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On June 5th, 1966, 200 kilometers above Australia, Gene Cernan was attempting to activate one of the most ill-conceived, badly designed, and dangerous pieces of equipment ever flown in space.

The Astronaut Maneuvering Unit (AMU) was a James Bond-style rocket-powered backpack. To put the AMU on, Cernan had to leave his Gemini capsule, spacewalk around to the rear section, unfold struts and arms, and activate a complex series of fuel and oxygen valves — most out of sight in the near-darkness — before strapping himself in.

“The spacewalk from hell.”

He was issued woven chain mail trousers for protection against the rocket’s exhaust.

“It was a real rocket, powered by hydrogen peroxide,” says Cernan. “They had one of these rocket nozzles coming right up between my legs — that should have raised some questions to start with.”

Breathing heavily, with his suit overheating and visor fogging up, the astronaut struggled to get the contraption to work. He later termed the experience “the spacewalk from hell.”

Painful squeeze

“You’re handicapped out there in a pressure suit that doesn’t give you much mobility, traveling around the world at 18,000mph (28,800km/h) and can’t see,” he says. “Needless to say, that got my attention.”

Fortunately for Cernan, preparing the AMU was so onerous that the procedure was aborted by mission control before the astronaut could switch the machine on. He eventually made it back into the spacecraft although, thanks to his cumbersome metal trousers, he had to painfully squeeze himself down into the seat to shut the hatch.

This was by no means the only near-miss during the short-lived Gemini space program. From malfunctioning thrusters that caused violent spinning, to docking modules that failed to deploy and ejector seats that shredded test dummies, the Gemini missions saw more than their fair share of mishaps.

All the more surprise then that the crews made it back to Earth safely. Cernan would return to space twice in Apollo and go down in history as the last man on the Moon.

The Gemini missions saw more than their fair share of mishaps

The 12 (10 of them crewed) Gemini missions were carried out over a period of just two and a half years — between NASA’s pioneering Mercury flights and the Moon landings — to test the technology to deliver people to the lunar surface and return them safely to Earth.

"Every flight was a challenge"

Gemini demonstrated spacewalks, how to dock two spacecraft, and the challenges of living in orbit for long periods of time. The flights tested space food, life support, and fuel cells. Together they rank among the most exciting seat-of-the-pants missions ever undertaken.

“Every flight was a new endeavor, every flight was a challenge,” says Cernan. “I thought my flight was a failure but it turns out we learned a great deal that allowed us to move forward and eventually be very successful at walking on the Moon.”

Designed to carry two astronauts into orbit, Gemini looks unlike any other spacecraft before or since. Inside the cone-shaped capsule are two seats, an instrument panel and central control console, much like the layout of a car. Above each seat is a hatch, which astronauts could open to leave the craft for spacewalks.

In fact the best way to appreciate what a Gemini mission was like is to put on a spacesuit and lock yourself in the front of your car with a work colleague for up to two weeks. There was no toilet, so you will need some plastic bags.

Fighter pilot's spacecraft

“It’s a very small space,” says Ed Stewart, director of exhibits and curation at the Space and Rocket Center in Huntsville, Alabama as we stand in front of one of the capsules. “You could equate it to the driver and passenger seat of a VW Beatle.”

The museum’s Gemini capsule is one of the original mission simulators. Every Gemini astronaut spent hours in this module to practice routines and drill procedures.

“They referred to this as the fighter pilot’s spacecraft,” says Stewart. “The way the seats and windows were oriented made it much more like an aircraft than the previous Mercury capsules.”

This was, after all, a spacecraft designed to be flown. Fitted with an electronic guidance computer (boasting some 12 kilobytes of memory and interchangeable programs on magnetic tape), multiple thrusters, and radar, one of the primary missions of Gemini was to prove that two spacecraft could rendezvous and dock in orbit. It’s a feat that involves complex calculations, precise navigation, and accurate positioning. Without this capability, Apollo was a non-starter.

