I view existence as a sentient being to be of prime importance and I see Human sentience as extraordinary, precious and all too fragile. I see it as tragic that our awareness is so limited much of the time and is cut short so quickly by biological death. Accelerating Technological Change may offer some options for significantly extending human lifetime in the near future, i.e. soon enough for human beings alive to benefit from it.
At this point we can only speculate about the extent of sentience in non-human beings, e.g. animals, extraterrestrial intelligences or artificial intelligences. I see value in further inquiry into non-human sentience but I will not do so here.
The quality of our being, our consciousness in each moment, is more important than the quantity of years we have. A practice of being present much or most of the time gives our life very great value, whether or not it is especially long in years. Without being very present, an extended lifetime would bring little benefit.
Our development as conscious beings, adding distinctions, growing the range, scope and power of our experience and expression of being is also of prime importance. Extending our lifetimes beyond our ability to grow and develop would be of limited benefit.
Given a high quality of being present, of ongoing growth and development and ongoing expression of and sharing of our experience of being sentient with others, it would then be very wonderful to extend our biological lifespans. Quantity AND quality of life.
At the biological level, we are already immortal. Every cell in my body is part of a continuous chain of existence since the beginning of life on Earth. Although that original cell has divided and divided and divided, and although the vast majority of the descendent cells have died, along the lines of descent leading to each of my cells, there has not been a single fatality. My biological body is somewhat more than 3.5 billion years old, and so is yours and so are the bodies of every other living thing of which we know. Evolution has already solved the problem of biological immortality.
With single-celled organisms (the vast majority of cellular life), death by aging is not an issue, it does not occur. With multi-cellular life forms, there is an appearance of aging and death but it is an illusion. Multi-cellular organisms consist of a group of cells which (generally and approximately) share the same DNA. Therefore, if one of those cells is immortal it is biologically the same as if they were all immortal. Germ cells, egg and sperm cells in familiar organisms, are immortal in the same sense that single-celled organisms are. Any one of our germ cells could conceivably live on for another 3.5 billion years with undiminished, even evolutionarily enhanced vitality,
The majority of our cells are called somatic cells. Although they are genetic clones of our germ cells, they express different parts of their DNA in order to perform a support function. Their function is to support the survival and passing on of their genetic clones, the germ cells. Because of sexual recombination of germ cells in each generation, any organism which is moderately successful in reproduction is likely to have a vast number of descendants. Therefore, if all we care about is immortality as a biological organism, it is not a big problem, it is a done deal. Nothing is lost when a cell dies in order that an identical clone survives.
The phenomena of disability, aging and death are only meaningful when we are addressing the experience of the life of the whole organism that goes beyond its biological function. Evolution treats the whole organism as merely a temporary means to an end - if the germ line has continued before the whole organism dies nothing is lost. Evolution does not register sentience except as it contributes to reproductive success. Sentience is a higher-order property of a large and highly organized central nervous system.
The most obvious way to continue our lives as conscious sentient beings is by extending the life of the whole biological organism which is our substrate. I'll address the distinction of substrate more later, but for now, let's just take the obvious viewpoint that if our body does not age then we will live longer as a sentient being.
The invention of the microscope and other technologies for examining biological structure has revealed that our bodies are built of cells and allowed us to discover that cells are a kind of machine we now call a molecular-scale replicator. Machines are physical systems built out of components with a specific structure. The nature of a machine's components and structure gives all of the machine's properties, including what we consider its function. Until recently human machines seemed to be very different from biological machines in many ways. This is no longer true.
We are rapidly acquiring the ability to modify the DNA of biological organisms and even to design biological organisms from scratch. Along a parallel track, we are acquiring the ability to create more general molecular machines which have a potential functional range much greater than that which is available to biological organisms. If you're having trouble applying the word "machine" to living things, it is because your concept of "machine" is out of date. Some human-designed machines made of precise molecular assemblies exist now more such machines are likely to be produced in vast numbers in a few years. Structures at the molecular size-scale are popularly called nano-structures.
I am assuming here that human technological capacities continue along their exponential growth path for another few decades. There are physical limits to such growth imposed by, e.g. the size of atoms but we are still comfortably far from those limits. I will be conservative here and ignore any potential technology which would require a change in our understanding of physical law or require any unpredictable or unlikely breakthroughs. I am addressing the kind of advances which go beyond today's technology but which are likely to be considered ordinary in a few decades.
