywhere else in the universe. This is the simplest assumption to make.

  The problem is, just because this assumption is simple and reasonable doesn’t mean it’s correct. The universe never gave any guarantee that it was going to make things easy for us. And we can never zoom out from our current position to get a grand view of the whole universe in order to confirm that the inflating-balloon model actually is correct. So, the possibility always remains that some other, weirder, more complex large-scale way in which the cosmos is structured could be producing all the effects we observe.*

  After abandoning the egg model, Ellis proceeded to make it something of a hobby to dream up alternative cosmic structures of this kind. One of his ideas is that we may live in a small universe. Astronomers believe the universe is infinite in size, but Ellis suggested that maybe it’s only several hundred million light years across, but it curves around so that, if you were able to travel that entire distance, you’d end up back where you started. This would mean there aren’t as many galaxies surrounding us as we think; it’s just an optical illusion. We’re seeing the same patch of galaxies over and over, as if we’re staring into two parallel mirrors producing an endless series of reflections. Ellis happily concedes that this probably isn’t how the universe really is, but there’s no way to know for sure.

  Another of his ideas is that while the Earth may not be at the centre of the entire universe, it may be at the centre of a ‘cosmic void’. This was suggested to him by the discovery, made in the late 1990s, that the expansion of the universe is accelerating. To account for this, astronomers have theorized that a mysterious ‘dark energy’ is causing the acceleration, but Ellis proposed instead that the distribution of matter in the universe may be uneven. In most of it, he said, the density of matter is very high, but there are bubbles of space, ‘cosmic voids’, in which it is low. Our galaxy might be floating in the centre of one of these low-density voids, and the galaxies trapped in this bubble with us are all falling outwards, pulled towards the surrounding regions of higher gravity at an ever-increasing rate. If this is the case, the apparent accelerating expansion of the universe would only be a phenomenon local to our region of space.

  Again, Ell
is doesn’t insist that this is the way the universe really is. It’s more of a philosophical point he’s trying to make about the nature of cosmology itself. It’s a curious discipline, because cosmologists study an object (the cosmos) the vast majority of which they can never see or interact with. It’s a bit like trying to figure out what a building looks like when all you’ve got to go on is one brick. The best you can do is to assume that the entire structure is similar to that one brick and proceed on that basis. But what if the brick isn’t actually representative of the building as a whole? What if it was part of a facade? You’d have no way of knowing.

  That’s what it’s like for cosmologists. They try to figure out what they can about the nature of the universe based on the assumption that the part of it we can see is representative of the whole thing. But there’s no way to be certain. There’s an absolute, hard limit on our knowledge, forcing us to live with the nagging suspicion that the universe might actually have some kind of large-scale structure completely different from anything we could ever imagine.

  What if planets can explode?

  If you’re the type that’s prone to worry, the natural world offers a lot to fret about: hurricanes, earthquakes, tsunamis, super-volcanoes, asteroid impacts and any number of other threats could spell your doom. But what about exploding planets? What if Mars suddenly exploded and showered us with meteors? Or, even worse, what if the Earth itself burst apart without warning in a fiery cataclysm? Could this happen? Should you add the possibility to your list of concerns?