“Movements on previous spacecraft, such as Mercury and Soviet spacecraft, were predetermined and not precise,” explains Stewart. “With Gemini, they required complete control of the vehicle, and this hadn’t been done before.”

Santa "sighting"

The first attempt at orbital rendezvous was supposed to take place between Gemini 5 and an uncrewed probe. This had to be abandoned after the spacecraft thrusters failed. The next, planned for Gemini 6, was scrubbed when the target-docking spacecraft exploded shortly after launch.

However, because the procedure was so important, NASA went ahead with the launch of Gemini 7 crewed by Frank Borman and Jim Lovell. Eleven days later on December 15th, 1965, Gemini 6 launched with Wally Schirra and Tom Stafford on a mission to intercept. Within six hours, the two spacecraft were flying in formation within 120 feet (36m) of each other.

As well as its unprecedented technical achievements, the mission is noted for the “Beat Army” card that US Naval Officer Schirra held at the window of his spacecraft for the benefit of Army man Borman in the adjacent capsule. The astronauts also celebrated a sighting of Santa Claus with a rendition of “Jingle Bells” on a harmonica. Not the greatest achievement of Gemini, perhaps, but another space first.

Fifty years ago this March, the first Gemini mission was launched. Four and a half years later, men were walking on the Moon.

Fifty years ago, the first Gemini mission was launched. Four and a half years later, men were walking on the Moon

“It was 1961 that Kennedy challenged us to go the Moon in a decade,” recalls Cernan. “We didn’t know beans about going to the Moon — we had to develop all that technology and answer all those questions we didn’t even have the questions for — but we did it.”

Gemini made the Moon, Shuttle, and life on the International Space Station possible and deserves more than a footnote in space history.

Stewart has nothing but admiration for the astronauts who flew Gemini into the unknown. “They were brave, tolerant, and brilliant,” he says. “Because it does take a special perspective on life to be able to do this kind of thing.”

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It’s been just over 45 years since the Apollo Moon landings, and some would have it that we are failing to build big anymore; that we’ve since become too fascinated with the small, too impressed by our tablet computers, games consoles, and smartphones that we don’t invest in grand, world-changing engineering projects.

Stand on the bridge of a container ship docked in a mega-port in Korea, however, and it’s clear that’s just not true. The global supply chain that brings us those tablets and phones, and pretty much everything else from our clothes and food to our toys and souvenirs, is nothing short of a moon shot itself — a vast, unprecedented engineering solution to a truly astronomical logistics problem. The fact that it’s hidden from most people’s sight, and that it has become so utterly reliable and efficient to the point of transparency, doesn’t make it any less of an achievement of human technical endeavor.

The fact that it’s hidden from most people’s sight doesn’t make it any less of an achievement of human technical endeavor

To find out more about this huge, invisible network, I accompanied a group of architects and designers called the Unknown Fields Division for a rare voyage on a container ship between Korea and China. The aim of the trip was to follow the supply chain back to some of the remotest parts of China and the source of our consumer goods — and what we saw as we travelled through mega-ports and across oceans looked closer to science fiction than reality.

Early rise

We’re picked up at 9AM from our guesthouse in the Korean city of Busan by a local “ground agent” for the shipping company Maersk, whose ship will be carrying us for the next week. They have at least one of these personnel handlers in every major port in the world, their job being to ensure crew members make their way through each country’s unique and complex maze of customs and immigration bureaucracy, and onto their ships on time.

If you were asked to name some multi-national corporate brands you could probably reel off half a dozen, from Apple to Coca-Cola, but chances are that Maersk wouldn’t spring to mind. Yet the Danish shipping giant is the very definition of a multi-national corporation, with over 25,000 employees, 345 offices in 125 countries, 600 active ships, and more than 2 million containers moved every year. The company is estimated to be responsible for 20 percent of Denmark’s GDP on its own. Maersk might not make any of the things you buy in shops, but it more than likely put a lot of them there.

