About 10 months ago, Elon Musk, CEO of Tesla Motors and Space X, asked some engineers from both of these companies to help him invent a new form of transportation. Today he unveiled the design for what he calls the “hyperloop,” which would convey passengers from San Francisco to LA in about half an hour. It involves propelling 28-passenger “pods” through a tube at speeds of up to 760 miles per hour. The pods would be propelled by something called a linear motor, and they would ride on a cushion of air to minimize friction.

“It’s far-fetched,” says John Hansman, a professor of aeronautics and astronautics at MIT. “But Musk is a smart guy, so some of the things he’s doing make reasonable sense.”

Musk is proposing the system as an alternative to the high-speed rail system planned in California. He’s no expert on high-speed mass transit, but he’s gained prominence through a series of successes with electric cars and private space flight. He says that at $6 billion, his hyperloop would cost a tenth as much as the planned California project, and be much faster and safer. His cost estimate is a guess, of course, since the concept hasn’t even been demonstrated at a small scale.

“I don’t see anything that violates fundamental laws of physics,” Hansman says. But he says Musk’s cost estimates are too optimistic. “It would be enormously expensive. And I think there are a huge number of technical challenges with the vehicle,” he says. One of the biggest questions is how much energy the system will require. “My questions aren’t could you do it, but could you do it in a way that makes sense from an energy efficiency standpoint and makes sense from an economic standpoint,” he says.

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