"The idea of eternal return is a mysterious one, and Nietzsche has often perplexed other philosophers with it: to think that everything recurs as we once experienced it, and that the recurrence itself recurs ad infinitum! What does this mad myth signify?"
Milan Kundera, The Unbearable Lightness of Being
If matter exists in finite quantities while time is infinite, everything has to occur more than once - this is more or less the modern scientific expression of the 'mad myth' that Nietzsche excavated from its Egyptian tomb.
However, contemporary cosmology has even more perplexing notions up its sleeve. For if space is infinite, the limited number of possible arrangements of protons means that many of these recurrences may actually be ocurring concurrently — your double is repeating your actions for you.
Fanciful? According to science writer Marcus Chown in The Never Ending Days of Being Dead, your nearest double is a long way away, but he or she is fully predicted by our current model of the universe.
"All that is really necessary is an assemblage of matter identical to you. Roughly speaking that means about 10˄28 particles. So how far would we have to travel to find another identical assemblage? Well, if there are 10˄28 possible locations for protons, there are 10˄10˄28 possible arrangements of those protons. And this means your nearest double is just 10˄10˄28 metres away."
This is of course a lot further than we can currently see. This spatial horizon 'is 13.7bn light years, a limit set by the age of our local universe. But we have grounds for considering that it goes on forever — technically at least —- because of what we have been able to deduce about its earliest moments.
It is often said of the Big Bang that it was neither big nor indeed a bang (it not being an explosion as such). It is also the case that it was probably neither the beginning nor indeed a singular event, according to our current standard model of cosmology.
Central to the latter is the idea of Inflation. This was the moment -—literally a fraction of a second — before what we call the Big Bang, when our universe doubled and redoubled its volume as many as eighty times over.
How did this happen? Well, one of the consequences of quantum theory is that there is no such thing as nothing. The vacuum isn't empty, in fact by default it contains energy which is constantly flicking into and out of existence, but in sum, comes to more than zero.
In a process still only partially understood, this initial 'void' acquired the propensity to inflate through the repulsive gravity which emerges when the pressure of a material is large and negative enough to cancel out the energy density which Einstein revealed as the 'cause' of gravity. Chown has a rather handy metaphor that can help lay persons, such as myself, grasp the implications of this:
"Imagine holding a stack of bank notes between your hands, pulling your hands apart and discovering that ever more bank notes fountain out of nothing to fill the gap. That was how the vacuum at the beginning of time was. As the universe grew, energy was literally conjured out of nothing. Inflation, as many physicists have remarked, was the 'ultimate free lunch'"
Big Bangs have been going off like a rastro of cohetes (without the bangs of course) whenever and wherever the inflation came to an end, dumping energy into local 'bubble' universes in the form of superheated matter.
But the false vacuum that drives inflation expands far faster than it can decay into a normal vacuum, so 'outside' the region of matter that we can see — the consequences, if you like, of our own Big Bang — the process continues. And this is why our own space is technically infinite — because the boundary between it and the continually inflating false vacuum — the pregnant void — is receding faster than light and is therefore unreachable; a technicality which allows us to speak of our own material universe as infinite to all intents and purposes.
And this is why there must be many other versions of me doing the same or similar things I'm doing right now, somewhere out there in spacetime.
And if that weren't enough, there are all those parallel universes predicted by the Many Worlds interpretation of quantum phenomena — knowing that a fundamental property of particles is the ability to be in more than one place at the same time, yet unable (barring one much-debated exception) to observe this in our own universe, a school of thought exists which holds to the notion that every time one of these tiny quantum 'choices' is made, a splitting or bifurcation of the universe into a pair of alternative streams occurs, resulting in a vast structure known as the Multiverse.
Oxford's David Deutsch has a great hypothesis covering how time travel works in the Multiverse — you can go back in time, but it's never the same quantum track you came from, so in a sense time-travel is a special form of space travel, only one moves between parallel realities. More on that another day...
I long wanted to write a novel that fictionally explored the framework of multiple universes, but it seems that bastard Iain Banks has beaten me to it.
Actually, the idea has been bouncing around serious science fiction for quite a while. In Larry Niven's All The Myriad Ways people able to jump between the various versions of reality have been topping themselves, unable to cope with the knowledge that nothing we ever do really matters, because every time we succede another version of us fails.
And this brings us back to The Unbearable Lightness of Being and the existential confusion served up by the idea of eternal recurrence. In practical terms even suicide is pointless because one of your not-so-hypothetical doubles in time, space or even quantum reality, is bound to choose life over death.