As we drive along, Busan’s dense mass of high-rise apartment blocks gives way to what will be one of the defining images of the next seven days; the giant cranes that line every major port in the world. Soon we’re into the depths of Busan New Port itself, and speeding past endless, towering stacks of shipping containers until we’re finally dwarfed by the huge, blue mass of the Maersk Seletar, the 320m-long (1,050ft), 80,000-ton, 9,000-container capacity ship that will be my home for the next seven days.

It’s not until we get out on to the towering balconies around the ship’s bridge and look back at Busan that we’re fully able to first comprehend the scale and nature of these Asian mega-ports. It feels like we’re being given a rare look into a usually hidden space, a peek at the intricate but city-scaled machinery of global capitalism.

From that viewpoint — essentially high above the sea, looking into land — it’s easiest to describe the ports as a sequence of layers. First, towering above and over the ship, are the loading cranes. Vast structures mounted on huge, four-legged frames, they resemble the naked scaffolding of unbuilt skyscrapers, and trigger nostalgic reminders of Saturn V rocket launch towers from the 1960s. Their sheer size makes them the first thing you see when you arrive at any port — whether from land or sea, and as staggering as they are they don’t make their full impact until you see them move.

Future gaze

Built on tracks in the surface of the harbor side, they slide left and right, parallel to the berthed ships, accompanied by a cacophony of warning sounds and robotic safety announcements. Once in port at night I saw one suddenly fire into life next to the ship in a stroboscopic explosion of lights, before it tracked slowly above my high vantage point, bathing me in the orange glow of a dozen small halogen suns. It was an intense experience.

The second layer in from the cranes are the trucks; a constant loop of circling, diesel-belching flatbeds. Endlessly, they arrive at the bottom of the vast cranes, seemingly one every minute when the port is at its busiest, stopping in precise locations so the crane drivers can either pluck their containers off their backs and on to the ships, or drop a freshly unloaded container straight on to them, before the trucks instantly pull away and head inland.

It’s a hypnotic, fascinating dance to watch: the cranes lifting containers off the ships, the trucks pulling up in time to catch them as they are elegantly lowered down on steel cables. The complex and precise orchestration behind every move is almost bewildering to comprehend. The ships never unload everything at just one port — that’d be hugely inefficient for these vast, globe-orbiting warehouses — so the crane drivers need to know which one to take off and when, just as the truck drivers need to know where to take each one they collect.

It’s the kind of logistical information that it’s hard to imagine any one human mind comprehending, and the truth is no single one does — this is distributed knowledge, managed by Maersk’s vast world-spanning computer network and shaped and interpreted by complex, similarly unknowable, algorithms. In a very real sense, the crane and truck drivers are little more than elements in a vast robotic system, receiving instructions in their cabs from their computerized managers, following orders on endless cycles until their shift ends.

The crane and truck drivers are little more than elements in a vast robotic system, receiving instructions in their cabs from their computerized managers

Not that there isn’t a certain amount of pride in their work, as regimented and alienating as it might seem — it’s not unusual to see the cranes decorated with awards and badges announcing record breaking container shifting performances. At the same time it’s also impossible not to be struck by the precariousness of their job security; with so much managed by the network it must surely only be a matter of time before the system evolves enough to remove the human element entirely. In fact, ports like Rotterdam in the Netherlands have already moved to fully automated systems, with driverless trucks and robotic cranes.

Stacked neighbors

Layer three, sat behind the cranes and trucks, is the most easily recognizable image from the whole process: the container stacks. From the bridge of the Seletar they look like row upon row of repeating, multicolored Lego bricks, six units high, each straddled by multiple cranes — miniature versions of the ship loaders, sliding back and forth on rails. These cranes continually shuffle the position of the containers within the stacks themselves, following the algorithmic wisdom of the network to ensure everything is in the most efficient position possible; cargo heading backwards and forwards, inland and out.

