Basic Overview
The goal of cryonics is to preserve the information in the human mind. This includes the memories, personality, emotions, and aspirations of the person; everything that makes them a unique individual. The image below shows a few neurons (brain cells). There are approximately 100,000,000,000 neurons in the human brain. As you can see, each neuron has many lacy extensions forming about 2000 synapses (connections) with other neurons.
It is well accepted by neuroscientists that the information in the mind is stored in the physical arrangement of the neurons. When a person forms a new memory or learns a new idea, it is the result of neurons actually changing shape, either strengthening or weakening the connections to other neurons. This change in shape is not subtle, but is quite dramatic and obvious. When electricity is flowing through the neurons, a person may be conscious. But a brain can just as easily shut down all electrical activity without losing more than a few hours of memory. Here's a quote from a medical textbook:
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Death
Physicians are well aware that death is not a sudden moment, but is rather a slow process. Death is a very gray area that medical ethicists struggle with constantly. Is a person "dead" if their heart stops? Or do we define it as no brain activity? The gradual nature of death is the very reason why organ transplant technology is so successful; the tissues are still alive. Living neurons can easily be extracted from cadavers as long as 8 to 12 hours after death.
The terms "life" and "death" are convenient labels that we have created, but they do not fully describe reality. A more accurate definition of death of the mind is a loss of the information in the mind. By this definition, the mind of a person with brain damage from an auto accident or Alzheimer's disease has partially died. Some of the information has been irretrievably lost. Also, by this definition, the mind of a person who has been pronounced "dead" is not at all dead yet. All of the wiring in the brain is still present in its full detail and it is only when this wiring detail is lost that the mind of the person begins to die. As long as the physical arrangement of the neurons is preserved, then there is still potential for that mind to regain consciousness.
Preservation
We are interested in preserving the physical arrangement of the connections of the neurons. This includes the basic shapes of the neurons as well as the size and shape of all the synapses. This has been done routinely for decades by brain banks who study brain tissue to learn about diseases. Brain banks use two general preservation methods. To preserve structure, they use chemical fixation with aldehydes. In addition, they can also use various freezing methods for better long-term storage and for situations where they are interested in studying chemically active molecules. Brain banks cannot usually afford to preserve entire brains, so they usually save representative tissue samples from different parts of the brain. Cryonics preservation is soundly based on existing brain banking technology. Some changes have been made in order to accommodate storage of the entire brain rather than just portions of it.
Storage
Long-term storage needs to be below -130 C to prevent molecules from moving. The most reliable way to do this is in a large dewar of liquid nitrogen at -196 C. The dewar must be frequently topped off with liquid nitrogen, but it is not dependent on electricity. Electrical blackouts are of absolutely no consequence.
Future Technology
Predicting the future is fraught with complications. A common mistake is to try to predict when something will happen instead of simply what is theoretically possible. But there is a lot we can accurately say about what is theoretically possible. A given future technology is theoretically possible if it does not defy the known laws of physics. For example, flying cars are theoretically possible by using jets of air. Robot servants are theoretically possible by using computers and mechanical actuators. This means that eventually, such technologies can be developed. But it does not say anything about when they will happen.
So what is theoretically possible? For starters, growing any new body part from stem cells is theoretically possible. This includes organs, limbs, and any other body tissue. This is known as regenerative medicine and is already being used on patients for simpler organs like skin, bladders, and ears. Intensive research is underway to grow more complex organs, and it is commonly accepted that we will eventually succeed. Given enough time, we will probably learn how to control the environments of stem cells to make them behave the way we want. Will regenerative medicine be enough to revive cryonics patients? Far from it. We could easily grow a new body around an existing brain, but the brain would not be repairable using regenerative medicine.
On a much much smaller scale, we know it is theoretically possible to manipulate individual molecules. It's very hard to explain this concept to someone who has not had training in biochemistry. There is a video called The Inner Life of the Cell. It's an absolutely amazing 8 minute animation that finally gives us a glimpse of what these molecules of life are possible of doing. One place to find it is here:
The point is that we are surrounded by "life". Life is simply collections of molecular machines that are busy manipulating other molecules. Nature has proven that molecular machines are possible and do not defy any laws of physics. Here are some molecular nanotechnology overviews:
Brief Nanotech Overview
Nanotech Overview - Alan Alda
Drexler Nanotech Factory
Regenerative medicine would be a huge revolution that would improve the quality of human life dramatically. Molecular nanotechnology could then take the quality of life to levels that are hard for us to imagine. If we have the toolset of molecular nanotechnology, we would be able to cure essentially every current medical disease including aging. We would also be able to repair the damage in a cryopreserved brain.
Repair Scenarios
It is clearly possible in theory to eventually repair the damage in a cryopreserved brain. So why aren't people signing up for cryonics in droves? A very common objection is that people don't think we will ever develop molecular nanotechnology. And this is in spite of the very extensive examples of natural molecular machines all around us, what we simplistically call "nature". People just have a hard time visualizing such an advanced technology. Many futurists write books that try to explain what such a world might look like. But the important thing to keep in mind is that it really doesn't matter at this point how the repair is done. We know that we are preserving the information, and we know that the repairs are possible. Therefore, the only sensible course of action is to get the brain cryopreserved instead of letting it get destroyed.
Irrational Objections
People are very slow to change their core beliefs. Fundamentally new ideas take a long time to become incorporated, if it happens at all.
Nearly everyone who has not accepted cryonics has a variety of irrational objections even once they accept that cryonics could work. Listed below are some of the more common irrational objections and some very brief responses:
It's not natural. No. Nothing about modern life is natural. Humans don't accept nature; we work to survive in spite of it.
My spirit will live on, so it's unnecessary. That doesn't stop people from taking advantage of modern medicine. It's still irrational to die when there is an alternative.
It hasn't been proven. Actually, it has. To the same degree that manned space flight was a proven science long before we actually traveled to space.
It will hurt people's feelings. Then educate them. Lives are at stake.
90 years is long enough. The oldest members of our society are the wisest and most knowledgeable. Our society suffers greatly from their loss as each younger generation repeats terrible mistakes of the past.
It's weird. That's just because you're not used to it yet.
It's gross. So is surgery, but we do what we have to do.
It's expensive. No, it's not. The price continues to drop and is approaching zero in some situations.
I want to live for the present. Fine, but most people also plan for retirement. The present and the future are not mutually exclusive.
People will come up with endless reasons to dismiss cryonics. They should rethink their objections. Cryonics is the only known way to survive, and avoiding cryonics can only be described as self destructive behavior.