Many questions surround Elon Musk's proposal for a super-high-speed Hyperloop transportation system that would use air cushioning to send passengers through a tube between San Francisco and Los Angeles in just 30 minutes.
The biggest question is whether it will ever get built. There are lots of opinions on that one, but no definite answer yet.
One question that it appears can be answered now, however, is being asked by plenty of people outside the scientific community: "Wouldn't the people inside the capsule traveling through the tube at hundreds of miles an hour get crushed by high G-forces?"
The answer is no, and the explanation involves relatively simple physics.
According to John Lindner, professor of physics at The College of Wooster in Ohio, "If Hyperloop works as planned, for most of the trip the passengers would move at constant velocity in a sealed container, so it would feel like riding a plane or train."
That's because G-forces, the sensation of pressing weight that's familiar to anyone who has taken a roller coaster ride, are the result of acceleration, not just speed.
The system proposed by Musk in a 57-page document released this week is specifically designed to link cities that are less than 900 miles apart. (Musk says that it would probably be faster and cheaper to use supersonic planes to cover longer distances.)
The proposal is not, as some had hopefully speculatedbefore the plan was released, a way to cover the 2,400 miles between New York and Los Angeles in 45 minutes. Such a trip would require speeds in the thousands of miles per hour, making acceleration more of a consideration.
At a top speed of 760 miles an hour (for aerodynamic considerations), Musk plans on relatively brief periods of acceleration limited to 0.5g—half of the force one feels from gravity in a normal situation—on passengers, interspersed by much longer periods of coasting.
By point of comparison, someone in a car going from 0 to 60 miles per hour in 6.4 seconds feels a G-force just below 0.5g. Typically, passengers in a commercial aircraft will sometimes experience G-forces up to around 1.5g at the most.
Where local geography requires significant bends in the tube, the speed of the capsule would be reduced to 300 miles an hour to prevent inertial accelerations (the pressure felt inside a vehicle in a turn) from exceeding 0.5g, since "this is deemed the maximum inertial acceleration that can be comfortably sustained by humans for short periods." Musk notes that careful planning would be needed to optimize the route to avoid sharp bends.
"To further reduce the inertial acceleration experienced by passengers," Musk said this week, "the capsule and/or tube will incorporate a mechanism that will allow a degree of 'banking.' "
He also envisions seats that "conform well to the body to maintain comfort during the high-speed accelerations experienced during travel."
(Satirical website The Onion isn't so sure. Its headline this week is "New Super-Fast Transport System Powered By Passengers' Screams.")
While getting crushed isn't a major problem, Musk's Hyperloop does present some technical challenges.
Sam Jaffe, writing in a Navigant Research blog post cited by USA Today, notes that the "pod will be compressing air and expelling it downwards and backwards. All that air compression creates an enormous amount of heat, which can damage the pod and its machinery." Jaffe isn't sure Musk's water-cooling solution will be enough to handle the heat. He also has concerns about wind stress.