At every port we arrived at, the three layers — cranes, trucks, containers — seemed fundamentally the same, standardized with only the occasional exception. The Taiwanese port of Kaohsiung uses small, fast moving, mini cranes instead of trucks — oddly top-heavy looking skeletal machines that bounce along on huge moon-buggy tires. They swarm endlessly like busy robots, their blue or yellow frames smeared in grease and grime, discharging black clouds of diesel smog. It’s only layer four that displays any individualization — the landscapes beyond the ports. In Busan it was housing blocks branded with corporation logos nestling beneath green mountains. And at Shanghai’s Yangshan Deep Water Port it was the six-lane, 20-mile-long Donghai Bridge, built to carry the constant flow of container trucks in and out from the port, built on a vast artificial island.

Dark skies

After three days at sea, we reach the city of Ningbo. Arriving into port at night we are presented with an almost nightmare vision, a Blade Runner-like landscape of glowing lights and smokestacks painting the low cloud ceiling orange. We’d been delayed going in as the black shadow of a ship owned by French giant CMA CGM slid gently past. It towers above ours, with a startling 18,000-container capacity. As soon as we hit port-side the cranes fired into life, the trucks queuing up as though impatient to make up the delayed time.

The next morning, unable to sleep, I rise at 4:30AM and head for the top of the ship again, eager to see the port in daylight. The industrial landscape in front of me is vast and awful: a huge coal burning power station fed directly by incoming bulk carrier ships sits right on the port-side, its towers filling the sky with black, while the landscape behind is filled with refineries, gas storage plants, and tightly huddled together housing blocks. You can not only see the pollution — he vast carbon footprint of this industrial network — but taste it in the air. As much talk and concern as there is in the West about the environmental impact of China’s economic dominance, it’s easy to forget how much of that impact the Chinese people are taking as a direct hit themselves, as much for the West’s benefit as their own.

It’s not just onshore environments that containerization has shaped and transformed. The surfaces of our planet’s oceans — for centuries a space of mystery and myth, of expanse and desolation — have been rationalized and shrunk. Once an enigmatic, awe-inspiring place, the sea has become a zone of efficiency, little more than another channel for the automated supply chain network.

Over the course of seven days there wasn’t a single time when I stood on the bridge of the Seletar, even when deep at sea, that I couldn’t look out and see other container ships, and a brief scan of the horizon with binoculars would usually reveal four or five more. The implications are even more startling when nearing a port, and seeing dozens of vast ships held at anchorage, lined up like trucks in a parking lot. The sea has become dominated by GPS tracking and auto-piloted navigation, where the shipping routes are more than just vague geographical gestures, but instead precise spaces traced out both by computerized charts and physical markers, with deep sea buoys marking lanes like the painted lines on road surfaces.

Human shift

Change the sea and change ships and it’s impossible to not also change seafarers, although perhaps the impact containerization has had on ship crews is a little more subtle. Certainly, like the crane and truck drivers back at the port, there’s a certain sense of alienation from the cargo. Nobody on the ship knows what lies inside the containers the ship carries.

There are exceptions of course; hazardous materials must be declared, as must the contents of the refrigerated containers — known in the industry as reefers. The reefers themselves are fascinating pieces of technology, basic containers outfitted to be advanced, climate-controlled, computer-monitored micro-environments. Checking the reefers are running is the crew’s only tangible, direct responsibility for the cargo beyond ensuring it arrives on time. Even so, it is still highly computer-controlled; the ship’s captain, Brian Argent, tells me that the crew’s emails piggy-back off a satellite uplink designed keep an eye on the reefers, transmitting status updates thousands of miles back to land so that the company’s computers know about a problem before the ship’s crew does.

The reefers themselves are fascinating pieces of technology, basic containers outfitted to be advanced, climate-controlled, computer-monitored micro-environments

And that’s not all the ship receives emails about: Maersk regularly sends Argent and his senior officers messages informing him of course changes or of what speed to take. Even the captain has become just another node in the network, the running of his ship dictated by the unseen algorithms.

