Researcher Rafal Smigrodzki (involved in the development of mitochondrial protofection, you may recall, a way to potentially replace age-damaged mitochondria in your cells) has made an interesting critique of the concept of actuarial escape velocity on the Gerontology Research Group mailing list.
I am warmly supportive of SENS in its general philosophy though I have many objections to various details, as may expected when dealing with ideas on the cutting edge. There is however one general issue related to SENS that keeps bothering me: the escape velocity.
If I understand it correctly, Aubrey, the escape velocity idea postulates that if the speed of average longevity increase from medical progress exceeds the rate of aging of the population, then the members of that population would be in effect immortal (modulo accidents &c).
Now, this may be true when you are talking about a population that doesn't have any age-related mortality to begin with - e.g. a cohort of babies who are born into a world where medicine continually races about 50 years ahead of their aging, eventually making them as likely to die from aging at age fifty as when they were babies, and maintaining this low mortality rate indefinitely (a simple shift of mortality curve without changing its shape).
But for the fifty year old, the situation is not so rosy anymore. At age fifty there is already an uptick in mortality, and as long as the speed of mortality reduction is not substantially higher than the speed of aging, the members of this cohort will indefinitely experience mortality at this (i.e. natural 50's) level, whether at age 70 or 100. Obviously, eventually you will run out of members of this cohort. The problem is even more severe for the 70-year olds, and hopeless for the really old ones - all this even with medical progress substantially faster than today.
This makes me doubt that SENS could lead to radically long survival in humans who are currently in their thirties or later - none of the proposed rejuvenation therapies is likely to kick in before they reach fifty or older, and to give them an average 5000 year span you would need to have much better than escape velocity progress.
Of course, if there are enough fifty-year olds being rejuvenated at escape velocity, some of them will live many hundreds, or even thousands of years - but the odds are, it would be neither you nor me.
This is not to say I am pessimistic about extreme longevity in general - I think that the first immortals are already alive, and most of them are babies, perhaps female, born recently in rich countries. It's just that for us, the old folks, something more than the escape velocity is needed.
Aubrey de Grey replies:
Your analysis is quite correct and your error is in your description of what the escape velocity idea postulates. Indeed, it is absolutely vital to avoid having even the average mortality rate of today's 50-year-olds for very long if one wants a good chance of living to 1000. But if you check my most widely-available description of EV (my PLoS Biol paper) you will see that that's exactly what I predict -- those who make it even by the skin of their teeth will be restored to truly young-adult health and mortality risk only a few decades therafter.
By the way, Chris Phoenix and I are collaborating on a computer analysis of the rate of arrival of rejuvenation advances that is required to keep a population's death rate from aging negligible, and the short answer is that for all reasonable assumptions that rate becomes absurdly slow in no time at all. I'll be presenting this work at the Biology of Aging Gordon Conference at the end of January.