The term "singularity" as applied to the medium-term future of technology/humanity bundles together a variety of predictions for purposes that suit advocates of continued technological development, but don't necessarily have to occur together. In this post, I want to briefly point out the different pieces, and comment on the strength of the connections. For readers wanting some more background on the concept, the Wiki article is a good place to start.
A) Human level machine intelligence. This is the idea that continued progress in computer science, computer engineering, neuroscience, etc, will ultimately result in computer systems that are equivalent in intelligence to humans.
B) Intelligence Explosion. This is the idea that as organisms/machines get smarter, they are able to improve their own intelligence faster, so that intelligence is in some sense accelerating and will accelerate faster and faster in the future to the point where infinitely smart machines (whatever that would mean) can be produced in a finite amount of history, or at least where the acceleration of intelligence would be so great that the consequences would be completely unforeseeable for beings of lesser intelligence (us) ahead of time.
C) Transcendental Singularity. Some people have argued that such a hypothetical super-intelligence (or collection of super-intelligences) would take over the entire universe and essentially become something equivalent to god.
D) Human Augmentation. For lack of a better term, I'm using this to denote a cluster of ideas that humans and machines will merge to varying degrees - that people will have brain implants to improve their memory and cognition, and eventually be able to upload their personalities and experiences up into a digital fabric of some kind and have a continued life entirely free of the constraints of current biological bodies.
E) Economic Singularity. The idea that intelligent machines will increasingly take over from humans in the workforce, and that people will not need to, or be able to, work in the future.
F) Growth Singularity. The idea that economic growth will increase faster and faster in the future as a result of trends in machine intelligence.
The basic view I have come to is that from amongst these possibilities, A) and E) are serious issues worth grappling with, while the rest range from highly speculative to extremely unlikely. Here are my arguments, in brief.
A) Machine intelligence. Clearly, we are all painfully aware that computers are not intelligent today, but rather show their heritage as digital calculating machines all too clearly - the computer is good at doing repetitive calculations accurately, but needs to be programmed by a human to do anything new. Still, we have seen that the number of previously "human only" skills that can now be done by computation are being knocked off one by one - chess, Jeopardy, understanding speech, generating speech, recognizing faces, driving (with the implied ability to perceive general roadway environments and make good decisions about what to do in a wide range of them). I'm well aware that my fellow computer scientists are furiously trying to improve each of these areas, as well as new ones, and it's not obvious to me that there's some fundamental barrier that will prevent the next 20-30 years from showing a similar level of progress to the last few decades.
Human abstract reasoning appears to me to consist of reuse, via the fundamental mechanism of metaphor/symbolism, of a large number of specialized circuits that were developed earlier in evolution for solving the problems of being an animal in an environment of plant food sources and other competing animals. Although I certainly wouldn't know how to build such a system in the next five years if you gave me a big team, given that we are essentially reproducing all the specialized circuits one by one in order to solve various problems in robotics, computer vision, etc, I don't see why we won't eventually be able to tie them altogether in some equivalent way. Just arguing from evolutionary/genetic distance, it's clear that human intelligence must only involve relatively minor tweaks on mechanisms that had already evolved to manage being a dog or an ape, and it would seem very difficult to be confident that computer science will not be able to develop something equivalent to the control system for a dog or an ape in coming decades.
Certainly, it appears that the amount of available computer power for running such systems will continue to increase rapidly for quite some time to come. So, while I cannot say it's a certainty, it appears to be more likely than not that machines will gradually approach human levels of intelligence in the present century.
B) Intelligence Explosion. The idea that intelligence will accelerate in future does not follow from the possibility of developing human-level machine intelligence. It assumes a completely unproven "law of intelligence" that the smarter you are, the easier it is to produce an intelligence even greater. Maybe it works the other way altogether - the smarter you are, the more complex and difficult it is to produce an even greater intelligence. Perhaps there's some fundamental limit to how intelligent it's possible for any agent to be. We have no clue. We haven't so far seen even a single generation of intelligences (us) producing a more intelligent entity, so the whole intelligence explosion idea consists of extrapolating from less than one data point. It's utter speculation.
C) Transcendental singularity. A fortiori, this is even more speculative. We have no idea whether some future super-intelligence descended from our efforts will be in a position to spread throughout the universe via yet unknown physical mechanisms, or will be condemned to sit here on earth living on solar power and contemplating its ultimate demise when the sun blows up.
D) Human augmentation. At the moment, a major economic problem for the United States, and, to slightly lesser degree, other developed countries, is that medicine is encountering diminishing returns. It's costing more and more to produce smaller and smaller gains in human health/longevity/wellbeing. While there are crude implants in use to treat things like Parkinson's disease, they involve brain surgery, which is incredible expensive, complex, and involves serious risks that one wouldn't undertake except for the most compelling of reasons.
So the idea that brain/computer interfaces can sometime soon enough to care about become as cheap and as rapidly evolving as computers themselves strikes me as implausible. It's less speculative, perhaps, than the idea of an intelligence explosion, but it certainly doesn't follow just from extrapolating existing trends.
Further, I think there are very strong psychological reasons why technology advocates are pushing this idea. If you take away this possibility, then the singularity basically just sucks from the perspective of a human being. It just becomes about your kids having no jobs, and creating a super-intelligence that we won't understand and therefore won't be able to control, and therefore will be an existential threat to us. If you are, say, an artificial intelligence or robotics researcher, it will be impossible to derive meaning from your work if you think this is what you are doing, so you have to come up with some psychological out that lets you continue the work you enjoy without feeling like you are being destructive to your own species. So, I think that's why this human augmentation thing gets so much play.
E) Economic Singularity. I've argued elsewhere that this is a serious concern. To the extent machine intelligence exists, or even very partial human skills can be replicated by computers, businesses are highly motivated to replace humans with it. Humans have rights, and machines don't. Humans insist on wasting a bunch of resources driving home and living in the biggest house they can afford, whereas computers will happily continue working all night on nothing more than a little electricity. Human workers can only reproduce very slowly and at great expense, whereas software can be replicated as often as necessary at almost no cost. This is exactly why you now talk to a computer when you call your phone company.
F) Growth singularity. At the moment, there's no evidence for this - global economic growth has been proceeding at a few percent a year, more-or-less, since the industrial revolution - and we haven't seen signs of a major acceleration yet. Future economic acceleration pretty much depends on assuming an intelligence explosion and is equally speculative in my view.
Hopefully, I'm wrong about all this, since these are seriously miserable conclusions. However, if so, my errors have not yet become clear to me.