Sorry, Han Solo and Mr Sulu. Based on everything we know right now, you'll never be able to punch a button and travel through "hyperspace", or go to warp speed. Travelling faster than light is almost certainly impossible. According to scientists, the only way you could personally visit other stars is by taking a long, long nap. The idea of cryogenic suspension, or cryosleep, has always been one of the markers of "hard" science fiction. Instead of hand-waving some way to cheat Einstein, movies like Alien and authors like Alastair Reynolds have shown people going to sleep and waking up months, years or even centuries later, in another part of the universe.
I've always been a little bit sceptical about the idea of being able to freeze someone for months or years, or any other ways to slow metabolic functions enough to allow a person to wake up — no older and none the worse for wear — years later. It's a pretty huge leap from what we know about hibernation among mammals on Earth. But after talking to a ton of scientists and science fiction authors, including Reynolds himself, I'm convinced: Cryosleep is the only hope we have of making an interstellar journey in a single lifetime.
Faster-than-light travel is a dream
Cryosleep involves huge challenges that we have no idea how to get around — but we already know that travelling faster-than-light (FTL) is plain impossible.
"Based on what we now know, FTL is either impossible, or would require energies that are flat out ridiculous. Storing brains for a long period of time is merely very, very hard," said Terry Johnson, a biology professor at UC Berkeley who co-wrote the book How to Defeat Your Own Clone.
"We cannot even come up with a physically consistent theory for how faster than light travel would work, much less conceive of engineering such a thing," said Karl Schroeder, author of Lockstep, a novel about a futuristic civilisation that uses cryogenic suspension to travel. "FTL is, in our universe as we understand it today, a complete fantasy rather than merely an 'as-yet undeveloped technology'."
The Millennium Falcon goes to hyperspace from Star Wars (20th Century Fox/Lucasfilm)
Even a tantalising idea such as the Alcubierre drive, a concept for a faster-than-light engine that NASA has been researching for years, "requires an impossible substance with negative mass to run it," said Schroeder.
But wait... what about wormholes? Wormholes basically tunnel from one part of spacetime to another, allowing us to leap across unimaginable distances.
But wormholes may actually be impossible too, according to Dave Goldberg, a physicist at Drexel University who wrote the book The Universe in the Rearview Mirror. The laws of the universe may not actually allow wormholes to exist, at least not in a form that we could take advantage of.
"For example, to be stable, wormholes need something known as 'exotic matter' to hold them open," said Goldberg. "Exotic matter violates what's known as the 'weak energy condition', meaning that there are frames of reference where the energy density is negative." Scientists have never actually observed the "exotic energy" this would require, in the real world, said Goldberg, "and there doesn't seem to be any natural way to produce it — or any way at all."
The Enterprise in a wormhole from Star Trek (Paramount)
And even if we could keep a wormhole open once we had opened one, there's no theory as to how we would create one in the first place. The whole idea of a wormhole requires punching a hole in spacetime, after all. There is some speculation that at the tiniest scales — or "about 10^20 times smaller than an atomic nucleus" — space creates natural wormhole-like structures, but we don't actually know that for sure yet. To figure that out, we'd require a working theory of quantum gravity, which we haven't got yet. And even if we could open up a microscopic wormhole, we have no clue how we would go about inflating it to the size of a spaceship.
Of course, you can't prove a negative — so we can't know for sure that travelling via wormholes would be impossible. "So I'll concede that a wormhole might be physically possible, at which point it would just be practically impossible, requiring enormous amounts of energy and engineering," said Goldberg. Plus even if we somehow pulled it off, you'd have to travel the "long way" to your destination star, to set up the other end of the short cut.
So if we can't magically travel faster than light, or build a large wormhole, then getting to other star systems is going to take a long, long time. Here's a pretty good rundown of just how long it would take to reach our nearest star, Proxima Centauri, with propulsion methods that we currently know are possible. Estimates range from 81,000 years (yikes!) to a mere 1000 years. There are a few potentially faster methods — but they involve huge logistical nightmares.
The Water Bear Necessities
"If faster-than-light travel is even possible, there will almost certainly be only one way to do it, and that way may not be accessible by any human technology," said Schroeder. But meanwhile, "we can sketch many different engineering solutions for cold sleep". Schroeder has put a lot of thought into this topic: his novel Lockstep is all about a space empire that's based on people hibernating for thousands of years and then all waking up at the same time.
