For people who fear death or simply want to defy it, being cryogenically frozen seems like the ultimate solution. You take a little nap at an extremely low temperature, so low that biological functions are slowed to the point of being effectively halted. Then when science has a cure for old age and other fatal conditions, they wake you up and make you whole again. You live a whole new life — in the magnificent future. It worked for Captain America and Philip J. Fry, right? The one hitch: We don’t really know if we can wake up someone from a cryogenic sleep and give them that fresh start.

Can humans be frozen and awoken 1,000 years in the future? Source: Tumblr

The technique is most popular in science-fiction, but the real-life practice has been thrust back into the spotlight after a 14-year-old girl who died of cancer was cryogenically frozen, just days after winning her court case for the right to freeze her body. In the emotional case, the British girl, known only as JS, had written to a judge that she didn’t want to die and hoped by being frozen in the United States, one of the two countries with cryogenic facilities, she could “be cured and woken up — even in hundreds of years’ time,” the BBC reported. Her father had tried to block the deep-freeze, worried about what would happen to a young teen brought back to life in a strange country without any relatives.

Read: Girl, 14, Who Died Of Cancer Is Cryogenically Frozen In Hopes Of Future Cure

The Telegraph said JS is the only British child to have been frozen, and one of just 10 Britons. That publication referred to cryogenic freezing as “teetering between science fiction and science fact,” adding that “cryonics is a leap of faith, relying entirely on future medical advances that may or may not happen. Its proponents frame it as a choice between ‘definitely’ dying and ‘maybe’ living on.”

According to the Conversation, it’s possible to cryopreserve animals like reptiles and insects, but humans have different needs. Although a wood frog “freezes during winter into a block of ice and hops around the following spring,” each time human tissue freezes and thaws it creates damage.

“One of the main objectives is to inhibit ice formation, which can destroy cells and tissues by displacing and rupturing them,” the article says. To avoid this, “rapid cooling, rather than ‘freezing,’ is the aim” and there are substances to prevent ice on a cellular level.

Likewise there are substances to prevent damage during thawing, but that process comes with its own issues. According to the Conversation, there are many ways our environment and lifestyle play on our genetics, and being frozen can change how our DNA manifests itself. Then there’s the matter of needing to restart internal organs at the same time.

Other issues include preserving brain function and memory, which is still a large unknown in cryogenics. And being cryogenically frozen is not for everybody who has a devastating illness — the Conversation notes that people with dementia who have already lost significant memory wouldn’t necessarily benefit from the procedure because those memories cannot be restored. Freezing people before they decline raises ethical questions.

While cryogenic preservation may seem farfetched, there are clues that it may someday be possible. For example, human organs are preserved in cold temperatures for use in transplants. And there have been cases in which freezing cold water has appeared to preserve a drowned person to the extent that they were able to be revived after long periods of being unresponsive.

For definitive answers, we will have to wait — just like the people who have been frozen are waiting, for a day and a cure they can’t be sure of but hope will come.