People often ask "What is the probability that cryonics will work?"
The short answer: discounting dystopian futures, assuming a good quality of cryopreservation, and assuming that MNT is developed more or less as expected, is that the overall probability of success seems quite high - likely exceeding 85%.
How we arrived at this answer is the subject of the rest of this page.
There are various fundamental issues that must be analyzed to answer this question but before we can even discuss these we need a framework within which to work. The simplest framework is to break down the probability of success into separate independent variables and then analyze each variable. This task is not as easy as it looks, so the following overview is intended to provide a framework within which to carry out the analysis.
1) You must avoid information theoretic death.
1a) Information theoretic death might occur before you are cryopreserved (you might be lost at sea, or suffer a neurological disease that destroys the brain).
1b) Information theoretic death might occur at the time you are cryopreserved (you have a heart attack and aren't found for a week).
1c) Information theoretic death might occur if you are prematurely thawed (civilization collapses, cryonics is outlawed, the universe comes to an end, etc.).
1d) Information theoretic death might occur because of the slow accumulation of damage (background radiation damages you).
2) Technology to revive you must be developed and applied..
This framework is intended to provide clearly independent probabilities, so that multiplication of the probabilities can be used to draw conclusions about the overall probability of success. It is all too common to hear analyses of "independent" variables that are in fact highly correlated. For example, someone might argue that for cryonics to succeed, (a) civilization must survive (b) your cryonics organization must survive and (c) someone must have the resources to revive you. All three of these issues are highly correlated, as the survival of your cryonics organization implies both that civilization has survived and that it is quite likely that the resources to revive you are available. Multiplying these three "independent" variables together will produce an erroneously low overall probability of success.
Even in our analysis, item (2) is not independent because, strictly speaking, if we never develop and apply the technology to revive you then either 1(c) or 1(d) will occur.
A brief summary of these variables (with a few links to further reading) follows.
Item 1(a) does not usually pose a major risk. Most people suffer legal death because of a heart attack or cancer. Perhaps the most significant risk of this type is caused by deterioration of the brain prior to legal death from neurodegenerative diseases (e.g., Alzheimer's, Parkinson's disease or other dementias). Accidental damage to the brain (e.g., as in the case of Phineas Gage) would also adversely influence this risk.
Data on the likely prevelance of such pre-mortem dementias is available from the medical literature. "For Alzheimer’s, the estimated lifetime risk was nearly one in five (17.2 percent) for women compared with one in 10 (9.1 percent) for men." The definition of Alzheimer's used was "moderate to severe disease as well as symptoms lasting a minimum of six months." Alzheimer's "accounts for an estimated 60 to 80 percent of cases" of dementia. Quoted from the 2011 Alzheimer’s Disease Facts and Figures . We can reasonably expect that the technology required to revive a cryopreserved patient and restore them to health will be able to reverse dementia, but associated amnesia caused by loss of the actual memory (rather than damage to memory retrieval mechanisms) would not be amenable to repair regardless of advances in technology. The extent of such irreparable memory loss (as opposed to damage to retrieval mechanisms) is not known at this time. In cases of advanced dementia with significant damage to the brain it is reasonable to expect that such irreparable memory loss would be extensive.
Overall risk of dementia prior to cryopreservation might range from little more than 10% to almost 30%, with the actual memory loss from such dementia itself being quite variable. Mild dementia caused by damage to neuronal mechanisms responsible for retrieval of the memory trace that left the memory trace itself relatively undamaged could be fully reversable by application of appropriate advanced technology. Severe dementia that destroyed the memory trace itself could not be reversed by any future technology. We cannot, at the present time, distinguish reliably between these two possibilities in most cases. Arbitrarily assigning a 50% probability to the risk that symptomatic dementia has in fact destroyed the underlying memory trace itself leaves an overall risk of significant memory loss from dementia of from 5% to 15%, or an average of ~10% with substantial uncertainty and variation between men and women. In men, this risk might range from almost negligible if we assume that the memory trace is preserved except in the most severe cases of dementia to a high of ~15% if even mild dementia causes loss of the memory trace. In women, the corresponding range is negligible to ~28%.
This analysis neglects the probability that medical technology in the next few years will find ways to reduce the risk of Alzheimer's. It also begs the question of how bad it is to suffer a "significant memory loss". If you had the option of brain surgery to cure a disease, but the surgery had a 10% risk of "significant memory loss" would you tolerate that risk? If the surgery offered complete recovery of all your mental faculties, but you had some degree of permanent memory loss that forced you to "relearn" some basic facts about yourself, how much damage would this do to you? How much memory loss would you accept before deciding that "significant memory loss" was "unacceptable memory loss"? The answers are likely to vary widely from person to person.
