Huffington Post has had a lot of articles about the Singularity lately. The most recent one is "Hogwash About the Singularity is Here" by Neil S. Greenspan, a Cleveland immunologist.
The article puts forward the usual "complexity of biology" and "exponential growth cannot continue forever" criticisms of Kurzweil's predictions. Most of these criticisms have already been addressed by Kurzweil at the end of his last book. I think there are good points on both sides, but critics like Greenspan are ultimately being too pessimistic.
What I find interesting in articles like this are not the specific criticisms, which I've heard many times before and somewhat agree with, but the moral valence and indignation present in the critique. Biologists like Greenspan are angry that Kurzweil is, in their view, glossing over the complexity of biology. The most morally valent part of the article are the comments, actually. I'm going to skip looking at the moral part this time, and look closer at a scientific statement that Greenspan makes.
Greenspan goes directly after "nanobots" in one part:
There is no basis at present for believing that medical interventions based on the postulated but not-yet-realized nanobots, often-invoked by Singularity enthusiasts for the resolution of all medical threats and malfunctions, will perform their duties without trade-offs and side-effects like those associated with every other therapeutic agent ever employed.
One could argue this, but I'll bet that the reason why Greenspan sees "no basis" is that he knows next to nothing about the postulated "nanobots" he is criticizing. Note how his argument is based simply on the generalization that there are "trade-offs and side-effects" with every therapeutic agent. This is true, but trivial. Some of the trade-offs are quite modest. Am I really trading much by letting my skin get pricked by a needle to inoculate me against a deadly disease? Is the recent rise of antibiotic-resistant bacteria really all that huge of a price to pay given the suffering that antibiotics have alleviated in the more than half-century since they started to be mass produced? Is using a condom for casual sex really that much of a bad tradeoff, given what our ancestors had to deal with without them?
I'm not going after the specific content of Greenspan's criticism here so much as the worldview it represents: that things will always be roughly the same in medicine as they are now. That's the default view of the future of medicine that non-specialists, like elementary school teachers, conveyed to me while I was growing up. Fortunately, I eventually met people working in biotechnology who said that the progress mankind has achieved so far in medicine is quite primitive in comparison to what we will one day achieve. Today's medicine will be viewed as medieval from the perspective of the future.
The fact that nanobots are indeed relied upon for the more extreme regeneration, life extension, and disease prevention scenarios does show a strong potential point of failure for the transhumanist vision. If nanobots turn out to be impossible, does that mean we will be stuck with the same old medicine forever? Not likely, because there are a variety of other approaches and techniques for fine-grained intervention in human biology that do not depend on nanobots.
Increasingly sophisticated bioMEMS already exist and have been used in the bloodstreams of animals, mostly as sensors. To be able to navigate the body effectively, "nanobots" are not likely to ever be used anyway -- they would just get tossed around by the blood and have to spend too much energy to make progress. Any robot that performs medical functions in the human body is likely to have a diameter greater than 1 micron (1000 nanometers), and probably more like 5 microns (5000 nanometers) making them microbots, rather than nanobots. Microbots already exist, the primary challenge is improving them; making them more durable, biocompatible, mass-produced, and sophisticated. Molecular assembly lines already exist, and it is only a matter of time until biomedical devices are created using them.
Greenspan closes with the following:
It is entirely reasonable to expect significant diagnostic and therapeutic progress to continue, but predicting complete conquest of disease is unrealistic in light of both the numerous deficiencies in our understanding of the subtleties of cellular and molecular function that are likely to persist in some measure for many years and the extremely-difficult-to-avoid trade-offs that afflict most medical interventions. Indefinite human lifespan remains wishful thinking well beyond the realm plausibility.
Again, maybe so, that these deficiencies will persist "in some measure" for many years, but the specifics mean the difference between a widespread adoption of enhancement and life extension or not. Reports funded by government agencies, such as Converging Technologies for Improving Human Performance, seem less pessimistic than Greenspan on human enhancement made possible by our "understanding of the subtleties of cellular and molecular function". And has Greenspan ever heard of the engineering approach to aging, where instead of trying to stop all possible sources of metabolic damage, focus is merely put on removing age-related damage faster than it can accumulate? I would particularly be interested in hearing his take on the latter, purely as a scientific matter rather than a moral one.
It is worth noting that even if we "conservatively" assume that average lifespan in the 21st century will be roughly the same as progress throughout the 20th -- that is, improving by about a fifth of a year per year for people in developed countries, and we assume today's average lifespan is about 75, then by the year 2100, people will live an average of 95 years. Not that radical of a number, but thinking of an entire society of active people in their 70s and 80s is probably more than many of today's unimaginative minds can handle. To them, this little patch of history in which they were born is considered typical of reality in general, and any major change will come as a surprise.