One of the things that is difficult to grasp about the many worlds interpretation is that it seems to belie our unitary experience of history and a largely integrated experience in the moment. The many worlds interpretation is the prevailing interpretation of quantum physics; I assume it to be valid in this article. A common way of discussing our experience in the many worlds hypothesis is to talk about how we’re in “just one” of the many worlds, or universes. This is not true, as it is observations of interference from adjacent worlds that has brought about the many worlds hypothesis. Therefore it is not the case that we are in one universe; we are somehow in many universes at once. It seems like, if there are an infinity of presents, that we should somehow feel multiplicitous, that we should be aware of the branching and recombination of all of these possible worlds. The solution to this seeming contradiction lies in understanding that we experience only great aggregates of phenomena.
Our phenomenal experience is truly a phenomenal quantity; teeming masses of interrelating entities on six orders of magnitude in scale. In order for you to hear your friend talking to you a meter away, 2.69×10^22 molecules interact to bring that signal to your ears, where about 4025 innervated hairs in your cochlea pick up the signal, and transfer it to your brain, where some significant fraction of the 10 billion neurons in your brain take 200 msec to form a synchronous pattern of activity that is your recognition of the event. That brain is ensconced inside a skull perching on a hundred trillion cells.
An interesting characteristic of these scalars is that they also come with what you might call “abstraction layers”; It is irrelevant to the atom which subatomic particles constitute it, much as a water molecule can be made up of any hydrogen and oxygen atoms and still be a water molecule, and as living things care not which specific molecules they are made of as long as they’re the right sort. In the above example, it could be any set of molecules in the air participating in the propagation of your friend’s voice and you would still recognise it; your body itself is a shifting mass of molecules, whose distribution and frequency can and do shift constantly over time, but you remain essentially whole. This fungibility shows that even from a classical perspective, many different circumstances in time could lead to similar phenomenal experiences.
So if many different circumstances are happening in this moment, we’re left to wonder how they’ve managed to conspire to make such improbable events extremely likely; that is, of all of the things that could possibly be happening now with the atoms that make up your body, why are they in you and reading this article? This becomes clear when you see that each layer “exposes” to the next a small fraction of its combinatorial repertoire: the space of molecular recombination possibilities that living things work within is a small but unlikely corner of it. This has the effect of concentrating patterns of expression in spacetime as well as in adjacent universes. This happens at every scale: only 4% of the universe is comprised of energy and matter; of the countless molecules atoms can make, all of life is comprised of only twenty different molecules, of the sextillion living beings on this earth only six billion are human – that’s one per trillion.
One of the consequences of being built on top of successive layers of finite combinatorial possibilities is that increasingly improbable events become probable in the reduced space. If you ran across someone reading a website floating about in the early universe’s quark-gluon plasma you’d be very surprised; on this earth there are a goodly number of website readers. Since we are ultimately only related by causality, our phenomenal space will therefore be those events that occur in proximate causal relation.
Imagine being in the chair at an eye exam, looking into the light. That light source is putting off light in all of its possible directions, but only those universes where photons fell on enough adjacent cones are causally proximate to your experience. In order for us to see, at least ninety photons have to have had a similar history. A cone or rod in the eye converts a photon into a moving wave of potential down to one of many dendrites of a nerve cell. That nerve cell only gets excited enough when several cones get stimulated; it then conveys the signal on. So for light to be sensed it has to be extremely probable; that is, present in a large number of adjacent universes.
Our mechanism of memory seems to involve the persistent shape of certain proteins that convert others like them over time. This creates long-lived patterns that persist even when some of the proteins change shape or degrade. Again, in most of the local universes there exist a quorum of proteins-in-neurons such that a memory persists.
So it is because the vast majority of the multiverses that you are in all have similar possibilities, and the differences are swamped both by the layer abstraction methods and by evolved adaptations to randomness that you have a phenomenal experience of unity in a dizzying multiplicity of worlds.