The myth of the seafarer as the drunken adventurer with “a girl in every port” couldn’t be further from the truth, with shore leave a rarity, and ports so far from urban centers that day leave excursions become little more than trips to out-of-town shopping malls to stock up on essentials such as shampoo, shower gel, and snacks. But for most of the Seletar’s crew — made up of mainly Indian, Filipino, and Chinese seafarers — the reasons for going to sea are the same as they ever were: to make money that can be sent home to their wives, children, and parents.

And even the sense of distance from their families shrunk, thanks to the Seletar’s satellite internet access. It’s slow and patchy, but it’s enough for the crew to maintain daily communication with home — with most of them spending a sizable chunk of their free time crouching in corridors and stairwells with smartphones and laptops, trying to find the elusive Wi-Fi signal so they can talk to home, send messages to loved ones, and connect to Facebook.

It’s a fascinating sight at first, and within a few days I find myself doing the exact same thing. I’d initially thought it might be fun — healthy even — to truly escape the internet and life back home for a few days. That was before I realized what the Seletar really was, saw how it was just another node in the network, another rationalized point in the global infrastructure, a bridge between the physical and digital. Once I saw that, I knew it was time to abandon my romantic notions of being away at sea, that it was pointless to resist the lure of the network, and found myself squatting in the stairwell between Decks C and D, trying to get reconnected.

This article was originally published on BBCFuture.

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Get in your car and drive. If there’s one thing that cinema and Bruce Springsteen songs have taught us, it’s that there’s nothing freer than the open road. Or is there? Although it might feel that way, behind the scenes a huge array of technologies will end up controlling the progress you make along your route. The same goes for train and boat travel. And should you head to the airport, you will fly subject to the demands of air traffic control, monitoring other aircraft and the ever-changing weather. Welcome to the hidden world that controls our transport infrastructure, and it’s getting more sophisticated all the time.

Behind the scenes a huge array of technologies will end up controlling the progress you make along your route

Some of the subtle transport management techniques seem to be the stuff of urban myths — but they’re very real. Take, for example, the “green wave.” A green wave is when all the traffic lights into town conveniently turn green as traffic approaches, opening up an unhindered route to motorists.

Synchronized lights

It was a popular concept in Germany, where motorists were sometimes told that traveling at a certain speed would let them encounter consecutive green signals — a Grüne Welle. Today, green waves have become more sophisticated, with adaptive systems able to synchronize traffic lights when it is most advantageous and safe to do so. And it doesn’t just stop at cars. In the Danish capital Copenhagen, the city has introduced green waves for its hordes of cyclists.

Green waves have become more sophisticated, with adaptive systems able to synchronize traffic lights when it is most advantageous and safe to do so

More accurate tracking of vehicles in real time is generally what makes this possible. In the UK, sensors hidden beneath roads such as Midas (Motorway Incident Detection and Automatic Signaling) allow traffic control rooms to monitor how congested a motorway has become. Midas detects the volume of vehicles passing over the tarmac in real time. At high volumes, traffic can be slowed down via variable speed limit signs, which allow cars to travel more closely together and therefore take advantage of the motorway’s full capacity. This is often done “upstream” to prevent extreme cases of congestion which may already be forming. That’s why seemingly smooth-moving traffic is sometimes gently forced to slow down, because that reduction in speed helps prevent congestion further behind.

In towns and cities, a similar and widely used technology called Scoot (Split Level Offset Optimization Technique) can tweak traffic signals for particular types of vehicles, such as buses, or those which are arriving along especially busy routes.

Nick Hounsell at the University of Southampton, an expert in transport research, says the dynamics of traffic movement are incredibly sensitive, which is why these clever, pseudo-intelligent systems are so useful. Hounsell points out one example, the “butterfly effect,” where a sudden slowing down — even something as simple as one driver braking a little too hard — can become amplified to a full-scale jam and even accidents happening further behind.

River register

“Once a shockwave forms you will get stationary traffic,” he says. “The shockwave moves and the potential then for a nose-to-tail accident is quite high.”

One classic cause of these shockwaves is “rubbernecking,” in which drivers slow down to look at accidents in nearby lanes. Easy solutions for this have been found, though. Britain’s Department of Transport simply decided to hide them from view, using screens.