Item 1(b), the risk that information theoretic death occurs when you are cryopreserved, depends heavily on the circumstances of your cryopreservation. While severe cases (unattended death followed by many days of deterioration prior to discovery) would likely result in information theoretic death, shorter periods (likely including many hours) should pose a much smaller risk. Short delays (cryopreservation under favorable conditions) would seem unlikely to pose a significant risk of information theoretic death. For further consideration of the kind of damage required to cause information theoretic death, see Cryonics, Cryptography, and Maximum Likelihood Estimation.
The assumption that you will be cryopreserved under reasonable conditions is perhaps the most open to question. Besides logistical issues, the wishes of those who elect cryopreservation can be and have been deliberately thwarted by relatives, medical personnel, coroners, or others. As the legal, medical and social standing of cryonics improves these risks will decrease, but they will likely remain significant for some time. Those who have most clearly and most publicly stated their wish to be cryopreserved are least likely to encounter cryopreservation injury from this cause. Those most vulnerable are those who are not in routine communication with other members of the cryonics community, those who have not clearly informed their doctor and any medical personnel they might encounter about their wishes, those who make a habit of concealing their wishes, and those who have chosen someone hostile to cryonics or simply indifferent to cryonics for a power of attorney or other legally important function. Make sure no one in a position to block your cryopreservation would benefit financially from such an action. The wealthy, terminally ill cryonicist with hostile relatives (or any relative or "friend" who feels they "should" receive a substantial part of your estate) who has left substantial sums that do not go to those relatives, and particularly if significant sums go to cryonics-related purposes, should take extreme precautions. Some jurisidictions commonly alter Trusts after the fact for reasons unrelated to the wishes of the decedant: avoid them. Control access to your hospital room or sick bed. Insure that neither the hostile relatives nor their attorneys can gain access to you. If such visits must be allowed, it is essential that neutral witnesses be present, preferably with video cameras, who cannot be bullied by an aggressive attorney. As your mind deteriorates you might say or sign almost anything, and if you are alone with several hostile relatives they might claim you said or signed almost anything regardless of what actually happened.
Item 1(c) can reasonably be summed up as "Your cryonics organization fails, for whatever reasons, to carry out its mission." Alcor was founded in 1972, and its continued existence and growth is a favorable indicator. Pragmatically, the more people who support cryonics the less likely that this failure mode will occur. Some argue that civilization is likely to collapse within the next century (and describe a diverse menu of possible causes). It is difficult to assign probabilities to these dystopian futures. Those who seriously believe that we are all doomed are invited to watch television, or otherwise amuse themselves. Those of us who think humanity has a future will continue to make plans based on this assumption. Alcor appears stable and the people involved are dedicated to carrying out its mission. Barring dystopian catastrophes, the probability that Alcor will fail seems quite small.
Item 1(d) is unlikely to occur in less than a few thousand years, as the only significant source of damage inside a stainless steel Dewar of liquid nitrogen protected from light and mechanical damage is background radiation of ~0.1 rads/year. It would take about 6,000 years to accumulate a lethal dose of 600 rads by today's standards (see the Alcor FAQ entry and Merkle's cryonet posting on this subject and the comments by a cryobiologist).
Item 2, the failure to develop the required technology despite the passage of multiple centuries, seems unlikely. More specifically, published analyses strongly support the feasibility of both nanotechnology and nanomedicine. Given sufficient time and the non-collapse of civilization, they should both be developed.
As nanotechnology, nanomedicine, and the ability to revive cryopreserved patients who have not suffered information theoretic death are based on the foundation provided by our understanding of physical law, and as that understanding, as it applies to the physics and chemistry that is relevant to human biology, biochemistry and neuroscience is highly unlikely to change in any significant fashion, let alone change in a way that would undermine the feasibility of cryonics, we can reasonably conclude that the development of the relevant technologies is primarily a matter of time. There are no fundamental issues of physics that stand between us and the recovery of the information present in the cryopreserved brain of a patient who has not suffered information theoretic death, and there are no fundamental issues of physics that prevent us from restoring the atoms in a cryopreserved human brain to an arrangement that corresponds to that person following their full and complete recovery.
Not only are these technological issues clearly feasible in principle, implementation pathways have been outlined and specific implementation strategies have been proposed and analyzed. Development of the relevant technologies is now well underway. The 4th Quarter 2008 issue of Cryonics magazine discusses the application of MNT (Molecular NanoTechnology) to cryonics. Feynman's classic talk There's Plenty of Room at the Bottom is required reading for anyone interested in this subject.
Discounting dystopian futures, assuming at least a reasonable quality of cryopreservation, and assuming that MNT is developed more or less as expected, the expected probability of success seems quite high - likely exceeding 85%. Claims that cryonics is "impossible" or "unlikely to work" are not supported by any published technical articles or analyses and are uniformly based on gross misunderstandings or misrepresentations of basic aspects of cryonics.
Many of the fundamental technical issues are discussed in The Molecular Repair of the Brain.
1. 2011 Alzheimer’s Disease Facts and Figures