ELON MUSK has made a career out of confounding the experts. When he promised to slash the cost of spaceflight he was told it could not be done; nowadays SpaceX, his rocketry firm, charges around half what its competitors do and has a bulging order book. With Tesla Motors he has helped transform electric cars from limp-but-worthy vehicles driven by muesli-knitters into the objects of consumer desire that they must become if they are to displace fossil-fuel cars. So when on August 12th the South-African born tech squillionaire announced a super-high-speed transport system called Hyperloop that would put California’s planned high-speed rail project in the shade, costing $6 billion instead of $60 billion and taking just half an hour to travel between Los Angeles and San Francisco, it generated plenty of buzz. Much of the commentary has been incredulous, arguing that the costs are impossibly low (a few have accused Mr Musk of acting in bad faith). But never mind the bean-counting, what about the technology? How exactly would the Hyperloop work, and is it at least theoretically plausible? The Hyperloop is designed to overcome the biggest single obstacle to high-speed travel: air. At walking speed, air is ephemeral stuff. But, as any child who has stuck his hand out of a car window at speed knows, the faster you go the more obvious its effects become. In fact, the grunt needed to counteract air resistance rises with the cube of speed. That is why a Citroen C1 with a mere 50kW under the bonnet is good for 158kph (98mph), but a Bugatti Veyron requires a whopping 763kW to go a bit more than two and a half times as fast. It is also why the all-time human speed record is held by astronauts: in space, there is no air to slow you down. For over a century engineers and science fiction authors have dreamed of running trains through their own little chunk of outer space, by sending them through tubes from which all the air has been evacuated. That would allow them to reach enormous speeds: one study from America’s RAND Corporation in the 1970s suggested a trans-continental underground “vac-train” with a top speed of 22,000kph that would ferry passengers across the United States in 20 minutes.

The trouble is that vacuums are finicky things, and maintaining them is tricky. Holding one in a long train-tunnel would be a serious technical challenge. Hence the Hyperloop’s key difference from many other vac-train proposals: there is no vacuum. Instead, the tunnels would be kept at around a thousandth of the normal atmospheric pressure at sea level. That would be low enough to cut air resistance drastically, but high enough to keep the pneumatics simple—or at least simple compared with maintaining a hard vacuum. It does introduce another problem, though: Hyperloop capsules are designed to sit snugly within their tubes. At high speeds, they would act like a plunger in a syringe, compressing the air ahead of them. That would require large amounts of power to overcome, undoing many of the advantages of a vac-train in the first place. Mr Musk's proposed solution is to fit each pod with a fan designed to blow what little air is present through a pipe in the capsule and out of the back—essentially drilling a hole in the plunger. As a useful side-benefit, some of that airflow could be blown out of the skis on which the pods travel. That would keep them from bumping into the tunnel walls, removing the need for the expensive magnetic-levitation systems that do the same job in most vac-train proposals.

So far, so good? Well, perhaps. Designing something that is plausible on the back of a napkin (or in a 58-page PDF) is one thing; actually turning it into a buildable machine is much harder, even ignoring cost questions. Take passenger comfort: Hyperloop proposes to subject its passengers to fairly severe accelerations as it goes around corners and up hills; from the published numbers those accelerations seem greatly to exceed the rules of thumb used to design ordinary railways. Riding a rollercoaster from San Francisco to Los Angeles may not be a universally popular proposition. The system’s capacity seems relatively low, too: each capsule would hold 28 people and one capsule would leave every 30 seconds. Assuming they were all completely full, that would be 3,360 people an hour—significantly less than a train can manage. And given the streamlined designs of the pods, there appears to be no room for an onboard toilet. Mr Musk has said repeatedly that this is merely the sketch of an idea, that he has no time to pursue it, and that others are welcome to build on it. Or, at least, that was the story: at the conference call following his announcement, he seemed to say that he might be amenable to building a small demonstration version after all. And there is no better way to prove or disprove an idea than to actually try it.