Ideally, managing traffic using all these techniques would help to reduce crashes and fatalities in the first place. Paul Unwin at the UK’s Highways Agency notes that since the introduction of a smart motorway system on one of the country’s busiest routes, safety has improved considerably. “We’ve not had a fatality on the M42 since 2006,” he claims.

Similar principles to these are found off the roads. The Port of London Authority (PLA), for example, has found that it makes sense to slow river traffic on the Thames so that vessels further up the river may turn sharply or berth. The PLA tracks which boats are where and what’s on them — be it passengers or cargo — using a register known as Thames AIS, which stands for Automatic Identification System.

It’s particularly useful on a river like the Thames, which has both a lot of traffic and a lot of bends, making it hard for captains to see what’s ahead and react accordingly. Having a complete overview of traffic is probably what you’d expect nowadays, but as recently as 1989 controllers had nowhere near this level of visibility. That year a Thames pleasure boat, the Marchioness, collided with a dredger and sank, resulting in the deaths of 51 people.

“All the investigations that followed changed forever the way we deal with safety and navigation on the Thames,” explains Kevin Gregory at the PLA. “Because we were determined that we would never have something like that happen again.”

Up in the air

Safety concerns are also behind the fastidious traffic control we see in the air. Tens of thousands of flights take off and land every day around the world and it’s largely down to air traffic controllers to get them up and down again without accidents. Nats, which used to be known as National Air Traffic Services, handles about 5,000 flights a day in the UK alone. It has recently been trying out its own clever technologies to help increase capacity and safety. A recent addition to the air traffic controller’s toolbox is iFACTS, which predicts aircraft trajectories and lets operators peer up to 18 minutes into the future to see where planes will be at that time.

A “conflict” in air traffic control is when a plane is predicted to move within a certain distance of another aircraft. iFACTS allows controllers to prevent this ahead of time, and more confidently schedule planes through sectors of airspace. “We had a 20 percent increase in overall capacity based on the introduction of iFACTS because the workloads on the controllers was that much less,” says Stuart McBride, customer and network service manager at Nats. This happened because controllers didn’t have to take the time to make such drastic adjustments to a plane’s course when conflict was getting close. Instead, minor adjustments which are easier to do could be made further ahead of time.

Another company, Airbus ProSky, has a product that calculates optimum arrival times for planes so that connections aren’t missed and less fuel is used. In order to do that, the software has to calculate data on all the flights coming to and leaving from a given airport. The company’s CEO, Paul-Franck Bijou, says the amount of information going into these calculations is gargantuan.

“For one airport, it’s about the size of the database for a bank like Credit Agricole in France for example, the size of a national bank,” he says. “This is what we’re talking about with regard to data we’re handling at just one airport. Imagine what we’re talking about for a whole country.”

Natural solutions?

That’s the thing about traffic; the greater the volume, the sooner capacities are reached. But capacity can become flexible if vehicles and the systems which control their movements become smarter, and don’t need to be constantly controlled by us.

That’s the thing about traffic; the greater the volume, the sooner capacities are reached

In the future, the way all of this is done could look quite different from today. Researchers are increasingly looking to the animal kingdom for inspiration. Ants, for instance, have helped researchers create “bio-inspired algorithms,” which allow traffic to move in a similar way ants hunting for food; the insects automatically find preferred routes, without any central control.

This approach allowed J.J. Merelo at the University of Granada to design a program that could tell the Spanish military how to route vehicles during a training simulation.

Bats, too, could prove useful teachers to help manage tomorrow’s air traffic. Rolf Mueller at Virginia Tech’s Department of Mechanical Engineering has been studying Chinese bats in order to better understand how the animals are able to fly in such dense swarms even in tight areas. A 3D scan of their cave was taken and an array of infrared cameras captured the bats’ movement. One day, Mueller believes a system similar to this could manage aircraft in busy pockets of airspace.