So Schroeder is optimistic that we'll figure out how to sleep for the long periods of time required for space travel — he's just not sure how we'll do it. "There are many different paths to long-term hibernation. We already know that living things can be revived unharmed after very long periods." For instance, microscopic "water bears", or tardigrades, can sleep for up to 30 years and then be revived without any harm.
The Doctor visits a massive cryogenic facility in Doctor Who, "The Ark in Space" (BBC)
"We already have several animal models for hibernation, so we know that it's at least distantly possible," said Reynolds, another novelist whose work frequently includes cryosleep. At the same time, he concedes that "extending it to humans is bound to be complicated. But there are efforts being made in that direction."
Ten years ago, when I wrote Pushing Ice, I was excited by an article I'd read in Scientific American concerning the use of hydrogen sulphide as a chemical pathway to inhibiting cellular processes, so much so that I worked it into the story as a relatively near-future hibernation technology which was risky but worth using in extreme situations. It's a bit disappointing that there hasn't been much reported progress in the last decade, but I'd guess that's as much down to funding as it is to any fundamental problems with the concept.
Reynolds pointed out that NASA has been spending more time lately looking into the idea of using hibernation, or "torpor". "I heard something about this only a week or two ago, but it's been bubbling under for a while," he said.
Marines wake up from stasis in Aliens (Fox)
At this point, Reynolds conceded, all the research is looking into "induced sleep rather than Ripley-style cryosleep. But anything that helps us along in that direction has to be worth pursuing, and presumably there would be any number of Earthbound applications for such a technology."
"I think a hibernation-like state may be possible, but the question is whether it will be under hypothermic conditions or not," said Marina Blanco, a Duke scientist who has studied hibernation in lemurs. "Remember, some lemur hibernate despite hot weather."
Space colonists could be like retired football players
"Right now, we are working to cool people for one to three days," said Kelly Drew, a professor at the Institute of Arctic Biology at the University of Alaska, Fairbanks, who has studied the mechanisms that allow Arctic ground squirrels to hibernate. "If we get this figured out, the next step will be to cool them for several days, maybe a month. We have a long ways to go to make it a hundred, or a thousand, years."
Cryosleep in Prometheus (Fox)
The tricky part, according to Drew? Figuring out how to freeze and thaw the human body without causing immense damage. "Even a ground squirrel that cools to around freezing doesn't stay there for more than three weeks," she said. "Mammalian tissues cannot freeze without bursting cells." Basically, when mammalian cells are frozen, ice crystals form inside the cells, causing damage to cell membranes.
And even if you manage to overcome the effects of freezing and thawing without casing cell damage — and that's a big if — you still have other problems.
"If you freeze a person for 100,000 years, there's likely going to be a lot of damage done," said Johnson. "Chemistry is slow at low temperatures, but it doesn't stop entirely, and you're giving it plenty of time to wreak havoc."
"Entropy marches on," he added. It might march slowly, with baby steps, at low temperatures, "but 100,000 years is a long march."
Cryosleep in Fallout 4 (Bethesda Game Studios)
One of the most likely outcomes of long-term cryogenic suspension is brain damage, according to Johnson: "Either we'd have to plan to do extensive rehab on the brain-damaged colonists post-thawing." Otherwise, he envisions space colonists having problems similar to retired football players, and you could have a whole planet full of people dealing with so-called "cryonic encephalopathy".
The other option is to engineer hardier human-like minds — via artificial consciousness, or cyborgs, or finding some way to engineer "neural-like networks that age better than ours," said Johnson.
Schroeder offered a few different ways we might be able to cope with the problems of cell damage and brain damage. We could genetically alter space travellers — for example, giving humans cell walls like plants. We could perform a drastic medical intervention, like infusing a person's cells with sugar as an "antifreeze". Or we could replace the cells altogether, with nanotechnology or artificial organs. We could also upload people's minds and store them digitally, then load them into new bodies when they reach the destination.
The bottom line is that putting people to sleep for centuries involves massive challenges that we have no real solution for right now. But at least it's not actually impossible according to all the laws of physics.
Top: Ripley in stasis, from Alien (20th Century Fox)