I must assume no global catastrophe stops our technological growth over the next few decades. Therefore, the reason any of the following possibilities will not occur is because a better option is found first and the possibility presented here becomes obsolete before it is developed. Any one of the possibilities listed here might scoop the others. And an unexpected breakthrough could easily scoop all of them. Despite my conservative assumptions, I find it very interesting to look at just how radical our future is likely to be.
Since biological cells already have the capacity for immortality, it is possible to create versions of multi-cellular organisms which are immortal at the whole organism level, including immortal versions of human beings. This could be done over time through selective breeding or very quickly using genetic engineering. In addition to creating immortal-from-birth humans, we could use gene "therapy" to give all such advantages to existing human beings.
Reversing aging and any other desired repairing and upgrading of existing humans is well within foreseeable engineering technology. It turns out that there is room in the human body to introduce extensive therapeutic machinery. Currently more than 90% of the cells within our body are not our cells containing our DNA, they are symbiotic organisms such as bacteria. We could introduce lots more symbiotic organisms and/or replace the existing ones without anyone knowing the difference. Symbiotic organisms and other nano-machines can be designed to carry out any desired therapeutic function on human cells and tissues in situ and ongoingly.
Even with extensive built-in immortality and healing, humans could still die from catastrophic accidents. Advanced biological technology, nano-technology or a combination of the two can offer us the ability to make backup copies of bodies, including all of the subtle structures in our nervous system which store our memories and personality. These backups can be updated daily or even continuously via, for example, built-in wireless Internet connections. You would never know that such backups were being made, unless or until you suffered a catastrophic accident, e.g. falling into a volcano! In such a case, you would simply wake up a few moments later in a backup body, none the worse for the experience.
Merely extending life as we know it is not revolutionary. Predictable technological developments offer much more.
We are more intelligent in many ways than the most powerful computers or computer networks, and this will be true for some time. However, current computers are in many ways more intelligent than us as well. Current computers and humans have very different cognitive limitations and are remarkably complementary. We don't currently know how to imbue computers with true sentience, but a good interface between a human mind and a computer system would produce a sentient being with extraordinary capacities. While we can imagine some of what would be possible for such a being, the true range of its consciousness and capacities go far beyond what we are able to forsee given our vastly inferior minds.
Over the next few years computers are going to get much more powerful and will be made of nano-structures at least as elegant as any biological nano-structures. This can be accomplished with reasonable extrapolations of existing technologies. Althrough vast computational and logical capacity are not the same as intelligence or consciousness, here are two different possible paths to transcendence: The AI Path and the Symbiosis Path.
In the AI Path, we somehow develop an advanced computer system which has all of the kinds of intelligence and insight which human beings have. it might or might not be conscious as well. Since its raw computational capacities go far beyond ours, such a system would seem super-human. Building such a system is not a mere extrapolation of current knowledge, it would require fundamental breakthroughs. I know at least three approaches to building such a system which seem promising, but I won't cover them here.
In the Symbiosis Path, we interface our minds to computer systems in such a way that the computer system seems like part of our mind. We retain all of our qualities of conscious experience along with all of our other human qualities but with a vast expansion of our perception, discrimination, knowledge, awareness and overall intelligence. Again we have a super-human. This path does not seem to require as manyf undamental breakthroughs as the AI Path does and it could be accomplished by modest incremental improvements over many years.
These two paths are not necessarily distinct. A super-human AI system would surely be able to create a Symbiosis Path super-human and a coalition of Symbiosis Path super-humans would surely be able to figure out how to create a super-human AI. These two paths might emerge together, or one might emerge first and quickly give rise to the other.
A computer system can share any part of its "mind" over a network. Computers have perfect "telepathy". Human/computer symbiotic persons would be able to share any parts of their minds with one another. Eventually the distinction "individual" breaks down in such a world, and the phenomenon of death no longer applies. How can "I" die when many versions of the parts of me I care about are distributed throughout vast numbers of physical brains and cyber minds?
These scenarios (and many alternatives which I have not addressed here) take us into a new world with sentient beings possessing capacities far beyond ours. Our problems are not their problems. Their problems - if they even think in terms of problems - would likely be beyond our comprehension. Although the path to such transcendent beings is entirely a result of continuing the lines of development we are currently on and involves no fundamental scientific breakthroughs, the result is the creation of a new order of being(s) which we cannot in principle comprehend. This is the Technological Singularity.
The rate of technological development continues to increase. The Technological Singularity is likely to happen in our lifetimes. It could happen within a few years, although I think a few decades more likely. A lot of very smart people have been thinking about and discussing these issues for many years - what I've written here only scratches the surface. I recommend looking up "technological singularity" in Wikipedia.org and Google.