“The bats are not afraid of touching each other. They come very close but in a very controlled manner, they know what they’re doing,” he says. “They don’t have to get in touch with air traffic flow control and say, ‘Should I fly now to the right of that guy or the left of that guy?’ They make those decisions automatically.”

Mueller observed that this remained true even when his team placed unfamiliar obstacles in the cave. Unflinching, the bats simply flew around them and carried on. What if planes were just as safe to fly in? Nats’ Stuart McBride says capacity could be increased if planes didn’t have to be so far apart from one another.

Man-made traffic has often reached a point where behind-the-scenes control is not just essential, but also increasingly automated

In all these areas — road, sea, or air — man-made traffic has often reached a point where behind-the-scenes control is not just essential, but also increasingly automated. It means that the traveller is ever further away from knowing why the traffic around him or her flows the way it does. The sense of being a cog in a distantly operated machine may be disarming to some, but it’s testament to the sophistication of systems that keep the world’s traffic moving. The “driver,” simply, has become split up. He or she is part human, part vehicle, part road, and computer.

This article was originally published on BBC Future.

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Here we go again. The movie Predestination, released in the UK this February, is the very latest in the long history of time travel films. There’s been more than 100 since the Terminator and Back to the Future franchises began 30 years ago — all of which are science fiction that have little to do with science facts.

Like many of them, Predestination has its own engaging twist: Ethan Hawke plays a time agent who nips back through history to snuff out crimes before they can be committed. Like all of them it’s chronologically illogical: the split second a movie has someone monkeying with time you are doomed — predestined, you might say — to abandon the science and enter a plea of temporal insanity.

Things very quickly get very brain boggling. Consider this — if someone builds a time machine, what’s to prevent them going back one minute and smashing it up before they first use it? Which means it never gets used — so how did it get smashed up? What prevents the whole parade of paradoxes thrown up by time travel to the past — becoming your own grandfather, killing Hitler before he starts World War II and so on — is that they trash big laws of physics. And the Universe, as far as we understand it, likes to play by the rules.

If someone builds a time machine what’s to prevent them going back one minute and smashing it up before they first use it?

One of the fundamentals that underpins not just physics but every aspect of existence is the law of cause and effect, always in that order. Changing the past would violate that: your actions would affect what caused you to go back in the first place — so if you did manage to kill Hitler he wouldn’t have done what led you to go back and kill him.

That doesn’t stop filmmakers from exploring the consequences if you could somehow drop in on history. For Hollywood, applause and special effects are more important than cause and effect, and time travel offers unlimited opportunities to push both imagination and CGI to their limits. Hence screen time machines have included a police box (Dr Who), a phone booth (Bill & Ted’s Excellent Adventure), a DeLorean sports car (Back to the Future) and a big nudes-only energy ball (Terminator).

Wormhole loophole

Unlike other favorite science fiction themes such as smarter-than-human robots or interstellar travel or encounters with aliens — all of which are somewhere on a scale of theoretically possible to eventually probable — time travel to the past is now and forever a complete scientific no-no.

Well, almost complete. There is a loophole. A tiny loophole called a wormhole. Stephen Hawking is one of many respected scientists who now believe that our entire Universe is permeated by wormholes, effectively shortcuts through time and space. While entirely compatible with Einstein’s theory of relativity and most other current big ideas about the nature of reality, they open up the possibility not only of journeys in time — popping in one end of a wormhole and popping out days, years or centuries earlier — but also of instantly linking distant parts of the cosmos, effectively enabling faster-than-light travel. No wonder the idea of wormholes is so often borrowed by a whole slew of so-called sci-fi (including Star Trek, Stargate, The Avengers and Interstellar).

There is a loophole. A tiny loophole called a wormhole

Just one word of warning before anyone attempts to build a spaceship and steer for the nearest wormhole: they may be real, they may be common and they may bridge time and space, but having a wormhole is one thing, having one you can use is quite another. Even Professor Hawking, who admits to being “obsessed by time” and to wanting to believe time travel is possible, points out that wormholes are only thought to exist in the “quantum foam” right down below even the scale of atoms. That’s way too small to squeeze a spaceship through. Or Arnold Schwarzenegger. Or even Michael J. Fox.

