LOS ANGELES — Imagine strapping into a car-sized capsule and hurtling through a tube at more than 700 miles per hour — not for the thrill of it, but to get where you need to go.
On Monday, billionaire entrepreneur Elon Musk unveiled a transportation concept that he said could whisk passengers the nearly 400 miles between Los Angeles and San Francisco in 30 minutes — half the time it takes an airplane.
If it’s ever built.
His ‘‘Hyperloop’’ system for travel between major cities is akin to the pneumatic tubes that transport capsules stuffed with paperwork in older buildings.
In this case, the cargo would be people, reclining for a ride that would start with a force of acceleration like an airplane but then be turbulence free.
Capsules would catapult through a large, nearly air-free tube. Inside, they would be pulled down the line by magnetic attraction.
Each capsule would float on a cushion of air it creates — like an air hockey table in which the puck produces the air instead of the surface. To minimize friction, a powerful fan at the front would suck what air is in the tube to the rear.
‘‘Short of figuring out real teleportation, which would of course be awesome (someone please do this), the only option for super fast travel is to build a tube over or under the ground that contains a special environment,’’ Musk wrote in his proposal, posted online. and the rocket-building company SpaceX.
Capsules could depart every 30 seconds, carrying 28 people, with a projected cost of about $20 each way, according to Musk’s plan, which was posted online at www.spacex.com/hyperloop.
On a conference call Monday, Musk, the cofounder of PayPal, Tesla Motors, and SpaceX, said that if all goes right, it could take seven to 10 years for the first passengers to make the journey between California’s two biggest metro areas. He put the price tag at around $6 billion — pointedly mentioning that’s about one-tenth the projected cost of a high-speed rail system that California has been planning to build.
In a written statement, California High-Speed Rail Authority chairman Dan Richard suggested that Musk was oversimplifying the challenges.
‘‘If and when Mr. Musk pursues his Hyperloop technology, we’ll be happy to share our experience about what it really takes to build a project in California, across seismic zones, minimizing impacts on farms, businesses, and communities and protecting sensitive environmental areas and species,’’ Richard said.
Like the bullet train, the Hyperloop didn’t take long to attract skepticism.
Musk had framed his concept as a fifth way — an alternative to cars, planes, trains, and boats. Citing barriers such as cost and the mountains that rim the Central Valley, one transportation expert called Musk’s idea novel, but not a breakthrough.