Cryonics has been in the press and the broader online media seemingly more often than not of late. Perhaps it is the season for it. As is the case for many other areas of science and technology in this era of rapid progress, matters in the cryonics industry are on the verge of moving from reasonable but unproven hypothesis to self-evident extrapolation of the uses of a new technology.
The reasonable but unproven hypothesis has been that suitable methods of low-temperature preservation, vitrification being the present standard, will maintain the fine structure of neurons and neural connections sufficiently well that the data of the mind is also preserved. Thus people can be preserved at death, and given the chance at a new life in a future in which restoration becomes technologically feasible. So long as the data is preserved, that restoration remains a possibility. That this data is overwhelmingly static, in the form of encoded structures, is demonstrated by the survival and recovery of cold water drowning victims, among other evidence. The core cryonics hypothesis has held up very well in the face of the past few decades of progress in our understanding of the mechanisms of memory, and progress towards being able to mount initial demonstrations of preserved structure in neural tissue and preserved memory in lower animals.
The phase in which cryonics moves to being an obvious extrapolation of a new medical technology starts when reversible vitrification is used to preserve donor organs indefinitely, removing many of the logistical issues in that industry. When organs are grown from cells to order rather than donated, again reversible vitrification will make the logistics of that tissue engineering a lot easier. This is actually pretty close to realization in the grand scheme of things. Experiments have been carried out in animals with some degree of success, and more researchers are now interested in fixing up the last set of issues in vitrification and thawing protocols that make reversible vitrification hard at present. In a world in which organs are regularly vitrified for storage, then used for transplant years later, it will become pretty obvious to the public at large that storing the brain or the body as a whole is similarly viable.
People who pay more attention to the state of technology still under development are already at that point, of course. If organized well, end of life cryopreservation is a very reasonable wager on the course of the future for someone who will age to death before the advent of sufficiently effective rejuvenation treatments. Few people realize this, unfortunately, and so near all of humanity let themselves fall into oblivion, thinking there is no alternative.
We think of the divide between life and death as a distinct boundary, and we believe that at any given point, a person is either definitively alive or definitively dead. Today, dead means the heart has been stopped for 4-6 minutes, because that's how long the brain can go without oxygen before brain death occurs. The brain 'dies' after several minutes without oxygen not because it is immediately destroyed, but because of a cascade of processes that commit it to destruction in the hours that follow restoration of warm blood circulation. Restoring circulation with cool blood instead of warm blood, reopening blocked vessels with high pressure, avoiding excessive oxygenation, and blocking cell death with drugs can prevent this destruction. With new experimental treatments, more than 10 minutes of warm cardiac arrest can now be survived without brain injury. Future technologies for molecular repair may extend the frontiers of resuscitation beyond 60 minutes or more, making today's beliefs about when death occurs obsolete.
In other words, what we think of as "dead" actually means "doomed, under the current circumstances." Someone fifty years ago who suffered from cardiac arrest wasn't dead, they were doomed to die because the medical technology at the time couldn't save them. Today, that person wouldn't be considered dead yet because they wouldn't be doomed yet. Instead, someone today "dies" 4-6 minutes after cardiac arrest, because that happens to be how long someone can currently go before modern technology can no longer help them. Cryonicists view death not as a singular event, but as a process - one that starts when the heart stops beating and ends later at a point called "the information-theoretic criterion for death" when the brain has become so damaged that no amount of present or future technology could restore it to its original state or have any way to retrieve its information.
Here's an interesting way to think about it: Imagine a patient arriving in an ambulance to Hospital A, a typical modern hospital. The patient's heart stopped 15 minutes before the EMTs arrived and he is immediately pronounced dead at the hospital. What if, though, the doctors at Hospital A learned that Hospital B across the street had developed a radical new technology that could revive a patient anytime within 60 minutes after cardiac arrest with no long-term damage? What would the people at Hospital A do? Of course, they would rush the patient across the street to Hospital B to save him. If Hospital B did save the patient, then by definition the patient wouldn't actually have been dead in Hospital A, just pronounced dead because Hospital A viewed him as entirely and without exception doomed.
What cryonicists suggest is that in many cases where today a patient is pronounced dead, they're not dead but rather doomed, and that there is a Hospital B that can save the day - but instead of being in a different place, it's in a different time. It's in the future. That's why cryonicists adamantly assert that cryonics does not deal with dead people - it deals with living people who simply need to be transferred to a future hospital to be saved.
The statement "a dead person cannot be revived" seems so obvious that it is hardly worth writing down, but when you look a little deeper, it is not so clear cut. A few decades ago, someone who suffered a cardiac arrest was considered irreversibly dead. Move forward to today, and we routinely bring those people back from the brink. So, in some regards, our definition of what constitutes "dead" has shifted. It is this kind of stance that cryonics researchers often take when faced with dissenters. Their argument, whether you are prepared to run with it or not, is that death has already had its goal posts moved, so who is to say that they cannot be moved again?
Today, brain death, rather than cardiac death, is considered the stamp of finality. But even this, it might be argued, is not entirely infallible. For instance, in 1955, James Lovelock cooled a rat to just above 0°C. Its brain completely stopped its normal business. However, Lovelock managed to reanimate the rodent by warming it back up. Even in a real life situation - cold water drowning - similar findings have been described. A young girl was resuscitated after an astonishing 66 minutes of total submersion in freezing water. After an hour under water, most people would consider the individual irreversibly dead. We now know this is not always the case. Importantly, her memories and personality were still intact. This is not the only case of people "surviving clinical death." Cases like these have spawned a saying in some emergency rooms: "Nobody is dead until warm and dead."