The afterlife, aka the Sideways, is a simulation created in the far-flung future in which all consciousness has been united and recovered. This point in time is known as Omega Point. Note that I'm pulling this notion out of my science fictional travels, so my discussion of it is not going to be very sciencey, but based on my collected encounters with the phenomenon (or situations which resemble the phenomenon) in various astounding tales. =)
If any reader out there has a more detailed handle on the philosophical, scientific/technological or futurist aspects of Omega Point, please do share, comment, or correct my para-remembered understanding.
The basic idea is that the universe and life in it progress over time toward an increasingly complex state, leading to greater and greater (approaching infinite) intelligence, knowledge, creativity, and altruism, allowing for more and more information to be saved, recovered, and restored. Ultimately, all things and their states over time, which would include human beings, their actions, their memories, and if you can accept that personalities, thoughts, and emotions are a manifestation of the arrangement of bioelectric energy and pathways, their consciousness, can be remembered and virtually reassembled, sometimes as a distinct individual, or as part of a massive collected consciousness. As virtual, bodiless, consciousness, an individual at Omega Point has as much time as one wishes, to do, live, or re-live whatever one may imagine.
The method and mechanisms involved vary. In Kubrick and Spielberg's A.I., sophisticated machine intelligences (the continually evolving offspring of humanity) are approaching Omega Point, and able to re-create a single human being, in body and soul (apparently), from material and memories preserved from her lifetime. At true Omega Point, as superintelligent minds, these further/fully evolved intelligences would be able to remember and replay everything, and restore any and all of their human preogenitors.
In Greg Bear's BLOOD MUSIC, a localized Omega Point is reached when a quantum physical event that "tilts" and rewrites macro/Newtonian physics in the region of Earth results in the creation of a biological astral plane inhabited and shaped by all living things on the planet.
In Robert Sawyer's FLASH FORWARD we get a glimpse of the ultimate possibility of an evolved humanity of disembodied energy and consciousness that can actually begin within our (extended) lifetimes.
LOST's season 6 Sideways doesn't let us in on the How (surprise surprise, it *is* LOST after all), but the Whos, Whats, Wheres, and Whens that we get to witness are exactly the kind of experience an Omega Point would enable. The restoration of individuals' consciousnesses, long after their original lifetimes (and deaths), in a simulated environment that would appear and feel familiar, shaped by their own memories, but would not have to adhere strictly to the laws of physics as we know them. And it follows that one could visit the Omega Point via a time-projected consciousness (not physical time travel, or reality-jumping), as both Desmond and Juliet demonstrate.
So, technically, yes, an after-life, in that it is a return to (apparent) temporal existence for our Losties long long long long long after their physical deaths. But, not a supernatural, spiritual one. Not one that defies science and reason. And yes, a reward, in the form of "moving on," which at Omega Point might entail joining others who have moved on, into a purer form, unconstrained by the notion of physical bodies, space, time, and the filter of five simple senses through which they experience the universe.
The experience of an after-life makes sense as a gateway to Omega Point for remembered/restored souls like those of our Losties. Only those beings who were alive upon reaching Omega Point experienced the evolution—of physicality, consciousness, emotions, society, civilization, and morality—to their ultimate state. Those beings who pre-date Omega Point and are revived then-there, would need a way to ascend those evolutionary steps and join their super-progeny as equals. Perhaps what we witnessed in the Sideways chapel is only a first tier. Next come lessons in telekinesis and temporal projection. And after that, the verbal.
Could the Heart of the Island (or the Island and its pocket dimension as a whole) be the extension of Omega Point thru time? Might a projection of Omega Point throughout time exhibit the properties and behavior we've seen associated with the Heart—responsible for life, death, and rebirth? Creation?
Is there an intelligence behind or within it? A morality? A collective one? A collective one that could program the physical laws of the matter, living and not-living, within the boundaries of the Island's snowglobe dimension in such ways that human minds might interpret as Rules? Does it make a kind of sense that when an unevolved being arrives at Omega Point without any transition (aka a living human being enters the Heart), it would be transformed or converted into a creature we've come to know as a Monster? Disembodied consciousness, bound by Rules to the realm of Omega Point? Sure, cuz... why not?
Keep on keepin on~
* August 7, 2010. Floated this ramble at The Fuselage.