There are those who argue that given enough technology, theoretical physicists, and, er, time, we might eventually develop a way to snare some of these infinitesimally tiny wormholes and then make them billions of times bigger so we could go where and when we wanted. It’s all colossally speculative at the moment, but just supposing such a traversable wormhole were someday constructed, and you carefully avoided all deliberate interference with the past, you would still run smack into another prohibitive paradox.

Beware the Butterfly Effect

It’s one nicely illustrated by Ray Bradbury’s classic early ’50s short story A Sound of Thunder, where time travelers to Earth’s prehistoric past are kept on a levitating walkway to minimize their chance of any contact with the past. Someone falls off and accidentally crushes a single butterfly. When they return to their present, all sorts of things, from spellings of words to the outcome of elections, are different, and they have created an alternative reality.

Bradbury’s story is the first incarnation of the “Butterfly Effect” often evoked in chaos theory: the idea that one tiny change now can result in all manner of large and often unforeseeable changes later. And that’s a real obstacle for going back in time. If anyone could overcome the enormous challenge of how to do it, they would still face the equally great challenge of how to do it without risk of affecting the past in the slightest. Alter one thing, and you could alter everything and end up rewriting reality.

Again, there are some who are bending their minds to ways round these restrictions: there are all sorts of theories involving configurations of multiple wormholes and ‘closed timelike curves’ and other elaborate alternatives. But sadly for those who like their film fantasies grounded in reality there could be one simple reason why all these problems and paradoxes appear insurmountable: they just are. Much as we love the idea of changing the past and erasing our mistakes — I’ve some deep regrets about a coat I bought in 2007 — it seems nothing, including wormholes, can take us back there.

A fraction younger

Traveling into the future, however, is not necessarily impossible. In fact, there are those who have already done it. The greatest of them is Sergei Krikalev, a cosmonaut now also a chrononaut by virtue of having spent so long in space that it’s been calculated he’s traveled into his own future by about one-two hundredths of a second.

Traveling into the future, however, is not necessarily impossible. In fact, there are those who have already done it

Okay, it’s not much. But it’s still enough to be tricky to get your head round. It’s all down to time dilation, something predicted by Einstein’s Theory of Relativity but which we can measure, whereby the faster someone goes (and Sergei spent over two years in orbit on Mir and the International Space Station traveling at over 17,000mph) the slower their clock goes relative to those back down on Earth. It’s more complicated than this because gravity is also involved, but Sergei has aged fractionally less than he would have if he’d not gone into space.

Start cranking up the velocity and the effects become much more pronounced: if chrononaut Krikalev had spent his two years in space traveling at a smidge below the speed of light (almost 40,000 times faster than he was orbiting at) he would have returned to find two centuries or more had passed on Earth.

That’s proper time travel. Of course it might be unfeasible to ever get up to such speeds and the trips are strictly one way, but unlike plunging into history we at least know it is possible. So while films about going back in time are pure fantasy, ones where people end up in the future have a smidge of science amongst the fiction. Which makes it a pity there are so few of them.

The two versions of HG Wells’ The Time Machine, the Back to the Future trilogy, and Planet of the Apes are among the few that at least involve some travel into the future. The only film I’m aware of to attempt to portray the realities of time travel is last year’s Interstellar. It draws on time dilation and the “Rip Van Winkle” experience for returning astronauts of finding themselves decades out of phase with the people they left behind.

Perhaps it’s the start of a move toward factual accuracy in time travel films. But I doubt it. Is it too much to wish for more movies that don’t take it so seriously, and have fun with the ludicrousness of leaping around in history? Films like Bill & Ted’s Excellent Adventure with its paradox-popping ending. Now there’s a film I can watch time and time again.

This article was first published on BBC Future.

Correction: the original version of this article stated that Predestination was released “this week,” it was released in February.