*Commuting days until retirement: ***91**

**91**

A theme underlying some of my recent posts has been what (if anything) happens to us when we die. I’d like to draw this together with some other thoughts from eight months ago, when I was gazing at the roof of Exeter Cathedral and musing on the possibility of multiple universes. This seemingly wild idea, beloved of fantasy and science fiction authors, is now increasingly taken seriously in the physics departments of universities as a serious model of reality. The idea of quantum immortality (explained below) is a link between these topics, and it was a book by the American physicist Max Tegmark, *The Mathematical Universe**, that got me thinking about it.

I won’t spend time looking at the theory of multiple universes – or the Multiverse – at any length. I did explain briefly in my earlier post how the notion originally arose from quantum physics, and if you have an appetite for more detail there’s plenty in Wikipedia. There are a number of theoretical considerations which lead to the notion of a multiple universe: Tegmark sets out four that he supports, with illustrations, in a Scientific American article. I’m just going to focus here on two of them, which as Tegmark and others have speculated, could ultimately be different ways of looking at the same one. I’ll try to explain them very briefly.

### The first approach: quantum divergence

It has been known since early in the last century that, where quantum physics allows a range of possible outcomes of some subatomic event, only one of these is actually observed. Experiments (for example the double slit experiment) suggest that the outcome is undetermined until an observation is made, whereupon one of the range of possibilities becomes the actual one that we find. In the phrase which represents the traditional ‘Copenhagen interpretation’ of this puzzle, *the wave function collapses*. Before this ‘collapse’, all the possibilities are simultaneously real – in the jargon, they exist ‘in superposition’.

But it was Hugh Everett in 1957 who first put forward another possibility which at first sight looks wildly outlandish now, and did so even more at the time: namely that the wave function never does collapse, but each possible outcome is realised in a different universe. It’s as if reality branches, and to observe a specific outcome is actually to find yourself in one of those branched universes.

### The second approach: your distant twin

According to the most widely accepted theory of the creation of the universe, a phenomenon known as ‘inflation’ has the mathematical consequence that the cosmic space we now live in is infinite – it goes on for ever. And infinite space allows infinite possibilities. Statistics and probability undergo a radical transformation and start delivering certainties – a certainty, for example, that there is someone just like you, an unimaginable distance away, reading a blog written by someone just like me. And of course the someone who is reading may be getting bored with it and moving on to something else (just like you? – I hope not). But I can reassure myself that for all the doppelgangers out there who are getting bored there are just as many who are really fired up and preparing to click away at the ‘like’ button and write voluminous comments. (You see what fragile egos we bloggers have – in most universes, anyway.)

### Pulling them together

But the point is, of course, that once again we have this bewildering multiplicity of possibilities, all of which claim a reality of their own; it all sounds strangely similar to the scenario posited by the first, quantum divergence approach. This similarity has been considered by Tegmark and other physicists, and Tegmark speculates that these two could be simply the same truth about the universe, but just approached from two different angles.

That is a very difficult concept to swallow whole; but for the moment we’ll proceed on the assumption that each of the huge variety of ramified possibilities that could follow from any one given situation does really exist, somewhere. And the differences between those possible worlds can have radical consequences for our lives, and indeed for our very existence. (As a previous post – *Fate, Grim or Otherwise* – illustrated.) Indeed, perhaps you could end up dead in one but still living in another.

### Quantum Russian roulette

So if your existence branches into one universe where you are still living, breathing and conscious, and another where you are not, where are you going to find yourself after that critical moment? Since it doesn’t make sense to suppose you could find yourself dead, then we suppose that your conscious life continues into one of the worlds where you are alive.

This notion has been developed by Tegmark into a rather scary thought experiment (another version of which was also formulated by Hans Moravec some years earlier). Suppose we set up a sort of machine gun that fires a bullet every second. Only it is modified so that, at each second, some quantum mechanism like the decay of an atom determines, with a 50/50 probability, whether the bullet is actually fired. If it is not, the gun just produces a click. Now it’s the job of the intrepid experimenter, willing to take any risk in the cause of his work, to put his head in front of the machine gun.

According to the theory we have been describing, he can only experience those universes in which he will survive. Before placing his head by the gun, he’ll be hearing:

* Bang… Click… Bang… Bang… Click… Click… Click… Bang… * …etc

But with his head in place, it’ll be:

*Click… Click… Click… Click… Click… Click… Click… Click… * …and so on.

Suppose he keeps his head there for half a minute, the probability of all the actions being clicks will be 2^{30}, or over a billion to one against. But it’s that one in a billion universe, with the sequence of clicks only, that he’ll find himself in. (Spare a thought for the billion plus universes in which his colleagues are dealing with the outcome, funerals are being arranged and coroners’ courts convened.)

### Real immortality

Things become more disconcerting still if we move outside the laboratory into the world at large. At the moment of any given person’s death, obviously things could have been different in such a way that they might have survived that moment. In other words, there is a world in which the person continues to live – and as we have seen, that’s the one they will experience. But if this applies to *every* death event, then – subjectively – we must continue to live into an indefinitely extended old age. Each of us, on this account, will find herself or himself becoming the oldest person on earth.

A natural reaction to this argument is that, intuitively, it can’t be right. What if someone finds themselves on a railway track with a train bearing down on them and no time to jump out of the way? Or, for that matter, terminally ill? And indeed Tegmark points out that, typically, death is the ultimate upshot of a series of non-fatal events (cars swerving, changes in body cells), rather than a single, once-and-for-all, dead-or-alive event. So perhaps we arrive at this unsettling conclusion only by considerably oversimpifying the real situation.

But it seems to me that what is compelling about considerations of this sort is that they do lead us to take a bracing, if slightly unnerving, walk on the unstable, crumbling cliff-edge which forms the limits of our knowledge. Which always leads me to the suspicion, as it did for JBS Haldane, that the world is ‘not only queerer than we suppose, but queerer than we *can* suppose’. And that’s a suitable thought on which to end this blogging year.

*Tegmark, Max, *Our Mathematical Universe: My Quest for the Ultimate Nature of Reality*. Allen Lane/Penguin, 2014